Bth 101 cell biology

PROF. BALASUBRAMANIAN SATHYAMURTHY 2014 EDITION BTH – 101: CELL BIOLOGY
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FOR MSC BIOTECHNOLOGY STUDENTS
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Biochemistry scanner
THE IMPRINT
BTH – 101 CELL BIOLOGY
(THEORY)
As per Bangalore University (CBCS) Syllabus
2014 Edition
BY: Prof. Balasubramanian Sathyamurthy
Supported By:
Ayesha Siddiqui
Kiran K.S.
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KAMALA KISHORE
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M. SC. BIOTECHNOLOGY - FIRST SEMESTER
BTH – 101: CELL BIOLOGY
TOTAL HOURS: 52 hrs
UNIT 1: BASIC CHARACTERISTICS OF THE CELL 8 HOURS
Structure, organization and composition of prokaryotic and eukaryotic cell. Plasma
membranestructure and functions, membrane models. Components of Blood & their
functions (Plasma, RBC, WBC, Platelets). Extracellular matrix (collagen, proteoglycans,
fibronectin, lamins).
UNIT 2: CYTOSKELETON 8 HOURS
Nature of cytoskeleton, Actin filaments, actin binding proteins, Intermediate filaments,
Microtubules, MAPs, Structure and functions of cilia and flagella.
UNIT 3: MEMBRANE TRANSPORT 8 HOURS
Transport across membrane- passive diffusion, osmosis, active transport, Ion Channels,
ABC transporters, Na+and K+pump, Ca2+ ATPase pump, co-transport, symport,
antiport, endocytosis and exocytosis. Membrane vesicular traffic.
UNIT 4: CELL SIGNALLING 8 HOURS
Cell to cell interactions, Cell adhesion-integrins, selectins, cadherins. Cell Junction-
Tight and gap junctions, Desmosomes, plasmodesmata. General principles of cell
signaling, signaling via G-protein coupled receptors, kinase receptors, role of secondary
messengers.
UNIT 5: CELL CYCLE 6 HOURS
Molecular events of cell division and cell cycle, regulation of cell cycle events- Cyclins,
Cyclin dependent kinases, inhibitors. Apoptosis, necrosis.
UNIT 6: SPECIALIZED CELLS (MUSCLE & NERVE CELLS) 8 HOURS
Structure & functions of muscles (Straited, nonstraited and cardiac). Molecular basis of
muscle contraction. Structure of neuron, neuroglia. Mechanism of nerve transmission-
Resting and action potential, electrical and chemical transmission, Neurotransmitters
and their receptors.
UNIT 7: ANTIOXIDANT DEFENCE SYSTEM AND SENESCENCE 6 HOURS
Free radicals- ROS, RNS. Effect of free radicals on Proteins, Lipids and Nucleic acids.
Mechanism of antioxidant defence system- enzymatic and non-enzymatic. Senescence-
theories and concepts of aging.

References:
1. Matthews, C.A. (2003). Cellular physiology of nerve and muscle. 4th Edn. Blackwell
publishers.
2. Alberts, B., Bray, D., Lewis, J., Raf, M., Roberts, K., Watson, J.D. (1994). Molecular
Biology of the Cell.
3. Cooper, G.M. (1997).The Cell: A molecular approach, ASM Press, USA.
4. Darnell, J., Lodish, H., Baltimore, D. (1990). Molecular Cell Biology. Scientific American
Books Inc. NY.
5. Edwards and Hassall (1980). Biochemistry and Physiology of cell, 2nd Edn. McGraw Hill
Company.
6. Garrett, R.H., Gresham, C.M. (1995). Molecular aspects of Cell Biology, International
edition, Saunders College Pub.
7. Holy Ahern (1992). Introduction to Experimental Cell Biology, Wm. C. Brown
Publishers.
8. Karp, G. (1996). Cell and Molecular Biology concepts and experiments, John Wiley and
Sons Inc. NY.
9. Lodish, H., Baltimore, D., Berk, A., Zipursky, B.L., Mastsydaira, P., Darnell, J. (2004).
10. Molecular Cell Biology, Scientific American Books Inc. NY.
11. Tobin and Morel (1997). Asking about “Cells” Saunders College Publisher.
12. Wolfe, S.L. (1991). Molecular and Cellular Biology, Wordsworth Pub.Co.
13. Hallwell, B., Gutteridge, J.M.C. (2002). Free Radicals Biology and Medicine. Oxford
Press. UK.
14. Kanugo, M.S. (2002) Genes and aging. Cambridge University Press.

UNIT – 1: BASIC CHARACTERISTICS OF THE CELL
Structure, organization and composition of prokaryotic and eukaryotic cell.
Plasma membrane structure and functions, membrane models. Components of
Blood & their functions (Plasma, RBC, WBC, Platelets). Extracellular matrix
(collagen, proteoglycans, fibronectin, lamins).
STRUCTURE, ORGANIZATION AND COMPOSITION OF PROKARYOTIC CELL
Prokaryotes and eukaryotes are chemically similar, in the sense that they both contain
nucleic acids, proteins, lipids, and carbohydrates. They use the same kinds of chemical
reactions to metabolize food, build proteins, and store energy. It is primarily the
structure of cell walls and membranes, and the absence of organelles (specialized
cellular structures that have specific functions), that distinguish prokaryotes from
eukaryotes. The chief distinguishing characteristics of prokaryotes (from the Greek
words meaning prenucleus) are as follows:
Their DNA is not enclosed within a membrane and is usually a singular circularly
arranged chromosome. (Some bacteria, such as Vibrio cholerae, have two chromosomes,
and some bacteria have a linearly arranged chromosome.)
Their DNA is not associated with histones (special chromosomal proteins found in
eukaryotes); other proteins are associated with the DNA.
They lack membrane-enclosed organelles.
Their cell walls almost always contain the complex polysaccharide peptidoglycan.
They usually divide by binary fission. During this process, the DNA is copied, and the
cell splits into two cells. Binary fission involves fewer structures and processes than
eukaryotic cell division.
Eukaryotes (from the Greek words meaning true nucleus) have the following
distinguishing characteristics:
Their DNA is found in the cell’s nucleus, which is separated from the cytoplasm by a
nuclear membrane, and the DNA is found in multiple chromosomes.
Their DNA is consistently associated with chromosomal proteins called histones and
with nonhistones.
They have a number of membrane-enclosed organelles, including mitochondria,
endoplasmic reticulum, Golgi complex, lysosomes, and sometimes chloroplasts.
Their cell walls, when present, are chemically simple.

Cell division usually involves mitosis, in which chromosomes replicate and an identical
set is distributed into each of two nuclei. This process is guided by the mitotic spindle,
a football-shaped assembly of microtubules. Division of the cytoplasm and other
organelles follows so that the two cells produced are identical to each other.
Procaryotic Cell Organization:
The Size, Shape, and Arrangement of Bacterial Cells
Bacteria come in a great many sizes and several shapes.Most bacteria range from 0.2 to
2.0 μm in diameter and from 2 to 8 μm in length. They have a few basic shapes:
spherical coccus (plural: cocci, meaning berries), rod-shaped bacillus (plural: bacilli,
meaning little staffs), and spiral. Cocci are usually round but can be oval, elongated, or
flattened on one side. When cocci divide to reproduce, the cells can remain attached to
one another. Cocci that remain in pairs after dividing are called diplococci; those that
divide and remain attached in chainlike patterns are called streptococci.
Those that divide in two planes and remain in groups of four are known as tetrads.
Those that divide in three planes and remain attached in cubelike groups of eight are
called sarcinae.

Those that divide in multiple planes and form grapelike clusters or broad sheets are
called staphylococci.
These group characteristics are frequently helpful in identifying certain cocci.
Bacilli divide only across their short axis, so there are fewer groupings of bacilli than of
cocci. Most bacilli appear as single rods.
Diplobacilli appear in pairs after division), and streptobacilli occur in chains.
Some bacilli look like straws. Others have tapered ends, like cigars. Still others are oval
and look so much like cocci that they are called coccobacilli.
“Bacillus” has two meanings in microbiology.As we have just used it, bacillus refers to
a bacterial shape.When capitalized and italicized, it refers to a specific genus. For

example, the bacterium Bacillus anthracis is the causative agent of anthrax. Bacillus
cells often form long, twisted chains of cells.
Spiral bacteria have one or more twists; they are never straight. Bacteria that look like
curved rods are called vibrios.
Others, called spirilla, have a helical shape, like a corkscrew, and fairly rigid bodies.
Yet another group of spirals are helical and flexible; they are called spirochetes.
Unlike the spirilla, which use propeller-like external appendages called flagella to move,
spirochetes move by means of axial filaments, which resemble flagella but are contained
within a flexible external sheath.
In addition to the three basic shapes, there are star-shaped cells (genus Stella);
rectangular, flat cells (halophilic archaea) of the genus Haloarcula); and triangular cells.

The shape of a bacterium is determined by heredity. Genetically, most bacteria are
monomorphic; that is, they maintain a single shape. However, a number of
environmental conditions can alter that shape. If the shape is altered, identification
becomes difficult. Moreover, some bacteria, such as Rhizobium (r¯ı-zo¯ _b¯e-um) and
Corynebacterium (kô-r¯ı-ne¯-bakti _r¯e-um), are genetically pleomorphic, which means
they can have many shapes, not just one.
The structure of a typical prokaryotic cell is shown in Figure.
We will discuss its components according to the following organization: (1) structures
external to the cell wall, (2) the cell wall itself, and (3) structures internal to the cell
wall.
Structures External to the Cell Wall
Glycocalyx

Many prokaryotes secrete on their surface a substance called glycocalyx. Glycocalyx
(meaning sugar coat) is the general term used for substances that surround cells. The
bacterial glycocalyx is a viscous (sticky), gelatinous polymer that is external to the cell
wall and composed of polysaccharide, polypeptide, or both. Its chemical composition
varies widely with the species. For the most part, it is made inside the cell and secreted
to the cell surface. If the substance is organized and is firmly attached to the cell wall,
the glycocalyx is described as a capsule. The presence of a capsule can be determined
by using negative staining.
If the substance is unorganized and only loosely attached to the cell wall, the glycocalyx
is described as a slime layer.
In certain species, capsules are important in contributing to bacterial virulence (the
degree to which a pathogen causes disease). Capsules often protect pathogenic bacteria
from phagocytosis by the cells of the host. (As you will see later, phagocytosis is the
ingestion and digestion of microorganisms and other solid particles.) For example,
Bacillus anthracis produces a capsule of D-glutamic acid. Because only encapsulated B.
anthracis causes anthrax, it is speculated that the capsule may prevent its being
destroyed by phagocytosis.
Another example involves Streptococcus pneumonia (strep-to¯ -kok_kus nü-mo¯ _n¯e-¯ı),
which causes pneumonia only when the cells are protected by a polysaccharide capsule.
Unencapsulated S. pneumoniae cells cannot cause pneumonia and are readily
phagocytized. The polysaccharide capsule of Klebsiella (kleb-s¯e-el_lä) also prevents
phagocytosis and allows.
A glycocalyx that helps cells in a biofilm attach to their target environment and to each
other is called an extracellular polymeric substance (EPS). The EPS protects the cells
within it, facilitates communication among them, and enables the cells to survive by
attaching to various surfaces in their natural environment.
Through attachment, bacteria can grow on diverse surfaces such as rocks in fast-
moving streams, plant roots, human teeth, medical implants, water pipes, and even
other bacteria. Streptococcus mutans (m¯u_tans), an important cause of dental caries,
attaches itself to the surface of teeth by a glycocalyx.
S. mutans may use its capsule as a source of nutrition by breaking it down and utilizing
the sugars when energy stores are low. Vibrio cholerae (vib_-r¯e-o kol_-er-¯ı), the cause
of cholera, produces a glycocalyx that helps it attach to the cells of the small intestine.

A glycocalyx also can protect a cell against dehydration, and its viscosity may inhibit
the movement of nutrients out of the cell.
Cytoplasm
For a prokaryotic cell, the term cytoplasm refers to the substance of the cell inside the
plasma membrane.
(see Figure 4.6).
Cytoplasm is about 80% water and contains primarily proteins (enzymes),
carbohydrates, lipids, inorganic ions, and many lowmolecular- weight compounds.
Inorganic ions are present in much higher concentrations in cytoplasm than in most
media. Cytoplasm is thick, aqueous, semitransparent, and elastic. The major structures
in the cytoplasm of prokaryotes are a nucleoid (containing DNA), particles called
ribosomes, and reserve deposits called inclusions. Protein filaments in the cytoplasm
are most likely responsible for the rod and helical cell shapes of bacteria.
Prokaryotic cytoplasm lacks certain features of eukaryotic cytoplasm, such as a
cytoskeleton and cytoplasmic streaming.
The Nucleoid
The nucleoid of a bacterial cell usually contains a single long, continuous, and
frequently circularly arranged thread of double-stranded DNA called the bacterial
chromosome. This is the cell’s genetic information, which carries all the information
required for the cell’s structures and functions. Unlike the chromosomes of eukaryotic
cells, bacterial chromosomes are not surrounded by a nuclear envelope (membrane) and
do not include histones. The nucleoid can be spherical, elongated, or dumbbell-shaped.
In actively growing bacteria, as much as 20% of the cell volume is occupied by DNA
because such cells presynthesize nuclear material for future cells. The chromosome is
attached to the plasma membrane. Proteins in the plasma membrane are believed to be
responsible for replication of the DNA and segregation of the new chromosomes to
daughter cells during cell division.
In addition to the bacterial chromosome, bacteria often contain small usually circular,
double-stranded DNA molecules called plasmids.
These molecules are extrachromosomal genetic elements; that is, they are not
connected to the main bacterial chromosome, and they replicate independently of
chromosomal DNA. Research indicates that plasmids are associated with plasma
membrane proteins. Plasmids usually contain from 5 to 100 genes that are generally

not crucial for the survival of the bacterium under normal environmental conditions;
plasmids may be gained or lost without harming the cell. Under certain conditions,
however, plasmids are an advantage to cells. Plasmids may carry genes for such
activities as antibiotic resistance, tolerance to toxic metals, the production of toxins,
and the synthesis of enzymes. Plasmids can be transferred from one bacterium to
another. In fact, plasmid DNA is used for gene manipulation in biotechnology.
Ribosomes
All eukaryotic and prokaryotic cells contain ribosomes, which function as the sites of
protein synthesis. Cells that have high rates of protein synthesis, such as those that are
actively growing, have a large number of ribosomes. The cytoplasm of a prokaryotic cell
contains tens of thousands of these very small structures, which give the cytoplasm a
granular appearance. Ribosomes are composed of two subunits, each of which consists
of protein and a type of RNA called ribosomal RNA (rRNA). Prokaryotic ribosomes differ
from eukaryotic ribosomes in the number of proteins and rRNA molecules they contain;
they are also somewhat smaller and less dense than ribosomes of eukaryotic cells.
Accordingly, prokaryotic ribosomes are called 70S ribosomes, and those of eukaryotic
cells are known as 80S ribosomes. The letter S refers to Svedberg units, which indicate
the relative rate of sedimentation during ultra-high-speed centrifugation. Sedimentation
rate is a function of the size, weight, and shape of a particle. The subunits of a 70S
ribosome are a small 30S subunit containing one molecule of rRNA and a larger 50S
subunit containing two molecules of rRNA.
Several antibiotics work by inhibiting protein synthesis on prokaryotic ribosomes.
Antibiotics such as streptomycin and gentamicin attach to the 30S subunit and
interfere with protein synthesis. Other antibiotics, such as erythromycin and
chloramphenicol, interfere with protein synthesis by attaching to the 50S subunit.
Because of differences in prokaryotic and eukaryotic ribosomes, the microbial cell can
be killed by the antibiotic while the eukaryotic host cell remains unaffected.
Inclusions
Within the cytoplasm of prokaryotic cells are several kinds of reserve deposits, known
as inclusions. Cells may accumulate certain nutrients when they are plentiful and use
them when the environment is deficient. Evidence suggests that macromolecules
concentrated in inclusions avoid the increase in osmotic pressure that would result if
the molecules were dispersed in the cytoplasm. Some inclusions are common to a wide

variety of bacteria, whereas others are limited to a small number of species and
therefore serve as a basis for identification.
Metachromatic Granules
Metachromatic granules are large inclusions that take their name from the fact that
they sometimes stain red with certain blue dyes such as methylene blue. Collectively
they are known as volutin. Volutin represents a reserve of inorganic phosphate
(polyphosphate) that can be used in the synthesis of ATP. It is generally formed by cells
that grow in phosphate-rich environments. Metachromatic granules are found in algae,
fungi, and protozoa, as well as in bacteria. These granules are characteristic of
Corynebacterium diphtheriae (kô-r¯ı-n¯e-bak-ti_r¯e-um difthi _r¯e-¯ı), the causative
agent of diphtheria; thus, they have diagnostic significance.
Polysaccharide Granules
Inclusions known as polysaccharide granules typically consist of glycogen and starch,
and their presence can be demonstrated when iodine is applied to the cells. In the
presence of iodine, glycogen granules appear reddish brown and starch granules appear
blue.
Lipid Inclusions
Lipid inclusions appear in various species of Mycobacterium, Bacillus, Azotobacter (ä-
zo¯-to¯-bak_tér), Spirillum (sp¯ı-ril_lum), and other genera. A common lipid-storage
material, one unique to bacteria, is the polymer poly-β-hydroxybutyric acid. Lipid
inclusions are revealed by staining cells with fat-soluble dyes, such as Sudan dyes.
Sulfur Granules
Certain bacteria—for example, the “sulfur bacteria” that belong to the genus
Thiobacillus—derive energy by oxidizing sulfur and sulfur-containing compounds. These
bacteria may deposit sulfur granules in the cell, where they serve as an energy reserve.
Carboxysomes
Carboxysomes are inclusions that contain the enzyme ribulose 1,5-diphosphate
carboxylase. Photosynthetic bacteria use carbon dioxide as their sole source of carbon
and require this enzyme for carbon dioxide fixation. Among the bacteria containing
carboxysomes are nitrifying bacteria, cyanobacteria, and thiobacilli.
Gas Vacuoles
Hollow cavities found in many aquatic prokaryotes, including cyanobacteria, anoxygenic
photosynthetic bacteria, and halobacteria are called gas vacuoles. Each vacuole

consists of rows of several individual gas vesicles, which are hollow cylinders covered by
protein. Gas vacuoles maintain buoyancy so that the cells can remain at the depth in
the water appropriate for them to receive sufficient amounts of oxygen, light, and
nutrients.
Magnetosomes
Magnetosomes are inclusions of iron oxide (Fe3O4), formed by several gram-negative
bacteria such as Magnetospirillum magnetotacticum, that act like magnets.
Bacteria may use magnetosomes to move downward until they reach a suitable
attachment site. In vitro, magnetosomes can decompose hydrogen peroxide, which
forms in cells in the presence of oxygen. Researchers speculate that magnetosomes may
protect the cell against hydrogen peroxide accumulation.
Endospores
When essential nutrients are depleted, certain gram-positive bacteria, such as those of
the genera Clostridium and Bacillus, form specialized “resting” cells called endospores.
Some members of the genus Clostridium cause diseases such as gangrene, tetanus,
botulism, and food poisoning. Some members of the genus Bacillus cause anthrax and
food poisoning. Unique to bacteria, endospores are highly durable dehydrated cells with
thick walls and additional layers. They are formed internal to the bacterial cell
membrane.
When released into the environment, they can survive extreme heat, lack of water, and
exposure to many toxic chemicals and radiation. For example, 7500-year-old
endospores of Thermoactinomyces vulgaris (th˙ er-mo¯-ak-tin-o¯-m¯ı_s¯es vul-ga_ris)
from the freezing muds of Elk Lake in Minnesota have germinated when rewarmed and
placed in a nutrient medium, and 25- to 40-million-year-old endospores found in the
gut of a stingless bee entombed in amber (hardened tree resin) in the Dominican

Republic are reported to have germinated when placed in nutrient media. Although true
endospores are found in gram-positive bacteria, one gram-negative species,Coxiella
burnetii (käks-¯e-el_lä b˙er-ne_t¯e-¯e), the cause of Q fever, forms endosporelike
structures that resist heat and chemicals and can be stained with endospore stains.
The process of endospore formation within a vegetative cell takes several hours and is
known as sporulation or sporogenesis.
Vegetative cells of endosporeforming bacteria begin sporulation when a key nutrient,
such as the carbon or nitrogen source, becomes scarce or unavailable. In the first
observable stage of sporulation, a newly replicated bacterial chromosome and a small

portion of cytoplasm are isolated by an ingrowth of the plasma membrane called a spore
septum.
The spore septum becomes a double-layered membrane that surrounds the
chromosome and cytoplasm. This structure, entirely enclosed within the original cell, is
called a forespore. Thick layers of peptidoglycan are laid down between the two
membrane layers. Then a thick spore coat of protein forms around the outside
membrane; this coat is responsible for the resistance of endospores to many harsh
chemicals. The original cell is degraded, and the endospore is released. The diameter of
the endospore may be the same as, smaller than, or larger than the diameter of the
vegetative cell.Depending on the species, the endospore might be located terminally (at
one end), subterminally, or centrally inside the vegetative cell.When the endospore
matures, the vegetative cell wall ruptures (lyses), killing the cell, and the endospore is
freed.(near one end)
Most of the water present in the forespore cytoplasm is eliminated by the time
sporulation is complete, and endospores do not carry out metabolic reactions. The
highly dehydrated endospore core contains only DNA, small amounts of RNA,
ribosomes, enzymes, and a few important small molecules. The latter include a
strikingly large amount of an organic acid called dipicolinic acid (found in the
cytoplasm), which is accompanied by a large number of calcium ions.
These cellular components are essential for resuming metabolism later.
Endospores can remain dormant for thousands of years. An endospore returns to its
vegetative state by a process called germination. Germination is triggered by physical
or chemical damage to the endospore’s coat. The endospore’s enzymes then break down
the extra layers surrounding the endospore, water enters, and metabolism resumes.
Because one vegetative cell forms a single endospore, which, after germination, remains
one cell, sporulation in bacteria is not a means of reproduction. This process does not
increase the number of cells. Bacterial endospores differ from spores formed by
(prokaryotic) actinomycetes and the eukaryotic fungi and algae, which detach from the
parent and develop into another organism and, therefore, represent reproduction.
Endospores are important from a clinical viewpoint and in the food industry because
they are resistant to processes that normally kill vegetative cells. Such processes
include heating, freezing, desiccation, use of chemicals, and radiation. Whereas most
vegetative cells are killed by temperatures above 70°C, endospores can survive in

boiling water for several hours or more. Endospores of thermophilic (heat-loving)
bacteria can survive in boiling water for 19 hours. Endospore-forming bacteria are a
problem in the food industry because they are likely to survive underprocessing, and, if
conditions for growth occur, some species produce toxins and disease.
STRUCTURE, ORGANIZATION AND COMPOSITION OF EUKARYOTIC CELL
As mentioned earlier, eukaryotic organisms include algae, protozoa, fungi, plants, and
animals. The eukaryotic cell is typically larger and structurally more complex than the
prokaryotic cell.
When the structure of the prokaryotic cell is compared with that of the eukaryotic cell,
the differences between the two types of cells become apparent.
The Cell Wall and Glycocalyx
Most eukaryotic cells have cell walls, although they are generally much simpler than
those of prokaryotic cells. Many algae have cell walls consisting of the polysaccharide
cellulose (as do all plants); other chemicals may be present as well. Cell walls of some
fungi also contain cellulose, but in most fungi the principal structural component of the
cell wall is the polysaccharide chitin, a polymer of N-acetylglucosamine (NAG) units.
(Chitin is also the main structural component of the exoskeleton of crustaceans and
insects.) The cell walls of yeasts contain the polysaccharides glucan and mannan. In
eukaryotes that lack a cell wall, the plasma membrane may be the outer covering;
however, cells that have direct contact with the environment may have coatings outside
the plasma membrane. Protozoa do not have a typical cell wall; instead, they have a
flexible outer protein covering called a pellicle. In other eukaryotic cells, including
animal cells, the plasma membrane is covered by a glycocalyx, a layer of material
containing substantial amounts of sticky carbohydrates. Some of these carbohydrates
are covalently bonded to proteins and lipids in the plasma membrane, forming
glycoproteins and glycolipids that anchor the glycocalyx to the cell. The glycocalyx
strengthens the cell surface, helps attach cells together, and may contribute to cell–cell
recognition.
Eukaryotic cells do not contain peptidoglycan, the framework of the prokaryotic cell
wall. This is significant medically because antibiotics, such as penicillins and
cephalosporins, act against peptidoglycan and therefore do not affect human eukaryotic
cells.

The Plasma (Cytoplasmic) Membrane
The plasma (cytoplasmic) membrane of eukaryotic and prokaryotic cells is very
similar in function and basic structure. There are, however, differences in the types of
proteins found in the membranes. Eukaryotic membranes also contain carbohydrates,
which serve as attachment sites for bacteria and as receptor sites that assume a role in
such functions as cell–cell recognition. Eukaryotic plasma membranes also contain
sterols, complex lipids not found in prokaryotic plasma membranes (with the exception
ofMycoplasma cells). Sterols seem to be associated with the ability of the membranes to
resist lysis resulting from increased osmotic pressure.
Substances can cross eukaryotic and prokaryotic plasma membranes by simple
diffusion, facilitated diffusion, osmosis, or active transport. Group translocation does
not occur in eukaryotic cells. However, eukaryotic cells can use a mechanism called
endocytosis. This occurs when a segment of the plasma membrane surrounds a
particle or large molecule, encloses it, and brings it into the cell.
Two very important types of endocytosis are phagocytosis and pinocytosis. During
phagocytosis, cellular projections called pseudopods engulf particles and bring them
into the cell.
Phagocytosis is used by white blood cells to destroy bacteria and foreign substances.

In pinocytosis, the plasma membrane folds inward, bringing extracellular fluid into the
cell, along with whatever substances are dissolved in the fluid. Pinocytosis is one of the
ways viruses can enter animal cells.
Cytoplasm
The cytoplasm of eukaryotic cells encompasses the substance inside the plasma
membrane and outside the nucleus.
The cytoplasm is the substance in which various cellular components are found. (The
term cytosol refers to the fluid portion of cytoplasm.) A major difference between
eukaryotic and prokaryotic cytoplasm is that eukaryotic cytoplasm has a complex
internal structure, consisting of exceedingly small rods (microfilaments and intermediate
filaments) and cylinders (microtubules). Together, they form the cytoskeleton. The
cytoskeleton provides support and shape and assists in transporting substances
through the cell (and even in moving the entire cell, as in phagocytosis). The movement
of eukaryotic cytoplasm from one part of the cell to another, which helps distribute
nutrients and move the cell over a surface, is called cytoplasmic streaming.
Another difference between prokaryotic and eukaryotic cytoplasm is that many of the
important enzymes found in the cytoplasmic fluid of prokaryotes are sequestered in the
organelles of eukaryotes.
Ribosomes
Attached to the outer surface of rough endoplasmic reticulum are ribosomes, which are
also found free in the cytoplasm.As in prokaryotes, ribosomes are the sites of protein
synthesis in the cell. The ribosomes of eukaryotic endoplasmic reticulum and cytoplasm
are somewhat larger and denser than those of prokaryotic cells. These eukaryotic
ribosomes are 80S ribosomes, each of which consists of a large 60S subunit containing
three molecules of rRNA and a smaller 40S subunit with one molecule of rRNA. The
subunits are made separately in the nucleolus and, once produced, exit the nucleus
and join together in the cytosol.
Chloroplasts and mitochondria contain 70S ribosomes, which may indicate their
evolution from prokaryotes.
Some ribosomes, called free ribosomes, are unattached to any structure in the
cytoplasm. Primarily, free ribosomes synthesize proteins used inside the cell. Other
ribosomes, called membranebound ribosomes, attach to the nuclear membrane and the
endoplasmic reticulum. These ribosomes synthesize proteins destined for insertion in

the plasma membrane or for export from the cell. Ribosomes located within
mitochondria synthesize mitochondrial proteins. Sometimes 10 to 20 ribosomes join
together in a stringlike arrangement called a polyribosome.
Organelles
Organelles are structures with specific shapes and specialized functions and are
characteristic of eukaryotic cells. They include the nucleus, endoplasmic reticulum,
Golgi complex, lysosomes, vacuoles, mitochondria, chloroplasts, peroxisomes, and
centrosomes. Not all of the organelles described are found in all cells. Certain cells have
their own type and distribution of organelles based on specialization, age, and level of
activity.
The Nucleus
The most characteristic eukaryotic organelle is the nucleus.

The nucleus is usually spherical or oval, is frequently the largest structure in the cell,
and contains almost all of the cell’s hereditary information (DNA). Some DNA is also
found in mitochondria and in the chloroplasts of photosynthetic organisms.
The nucleus is surrounded by a double membrane called the nuclear envelope. Both
membranes resemble the plasma membrane in structure. Tiny channels in the
membrane called nuclear pores allow the nucleus to communicate with the cytoplasm.
Nuclear pores control the movement of substances between the nucleus and cytoplasm.
Within the nuclear envelope are one or more spherical bodies called nucleoli (singular:
nucleolus). Nucleoli are actually condensed regions of chromosomes where ribosomal
RNA is being synthesized. Ribosomal RNA is an essential component of ribosomes.
The nucleus also contains most of the cell’s DNA, which is combined with several
proteins, including some basic proteins called histones and nonhistones. The
combination of about 165 base pairs of DNA and 9 molecules of histones is referred to
as a nucleosome. When the cell is not reproducing, the DNA and its associated proteins
appear as a threadlike mass called chromatin. During nuclear division, the chromatin
coils into shorter and thicker rodlike bodies called chromosomes.
Prokaryotic chromosomes do not undergo this process, do not have histones, and are
not enclosed in a nuclear envelope. Eukaryotic cells require two elaborate mechanisms:
mitosis and meiosis to segregate chromosomes prior to cell division. Neither process
occurs in prokaryotic cells.
Endoplasmic Reticulum
Within the cytoplasm of eukaryotic cells is the endoplasmic reticulum, or ER, an
extensive network of flattened membranous sacs or tubules called cisterns.

The ER network is continuous with the nuclear envelope. Most eukaryotic cells contain
two distinct, but interrelated, forms of ER that differ in structure and function. The
membrane of rough ER is continuous with the nuclear membrane and usually unfolds
into a series of flattened sacs. The outer surface of rough ER is studded with ribosomes,
the sites of protein synthesis. Proteins synthesized by ribosomes that are attached to
rough ER enter cisterns within the ER for processing and sorting. In some cases,
enzymes within the cisterns attach the proteins to carbohydrates to form glycoproteins.
In other cases, enzymes attach the proteins to phospholipids, also synthesized by rough
ER. These molecules may be incorporated into organelle membranes or the plasma
membrane. Thus, rough ER is a factory for synthesizing secretory proteins and
membrane molecules.
Smooth ER extends from the rough ER to form a network of membrane tubules.
Unlike rough ER, smooth ER does not have ribosomes on the outer surface of its
membrane. However, smooth ER contains unique enzymes that make it functionally
more diverse than rough ER. Although it does not synthesize proteins, smooth ER does
synthesize phospholipids, as does rough ER. Smooth ER also synthesizes fats and
steroids, such as estrogens and testosterone. In liver cells, enzymes of the smooth ER
help release glucose into the bloodstream and inactivate or detoxify drugs and other
potentially harmful substances (for example, alcohol). In muscle cells, calcium ions
released from the sarcoplasmic reticulum, a form of smooth ER, trigger the contraction
process.
Golgi Complex
Most of the proteins synthesized by ribosomes attached to rough ER are ultimately
transported to other regions of the cell. The first step in the transport pathway is

through an organelle called the Golgi complex. It consists of 3 to 20 cisterns that
resemble a stack of pita bread.
The cisterns are often curved, giving the Golgi complex a cuplike shape. Proteins
synthesized by ribosomes on the rough ER are surrounded by a portion of the ER
membrane, which eventually buds from the membrane surface to form a transport
vesicle.
The transport vesicle fuses with a cistern of the Golgi complex, releasing proteins into
the cistern. The proteins are modified and move from one cistern to another via
transfer vesicles that bud from the cisterns’ edges. Enzymes in the cisterns modify the
proteins to form glycoproteins, glycolipids, and lipoproteins.
Some of the processed proteins leave the cisterns in secretory vesicles, which detach
from the cistern and deliver the proteins to the plasma membrane, where they are
discharged by exocytosis.
Other processed proteins leave the cisterns in vesicles that deliver their contents to the
plasma membrane for incorporation into the membrane. Finally, some processed
proteins leave the cisterns in vesicles that are called storage vesicles. The major
storage vesicle is a lysosome, whose structure and functions are discussed next.
Lysosomes
Lysosomes are formed from Golgi complexes and look like membrane-enclosed
spheres. Unlike mitochondria, lysosomes have only a single membrane and lack
internal structure. But they contain as many as 40 different kinds of powerful digestive
enzymes capable of breaking down various molecules. Moreover, these enzymes can
also digest bacteria that enter the cell. Human white blood cells, which use
phagocytosis to ingest bacteria, contain large numbers of lysosomes.
Vacuoles
A vacuole is a space or cavity in the cytoplasm of a cell that is enclosed by a membrane
called a tonoplast. In plant cells, vacuoles may occupy 5–90% of the cell volume,
depending on the type of cell. Vacuoles are derived from the Golgi complex and have
several diverse functions. Some vacuoles serve as temporary storage organelles for
substances such as proteins, sugars, organic acids, and inorganic ions. Other vacuoles
form during endocytosis to help bring food into the cell. Many plant cells also store
metabolic wastes and poisons that would otherwise be injurious if they accumulated in

the cytoplasm. Finally, vacuoles may take up water, enabling plant cells to increase in
size and also providing rigidity to leaves and stems.
Mitochondria
Spherical or rod-shaped organelles called mitochondria (singular: mitochondrion)
appear throughout the cytoplasm of most eukaryotic cells. The number of mitochondria
per cell varies greatly among different types of cells. For example, the protozoan Giardia
has no mitochondria, whereas liver cells contain 1000 to 2000 per cell. A mitochondrion
consists of a double membrane similar in structure to the plasma membrane.
The outer mitochondrial membrane is smooth, but the inner mitochondrial membrane
is arranged in a series of folds called cristae (singular: crista). The center of the
mitochondrion is a semifluid substance called the matrix. Because of the nature and
arrangement of the cristae, the inner membrane provides an enormous surface area on
which chemi cal reactions can occur. Some proteins that function in cellular
respiration, including the enzyme that makes ATP, are located on the cristae of the
inner mitochondrial membrane, and many of the metabolic steps involved in cellular

respiration are concentrated in the matrix. Mitochondria are often called the
“powerhouses of the cell” because of their central role in ATP production.
Mitochondria contain 70S ribosomes and some DNA of their own, as well as the
machinery necessary to replicate, transcribe, and translate the information encoded by
their DNA. In addition, mitochondria can reproduce more or less on their own by
growing and dividing in two.
Chloroplasts
Algae and green plants contain a unique organelle called a chloroplast, a membrane-
enclosed structure that contains both the pigment chlorophyll and the enzymes
required for the light-gathering phases of photosynthesis.
The chlorophyll is contained in flattened membrane sacs called thylakoids; stacks of
thylakoids are called grana (singular: granum).
Like mitochondria, chloroplasts contain 70S ribosomes, DNA, and enzymes involved in
protein synthesis. They are capable of multiplying on their own within the cell. The way
both chloroplasts and mitochondria multiply—by increasing in size and then dividing in
two—is strikingly reminiscent of bacterial multiplication.

Peroxisomes
Organelles similar in structure to lysosomes, but smaller, are called peroxisomes.
Although peroxisomes were once thought to form by budding off the ER, it is now
generally agreed that they form by the division of preexisting peroxisomes.
Peroxisomes contain one or more enzymes that can oxidize various organic substances.
For example, substances such as amino acids and fatty acids are oxidized in
peroxisomes as part of normal metabolism. In addition, enzymes in peroxisomes oxidize
toxic substances, such as alcohol. A by-product of the oxidation reactions is hydrogen
peroxide (H2O2), a potentially toxic compound. However, peroxisomes also contain the
enzyme catalase, which decomposes H2O2. Because the generation and degradation of
H2O2 occurs within the same organelle, peroxisomes protect other parts of the cell from
the toxic effects of H2O2.
Centrosome
The centrosome, located near the nucleus, consists of two components: the
pericentriolar area and centrioles. The pericentriolar material is a region of the cytosol
composed of a dense network of small protein fibers.
PLASMA MEMBRANE STRUCTURE AND FUNCTIONS
The Plasma (Cytoplasmic) Membrane
The plasma (cytoplasmic) membrane (or inner membrane) is a thin structure lying
inside the cell wall and enclosing the cytoplasm of the cell.
The plasma membrane of prokaryotes consists primarily of phospholipids, which are
the most abundant chemicals in the membrane, and proteins. Eukaryotic plasma
membranes also contain carbohydrates and sterols, such as cholesterol. Because they
lack sterols, prokaryotic plasma membranes are less rigid than eukaryotic membranes.
One exception is the wall-less prokaryote Mycoplasma, which contains membrane
sterols.
Structure:
In electron micrographs, prokaryotic and eukaryotic plasma membranes (and the outer
membranes of gram-negative bacteria) look like two-layered structures; there are two
dark lines with a light space between the lines.
The phospholipid molecules are arranged in two parallel rows, called a lipid bilayer.

Each phospholipid molecule contains a polar head, composed of a phosphate group and
glycerol that is hydrophilic (water-loving) and soluble in water, and nonpolar tails,
composed of fatty acids that are hydrophobic (water-fearing) and insoluble in water.
The polar heads are on the two surfaces of the lipid bilayer, and the nonpolar tails are
in the interior of the bilayer. The protein molecules in the membrane can be arranged in
a variety of ways. Some, called peripheral proteins, are easily removed from the
membrane by mild treatments and lie at the inner or outer surface of the membrane.
They may function as enzymes that catalyze chemical reactions, as a “scaffold” for
support, and as mediators of changes in membrane shape during movement. Other
proteins, called integral proteins, can be removed from the membrane only after
disrupting the lipid bilayer (by using detergents, for example). Most integral proteins
penetrate the membrane completely and are called transmembrane proteins. Some
integral proteins are channels that have a pore, or whole, through which substances
enter and exit the cell.
Many of the proteins and some of the lipids on the outer surface of the plasma
membrane have carbohydrates attached to them. Proteins attached to carbohydrates

are called glycoproteins; lipids attached to carbohydrates are called glycolipids. Both
glycoproteins and glycolipids help protect and lubricate the cell and are involved in cell-
to-cell interactions. For example, glycoproteins play a role in certain infectious diseases.
The influenza virus and the toxins that cause cholera and botulism enter their target
cells by first binding to glycoproteins on their plasma membranes.
Studies have demonstrated that the phospholipid and protein molecules in membranes
are not static but move quite freely within the membrane surface. This movement is
most probably associated with the many functions performed by the plasma membrane.
Because the fatty acid tails cling together, phospholipids in the presence of water form a
self-sealing bilayer; as a result, breaks and tears in the membrane heal themselves. The
membrane must be about as viscous as olive oil, which allows membrane proteins to
move freely enough to perform their functions without destroying the structure of the
membrane. This dynamic arrangement of phospholipids and proteins is referred to as
the fluid mosaic model.
Functions
The most important function of the plasma membrane is to serve as a selective barrier
through which materials enter and exit the cell. In this function, plasma membranes
have selective permeability (sometimes called semipermeability).
This term indicates that certain molecules and ions pass through the membrane, but
that others are prevented from passing through it. The permeability of the membrane
depends on several factors. Large molecules (such as proteins) cannot pass through the
plasma membrane, possibly because these molecules are larger than the pores in
integral proteins that function as channels. But smaller molecules (such as water,
oxygen, carbon dioxide, and some simple sugars) usually pass through easily. Ions
penetrate the membrane very slowly.
Substances that dissolve easily in lipids (such as oxygen, carbon dioxide, and nonpolar
organic molecules) enter and exit more easily than other substances because the
membrane consists mostly of phospholipids. The movement of materials across plasma
membranes also depends on transporter molecules, which will be described shortly.
Plasma membranes are also important to the breakdown of nutrients and the
production of energy. The plasma membranes of bacteria contain enzymes capable of
catalyzing the chemical reactions that break down nutrients and produce ATP. In some
bacteria, pigments and enzymes involved in photosynthesis are found in infoldings of

the plasma membrane that extend into the cytoplasm. These membranous structures
are called chromatophores or thylakoids.
When viewed with an electron microscope, bacterial plasma membranes often appear to
contain one or larger, irregular folds called mesosomes.Many functions have been
proposed for mesosomes.However, it is now known that they are artifacts, not true cell
structures. Mesosomes are believed to be folds in the plasma membrane that develop by
the process used for preparing specimens for electron microscopy.
MEMBRANE MODELS
There are eight different models of membranes:
Models of membranes:
1. Langmuir's monolayers (1920)
2. Micelles
3. Gortner & Grendel's bilayers (1925)
4. Liposomes
5. Black lipid membranes (1930)
6. Davison & Danielli's sandwich (1935)
7. Robertson's unit membrane
8. Singer-Nicholson fluid mosaic (1972)
1. Langmuir's monolayers (1920):
If phospholipids are dissolved in benzene they
could be dispersed as a monolayer on the
surface of water in a Langmuir trough.
2. Micelles:

If shaken with water, phospholipids (like detergents) will
form micelles. These are colloids in an aqueous
suspension.
Micelles have a hydrophilic outside and hydrophobic
inside.
Water is inherently disordered, but when lipids are
present, the water molecules have to arrange themselves
in a more ordered way.
The less order there is in a arrangement, the more likely
(∆G = ∆H + T∆S) the arrangement is to happen.
The clumping together of fat molecules reduces the amount of water that has to be
ordered.
3. Gortner & Grendel's bilayers (1925):
The lipid extracted from the plasma membrane of RBCs and applied them to a
Langmuir trough. They covered twice the area of the original membrane showing that
natural membranes are bilayers.
4. Liposome:
LIPOSOME is derived from two Greek words.
'Lipid' meaning fat and 'Soma' meaning body.
If lipids are sonicated at 20 kHz they form vesicles
(liposomes) with an internal space.
5. Black lipid membranes (1930):
Produced by forcing a membrane with a small
hole in it through a monolayer in a Langmuir
trough.
Natural and black lipid membranes have similar
thicknesses (c. 7 nm), but natural membranes
are generally far more conductive.
This indicates there's something else in natural
membranes besides lipid
6. Davison & Danielli's sandwich (1935):

The earliest true model of membranes
proposed a phospholipid bilayer covered in a
globular protein coat.
7. Robertson's unit membrane:
Under the electron micrograph, this model appears acceptable: 7 nm thick, with lipid in
the middle (white) and protein on outside (black).
But freeze-fracture scanning electron micrographs showed things inconsistent with the
unit membrane, such as pores and pits.
This micrograph was prepared by freezing a cell, then fracturing it with a sharp blow.
The forces holding the leaflets of a membrane together are quite weak, so freeze fracture
often pulls the two leaflets apart, allowing you to see the proteins that span the
membrane very clearly.
8. Singer-Nicholson fluid mosaic (1972):
 The fluid mosaic model pictures the membrane as a phospholipid bilayer with many
proteins, some integral to the membrane, others attached more loosely.
The many other components, such as cholesterol; and the attachement sites for the
extracellular environment (via glycoproteins) and intracellular cytoskeleton.
COMPONENTS OF BLOOD & THEIR FUNCTIONS
(PLASMA, RBC, WBC, PLATELETS)
PLASMA COMPOSITION AND FUNCTIONS

The important plasma proteins along with their characteristics (based on
electrophoretic pattern) and major functions are given, some selected plasma proteins
are given below.
Albumin:
Albumin is the major constituent (60%) of plasma proteins with a concentration of 3.5-
5.0g/dl. Human albumin has a molecular weight of 69,000 and consists of a single
polypeptide chain of 585 amino acids with 17 disulfide bonds.
Synthesis of albumin: Albumin is exclusively synthesized by the liver. Liver produces
about 12g albumin per day. Albumin has a half-life of 20 days.
Osmotic function: Due to its high concentration and low molecular weight, albumin
contributes to 75-80% of the total plasma osmotic pressure(25mm Hg).Thus albumin
plays a predominant role in maintaining blood volume and body fluid distribution.
Decrease in plasma albumin level results in a fall in osmotic pressure, leading to
enhanced fluid retention in tissue spaces, causing edema.
Nutritive function: Albumin serves as a source of amino acids for tissue protein
synthesis to limited extent, particularly in nutritional deprivation of amino acids.
Buffering function: Among the plasma proteins, albumin has the maximum buffering
capacity.
Globulins:
Globulins constitute several proteins that are separated into four distinct bands (α1, α2,
β and γ globulins) on electrophoresis. Globulins, in general, are bigger in size than
albumin. They perform a variety of functions which include transport and immunity.
α1 –Antitrypsin:
α1 – antitrypsin, more recently called as α-anti-proteinase, is a glycoprotein with 394
amino acids and a molecular weight of 54,000. It is a major constituent of α1-globulin
fraction of plasma proteins with a normal concentration of about 200 mg/dl. α1 –
antitrypsin is a serine protease inhibitor. It combines with trypsin, elastate and other
protease enzymes and inhibits their activity.
Clinical significance of α1 –Antitrypsin:
α1 – antitrypsin deficiency has been implicated in two diseases, namely, emphysema
and α1-AT deficiency liver disease.
α2 – Macroglobulin:

It is a high molecular weight (8, 00,000) protein and is a major constituent of α2-
fraction. α2-Macroglobulin inhibits protease activity and serves as an anticoagulant.
Haptoglobin:
Haptoglobin (Hp) is a plasma glycoprotein with an approximate molecular weight of
90,000. Hp is an acute phase protein since its plasma concentration is increased in
several inflammatory conditions.
Functions of haptoglobin:
Haptoglobin binds with the free hemoglobin (known as extra-corpuscular hemoglobin)
that spills into the plasma due to hemolysis.
Fibrinogen (factor1):
It is asoluble glycoprotein that constitutes 2-3% of plasma proteins (plasma
concentration 0.3g/dl.).Fibrinogen consists of 6polypeptide chain two A α, two B β and
two gama making the structure (A α)2 (B β)2 gama2.
Prothrombin (II):
It is an inactive zymogen form of thrombin (IIa). The activation of prothrombin occurs
on the platelets and requires the presence of factors Va and Xa, besides phospholipids
and Ca2+.
Albumin is the major constituent (60%) of plasma proteins with a concentration of 3.5-
5.0g/dl. Human albumin has a molecular weight of 69,000 and consists of a single
polypeptide chain of 585 amino acids with 17 disulfide bonds.
Low density lipoproteins (LDL): They are formed from VLDL in the blood circulation.
They transport cholesterol from liver to other tissues.
Characteristic of LDL in human plasma:
Electrophoretic mobility is beta.
Density of LDL is 1.006-1.063.
Diameter (nm) is 20-25.
Apo proteins are B100.
Composition:
Proteins 20(%), lipids (total) 80% (triacylglycerol 12%, cholesterol (free and ester) 59%,
phospholipids 28%, free fatty acids 1%).

COMPOSITION AND FUNCTIONS OF PLASMA LIPOPROTEINS
Lipoproteins are molecular complexes that consist of lipids and proteins (conjugated
proteins). They function as transport vehicles for lipids in blood plasma. Lipoproteins
deliver the lipid components (cholesterol, triacylglycerol etc) to various tissues for
utilization.
Structure of lipoproteins:
A lipoprotein basically consists of a neutral lipid core (with triacylglycerol and/or
cholesteryl ester) surrounded by a coat shell of phospholipids, apoproteins and
cholesterol. The polar portions (amphiphilic) of phospholipids and cholesterol are
exposed on the surface of lipoproteins so that lipoprotein is soluble in aqueous solution.
Classification of lipoproteins:
Five major classes of lipoproteins are identified in human plasma, based on their
separation by electrophoresis.
Chylomicrons: They are synthesized in the intestine and transport exogenous (dietary)
triacylglycerol to various tissues. They consist of highest (99%) quantity of lipid and
lowest (1%) concentration of protein. The chylomicrons are the least in density and the
largest in size, among the lipoproteins.
Very low density lipoproteins (VLDL):
They are produced in liver and intestine and are responsible for the transport of
endogenously synthesized triacylglycerols.
Low density lipoproteins (LDL):
They are formed from VLDL in the blood circulation. They transport cholesterol from
liver to other tissues.
High density lipoproteins (HDL):
They are mostly synthesized in liver. Three different fractions of HDL (1, 2 and 3) can be
identified by ultracentrifugation. HDL particles transport cholesterol from peripheral
tissues to liver (reverse cholesterol transport).
Free fatty acids-albumin:
Free fatty acids in the circulation are in a bound form to albumin. Each molecule of
albumin can hold about 20-30 molecules of free fatty acids. This lipoprotein cannot be
separated by electrophoresis.

Apo lipoproteins (apoproteins):
The protein components of lipoproteins are known as apolipoproteins or simply
apoproteins. They perform the following functions
Act as structural components of lipoproteins.
Recognize the cell membrane surface receptors.
Active enzymes involved in lipoprotein metabolism.
The comparative characteristic features of different lipoproteins with regard to
electrophoretic patterns, size, composition etc are given below.
RBC OR ERYTHROCYTES:
RBC is the non-nucleated formed elements in the blood. The red color of these cells is
due to the coloring matter hemoglobin. The RBC count ranges from 4 to 5.5 million per
cubic millimeter of blood.
Morphology of RBC:
Normal size
Diameter 7.2 microns
Thickness Periphery 2.2 microns

Centre 1 microns
Surface area 120 sq microns
Volume 85 to 90 cubic micron
Normal shape
Normally RBC are disc shaped and biconcave
Properties of RBC
Rouleaux formation: When blood is taken out , the RBC pile up one above another like
the pile of coins known as rouleaux formation
Specific gravity:
The specific gravity of RBC is 1.092 to 1.101
Suspension stability:
During circulation, the red blood cells remain suspended uniformly in the blood. This
property of the RBC is called the suspension stability
Life span and fate of RBC:
Average life span of RBC is about 120 days. The senile RBC are destroyed in reticulo
endothelial system
Determination of RBC life span: The life span of the RBC is determined by radioisotope
method.
Function of the RBC
Erythrocytes transport oxygen from the lungs to the tissues
RBC transport carbon dioxide from the tissue to lungs
Hemoglobin in RBC also functions as a good buffer
RBC carries the blood group antigens like A agglutinogen, B agglutinogen and Rh factor
which helps in determination of blood group
WHITE BLOOD CELLS
White Blood Cells or leukocyte are the colorless and nucleated formed element of blood.
Depending upon the absence or presence of granules in the cytoplasm they are
classified into two types
Granulocytes (with granules) : neutrohils, eosinophils and basophils
Agranulocytes (without granules) : monocytes and lymphocytes
Life span of WBC

Neutrophils 2 to 5 days
Eosinophils 7 to 12 days
Basophils 12 to 15
days
Monocytes 2 to 5 days
Lymphocytes ½ to 1 day
Neutrophils
Neutrophil or polymorphs have fine or small granules in the cytoplasm. The granules
take both acidic and basic stains. So the granules are violet in color. The nucleus is
multilobed. They play an important role in the defence of the body. The granule
contains antibody like substance called defensins, they act against bacteria and fungi
Eosinophils
They have larger granules, which stain bright red or orange with eosin. The nucleus is
bilobed. They play an important role in the defence of the body, especially acting against
the parasites. The major function includes detoxification, disintegration and removal of
foreign proteins
Basophils
They also have larger granules in the cytoplasm. The granules stain purple blue with
basic dyes like methylene blue. Nucleus is bilobed. They play an important role in
healing processes after inflammation and in acute hypersensitivity reactions (allergy).
Monocytes
They are the largest leukocytes with diameter of 14 to 18 microns. The cytoplasm is
clear without granules. The nucleus is round, oval, and horse shoe or kidney shape.
Along with neutrophils they constitute the first line of defence.
Lymphocytes
They have less amount of cytoplasm. The nucleus is oval or kidney shaped occupying
the whole of the cytoplasm. They play an important role in immunity functionally
lymphocytes are classified into T- lymphocytes and B- lymphocytes. T-lymphocytes are
responsible for the development of cellular immunity, and B-lymphocytes for humoral
immunity.

PLATELETS
Platelets or thrombocytes are small colorless, non-nucleated and moderately refractive
bodies. Normally they are spherical or rod shaped.
Diameter 2.5 microns
Volume 7.5 cubic microns
Normal count 2.5 lacs per cubic mm of
blood
Functions:
Role in blood clotting
Role in clot retraction
Role in prevention of blood loss (hemostasis)
Role in repair of ruptured blood vessels
Role in defence mechanism.
Formation of lymphocytes from pluripotent cells

The blood cells begin their lives in the bone marrow from a single type of cell called the
pluripotential hematopoietic stem cell, from which all the cells of the circulating blood are
eventually derived.
As these cells reproduce, a small portion of them remains exactly like the original
pluripotential cells and is retained in the bone marrow to maintain a supply of these,
although their numbers diminish with age.
Most of the reproduced cells, however, differentiate to form the other cell types shown to
the right in Figure. The intermediate stage cells are very much like the pluripotential
stem cells, even though they have already become committed to a particular line of cells
and are called committed stem cells.
The different committed stem cells, when grown in culture, will produce colonies of
specific types of blood cells. A committed stem cell that produces erythrocytes is called
a colony-forming unit–erythrocyte, and the abbreviation CFU-E is used to designate this
type of stem cell. Likewise, colony-forming units that form granulocytes and monocytes
have the designation
CFU-GM, and so forth. Growth and reproduction of the different stem cells are
controlled by multiple proteins called growth inducers. Four major growth inducers have
been described, each having different characteristics.
One of these, interleukin-3, promotes growth and reproduction of virtually all the
different types of committed stem cells, whereas the others induce growth of only
specific types of cells. The growth inducers promote growth but not differentiation of the
cells. This is the function of another set of proteins called differentiation inducers. Each
of these causes one type of committed stem cell to differentiate one or more steps
toward a final adult blood cell.
Formation of the growth inducers and differentiation inducers is itself controlled by
factors outside the bone marrow. For instance, in the case of erythrocytes (red blood
cells), exposure of the blood to low oxygen for a long time results in growth induction,
differentiation, and production of greatly increased numbers of erythrocytes
In the case of some of the white blood cells, infectious diseases cause growth,
differentiation, and eventual formation of specific types of white blood cells that are
needed to combat each infection.

EXTRACELLULAR MATRIX
(COLLAGEN, PROTEOGLYCANS, FIBRONECTIN, LAMINS).
COLLAGENS - TYPES, COMPOSITION, STRUCTURE AND SYNTHESIS
Introduction:
More than 20 types of collagen participate in the formation of the extracellular matrix in
various tissues.
Although they differ in certain structural features and tissue distribution, all collagens
are trimeric proteins made from three polypeptides called collagen α chains.
Types and composition:
Structure:

Characteristic features:
All three α chains can be identical (homo trimeric) or different (hetero trimeric).
A trimeric collagen molecule contains one or more three-stranded segments, each with
a similar triplehelical structure.
Each strand contributed by one of the α chains is twisted into a left-handed helix, and
three such strands from the three α chains wrap around each other to form a right-
handed triple helix.
The collagen triple helix can form because of an unusual abundance of three amino
acids: glycine, proline, and a modified form of proline called hydroxyproline.
They make up the characteristic repeating motif Gly-X-Y, where X and Y can be any
amino acid but are often proline and hydroxyproline and less often lysine and
hydroxylysine.
Glycine is essential because its small side chain, a hydrogen atom, is the only one that
can fit into the crowded center of the three stranded helix
Hydrogen bonds help holds the three chains together. Although the rigid peptidylproline
and peptidyl-hydroxyproline linkages are not compatible with formation of a classic
single-stranded α helix, they stabilize the distinctive three-stranded collagen helix. The
hydroxyl group in hydroxyproline helps hold its ring in a confirmation that stabilizes
the three stranded helix.
The unique properties of each type of collagen are due mainly to differences in

1. The number and lengths of the collagenous, triple-helical segments
2. The segments that flank or interrupt the triple-helical segments and that fold into other
kinds of three-dimensional structures; and
3. The covalent modification of α chains (e.g., hydroxylation, glycosylation, oxidation,
cross-linking).
For example, the chains in type IV collagen, which is unique to basal laminae, are
designated IVα chains. Mammals express six homologous IV α chains, which assemble
into a series of type IV collagens with distinct properties.
All subtypes of type IV collagen, however, form a 400-nm-long triple helix that is
interrupted about 24 times with non helical segments and flanked by large globular
domains at the C-termini of the chains and smaller globular domains at the N-termini.
The non helical regions introduce flexibility into the molecule. Through both lateral
associations and interactions entailing the globular N- and C-termini, type IV collagen
molecules assemble into a branching, irregular two-dimensional fibrous network that
forms the lattice on which the basal lamina is built
Post translational modification of collagen fibers
After transcription, mRNA is extensively processed and then translated in rough
endoplasmic reticulum.
The first step in intracellular processing of the polypeptide chain is the cleavage of
signal peptides.
Then the proline and lysine residues in Y-position are hydroxylated to 4-hydroxyproline
and hydroxylysine by prolyl 4-hydroxylase and lysyl hydroxylase.
A few X-position proline residues are hydroxylated to 3-hydroxyproline.
Galactose and/or glucose are added to some of the hydroxylysine by hydroxylysyl
galactosyl transferase and galactosyl hydroxylysyl glucosyltransferase (GGT).
Fibrillar collagens have C- and N-terminal propeptides, in which mannose-rich
oligosaccharides are added while processing.
Formation of the collagen trimer begins by association of the C-terminal propeptides.
Triple helix is formed from a C-terminal nucleus towards the N-terminus in a zipper-like
manner.
Formation of intra- and interchain disulfide bonds stabilizes the structure.

The processing and assembly of fibrillar and nonfibrillar collagens is principally the
same, although many nonfibrillar collagens contain N- and /or C-terminal domains that
are not removed and therefore not called propeptides.
Different collagens may have special features in their synthesis, e.g. chain association
and folding of type I and IV collagens may involve a collagen-specific stress protein, heat
shock protein 47 (Hsp47).
PROTEOGLYCANS
Proteoglycans are proteins that are heavily glycosylated.
The basic proteoglycan unit consists of a "core protein" with one or
more covalently attached glycosaminoglycan (GAG) chain(s). The point of attachment is
a Ser residue to which the glycosaminoglycan is joined through a tetrasaccharide bridge
(e.g. chondroitin sulfate-GlcA-Gal-Gal-Xyl-PROTEIN). The Ser residue is generally in the
sequence -Ser-Gly-X-Gly- (where X can be any amino acid residue, but Proline),
although not every protein with this sequence has an attached glycosaminoglycan. The

chains are long, linear carbohydrate polymers that are negatively charged under
physiological conditions, due to the occurrence of sulfate and uronic acid groups.
Proteoglycans occur in the connective tissue.
Functions
Proteoglycans are a major component of the animal extracellular matrix, the "filler"
substance existing between cells in an organism. Here they form large complexes, both
to other proteoglycans, to hyaluronan and to fibrous matrix proteins (such as collagen).
They are also involved in binding cations (such as sodium, potassium and calcium)
and water, and also regulating the movement of molecules through the matrix.
Evidence also shows they can affect the activity and stability of proteins and signalling
molecules within the matrix. Individual functions of proteoglycans can be attributed to
either the protein core or the attached GAG chain and serve as lubricants.
FIBRONECTINS
 Fibronectin is an extracellular protein that helps cells attach to the matrix.
 The extra cellular matrix contains a number of non collagen proteins that typically have
multiple domains, each with specific binding sites for other matrix macromolecule and
for receptors on the surface of cells.
 These proteins therefore contribute to both organizing the matrix and helping cells
attach to it.
Structure of fibronectin:

 Fibronectin is a dimer composed of two very large subunits joined by disulfide bonds at
one end.
 The two polypeptide chains are similar but generally not identical (being made from the
same gene but from differently spliced mRNAs).
 They are joined by two disulfide bonds near the C – termini.
 Each chain is almost 2500 amino acids long and is folded into five or six domains
connected by flexible polypeptide segments.
 Individual domains are specialized for binding to a particular molecule or to a cell, as
indicated for five of the domains.
 The three dimensional structure of two type 3 fibronectin repeats as determined by X –
ray crystallography.
 The type 3 repeat is the main repeating module in fibronectin.
 Both the Arg – Gly – Asp (RGD) and the “synergy” sequence form part of the major cell
binding site.
 All forms of fibronectin are encoded by a single large gene that contains about 50 exons
of similar size.
 Transcription produces a single large RNA molecule that can be alternatively spliced to
produce the various isoforms of fibronectin.
 The main type of module, called the type 3 fibronectin repeat, binds to integrins.
 It is about 90 amino acids long and occurs at least 15 times in each subunit.
 The type 3 fibronectin repeat is amount the most common of all protein domains in
vertebrates.
 Even very short peptides containing this RGD sequence can compete with fibronectin
for the binding site on cells, thereby inhibiting the attachment of the cells to a
fibronectin matrix.

 If these peptides are coupled to a solid surface, they cause cells to adhere to it.
 It is found in a number of extracellular proteins, including, for example, the blood
clotting factor fibrinogen.
 Fibrinogen peptides containing this RGD sequence have been useful in the development
of anti clotting drugs that mimic these peptides.
 Snakes use a similar strategy to cause their victims to bleed: they secrete RGD
containing anti clotting proteins called disintegrins into their venom.
 RGD sequences are recognized by several members of the integrin family of cell surface
matrix receptors.
Components:
Role of integrins in binding of fibronectin:-
 Each integrin, however, specifically recognizes its own small set of matrix molecules,
indicating that tight binding requires more than just the RGD sequence.
 There are multiple isoforms of fibronectin. Fibronectin exists in both soluble and
fibrillar forms .
 One, called plasma fibtonectin, is soluble and circulates in the blood and other body
fluids, where it is thought to enhance blood clotting, wound healing, and phagocytosis.
 All of the other forms assemble on the surface of cells and are deposited in the
extracellular matrix as highly insoluble fibronectin fibrils.
Role of actin and myosin in fibronectin polymerization:
 The contractile actin and myosin cytoskeleton thereby pulls on the fibronectin matrix to
generate tension.
 In addition, the stretching exposes more binding sites for integrins.
 In this way, the actin cytoskeleton promotes fibronectin polymerization and matrix
assembly.
 Extracellular signals can regulate the assembly process by altering the actin
cytoskeleton and thereby the tension on the fibrils.
 Glycoproteins in the matrix help guide cell migration.
 Fibronectin is important not only for cell adhesion to the matrix but also for guiding cell
migrations in vertebrate embryos.

LAMININS, AND ASSOCIATED PROTEINS AND THEIR FUNCTIONS
Introduction:
Extracellular matrix is scanty and consists mainly of a thin mat called the basal lamina,
which underlies the cellular sheet; most of the volume is occupied by cells. Here the
cells themselves, rather than the matrix, bear most of the mechanical stresses, by
means of strong intracellular protein filaments (components of the cytoskeleton) that
criss-cross the cytoplasm of each epithelial cell; to transmit mechanical stress from one
cell to the next, the filaments are directly or indirectly attached to transmembrane
proteins in the plasma membrane, where specialized junctions are formed between the
surfaces of adjacent cells and with the underlying basal lamina.
Site:
Basal laminae are flexible thin (40-120 nm thick) mats of specialized extracellular
matrix that underlie all epithelial cell sheets and tubes; they also surround individual
muscle cells, fat cells, and Schwann cells (which wrap around peripheral nerve cell
axons to form myelin).
Composition:
Its composition varies from tissue to tissue and even from region to region in the same
lamina; most mature basal laminae contain type IV collagen, the large heparan sulfate
proteoglycan perlecan, and the glycoproteins laminin and entactin.
Structure:
Laminin is one of the first extracellular matrix proteins synthesized in a developing
embryo, and early in development basal laminae contain little or no type IV collagen
and consist mainly of a laminin network. Laminin is a large (~850,000 daltons) flexible
complex of three very long polypeptide chains arranged in the shape of an asymmetric
cross and held together by disulfide bonds.Like many other proteins in the extracellular
matrix, it consists of a number of functional domains: one binds to type IV collagen, one
to heparan sulfate, one to entactin, and two or more to laminin receptor proteins on the
surface of cells. Like type IV collagen, laminin molecules can self-assemble in vitro into
a feltlike sheet, largely through interactions between the ends of the laminin arms. A
single dumbbell-shaped entactin molecule binds tightly to each laminin molecule where
the short arms meet the long one; as entactin also binds to type IV collagen, it is

thought to act as an additional bridge between the type IV collagen and laminin
networks in basal laminae.
Components of basal lamina
Basal laminae are flexible, thin (40–120 nm thick) mats of specialized extracellular
matrix that underlie all epithelial cell sheets and tubes.
Precise composition varies from tissue to tissue and even from region to region in the
same lamina; most mature basal laminae contain type IV collagen, the large heparan
sulfate proteoglycan perlecan, and the glycoproteins laminin and nidogen (also called
entactin).cell survival, proliferation, or differentiation, and serve as specific highways for
cell migration.
Early in development, basal laminae contain little or no type IV collagen and consist
mainly of laminin molecules.
Laminin-1 (classical laminin) is a large, flexible protein composed of three very long
polypeptide chains (α, β, and γ) arranged in the shape of an asymmetric cross and held
together by disulfide bonds.

Laminin
It is a family of multi adhesive proteins that form a fibrous two-dimensional network
with type IV collagen and that also bind to integrins.
Laminin, the principal multi adhesive matrix protein in basal laminae, is a
heterotrimeric, cross-shaped protein with a total molecular weight of 820,000.
Globular LG domains at the C terminus of the laminin α subunit mediate Ca2+-
dependent binding to specific carbohydrates on certain cell-surface molecules such as
syndecan and dystroglycan.
LG domains are found in a wide variety of proteins and can mediate binding to steroids
and proteins as well as carbohydrates.
For example, LG domains in α chain of laminin can mediate binding to certain
integrins, including α6β4 integrin on epithelial cells.
Entactin (also called nidogen), a rod like molecule that cross-links type IV collagen and
laminin and helps incorporate other components into the ECM
Perlecan, a large multidomain proteoglycan that binds to and cross-links many ECM
components and cell-surface molecules
Functions:
Separates cells and cell sheets from the underlying or surrounding connective tissue.
In other locations, such as the kidney glomerulus and lung alveolus, a basal lamina lies
between two cell sheets and functions as a highly selective filter.
Basal laminae serve more than simple structural and filtering roles.

They are able to determine cell polarity, influence cell metabolism, organize the proteins
in adjacent plasma membranes, induce cell differentiation, and serve as specific
highways for cell migration.
In the kidney glomerulus an unusually thick basal lamina acts as a molecular filter,
preventing the passage of macromolecules from the blood into the urine as urine is
formed.The heparan sulfate proteoglycan seems to be important for this function: when
the GAG chains are removed by specific enzymes, the filtering properties of the lamina
are destroyed.
The basal lamina can also act as a selective barrier to the movement of cells. The
lamina beneath an epithelium, for example, usually prevents fibroblasts in the
underlying connective tissue from making contact with the epithelial cells. It does not,
however, stop macrophages, lymphocytes, or nerve processes from passing through it.
The basal lamina plays an important part in tissue regeneration after injury. When
tissues such as muscles, nerves, and epithelia are damaged, the basal lamina survives
and provides scaffolding along which regenerating cells can migrate. In this way the
original tissue architecture is readily reconstructed. In some cases, as in the skin or
cornea, the basal lamina becomes chemically altered following injury - for example, by
the addition of fibronectin, which promotes the cell migration required for wound repair.
The junctional basal lamina apparently coordinates the local spatial organization of the
components in each of the two cells that form a neuromuscular junction.
Basal laminae also play a sophisticated part in guiding cell migrations during
embryonic development.

UNIT – 2: CYTOSKELETON
Nature of cytoskeleton, Actin filaments, actin binding proteins, Intermediate
filaments, Microtubules, MAP’s Structure and functions of cilia and flagella.
NATURE OF CYTOSKELETON
The distinctive shape of a cell depends on the organization of actin filaments and
proteins that connect microfilaments to the membrane. These proteins, called as spot
welds that tack the actin cytoskeleton framework to the overlying membrane.
When attached to a bundle of filaments, the membrane acquires the fingerlike shape of
a microvillus or similar projection. When attached to red blood cell membrane.
The simplest membrane–cytoskeleton connections entail the binding of integral
membrane proteins directly to actin filaments. More common are complex linkages
those membrane proteins through peripheral membrane proteins that function as
adapter proteins.
The richest area of actin filaments in many cells lies in the plasma membrane. In this
region, most actin filaments are arranged in a network that excludes most organelles
from the cortical cytoplasm.
Perhaps the simplest cytoskeleton is the two erythrocyte plasma membrane. In more
complicated cortical cytoskeletons, such as those in platelets, epithelial cells, and
muscle, actin filaments are part of a three and anchors the cell to the substratum.

A red blood cell must squeeze through narrow blood capillaries without rupturing its
membrane. The strength and flexibility of the erythrocyte plasma membrane depend on
a dense cytoskeletal network that underlies the entire membrane and is attached
The primary component of the erythrocyte cytoskeleton is entire cytoskeleton is
arranged in a spoke each spoke is composed of a single spectrin molecule, which
extends from two hubs and cross – links them. Each hub comprises a short (14
tropomodulin. The last two proteins strengthen the network by preventing the actin
filament from depolymerizing.
Six or seven spokes radiate from each hub, suggesting that six or seven spectrin
molecules are bound to the same actin filament.
To ensure that the erythrocyte retains its characteristic shape, the spectrin-actin
cytoskeleton is firmly attached to the overlying erythrocyte plasma membrane by two
peripheral membrane proteins, each of which binds to a specific integral membrane
protein and to membrane phospholipids.
Ankyrin connects the center of spectrin to band 3 protein, an anion-transport protein in
the membrane.
Band 4.1 protein, a component of the hub, binds to the integral membrane protein
glycophorin, whose structure was discussed previously.
Both ankyrin and band 4.1 protein also contain lipid-binding motifs, which help bind
them to the membrane. The dual binding by ankyrin and band 4.1 ensures that the
membrane is connected to both the spokes and the hubs of the spectrin-actin
cytoskeleton.
ACTIN FILAMENTS
Structure and location:
Microfilaments are long tubular organelles present throughout the cell.
The microfilament of the ectoplasm contain only actin and endoplasm contain both
actin and myosin.
Each microfilament is made up of many tiny molecules of a protein called actin, which
is manufactured inside the cell in the cytoplasm.
Individual actin molecules are rounded and they are stranded together into long chains.
Two long chains twist around each other to form an elongated spiral known as a helix,
and this creates one microfilament with a diameter of around five nanometers.

ACTIN BINDING PROTEINS
Types:
 Actin occurs in two forms—a mono molecular form(G actin, globular actin) and a
polymer (F actin, filamentous actin).
 G actin is an asymmetrical molecule with a mass of 42 kDa, consisting of two domains.
 As the ionic strength increases, G actin aggregates reversibly to form F actin, a helical
homopolymer.
 G actin carries a firmly bound ATP molecule that is slowly hydrolyzed in F actin to form
ADP.
 Actin therefore also has enzyme properties (ATPase activity).
 Individual G actin molecules are always oriented in the same direction relative to one
another
 F actin consequently has polarity. It has two different ends, at which polymerization
takes place at different rates.
 If the ends are not stabilized by special proteins (as in muscle cells), then at a critical
concentration of G actin the (+) end of F actin will constantly grow, while the (–) end
simultaneously decays.
 These partial processes can be blocked by fungal toxins experimentally. Phalloidin, a
toxin contained in the Amanita phalloides mushroom, inhibits decay by binding to the (–
) end.

 By contrast, cytochalasins, mold toxins with cytostatic effects, block polymerization by
binding to the (+) end.
Actin binding proteins or ABP are the protein which bind to actin. It has ability to bind
actin monomers, or polymers, or both. Many actin binding protein including α-actinin,
distriphin, eutrophin, and fimbrin, do this through the actin-binding calponin,
homology domain.
Role of ABP:
The modular structure of four actin-binding proteins, each of the proteins shown has
two actin-binding sites (red) that are related in sequence. Fimbrin has two directly
adjacent actin-binding sites, so that it holds its two actin filaments very close together
(14 nm apart), aligned with the same polarity . The two actin-binding sites in _-actinin
are more widely separated and are linked by a somewhat flexible spacer 30 nm long, so
that it forms actin filament bundles with a greater separation between the filaments (40
nm apart) than does fimbrin. Filamin has two actin-binding sites that are very widely
spaced, with a Vshaped linkage between them, so that it cross-links actin filaments into
a network with the filaments oriented almost at right angles to one another.

Spectrin:
Spectrin is a tetramer f two a and two b subunits, and the tetramer has two actin-
binding sites spaced about 200 nm apart. The spacer regions of these various proteins
are built in a modular fashion from repeating units that include a- helical motifs (light
green), b-sheet motifs ( dark green), and Ca2+- binding domains ( blue ovals).
Function:
 It forms a band just beneath the cell membrane that Provides mechanical strength to
the cell
 Links transmembrane proteins (e.g., cell surface receptors) to cytoplasmic proteins
 Anchors the centrosomes at opposite poles of the cell during mitosis
 Pinches dividing animal cells apart during cytokinesis
 Generate cytoplasmic streaming in some cells;
 Generate locomotion in cells such as some leukocytes (white blood cells) and the
amoeba
 Interact with myosin ("thick") filaments in skeletal muscle fibers to provide the force of
muscular contraction.
INTERMEDIATE FILAMENTS
 Intermediate filaments are found in nearly all animals but not in plants and fungi.
 The association of intermediate filaments with the nuclear and plasma membranes
suggests that their principal function is structural
 In epithelium, for instance, intermediate filaments provide mechanical support for the
plasma membrane where it comes into contact with other cells or with the extracellular
matrix.
 In epidermal cells (outer layer of skin) and the axons of neurons, intermediate filaments
are at least 10 times as abundant as microfilaments or microtubules, the other
components of the cytoskeleton.
 Several physical and biochemical properties distinguish intermediate filaments from
microfilaments and microtubules.
 Intermediate filaments are extremely stable. Even after extraction with solutions
containing detergents and high concentrations of salts, most intermediate filaments in a

cell remain intact, whereas microfilaments and microtubules depolymerize into their
soluble subunits.
 The assembly of intermediate filaments probably proceeds through several intermediate
structures, which associate by lateral and end-to-end interactions.
 The organization of intermediate filaments into networks and bundles, mediated by
various IFAPs, provides structural stability to cells. IFAPs also cross-link intermediate
filaments to the plasma and nuclear membranes, microtubules, and microfilaments.
 Unlike microtubules and microfilaments, intermediate filaments are assembled from a
large number of different IF proteins. These proteins are divided into four major types
based on their sequences and tissue distribution. The lamins are expressed in all cells,
whereas the other types are expressed in specific tissues.
There are five different types of Intermediate filaments:
Types I and II(keratins): Acidic Keratin and Basic Keratin, respectively. Produced by
different types of epithelial cells (bladder, skin, etc).

Type III(vimentin like proteins): Intermediate filaments are distributed in a number of
cell types, including: Vimentin in fibroblasts, endothelial cells and leukocytes; desmin
in muscle; glial fibrillary acidic factor in astrocytes and other types of glia, and
peripherin in peripheral nerve fibers.
Type IV (neuronal intermediate filaments): Neurofilament H (heavy), M (medium) and L
(low).
Modifiers refer to the molecular weight of the NF proteins. Another type IV is
"internexin" and some nonstandard IV's are found in lens fibers of the eye (filensin and
phakinin).
Type V(nuclear lamins): these are the lamins which have a nuclear signal sequence so
they can form a filamentous support inside the inner nuclear membrane. Lamins are
vital to the re-formation of the nuclear envelope after cell division.
MICROTUBULES AND MAPS ASSEMBLY AND REGULATION
Characteristics:
 Straight, hollow cylinders are found throughout the cytoplasm of all eukaryotic cells
(prokaryotes don't have them) and carry out a variety of functions, ranging from
transport to structural support.
 25 nanometers in diameter, form part of the cytoskeleton that gives structure and
shape to a cell, and also serve as conveyor belts moving other organelles throughout the
cytoplasm.
 Are the major components of cilia and flagella, and participate in the formation of
spindle fibers during cell division (mitosis).
 The length of microtubules in the cell varies between 200 nanometers and 25
micrometers, depending upon the task of a particular microtubule and the state of the
cell's life cycle.

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