PSC পরিচালিত স্কু লে নবম থেকে দ্বাদশ শ্রেণী পর্যন্ত শিক্ষক নিয়োগ পরীক্ষা

S U B J E C T : Botany

CELL is the fundamental structural and functional unit of all living organisms

History

• Robert Hooke first saw dead cell in 1665.

• Anton Von Leeuwenhoek first saw and described a live cell.

• Robert Brown later discovered the nucleus.

Cells can be eukaryotic and prokaryotic.

Prokaryotic cells

• No Nucleus: Prokaryotic cells lack a true nucleus. Instead, their genetic material is found in a
single circular chromosome that floats freely in the cytoplasm.

• Simple Structure: Prokaryotic cells are structurally simpler compared to eukaryotic cells.
They lack membrane-bound organelles such as mitochondria, endoplasmic reticulum, and
Golgi apparatus.

• Ribosomes: Prokaryotic cells contain ribosomes(70s), which are responsible for protein
synthesis. These ribosomes are smaller than those found in eukaryotic cells.

• Cell Wall: Most prokaryotic cells have a cell wall composed of peptidoglycan. This provides
structural support and protection.

• Flagella and Pili: Prokaryotic cells may have flagella for movement and pili for attachment to
surfaces or other cells. These structures are simpler than those found in eukaryotic cells.

• Circular DNA: Prokaryotic cells typically have a single circular DNA molecule that contains
most of their genetic information. This DNA is not enclosed within a nucleus.

• Binary Fission: Prokaryotic cells reproduce through a process called binary fission, where the
cell divides into two daughter cells, each containing a copy of the genetic material.

• Small Size: Prokaryotic cells are generally smaller in size compared to eukaryotic cells,
typically ranging from 0.2 to 2.0 micrometers in diameter.

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Eukaryotic cells

• Nucleus: Eukaryotic cells have a distinct nucleus enclosed within a double membrane. The
nucleus houses the cell's genetic material, organized into linear chromosomes.

• Membrane-Bound Organelles: Eukaryotic cells contain various membrane-bound organelles,

each with specific functions. These organelles include the endoplasmic reticulum, Golgi
apparatus, mitochondria, lysosomes, and chloroplasts (in plant cells).

• Complex Structure: Eukaryotic cells exhibit a more complex internal structure compared to
prokaryotic cells due to the presence of organelles.

• Large Size: Eukaryotic cells are generally larger in size compared to prokaryotic cells, typically
ranging from 10 to 100 micrometres in diameter.

• Linear DNA: The genetic material in eukaryotic cells is organized into multiple linear
chromosomes within the nucleus. These chromosomes are associated with histone proteins,
forming chromatin.

• Cytoplasm: Eukaryotic cells have a cytoplasmic matrix known as cytosol, where organelles
are suspended. Cellular organelles are responsible for specific functions such as protein
synthesis, energy production, and waste management.

• Endomembrane System: Eukaryotic cells possess an endomembrane system, consisting of

the endoplasmic reticulum, Golgi apparatus, vesicles, and lysosomes.

• Mitochondria: Eukaryotic cells typically contain mitochondria, organelles responsible for

generating energy in the form of ATP through cellular respiration.

• Reproduction: Eukaryotic cells reproduce through mitosis (for somatic cells) or meiosis (for
gametes), processes that ensure the faithful transmission of genetic material during cell
division.

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Properties Prokaryotic cell Eukaryotic cell

Example- Bacteria and Archaea Protists, fungi, animals, and plants

Meaning prokaryotic means "before nucleus" eukaryotic means "true nucleus"

Location of DNA DNA is concentrated in a region that is not DNA is in an organelle called the nucleus,
membrane-enclosed, called the nucleoid which is bounded by a double membrane

Membrane-bounded Absent Present (+); Lysosome, Golgi complex, ER,

structures Mitochondria and Chloroplast

Size Small Large

Nucleus Absent Present

Cell wall Present Present in plant and fungi

Nuclear membrane Absent Present

Ribosome Present (70s) Present (70s and 80s)

Cell membrane Peptidoglycan, LPS Lipid bilayer and cellulose

Histone Absent Present

Cellular organization Singled celled Multicellular mostly

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Terms-

1. Protoplasm: The living substance of cell. Contain cytoplasm and nucleus.

2. Cytoplasm: The jelly-like fluid within a cell, excluding the nucleus.

3. Cytosol- The jelly-like fluid within a cell, excluding the nucleus and cell organelle.

4. Somatoplasm: The Protoplasm of somatic cells.

5. Germplasm: The Protoplasm of germ cells.

6. Ectoplasm: The outer, more fluid portion of the cytoplasm, closer to the cell membrane.

7. Endoplasm: The inner, denser portion of the cytoplasm, closer to the nucleus.

8. Hyaloplasm: The clear, gel-like substance in the cytoplasm, containing various organelles.

9. Neoplasm: Abnormal growth of cells, typically forming a tumor.

10. Nucleoplasm- The gel-like substance present within the nucleus.

11. Periplasm- The gel-like region between the inner and outer membranes.

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ORGANELLES Location(plant/ FUNCTIONS

animal)

1.Nucleus Both DNA storage and control of cells activity, genetic information

2.Mitochondria Both Energy currency of cell; ATP production

3.Chloroplast Plant Capture energy from sun, food production; Photosynthesis

4.Ribosome Both Protein synthesis

5.Peroxisome Both Oxidation of Fatty acid and other component

6.cytoskeleton Both Structure and shape;

7.Vacoule Both Storage area

8.Cytoplasm Both Home of cell organelles

9.Cell wall Plant Protection and support

10.Golgi bodies Both Packaging, protein processing

11.Lysosome Animals mainly, Breakdown/digest food and waste material.

plant rare

12.Cell membrane Both Control inward and outward movement of substance; selective permeability

13.Endoplasmic Both protein synthesis(RER); lipid synthesis(SER)

reticulum

14.Centriole Animals Cell division and mitosis

15.Flagella Some Animals Movement

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Plasma membrane

• Other name of Plasma membrane- Bio membrane, Cell membrane, living membrane.

• It forms the barrier that separates the cytoplasm from the exterior environment, thus
defining a cell's physical and chemical boundaries.

• All cellular membranes, both plasma membranes and organelle membranes, consist of a
bilayer of phospholipids in which other lipids and specific types of proteins are embedded.

• It is semipermeable and selective permeable in nature.

• Dynamic, fluid structures, and most of their molecules move in the plane of membrane

Functions of Plasma membrane

• Protective layer

• Separate internal environment of cell from surrounding.

• Semipermeable

• Selectively permeable

• Help in transport

• Contains receptors

• Cell signaling

Composition of Plasma membrane

 Phospholipid Bilayer: The fundamental structural component of the plasma membrane is a

double layer of phospholipid molecules. Each phospholipid molecule consists of a
hydrophilic (water-attracting) head and two hydrophobic (water-repelling) tails. These
molecules arrange themselves in a bilayer with their hydrophilic heads facing outward
towards the aqueous environments inside and outside the cell, while their hydrophobic tails
face inward, shielding themselves from water.
 Proteins: Proteins are embedded within or associated with the phospholipid bilayer. There
are two main types of proteins found in the plasma membrane:
Integral Proteins: These proteins are firmly embedded within the lipid bilayer. They may
span the entire width of the membrane (transmembrane proteins) or partially penetrate it.
Transmembrane proteins often serve as channels or carriers for specific molecules or ions,
facilitating their transport across the membrane. Other integral proteins may function as
receptors, participating in cell signalling, or as enzymes, catalysing specific biochemical
reactions.

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Peripheral Proteins: These proteins are not embedded within the lipid bilayer but are
instead attached to the membrane's surface. They interact with integral proteins or with the
polar heads of phospholipids. Peripheral proteins play various roles, including providing
structural support, participating in cell signaling, and facilitating cell-cell communication.

 Carbohydrates: Carbohydrates are often found attached to proteins (glycoproteins) or lipids

(glycolipids) on the extracellular surface of the plasma membrane. These carbohydrate
chains form the glycocalyx, which helps in cell recognition, cell-cell communication, and
immune response.
 Cholesterol: Cholesterol molecules are interspersed within the phospholipid bilayer.
Cholesterol helps to regulate the fluidity and stability of the membrane by preventing the
phospholipid molecules from packing too closely together or becoming too loosely packed.

Some models of plasma membrane-

Lipid Models-
The Overton model, proposed by scientist Overton in 1902, suggests that the cell membrane
is primarily composed of lipids, arranged in a single layer.
The Gorter and Grendel model, developed in 1926,
expanded on the Overton model by proposing that the
lipid of the cell membrane consists of two layers of
lipids. This led to the concept of the lipid bilayer
membrane. where hydrophilic heads of phospholipids
face outward towards the aqueous environment while
hydrophobic tails are present in middle position of
plasma membrane.

Sandwich Model
The Danielli and Davson model-
It proposed in 1935, introduced the idea of Sandwich Model. According to their model single
layered globular proteins are present in both side of lipid bilayer.

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The Robertson model-

• The Robertson model, also known as the unit membrane model, was proposed by J. D.
Robertson in 1959. This model represents a significant advancement in the understanding of
cell membrane structure.
• According to the unit membrane model, the cell membrane is composed of a trilaminar
structure consisting of three layers: two dense outer layers and a less dense central layer.
The outer layers are believed to be composed of proteins, while the central layer is primarily
made up of phospholipids.
• This model proposed that the membrane structure is consistent across different types of
cells and organelles.

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Micellar Theory. "Hilleir and Hoffman (1953)

1) The Plasma membrane is formed by the arrangement of globular Sub-units or Micelles in

Mosaic fashion.

2) When fatty acid molecule are completely enclosed by water and organized then they are
called Micelles.

(3) In each micelle the hydrophilic end of lipid is towards outside while hydrophobic ends
towards inside.

(4) A single layer of Protein is present on either side of lipid micelle.

The Singer and Nicolson model-

It also known as the fluid mosaic model, was proposed by Singer and Nicolson in 1972.
According to the fluid mosaic model:
• The membrane is primarily composed of a lipid bilayer, consisting of phospholipids arranged
with their hydrophilic heads facing outward towards the aqueous environment and their
hydrophobic tails facing inward.
• The membrane is considered a mosaic of various components, primarily lipids and proteins,
arranged in a non-uniform manner. Proteins of different types and functions are embedded
within the lipid bilayer, creating a mosaic-like pattern.
Types of proteins present in membrane-
• Integral membrane proteins- Integral membrane proteins are embedded within the lipid
bilayer.
• Peripheral membrane proteins-Peripheral membrane proteins are attached to the
membrane surface.

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MEMBRANE TRANSPORT
ACTIVE TRANSPORT
• Against the concentration gradient(low concentration→ high concentration)
• ATP(energy) required.
• With the help of Transport with is ATP Dependent

PASSIVE TRANSPORT

• Along the concentration gradient(high concentration→ low concentration)

• No ATP(energy) required.
• With protein (facilitated diffusion) and Without protein (simple diffusion)

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Factor affect simple diffusion are-

1. Temperature ∝ Diffusion

2. Solute concentration ∝ Diffusion

3. Distance ∝ 1/Diffusion

4. Molecule size ∝ 1/Diffusion

5. Hydrophobic molecule ∝ Diffusion

Transport systems

1. Uniport system: This involves the movement of a single molecule through the membrane
e.g. transport of glucose to the erythrocytes.
2. Symport system : The simultaneous transport of two different molecules in the same
direction e.g. transport of Na+ and glucose to the intestinal mucosal cells from the gut.
3. Antiport system: The simultaneous trans- port of two different molecules in the opposite
direction e.g exchange of Cl- and HCO3 in the erythrocytes. Uniport, symport and antiport
systems are considered as secondary active transport systems.
Cotransport system : In cotransport, the transport of a substance through the membrane is
coupled to the spontaneous movement of another substance. The symport and antiport
systems referred to above are good examples of cotransport system.

Transport of macromolecules
The transport of macromolecules such as proteins, polysaccharides and polynucleotides
across the membranes is equally important. This is brought about by two independent
mechanisms namely endocytosis intake of macromolecules by the cells (e.g. uptake of LDL
by cells) and exocytosis release of macromolecules from the cells to the outside (e.g.
secretion of hormones-insulin, PTH)

What is the need of Endocytosis?

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■ Carrier and channel proteins discussed in the preceding section transport small molecules
through the phospholipid bilayer. Eukaryotic cells are also able to take up macromolecules
and particles from the surrounding medium
■ Small molecules such as sugar, ions, amino acids can be transported through plasma
membrane with the help of integral membrane protein form (channel, carriers and pump)
Endocytosis is needed to transport macromolecules.

The former of these activities is known as phagocytosis (cell eating) and occurs largely in
specialized types of cells. Other forms of endocytosis take place in all eukaryotic cells.

Endocytosis
1. Phagocytosis
2. Pinocytosis (Fluid phase endocytosis)
3. Receptor mediated endocytosis (Clathrin-mediated endocytosis)

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Phagocytosis
Reported by Metchikoff
. During phagocytosis cells engulf large particles such as bacteria, cell debris, or even intact
cells.
Binding of the particle to receptors on the surface of the phagocytic cell triggers the
extension of pseudopodia-an actin- based movement of the cell surface.
• The pseudopodia surround the particle and their membranes fuse to form a large
intracellular vesicle called a phagosome(intracellular vesicle).
• The phagosomes then fuse with lysosomes, producing phagolysosomes, in which the
ingested material is digested by the action of lysosomal acid hydrolases.
Many amoebas use phagocytosis to capture food particles, such as bacteria or other
protozoans

Role of Phagocytosis in multicellular Organism

In multicellular animals, the major roles of phagocytosis are to provide a defense against
invading microorganisms and to eliminate aged or damaged cells from the body.

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In mammals, phagocytosis of microorganisms is the function of three types of white blood

cells macrophages, neutrophils, and dendritic cells- which are frequently referred to as
"professional phagocytes.“
These cells play critical roles in the body's defense systems by eliminating microorganisms
from infected tissues.
In addition, macrophages as well as other cell types, including fibroblasts and epithelial cells,
eliminate aged or dead cells from tissues throughout the body.

Pinocytosis
Fluid phase endocytosis.
Invagination of solute or fluid.
Can occur selectively as well as non-selectively.

Exocytosis
Transport vesicle destined for plasma membrane undergo fusion with the plasma membrane
and release the contents outside the cell in the process called exocytosis.
The process of exocytosis involves several steps:
 Vesicle Formation: Specialized vesicles containing molecules to be released are
formed within the cell's cytoplasm. These vesicles are typically produced by the
Golgi apparatus or endoplasmic reticulum.
 Vesicle Transport: The vesicles move towards the cell membrane along cytoskeletal
elements such as microtubules or actin filaments.
 Membrane Fusion: When the vesicle reaches the cell membrane, it fuses with it.
This fusion is mediated by specific proteins present on both the vesicle membrane

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and the target membrane. The interaction between these proteins helps to bring the
membranes into close proximity and initiate fusion.
 Release of Contents: Once fusion occurs, the contents of the vesicle are released
into the extracellular space. These contents may include neurotransmitters,
hormones, enzymes, or other signaling molecules, depending on the cell type and
function.
 Membrane Recycling: After exocytosis, the cell membrane must be recycled to
maintain its integrity.

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When amphiphilic molecules are exposed to an aqueous environment, how they behave?
■ When amphiphilic molecules are exposed to an aqueous environment, They
spontaneously aggregate to bury their Hydrophobic tails in the interior, where they are
shielded from the water, and they expose their hydrophilic heads to water.
. Depending on their shape, they can do this in either of two ways:
1. they can form spherical micelles, with the tails inward(with single hydrocarbon chain), or
2. they can form double-layered sheets, or bilayers, with the hydrophobic tails sandwiched
between the hydrophilic head groups
When cellular membranes form, phospholipids assemble into two layers because of these
hydrophilic and hydrophobic properties. The phosphate heads in each layer face the
aqueous or watery environment on either side, and the tails hide away from the water
between the layers of heads, because they are hydrophobic.

Role of Cholesterol in membrane fluidity

The cholesterol molecules are randomly distributed across the phospholipid bilayer, helping
the bilayer stay fluid in different environmental conditions. Cholesterol modulates the
properties lipid bilayers
✓ Cholesterol acts as a bidirectional regulator of membrane fluidity because
1. At high temperatures, it stabilizes the membrane and raises its melting point, whereas
2. At low temperatures it intercalates between the phospholipids and prevents them from
clustering together and stiffening and prevents the hydrocarbon chains from coming
together and crystallizing.
The cholesterol holds the phospholipids together so that
1. They don't separate too far, letting unwanted substances in, or
2. Compact too tightly, restricting movement across the membrane.
Membrane without Cholesterol
Without cholesterol, the phospholipids in your cells will start to get closer together when
exposed to cold, making it more difficult for small molecules, like gases to squeeze in
between the phospholipids like they normally do. Without cholesterol, the phospholipids
start to separate from each other, leaving large gaps.

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MEMBRANE PROTEINS

Membrane proteins are amphiphilic, hydrophobic and hydrophilic regions demy

PERIPHERAL PROTEINS /EXTRENSIC
Peripheral membrane proteins(cytosolic)
Interact with the surface of the lipid bilayer of cell membranes.
Do not enter into the hydrophobic space within the cell membrane
EXAMPLE: cytochrome c, high potential iron protein, adrenodoxin reductase, some
flavoproteins, Spectrin, Ankyrin
INTEGRAL PROTEINS INTRENSIC
Membrane proteins (All transmembrane proteins) Include transporters, linkers, channels,
receptors, enzymes, structural membrane-anchoring domains
EXAMPLE: Glycophorin, rhodopsin, Band3, GPCR, etc.

Gap junctions
• These are specialized intercellular channels that allow direct communication and exchange
of small molecules, ions, and electrical signals between adjacent cells.
• They are formed by connexin proteins, which assemble into hexameric structures called
connexons. These connexons on one cell membrane align with connexons on the membrane
of an adjacent cell, forming a continuous channel called a gap junction.

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• Gap junctions are crucial for coordinating cellular activities in tissues such as cardiac muscle,
where they facilitate rapid electrical signaling, and in nervous tissue, where they allow for
the passage of neurotransmitters and signaling molecules between neurons.

Tight junctions
• These are specialized membrane structures that form a barrier between adjacent cells,
sealing the intercellular space.
• They are composed of transmembrane proteins, such as claudins and occludins.
• Tight junctions play a critical role in maintaining the integrity of epithelial and endothelial
cell layers in tissues like the intestinal epithelium and blood-brain barrier, controlling the
selective permeability of these barriers and preventing leakage of harmful substances.

Desmosomes-
• These are specialized junctions between cells that provide strong adhesion and mechanical
strength, particularly in tissues subjected to mechanical stress, like skin and heart muscle.
• They consist of proteins that anchor cells together and form a strong bond, contributing to
tissue integrity and stability.

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Hemidesmosomes-
• these are similar to desmosomes in that they provide adhesion between cells, but they
differ in their structure and function.
• Hemidesmosomes anchor epithelial cells to the basement membrane, providing stability and
resistance to mechanical stress.
• They consist of integrins and other proteins that link the cell's cytoskeleton to proteins in
the extracellular matrix.

Glycocalyx/ cell coat

• The glycocalyx is a layer of carbohydrates (sugars) that coats the outer surface of many
animal cells, particularly epithelial cells. It consists of mucopolysaccharide, glycoproteins and
glycolipids that are attached to the cell membrane.
• It also present in bacteria. In bacteria it is made-up of protein-lipid polysaccharide.
• The glycocalyx plays several important roles, including cell-cell recognition, protection
against pathogens, cell adhesion and signaling.

Types of Membrane
 Impermeable Membrane:
This type of membrane that does not allow the passage of any substances through it except
some gases.
Example –The unfertilized ovum (egg) of certain fish species often has an impermeable
plasma membrane, which serves to protect the egg from external influences such as water
penetration or the entry of harmful substances.
 Semipermeable Membrane:
A semipermeable membrane allows certain substances(solvent) to pass through while
blocking others(solute).
This selective permeability is often based on factors like size, charge, or solubility.
Biological membranes, such as the cell membrane, are typically semipermeable.
 Selectively Permeable Membrane:
This term is essentially synonymous with semipermeable membrane. A selectively
permeable membrane is one that permits the passage of certain substances while blocking
others, based on specific properties of the molecules or ions involved.
Example – cell membrane of animal cell

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Mitochondria
 Popularly known as the “Powerhouse of the cell,” mitochondria (singular: mitochondrion) are
a double membrane-bound organelle found in most eukaryotic organisms. They are found
inside the cytoplasm and essentially function as the cell’s “digestive system.”
 They play a major role in breaking down nutrients and generating energy-rich molecules for
the cell. Many of the biochemical reactions involved in cellular respiration take place within
the mitochondria. The term ‘mitochondrion’ is derived from the Greek words “mitos” and
“chondrion” which means “thread” and “granules-like”, respectively. It was first described by
a German pathologist named Richard Altmann in the year 1890.

 Mitochondria (term coined by C. Benda) are energy-converting organelles, which are present
in virtually all eukaryotic cells. They are the sites of aerobic respiration. They produce
cellular energy in the form of ATP, hence they are called ‘power houses’ of the cell.
 Mitochondria are double membrane-bound organelle. Each mitochondrion is a double
membrane-bound structure with outer and inner membranes. The outer membrane is fairly
smooth. But the inner membrane is highly convoluted; forming folds called cristae. The inner
membrane is also very impermeable to many solutes due to very high content of a
phospholipid called cardiolipin. The cristae greatly increase the inner membrane’s surface
area. The two faces of this membrane are referred to as the matrix side (N-side) and the
cytosolic side (P-side). Inner membrane contains enzyme complex called ATP synthase (or
F0-F1 ATPase or oxysome) that makes ATP. The outer membrane protects the organelle, and
contains specialized transport proteins such as porin which allows free passage for various
molecules into the intermembrane space (the space between the inner and outer membranes)
of the mitochondria. Mitochondrial porins, or voltage-dependent anion-selective channels
(VDAC) allow the passage of small molecules across the mitochondrial outer membrane.

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A mitochondrion has double-membraned organization and contains:

 the outer mitochondrial membrane, the intermembrane space (the space between the
outer and inner membranes),
 the inner mitochondrial membrane, and the matrix (space within the inner membrane)

The matrix (large internal space) contains several identical copies of the dsDNA (as genetic material),
mitochondrial ribosomes (ranging from 55S-75S), tRNAs and various proteins. Mitochondrial dsDNA
is mostly circular. The size of mitochondrial DNA also varies greatly among different species.

Organisms Size (kb)

Human 16.6
Xenopus (frog) 18.4
Drosophila (fruit fly) 18.4
Saccharomyces (yeast) 75.0
Arabidopsis (mustard plant) 367.0

Cristae
The inner membrane of mitochondria is rather complex in structure. It has many folds that form a
layered structure called cristae, and this helps in increasing the surface area inside the organelle. The
cristae and the proteins of the inner membrane aid in the production of ATP molecules. The inner
mitochondrial membrane is strictly permeable only to oxygen and ATP molecules. A number of
chemical reactions take place within the inner membrane of mitochondria.

Mitochondrial Matrix
The mitochondrial matrix is a viscous fluid that contains a mixture of enzymes and proteins. It also
comprises ribosomes, inorganic ions, mitochondrial DNA, nucleotide cofactors, and organic
molecules. The enzymes present in the matrix play an important role in the synthesis of ATP
molecules.

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F0-F1 particle structure and Function

• is a spherical structure composed largely of
alternating α and β subunits; the three active sites are
on the β subunits.
• The γ subunit extends through the center of the
sphere and can rotate.
• The stalk(γ and ε subunits) connects the sphere to F0,
the membrane embedded complex that serves as a
proton channel.
• F0 contains one a subunit, two b subunits, and nine
to 12 c subunits.

• The stator arm is composed of subunit a, two b

subunits, and the δ subunit; it is embedded in the
membrane and attached to F1.
• A ring of c subunits in F0 is connected to the stalk
and may act as a rotor (inset) and move past the a
subunit of the stator. As the c subunit ring turns, it
rotates the shaft (γ
• ε subunits).
• The inset is an atomic force micrograph of the F0
rotor.

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Enzymes present in Mitochondria-

(a) Enzymes present in outer membrane of Mitochondria- monoamine oxidase, rotenone-insensitive
NADH-cytochrome c-reductase, kynurenine hydroxylase and fatty acid Co-A ligase.
(b) Enzymes present in outer chamber of Mitochondria- adenylate kinaseand nucleoside
diphosphokinase
(c) Enzymes present in inner membrane of Mitochondria- NAD (nicotinamide adenine dinucleotide),
FAD (flavin adenine dinucleotide), DPN (diposphopyridine nucleotide), adenosine triphosphate
synthetase, Succinatedehydrogenase, Cartine fatty acid acyl transferase, B-hydroxybutyrate
dehydrogenase etc.
(d) Enzymes present in matrix of Mitochondria -malate dehydrogenase, isocitrate dehydrogenase),
fumerase, aconitase, citrate synthetase,etc.

Functions of Mitochondria
The most important function of mitochondria is to produce energy through the process
of oxidative phosphorylation. It is also involved in the following process:

1. Regulates the metabolic activity of the cell

2. Promotes the growth of new cells and cell multiplication
3. Helps in detoxifying ammonia in the liver cells
4. Plays an important role in apoptosis or programmed cell death
5. Responsible for building certain parts of the blood and various hormones like testosterone and
oestrogen
6. Helps in maintaining an adequate concentration of calcium ions within the compartments of
the cell
7. It is also involved in various cellular activities like cellular differentiation, cell signalling, cell
senescence, controlling the cell cycle and also in cell growth.

Hydrogenosome and mitosomes

 Some protists (such as Trichomonas vaginalis) adapted to anoxic or microaerobic
environments lack mitochondria. These protists contain a specialized organelle, involved in
synthesis of ATP and hydrogen, called the hydrogenosome. Hydrogenosomes are the site of
pyruvate fermentation, coupled with ATP production via substrate level phosphorylation.
Pyruvate is broken down to acetate, CO2 and molecular hydrogen. Like mitochondria,
hydrogenosomes are surrounded by a double-membrane and produce ATP. In contrast to
mitochondria, hydrogenosomes produce molecular hydrogen through fermentations, lack
cytochromes and usually lack DNA.
 Mitosomes, organelles of mitochondrial origin, are double-membrane bound organelles found
in some unicellular eukaryotes, including Entamoeba histolytica and microsporidia. The name
‘mitosome’ was proposed to indicate that the organelles are highly reduced (cryptic)
mitochondria. The mitosome has been detected only in anaerobic organisms that do not have
mitochondria

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Endosymbiotic theory
Mitochondria are semi-autonomous organelle and divide by binary fission just like bacteria. The
similarities with the prokaryotic characters (especially with genetic systems), suggest that
mitochondria evolved from bacteria. According to endosymbiotic theory (proposed by Lynn
Margulis), mitochondria are supposed to have evolved in eukaryotes from endosymbiotic association
of purple photosynthetic bacteria about 1.5×109 years ago. The captured cell (the endosymbiont) was
then reduced to a functional organelle bound by two membranes, and was transmitted cytoplasmically
to subsequent generations. Mitochondrial inheritance in higher eukaryotes is nearly uniparental (and
more precisely maternal inheritance because female gametes mostly contribute mitochondria to the
zygote). In lower eukaryotes such as yeasts, both parents contribute equal amounts of mitochondria to
the zygote. Thus, mitochondrial inheritance in yeasts is therefore biparental.

Evidences to support endosymbiotic theory

 Mitochondria are self-replicating bodies like bacteria and divide in a manner resembling
binary fission in bacteria.
 Mitochondria are surrounded by two membranes and the innermost of these membranes is
very similar in composition to bacteria.
 Mitochondria have their own DNA, which is structurally similar to bacterial DNA.
 Mitochondrial ribosomes, enzymes and transport systems are all similar to those of bacteria.
 Mitochondria are of approximately the same size as bacteria.
 Protein synthesis in mitochondria is inhibited by a variety of antibiotics (e.g.
chloramphenicol, tetracycline, erythromycin) that also inhibit protein synthesis by bacterial
ribosomes, but have little effect on the cytosolic ribosome of eukaryotic cells

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S U B J E C T : BOTANY

Endomembrane System
• While each of the membranous organelles is distinct in terms of its structure and function,
many of these are considered together as an endomembrane system because their functions
are coordinated.

• The endomembrane system include

 endoplasmic reticulum (ER),
 golgi complex,
 lysosomes and
 vacuoles.
• Since the functions of the mitochondria, chloroplast and peroxisomes are not coordinated with
the above components, these are not considered as part of the endomembrane system.

Endoplasmic reticulum
 In 1945, lace like membrane of ER were first seen in cytoplasm of chick embryo.
 It is the largest single membrane bound intracellular compartment.
 It is an extensive network of closed and flattened membrane bound structure. The enclosed
compartment is called ER lumen.
 ER membrane are physiologically active, interact with the cytoskeleton and contain
differentiated domains specialized for different functions.
 ER are considered as one of the components of cytoskeleton along with microtubules,
microfilaments and intermediate filaments.
 ER is found in almost all eukaryotic cells except mature erythrocytes, ova, embryonic cells and
prokaryotic cells too,
 The amount of ER vary from cell to cell according to the function of cell.
 In spermatocytes, it is represented by few vacuoles only.
 In the cells of adipose tissues, it has only few tubules
 The cells that are actively synthesizing proteins, such as liver and pancreatic cells and fibroblast
have abundant ER.

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 At present, origin of ER is not definitely known. The most concrete hypothesis is that the ER is
budded off from the nuclear Envelope.
 There are 2 types of ER-

1) Rough endoplasmic reticulum has ribosomes embedded within its structure, giving a
“rough” appearance.
2) The smooth endoplasmic reticulum does not have these ribosomes, hence appearing
“smooth.”

 The ER appears to arise from outer membrane of nuclear envelope by out folding or from the
plasma membrane by in folding.

 SER seem to arise from RER by detachment of ribosome.

 Endoplasmic reticulum has three different structures -(i) cisternae, (ii) vesicles and (iii) tubules.
(i) Cisternae: They look like long, flat sacs. Their diameter is usually 40-50 nm. They often contain
ribosomes.
(ii) Vesicles: These are generally oval or round and membrane bound cavities. Their diameter ranges
from 25-500 nm. They are sometimes clustered in the cytoplasm. They are especially observed in
pancreatic cells. Vesicles are called microsome.
(iii) Tubules: These are specialized branched tubules. They are connected to each other and form a net
like structure. The diameter of these tubules usually ranges from 50-190 nm. Their nucleus usually
does not contain ribosomal granules.

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Enzymes present in ER membranes-

• stearates,
• NADH-cytochrome C reductase,
• NADH-cytochrome b5 reductase,
• glucose-6-phosphatases,
• glycosyl transferases,
• NADH-diaphoresis,
• Mg++ activated ATPase.

 When cells are disrupted by homogenization, the ER breaks into fragments and reseals into small
vesicles called microsomes. Microsome derived from RER are studded with ribosomes on the
outer surface are called rough microsomes and one those lacking ribosomes are smooth
microsome

N-LINKED GLYCOSYLATION

 Glycosylation is an important modification to eukaryotic proteins because the added sugar

residues are often used as molecular flags or recognition signals to other cells than come in
contact with them.
 There are two types of protein glycosylation, both of which require import of the target 2
polypeptide into the ER. N-linked glycosylation actually begins in the endoplasmic reticulum,
but O-linked glycosylation does not occur until the polypeptide has been transported into the
Golgi apparatus.
 Therefore, it is also the case that N-linked glycosylation can (and is) usually beginning as a
co-translational mechanism, whereas O-linked glycosylation must be occurring post-
translationally.

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FUNCTION OF GLYCOSYLATION
1. Some proteins require N-linked oligosaccharide in order to fold properly in the ER
2. N-linked oligosaccharide confer stability (proteases resistance) on many secreted glycoproteins.
3. Oligosaccharide on certain glycoproteins play a role in cell-cell adhesion.
4. Oligosaccharide present on glycoprotein also serves as antigens.

Function of ER-
• Mechanical support and distribution of cytoplasm.
• Surface area for chemical reactions.
• Intracellular transport system.
• Storage of synthesized molecules.
• Protein (RER) & lipid synthesis by SER.
• Detoxification of certain molecules.
• Release of calcium ions in muscles.
Function of RER-
• Proteins synthesized by ribosomes associated with RER enters into the lumen and membrane
of RER by the process of co-translation translocation
• In the lumen of RER, five principal modification of proteins occur before they reach their
final destination. These modification involves:
• 1. Addition & processing of carbohydrate (N-linked glycosylation).
• 2. Formation of disulfide bonds) Strong
• 3. Proper folding
• 4. Specific proteolytic cleavage
• 5. Assembly into multimeric proteins.
Functions of SER
• SER acts as a site of:
1. Lipid biosynthesis
2 Detoxification
3 Calcium regulation

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GOLGI BODIES/ GOLGI COMPLEX

 The golgi complex was discovered by an Italian physician and Camillo Golgi in 1898 during an
investigation on nervous system.
 This term given to groups of flattened disc-like structures located dose to the endoplasmic
reticulum.
 The golgi apparatus is noticeable with both light and electron microscope.
 The Golgi apparatus occurs in all eukaryotic cells except male gametes of bryophytes and
pteridophytes, mature sieve tubes, some fungal cells, and mature sperms and RBCs of animals.
It is absent in prokaryotes.
 The number of Golgi apparatus' within a cell is variable. Animal cells tend to have fewer and
larger Golgi apparatus. Plant cells can contain as many as several hundred smaller versions.

✓ The number of compartments in any one Golgi apparatus is usually between 3 and 8.
✓ The number of sets of Golgi apparatus in a cell can be as few as 1, as in many animal cells, or
many hundreds as in some plant cells.
✓ Specialized secretory cells contain more sets of Golgi apparatus than do other cells.
GOLGI APPARATUS - MANUFACTURING & SUPPLY CHAIN
In non-biological terms the Golgi apparatus can be divided into three main sections:
1) Goods inwards
2) Main processing area
3) Goods outwards
1) Cis Golgi network (Goods inwards):
• Also called the cis Golgi reticulum it is the entry area to the Golgi apparatus. It follows the
'transitional elements' which are smooth areas of the RER that are also known as the
'endoplasmic reticulum Golgi intermediate compartments' (ERGIC).
2) Golgi stack (Main processing area)
• This section is composed of a variable number, typically 3-6, of flattened sacs called cisternae
(sing. cisterna).
• The cisternae of the Golgi stack are divided into three working areas: cis cisternae, medial
cisternae and trans cisternae
3) Trans Golgi network (Goods outwards)
• This section is directly connected to the trans cisternae and it is here that final reactions and
sorting takes place.
• The concentrated biochemical are packed into sealed droplets or vesicles that form by
budding off from the trans Golgi surface. The vesicles are then transported away for use in the
cell and beyond.

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• There are three main destinations for biochemicals released from the trans Golgi network:
(1) inside the cell to the lysosomes
(2) the plasma membrane
(3) outside of the cell.
• In each case the destination is clearly linked to function. all the biochemicals transported
away from the trans Golgi network have labels and barcodes built into them. They are all
packed in vesicles and the construction of the vesicle or vessel is largely related to the vesicle
contents, its destination and end use.

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FUNCTIONS OF GOLGI COMPLEX

• Although the golgi apparatus is involved in many different cellular processes it's principle
function in many cells is in secretion.
• Golgi also plays an important role in synthesis of proteoglycans, which are molecules present
in the extracellular matrix of animals.
• A major function is the modifying, sorting and packaging of proteins for secretion. It is also
involved in the transport of lipids around the cell, and the creation of lysosomes
• In specialist secretory cells, the Golgi complex is responsible for the sorting and packing of
such well-known items as insulin, digestive enzymes and pectin.

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• O-linked glycosylation of secreted and membrane bound proteins is a post- translational event
that takes place in the cis-Golgi compartment after N- glycosylation and folding of the
protein.

O-LINKED GLYCOSYLATION
• Glycosylation is a very common modification of protein and lipid, and most glycosylation
reactions occur in the Golgi.
• Although the transfer of initial sugar(s) to glycoproteins or glycolipids occurs in the ER or on
the ER membrane, the subsequent addition of the many different sugars that make up a
mature glycan is accomplished in the Golgi.

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LYSOSOMES
• Single membrane bound organelles present in animal cells. This single membrane is unique in
composition.
• Lysosomes contain digestive enzymes.
• They are found in animal cells while in plant cells same role is performed by vacuoles.
• Lysosomes were discovered by Belgium cytologists Christian de Duve in 1955, & named as
suicide bags.
• They are most abundant in cells which are related with the enzymatic reactions such as liver
cells, pancreatic cells, kidney cells, spleen cells, leucocytes, macrophages, etc.

FEW IMPORTANT POINTS ABOUT LYSOSOMES

• The lysosomal membrane also harbor a proton pump that utilizes the energy from ATP
hydrolysis to pump H+ into the lumen, there by creating the acidic pH in the lysosomes.
• The primary function of lysosome is degradation of extra and intracellular material.
• For this purpose, lysosomes are filled with hydrolases, which may contain about 40
varieties of enzymes.
 Proteases: which digest proteins.
 Lipases: which digest lipids.
 Amylase: which digest carbohydrates (Sugars).
 Nucleases: which digest nucleic acid.
 Lysosomes are manufactured and budded into the cytoplasm by the golgi apparatus by
enzymes in it.
 The enzymes that are within lysosomes are made up in the rough endoplasmic reticulum,
which are then delivered to the golgi apparatus via transport vesicles.

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Lysosomes are responsible for the degradation of large particles taken up by phagocytosis and for the
gradual digestion of the cell's own components by autophagy. On this bases lysosome can be divided
into

 Hetrophagic vacuole or hetrolysosome or phagolysosome

They are formed by the fusion of primary lysosome with cytoplasmic vacuoles containing
extracellular substances brought into the cell by any of a variety of endocytic process.

 Autophagic vacuoles or autolysosomes

Autophagic vacuoles contain particles isolated from the cells own cytoplasm including
mitochondria, microbodies etc.

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1. HETEROPHAGY: lysosomal degradation of extracellular material by the process of

endocytosis. Heterophagy is of two types: phagocytosis & pinocytosis.
2. PCD: due to high level of hydrolytic enzymes, lysosomes are potentially capable of inducing
cell death (Programmed cell death). The proteases implicated in cell death are cathepsins, which
remain active at neutral pH, such as cathepsin B, C, D, L.
3. AUTOLYSIS: it refers to digestion of own cells by the lysosomes, it occurs during amphibian
& insect metamorphosis
Lysosomes are responsible for the breakdown of tadpole tail as the tadpole develops frog. into the
In the process, lysosome release the hydrolases to the cytoplasm to digest the cells of oneself.

4. FERTILIZATION: during fertilization process acrosome (giant lysosome) of sperm head

ruptures and release enzyme on the surface of egg. These enzymes rupture the egg and provide
way for the entry of sperm nucleus into the egg.

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VACUOLE
 A vacuole is a membrane-bound organelle which is present in all plant and fungal cells and
some protist, animal and bacterial cells
 Vacuoles are acidic in nature and share some basic properties with lysosomes that are
predominantly found in plant cells. Depending on the type of plant, there are different types of
vacuoles with specific properties that are crucial to their functions.
 In plant cells, the vacuoles are much larger than in animal cells. When a plant cell has stopped
growing, there is usually one very large vacuole. Sometimes that vacuole can take up more
than half of the cell's volume. The vacuole holds large amounts of water or food.

 Most plants and fungal cells contain one or several very large, fluid-filled vesicles called
vacuoles. They are surrounded by single membrane called tonoplast and related to the
lysosomes of animal cells, containing a variety of hydrolytic enzymes, but their functions are
remarkably diverse.

 Like a lysosome, the lumen of a vacuole has an acidic pH, which is maintained by similar
transport proteins in the vacuolar membrane. The plant vacuole contains water and dissolved
inorganic ions, organic acids, sugars, enzymes and a variety of secondary metabolites. Solute
accumulation causes osmotic water uptake by the vacuole, which is required for plant cell
enlargement. This water uptake generates the turgor pressure.

 The vacuole is different from contractile vacuole. A contractile vacuole is an organelle

involved in osmoregulation. It pumps excess water out of the cell. It is found predominantly
in protists (such as Paramecium, Amoeba) and in unicellular algae (Chlamydomonas).

DIFFERENT VACUOLE TYPES FOUND IN DIFFERENT CELLS:

• FOOD VACUOLE: storage for molecules that is a food source for the cell .
• GAS VACUOLES- help bacteria in buoyancy and enable them to float at the desired
depth. They help the photosynthetic bacteria in getting optimal light and oxygen.
• CONTRACTILE VACUOLE : found in some protists, can pump water and used to maintain
water balance.
• EXCRETORY VACUOLE- storage or split toxic material from cytoplasm.

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RIBOSOME
• The ribosomes are large ribonucleoproteins consisting of RNAs and proteins, ubiquitous in all
cells, that translates genetic information stored on the messenger RNA into polypeptides.
• The ribosome are approximately globular structure, its average diameter ranging from 2.5 nm
(Escherichia coli) to 2.8 nm (mammalian).
• The functional ribosomes consist of two subunits of unequal size, known as the large and
small subunits.
• Ribosomes are consist of rRNA and r-proteins.
• The names of the r-proteins are composed of L or S (depending on whether the protein is from
the large or small subunit).

Ribosome-
Ribosomes are ribonucleoprotein particles that contain rRNA and r-proteins. It acts like a small
migrating factory, which physically moves along an mRNA molecule in 5' to 3' direction, catalyze
the assembly of amino acids into protein chain.

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Ribosomes can be found in two main locations within a cell:

• Free Ribosomes: These are ribosomes that float freely in the cytoplasm, not attached to any
organelles. They synthesize proteins that are used within the cytoplasm itself or targeted to
other organelles within the cell.
• Bound Ribosomes: These ribosomes are attached to the endoplasmic reticulum (ER) or
nuclear envelope. They synthesize proteins that are either inserted into the membranes (e.g.,
integral membrane proteins) or secreted from the cell (e.g., enzymes, hormones).
Monosome:
• A monosome refers to a single ribosome attached to a single mRNA molecule.
Polyribosome (or polysome):
• A polyribosome refers to a cluster of several ribosomes simultaneously translating a single
mRNA molecule.

• Each ribosome is made up of two subunits - large and small subunits. Each of the ribosome
subunits contains rRNA and a number of small proteins.
• In the 70S ribosome, the 30S subunit consists of 16S rRNA and 21 r-proteins whereas the
50S subunit contains 23S rRNA, 5S rRNA and 31 r-proteins.
• The cytosolic 80S ribosomes of eukaryotes are larger than 70S ribosomes. The total content
of both RNA and protein is greater.
• In the 80S ribosome, the larger 60S subunit contains 28S rRNA, 5S rRNA, 5.8S rRNA and
46 r-proteins whereas the 40S subunit consists of the 185 rRNA and 33 r-proteins.

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The 70S ribosome has three tRNA-binding sites:

• P-site
P-site, also called the peptidyl-tRNA-binding site, holds the tRNA molecule that is linked to the
growing polypeptide chain.
• A-site
A-site, also called the aminoacyl-tRNA-binding site, holds the incoming tRNA molecule charged
with an amino acid.
• E-site
Deacylated tRNA (lacking any amino acid) exits via the E-site, also called the exit site

Structure of Ribosomes
• A ribosome is in the subspherical structure and consists of two unequal parts. One is a larger
unit, and the next is a smaller unit.
• The larger subunit is dome-shaped, whereas the smaller subunit is in the oblate-ellipsoid cap
shape.
• In the larger subunit, protuberance, stalk, and ridge are present. In the larger subunits, two
sites, the peptidyl (P) and the aminoacyl (A) site, are present.
• The smaller subunit consists of the head, cleft, and platform. The smaller unit fits over, the
larger subunit like the cap. The cleft provides space for the mRNA in between these two
subunits.

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These two subunits remain attached to each other due to the higher concentration of the Mg ++ ions.
When there is a decrease in the concentration of the Mg++, then these larger and smaller subunits
get separated.
In the bacterial cell, these two subunits remain freely in the cytoplasm, and only during the
protein synthesis do they unite. During the higher concentration of the Mg++ ions.
The important metallic ions in the ribosome are Mg++, Ca++, and Mn++.
Types of ribosomes
Based on the sedimentation coefficient, these types of ribosomes are found.
70S Ribosome
• 70S ribosome is smaller in size with a molecular weight of 2.7×106 daltons.
• In the prokaryotic cell, 70S ribosome is found in bacteria and blue-green algae.
• 70S ribosomes consist of two subunits: 50S (larger subunit) and 30 S (smaller subunit)
• the RNA content is high than that of the protein. E.g., in the ribosome of E. coli, rRNA is
63%, and protein is 37 %.
55S and 77S Ribosome
• The mitochondrial ribosome of yeast is 77S.
• And the mitochondrial ribosome found in mammals is 55S.
80S Ribosome
• It is larger in size with a molecular weight of 40×106 daltons.
• 80S ribosome is absent in the prokaryotic cell.
• In the eukaryotic cell, 80S ribosome is found either in free form in the cytoplasm or bound
with the endoplasmic reticulum and nuclear membrane.
• 80S ribosome consists of two subunits: 60 S (larger subunit) and 40 S (smaller subunit).
• The RNA content is less than that of the protein. E.g., in the ribosome of a pea seedling, RNA
is 40%, and protein is 60%.

Eukaryotic ribosomes

• Most eukaryotes contain two distinct types of ribosomes: cytoplasmic and organellar. The
ribosomes of eukaryotic cells (other than mitochondrial and chloroplast ribosomes) are
substantially larger and more complex than bacterial ribosomes.
• Cytoplasmic ribosomes show marked variation with the complexity of the organism: the yeast
ribosome is relatively small and globular, resembling a eubacterial ribosome except for its
greater size, while higher eukaryotes have larger and more ellipsoidal ribosomes.
• Organelle ribosomes are distinct from the ribosomes of the cytosol and take varied forms.
Organellar ribosomes from mitochondria (found both in animals and plants) and plastids
(found in plants only) are smaller than cytoplasmic, in general, and bear some resemblance to
the eubacterial ribosomes, consistent with the hypothesis that these organelles have a
eubacterial origin (endosymbiosis). While chloroplast ribosomes have been measured at 67-
68S, mitochondrial ribosomes show a wide variation, from 55S (mammalian) to 77S (yeast).

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Functions of Ribosomes
• Ribosomes are the sites of protein synthesis. So ribosomes are called the protein factories or
workbench of proteins.
• Free ribosomes synthesize the structural and enzymatic proteins for intracellular use.
• The ribosomes, bound with the endoplasmic reticulum, produce secretory, lysosomal, and
membrane proteins.
• The ribosome provides enzymes (peptidyl transferase) and initiation factors for the
condensation of the amino acids to form polypeptides.
• Ribosomes contain the rRNA for attachments of mRNA and tRNA.

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PEROXISOMES
• Single membrane bound small organelle, present in all eukaryotes.
• It produced and consume hydrogen peroxide.
• Peroxisome lack DNA & ribosomes.
• Thus all peroxisomal proteins are encoded by nuclear genes, synthesized on ribosomes
present in the cytosol and then incorporated into pre existing peroxisomes.
• 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.
• Peroxisomes are membrane-bound packets of oxidative enzymes.
• In plant cells, peroxisomes play a variety of roles including converting fatty acids to sugar
and assisting chloroplasts in photorespiration.
• In animal cells, peroxisomes protect the cell from its own production of toxic hydrogen
peroxide.
• Matrix of peroxisome contains oxidative enzymes, such as catalase, oxidase, etc.
• The enzyme catalase present in peroxisome converts H2O2 to 02.
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.
• Peroxisomes do not have their own DNA. Proteins are transported from the cytosol after
translation.
• Peroxisomes are also called microbodies.

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Transport into PEROXISOMES

• Proteins are synthesized and fully folded into cytosol.
• Fully functional and folded protein is transported.
• Import requires ATP hydrolysis.
Origin
• The ability of peroxisomes to divide themselves suggest that peroxisome may have had an
endosymbiotic origin similar to mitochondria.
• However the localization of peroxisomal proteins to ER and similarity of some peroxisomal
proteins to those localized in the ER suggest an alternative hypothesis, that peroxisomes are
synthesized from ER.
• Aspects of both views may be true.
Peroxisome Function
• The main function of peroxisome is the lipid metabolism and the processing of reactive
oxygen species.
• Other peroxisome functions include:
• They take part in various oxidative processes.
• They take part in lipid metabolism and catabolism of D-amino acids, polyamines and bile
acids.
• The reactive oxygen species such as peroxides produced in the process is converted to water
by various enzymes like peroxidase and catalase.
• In plants, peroxisomes facilitate photosynthesis and seed germination. They prevent loss of
energy during photosynthesis carbon fixation.

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