CELLULAR STRUCTURES AND FUNCTIONS
A cell is the structural and functional unit of all living organisms.
Based on internal structures, the cells can be broadly categorized into two major types; Prokaryotic cells and
Eukaryotic cells.
Concept Map:
EXAMPLE: Bacteria EXAMPLES: Plants, Animals and Fungi
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CELLULAR STRUCTURES AND FUNCTIONS
A cell is made by the assemblage of organelles. There are some structures in cell that are not organelles, but
are still very important for cell.
These structures are cell wall, cell membrane, cytoplasm, and cytoskeleton
CELL WALL
We know that not all living organisms have cell walls around their
cells, e.g. animals and many animal-like protists. Cell wall is a
non-living and strong component of cell, located outside plasma
membrane in cells of plants, fungi and many protists. It provides
shape, strength, protection and support to the inner living matter
(protoplasm) of cell.
Cell wall consists of many layers. They are middle lamella,
primary cell wall and secondary cell wall. Sometimes, tertiary cell
wall may also be present.
CELL MEMBRANE
All prokaryotic and eukaryotic cells have a thin and elastic covering that serves as the boundary of the
cytoplasm. In the plant cells, cell wall is present outside the cell or plasma membrane and thus, cell
membrane is present between the cytoplasm and the cell wall. In animal cells, it forms the outermost
boundary, covering the cytoplasm.
Cell membrane functions as a semi-permeable barrier, allowing a very few molecules across it while fencing
the majority of chemicals inside cell. In this way, cell membrane maintains the internal composition of cell.
In addition to this vital role, cell membrane can also sense chemical messages and can identify other cells.
In eukaryotic cell many organelles e.g. mitochondria, chloroplasts, Golgi apparatus, and endoplasmic
reticulum are also bounded by cell membranes.
When we talk about all the membranes of a cell, we say them as cell membranes.
When we talk about only the outer membrane of cell, we say it as plasma membrane.
CYTOPLASM
Cytoplasm occupies the major part of the cell. It is a living component, semi-viscous and semi-transparent
substance between plasma membrane (cell membrane) and nuclear envelope. It contains water (upto 90%) in
which many organic molecules (proteins, carbohydrates, lipids) and inorganic salts are completely or
partially dissolved.
Cytoplasm provides space for the proper functioning of
organelles and also acts as the site for various biochemical
(metabolic) reactions.
CYTOSKELETON
Cytoskeleton is a network of interconnected filaments and
tubules that extends from the nucleus to the plasma
membrane in a eukaryotic cell.
The three main types of fibres make up the cytoskeleton-
microfilaments (also called actin filaments),
microtubules, and intermediate filaments. Microtubules
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are made of tubulin protein and are used by cells to hold their shape. They are also the major component of
cilia and flagella. Microfilaments are thinner and are made of actin protein. They help cells to change their
shapes.
CELL ORGANELLES
Organelles are small structures within cells that perform dedicated functions. There are about a dozen types
of organelles commonly found in eukaryotic cells. We will go through the basic facts about important cell
organelles.
NUCLEUS
A prominent nucleus occurs in eukaryotic cells. In animal cells it
is present in the centre while in mature plant cells, due to the
formation of large central vacuole, it is pushed to sideA nucleus
shows following components.
(i) Nuclear envelope or nuclear membrane
(ii) Nucleoplasm
(iii) Nucleolus, and
(iv) Chromatin
Nucleus is bounded by a double membrane known as nuclear envelope. Nuclear envelope contains many
small pores that enable it to act as a semi-permeable membrane. Inside nuclear envelope, a granular fluid i.e.
nucleoplasm is present. Nucleoplasm contains one or two nucleoli (singular; nucleolus) and chromosomes.
Nucleolus is usually visible as a dark spot and it is the site where ribosomal RNA are formed and assembled
as ribosomes. Chromosomes are visible only during cell division while during interphase (non-dividing
phase) of cell they are in the form of fine thread-like structures known as chromatin. Chromosomes are
composed of Deoxyribonucleic acid (DNA) and histone proteins. Histone proteins provide structural support
to DNA for making chromosomes.
The prokaryotic cells do not contain prominent nucleus. Their chromosome is made of DNA only and is
submerged in cytoplasm.
RIBOSOMES
Ribosomes are tiny granular structures that are either floating freely
in cytoplasm or are bound to endoplasmic reticulum (ER).
Mitochondria and chloroplasts also contain ribosomes. They are
perhaps the smallest cell organelles and can be seen only with
electron microscope. Ribosomes are made up of two different
subunits, one large and the other small. Ribosomes are not bound by
membranes and so are also found in prokaryotes. Eukaryotic
ribosomes are slightly larger than prokaryotic ones.
Each ribosome is made up of almost equal amounts of proteins and ribosomal RNA (rRNA). Ribosomes
arise in the nucleolus. Ribosomal protein from cytoplasm migrates to nucleolus for formation of ribosomes.
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Ribosomes are the sites of protein synthesis in a cell. Protein synthesis is extremely important to cells, and
so large numbers of ribosomes are found throughout cells. When a ribosome is not working, it disassembles
into two smaller units as shown in the above figure.
MITOCHONDRIA
Mitochondria (singular: mitochondrion) are double membrane
bounded structures found in almost all eukaryotic cells, both plants
and animals. These are absent from prokaryotic cells. Some of the
eukaryotic cells, like mammalian R.B.C. also lack mitochondria.
These are the sites of aerobic respiration, and are the major energy
production centres of the cell. Energy so produced is used for
performing various cellular activities.
The outer membrane of a mitochondrion is smooth but the inner
membrane forms many finger like infoldings, called cristae (singular crista) in the inner mitochondrial
matrix.
The cristae increase the surface area of inner membrane on which membrane-bound reactions can take place.
The mitochondrial matrix contains proteins, lipids , DNA, ribosomes and calcium phosphate granules. The
ribosomes of mitochondria are smaller and more similar to bacterial ribosomes than to eukaryotic
ribosomes. Mitochondrial DNA is a closed circular molecule similar to bacterial DNA.
PLASTIDS
Plastids are the largest cytoplasmic, double membrane bound organelles that only occur in the cells of plants
and photosynthetic protists (algae). These are readily visible under light microscope. On the basis of
pigments present in plastids, plastids are of three types:
(a) Leucoplasts (colorless plastids)
(b)Chloroplasts (green plastids)
(c) Chromoplasts (colored plastids, other than green).
Leucoplasts store starch, proteins and lipids. They are present in the cells of those parts where food is
stored.
Chromoplasts contain pigments associated with bright colors and are present in the cells of flower petals
and fruits. Their function is to give colors to these parts
and thus help in pollination and dispersal of fruit.
CHLOROPLASTS
Chloroplasts are large, spherical, ovoid or disc- shaped
organelles. Like mitochondria, each chloroplast is also
bound by a double membrane. The outer membrane is
smooth while the inner membrane gives rise to sacs called
thylakoids. The stack of thylakoids is called granum
(plural = grana). Grana float in the inner fluid of
chloroplast i.e. stroma, which is a colourless matrix. The stroma contains DNA, ribosomes, proteins and
enzymes that take part in synthesis of carbohydrates .
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Chloroplasts are the sites of photosynthesis in eukaryotes. They contain chlorophyll (the green pigment
necessary for photosynthesis) and associated pigments. These pigments are present in the thylakoids of
grana.
ENDOPLASMIC RETICULUM (ER)
Endoplasmic reticulum is a double walled, irregular
network of interconnected channels that extends
from cell membrane to nuclear envelope.
Endoplasmic reticulum is found in all eukaryotic
cells except mammalian RBCs. Prokaryotes also
lack this organelle.
The network exists in two forms:
Rough Endoplasmic Reticulum (RER) is so-named because of its rough appearance due to numerous
ribosomes that are attached to it.
Smooth Endoplasmic Reticulum (SER) lacks ribosomes on its surface.
Being a membranous network, endoplasmic reticulum provides mechanical support to the cytoplasm. Due to
the presence of ribosomes, Rough ER serves a function in protein synthesis . Smooth ER is involved takes
part in the synthesis of lipids and in the transport of materials from one part of cell to other. It also detoxifies
the harmful chemicals that have entered cell.
GOLGI APPARATUS
An Italian physician Camillo Golgi discovered a set of flattened sacs (cisternae) in cell, and named them
after him as Golgi body or Golgi complex. It is found in both plant and animal cells. Prokaryotic cells lack
Golgi apparatus.
Golgi apparatus is a membranous organelle consisting of flattened, sac- like structures called cisternae
stacked one above the other. The complete set of cisternae is called Golgi apparatus or Golgi complex. It is
rich in proteins and phospholipids.
It modifies molecules coming from rough ER and packs them into small membrane bound sacs called Golgi
vesicles. These sacs can be transported to various locations in cell or to its exterior, in the form of secretions.
Lysosomes originate from Golgi apparatus.
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LYSOSOMES
Lysosomes are small, spherical organelles surrounded by a single- walled membrane.
They are present in animal cells (except mammalian RBCs).prokaryotic cells lack lysosomes. Plant cells
generally lack lysosomes; some plant cells (yeast and some
fungi) do possess lysosomes.
Lysosomes contain strong digestive enzymes for the
breakdown (digestion) of food and waste materials within
cell. During its function, a lysosome fuses with the vacuole
that contains the targeted material and its enzymes break
down the material.
Lysosomes also help in cellular digestion; when the cell
dies, the lysosome rupture liberating the hydrolytic enzymes which digest the entire cell. This is a
mechanism for removing dead cells.
CENTRIOLES
Animals and many unicellular organisms have
hollow and cylindrical organelles known as
centrioles. They are absent from plant cells. Each
centriole is made of nine triplets of microtubules
(made up of tubulin protein).
Animal cells have two centrioles located near the
exterior surface of nucleus. The two centrioles are
collectively called a centrosome. Their function is to help in the formation of spindle fibers during cell
division. In some cells, centrioles are involved in the formation of cilia and flagella.
VACUOLES
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Vacuoles are fluid filled single-membrane bound organelles. Cells have
many small vacuoles in their cytoplasm. However, when a plant cell
matures its small vacuoles absorb water and fuse to form a single large
vacuole in centre. Cell in this state becomes turgid. Many cells take in
materials from outside in the form of food vacuole and then digest the
material with the help of lysosomes. Some unicellular organisms use
contractile vacuole for the elimination of wastes from their bodies. In
plant cells, the vacuoles contain dissolved food materials, waste
products and pigments, whereas in animal cells vacuoles contain
engulfed solids or liquids.
DIFFERENCES BETWEEN PROKARYOTIC AND EUKARYOTIC CELLS
Differences between prokaryotic cell / Eukaryotic cell
Prokaryotic cells Eukaryotic cells
Prokaryotic cell have no Eukaryotic cells have a prominent nucleus.
prominent nucleus. (Think pro
rhymes with No).
DNA exists as a single, circular DNA exists as many strands.
strand.
DNA located in the nucleoid an The nuclear envelope surrounds the nucleus, regulating what does in
area without a protective and out.
membrane.
(Only non-membrane bounded Membrane and non-membrane bounded organelles are present. Such
organelles are present e.g. as mitochondria, Golgi apparatus etc…
ribosomes...
Prokaryotic cells are smaller Eukaryotic cells are bigger than prokaryotic cells.
than eukaryotic cells.
Ribosomes are smaller in size. Ribosomes are larger in size.
Cell wall is made up of Cell wall is made up of cellulose (in plants) or chitin (in fungi).
peptidoglycan (a large polymer
of amino acids and sugars).
Similarities:
They both have DNA as their genetic material.
They both have ribosomes.
They have similar basic metabolism.
They both are amazing diverse in form.
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