SCHOOL OF BIO AND CHEMICAL ENGINEERING

DEPARTMENT OF BIOTECHNOLOGY

UNIT – I – CELL BIOLOGY – SBB1101

 UNIT:1

CELL THEORY

The microscopes we use today are far more complex than those used in the 1600s by Antony van
Leeuwenhoek, a Dutch shopkeeper who had great skill in crafting lenses. Despite the limitations of
his now-ancient lenses, van Leeuwenhoek observed the movements of protista (a type of single-
celled organism) and sperm, which he collectively termed “animalcules. ”

In a 1665 publication called Micrographia, experimental scientist Robert Hooke coined the term
“cell” for the box-like structures he observed when viewing cork tissue through a lens. In the 1670s,
van Leeuwenhoek discovered bacteria and protozoa. Later advances in lenses, microscope
construction, and staining techniques enabled other scientists to see some components inside cells.

Structure of an Animal Cell: The cell is the basic unit of life and the study of the cell led to the
development of the cell theory.

By the late 1830s, botanist Matthias Schleiden and zoologist Theodor Schwann were studying
tissues and proposed the unified cell theory. The unified cell theory states that: all living things are
composed of one or more cells; the cell is the basic unit of life; and new cells arise from existing
cells. Rudolf Virchow later made important contributions to this theory.

Schleiden and Schwann proposed spontaneous generation as the method for cell origination, but
spontaneous generation (also called abiogenesis) was later disproven. Rudolf Virchow famously
stated “Omnis cellula e cellula”… “All cells only arise from pre-existing cells. “The parts of the
theory that did not have to do with the origin of cells, however, held up to scientific scrutiny and
are widely agreed upon by the scientific community today. The generally accepted portions of the
modern Cell Theory are as follows:

1. The cell is the fundamental unit of structure and function in living things.
2. All organisms are made up of one or more cells.
3. Cells arise from other cells through cellular division.

The expanded version of the cell theory can also include:

 Cells carry genetic material passed to daughter cells during cellular division
 All cells are essentially the same in chemical composition
 Energy flow (metabolism and biochemistry) occurs within cells

What are the 9 basic properties of cell?

1. Cells are Highly Complex and Organized
2. Cells Possess a Genetic Program and the Means to Use it
3. Cells need Organic Compounds to Generate Essential Macromolecules
4. Cells are dependent to Water, making up more than 70% of the cell
5. Cells are Capable of Producing More of Themselves
6. Cells Acquire and Utilize Energy
7. Cells Carry Out a Variety of Chemical Reactions
8. Cells Engage in Mechanical Activities
9. Cells Die very Fast and Regenerate very Fast to Make New Cells
10. Cells are Able to Respond to Stimuli
11. Cells Are Capable of Self-Regulation
12. Cells Evolve

Prokaryotic Cells

All cells share four common components:

1. a plasma membrane: an outer covering that separates the cell’s interior from its surrounding
environment.
2. cytoplasm: a jelly-like cytosol within the cell in which other cellular components are found
3. DNA: the genetic material of the cell
4. ribosomes: where protein synthesis occurs

However, prokaryotes differ from eukaryotic cells in several ways.

A prokaryote is a simple, single-celled (unicellular) organism that lacks an organized nucleus or

any other membrane-bound organelle. We will shortly come to see that this is significantly different
in eukaryotes. Prokaryotic DNA is found in a central part of the cell: the nucleoid.

Most prokaryotes have a peptidoglycan cell wall and many have a polysaccharide capsule. The cell
wall acts as an extra layer of protection, helps the cell maintain its shape, and prevents dehydration.
The capsule enables the cell to attach to surfaces in its environment. Some prokaryotes have
flagella, pili, or fimbriae. Flagella are used for locomotion. Pili are used to exchange genetic
material during a type of reproduction called conjugation. Fimbriae are used by bacteria to attach to
a host cell.

Figure 4.2A.14.2A.1: General

Structure of a Prokaryotic Cell: This figure shows the generalized structure of a prokaryotic
cell.All prokaryotes have chromosomal DNA localized in a nucleoid, ribosomes, a cell membrane,
and a cell wall.The other structures shown are present in some, but not all, bacteria.

Cell Size

At 1 μm to 5 μm in diameter, prokaryotic cells are significantly smaller than eukaryotic cells,

which have diameters ranging from 10 to 100 μm. The small size of prokaryotes allows ions and
organic molecules that enter them to quickly diffuse to other parts of the cell. Similarly, any wastes
produced within a prokaryotic cell can quickly diffuse out. This is not the case in eukaryotic cells,
which have developed different structural adaptations to enhance intracellular transport.
Figure 4.2A.14.2A.1: Microbial Size: This figure shows relative sizes of microbes on a logarithmic
scale (recall that each unit of increase in a logarithmic scale represents a 10-fold increase in the
quantity being measured).

Small size, in general, is necessary for all cells, whether prokaryotic or eukaryotic. Let’s examine
why that is so. First, we’ll consider the area and volume of a typical cell. Not all cells are spherical
in shape, but most tend to approximate a sphere. You may remember from your high school
geometry course that the formula for the surface area of a sphere is 4πr2, while the formula for its
volume is 4/3πr3. Thus, as the radius of a cell increases, its surface area increases as the square of
its radius, but its volume increases as the cube of its radius (much more rapidly). Therefore, as a
cell increases in size, its surface area-to-volume ratio decreases. This same principle would apply if
the cell had the shape of a cube. If the cell grows too large, the plasma membrane will not have
sufficient surface area to support the rate of diffusion required for the increased volume. In other
words, as a cell grows, it becomes less efficient. One way to become more efficient is to divide;
another way is to develop organelles that perform specific tasks. These adaptations led to the
development of more sophisticated cells called eukaryotic cells.

Figure 4.2A.14.2A.1: Cell

Surface Size: Notice that as a cell increases in size, its surface area-to-volume ratio
decreases.When there is insufficient surface area to support a cell’s increasing volume, a cell will
either divide or die.The cell on the left has a volume of 1 mm3 and a surface area of 6 mm2, with a
surface area-to-volume ratio of 6 to 1, whereas the cell on the right has a volume of 8 mm3 and a
surface area of 24 mm2, with a surface area-to-volume ratio of 3 to 1

Eukaryotic Cell Structure

Like a prokaryotic cell, a eukaryotic cell has a plasma membrane, cytoplasm, and ribosomes.
However, unlike prokaryotic cells, eukaryotic cells have:

1. a membrane-bound nucleus
2. numerous membrane-bound organelles (including the endoplasmic reticulum, Golgi
apparatus, chloroplasts, and mitochondria)
3. several rod-shaped chromosomes

Because a eukaryotic cell’s nucleus is surrounded by a membrane, it is often said to have a “true
nucleus. ” Organelles (meaning “little organ”) have specialized cellular roles, just as the organs of
your body have specialized roles. They allow different functions to be compartmentalized in
different areas of the cell.

The Nucleus & Its Structures

Typically, the nucleus is the most prominent organelle in a cell. Eukaryotic cells have a true
nucleus, which means the cell’s DNA is surrounded by a membrane. Therefore, the nucleus houses
the cell’s DNA and directs the synthesis of proteins and ribosomes, the cellular organelles
responsible for protein synthesis. The nuclear envelope is a double-membrane structure that
constitutes the outermost portion of the nucleus. Both the inner and outer membranes of the nuclear
envelope are phospholipid bilayers. The nuclear envelope is punctuated with pores that control the
passage of ions, molecules, and RNA between the nucleoplasm and cytoplasm. The nucleoplasm is
the semi-solid fluid inside the nucleus where we find the chromatin and the nucleolus. Furthermore,
chromosomes are structures within the nucleus that are made up of DNA, the genetic material. In
prokaryotes, DNA is organized into a single circular chromosome. In eukaryotes, chromosomes are
linear structures.
Eukaryotic Nucleus: The nucleus stores chromatin (DNA plus proteins) in a gel-like substance
called the nucleoplasm.The nucleolus is a condensed region of chromatin where ribosome synthesis
occurs.The boundary of the nucleus is called the nuclear envelope.It consists of two phospholipid
bilayers: an outer membrane and an inner membrane.The nuclear membrane is continuous with the
endoplasmic reticulum.Nuclear pores allow substances to enter and exit the nucleus.

Other Membrane-Bound Organelles

Mitochondria are oval-shaped, double membrane organelles that have their own ribosomes and
DNA. These organelles are often called the “energy factories” of a cell because they are responsible
for making adenosine triphosphate (ATP), the cell’s main energy-carrying molecule, by conducting
cellular respiration. The endoplasmic reticulum modifies proteins and synthesizes lipids, while the
golgi apparatus is where the sorting, tagging, packaging, and distribution of lipids and proteins
takes place. Peroxisomes are small, round organelles enclosed by single membranes; they carry out
oxidation reactions that break down fatty acids and amino acids. Peroxisomes also detoxify many
poisons that may enter the body. Vesicles and vacuoles are membrane-bound sacs that function in
storage and transport. Other than the fact that vacuoles are somewhat larger than vesicles, there is a
very subtle distinction between them: the membranes of vesicles can fuse with either the plasma
membrane or other membrane systems within the cell. All of these organelles are found in each and
every eukaryotic cell.

Animal Cells Versus Plant Cells

While all eukaryotic cells contain the aforementioned organelles and structures, there are some
striking differences between animal and plant cells. Animal cells have a centrosome and lysosomes,
whereas plant cells do not. The centrosome is a microtubule-organizing center found near the nuclei
of animal cells while lysosomes take care of the cell’s digestive process.
 Figure 4.3A.14.3A.1: Animal Cells:
Despite their fundamental similarities, there are some striking differences between animal and plant
cells. Animal cells have centrioles, centrosomes, and lysosomes, whereas plant cells do not.

In addition, plant cells have a cell wall, a large central vacuole, chloroplasts, and other specialized
plastids, whereas animal cells do not. The cell wall protects the cell, provides structural support,
and gives shape to the cell while the central vacuole plays a key role in regulating the cell’s
concentration of water in changing environmental conditions. Chloroplasts are the organelles that
carry out photosynthesis.
 Figure 4.3A.14.3A.1: Plant
Cells: Plant cells have a cell wall, chloroplasts, plasmodesmata, and plastids used for storage, and a
large central vacuole, whereas animal cells do not.

CELL CYCLE: PHASES OF CELL CYCLE

 In both unicellular and multicellular eukaryotes, the cell reproduction is a cyclic process of
growth, nuclear division and usually cytoplasmic division called cell cycle.
 Cell cycle is a series of macromolecular events that lead to cell division and the
production of two daughter cells, each containing chromosomes identical with those of
the parental cell.
  Two main molecular processes take place during the cell cycle are duplication of parental
chromosome during S phase and separation of chromosome equally to daughter cell during
M phase.
 In somatic cell, the cell cycle consists of following four phase;
 G1 (gap 1) phase
 S (synthesis) phase
 G2 (gap2) phase
 M (mitosis) phase

1. G1 (gap1) phase:

 The first stage of interphase is called the G1 phase (first gap) because, from a microscopic
aspect, little change is visible. However, during the G1 stage, the cell is quite active at the
biochemical level.
 It is characterized by a change in chromosome from condensed state to more extended state
and series of metabolic events that leads to initiation of DNA replication. During G1 phase,
chromatin fibres become slender, less coiled and fully extended and more active for
transcription. The transcription results in synthesis of RNAs (tRNA, mRNA and rRNA) ad
series of proteins molecules required for initiation of DNA replication.
 The length of G1 phase varies from cell to cell and also the length of G1 phase is more than
other three phase in cell cycle.
 G1 phase represents 25-40% of generation time of a cell.
 G1 phase is very significant phase of cell cycle as the cell grows and accumulates the
building blocks of chromosomal DNA and the associated proteins as well as sufficient energy
reserves to complete the task of replicating each chromosome.
 Within G1 phase there is a definite check point at which DNA synthesis is initiated and once
the biochemical events associated with that point have occurred cell proceeds towards
division.

2. S (synthesis) phase:

 The synthesis phase of interphase is biochemically a phase of active DNA synthesis and
histone synthesis.
 In the S phase, chromosome numbers doubles which is accomplished by DNA replication
and associated proteins. Although some of the histone protein synthesis occurs in G1 phase,
most of it is synthesized during S phase.
 DNA replication is semi conservative and discontinuous type which results in the formation
of identical pairs of DNA molecules.
 After doubling of chromosome, sister chromatids are still firmly attached to the centromeric
region.
 At the center of each animal cell, the centrosomes of animal cells are associated with a pair of
rod-like objects, the centrioles, which are at right angles to each other. Centrioles help
organize cell division. Centrioles are absent in plants and most fungi.
 The centrosome (centriole) is also duplicated during the S phase. The two centrosomes will
give rise to the mitotic spindle, the apparatus that mediate the movement of chromosomes
during mitosis.
3. Gap2 (gap2) phase:

 G2 phase follows S phase. This phase represents 10-25% of generation time of cell.
 In G2 phase chromosome consists of two chromatids ie the cell has twice the amount of DNA
content.
 In the G2 phase, the cell restore its energy stores and synthesizes proteins necessary for
chromosome manipulation.
 Some cell organelles are duplicated, and the cytoskeleton is dismantled to provide resources
for the mitotic phase.
 There may be additional cell growth during G2. The final preparations for the mitotic phase
must be completed before the cell is able to enter the first stage of mitosis

4. M (mitotic) phase:

M phase follows G2 phase. During this phase cell divides into two daughter cell with equal
distribution of chromosome among daughter cells. After M phase cell enter into G1 phase
and next cell cycle is repeated. However, some cell after completion of mitosis do not enter
into G1 phase, those cell are referred as G0 cells.
 M phase consists of following sub –phases;
 During prophase, the nuclear membrane disappears, spindle fibers form, and DNA
condenses into chromosomes (sister chromatids ).
 During metaphase, the sister chromatids align along the equator of the cell by attaching
their centromeres to the spindle fibers.
 During anaphase, sister chromatids are separated at the centromere and are pulled towards
opposite poles of the cell by the mitotic spindle.
 During telophase, chromosomes arrive at opposite poles and unwind into thin strands of
DNA, the spindle fibers disappear, and the nuclear membrane reappears.
 Cytokinesis is the actual splitting of the cell membrane; animal cells pinch apart, while
plant cells form a cell plate that becomes the new cell wall.
 Cells enter the G0 (inactive) phase after they exit the cell cycle when they are not actively
preparing to divide; some cells remain in G0 phase permanently.

MITOSIS: MITOTIC CELL DIVISION, STAGES AND SIGNIFICANCE

 Mitosis is a type of cell division in which single haploid cell (n) or diploid cell (2n) divides
into two haploid or diploid daughter cells that are same as parent.
 Mitosis occurs in somatic cells of plants and animals. In this cell division, the two daughter
cells have same number of chromosomes as that in the parent cells.

The process of mitosis consists of the following stages or phases:

1. Interphase or Interkinesis
2. Karyokinesis
3. Cytokinesis
1. Interphase or interkinesis

 Interphase is the phase between two successive cell division (end of one cell division to the
beginning of next cell division).
 It is the longest phase in the cell cycle.
 Interphase looks dormant but it is metabolically active stage.

It is divided into 3 sub-stages viz. G1-phase, S-phase and G2-phase.

i. G1-Phase or Gap-1 phase

 The cell grows in size due to active biosynthesis.

 Formation of structural and functional proteins.
 Synthesis of mRNA, tRNA and rRNA takes place.

ii. S-Phase or Synthetic phase

 Replication of DNA takes place.

 Synthesis of histone proteins takes place which covers DNA.

iii. G2-Phase or Gap-two phase or Second growth phase

 RNA and protein is synthesize.

 Centrioles get replicate (in case of animal cell)
 Synthesis of spindle proteins takes place.
2. Karyokinesis

 Karyokinesis is the division of the nucleus.

 It consists of the following four phases.

i. Prophase

 It is the first visible stage in karyokinesis.

 The chromosomes appear as long coiled threads called chromatids.
 The chromatin becomes shorter, thicker and visible due to the condensation of DNA.
 The chromatins are now called chromosomes.
 Stainability of nucleus increase.
 Each chromosome starts to splits longitudinally into two sister chromatids. These sister
chromatids are attached with each other at centromere.
 The nuclear membrane and nucleolus starts to disappear and by the end it will completely
disappeared.

ii. Metaphase

 Nuclear membrane and nucleolus completely disappears and simultaneously appearance of

spindle fibres
 Spindle fibres attached to the centromere of chromosome.
 The chromosomes are arranged on the equatorial plane.
 The process of gathering of chromosomes in equator is called congression and plate formed
is called metaphasic plate.

iii. Anaphase

 The centromere of each chromosome splits into two sister chromatids and forms two
daughter chromosomes.
 The daughter chromosomes are pulled towards the poles due to the contraction of spindle
fibres and stretching of inter zonal fibres.
 During polar movement, the chromosomes shows different shapes i.e. J,U,V,L or I shaped in
appearance.
 At the end of anaphase, each pole will get one set of daughter chromosomes.
 It is shortest phase and is also known as migratory phase.

iv. Telophase

 The daughter chromosomes reach respective poles and uncoil and become thin, long and
visible.
 The spindle fibres start disappearing and finally disappear.
 The nuclear membrane and the nucleolus reappear.
 Two nuclei are formed at the end of telophase. Both the nuclei have the same number of
chromosome as parent cell.
  It is the last visible stage of karyokinesis and is also known as reorganization phase.

3. Cytokinesis

 Cytokinesis is the division of the cytoplasm.

 In plant cells, cytokinesis occurs by cell plate formation.
 During cytokinesis, many granular matrix formed by the golgibody and endoplasmic
reticulum accumulates in the equatorial region. These granular matrix form cell plate. This
plate divides the cell and by the end of telophase, cytokinesis is completed.
 In animal cells, cytokinesis occurs by cleavage or furrow formation.

SIGNIFICANCE OF MITOSIS:

 Mitosis produces 2 genetically identical cells, so mitosis maintains the genetic stability of
organisms.
 DNA remains constant, so mitosis keeps the chromosomes number constant in a species.
 Mitosis helps in the development of multicellular organism.
 Mitosis helps to replacement of old, dead or damaged cells by new one.
 It helps in the recovery of wounds and injury of the body by formation of new cells.
 In unicellular organisms like Yeast, Paramecium , mitosis is a means of asexual
reproduction.
 Mitosis causes maturation and multiplication of germ cells and makes them ready for
meiosis.

MEIOSIS: MEIOTIC CELL DIVISION, STAGES AND SIGNIFICANCE

 Meiosis is a cell division in which four haploid cells are formed from a single diploid cell.
 It usually occurs in reproductive organs or gonads of the organisms.
 Meiosis is also known as reductional cell division because four daughter cells produced
contain half the number of chromosomes than that of their parent cell.

Meiosis has two nuclear division phases:

1. Meiosis-I (Reductional or Heterotypic division)

2. Meiosis-II (Equational or Homotypic division)

Meiosis-I (heterolytic or Reductional division)

Meiosis-I has four different phases or stages:

1. Prophase-I
2. Metaphase-I
3. Anaphase-I
4. Telophase-I
1. Prophase-I

 It occupies the longest duration in Meiosis-I.

 It is divided into five sub-stages or sub-phases.
i. Leptotene
 This phase starts immediately after interphase.
 The size of cell and nucleus increases
 The chromosomes appear long, uncoiled thread-like in structure bearing many bead-like
structures called chromomeres.
 The nuclear membrane and nucleolus remain as it is.
ii. Zygotene
 Homologous chromosomes come closer and starts to pair up along their length.
 The pairing of homologous chromosomes is called Synapsis and the paired homologous
chromosomes are referred as bivalents.
 The homologous chromosomes are held together by ribonuclear protein between them.
iii. Pachytene
 The chromosome become shorter and thicker.
 Each chromosome of the bivalents splits longitudinally to form two chromatids such that
bivalents is composed of four strands and is known as a tetrad.
 The process of crossing over starts (crossing over; a small fragment of chromosome
exchange between two non-sister chromatids of bivalent by breakage and rejoining).
 Crossing over is the most important genetic phenomenon of meiosis which causes variation
in genetic characters in offspring.
iv. Diplotene
 In this stage crossing over takes place.
 Bivalents (chromatids) repel each other.
 Homologous chromosome (two non-sister chromatids) begins to separates but separation is
not complete, they remains attached to a point with a knot like structure called chiasmata
(singular – chiasma).
 The number of chiasmata varies. Depending upon the number of chiasmata, chromosome
appears different shape.
 1 chiasmata: cross like
 2 chiasmata: ring like
 Many chiasmata: series of loop
 Nuclear membrane and nucleolus begins to disappear.
v. Diakinesis
 The chiasma moves towards the end of the chromosomes (tetrad) due to contraction of
chromosomelastly slips over separating the homologous chromosome. This movement of the
chiasmata towards the end of chromosome is called terminalization.
 By the end of diakinesis the nuclear membrane and nuleolus get completely disappeared and
the chromosomes are free in the cytoplasm.
 Spindle fibres begin to form
2. Metaphase-I

 The spindle fibres organized between two poles and get attached to the centromere of
chromosomes.
 Chromosome moves to equator
 The bivalent chromosomes are arranged in the equatorial plate in such a way that 2
metaphasic plates are formed.

3. Anaphase-I

 Spindle fibres contracts and pulls the whole chromosomes to the polar region.
 The separated chromosome is known as dyads
 No splitting of chromosomes occurs so the centromere of each homologous chromosome
does not divide. Thus, the chromosome number of the daughter nuclei is reduced to half.
 Now the separated chromosome moves toward opposite poles.

4. Telophase-I

 Two groups of chromosome formed at each pole and organized into nuclei.
 The nuclear membrane and nucleolus reappears.
 The chromosomes get uncoiled into chromatin thread.
 The spindle fibres disappear totally.

Cytokinesis I

 Cytokinesis may or may not follow nuclear division (meiosis-I Cytokinesis occurs by cell
plate formation method in plant cell and furrowing method in animal cells.

Interphase II or Interkinesis

 The two cells or nuclei thus formed pass through a short stage called interphase-II.
Sometimes, interphase-II is absent.
 It is the resting phase between meiosis-I and meiosis-II.
 It is either very short or may be absent
 No DNA synthesis occurs.

Meiosis-II ( Homolytic or equational division)

 Meiosis-II is exactly similar to mitosis, so it is also known as meiotic mitosis.

 In this division, two haploid chromosome splits longitudinally and distributed equally to form
4 haploid cells.
 It completes in 4 stages.
1. Prophase-II
2. Metaphase-II
3. Anaphase-II
4. Telophase-II
1. Prophase-II:

 The dyads chromosome becomes thicker and shorter

 Nuclear membrane and nucleolus disappear
 Spindle fibre starts to form

2. Metaphase-II:

 The dyads chromosomes comes to equatorial plane

 Spindle fibres organize between poles and attaches to centromere of chromosome.

3. Anaphase-II:

 Centromere of each chromosome divides and sister chromatids separates to form two
daughter chromosome
 Spindle fibre contracts and pull the daughter chromosome apart towards opposite pole.

4. Telophase-II:

 Chromosome become organize at respective pole into nuclei

 Chromosome elongates to form thin networks of chromatin
 Nuclear membrane and nucleolus reappears

Cytokinesis-II:

 The result of cytokinesis is four haploid daughter cells (gametes or spores).

 Cytokinesis takes place by cell plate formation in plant cell
 Successive methods: cytokinesis followed by each nuclear division resulting in 4 haploid
cells. Eg. Monocot plants
 Simultaneous methods: cytokinesis occurs only after meiosis-II to form 4 haploid cells. Eg.
Dicot plants
 In animal cells, cytokinesis occurs by furrow formation or depression.
SIGNIFICANCE OF MEIOSIS

1. Meiosis helps to maintain a constant number of chromosomes by reducing the chromosome

number in the gametes
2. Essential for sexual reproduction in higher animals and plants
3. Meiosis helps in the formation haploid gametes and spores for sexual reproduction.
4. Number of chromosome remain fixed in a species from generation to generation
5. Crossing over occurring brings genetic variations in offspring which helps in evolution of
organisms.
6. Failure disjunction in Meiosis leads mutation to the formation of polypoid forms.
7. The random distribution of maternal and paternal chromosomes takes place into daughter
cells during meiosis and it is a sort of independent assortment which leads to variation.
 BINARY FISSION
Binary fission is a type of asexual reproduction most commonly seen in prokaryotes and some
single-celled eukaryotes. In this method of asexual reproduction, there is a separation of the parent
cell into two new daughter cells. This process happens with the duplication and division of the
parent’s genetic material into two parts, where, each daughter cells receive one copy of its parent
DNA.
It is a primary method of reproduction in prokaryotic organisms. Binary Fission occurs without any
spindle apparatus formation in the cell. In this process, the single DNA molecule begins replication
and then attaches each copy to various parts of the cell membrane. When the cell starts to get pulled
apart, the original and replicated chromosomes get separated.
However, asexual mode of reproduction has a significant drawback. All resultant cells are
genetically identical mirror copies of each other and the parent cell. Most antibiotics work on this
principle. If a parent cell is vulnerable to an antibiotic, then all resultant daughter cells are also
vulnerable too. If a mutation occurs in their genes, then it can render a particular strain resistant to
antibiotics.
Prokaryotes such as E. coli, Archaea as well as eukaryotes such as euglena reproduce through
binary fission.
Binary fission is a form of cell division in eukaryotes. In prokaryotes, it is a form of asexual
reproduction

Binary Fission in Bacteria

The process of binary fission is usually rapid, and its speed varies among species. The time required
by bacteria to double the number of cells it has is called doubling time. Furthermore, each species
requires specific conditions for its growth. These conditions include pH levels, temperature,
oxygen, light, moisture, osmotic pressure.
For instance, mesophiles thrive at moderate temperatures ranging from 20°C to 45 °C. The ambient
temperature of the human body is 37 °C, which means many of the disease-causing bacteria are
mesophiles. Mycobacterium tuberculosis is the bacterium that causes tuberculosis in humans. It
divides every 15 to 20 hours, which is very slow when compared to other pathogenic bacteria such
as Escherichia coli, which can divide every 20 minutes.
On the other end of the spectrum are the extremophiles. These bacteria can survive extremely harsh
conditions such as high temperatures, high salinity, highly acidic environments and more. For
instance, the Deinococcus radiodurans is an extremophilic bacteria that can survive a thousand
times more radiation than a person can. Under normal circumstances, it can divide every 48 hours.
However, when exposed to harsh conditions like drought, it can slow down its growth rate until
more favourable conditions arise.
Read more: Bacteria
The steps involved in the bacterial binary fission are:

Step 1- Replication of DNA

The bacterium uncoils and replicates its chromosome, essentially doubling its content.

Step 2- Growth of a Cell

After copying the chromosome, the bacterium starts to grow larger in preparation for binary
fissions. It is followed by an increase in cytoplasm volume as well as an increase in the number of
organelles. Another prominent trait of this stage is that the two strands migrate to opposite poles of
the cell.
Step 3-Segregation of DNA
The cell elongates with a septum forming at the middle. The two chromosomes are also separated
in this phase.

Step 4- Splitting of Cells

A new cell wall is formed at this phase, and the cell splits at the centre, dividing the parent cell into
two new daughter cells. Each of the daughter cells contains a copy of the nuclear materials as
necessary organelles.

Amoeba Reproduction-Binary Fission in Amoeba

Amoeba is a unicellular organism, and just like bacteria, it reproduces through binary fission. After
replicating its genetic material through mitotic division, the cell divides into two equal-sized
daughter cells. In this method, two similar individuals are produced from a single parent cell.
An amoeba that is about to undergo division grows larger, and eventually, its nucleus extends and
divides into two. The division of cytoplasm follows the division of the nucleus. So, two amoebae
are produced from a single parent, and the parent’s identity is technically “lost.”

1. How does Amoeba reproduce?

Amoeba reproduces asexually through binary fission. In this process, an individual divides itself
into two daughter cells. These are genetically identical to each other.

2. List out the similarities and differences between binary fission and mitosis
Similarities:

 During binary fission as well as mitosis, chromosomes are copied before a cell
divides and forms two new daughter cells.
Differences:

 Mitosis is a process of cell division, observed in all eukaryotes -organisms with a

true nucleus.
Binary fission is a process of asexual reproduction carried out by all prokaryotes-
organisms without a true nucleus.
 SCHOOL OF BIO AND CHEMICAL ENGINEERING
DEPARTMENT OF BIOTECHNOLOGY

UNIT – II – CELL BIOLOGY – SBB1101

 PlASMAMEMBRANE
Plasma membrane or plasma-lemma is a bio membrane that occurs on the outside of the cytoplasm
in both prokaryotes and eukaryotic cells.
It separates the cellular protoplasm from its external environment. Prokaryotic cells do not have
internal membranous partitions. The latter occur in eukaryotic cells as covering of several cell
organelles like nucleus, mitochondria, plastids, lysosomes, Golgi bodies, peroxisomes, etc.

Bio membranes line the endoplasmic reticulum. They also occur on thylakoids inside plastids or
cristae inside the mitochondria. Vacuoles are separated from cytoplasm by a membrane called
tonoplast. All bio membranes are dynamic in nature, continually showing changes in their form,
size, structure and function. Plasma membrane was discovered by Schwann (1838). It was named
as cell membrane by Nageli and Cramer (1855). The membrane was given the name of plasma
lemma by Plowe (1931).

Chemical Nature of Membranes:

Chemically a bio membrane consists of lipids (20—40%), proteins (59—75%) and carbohydrates
(1—5%). The important lipids of the membrane are phospholipids, sterols (e.g., cholesterol),
glycolipids, sphingolipids (e.g., sphingomyelin, cerebrosides).

Carbohydrates present in the membrane are branched or un-branched oligosaccharides, e.g., hexose,
fucose, hexoamine, sialic acid, etc. Proteins can be fibrous or globular, structural, carrier, receptor
or enzymatic. About 30 kinds of enzymes have been recorded in different bio membranes, e.g.
phosphatases, ATP-aseesterases, nucleases, etc.

The lipid molecules are amphiatic or amphipathic, that is, they possess both polar hydro- philic
(water loving) and nonpolar hydrophobic (water repelling) ends. The hydrophilic region is in the
form of a head while the hydrophobic part contains two tails of fatty acids.

Hydrophobic tails usually occur towards the centre of the membrane. Protein molecules also
possess both polar and nonpolar side chains. Usually their polar hydrophilic linkages are towards
the outer side. The nonpolar or hydrophobic linkages are either kept folded inside or used to
establish connections with hydrophobic part of the lipids. Several types of models have been put
forward to explain the structure of a biomembrane. The more important are Lamellar and Mosaic.

Lamellar Models (= Sandwich Models):

They are the early molecular models of bio membranes. According to these models, bio membranes
are believed to have a stable layered structure.

Danielli and Davson Model:

The first lamellar model was proposed by James Danielli and Hugh Davson in 1935 on the basis of
their physiological studies. According to Danielli and Davson, a biomembrane contains four
molecular layers, two of phospholipids and two of proteins. Phospholipids form a double layer.

The phospholipids bilayer is covered on either side by a layer of hydrated globular or a-protein
molecules. The hydrophilic polar heads of the phospholipid molecules are directed towards the
proteins. The two are held together by electrostatic forces. The hydrophobic nonpolar tails of the
two lipid layers are directed towards the centre where they are held together by hydrophobic bonds
and van der Waals forces.

Robertson Model:

J. David Robertson (1959) modified the model of Danielli and Davson by proposing that the lipid
bilayer is covered on the two surfaces by extended or 3-protein molecules. A difference in the
proteins of the outer and inner layers was also proposed, e.g., mucoprotin on the outer side and non-
mucoid protein on the inner side.

Robertson worked on the plasma membrane of red blood cells under electron microscope. He gave
the concept of unit membrane which means that:
(i) All cytoplasmic membranes have a similar structure of three layers with an electron transparent
phospholipid bilayer being sand-witched between two electrons dense layers of proteins,

(ii) All bio membranes are either made of a unit membrane or a multiple of unit membrane. The
unit membrane of Robertson is also called trailaminar membrane. It has a thickness of about 75 Å
with a central lipid layer of 35 A thick and two peripheral protein layers of 20Aeach. According to
Robertson, if a membrane contains more than three layers, or is thicker than 75A, it must be a
multiple of unit membrane.

Mosaic Model:

Fluid-Mosaic Model. It is the most recent model of a bio membrane proposed by Singer and
Nicolson in 1972.

1. According to this model, the membrane does not have a uniform disposition of lipids and
proteins but is instead a mosaic of the two. Further, the membrane is not solid but is quasi-fluid.

2. It postulates that the lipid molecules are present in a viscous bilayer as in lamellar model. Protein
molecules occur at places both inside and on the outer side of lipid bilayer. The internal proteins are
called intrinsic or integral proteins while the external ones are known as extrinsic or peripheral
proteins.

The integral or intrinsic proteins account for 70% of the total membrane proteins and pass into the
lipid bilayer to different depths. Some of them run throughout the lipid bilayer. They are called
tunnel proteins which individually or in a group form channels for the passage of water and water
soluble substances.

3. The proteins provide the structural and functional specificity to the membranes. Further since the
lipid bilayer is quasifluid, the membrane proteins may shift laterally and thence provide flexibility
and dynamism to the membrane.

Many membrane proteins function as enzymes, some of them behave as per-meases for facilitated
diffusion and a few proteins act as carriers because they actively transport different substances
across the membrane. Certain other protein functions as receptors for hormones, recognition centres
and antigens. Some of the lipid at the outer surface is complexed with carbo hydrates to form
glycolipids or glycocalyx.
Modifications of Cell Membrane:

1. Microvilli:

They are finger like evaginations of 0.6—0.8 µm length and 0.1 µm diameter which are found on
the free surface of cells engaged in absorption, e.g. intestinal cells, hepatic cells, mesothelial cells,
uriniferous tubules. The surface having microvilli is called striated border or brush border.

Microvilli increase the surface area several times. They are supported by a web of microfilaments,
actin along with myosin, tropomysoin, spectrin, etc. The narrow spaces in between microvilli take
part in pinocytosis.
2. Mesosomes:

They are plasmalemmainfoldings found in bacteria. One type of mesosome is attached internally to
the nucleoid. It is required for nucleoid replication and cell division.

3. Junctional Complexes:

They are contacts between adjacent cells which in case of animal cells are separated by spaces
of 150-200 Å filled with tissue fluid. The important ones are:
(i) Interdigitations:
There is interlocking of finger-like membrane outgrowths between two adjacent cells.
Interdigitations increase the area of the contact between two cells for exchange of materials.

(ii) Intercellular Bridges:

Projections fro m adjacent cells make contact for rapid conduction of stimuli.
(iii) Tight junctions:
(Zonulae Occludentes, singular— Zonula Occludens). Here plasma membranes of two adjacent
cells are fused at a series of points with a network of ridges or sealing strands. Tight junctions occur
in epithelia with high electrical resistance and where filtration is to occur through the cells, e.g.,
capillaries, brain cells, collecting tubules of kidneys.
(iv) Gap Junctions:
The adjacent cells have protoplasmic connections through special protein cylinders called
connexons. Each connexon is made of six identical protein subunits around a hydrophilic channel.

(v) Plasmodesmata:
They are protoplasmic bridges amongst plant cells which occur in the areas of cell wall pits or
pores.

(vi) Desmosomes:
(Maculae Adherentes, singular—Macula Adherens). Adjacent membranes possess disc-shaped
thickenings of about 0.5 (am diameter, a number of tonofibrils (= tonofilaments) and trans-
membrane linkers embedded in dense intercellular material. Desmosomes function as spot welds
and are hence called spot desmosomes. They occur in epithelia subjected to disruption.
(vii) Terminal Bars:
(Belt Desmosomes, Zonulae Adherentes, singular—Zonula Adherens. Intermediary Junction).
Terminal bars are desmosomes without tonofibrils. Bands of thickenings occur on the inner surface
of membrane. The bands contain microfilaments and intermediate filaments.

Functions of Cell Membranes:

1. The major function of cellular membranes is compartmentalisation. As plasma membranes they
separate the cells from their external environment. As organelle coverings, they allow the cell
organelles to maintain their identity, specific internal environment and functional individuality.

2. The membranes allow the flow of materials and information between different organelles of the
same cell as well as between one cell and another.

3. As plasmodesmata and gap junctions, the bio membranes provide organic connections between
adjacent cells.

4. Plasma membranes as well as other membranes of the organelles have selective permeability,
that is, they allow only selected substances to pass inwardly to selected degrees. The membranes
are impermeable to others.

5. Bio membranes have the property of retentivity, that is, they do not allow the outward passage of
substances already permitted entry.

6. Plasma membrane possesses specific substances at its surface which function as recognition
centres and points of attachment.
7. Substances attached to cell membrane determine antigen specificity. Glycophorins present on the
surface of erythrocytes function as antigen determinants. Histocompatibility antigens signify
whether a foreign cell or tissue should be incorporated or rejected.

8. Cell membrane has receptors for certain hormones. The hormone combines with its particular
receptors and either changes membrane permeability or activates enzyme adenylate cyclase to
produce cyclic AMP from ATP. cAMP then triggers a set of enzymes to perform a particular
function.

9. Membranes have carrier proteins for active transport.

10. Cell membranes contain enzymes for performing certain reaction on their surface, e.g., ATP-ase
(for ATP synthesis and release of energy from ATP), phosphatases, esterases etc.

11. Certain cell membranes (e.g. plasma membrane in bacteria, thylakoid membranes of
chloroplasts, inner mitochondrial membrane) possess electron transport systems.

12. Membrane infolds are used for bulk intake of materials by endocytosis.

MEMBRANE TRANSPORT
Passage of substances across bio membranes or cell membrane is brought about by the
following methods:

A. Transport of Water:

(I) Osmosis:
Osmosis is the diffusion of water or solvent molecules through plasma membrane from low osmotic
pressure to high osmotic pressure, i.e., from high water contents to low water content. Plasma
membrane acts as a differential membrane permitting the movement of water molecules in and out
retaining the metabolites.

B. Transport of Ions and Small Molecules:

(II) Passive Transport:

It is a mode of membrane transport where the cell does not spend any energy nor shows any special
activity. The transport is according to concentration gradient. It is of two types, passive diffusion
and facilitated diffusion.
(a) Passive Diffusion or Transport across Cell Membranes:
Here the cell membrane plays a passive role in the transport of substances across it. Passive
diffusion can occur either through lipid matrix of the membrane or with the help of channels.

(i) Lipid Soluble Substances:

It was found out by Overton (1900) that lipid soluble substances pass rapidly across the cell
membrane according to their concentration gradient. Based on this finding, Overton proposed that
cell membranes are made of lipids.

(ii) Channel Transport:

Membrane possesses channels in the form of tunnel proteins which do not carry any charge. They
allow water and soluble gases (CO2 and O2) to pass according to their concentration gradient.
Osmosis is an example of such a transport.
If two solutions of different concentrations are separated by a semi-permeable membrane, the
solvent molecules move across the membrane from the less concentrated into the more concentrated
solution. This process- the diffusion of solvent molecules into a region in which there is a higher
concentration of solute to which the membrane is impermeable, is called osmosis.

Filtration is diffusion under pressure across a membrane having minute pores. Ultra filtration
occurs during glomerular filtration inside kidneys. Dialysis is the process of separating small
particles (e.g., crystalline solutes) from larger ones (e.g., colloids) due to difference in the rate of
diffusion across a membrane having very minute pores.

(b) Facilitated Diffusion:

It occurs through the agency of special membrane proteins called permeases. When such carrier-
mediated transport is from an area of greater concentration, energy is not required and the process is
called facilitated diffusion. As a result, the rate of transport is stereo-specific.
Entry of glucose into red blood corpuscles is a facilitated diffusion.

The process of facilitated diffusion involves following steps:

1. Diffusing molecules combine with the specific carrier protein molecules forming carrier-protein
complexes.

2. The shape of carrier protein molecule changes in response to the diffusing molecule so that the
membrane – bound carrier protein complexes form, channels.

3. The shape of carrier protein molecule changes in response to diffusing molecule, allowing the
molecule to cross the plasma membrane.

4. Once the diffusing molecule has reached the other side, the change in shape of carrier molecule
(conformational change) lowers its affinity with the diffusing molecule, and allows it to be released.

5. After the release of diffusing molecule, the carrier-protein molecule resumes original shape.

The facilitated diffusion enables molecules to cross the otherwise impermeable or poorly permeable
membrane.

Facilitated diffusion differs from simple diffusion in following features:

(i) Facilitated diffusion is stereo-specific (either L or D isomer is transported).

(ii) It shows saturation kinetics.

(iii) Facilitated diffusion requires a carrier for transport across the membrane. The carrier protein
molecules move to and fro across the membrane by thermal diffusion.
(III) Active Transport:
It is uphill movement of materials across the membranes where the solute particles move against
their chemical concentration or electro-chemical gradient. This form of transport requires energy
which is provided almost exclusively by hydrolysis of ATP.

Active transport occurs in case of both ions and non-electrolytes, e.g., salt uptake by plant cells,
ions, glucose and phenolphthalein in case of renal tubules, sodium and potassium in case of nerve
cells, etc. It is supported by various evidences:

(a) Absorption is reduced or stopped with the decrease in oxygen content of the surrounding
environment.

(b) Metabolic inhibitors like cyanides inhibit absorption.

(c) Cells often accumulate salts and other substances against their concentration gradient.

(d) Active transport shows saturation kinetics that is, the rate of transport increases with increase in
solute concentration till a maximum is achieved. Beyond this value the rate of membrane transport
does not increase indicating that it takes place through the agency of special organic molecules
called carrier molecules, carrier particles or carrier proteins.

The carrier molecules are ATP ases, enzymes that catalyze the hydrolysis of ATP. The most
important of these ATPases is Na+-K+ ATP ase which is also known as the Na+-K+ pump. There are
in addition H+-K+ ATPases in the gastric mucosa and the renal tubules.
There is a special carrier molecule for each solute particle. The carrier has its binding site on two
surfaces of the membrane. The solute particles combine with the carrier to form carrier-solute
complex. In the bound state the carrier undergoes a conformational change which transports the
solute to the other side of the membrane. Energy is used in bringing about the conformational
change in the carrier. It is provided by ATP. In the process ATP is dephosphorylated to form ADP.
Carrier proteins are of three types.

1. Uniport:
They transport only one substance.

2. Symports:
In some cases, transport requires the binding of more than one substance to the transport protein
and the substances are transported across the membrane together. An example is the symport in the
intestinal mucosa that is responsible for the co-transport by facilitated diffusion of Na+ and glucose
from the intestinal lumen into mucosal cells.
3. Antiports:
They exchange one substance for another. The Na+-K+ ATPase is a typical antiport.

Many animal cells operate a sodium-potassium exchange pump at their plasma membrane. A
similar proton pump operates in chloroplasts, mitochondria and bacteria Na+—K+ exchange pump
operates with the help of enzyme ATP-ase which also functions as a carrier molecule.

The enzyme hydrolyses ATP to release energy. The energy is used in bringing about
conformational changes in the carrier. For every ATP molecule hydrolysed, three Na+ ions are
pumped outwardly and two K+ ions are pumped inwardly.
Na+– K+ exchange pump performs the following functions: (i) Maintains a positive potential on the
outer side of the membrane and relatively electronegative potential on the inner side,
(ii) The pump creates a resting potential in the nerve cells,
(iii) The pump maintains water balance of living cells.

(iv) It helps in urine formation,

(v) It takes part in excretion of salt as in marine animals. Sea gulls and penguins drink sea water.
They excrete excess salt through nasal glands. The nasal salt glands have sodium-potassium pump
in the plasma membranes of their cells. Na+ ions are pumped out actively. Chlorine ions pass out
passively. Nasal secretion of the two birds possesses 1.5—3.0 times more NaCl concentration than
the one present in the blood.
(vi) The unsecreted and unmetabolised excess Na+ ions present in the extracellular fluid have a
tendency to pass back into the cells. Other substances combine with sodium ions and pass inwardly
along-with them, e.g., glucose, amino acids in intestine. The phenomenon is called secondary active
transport as compared to Na+—K+ exchange pump which is called primary active transport.

Other important pumps include Calcium pump (RBCs, muscles), K+ pump, CP pump, K+—
H+ exchange pumps. The last one occurs in guard cells.
Active transport is a means of (i) absorption of most nutrients from the intestine (ii) reabsorption of
useful material from the uriniferous tubules (iii) rapid and selective absorption of nutrients by cells
(iv) maintaining a membrane potential (v) maintenance of resting potential in nerve cells (vi)
maintaining water and ionic balance between cells and extracellular fluid, (vii) excretion of salt
glands.

C. Transport of Solid Particles (Bulk transport):

Bulk transport inwards as well as outwards occurs across the plasma membrane by invagination and
evagination of the membrane. Bulk transport is useful in carrying large molecules which would
have difficulty in passing through the cell membrane normally. Endocytosis and exocytosis are the
two ways in which bulk transport is accomplished.

(IV) Endocytosis is the process of engulfing large sized particles of food substances or of foreign
substances. According to the nature of substances, the endocytosis may be:

(i) Pinocytosis or cells drinking is the process of intake of fluid material by the cell.

(ii) Micro pinocytosis is the pinocytosis of subcellular or sub microscopic level.

(iii) Rhophaeocytosis is the transfer of small quantities of cytoplasm together with their inclusion.

(iv) Phagocytosis is engulfing of large sized particles of solid food or solid matter by the cell.

(V) Exocytosis is the process of exudating the secretary products to outside the cell cytoplasm. It is
also known as emeicytosis or cell vomiting. In cells of pancreas, the vacuoles containing enzymes
move from the interior of the cytoplasm towards the surface. Here they fuse with plasma membrane
and discharge their contents to the exterior.

CELL MEMBRANE FLUIDITY

The motion of phospholipid molecules within the lipid bilayer, dependent on the classes of
phospholipids present, their fatty acid composition and degree of unsaturation of the acyl chains,
the cholesterol concentration, and temperature are called as membrane fluidity
 1. If the length of the phospholipids decreased, then individually, other molecules
within the cell membrane, such as transmembrane proteins, can move around more
easily inside.

2. Less Cholesterol: Heads up! This will depend on temperature. More cholesterol in
the cell membrane causes the membrane to become more rigid and less permeable to
other molecules or ions in high temperatures. However, more cholesterol increases
fluidity in low temperatures. Why? In higher temperatures, phospholipids and other
cell membrane molecules are already moving wildly. Cholesterol itself is a chunky
lad, so its bulky size will restrict the molecule movement within the membrane
more. Thus, in high temperatures, more cholesterol decreases fluidity. However, in
low temperatures, this is the opposite case. Imagine in cooler temperatures, the
molecules are freezing and pack closer together and thus move less. With the
presence of cholesterol, like double bonds and unsaturated fat, the tight packing is
inhibited more.
3. Less Packed Together:

This is correlated with the double bonds idea. If the molecules, proteins, and
individual
phospholipids are
packed together so
closely, this restricts
internal molecule
movement within
the membrane.

Cholesterols, shown as pink molecules in this diagram, take up large spaces in the cell membrane
and thus reduce cell membrane fluidity in high temperatures.
Summary of Cell Membrane Fluidity and Factors That Affect Cell Membrane Fluidity

Increase Fluidity Decrease Fluidity

Phospholipids with Shorter Tail

Phospholipids with Longer Tail Lengths
Lengths

More Double Bonds (Unsaturated) Fewer Double Bonds

More Cholesterol (generally in high

Less Cholesterol
temperatures)

Less Packed Together Closer Together/ More Packed together

High Temperature Low Temperature

Extracellular Matrix
Animal tissue is not only composed of cells but also contains many types of extracellular space or
intercellular space. These spaces are again filled up by many types of macromolecules constituting
the extracellular matrix.

The extracellular matrix has some specialised functions such as, strength, filtration, adhesion etc.
The macromolecules that constitute the extracellular matrix are mainly secreted locally by the cells.
In most of the connective tissues the macromolecules are secreted by fibroblast (Fig. 4.9). In some
specialised connective tissues, such as cartilage and bone, they are secreted by chondroblasts and

osteoblasts, respectively.
Types of Extracellular Matrix:
The extracellular matrix is made of three main types of extracellular macromolecules:
(i) Polysaccharide glycosaminoglycan’s (commonly known as mucopolysaccharides) or GAGs
which are usually linked covalently to proteins in the form of proteoglycans;

(ii) Fibrous proteins of two functional types:

(a) Mainly structural (e.g., collagen and elastin) and

(b) Mainly adhesive (e.g., fibronectin and laminin);

(iii) Specialised extracellular matrix or basal lamina.

(i) Glycosaminoglycan:

It is a long, un-branched linear polysaccharide chains and consists of repeating disaccharide units in
which one of two sugars is always either N-acetyl glucosamine or N-acetylgalactosamine. Hence it
is named glycosaminoglycan.

The second sugar of glycosaminoglycan is a uronic acid. In most of the cases, the amino sugar is
sulfated. Due to presence of large numbers of carboxyl and sulfate group on most of their sugar
residues, glycosaminoglycan’s are highly acidic and negatively charged.
There are four main classes of glycosaminoglycan’s:
(i) Hyaluronic acid,

(ii) Chondroitin sulfate and dermatan sulfate,

(iii) Heparan sulfate and heparin, and

(iv) Keratan sulfate.

These can be distinguished on the basis of sugar residue, the type of linkage and number and
location of sulfate groups (Table 4.1). These are distributed in the extracellular matrix of different
tissues. The amount of glycosaminoglycan is usually less than 10% by the weight of the amount of
the fibrous proteins.

Hyaluronic acid consisting of several thousand simple sugar residues, is made for regular repeating
sequence of non-sulphated disaccharide units. Each unit contains glucuronic acid and N-acetyl
glucosamine. Hyaluronic acid is thought to facilitate cell migration during tissue morphogenesis
and wound repair.

It is also an important constituent of joint fluid where it serves as a lubricant. It is also evident that
excess hyaluronic acid is degraded by the enzyme hyaluronidase.

In most cases, glycosaminoglycan’s exist in combination with proteins, the complex being termed a
proteoglycans. It is made of core protein linked with numerous un-branched glycosaminoglycan’s.
A serine residue of the polypeptide chain of core protein is first linked with three sugar residues
such as xylose, galactose, galactose (known as link trisaccharide) which, in turn, are attached with
glycosamino-glycan.

A proteoglycan aggregate from fetal bovine cartilage is made of 100 proteoglycan monomers which
are non-covalently bound to a single hyaluronic acid chain through two link proteins that bind to
both the core protein of the proteoglycan and to the hyaluronic acid chain.

Individual proteoglycan monomers consist of a central core protein to which large number of the
sulfated glycosaminoglycan’s chondroitin sulfate and keratan sulphate are attached.

(ii) Fibrous Protein:

A. Structural Fibrous Protein:
(a) Collagen:
The major fibre-forming structural proteins of the extracellular matrix are collagens. The fibrillar
collagens are generally rope-like, triple- stranded helical molecules that aggregate into long cable-
like fibrils in the extracellular space.

It is a hydrophobic protein. This protein is found in all multicellular animals and is secreted mainly
by connective tissue cells. The basic molecular unit of collagen is tropocollagen or pro-collagen
which is 300 nm in length and 1.5 nm wide. It is made of three polypeptide chains that are coiled
together to form a triple helical structure.

The major portion of three polypeptide chains of tropocollagen called a-chain (about 1.000 amino
acid long) has an a-helix organisation with short non-helical segments of 16-25 residues at both
ends that are called tclopeptides.

The amino acid composition of the polypeptide chain of collagen is very simple; they have a large
amount of proline and many of the proline and lysine residues are hydroxylated. So far, about 20
distinct a-chains of collagen have been identified. These are encoded by separate genes.

Different combination, and permutations of these genes are expressed in different tissues. So,
various combinations of the 20 types of a-chain will theoretically constitute more than thousand
types of collagen molecules.

So far, about five isotypes of collagen based on slight differences (Table 4.2) in the organisation of
the polypeptide and association with other molecules—such as polysaccharide and glycoprotein—
have been found.

These are types I, II, III, IV, and V. Types I, II, III, and V are fibrillar collagens, while type IV is
non-fibrillar and assemble into a sheet-like meshwork that constitutes a major part of all basal
laminae along with fibronectin and laminin.

(b) Elastin:
Elastin is a fibrillar cross-linked, random-coil, hydrophobic, non-glycosylated protein that gives the
elasticity of the tissues—such as skin, blood vessels and lungs—in order to function. This protein is
rich in proline and glycine and contains little amount of hydroxyproline and hydroxyserine.
It is secreted into the extracellular space and forms an extensive cross-linked network of fibres and
sheets that can stretch and recoil like a rubber band and imparts the elasticity to the extracellular
matrix. Elastin fibre also contains a glycoprotein which is distributed as micro-fibrils on the elastin
fibre surface.

B. Adhesive Fibrous Protein:

The extracellular matrix contains several adhesive fibrous glycoproteins that bind to both cells and
other matrix macromolecules and, ultimately, help cells stick to the extracellular matrix.
Fibronectin and laminin are the examples of best characterised large adhesive glycoproteins in the
extracellular matrix.

(a) Fibronectin:
Fibronectin is a glycoprotein. It is made of two polypeptide chains which are similar but not
identical. The two polypeptides are joined by two disulfide bonds near the carboxyl terminus. Each
chain is folded into a series of globular domains joined by a flexible polypeptide segments (Fig.
4.14).

Individual domains are specialised for binding to a particular molecule or to a cell. For example,
one domain binds to collagen, another to heparin, another to specific receptors on the surface of
various types of cells, and so on. In this way fibronectin builds up the close organisation of the
matrix and help cells attach to it.

Fibronectin occurs in three forms:

1. A Soluble Dineric Form:
Called plasma fibronectin—which circulates in the blood and other body fluid. The main function
of this fibronectin is to enhance blood clotting, wound healing and phagocytosis.

2. Oligomers of Fibronectin:
Called cell-surface fibronectin—which are occasionally found to attach on the cell surface and
helps cell to cell attachment.

3. Highly Insoluble Fibrillar Fibronectin:

Called matrix fibronectin—which help cell adhere to the matrix.

(b) Lamina:
Laminin is an adhesive glycoprotein. It is secreted specially by epithelial cells. This protein is a
major part of all basal laminae. It binds the epithelial cells to type IV collagen of basal Lamina.
Laminin is composed of three multi-domain polypeptide chains, such as A chain, B1 chain and
B2 chain (Fig. 4.15).

Laminin is the first extracellular matrix protein to appear in the embryo. In the kidney it acts a
major barrier to filtration. When this protein deposits in the glomerular basement membrane,
antibodies are produced against laminin and severely affect the kidney functions. Laminin is
increased in basement membranes of diabetic patients. Antibodies are also found in Chagas disease.
(iii) Specialised Extracellular Matrix Basal Laminae:

Basal lamina is a continuous thin mat or sheet like specialised extracellular structure that underlies
all epithelial cells. Individual muscle cells, fat cells, Schwann cells are wrapped by basal lamina. It
is actually linked to the plasma membranes of different types of cell by specific receptors.

The basal lamina separate these cells from the connective tissue. In the glomerulus of the kidney,
the basal lamina lie between two cell sheets and forms a porous filter that allows water, ions and
small molecules in blood to cross into the urinary space while retaining protein and cells in the
blood.

Basal lamina is also able to determine cell polarity, influence cell metabolism, organise the proteins
in neighbouring plasma membrane, induces cell differentiation and also facilitate cell migration.

Cytoskeleton

 The cytosol of cells contains fibers that help to maintain cell shape and mobility and that
probably provide anchoring points for the other cellular structures.
 Collectively, these fibers are termed as the cytoskeleton.
 The cytoskeleton gives cells structure and shape and allows them to move around. It’s also
important for intracellular transport.
 At least three general classes of such fibers have been identified in eukaryotic cells. Each of
these filaments is a polymer.
 All three filament systems are highly dynamic, altering their organization in response to the
needs of the cell.
A. Microtubules
 The thickest are the microtubules (20 nm in diameter) which consist primarily of the tubulin
protein.
 Each tubulin subunit is made up of one alpha and one beta-tubulin that are attached to each
other, so technically tubulin is a heterodimer, not a monomer. Since it looks like a tube, it is
named as microtubule.
 In a microtubule structure, tubulin monomers are linked both at their ends and along their
sides (laterally). This means that microtubules are quite stable along their lengths.
 Since the tubulin subunits are always linked in the same direction, microtubules have two
distinct ends, called the plus (+) and minus (-) ends.
 On the minus end, alpha-tubulin is exposed, and on the plus end, beta-tubulin is exposed.
 Microtubules preferentially assemble and disassemble at their plus ends.
Functions
1. Transportation of water, ions or small molecules.
2. Cytoplasmic streaming (cyclosis).
3. Formation of fibers or asters of the mitotic or meiotic spindle during cell division.
4. Formation of the structural units of the centrioles, basal granules, cilia, and flagella.
B. Microfilaments
 The thinnest are the microfilaments (7 nm in diameter) which are solid and are principally
made of two intertwined strands of a globular protein called actin. For this reason,
microfilaments are also known as actin filaments.
  Actin is powered by ATP to assemble its filamentous form, which serves as a track for the
movement of a motor protein called myosin.
 This enables actin to engage in cellular events requiring motion such as cell division in
animal cells and cytoplasmic streaming, which is the circular movement of the cell
cytoplasm in plant cells.
Functions
1. They maintain the shape of the cell.
2. Form contractile component of cells, mainly of the muscle cells.
3. White blood cells can move to the site of an infection and engulf the pathogen due to
microfilaments.
C. Intermediate Filaments
 The fibers of the middle-order are called the intermediate filaments (IFs) having a diameter
of 10 nm.
 They are composed of a family of related proteins sharing common structural and sequence
features.
 They having been classified according to their constituent protein such as desmin filaments,
keratin filaments, neurofilaments, vimentin, and glial filaments.
Functions
1. Intermediate filaments contribute to cellular structural elements and are often crucial in
holding together tissues like skin.
D. Microtrabecular Lattice
Recently, cytoplasm has been found to be filled with a three-dimensional network of interlinked
filaments of cytoskeletal fibers, called a micro-trabecular lattice. Various cellular organelles such
as ribosomes, lysosomes, etc., are found anchored to this lattice. The micro-trabecular lattice being
flexible changes its shape and results in the change of cell shape during cell movement.
Functions of Cytoskeleton

The cytoskeleton is responsible for lots of important cellular functions:

 In animal cells, which lack a rigid cell wall, it is the cytoskeleton that determines cell shape.
 It allows cells to move.
 Engulf particles.
 Brace themselves against pulling forces.
 Transport vesicles through the cytosol.
 Separate chromosomes during cell division.
 It allows our muscles to contract.
 SCHOOL OF BIO CHEMICAL ENGINEERING
DEPARTMENT OF BIOTECHNOLOGY

UNIT – III – CELL BIOLOGY – SBB1101

 UNIT: 3

CELLULAR ORGANELLES IN ENERGY METABOLISM

Mitochondria-structure and function, Chloroplast- structure and function,

Structure of nucleus-nuclear membrane, nucleolus, chromatin, structure of
nucleic acids.

MITOCHONDRIA DEFINITION

Mitochondria are oxygen-consuming ribbon-shaped cellular organelles of immense

importance floating free throughout the cell.
They are known as the “powerhouse of the cell” since these organelles supply all the
necessary biological energy to the cell by oxidizing the substrates available.
The enzymatic oxidation of chemical compounds in the mitochondria releases
energy.
Since mitochondria act as the power-houses, they are abundantly found on those sites
where energy is earnestly required such as sperm tail, muscle cell, liver cell (up to
1600 mitochondria), microvilli, oocyte (more than 300,000 mitochondria), etc.
Typically, there are about 2000 mitochondria per cell, representing around 25% of
the cell volume.
In 1890, mitochondria were first described by Richard Altmann and he called them
bioblasts. Benda in the year 1897 coined the term ‘mitochondrion’.

Mitochondria are mobile, plastic organelles that have a double-membrane structure.

It ranges from 0.5 to 1.0 micrometer in diameter. It has four distinct domains: the
outer membrane, the inner membrane, the intermembrane space, and the matrix.
Figure: Structure of Mitochondria
 The organelle is enclosed by two membranes—a smooth outer membrane and
a markedly folded or tubular inner mitochondrial membrane, which has a large
surface and encloses the matrix space.
 The intermembrane space is located between the inner and outer membranes.
 The number and shape of the mitochondria, as well as the numbers of cristae
they have, can differ widely from cell type to cell type.
 Tissues with intensive oxidative metabolism— e. g., heart muscle—have
mitochondria with particularly large numbers of cristae.
 Even within one type of tissue, the shape of the mitochondria can vary
depending on their functional status.
 Both mitochondrial membranes are very rich in proteins.
Outer Mitochondrial Membrane
 The outer mitochondrial membrane resembles more with the plasma
membrane in structure and chemical composition.
 Porins in the outer membrane allow small molecules to be exchanged between
the cytoplasm and the intermembrane space.
Inner Mitochondrial Membrane
 The inner mitochondrial membrane is rich in many enzymes, coenzymes, and
other components of electron transport chain. It also contains proton pumps
and many permease proteins for the transport of various molecules such as
citrates, ADP, phosphate, and ATP.
 The inner mitochondrial membrane gives out finger-like outgrowths (cristae)
towards the lumen of the mitochondrion and contains tennis-racket shaped F1
particles that contain ATP-ase enzyme for ATP synthesis.
 The inner mitochondrial membrane is completely impermeable even to small
molecules (with the exception of O2, CO2, and H2O).
 Numerous transporters in the inner membrane ensure the import and export of
important metabolites.
Intermembrane Space
 It is the space between the outer and inner membrane of the mitochondria, it
has the same composition as that of the cell’s cytoplasm.
 There is a difference in the protein content in the intermembrane space.
Mitochondrial Matrix
 The mitochondrial matrix which is the liquid (colloidal) area encircled by the
inner membrane, contains the soluble enzymes of the Krebs cycle which
completely oxidize the acetyl-CoA to produce CO2, H2O and hydrogen ions.
Hydrogen ions reduce the molecules of NAD and FAD, both of which pass on
hydrogen ions to respiratory or electron transport chain where oxidative
phosphorylation takes place to generate energy-rich ATP molecules.
 Mitochondria also contain in their matrix single or double circular and double-
stranded DNA molecules called mt DNA and also the 55S ribosomes, called
mitoribosomes. Since mitochondria can synthesize 10 percent of their proteins
in their own protein-synthetic machinery, they are considered as semi-
autonomous organelles.
Functions of Mitochondria
1. The most important function of mitochondria is to produce energy.
Mitochondria produce the molecule adenosine triphosphate (ATP), one of the
cell’s energy currencies that provide the energy to drive a host of cellular
reactions and mechanisms.
2. The simpler molecules of nutrition are sent to the mitochondria to be processed
and to produce charged molecules. These charged molecules combine with
oxygen and produce ATP molecules. This process is known as oxidative
phosphorylation.
3. Mitochondria may also produce heat (brown fat), and accumulate iron-
containing pigments (Heme ferritin), ions of Ca2+ and HPO42– (or phosphate;
e.g., osteoblasts of bones or yolk proteins).
4. Mitochondria help the cells to maintain the proper concentration of calcium
ions within the compartments of the cell.
5. The mitochondria also help in building certain parts of blood and hormones
like testosterone and estrogen.
6. The liver cell’s mitochondria have enzymes that detoxify ammonia.
7. The mitochondria also play an important role in the process of apoptosis or
programmed cell death.
8. Abnormal death of cells due to the dysfunction of mitochondria can affect the
function of an organ.

Mitochondrial Electron Transport Chain

The mitochondrial electron transport chain is composed of three main membrane-

associated electron carriers flavoproteins (FMN, FAD), cytochromes, and quinones
(coenzyme Q, also known as ubiquinone because it is a ubiquitous quinone in
biological systems).
All these electron carriers reside within the inner membrane of the mitochondria and
operate together to transfer electrons from donors, like NADH and FADH 2, to
acceptors, such as O2. The, electrons flow from carriers with more negative reduction
potentials to those with more positive reduction potentials and eventually combine
with O2 and H to form water.
However, the mitochondrial electron transport system is arranged into four enzyme
complexes of carriers, each capable of transporting electrons part of the way to
O2 (Fig. 24.5). Coenzyme Q and cytochrome c connect the complexes with each
other.
:

The four enzyme complexes of carriers are: NADH-Q oxidoreductase, succinate-Q-

reductase, Q-cytochrome c oxidoreductase, and cytochrome c oxidase. These
complexes are the enzyme complex and each of them consists of different prosthetic
groups (Table 24.2).
The process of mitochondrial electron transport chain is summarized in Figure 24.6,
which shows the flow of electrons and protons through the four enzyme complexes
of the transport chain.

The whole process of mitochondrial electron transport can be represented in brief in the
following manner:
1. Electrons donated by NADH enter the chain at complex I (NADH-Q-
oxidoreductase) and pass through a flavoprotein (FMN) to a series of iron-sulphur-
proteins (FeS) and then to ubiquinone (Q).

2. Electrons donated by succinate enter the chain at Complex II (succinate-Q-

reductase) and pass through a flavoprotein (FAD) and FeS centres and then to
ubiquinone (Q).

3. Ubiquinone (Q) serves as a mobile carrier of electrons received from complexes I

and II and passes them to complex III (Q-cytochrome c oxidoreductase).

4. Complex III called Q-cytochrome c oxidoreductase or cytochrome bc1 complex

passes the electrons through its prosthetic groups Cyt bL (Heme bL), Cyt bH (heme
bH), FeS, and Cyt cL (Heme cL) to cytochrome c.
5. Cytochrome c (Cyt c), a mobile connecting link between complex III and IV,
passes electrons to complex IV (cytochrome c oxidase). The latter carries electrons
through its prosthetic groups Cyt a (Heme a), Cyt a3 (Heme a3) CuA and CuB and
transfers them to molecular oxygen, reducing it to H2O.
6. Electron flow through complexes I, III and IV is accompanied by proton flow from
the mitochondrial matrix (which becomes negatively charged) to inter membrane
space or cytosolic side (which becomes positively charged). The number of protons
(H+) moved across the membrane at each site per pair of electrons transported is still
somewhat uncertain; the current consensus is that at least 10 protons move outward
during NADH oxidation.

CHLOROPLAST

Plants form the basis of all life on earth and are known as producers. Plant cells
contain structures known as plastids which are absent in animal cells. These plastids
are double-membraned cell organelles which play a primary role in the
manufacturing and storing of food. There are three types of plastids –

 Chromoplasts- They are the colour plastids, found in all flowers, fruits and are
mainly responsible for their distinctive colours.
 Chloroplasts- They are green coloured plastids, which comprises of green-
coloured pigments within the plant cell and are called as the chlorophyll.
 Leucoplasts- They are colourless plastids and are mainly used for the storage
of starch, lipids, and proteins within the plant cell.

Chloroplast Definition

“Chloroplast is an organelle that contains the photosynthetic pigment chlorophyll that captures
sunlight and converts it into useful energy, thereby, releasing oxygen from water. “

What is a Chloroplast?

Chloroplasts are found in all green plants and algae. They are the food producers of
plants. These are found in the guard cells located in the leaves of the plants. They
contain a high concentration of chlorophyll that traps sunlight. This cell organelle is
not present in animal cells.

Chloroplast has its own DNA and can reproduce independently from the rest of the
cell. They also produce amino acids and lipids required for the production of
chloroplast membrane.

DIAGRAM OF CHLOROPLAST

The chloroplast diagram below represents the chloroplast structure mentioning the
different parts of the chloroplast. The parts of a chloroplast such as the inner
membrane, outer membrane, intermembrane space, thylakoid membrane, stroma and
lamella can be clearly marked out.

Diagram representing Chloroplast Structure

Chloroplasts are found in all higher plants. It is oval or biconvex, found within the
mesophyll of the plant cell. The size of the chloroplast usually varies between 4-6 µm
in diameter and 1-3 µm in thickness. They are double-membrane organelle with the
presence of outer, inner and the thin intermembrane space. There are two distinct
regions present inside a chloroplast known as the grana and stroma.

 Grana is made up of stacks of disc-shaped structures known as

thylakoids. The grana of the chloroplast consists of chlorophyll pigments and
are the functional units of chloroplasts.
 Stroma is the homogenous matrix which contains grana and is similar to the
cytoplasm in cells in which all the organelles are embedded. Stroma also
contains various enzymes, DNA, ribosomes, and other substances. Stroma
lamellae function by connecting the stacks of thylakoid sacs.
The chloroplast structure consists of the following parts:

Membrane Envelope

It comprises inner and outer lipid bilayer membranes. The inner membrane separates
the stroma from the intermembrane space.

Intermembrane Space

The space between inner and outer membranes.

Thylakoid System

The system is suspended in the stroma. It is a collection of membranous sacs called

thylakoids. The green coloured pigments called chlorophyll are found in the
thylakoid membranes. It is the sight for the process of light-dependent reactions of
the photosynthesis process. The thylakoids are arranged in stacks known as grana and
each granum contains around 10-20 thylakoids.

Stroma

It is a colourless, alkaline, aqueous, protein-rich fluid present within the inner

membrane of the chloroplast present surrounding the grana.
Grana

These are the sites of conversion of light energy into chemical energy.

Chlorophyll

It is a green photosynthetic pigment that helps in the process of photosynthesis.

FUNCTIONS OF CHLOROPLAST

Following are the important chloroplast function:

 The most important function of the chloroplast is to synthesize food by the

process of photosynthesis.
 Absorbs light energy and converts it into chemical energy.
 Chloroplast has a structure called chlorophyll which functions by trapping the
solar energy and used for the synthesis of food in all green plants.
 Produces NADPH and molecular oxygen(O2) by photolysis of water.
 Produces ATP – Adenosine triphosphate by the process of photosynthesis .
 The carbon dioxide (CO2) obtained from the air is used to generate carbon and
sugar during the Calvin Cycle or dark reaction of photosynthesis.
Photosynthesis

The process by which green plants and some other organisms use sunlight to
synthesize nutrients from carbon dioxide and water. Photosynthesis in plants
generally involves the green pigment chlorophyll and generates oxygen as a by-
product.

Photosynthesis Equation

Photosynthesis reaction involves two reactants, carbon dioxide, and water. These two
reactants yield two products, namely, oxygen and glucose. Hence, the photosynthesis
reaction is considered to be an endothermic reaction. Following is the photosynthesis
formula:
6CO2 + 6H2O —> C6H12O6 + 6O2

Unlike plants, certain bacteria that perform photosynthesis do not produce oxygen as
the by-product of photosynthesis. Such bacteria are called anoxygenic
photosynthetic bacteria. The bacteria that do produce oxygen as a by-product of
photosynthesis are called oxygenic photosynthetic bacteria.

Photosynthetic Pigments

There are four different types of pigments present in leaves:

1. Chlorophyll a
2. Chlorophyll b
3. Xanthophylls
4. Carotenoids

Structure Of Chlorophyll

The structure of Chlorophyll consists of 4 nitrogen atoms that surround a magnesium

atom. A hydrocarbon tail is also present. Pictured above is chlorophyll-f, which is
more effective in near-infrared light than chlorophyll-a
Chlorophyll is a green pigment found in the chloroplasts of the plant cell and in the
mesosomes of cyanobacteria. This green colour pigment plays a vital role in the
process of photosynthesis by permitting plants to absorb energy from sunlight.
Chlorophyll is a mixture of chlorophyll-a and chlorophyll-b.

Besides green plants, other organisms that perform photosynthesis contain various
other forms of chlorophyll such as chlorophyll-c1, chlorophyll-c2, chlorophyll-
d and chlorophyll-f.

Process Of Photosynthesis

At the cellular level, the photosynthesis process takes place in cell organelles called
chloroplasts. These organelles contain a green-coloured pigment called chlorophyll,
which is responsible for the characteristic green colouration of the leaves.

As already stated, photosynthesis occurs in the leaves and the specialized cell
organelles responsible for this process is called the chloroplast. Structurally, a leaf
comprises a petiole, epidermis and a lamina. The lamina is used for absorption of
sunlight and carbon dioxide during photosynthesis.

“Photosynthesis Steps:”

 During the process of photosynthesis, carbon dioxide enters through the

stomata, water is absorbed by the root hairs from the soil and is carried to the
leaves through the xylem vessels. Chlorophyll absorbs the light energy from
the sun to split water molecules into hydrogen and oxygen.
 The hydrogen from water molecules and carbon dioxide absorbed from the air
are used in the production of glucose. Furthermore, oxygen is liberated out into
the atmosphere through the leaves as a waste product.
 Glucose is a source of food for plants that provide energy for growth and
development, while the rest is stored in the roots, leaves, and fruits for their
later use.
  Pigments are other fundamental cellular components of photosynthesis. They
are the molecules that impart colour and they absorb light at some specific
wavelength and reflect back the unabsorbed light. All green plants mainly
contain chlorophyll a, chlorophyll b and carotenoids which are present in the
thylakoids of chloroplasts. It is primarily used to capture light energy.
Chlorophyll-a is the main pigment.
The process of photosynthesis occurs in two stages:

 Light-dependent reaction or light reaction

 Light independent reaction or dark reaction

Stages of Photosynthesis in Plants de

Diagram depicting the two phases – Light reaction and Dark reaction

Light Reaction of Photosynthesis (or) Light-dependent Reaction

 Photosynthesis begins with the light reaction which is carried out only during
the day in the presence of sunlight. In plants, the light-dependent reaction takes
place in the thylakoid membranes of chloroplasts.
 The Grana, membrane-bound sacs like structures present inside the thylakoid
functions by gathering light and is called photosystems.
  These photosystems have large complexes of pigment and proteins molecules
present within the plant cells which plays the primary role during the process
of light reactions of photosynthesis.
 There are two types of photosystems: photosystem I and photosystem II.
 Under the light-dependent reactions, the light energy is converted to ATP and
NADPH which are used in the second phase of photosynthesis.
 During the light reactions, ATP and NADPH are generated by two electron-
transport chains, water is used and oxygen is produced.
The chemical equation in the light reaction of photosynthesis can be reduced to:

2H2O + 2NADP+ + 3ADP + 3Pi → O2 + 2NADPH + 3ATP

Dark Reaction of Photosynthesis (or) Light-independent Reaction

 Dark reaction is also called carbon-fixing reaction.

 It is a light-independent process in which sugar molecules are formed from the
water and carbon dioxide molecules.
 The dark reaction occurs in the stroma of the chloroplast where they utilize the
NADPH and ATP products of the light reaction.
 Plants capture the carbon dioxide from the atmosphere through stomata and
proceed to the Calvin photosynthesis cycle.
 In the Calvin cycle, the ATP and NADPH formed during light reaction drive
the reaction and convert 6 molecules of carbon dioxide into one sugar
molecule or glucose.
The chemical equation for the dark reaction can be reduced to:

3CO2 + 6 NADPH + 5H2O + 9ATP → G3P + 2H+ + 6 NADP+ + 9 ADP + 8 Pi

* G3P – glyceraldehyde-3-phosphate
Calvin photosynthesis Cycle (Dark Reaction)

Importance of Photosynthesis

 Photosynthesis is essential for the existence of all life on earth. It serves a

crucial role in the food chain – the plants create their food using this process,
thereby, forming the primary producers.
 Photosynthesis is also responsible for the production of oxygen – which is
needed by most organisms for their survival.

Frequently Asked Questions

1. What is Photosynthesis? Explain the process of photosynthesis.

Photosynthesis is a biological process utilized by all green plants to synthesize their

own nutrients. The process of photosynthesis requires solar energy, water and carbon
dioxide. The by-product of this process is oxygen.

2. What is the significance of Photosynthesis?

During photosynthesis, oxygen gas is liberated out into the environment and is
utilized by humans, animals and other living species during the process of
respiration.

3. List out the factors influencing Photosynthesis?

There are several factors that affect the rate of photosynthesis. Light intensity, water,
soil pH, carbon dioxide concentration, temperature and other climatic conditions are
the main factors affecting the rate of photosynthesis.

4. What are the different stages of Photosynthesis?

Photosynthesis takes place in two stages, namely light-dependent reactions and light-
independent reactions. Light-dependent reactions are also called light reactions and
occur during the day time. Light-independent reaction is also called the dark reaction
or the Calvin cycle.

5. What is the Calvin Cycle?

The Calvin cycle is also called the light-independent reaction. The complete process
of the Calvin cycle takes place in the stroma of the chloroplasts.

6. Write down the Photosynthesis Equation.

6CO2 + 6H2O —> C6H12O6 + 6O2

NUCLEUS STRUCTURE AND FUNCTIONS

 The cell nucleus is a membrane-bound structure that contains the cell’s

hereditary information and controls the cell’s growth and reproduction.
 It is the command center of a eukaryotic cell and is commonly the most
prominent organelle in a cell accounting for about 10 percent of the cell’s
volume.
 In general, a eukaryotic cell has only one nucleus. However, some eukaryotic
cells are enucleated cells (without a nucleus), for example, red blood cells
(RBCs); whereas, some are multinucleate (consists of two or more nuclei), for
example, slime molds.
  The nucleus is separated from the rest of the cell or the cytoplasm by a nuclear
membrane.
 As the nucleus regulates the integrity of genes and gene expression, it is also
referred to as the control center of a cell.

Figure: Diagram of Nucleus

Structure of Nucleus
The structure of a nucleus encompasses the nuclear membrane, nucleoplasm,
chromosomes, and nucleolus.
Nuclear Membrane
 The nuclear membrane is a double-layered structure that encloses the contents
of the nucleus. The outer layer of the membrane is connected to the
endoplasmic reticulum.
 Like the cell membrane, the nuclear envelope consists of phospholipids that
form a lipid bilayer.
 The envelope helps to maintain the shape of the nucleus and assists in
regulating the flow of molecules into and out of the nucleus through nuclear
pores. The nucleus communicates with the remaining of the cell or the
cytoplasm through several openings called nuclear pores.
 Such nuclear pores are the sites for the exchange of large molecules (proteins
and RNA) between the nucleus and cytoplasm.
 A fluid-filled space or perinuclear space is present between the two layers of a
nuclear membrane.
Nucleoplasm
 Nucleoplasm is the gelatinous substance within the nuclear envelope.
 Also called karyoplasm, this semi-aqueous material is similar to the cytoplasm
and is composed mainly of water with dissolved salts, enzymes, and organic
molecules suspended within.
 The nucleolus and chromosomes are surrounded by nucleoplasm, which
functions to cushion and protect the contents of the nucleus.
  Nucleoplasm also supports the nucleus by helping to maintain its shape.
Additionally, nucleoplasm provides a medium by which materials, such as
enzymes and nucleotides (DNA and RNA subunits), can be transported
throughout the nucleus. Substances are exchanged between the cytoplasm and
nucleoplasm through nuclear pores.
Nucleolus
 Contained within the nucleus is a dense, membrane-less structure composed
of RNA and proteins called the nucleolus.
 Some of the eukaryotic organisms have a nucleus that contains up to four
nucleoli.
 The nucleolus contains nucleolar organizers, which are parts of chromosomes
with the genes for ribosome synthesis on them. The nucleolus helps to
synthesize ribosomes by transcribing and assembling ribosomal RNA subunits.
These subunits join together to form a ribosome during protein synthesis.
 The nucleolus disappears when a cell undergoes division and is reformed after
the completion of cell division.
Chromosomes
 The nucleus is the organelle that houses chromosomes.
 Chromosomes consist of DNA, which contains heredity information and
instructions for cell growth, development, and reproduction.
 Chromosomes are present in the form of strings of DNA and histones (protein
molecules) called chromatin.
 When a cell is “resting” i.e. not dividing, the chromosomes are organized into
long entangled structures called chromatin.
 The chromatin is further classified into heterochromatin and euchromatin
based on the functions. The former type is a highly condensed,
transcriptionally inactive form, mostly present adjacent to the nuclear
membrane. On the other hand, euchromatin is a delicate, less condensed
organization of chromatin, which is found abundantly in a transcribing cell.
Besides the nucleolus, the nucleus contains a number of other non-membrane-
delineated bodies. These include Cajal bodies, Gemini of coiled bodies, polymorphic
interphase karyosome association (PIKA), promyelocytic leukemia (PML) bodies,
paraspeckles, and splicing speckles.
Functions of Nucleus
The nucleus provides a site for genetic transcription that is segregated from the
location of translation in the cytoplasm, allowing levels of gene regulation that are
not available to prokaryotes. The main function of the cell nucleus is to control gene
expression and mediate the replication of DNA during the cell cycle.
 It controls the hereditary characteristics of an organism.
 The organelle is also responsible for protein synthesis, cell division, growth,
and differentiation.
 Storage of hereditary material, the genes in the form of long and thin DNA
(deoxyribonucleic acid) strands, referred to as chromatin.
  Storage of proteins and RNA (ribonucleic acid) in the nucleolus.
 The nucleus is a site for transcription in which messenger RNA (mRNA) are
produced for protein synthesis.
 During the cell division, chromatins are arranged into chromosomes in the
nucleus.
 Production of ribosomes (protein factories) in the nucleolus.
 Selective transportation of regulatory factors and energy molecules through
nuclear pores.

Nucleic Acids- Nucleosides and Nucleotides

 Nucleotide is any member of the class of organic compounds in which the
molecular structure comprises a nitrogen-containing unit (base) linked to a
sugar and a phosphate group.
 They are monomeric units of nucleic acids and also serve as sources of
chemical energy (ATP, GTP), participate in cellular signalling (cAMP, cGMP)
and function as important cofactors of enzymatic reactions (coA, FAD, FMN,
NAD+).
 The molecule without the phosphate group of nucleotides is called as
nucleoside.
 Nucleosides are glycosylamines consisting simply of a nitrogenous base and a
five-carbon sugar (either ribose or deoxyribose).

Structure of Nucleotides
A single nucleotide is made up of three components: a nitrogen-containing base, a
five-carbon sugar (pentose), and at least one phosphate group With all three joined, a
nucleotide is also termed a “nucleoside phosphate”.
Individual phosphate molecules repetitively connect the sugar-ring molecules in two
adjacent nucleotide monomers, thereby connecting the nucleotide monomers of a
nucleic acid end-to-end into a long chain.
Unlike in nucleic acid nucleotides, singular cyclic nucleotides are formed when the
phosphate group is bound twice to the same sugar molecule, i.e., at the corners of the
sugar hydroxyl groups
Nitrogenous bases
 The nitrogenous base is either a purine or a pyrimidine.
 There are five major bases found in cells. The derivatives of purine are called
adenine and guanine, and the derivatives of pyrimidine are called thymine,
cytosine and uracil.
 Purines include adenine and guanine and have two rings.
 Adenine has an ammonia group on its rings, whereas guanine has a ketone
group.
 Pyrimidines include cytosine, thiamine, and uracil and have one ring.
 Thymine (found in DNA) and uracil (found in RNA) are similar in that they
both have ketone groups, but thymine has an extra methyl group on its ring.
 Bonds between guanine and cytosine (three hydrogen bonds) are stronger than
bonds between adenine and thymine (two hydrogen bonds).
Pentose Sugar
 The five-carbon sugar is either a ribose (in RNA) or a deoxyribose (in DNA)
molecule.
 In nucleotides, both types of pentose sugars are in their beta-furanose (closed
five-membered ring) form.
Structure of Nucleosides
 While a nucleotide is composed of a nucleobase, a five-carbon sugar, and one
or more phosphate groups, a nucleoside has only a nitrogenous base and a five-
carbon sugar.
 In a nucleoside, the base is bound to either ribose or deoxyribose via a beta-
glycosidic linkage at 1’ position.
 Examples of nucleosides include cytidine, uridine, adenosine, guanosine,
thymidine and inosine.
Properties of Nucleotides
Properties of purine bases
 Sparingly soluble in water
 Absorb light in UV region at 260 nm. (detection & quantitation of nucleotides)
 Capable of forming hydrogen bond
 Aromatic base atoms numbered 1 to 9
 Purine ring is formed by fusion of pyrimidine ring with imidazole ring.
 Numbering is anticlockwise.
Adenine : Chemically it is 6-aminopurine
Guanine : Chemically it is 2-amino,6-oxy purine
Can be present as lactam & lactim form
Properties of pyrimidine bases
 Soluble at body pH
  Also absorb UV light at 260 nm
 Capable of forming hydrogen bond
 Aromatic base atoms are numbered 1 to 6 for pyrimidine.
 Atoms or group attached to base atoms have same number as the ring atom to
which they are bonded.
Cytosine: Chemically is 2-oxy ,4-amino pyrimidine
Exist both lactam or lactim form
Thymine: Chemically is 2,4 dioxy ,5-methyl pyrimidine
Occurs only in DNA
Uracil: Chemically is 2,4 dioxy pyrimidine
Found only in RNA
Properties of Pentose Sugars
 A pentose is a monosaccharide with five carbon atoms.
 Ribose is the most common pentose with one oxygen atom attached to each
carbon atom.
 Deoxyribose sugar is derived from the sugar ribose by loss of an oxygen atom.
 The aldehyde functional group in the carbohydrates react with
neighbouring hydroxyl functional groups to form intramolecular hemiacetals.
 The resulting ring structure is related to furan, and is termed a furanose.
 The ring spontaneously opens and closes, allowing rotation to occur about the
bond between the carbonyl group and the neighboring carbon atom yielding
two distinct configurations (α and β). This process is termed mutarotation.

Classification of Nucleotides
On the basis of the type of sugar present, nucleotides may be:
1. Ribonucleotides if the sugar is ribose.
2. Deoxyribonucleotides if the sugar is deoxyribose.
Classification of Nucleosides
On the basis of type of nitrogenous bases present, nucleoside derivatives may be also
grouped as following:
1. Adenosine nucleotides: ATP, ADP, AMP, Cyclic AMP
2. Guanosine nucleotides: GTP, GDP, GMP, Cyclic GMP
3. Cytidine nucleotides: CTP, CDP, CMP and certain deoxy CDP derivatives of
glucose, choline and ethanolamine
4. Uridine nucleotides: UDP
5. Miscellaneous : PAPS (active sulphate), SAM (active methionine), certain
coenzymes like NAD+, FAD, FMN, Cobamide coenzyme, CoA
Functions of Nucleotides
 The nucleotides are of great importance to living organisms, as they are the
building blocks of nucleic acids, the substances that control all hereditary
characteristics.
 Polynucleotides consist of nucleosides joined by 3′,5′-phosphodiester bridges.
The genetic message resides in the sequence of bases along the polynucleotide
chain.
 Nucleotides have a variety of roles in cellular metabolism. They are the energy
currency in metabolic transactions.
 They act as essential chemical links in the response of cells to hormones and
other extracellular stimuli.
 They are the structural components of an array of enzyme cofactors and
metabolic intermediates.
 The structure of every protein, and ultimately of every biomolecule and
cellular component, is a product of information programmed into the
nucleotide sequence of a cell’s nucleic acids.
 Serving as energy stores for future use in phosphate transfer reactions. These
reactions are predominantly carried out by ATP.
 Forming a portion of several important coenzymes such as NAD+, NADP+,
FAD and coenzyme A.
 Serving as mediators of numerous important cellular processes such as second
messengers in signal transduction events. The predominant second messenger
is cyclic-AMP (cAMP), a cyclic derivative of AMP formed from ATP.
 Serving as neurotransmitters and as signal receptor ligands. Adenosine can
function as an inhibitory neurotransmitter, while ATP also affects synaptic
neurotransmission throughout the central and peripheral nervous systems. ADP
is an important activator of platelet functions resulting in control of blood
coagulation.
 Controlling numerous enzymatic reactions through allosteric effects on
enzyme activity.
 Serving as activated intermediates in numerous biosynthetic reactions. These
activated intermediates include S-adenosylmethionine (S-AdoMet or
 SAM) involved in methyl transfer reactions as well as the many sugar coupled
nucleotides involved in glycogen and glycoprotein synthesis.

WATSON AND CRICK DNA MODEL

 DNA stands for Deoxyribonucleic acid which is a molecule that contains the
instructions an organism needs to develop, live and reproduce.
 It is a type of nucleic acid and is one of the four major types of
macromolecules that are known to be essential for all forms of life.
DNA Model
 The three-dimensional structure of DNA, first proposed by James D. Watson
and Francis H. C. Crick in 1953, consists of two long helical strands that are
coiled around a common axis to form a double helix.
 Each DNA molecule is comprised of two biopolymer strands coiling around
each other.
 Each strand has a 5′end (with a phosphate group) and a 3′end (with a hydroxyl
group).
 The strands are antiparallel, meaning that one strand runs in a 5′to 3′direction,
while the other strand runs in a 3′to 5′direction.
 The diameter of the double helix is 2nm and the double helical structure
repeats at an interval of 3.4nm which corresponds to ten base pairs.
 The two strands are held together by hydrogen bonds and are complementary
to each other.
 The two DNA strands are called polynucleotides, as they are made of simpler
monomer units called nucleotides. Basically, the DNA is composed of
deoxyribonucleotides.
 The deoxyribonucleotides are linked together by 3′- 5′phosphodiester bonds.
 The nitrogenous bases that compose the deoxyribonucleotides include adenine,
cytosine, thymine, and guanine.
 The structure of DNA -DNA is a double helix structure because it looks like a
twisted ladder.
 The sides of the ladder are made of alternating sugar (deoxyribose) and
phosphate molecules while the steps of the ladder are made up of a pair of
nitrogen bases.
 As a result of the double helical nature of DNA, the molecule has two
asymmetric grooves. One groove is smaller than the other.
 This asymmetry is a result of the geometrical configuration of the bonds
between the phosphate, sugar, and base groups that forces the base groups to
attach at 120-degree angles instead of 180 degrees.
 The larger groove is called the major groove, occurs when the backbones are
far apart; while the smaller one is called the minor groove, and occurs when
they are close together.
 Since the major and minor grooves expose the edges of the bases, the grooves
can be used to tell the base sequence of a specific DNA molecule.
 The possibility for such recognition is critical since proteins must be able to
recognize specific DNA sequences on which to bind in order for the proper
functions of the body and cell to be carried out.
 SCHOOL OF BIO AND CHEMICAL ENGINEERING
DEPARTMENT OF BIOTECHNOLOGY

UNIT – IV – CELL BIOLOGY – SBB1101

 UNIT: 4
CELLULAR ORGANELLES IN METABOLISM
Endoplasmic reticulum-smooth & rough, function of endoplasmic reticulum; Golgi
complex-structure and function: Ribosomes-Types, structure and function,
Morphology and functions of peroxisomes and glyoxisomes, Plant cell Vacuoles.
ENDOPLASMIC RETICULUM (ER)- DEFINITION, STRUCTURE,
FUNCTIONS AND DIAGRAM
Within the cytoplasm of most animal cells is an extensive network (reticulum) of
membrane-limited channels, collectively called the endoplasmic reticulum (or ER).
The endoplasmic reticulum is a name derived from the fact that in the light
microscope it looks like a “net in the cytoplasm.”
The endoplasmic reticulum is only present in the eukaryotic cells. However, the
occurrence of the endoplasmic reticulum varies from cell to cell.
For example, the erythrocytes (RBC), egg and embryonic cells lack in the
endoplasmic reticulum.
Some portion of ER membranes remains continuous with the plasma membrane and
the nuclear envelope.
ER may be rough or smooth. The outer surface of rough ER has attached ribosomes,
whereas smooth ER does not have attached ribosomes.
The endoplasmic reticulum acts as secretory, storage, circulatory and nervous system
for the cell. It is also the site of the biogenesis of cellular membranes.
Figure: Diagram of Endoplasmic Reticulum (ER)
The membrane of the endoplasmic reticulum is 50 to 60 Aº thickness and fluid-
mosaic like the unit membrane of the plasma membrane.
The membranes of the endoplasmic reticulum are found to contain many kinds of
enzymes that are needed for various important synthetic activities. The most
important enzymes are the stearases, NADH-cytochrome C reductase, NADH
diaphorase, glucose-6-phosphatase, and Mg++ activated ATPase.
The membrane of endoplasmic reticulum remains continuous with the membranes of
the plasma membrane, nuclear membrane, and Golgi apparatus.
The cavity of the endoplasmic reticulum is well developed and acts as a passage for
the secretory products.
The endoplasmic reticulum may occur in the following three forms:
Lamellar form or cisternae
Vesicular form or vesicle and
Tubular form or tubules.
The Cisternae
RER usually exists as cisternae that occur in those cells which have synthetic roles as
the cells of the pancreas, notochord, and brain.
The cisternae are long, flattened, sac-like, unbranched tubules having a diameter of
40 to 50 μm.
They remain arranged parallelly in bundles or stakes.
The Vesicles
The vesicles are oval; membrane-bound vacuolar structures having a diameter of 25
to 500 μm.
They often remain isolated in the cytoplasm and occur in most cells but especially
abundant in the SER.
The Tubules
The tubules are branched structures forming the reticular system along with the
cisternae and vesicles.
They usually have a diameter from 50 to 190 μm and occur almost in all the cells.
Tubular form of ER is often found in SER and is dynamic in nature, i.e., it is
associated with membrane movements, fission and fusion between membranes of
cytocavity network.
Types of Endoplasmic Reticulum (ER)
1. Smooth Endoplasmic Reticulum
They are also called as the agranular endoplasmic reticulum.
This type of endoplasmic reticulum possesses smooth walls because the ribosomes
are not attached to its membranes.
The smooth type of endoplasmic reticulum occurs mostly in those cells, which are
involved in the metabolism of lipids (including steroids) and glycogen. Eg. adipose
cells, interstitial cells, glycogen storing cells of the liver, conduction fibers of heart,
spermatocytes, and leucocytes.
2. Rough Endoplasmic Reticulum
It possesses rough walls because the ribosomes remain attached to its membranes.
On their membranes, rough ER (RER) contains certain ribosome specific,
transmembrane glycoproteins, called ribophorins I and II, to which are attached the
ribosomes while engaged in polypeptide synthesis.
The rough type of endoplasmic reticulum is found abundantly in those cells which
are active in protein syntheses such as pancreatic cells, plasma cells, goblet cells, and
liver cells.
Functions
 Functions of smooth ER include lipid metabolism (both catabolism and
anabolism; they synthesize a variety of phospholipids, cholesterol, and
steroids).
 Glycogenolysis (degradation of glycogen; glycogen being polymerized in the
cytosol).
 Drug detoxification (by the help of the cytochrome P-450).
 The endoplasmic reticulum provides an ultrastructural skeletal framework to
the cell and gives mechanical support to the colloidal cytoplasmic matrix.
 The exchange of molecules by the process of osmosis, diffusion and active
transport occurs through the membranes of the endoplasmic reticulum.
 The endoplasmic reticulum is the main component of the endomembrane
system, also called the cytoplasmic vacuolar system or cytocavity network.
  The endoplasmic membranes contain many enzymes that perform various
synthetic and metabolic activities. Further, the endoplasmic reticulum provides
an increased surface for various enzymatic reactions.
 The endoplasmic reticulum acts as an intracellular circulatory or transporting
system.
 As a growing secretory polypeptide emerges from the ribosome, it passes
through the RER membrane and gets accumulated in the lumen of RER. Here,
the polypeptide chains undergo tailoring, maturation, and molecular folding to
form functional secondary or tertiary protein molecules.
 RER pinches off certain tiny protein-filled vesicles which ultimately get fused
to cis Golgi.
 The ER membranes are found to conduct intra-cellular impulses. For example,
the sarcoplasmic reticulum transmits impulses from the surface membrane into
the deep region of the muscle fibers.
 The ER membranes form the new nuclear envelope after each nuclear division.
 The SER contains several key enzymes that catalyze the synthesis of
cholesterol which is also a precursor substance for the biosynthesis of two
types of compounds— the steroid hormones and bile acids.
 RER also synthesize membrane proteins and glycoproteins which are
cotranslationally inserted into the rough ER membranes. Thus, the
endoplasmic reticulum is the site of the biogenesis of cellular membranes.

GOLGI COMPLEX STRUCTURE AND FUNCTION

 The Golgi apparatus or the Golgi body or Golgi complex or simply Golgi is a
cellular organelle present in most of the cells of the eukaryotic organisms.
 It is referred to as the manufacturing and the shipping center of the cell.
 Golgi is involved in the packaging of the protein molecules before they are
sent to their destination. These organelles help in processing and packaging the
macromolecules like proteins and lipids that are synthesized by the cell and
hence act as the ‘post office’ of the cell.
 Golgi apparatus was discovered in
 the year 1898 by an Italian biologist Camillo Golgi.
Figure: Diagram of the Golgi Apparatus
Structure of Golgi Apparatus
 Under the electron microscope, the Golgi apparatus is seen to be composed of
stacks of flattened structures that contain numerous vesicles containing
secretory granules.
 The Golgi apparatus is morphologically very similar in both plant and animal
cells. However, it is extremely pleomorphic: in some cell types it appears
compact and limited, in others spread out and reticular (net-like).
 Typically, however, Golgi apparatus appears as a complex array
of interconnecting tubules, vesicles, and cisternae.
A. Cisternae
 It is the simplest unit of the Golgi apparatus is the cisterna.
 Cisternae (about 1 μm in diameter) are central, flattened, plate-like or saucer-
like closed compartments that are held in parallel bundles or stacks one above
the other.
 In each stack, cisternae are separated by a space of 20 to 30 nm which may
contain rod-like elements or fibers.
 Each stack of cisternae forms a dictyosome which may contain 5 to 6 Golgi
cisternae in animal cells or 20 or more cisternae in plant cells.
 Each cisterna is bounded by a smooth unit membrane (7.5 nm thick), having a
lumen varying in width from about 500 to 1000 nm.
 The margins of each cisterna are gently curved so that the entire dictyosome of
the Golgi apparatus takes on a bow-like appearance.
 The cisternae at the convex end of the dictyosome comprise proximal, forming
or cis-face and cisternae at the concave end of the dictyosome comprise the
distal, maturing or trans-face.
B. Tubules
  A complex array of associated vesicles and anastomosing tubules (30 to 50 nm
diameter) surround the dictyosome and radiate from it. In fact, the peripheral
area of the dictyosome is fenestrated (lace-like) in structure.
C. Vesicles
The vesicles (60 nm in diameter) are of three types:
(i) Transitional vesicles are small membrane limited vesicles which are thought to
form as blebs from the transitional ER to migrate and converge to cis face of Golgi,
where they coalesce to form new cisternae.
(ii) Secretory vesicles are varied-sized membrane-limited vesicles that discharge
from margins of cisternae of Golgi. They, often, occur between the maturing face of
Golgi and the plasma membrane.
(iii) Clathrin-coated vesicles are spherical protuberances, about 50 μm in diameter
and with a rough surface. They are found at the periphery of the organelle, usually at
the ends of single tubules, and are morphologically quite distinct from the secretory
vesicles. The clathrin-coated vesicles are known to play a role in intracellular traffic
of membranes and of secretory products, i.e., between ER and Golgi, as well as,
between the GELR region and the endosomal and lysosomal compartments.
Functions of Golgi Apparatus
1. Golgi vesicles are often, referred to as the “traffic police” of the cell. They play a
key role in sorting many of the cell’s proteins and membrane constituents, and in
directing them to their proper destinations.
 To perform this function, the Golgi vesicles contain different sets of enzymes
in different types of vesicles— cis, middle and trans cisternae—that react with
and modify secretory proteins passing through the Golgi lumen or membrane
proteins and glycoproteins that are transiently in the Golgi membranes as they
are en route to their final destinations.
 The Golgi apparatus hence acts as the assembly factory of the cell where the
raw materials are directed to the Golgi apparatus before being passed out from
the cell.
2. In animals, the Golgi apparatus is involved in the packaging and exocytosis of the
following materials :
 Zymogen of exocrine pancreatic cells;
 Mucus (=a glycoprotein) secretion by goblet cells of the intestine ;
 Lactoprotein (casein) secretion by mammary gland cells (Merocrine secretion)
;
 Secretion of compounds (thyroglobulins) of thyroxine hormone by thyroid
cells;
 Secretion of tropocollagen and collagen ;
 Formation of melanin granules and other pigments; and
 Formation of yolk and vitelline membrane of growing primary oocytes.
3. It is also involved in the formation of certain cellular organelles such as plasma
membrane, lysosomes, acrosome of spermatozoa and cortical granules of a variety of
oocytes.
4. They are also involved in the transport of lipid molecules around the cell.
5. The Golgi complex also plays an important role in the production of
proteoglycans. The proteoglycans are molecules that are present in the extracellular
matrix of the animal cells.
6. It is also a major site of synthesis of carbohydrates. These carbohydratres include
the synthesis of glycosaminoglycans, Golgi attaches to these polysaccharides which
then attaches to a protein produced in the endoplasmic reticulum to form
proteoglycans.
7. The Golgi involves in the sulfation process of certain molecules.
8. The process of phosphorylation of molecules by the Golgi requires the import of
ATP into the lumen of the Golgi.
9. In plants, Golgi apparatus is mainly involved in the secretion of materials of
primary and secondary cell walls (e.g., formation and export of glycoproteins, lipids,
pectins and monomers for hemicellulose, cellulose, lignin, etc.)

RIBOSOMES-TYPES, STRUCTURE AND FUNCTION

 The ribosome word is derived – ‘ribo’ from ribonucleic acid and ‘somes’ from
the Greek word ‘soma’ which means ‘body’.
 Ribosomes are tiny spheroidal dense particles (of 150 to 200 A0 diameters) that
are primarily found in most prokaryotic and eukaryotic.
 They are sites of protein synthesis.
 They are structures containing approximately equal amounts of RNA and
proteins and serve as a scaffold for the ordered interaction of the numerous
molecules involved in protein synthesis.
 The ribosomes occur in cells, both prokaryotic and eukaryotic cells.
 In prokaryotic cells, the ribosomes often occur freely in the cytoplasm.
 In eukaryotic cells, the ribosomes either occur freely in the cytoplasm or
remain attached to the outer surface of the membrane of the endoplasmic
reticulum.
 The location of the ribosomes in a cell determines what kind of protein it
makes.
 If the ribosomes are floating freely throughout the cell, it will make proteins
that will be utilized within the cell itself.
 When ribosomes are attached to the endoplasmic reticulum, it is referred to as
rough endoplasmic reticulum or rough ER.
 Proteins made on the rough ER are used for usage inside the cell or outside the
cell.
 The number of ribosomes in a cell depends on the activity of the cell.
 On average in a mammalian cell, there can be about 10 million ribosomes.
Figure: Diagram of Ribosomes
Structure of Ribosomes
 A ribosome is made from complexes of RNAs and proteins and is, therefore, a
ribonucleoprotein.
 Around 37 to 62% of RNA is comprised of RNA and the rest is proteins.
 Each ribosome is divided into two subunits:
1. A smaller subunit which binds to a larger subunit and the mRNA pattern, and
2. A larger subunit which binds to the tRNA, the amino acids, and the smaller
subunit.
 Prokaryotes have 70S ribosomes respectively subunits comprising the little
subunit of 30S and the bigger subunit of 50S.
 Their small subunit has a 16S RNA subunit (consisting of 1540 nucleotides)
bound to 21 proteins.
 The large subunit is composed of a 5S RNA subunit (120 nucleotides), a 23S
RNA subunit (2900 nucleotides) and 31 proteins.
 Eukaryotes have 80S ribosomes respectively comprising of little (40S) and
substantial (60S) subunits.
 The smaller 40S ribosomal subunit is prolate ellipsoid in shape and consists of
one molecule of 18S ribosomal RNA (or rRNA) and 30 proteins (named as S1,
S2, S3, and so on).
 The larger 60S ribosomal subunit is round in shape and contains a channel
through which growing polypeptide chain makes its exit.
 It consists of three types of rRNA molecules, i.e., 28S rRNA, 5.8 rRNA and 5S
rRNA, and 40 proteins (named as L1, L2, L3 and so on).
 The differences between the ribosomes of bacterial and eukaryotic are used to
create antibiotics that can destroy bacterial infection without harming human
cells.
 The ribosomes seen in the chloroplasts of mitochondria of eukaryotes are
comprised of big and little subunits composed of proteins inside a 70S particle.
  The ribosomes share a core structure that is similar to all ribosomes despite
differences in its size.
 The two subunits fit together and work as one to translate the mRNA into a
polypeptide chain during protein synthesis.
 Because they are formed from two subunits of non-equal size, they are slightly
longer in the axis than in diameter.
 During protein synthesis, when multiple ribosomes are attached to the same
mRNA strand, this structure is known as polysome.
 The existence of ribosomes is temporary, after the synthesis of polypeptide the
two sub-units separate and are reused or broken up.
Functions of Ribosomes
 The ribosome is a complex molecular machine, found within all living cells,
that serves as the site of biological protein synthesis (translation).
 Ribosomes link amino acids together in the order specified by messenger
RNA (mRNA) molecules.

MORPHOLOGY AND FUNCTIONS OF PEROXISOMES

Peroxisomes are small vesicles, single membrane-bound organelles found around the
eukaryotic cells. They contain digestive enzymes for breaking down toxic materials
in the cell and oxidative enzymes for metabolic activity. They are a heterogeneous
group of organelles and the presence of the marker enzymes distinguished them from
other cell organelles.
Peroxisomes play an important role in lipid production and are also involved in the
conversion of reactive oxygen species such as hydrogen peroxide into safer
molecules like water and oxygen.
The peroxisome derives its name from the fact that many metabolic enzymes that
generate hydrogen peroxide as a by-product are sequestered here because peroxides
are toxic to cells. Within peroxisomes, hydrogen peroxide is degraded by the enzyme
catalase to water and oxygen.
Peroxisomes are surrounded by a single membrane and they range in the diameter
from 0.1 to 1 mm. They exist either in the form of a network of interconnected
tubules (peroxisome reticulum) as in liver cells or as individual micro peroxisomes in
other cells such as tissue culture fibroblasts.

Peroxisome Structure
Peroxisomes vary in shape, size and number depending upon the energy requirements
of the cell. These are made of a phospholipid bilayer with many membrane-bound
proteins.
The enzymes involved in lipid metabolism are synthesised on free ribosomes and
selectively imported to peroxisomes. These enzymes include one of the two
signalling sequences- Peroxisome Target Sequence 1 being the most common one.
The phospholipids of peroxisomes are usually synthesised in smooth Endoplasmic
reticulum. Due to the ingress of proteins and lipids, the peroxisome grows in size and
divides into two organelles.

Peroxisome Function
The main function of peroxisome is the lipid metabolism and the processing of
reactive oxygen species. Other peroxisome functions include:

 The synthesis of ether glycerolipids of plasmalogens.

 The formation of bile acids, dolichol, and cholesterol.
 The catabolism of purines, polyamines, and amino acids, and the detoxification
of reactive oxygen species
 In methylotrophic yeasts, peroxisomes are also involved in the metabolism of
methanol and methylamines.
In plants, peroxisomes facilitate photosynthesis and seed germination. They prevent
loss of energy during photosynthesis carbon fixation.

Metabolism of Peroxisomes
Isolated peroxisomes are permeable to small molecules such as sucrose. During the
isolation process, they often lose proteins that are normally confined to the
peroxisomal matrix. In all living cells, peroxisomes are the sealed vesicles
surrounded by a single membrane.
Biogenesis of Peroxisomes
As peroxisomes have no DNA, all their proteins must be imported from genes
encoded in the nucleus. Most of the proteins that reside in the peroxisome matrix and
membrane are synthesized in the cytosol and then imported posttranslationally to the
organelle.
About 25 PEX genes, encoding proteins called peroxins are necessary for the
biogenesis of the organelle. Most of these genes are found in multiple organisms and
13 are conserved in humans.

MORPHOLOGY AND FUNCTIONS OF GLYOXYSOMES,

Glyoxysomes are specialized peroxisomes found in plants (particularly in
the fat storage tissues of germinating seeds) and also in filamentous fungi. Seeds that
contain fats and oils include corn, soybean, sunflower, peanut and pumpkin. [1] As in
all peroxisomes, in glyoxysomes the fatty acids are oxidized to acetyl-CoA by
peroxisomal β-oxidation enzymes. When the fatty acids are oxidized hydrogen
peroxide (H2O2) is produced as oxygen (O2) is consumed.[1] Thus the seeds need
oxygen to germinate. Besides peroxisomal functions, glyoxysomes possess
additionally the key enzymes of glyoxylate cycle (isocitrate lyase and malate
synthase) which accomplish the glyoxylate cycle bypass.
Thus, glyoxysomes (as all peroxisomes) contain enzymes that initiate the breakdown
of fatty acids and additionally possess the enzymes to produce intermediate products
for the synthesis of sugars by gluconeogenesis. The seedling uses these sugars
synthesized from fats until it is mature enough to produce them by photosynthesis.
Plant peroxisomes also participate in photorespiration and nitrogen metabolism in
root nodules.

MORPHOLOGY AND FUNCTIONS OF PLANT CELL VACUOLES.

 A vacuole is a membrane-bound organelle that is present in all plant and

fungal cells and some protist, animal and bacterial cells.
 The most conspicuous compartment in most plant cells is a very large, fluid-
filled vacuole. Large vacuoles are also found in three genera of
filamentous sulfur bacteria, the Thioploca, Beggiatoa, and Thiomargarita.
 However, the function and significance of vacuoles vary greatly according to
the type of cell having much greater prominence in the cells of plants, fungi,
and certain protists than those of animals and bacteria.
 There may be several vacuoles in a single cell. Each vacuole is separated from
the cytoplasm by a single unit membrane, called the tonoplast.
 Generally, they occupy more than 30 percent of the cell volume; but this may
vary from 5 percent to 90 percent, depending on the cell type.

Figure: Diagram of Vacuoles

Structure of Vacuoles
 They generally have no basic shape or size; its structure varies according to the
requirements of the cell.
  In immature and actively dividing plant cells the vacuoles are quite small.
These vacuoles arise initially in young dividing cells, probably by the
progressive fusion of vesicles derived from the Golgi apparatus.
 A vacuole is surrounded by a membrane called the tonoplast or vacuolar
membrane and filled with cell sap.
 The tonoplast is the cytoplasmic membrane surrounding a vacuole, separating
the vacuolar contents from the cell’s cytoplasm. As a membrane, it is mainly
involved in regulating the movements of ions around the cell, and isolating
materials that might be harmful or a threat to the cell.
 Vacuoles are structurally and functionally related to lysosomes in animal cells
and may contain a wide range of hydrolytic enzymes. In addition, they usually
contain sugars, salts, acids and nitrogenous compounds such as alkaloids and
anthocyanin pigments in their cell sap.
 The pH of plant vacuoles may be as high as 9 to 10 due to large quantities of
alkaline substances or as low as 3 due to the accumulation of quantities of
acids (e.g., citric, oxalic and tartaric acids).
Types
Sap Vacuoles:
 It has a number of transport systems for the passage of different substances. A
number of small sap vacuoles occur in animal cells and young plant cells. In
mature plant cells, the small vacuoles fuse to form a single large central
vacuole which occupies up to 90% of the volume of the cell.
 The large central vacuole spreads the cytoplasm in the form of a thin
peripheral layer.
 This is a device to facilitate rapid exchange between the cytoplasm and the
surrounding environment. The fluid present in the sap vacuoles is often called
a sap or vacuolar sap.
Contractile Vacuoles:
 They occur in some protistan and algal cells found mostly in freshwater.
 A contractile vacuole has a highly extensible and collapsible membrane. It is
also connected to a few feeding canals (e.g., Paramecium). The feeding canals
obtain water with or without waste products from the surrounding cytoplasm.
They pour the same into the contractile vacuole.
 The vacuole swells up. The process is called diastole. The swollen contractile
vacuole comes in contact with the plasma membrane and collapses. Collapsing
is called systole. This throws the vacuolar contents to the outside.
 Contractile vacuoles take part in osmoregulation and excretion.
Food Vacuoles:
 They occur in the cells of protozoan protists, several lower animals and
phagocytes of higher animals.
 A food vacuole is formed by the fusion of a phagosome and a lysosome. The
food vacuole contains digestive enzymes with the help of which nutrients are
digested. The digested materials pass out into the surrounding cytoplasm.
Air Vacuoles (Pseudo-vacuoles, Gas vacuoles):
 They have been reported only in prokaryotes.
 An air vacuole is not a single entity, neither it is surrounded by a common
membrane. It consists of a number of smaller sub-microscopic vesicles. Each
vesicle is surrounded by a protein membrane and encloses metabolic gases.
 Air vacuoles not only store gases but provide buoyancy, mechanical strength
and protection from harmful radiations.
Functions
A plant vacuole has a variety of functions. Different vacuoles with distinct functions
are also often present in the same cell.
 Plant vacuoles can store many types of molecules. It can act as a storage
organelle for both nutrients and waste products.
 Some of the products stored by vacuoles have a metabolic function. For
example, succulent plants open their stomata and take up carbon dioxide at
night (when transpiration losses are less than in the day) and convert it to malic
acid. This acid is stored in vacuoles until the following day when light energy
can be used to convert it to sugar while the stomata are kept shut.
 In particular, they can sequester substances that are potentially harmful to the
plant cell, if they are present in bulk in the cytoplasm.
 The vacuole has an important homeostatic function in plant cells that are
subjected to wide variations in their environment. For example, when the pH in
the environment drops, the flux of H+ into the cytoplasm is buffered by
increased transport of H+ into the vacuole.
 Many plant cells maintain turgor pressure at remarkable constant levels in the
face of large changes in the tonicity of the fluids in their immediate
environment by changing the somatic pressure of the cytoplasm and vacuole—
in part by controlled breakdown and resynthesis of polymers such as
polyphosphate in the vacuole, and in part by altering
 By increasing in size, vacuoles allow the germinating plant or its organs (such
as leaves) to grow very quickly and using up mostly just water.
 In seeds, stored proteins needed for germination are kept in ‘protein bodies’,
which are modified vacuoles.
In Other Cells
 In fungal cells, they are involved in many processes including
the homeostasis of cell pH and the concentration of ions, osmoregulation,
storing amino acids and polyphosphate and degradative processes.
 In animal cells, vacuoles perform mostly subordinate roles, assisting in larger
processes of exocytosis and endocytosis.
 SCHOOL OF BIO AND CHEMICAL ENGINEERING
DEPARTMENT OF BIOTECHNOLOGY

UNIT – V – CELL BIOLOGY – SBB1101

 UNIT: 5
Passive and Active Transport, Sodium Potasssium Pumps, Ca 2+ATPase pump, ATP
dependant proton pumps, co-transport, symport, antiport, Endocytosis and
Exocytosis. Introduction to intra and extracellular products of medicinal use.

Passage of substances across bio membranes or cell membrane is brought about

by the following methods:
A. Transport of Water:
(I) Osmosis:
Osmosis is the diffusion of water or solvent molecules through plasma membrane
from low osmotic pressure to high osmotic pressure, i.e., from high water contents to
low water content. Plasma membrane acts as a differential membrane permitting the
movement of water molecules in and out retaining the metabolites.

B. Transport of Ions and Small Molecules:

(II) Passive Transport:
It is a mode of membrane transport where the cell does not spend any energy nor
shows any special activity. The transport is according to concentration gradient. It is
of two types, passive diffusion and facilitated diffusion.

(a) Passive Diffusion or Transport across Cell Membranes:

Here the cell membrane plays a passive role in the transport of substances across it.
Passive diffusion can occur either through lipid matrix of the membrane or with the
help of channels.

(i) Lipid Soluble Substances:

It was found out by Overton (1900) that lipid soluble substances pass rapidly across
the cell membrane according to their concentration gradient. Based on this finding,
Overton proposed that cell membranes are made of lipids.

(ii) Channel Transport:

Membrane possesses channels in the form of tunnel proteins which do not carry any
charge. They allow water and soluble gases (CO2 and O2) to pass according to their
concentration gradient. Osmosis is an example of such a transport.
If two solutions of different concentrations are separated by a semi-permeable
membrane, the solvent molecules move across the membrane from the less
concentrated into the more concentrated solution. This process- the diffusion of
solvent molecules into a region in which there is a higher concentration of solute to
which the membrane is impermeable, is called osmosis.

Filtration is diffusion under pressure across a membrane having minute pores. Ultra
filtration occurs during glomerular filtration inside kidneys. Dialysis is the process of
separating small particles (e.g., crystalline solutes) from larger ones (e.g., colloids)
due to difference in the rate of diffusion across a membrane having very minute
pores.

(b) Facilitated Diffusion:

It occurs through the agency of special membrane proteins called permeases. When
such carrier-mediated transport is from an area of greater concentration, energy is not
required and the process is called facilitated diffusion. As a result, the rate of
transport is stereo-specific.
Entry of glucose into red blood corpuscles is a facilitated diffusion.

The process of facilitated diffusion involves following steps:

1. Diffusing molecules combine with the specific carrier protein molecules forming
carrier-protein complexes.

2. The shape of carrier protein molecule changes in response to the diffusing

molecule so that the membrane – bound carrier protein complexes form, channels.

3. The shape of carrier protein molecule changes in response to diffusing molecule,

allowing the molecule to cross the plasma membrane.

4. Once the diffusing molecule has reached the other side, the change in shape of
carrier molecule (conformational change) lowers its affinity with the diffusing
molecule, and allows it to be released.

5. After the release of diffusing molecule, the carrier-protein molecule resumes

original shape.

The facilitated diffusion enables molecules to cross the otherwise impermeable or

poorly permeable membrane.

Facilitated diffusion differs from simple diffusion in following features:

(i) Facilitated diffusion is stereo-specific (either L or D isomer is transported).

(ii) It shows saturation kinetics.

(iii) Facilitated diffusion requires a carrier for transport across the membrane. The
carrier protein molecules move to and fro across the membrane by thermal diffusion.

(III) Active Transport:

It is uphill movement of materials across the membranes where the solute particles
move against their chemical concentration or electro-chemical gradient. This form of
transport requires energy which is provided almost exclusively by hydrolysis of ATP.

Active transport occurs in case of both ions and non-electrolytes, e.g., salt uptake by
plant cells, ions, glucose and phenolphthalein in case of renal tubules, sodium and
potassium in case of nerve cells, etc. It is supported by various evidences:

(a) Absorption is reduced or stopped with the decrease in oxygen content of the
surrounding environment.

(b) Metabolic inhibitors like cyanides inhibit absorption.

(c) Cells often accumulate salts and other substances against their concentration
gradient.

(d) Active transport shows saturation kinetics that is, the rate of transport increases
with increase in solute concentration till a maximum is achieved. Beyond this value
the rate of membrane transport does not increase indicating that it takes place through
the agency of special organic molecules called carrier molecules, carrier particles or
carrier proteins.

The carrier molecules are ATP ases, enzymes that catalyze the hydrolysis of ATP.
The most important of these ATPases is Na+-K+ ATP ase which is also known as the
Na+-K+ pump. There are in addition H+-K+ ATPases in the gastric mucosa and the
renal tubules.
There is a special carrier molecule for each solute particle. The carrier has its binding
site on two surfaces of the membrane. The solute particles combine with the carrier
to form carrier-solute complex. In the bound state the carrier undergoes a
conformational change which transports the solute to the other side of the membrane.
Energy is used in bringing about the conformational change in the carrier. It is
provided by ATP. In the process ATP is dephosphorylated to form ADP. Carrier
proteins are of three types.

1. Uniport:
They transport only one substance.

2. Symports:
In some cases, transport requires the binding of more than one substance to the
transport protein and the substances are transported across the membrane together.
An example is the symport in the intestinal mucosa that is responsible for the co-
transport by facilitated diffusion of Na+ and glucose from the intestinal lumen into
mucosal cells.
3. Antiports:
They exchange one substance for another. The Na+-K+ ATPase is a typical antiport.
Many animal cells operate a sodium-potassium exchange pump at their plasma
membrane. A similar proton pump operates in chloroplasts, mitochondria and
bacteria Na+—K+ exchange pump operates with the help of enzyme ATP-ase which
also functions as a carrier molecule.

The enzyme hydrolyses ATP to release energy. The energy is used in bringing about
conformational changes in the carrier. For every ATP molecule hydrolysed, three
Na+ ions are pumped outwardly and two K+ ions are pumped inwardly.
Na+– K+ exchange pump performs the following functions: (i) Maintains a positive
potential on the outer side of the membrane and relatively electronegative potential
on the inner side,
(ii) The pump creates a resting potential in the nerve cells,

(iii) The pump maintains water balance of living cells.

(iv) It helps in urine formation,

(v) It takes part in excretion of salt as in marine animals. Sea gulls and penguins
drink sea water. They excrete excess salt through nasal glands. The nasal salt glands
have sodium-potassium pump in the plasma membranes of their cells. Na + ions are
pumped out actively. Chlorine ions pass out passively. Nasal secretion of the two
birds possesses 1.5—3.0 times more NaCl concentration than the one present in the
blood.
(vi) The unsecreted and unmetabolised excess Na+ ions present in the extracellular
fluid have a tendency to pass back into the cells. Other substances combine with
sodium ions and pass inwardly along-with them, e.g., glucose, amino acids in
intestine. The phenomenon is called secondary active transport as compared to Na +—
K+ exchange pump which is called primary active transport.

Other important pumps include Calcium pump (RBCs, muscles), K + pump, CP pump,
K+—H+ exchange pumps. The last one occurs in guard cells.
Active transport is a means of (i) absorption of most nutrients from the intestine (ii)
reabsorption of useful material from the uriniferous tubules (iii) rapid and selective
absorption of nutrients by cells (iv) maintaining a membrane potential (v)
maintenance of resting potential in nerve cells (vi) maintaining water and ionic
balance between cells and extracellular fluid, (vii) excretion of salt glands.

C. Transport of Solid Particles (Bulk transport):

Bulk transport inwards as well as outwards occurs across the plasma membrane by
invagination and evagination of the membrane. Bulk transport is useful in carrying
large molecules which would have difficulty in passing through the cell membrane
normally. Endocytosis and exocytosis are the two ways in which bulk transport is
accomplished.
(IV) Endocytosis is the process of engulfing large sized particles of food substances
or of foreign substances. According to the nature of substances, the endocytosis may
be:

(i) Pinocytosis or cells drinking is the process of intake of fluid material by the cell.

(ii) Micro pinocytosis is the pinocytosis of subcellular or sub microscopic level.

(iii) Rhophaeocytosis is the transfer of small quantities of cytoplasm together with

their inclusion.

(iv) Phagocytosis is engulfing of large sized particles of solid food or solid matter by
the cell.

(V) Exocytosis is the process of exudating the secretary products to outside the cell
cytoplasm. It is also known as emeicytosis or cell vomiting. In cells of pancreas, the
vacuoles containing enzymes move from the interior of the cytoplasm towards the
surface. Here they fuse with plasma membrane and discharge their contents to the
exterior.