CAL PROGRAM - IA MVST
TEXT FOR CAL MODULE ON CYTOLOGY
A programme introducing various features of a typical cell
Cells
Cells are the basic structural and functional units of life.
Based upon their structural and functional characteristics, cells are divided into prokaryotes and
eukaryotes.
The cells of animals and plants are eukaryotes.
This programme will focus on animal eukaryotic cells.
----------------------------------------------------------------------------------------------------------------------Under the light microscope, you should be able to identify individual cells with their nucleus and
cytoplasm. In some cases, you would be able to see the nucleolus, chromatin and the cell
boundary.
The electron microscope reveals additional cellular structures.
The rest of this programme will deal with each of them individually
Plasma membrane (EM of a liver cell)
The plasma membrane delimits the cell. You can see the membrane as a black line in this EM.
Two cell membranes delineate adjacent cells which can be linked to each other by various types of
intercellular junctions.
Using electron microscopy, the plasma membrane was found to have a trilaminar (three-layer)
appearance. This gave rise to the view that the membrane was like a sandwich of protein (outer
layers) and lipid (middle layer). This simple model is no longer valid. The fluid mosaic model
is the one currently accepted.
Biological membranes are now considered to be a fluid bilayer of lipid with associated protein
molecules, giving the impression of a two-dimensional mosaic.
The protein compositon of a membrane determines its specific function.
Most of the protein and lipid molecules are freely mobile within the plane of the cell membrane.
Remember , cell membranes are dynamic structures!
Nucleus
This light micrograph (LM) of the liver shows a number of nuclei. Nuclei are membrane-bound
structures where the genetic processes of DNA replication and RNA synthesis occur.
In most cases, it is possible to distinguish the nucleolus (where ribosomes are assembled) and
heterochromatin (a condensed mass of coiled chromosomes).
The electron micrograph (EM) of a liver cell shows additional nuclear features: the nuclear
envelope (a double membrane) and a nuclear pore.
Euchromatin (uncoiled chromosomes) cannot be easily resolved from the general background of
the nuclear matrix.
In a freeze-fracture EM, nuclear pores through the surface of the nuclear membrane can be seen
with excellent clarity.
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Mitochondria
Mitochondria are mobile organelles, bound by a double membrane, similar in size to bacteria.
Mitochondria are the power house of the cell, responsible for cell respiration by combining oxygen
with cell nutrients to generate ATP.
The inner membrane is folded into ridges called cristae. It contains the enzymes involved in
oxidative phosphorylation.
The matrix is the space surrounded by the inner membrane. It contains a high concentration of
many enzymes, including those involved in both fatty acid and pyruvate oxidation.
Mitochondria contain their own DNA and are capable of self-replication!!
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Endoplasmic reticulum
The cytoplasm of most cells contains an extensive membrane system made up of tubular
membrane structures, flattened sheets and sacs, called the endoplasmic reticulum (ER).
It consists of two portions, the rough ER (rER) and the smooth ER (sER), both of which are
visible under EM.
Rough ER occurs mainly as flattened sheets of membrane called cisternae. It is 'rough' because of
the ribosomes associated with it.
Ribosomes play a vital role in protein synthesis. The rER synthesizes the proteins of cell
membranes as well as protein for export from the cell. Its membrane is continuous with the outer
nuclear membrane.
The sER, without ribosomes, appears smooth and is more tubular. It is mainly involved in the
synthesis and transport of lipids, lipoproteins and steroid hormones, as well as in intracellular
calcium storage.
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Golgi
The Golgi apparatus (GA, or simply "the Golgi") consists of stacked, flattened sacs of membrane
usually surrounded by numerous vesicles, most of which are pinched off from the GA membrane.
The GA is involved in the concentration, chemical modification and packaging of the protein/
polypeptide products synthesized in the ER, for export to intracellular and extracellular
destinations.
The GA also sorts and directs newly synthesized proteins to the correct cellular compartment.
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Ribosomes
Ribosomes are the site of protein synthesis. They are composed of RNA molecules (rRNA) and
many different proteins.
Ribosomes can be found free, or associated with ER as rough ER.
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Cytoskeleton
Cytoskeleton is the structural framework of the cell. It is composed of a network of microtubules,
actin microfilaments and intermediate filaments.
The cytoskeleton is responsible for the cell's shape, movement, and ability to organise and
transport its organelles from one part of the cell to another.
The various components of the cytoskeleton consist of protein and peptide subunits.
Microtubules are long, hollow cylinders 25nm in diameter, composed of the protein tubulin and
various other microtubule associated protein.
This EM illustrates, in cross-section, microtubules inside axonal processes where they participate
in axonal transport of organelles and vesicles.
Microtubules support cilia, form the centrioles and mitotic spindles, and serve as tracks along
which organelles move.
Centrioles are cylinders often described as "9 triplets of microtubule", 0.1 x 0.3 micrometres in
size. Centrioles are often found in a pair, perpendicular to each other, in the middle of the
centrosomes. They are thought to organise mitotic spindles during mitosis. Cells without
centrioles are, however, capable of mitosis, though with a higher incidence of errors in
chromosome distribution.
Microfilaments are fine strands of actin, 7nm in diameter. They are essential for cellular
movement, support microvilli and allow for changes in cell shape.
Together with myosin, actin microfilaments serve as contractile elements in muscles, and are also
involved in the processes of phagocytosis, cytokinesis and cell crawling.
Intermediate filaments are 8-10nm in diameter, maintain the structural integrity of the cell and
coordinate those filaments involved in cell movement.
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Secretory granules, lysosomes and peroxisomes
Secretory granules can be seen as specialised forms of membrane-bound intracellular vesicles,
mostly seen in protein secreting cells.
Lysosomes are rich in hydrolytic enzymes required for intracellular digestion of macromolecules,
membranes and organelles. Lysosomes have a variable shape and size, and are thought to be
formed by budding from the Golgi apparatus. Lysosomes are seen more abundantly in
macrophages.
Peroxisomes, also called microbodies, have a high concentration of catalase and are the site for
generation and degradation of reactive peroxides. Peroxisomes are often indistinguisable from
lysosomes under EM. However, sometimes peroxisomes show a crystalline electron-dense
inclusion which contains a high concentration of peroxisomal enzymes. Peroxisomes are found in
association with the ER from which they are thought to originate.
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Fuel storage
Lipid and glycogen are the two forms of fuel stored by cells. Cells vary greatly in the extent and
form of fuel stored.
Adipose tissue is made up of cells known as adipocytes, specialised in lipid storage.
Liver cells store glycogen, but that is not their exclusive function.
Glycogen is stored as granules which often associate with each other, forming aggregates of 'black
dots' seen under EM in the cytosol.
Lipid forms homogenous circular (globular in 3D) structures seen under EM. They are not
bounded by membrane. However, as lipid does not mix well with the aqueous cytosol, it forms
droplets.
This micrograph comes from a steroid-secreting cell. The lipid droplet stores the steroid precursor,
cholesterol.