Chapter

Cell
1
„ INTRODUCTION
„ STRUCTURE OF THE CELL
„ CELL MEMBRANE
„ CYTOPLASM
„ ORGANELLES IN CYTOPLASM
„ ORGANELLES WITH LIMITING MEMBRANE
„ ORGANELLES WITHOUT LIMITING MEMBRANE
„ NUCLEUS
„ DEOXYRIBONUCLEIC ACID
„ GENE
„ RIBONUCLEIC ACID
„ GENE EXPRESSION
„ GROWTH FACTORS
„ CELL DEATH
„ CELL ADAPTATION
„ CELL DEGENERATION
„ CELL AGING
„ STEM CELLS

„ INTRODUCTION 5. Shows immediate response to the entry of invaders

like bacteria or toxic substances into the body
„ CELL 6. Reproduces by division. There are some exceptions
like neuron, which do not reproduce.
All the living things are composed of cells. A single cell
is the smallest unit that has all the characteristics of life.
„ TISSUE
Cell is defined as the structural and functional unit of the
living body. Tissue is defined as the group of cells having similar
function. There are many types of tissues in the body. All
General Characteristics of Cell the tissues are classified into four major types which are
called the primary tissues. The primary tissues include:
Each cell in the body: 1. Muscle tissue (skeletal muscle, smooth muscle and
1. Needs nutrition and oxygen cardiac muscle)
2. Produces its own energy necessary for its growth, 2. Nervous tissue (neurons and supporting cells)
repair and other activities 3. Epithelial tissue (squamous, columnar and cuboidal
3. Eliminates carbon dioxide and other metabolic wastes epithelial cells)
4. Maintains the medium, i.e. the environment for its 4. Connective tissue (connective tissue proper, cartil-
survival age, bone and blood).
 4 Section 1 t General Physiology

„ ORGAN „ CELL MEMBRANE

An organ is defined as the structure that is formed by Cell membrane is a protective sheath, enveloping the
two or more primary types of tissues, which execute the cell body. It is also known as plasma membrane or
functions of the organ. Some organs are composed of all plasmalemma. This membrane separates the fluid out­
the four types of primary tissues. The organs are of two side the cell called extracellular fluid (ECF) and the fluid
types, namely tubular or hollow organs and compact or inside the cell called intracellular fluid (ICF). The cell
parenchymal organs. Some of the organs in the body are membrane is a semipermeable membrane. So, there is
brain, heart, lungs, stomach, intestine, liver, gallbladder, free exchange of certain substances between ECF and
pancreas, kidneys, endocrine glands, etc. ICF. Thickness of the cell membrane varies from 75 to
111Å (Fig. 1.2).
„ SYSTEM
„ COMPOSITION OF CELL MEMBRANE
The organ system is defined as group of organs that work
together to carry out specific functions of the body. Cell membrane is composed of three types of substances:
Each system performs a specific function. Digestive 1. Proteins (55%)
system is concerned with digestion of food particles. 2. Lipids (40%)
Excretory system eliminates unwanted substances. 3. Carbohydrates (5%).
Cardiovascular system is responsible for transport of
substances between the organs. Respiratory system „ STRUCTURE OF CELL MEMBRANE
is concerned with the supply of oxygen and removal of
carbon dioxide. Reproductive system is involved in the On the basis of structure, cell membrane is called a unit
reproduction of species. Endocrine system is concerned membrane or a three-layered membrane. The electron
with growth of the body and regulation and maintenance microscopic study reveals three layers of cell membrane,
of normal life. Musculoskeletal system is responsible for namely, one central electron-lucent layer and two elec-
stability and movements of the body. Nervous system tron-dense layers. The two electron-dense layers are
controls the locomotion and other activities including the placed one on either side of the central layer. The central
intellectual functions. layer is a lipid layer formed by lipid substances. The
other two layers are protein layers formed by proteins.
„ STRUCTURE OF THE CELL Cell membrane contains some carbohydrate molecules
Each cell is formed by a cell body and a membrane also.
covering the cell body called the cell membrane. Cell
body has two parts, namely nucleus and cytoplasm Structural Model of the Cell Membrane
surrounding the nucleus (Fig. 1.1). Thus, the structure 1. Danielli-Davson model
of the cell is studied under three headings:
1. Cell membrane ‘Danielli­Davson model’ was the first proposed basic
2. Cytoplasm model of membrane structure. It was proposed by
3. Nucleus. James F Danielli and Hugh Davson in 1935. And it was
accepted by scientists for many years. This model was
basically a ‘sandwich of lipids’ covered by proteins on
both sides.

FIGURE 1.1: Structure of the cell FIGURE 1.2: Diagram of the cell membrane
 Chapter 1 t Cell 5

2. Unit membrane model oily structures and cholesterol helps to ‘pack’ the
phospholipids in the membrane. So, cholesterol is
In 1957, JD Robertson replaced ‘Danielli­Davson model’
responsible for the structural integrity of lipid layer of the
by ‘Unit membrane model’ on the basis of electron
cell membrane.
microscopic studies.
3. Fluid mosaic model Functions of Lipid Layer in Cell Membrane
Later in 1972, SJ Singer and GL Nicholson proposed ‘The Lipid layer of the cell membrane is a semipermeable
fluid mosaic model’. According to them, the membrane membrane and allows only the fat-soluble substances
is a fluid with mosaic of proteins (mosaic means pattern to pass through it. Thus, the fat-soluble substances like
formed by arrangement of different colored pieces of oxygen, carbon dioxide and alcohol can pass through
stone, tile, glass or other such materials). This model this lipid layer. The water-soluble substances such as
is accepted by the scientists till now. In this model, the glucose, urea and electrolytes cannot pass through this
proteins are found to float in the lipid layer instead of layer.
forming the layers of the sandwich-type model.
Protein Layers of the Cell Membrane
Lipid Layers of the Cell Membrane
Protein layers of the cell membrane are electron-dense
The central lipid layer is a bilayered structure. This is layers. These layers cover the two surfaces of the
formed by a thin film of lipids. The characteristic feature central lipid layer. Protein layers give protection to the
of lipid layer is that, it is fluid in nature and not a solid central lipid layer. The protein substances present in
structure. So, the portions of the membrane move from these layers are mostly glycoproteins.
one point to another point along the surface of the cell. Protein molecules are classified into two categories:
The materials dissolved in lipid layer also move to all 1. Integral proteins or transmembrane proteins.
areas of the cell membrane. 2. Peripheral proteins or peripheral membrane
Major lipids are: proteins.
1. Phospholipids
2. Cholesterol. 1. Integral proteins
1. Phospholipids Integral or transmembrane proteins are the proteins that
pass through entire thickness of cell membrane from one
Phospholipids are the lipid substances containing phos- side to the other side. These proteins are tightly bound
phorus and fatty acids. Aminophospholipids, sphingo- with the cell membrane.
myelins, phosphatidylcholine, phosphatidyletholamine, Examples of integral protein:
phosphatidylglycerol, phosphatidylserine and phos- i. Cell adhesion proteins
phatidylinositol are the phospholipids present in lipid
ii. Cell junction proteins
layer of cell membrane.
iii. Some carrier (transport) proteins
Phospholipid molecules are arranged in two layers
(Fig. 1.3). Each phospholipid molecule resembles the iv. Channel proteins
headed pin in shape. The outer part of the phospholipid v. Some hormone receptors
molecule is called the head portion and the inner portion vi. Antigens
is called the tail portion. vii. Some enzymes.
Head portion is the polar end and it is soluble in
water and has strong affinity for water (hydrophilic). Tail
portion is the non-polar end. It is insoluble in water and
repelled by water (hydrophobic).
Two layers of phospholipids are arranged in such a
way that the hydrophobic tail portions meet in the center
of the membrane. Hydrophilic head portions of outer
layer face the ECF and those of the inner layer face ICF
(cytoplasm).
2. Cholesterol
Cholesterol molecules are arranged in between the
phospholipid molecules. Phospholipids are soft and FIGURE 1.3: Lipids of the cell membrane
 6 Section 1 t General Physiology

2. Peripheral proteins 3. Some carbohydrate molecules function as the

receptors for some hormones.
Peripheral proteins or peripheral membrane proteins
are the proteins which are partially embedded in the
„ FUNCTIONS OF CELL MEMBRANE
outer and inner surfaces of the cell membrane and do
not penetrate the cell membrane. Peripheral proteins 1. Protective function: Cell membrane protects the
are loosely bound with integral proteins or lipid layer of cytoplasm and the organelles present in the cyto-
cell membrane. So, these protein molecules dissociate plasm
readily from the cell membrane. 2. Selective permeability: Cell membrane acts as a
Examples of peripheral proteins: semipermeable membrane, which allows only some
i. Proteins of cytoskeleton substances to pass through it and acts as a barrier
ii. Some carrier (transport) proteins for other substances
iii. Some enzymes. 3. Absorptive function: Nutrients are absorbed into the
cell through the cell membrane
Functions of Proteins in Cell Membrane 4. Excretory function: Metabolites and other waste
products from the cell are excreted out through the
1. Integral proteins provide the structural integrity of cell membrane
the cell membrane 5. Exchange of gases: Oxygen enters the cell from the
2. Channel proteins help in the diffusion of water- blood and carbon dioxide leaves the cell and enters
soluble substances like glucose and electrolytes the blood through the cell membrane
3. Carrier or transport proteins help in the transport of 6. Maintenance of shape and size of the cell: Cell mem-
substances across the cell membrane by means of brane is responsible for the maintenance of shape
active or passive transport and size of the cell.
4. Pump: Some carrier proteins act as pumps, by
which ions are transported actively across the cell „ CYTOPLASM
membrane
Cytoplasm of the cell is the jelly­like material formed by
5. Receptor proteins serve as the receptor sites for
80% of water. It contains a clear liquid portion called
hormones and neurotransmitters
cytosol and various particles of different shape and
6. Enzymes: Some of the protein molecules form the
size. These particles are proteins, carbohydrates, lipids
enzymes and control chemical (metabolic) reactions
or electrolytes in nature. Cytoplasm also contains many
within the cell membrane
organelles with distinct structure and function.
7. Antigens: Some proteins act as antigens and induce
Cytoplasm is made up of two zones:
the process of antibody formation
1. Ectoplasm: Peripheral part of cytoplasm, situated
8. Cell adhesion molecules or the integral proteins are just beneath the cell membrane
responsible for attachment of cells to their neighbors 2. Endoplasm: Inner part of cytoplasm, interposed
or to basal lamina. between the ectoplasm and the nucleus.
Carbohydrates of the Cell Membrane
„ ORGANELLES IN CYTOPLASM
Some of the carbohydrate molecules present in
Cytoplasmic organelles are the cellular structures
cell membrane are attached to proteins and form embedded in the cytoplasm. Organelles are considered
glycoproteins (proteoglycans). Some carbohydrate as small organs of the cell. Some organelles are bound
molecules are attached to lipids and form glycolipids. by limiting membrane and others do not have limiting
Carbohydrate molecules form a thin and loose membrane (Box 1.1). Each organelle is having a definite
covering over the entire surface of the cell membrane structure and specific functions (Table 1.1).
called glycocalyx.
„ ORGANELLES WITH LIMITING MEMBRANE
Functions of Carbohydrates in Cell Membrance
„ ENDOPLASMIC RETICULUM
1. Carbohydrate molecules are negatively charged and
do not permit the negatively charged substances to Endoplasmic reticulum is a network of tubular and
move in and out of the cell microsomal vesicular structures which are interconnect-
2. Glycocalyx from the neighboring cells helps in the ed with one another. It is covered by a limiting membrane
tight fixation of cells with one another which is formed by proteins and bilayered lipids. The lumen
 Chapter 1 t Cell 7

BOX 1.1: Cytoplasmic organelles between nucleus and cell membrane by connecting the
cell membrane with the nuclear membrane.
Organelles with limiting membrane
1. Endoplasmic reticulum Types of Endoplasmic Reticulum
2. Golgi apparatus
3. Lysosome Endoplasmic reticulum is of two types, namely rough
4. Peroxisome endoplasmic reticulum and smooth endoplasmic reti-
5. Centrosome and centrioles culum. Both the types are interconnected and continuous
6. Secretory vesicles with one another. Depending upon the activities of the
7. Mitochondria
cells, the rough endoplasmic reticulum changes to
8. Nucleus
smooth endoplasmic reticulum and vice versa.
Organelles without limiting membrane
1. Ribosomes Rough Endoplasmic Reticulum
2. Cytoskeleton
It is the endoplasmic reticulum with rough, bumpy or
of endoplasmic reticulum contains a fluid medium called bead-like appearance. Rough appearance is due to the
endoplasmic matrix. The diameter of the lumen is about attachment of granular ribosomes to its outer surface.
400 to 700Å. The endoplasmic reticulum forms the link Hence, it is also called the granular endoplasmic

TABLE 1.1: Functions of cytoplasmic organelles

Organelles Functions
Rough endoplasmic reticulum 1. Synthesis of proteins
2. Degradation of worn­out organelles
Smooth endoplasmic reticulum 1. Synthesis of lipids and steroids
2. Role in cellular metabolism
3. Storage and metabolism of calcium
4. Catabolism and detoxification of toxic substances
Golgi apparatus 1. Processing, packaging, labeling and delivery of proteins and lipids
Lysosomes 1. Degradation of macromolecules
2. Degradation of worn­out organelles
3. Removal of excess of secretory products
4. Secretion of perforin, granzymes, melanin and serotonin
Peroxisomes 1. Breakdown of excess fatty acids
2. Detoxification of hydrogen peroxide and other metabolic products
3. Oxygen utilization
4. Acceleration of gluconeogenesis
5. Degradation of purine to uric acid
6. Role in the formation of myelin
7. Role in the formation of bile acids
Centrosome 1. Movement of chromosomes during cell division
Mitochondria 1. Production of energy
2. Synthesis of ATP
3. Initiation of apoptosis
Ribosomes 1. Synthesis of proteins
Cytoskeleton 1. Determination of shape of the cell
2. Stability of cell shape
3. Cellular movements
Nucleus 1. Control of all activities of the cell
2. Synthesis of RNA
3. Sending genetic instruction to cytoplasm for protein synthesis
4. Formation of subunits of ribosomes
5. Control of cell division
6. Storage of hereditary information in genes (DNA)
 8 Section 1 t General Physiology

reticulum (Fig. 1.4). Rough endoplasmic reticulum is

vesicular or tubular in structure.

Functions of Rough Endoplasmic Reticulum

1. Synthesis of proteins
Rough endoplasmic reticulum is concerned with the
synthesis of proteins in the cell. It is involved with the
synthesis of mainly those proteins which are secreted
from the cells such as insulin from β­cells of islets
of Langerhans in pancreas and antibodies from B
lymphocytes.
Ribosomes arrange the amino acids into small
units of proteins and transport them into the rough
endoplasmic reticulum. Here, the carbohydrates are
added to the protein units forming the glycosylated
proteins or glycoproteins, which are arranged in the FIGURE 1.4: Endoplasmic reticulum
form of reticular vesicles. These vesicles are transported
mainly to Golgi apparatus for further modification and 3. Storage and metabolism of calcium
processing. Few vesicles are transported to other cyto- Smooth endoplasmic reticulum is the major site of
plasmic organelles. storage and metabolism of calcium. In skeletal muscle
2. Degradation of worn-out organelles fibers, it releases calcium which is necessary to trigger
the muscle contraction.
Rough endoplasmic reticulum also plays an important
role in the degradation of worn-out cytoplasmic orga- 4. Catabolism and detoxification
nelles like mitochondria. It wraps itself around the worn- Smooth endoplasmic reticulum is also concerned
out organelles and forms a vacuole which is often called with catabolism and detoxification of toxic substances
the autophagosome. Autophagosome is digested by like some drugs and carcinogens (cancer-producing
lysosomal enzymes (see below for details). substances) in the liver.

Smooth Endoplasmic Reticulum „ GOLGI APPARATUS

It is the endoplasmic reticulum with smooth appearance. Golgi apparatus or Golgi body or Golgi complex is a
It is also called agranular reticulum. It is formed by many membrane-bound organelle, involved in the processing
interconnected tubules. So, it is also called tubular of proteins. It is present in all the cells except red blood
endoplasmic reticulum. cells. It is named after the discoverer Camillo Golgi.
Usually, each cell has one Golgi apparatus. Some of the
Functions of Smooth Endoplasmic Reticulum cells may have more than one Golgi apparatus. Each
Golgi apparatus consists of 5 to 8 flattened membranous
1. Synthesis of non-protein substance sacs called the cisternae.
Smooth endoplasmic reticulum is responsible for syn- Golgi apparatus is situated near the nucleus. It has
thesis of non-protein substances such as cholesterol two ends or faces, namely cis face and trans face. The
and steroid. This type of endoplasmic reticulum is cis face is positioned near the endoplasmic reticulum.
abundant in cells that are involved in the synthesis of Reticular vesicles from endoplasmic reticulum enter
lipids, phospholipids, lipoprotein substances, steroid the Golgi apparatus through cis face. The trans face
hormones, sebum, etc. In most of the other cells, smooth is situated near the cell membrane. The processed
endoplasmic reticulum is less extensive than the rough substances make their exit from Golgi apparatus through
trans face (Fig. 1.5).
endoplasmic reticulum.
2. Role in cellular metabolism Functions of Golgi Apparatus
Outer surface of smooth endoplasmic reticulum contains Major functions of Golgi apparatus are processing,
many enzymes which are involved in various metabolic packing, labeling and delivery of proteins and other
processes of the cell. molecules like lipids to different parts of the cell.
 Chapter 1 t Cell 9

Types of Lysosomes
Lysosomes are of two types:
1. Primary lysosome, which is pinched off from Golgi
apparatus. It is inactive in spite of having hydrolytic
enzymes
2. Secondary lysosome, which is the active lysosome.
It is formed by the fusion of a primary lysosome with
phagosome or endosome (see below).

Functions of Lysosomes
Lysosomes are often called ‘garbage system’ of the cell
because of their degradation activity. About 50 different
hydrolytic enzymes, known as acid hydroxylases are
present in the lysosomes, through which lysosomes
execute their functions.
FIGURE 1.5: Golgi apparatus Important lysosomal enzymes
1. Proteases, which hydrolyze the proteins into amino
1. Processing of materials acids
Vesicles containing glycoproteins and lipids are 2. Lipases, which hydrolyze the lipids into fatty acids
transported into Golgi apparatus. Here, the glycoproteins and glycerides
and lipids are modified and processed. 3. Amylases, which hydrolyze the polysaccharides
into glucose
2. Packaging of materials
4. Nucleases, which hydrolyze the nucleic acids into
All the processed materials are packed in the form of mononucleotides.
secretory granules, secretory vesicles and lysosomes,
which are transported either out of the cell or to another Mechanism of lysosomal function
part of the cell. Because of this, Golgi apparatus is called Lysosomal functions involve two mechanisms:
the ‘post office of the cell’. 1. Heterophagy: Digestion of extracellular materials
3. Labeling and delivery of materials engulfed by the cell via endocytosis
2. Autophagy: Digestion of intracellular materials such
Finally, the Golgi apparatus sorts out the processed and
as worn-out cytoplasmic organelles.
packed materials and labels them (such as phosphate
group), depending upon the chemical content for delivery Specific functions of lysosomes
(distribution) to their proper destinations. Hence, the
Golgi apparatus is called ‘shipping department of the 1. Degradation of macromolecules
cell’. Macromolecules are engulfed by the cell by means of
endocytosis (phagocytosis, pinocytosis or receptor-
„ LYSOSOMES mediated endocytosis: Chapter 3). The macromolecules
Lysosomes are the membrane-bound vesicular such as bacteria, engulfed by the cell via phagocytosis
organelles found throughout the cytoplasm. The lyso- are called phagosomes or vacuoles. The other
somes are formed by Golgi apparatus. The enzymes macromolecules taken inside via pinocytosis or
synthesized in rough endoplasmic reticulum are receptor-mediated endocytosis are called endosomes.
processed and packed in the form of small vesicles in The primary lysosome fuses with the phagosome or
the Golgi apparatus. Then, these vesicles are pinched endosome to form the secondary lysosome. The pH in the
off from Golgi apparatus and become the lysosomes. secondary lysosome becomes acidic and the lysosomal
Among the organelles of the cytoplasm, the enzymes are activated. The bacteria and the other
lysosomes have the thickest covering membrane. The macromolecules are digested and degraded by these
membrane is formed by a bilayered lipid material. It has enzymes. The secondary lysosome containing these
many small granules which contain hydrolytic enzymes. degraded waste products moves through cytoplasm and
 10 Section 1 t General Physiology

fuses with cell membrane. Now the waste products are ii. Degrade the toxic substances such as hydrogen
eliminated by exocytosis. peroxide and other metabolic products by means
of detoxification. A large number of peroxisomes
2. Degradation of worn-out organelles
are present in the cells of liver, which is the major
The rough endoplasmic reticulum wraps itself around organ for detoxification. Hydrogen peroxide is
the worn-out organelles like mitochondria and form formed from poisons or alcohol, which enter the
the vacuoles called autophagosomes. One primary cell. Whenever hydrogen peroxide is produced
lysosome fuses with one autophagosome to form the in the cell, the peroxisomes are ruptured and
secondary lysosome. The enzymes in the secondary the oxidative enzymes are released. These
lysosome are activated. Now, these enzymes digest the oxidases destroy hydrogen peroxide and the
contents of autophagosome. enzymes which are necessary for the production
3. Removal of excess secretory products in the cells of hydrogen peroxide
iii. Form the major site of oxygen utilization in the
Lysosomes in the cells of the secretory glands remove
cells
the excess secretory products by degrading the secretory
granules. iv. Accelerate gluconeogenesis from fats
v. Degrade purine to uric acid
4. Secretory function – secretory lysosomes vi. Participate in the formation of myelin
Recently, lysosomes having secretory function viii. Play a role in the formation of bile acids.
called secretory lysosomes are found in some of the
cells, particularly in the cells of immune system. The „ CENTROSOME AND CENTRIOLES
conventional lysosomes are modified into secretory
lysosomes by combining with secretory granules (which Centrosome is the membrane-bound cellular organelle
contain the particular secretory product of the cell). situated almost in the center of cell, close to nucleus.
Examples of secretory lysosomes: It consists of two cylindrical structures called centrioles
i. Lysosomes in the cytotoxic T lymphocytes and which are made up of proteins. Centrioles are responsible
natural killer (NK) cells secrete perforin and for the movement of chromosomes during cell division.
granzymes, which destroy both viral-infected
cells and tumor cells. Perforin is a pore-forming „ SECRETORY VESICLES
protein that initiates cell death. Granzymes belong Secretory vesicles are the organelles with limiting
to the family of serine proteases (enzymes that membrane and contain the secretory substances. These
dislodge the peptide bonds of the proteins) and vesicles are formed in the endoplasmic reticulum and
cause the cell death by apoptosis are processed and packed in Golgi apparatus. Secretory
ii. Secretory lysosomes of melanocytes secrete vesicles are present throughout the cytoplasm. When
melanin necessary, these vesicles are ruptured and secretory
iii. Secretory lysosomes of mast cells secrete substances are released into the cytoplasm.
serotonin, which is a vasoconstrictor substance
and inflammatory mediator. „ MITOCHONDRION

„ PEROXISOMES Mitochondrion (plural = mitochondria) is a membrane-

bound cytoplasmic organelle concerned with production
Peroxisomes or microbodies are the membrane of energy. It is a rod-shaped or oval-shaped structure
limited vesicles like the lysosomes. Unlike lysosomes, with a diameter of 0.5 to 1 μ. It is covered by a bilayered
peroxisomes are pinched off from endoplasmic reticulum membrane (Fig. 1.6). The outer membrane is smooth and
and not from the Golgi apparatus. Peroxisomes contain encloses the contents of mitochondrion. This membrane
some oxidative enzymes such as catalase, urate oxidase contains various enzymes such as acetyl-CoA synthetase
and D­amino acid oxidase. and glycerolphosphate acetyltransferase.
The inner membrane is folded in the form of shelf-like
Functions of Peroxisomes inward projections called cristae and it covers the inner
Peroxisomes: matrix space. Cristae contain many enzymes and other
i. Breakdown the fatty acids by means of a process protein molecules which are involved in respiration and
called beta­oxidation: This is the major function synthesis of adenosine triphosphate (ATP). Because of
of peroxisomes these functions, the enzymes and other protein molecules
 Chapter 1 t Cell 11

4. Other functions
Other functions of mitochondria include storage of
calcium and detoxification of ammonia in liver.

„ ORGANELLES WITHOUT
LIMITING MEMBRANE
FIGURE 1.6: Structure of mitochondrion „ RIBOSOMES
Ribosomes are the organelles without limiting mem-
in cristae are collectively known as respiratory chain or
brane. These organelles are granular and small dot-like
electron transport system.
structures with a diameter of 15 nm. Ribosomes are
Enzymes and other proteins of respiratory chain made up of 35% of proteins and 65% of ribonucleic acid
i. Succinic dehydrogenase (RNA). RNA present in ribosomes is called ribosomal
ii. Dihydronicotinamide adenine dinucleotide (NADH) RNA (rRNA). Ribosomes are concerned with protein
dehydrogenase synthesis in the cell.
iii. Cytochrome oxidase
Types of Ribosomes
iv. Cytochrome C
v. ATP synthase. Ribosomes are of two types:
Inner cavity of mitochondrion is filled with matrix which i. Ribosomes that are attached to rough endo-
contains many enzymes. Mitochondrion moves freely in plasmic reticulum
the cytoplasm of the cell. It is capable of reproducing ii. Free ribosomes that are distributed in the cyto-
itself. Mitochondrion contains its own deoxyribonucleic plasm.
acid (DNA), which is responsible for many enzymatic
actions. In fact, mitochondrion is the only organelle other
Functions of Ribosomes
than nucleus, which has its own DNA.
Ribosomes are called ‘protein factories’ because of
Functions of Mitochondrion their role in the synthesis of proteins. Messenger RNA
1. Production of energy (mRNA) carries the genetic code for protein synthesis
from nucleus to the ribosomes. The ribosomes, in turn
Mitochondrion is called the ‘power house’ or ‘power arrange the amino acids into small units of proteins.
plant’ of the cell because it produces the energy required Ribosomes attached to rough endoplasmic reticulum
for cellular functions. The energy is produced during the are involved in the synthesis of proteins such as the
oxidation of digested food particles like proteins, carbo-
enzymatic proteins, hormonal proteins, lysosomal pro-
hydrates and lipids by the oxidative enzymes in cristae.
teins and the proteins of the cell membrane.
During the oxidative process, water and carbon dioxide
Free ribosomes are responsible for the synthesis of
are produced with release of energy. The released ener-
gy is stored in mitochondria and used later for synthesis proteins in hemoglobin, peroxisome and mitochondria.
of ATP.
„ CYTOSKELETON
2. Synthesis of ATP
Cytoskeleton is the cellular organelle present throughout
The components of respiratory chain in mitochondrion the cytoplasm. It determines the shape of the cell and gives
are responsible for the synthesis of ATP by utilizing the support to the cell. It is a complex network of structures
energy by oxidative phosphorylation. ATP molecules
with varying sizes. In addition to determining the shape of
diffuse throughout the cell from mitochondrion. Whenever
the cell, it is also essential for the cellular movements and
energy is needed for cellular activity, the ATP molecules
are broken down. the response of the cell to external stimuli.
Cytoskeleton consists of three major protein
3. Apoptosis components:
Cytochrome C and second mitochondria-derived activator 1. Microtubule
of caspases (SMAC)/diablo secreted in mitochondria are 2. Intermediate filaments
involved in apoptosis (see below). 3. Microfilaments.
 12 Section 1 t General Physiology

1. Microtubules Microfilaments are present throughout the cytoplasm.

The microfilaments present in ectoplasm contain
Microtubules are the straight, hollow and tubular
only actin molecules (Fig. 1.9) and those present in
structures of the cytoskeleton. These organelles without
the limiting membrane are arranged in different bundles. endoplasm contain both actin and myosin molecules.
Each tubule has a diameter of 20 to 30 nm. Length of Functions of microfilaments
microtubule varies and it may be 1000 times more than
the thickness. Microfilaments:
Structurally, the microtubules are formed by bundles i. Give structural strength to the cell
of globular protein called tubulin (Fig. 1.7). Tubulin has ii. Provide resistance to the cell against the pulling
two subunits, namely α­subunit and β­subunit. forces
iii. Are responsible for cellular movements like
Functions of microtubules
contraction, gliding and cytokinesis (partition of
Microtubules may function alone or join with other cytoplasm during cell division).
proteins to form more complex structures like cilia,
flagella or centrioles and perform various functions. „ NUCLEUS
Microtubules:
i. Determine the shape of the cell Nucleus is the most prominent and the largest cellular
ii. Give structural strength to the cell organelle. It has a diameter of 10 µ to 22 µ and occupies
iii. Act like conveyer belts which allow the movement about 10% of total volume of the cell.
of granules, vesicles, protein molecules and
some organelles like mitochondria to different
parts of the cell
iv. Form the spindle fibers which separate the
chromosomes during mitosis
v. Are responsible for the movement of centrioles
and the complex cellular structures like cilia.

2. Intermediate Filaments
Intermediate filaments are the structures that form a FIGURE 1.7: Microtubule
network around the nucleus and extend to the periphery
of the cell. Diameter of each filament is about 10 nm. The
intermediate filaments are formed by rope­like polymers,
which are made up of fibrous proteins (Fig. 1.8).
Subclasses of intermediate filaments
Intermediate filaments are divided into five subclasses:
i. Keratins (in epithelial cells)
ii. Glial filaments (in astrocytes)
iii. Neurofilaments (in nerve cells)
iv. Vimentin (in many types of cells)
v. Desmin (in muscle fibers). FIGURE 1.8: Intermediate filament
Functions of intermediate filaments
Intermediate filaments help to maintain the shape of the
cell. These filaments also connect the adjacent cells
through desmosomes.

3. Microfilaments
Microfilaments are long and fine thread­like structures
with a diameter of about 3 to 6 nm. These filaments are
made up of non-tubular contractile proteins called actin
and myosin. Actin is more abundant than myosin. FIGURE 1.9: Microfilament of ectoplasm
 Chapter 1 t Cell 13

Nucleus is present in all the cells in the body except packing unit of chromatin called nucleosome. Nucleo-
the red blood cells. The cells with nucleus are called somes are packed together tightly with the help of a
eukaryotes and those without nucleus are known as histone molecule to form a chromatin fiber.
prokaryotes. Presence of nucleus is necessary for cell Just before cell division, the chromatin condenses to
division. form chromosome.
Most of the cells have only one nucleus (uninucleated
cells). Few types of cells like skeletal muscle cells have Chromosomes
many nuclei (multinucleated cells). Generally, the
nucleus is located in the center of the cell. It is mostly Chromosome is the rod-shaped nuclear structure
spherical in shape. However, the shape and situation of that carries a complete blueprint of all the hereditary
nucleus vary in some cells. characteristics of that species. A chromosome is formed
from a single DNA molecule coiled around histone
„ STRUCTURE OF NUCLEUS molecules. Each DNA contains many genes.
Normally, the chromosomes are not visible in the
Nucleus is covered by a membrane called nuclear mem- nucleus under microscope. Only during cell division,
brane and contains many components. Major components the chromosomes are visible under microscope. This is
of nucleus are nucleoplasm, chromatin and nucleolus. because DNA becomes more tightly packed just before
cell division, which makes the chromosome visible
Nuclear Membrane during cell division.
Nuclear membrane is double layered and porous in All the dividing cells of the body except reproductive
nature. This allows the nucleoplasm to communicate with cells contain 23 pairs of chromosomes. Each pair consists
the cytoplasm. The outer layer of nuclear membrane is of one chromosome inherited from mother and one from
continuous with the membrane of endoplasmic reticulum. father. The cells with 23 pairs of chromosomes are called
The space between the two layers of nuclear membrane diploid cells. The reproductive cells called gametes or
is continuous with the lumen of endoplasmic reticulum. sex cells contain only 23 single chromosomes. These
Pores of the nuclear membrane are guarded (lined) cells are called haploid cells.
by protein molecules. Diameter of the pores is about
80 to 100 nm. However, it is decreased to about 7 to Nucleolus
9 nm because of the attachment of protein molecules
Nucleolus is a small, round granular structure of the
with the periphery of the pores. Exchange of materials
nucleus. Each nucleus contains one or more nucleoli.
between nucleoplasm and cytoplasm occurs through
The nucleolus contains RNA and some proteins, which
these pores.
are similar to those found in ribosomes. The RNA is
synthesized by five different pairs of chromosomes and
Nucleoplasm
stored in the nucleolus. Later, it is condensed to form
Nucleoplasm is a highly viscous fluid that forms the the subunits of ribosomes. All the subunits formed in
ground substance of the nucleus. It is similar to cytoplasm the nucleolus are transported to cytoplasm through the
present outside the nucleus. pores of nuclear membrane. In the cytoplasm, these
Nucleoplasm surrounds chromatin and nucleolus. subunits fuse to form ribosomes, which play an essential
It contains dense fibrillar network of proteins called the role in the formation of proteins.
nuclear matrix and many substances such as nucleotides
and enzymes. The nuclear matrix forms the structural „ FUNCTIONS OF NUCLEUS
framework for organizing chromatin. The soluble liquid
part of nucleoplasm is known as nuclear hyaloplasm. Major functions of nucleus are the control of cellular
activities and storage of hereditary material. Several
processes are involved in the nuclear functions.
Chromatin
Functions of nucleus:
Chromatin is a thread-like material made up of large 1. Control of all the cell activities that include metabolism,
molecules of DNA. The DNA molecules are compactly protein synthesis, growth and reproduction (cell
packed with the help of a specialized basic protein division)
called histone. So, chromatin is referred as DNA-histone 2. Synthesis of RNA
complex. It forms the major bulk of nuclear material. 3. Formation of subunits of ribosomes
DNA is a double helix which wraps around central 4. Sending genetic instruction to the cytoplasm for
core of eight histone molecules to form the fundamental protein synthesis through messenger RNA (mRNA)
 14 Section 1 t General Physiology

5. Control of the cell division through genes from amino acids. It is like a book that contains the
6. Storage of hereditary information (in genes) information necessary for protein synthesis. Gene is
and transformation of this information from one considered as the basic hereditary unit of the cell.
generation of the species to the next. In the nucleotide of DNA, three of the successive
base pairs are together called a triplet or a codon. Each
„ DEOXYRIBONUCLEIC ACID codon codes or forms code word (information) for one
amino acid. There are 20 amino acids and there is
Deoxyribonucleic acid (DNA) is a nucleic acid that carries
the genetic information to the offspring of an organism. separate code for each amino acid. For example, the
DNA forms the chemical basis of hereditary characters. triplet CCA is the code for glycine and GGC is the code
It contains the instruction for the synthesis of proteins in for proline.
the ribosomes. Gene is a part of a DNA molecule. Thus, each gene forms the code word for a particular
DNA is present in the nucleus (chromosome) protein to be synthesized in ribosome (outside the
and mitochondria of the cell. The DNA present in the nucleus) from amino acids.
nucleus is responsible for the formation of RNA. RNA
regulates the synthesis of proteins by ribosomes. DNA „ GENETIC DISORDERS
in mitochondria is called non-chromosomal DNA.
A genetic disorder is a disorder that occurs because
of the abnormalities in an individual’s genetic material
„ STRUCTURE OF DNA
(genome). Genetic disorders are either hereditary dis­
DNA is a double­stranded complex nucleic acid. It orders or due to defect in genes.
is formed by deoxyribose, phosphoric acid and four
types of bases. Each DNA molecule consists of two Causes of Gene Disorders
polynucleotide chains, which are twisted around one
Genetic disorders occur due to two causes:
another in the form of a double helix. The two chains are
1. Genetic variation: Presence of a different form of
formed by the sugar deoxyribose and phosphate. These
gene
two substances form the backbone of DNA molecule.
2. Genetic mutation: Generally, mutation means an
Both chains of DNA are connected with each other by
some organic bases (Fig. 1.10). alteration or a change in nature, form, or quality.
Each chain of DNA molecule consists of many Genetic mutation refers to change of the DNA
nucleotides. Each nucleotide is formed by: sequence within a gene or chromosome of an
1. Deoxyribose – sugar organism, which results in the creation of a new
2. Phosphate character.
3. One of the following organic (nitrogenous) bases:
Purines – Adenine (A) Classification of Genetic Disorders
– Guanine (G) Genetic disorders are classified into four types:
Pyrimidines – Thymine (T) 1. Single gene disorders
– Cytosine (C) 2. Multifactorial genetic disorders
The strands of DNA are arranged in such a way that 3. Chromosomal disorders
both are bound by specific pairs of bases. The adenine 4. Mitochondrial DNA disorders.
of one strand binds specifically with thymine of opposite
strand. Similarly, the cytosine of one strand binds with 1. Single Gene Disorders
guanine of the other strand.
Single gene disorders or Mendelian or monogenic
DNA forms the component of chromosomes, which
disorders occur because of variation or mutation in one
carries the hereditary information. The hereditary infor-
single gene. Examples include sickle cell anemia and
mation that is encoded in DNA is called genome. Each
Huntington’s disease.
DNA molecule is divided into discrete units called
genes. 2. Multifactorial Genetic Disorders
Multifactorial genetic disorders or polygenic disorders
„ GENE
are caused by combination of environmental factors and
Gene is a portion of DNA molecule that contains the mutations in multiple genes. Examples are coronary heart
message or code for the synthesis of a specific protein disease, Alzheimer’s disease, arthritis and diabetes.
 Chapter 1 t Cell 15

FIGURE 1.10: Structure of DNA. A. Double helical structure of DNA; B. Magnified view of the components of DNA.
A = Adenine, C = Cytocine, G= Guanine, P = Phosphate, S = Sugar, T = Thymine.

3. Chromosomal Disorders syndrome, which is characterized by physical

disabilities
Chromosomal disorder is a genetic disorder caused
b. Trisomy due to the presence of one extra
by abnormalities in chromosome. It is also called chromosome along with normal pair of chromo-
chromosomal abnormality, anomaly or aberration. It often somes in the cells. Example is Down syndrome,
results in genetic disorders which involve physical or which is characterized by physical disabilities
mental abnormalities. Chromosomal disorder is caused and mental retardation.
by numerical abnormality or structural abnormality.
Chromosomal disorder is classified into two types: 4. Mitochondrial DNA Disorders
i. Structural abnormality (alteration) of chromosomes Mitochondrial DNA disorders are the genetic disorders
which leads to disorders like chromosome instability caused by the mutations in the DNA of mitochondria
syndromes (group of inherited diseases which (non­chromosomal DNA). Examples are Kearns-Sayre
cause malignancies) syndrome (neuromuscular disorder characterized by
ii. Numerical abnormality of chromosomes which is of myopathy, cardiomyopathy and paralysis of ocular mus-
two types: cles) and Leber’s hereditary optic neuropathy (disease
a. Monosomy due to absence of one chromosome characterized by degeneration of retina and loss of
from normal diploid number. Example is Turner’s vision).
 16 Section 1 t General Physiology

„ RIBONUCLEIC ACID „ TRANSCRIPTION OF GENETIC CODE

Ribonucleic acid (RNA) is a nucleic acid that contains The word transcription means copying. It indicates the
a long chain of nucleotide units. It is similar to DNA but copying of genetic code from DNA to RNA. The proteins
contains ribose instead of deoxyribose. Various functions are synthesized in the ribosomes which are present in the
coded in the genes are carried out in the cytoplasm of cytoplasm. However, the synthesis of different proteins
the cell by RNA. RNA is formed from DNA. depends upon the information (sequence of codon)
encoded in the genes of the DNA which is present in the
„ STRUCTURE OF RNA nucleus. Since DNA is a macromolecule, it cannot pass
through the pores of the nuclear membrane and enter
Each RNA molecule consists of a single strand of
the cytoplasm. But, the information from DNA must be
polynucleotide unlike the double­stranded DNA. Each
sent to ribosome. So, the gene has to be transcribed
nucleotide in RNA is formed by:
(copied) into mRNA which is developed from DNA.
1. Ribose – sugar.
Thus, the first stage in the protein synthesis is
2. Phosphate. transcription of genetic code, which occurs within
3. One of the following organic bases: the nucleus. It involves the formation of mRNA and
Purines – Adenine (A) simultaneous copying or transfer of information from
– Guanine (G) DNA to mRNA. The mRNA enters the cytoplasm from
Pyrimidines – Uracil (U) the nucleus and activates the ribosome resulting in
– Cytosine (C). protein synthesis. The formation of mRNA from DNA is
Uracil replaces the thymine of DNA and it has similar facilitated by the enzyme RNA polymerase.
structure of thymine.
„ TRANSLATION OF GENETIC CODE
„ TYPES OF RNA Translation is the process by which protein synthesis
RNA is of three types. Each type of RNA plays a specific occurs in the ribosome of the cell under the direction
role in protein synthesis. The three types of RNA are: of genetic instruction carried by mRNA from DNA. Or, it
is the process by which the mRNA is read by ribosome
1. Messenger RNA (mRNA) to produce a protein. This involves the role of other two
types of RNA, namely tRNA and rRNA.
Messenger RNA carries the genetic code of the amino
The mRNA moves out of nucleus into the cytoplasm.
acid sequence for synthesis of protein from the DNA to
the cytoplasm. Now, a group of ribosomes called polysome gets
attached to mRNA. The sequence of codons in mRNA
2. Transfer RNA (tRNA) are exposed and recognized by the complementary
sequence of base in tRNA. The complementary
Transfer RNA is responsible for decoding the genetic sequence of base is called anticodon. According to the
message present in mRNA. sequence of bases in anticodon, different amino acids
3. Ribosomal RNA (rRNA) are transported from the cytoplasm into the ribosome
by tRNA that acts as a carrier. With the help of rRNA,
Ribosomal RNA is present within the ribosome and forms the protein molecules are assembled from amino acids.
a part of the structure of ribosome. It is responsible for the The protein synthesis occurs in the ribosomes which are
assembly of protein from amino acids in the ribosome. attached to rough endoplasmic reticulum.

„ GENE EXPRESSION „ GROWTH FACTORS

Gene expression is the process by which the information Growth factors are proteins which act as cell signaling
(code word) encoded in the gene is converted into molecules like cytokines (Chapter 17) and hormones
functional gene product or document of instruction (Chapter 65). These factors bind with specific surface
(RNA) that is used for protein synthesis. receptors of the target cell and activate proliferation,
Gene expression involves two steps: differentiation and/or maturation of these cells.
1. Transcription. Often, the term growth factor is interchangeably used
2. Translation. with the term cytokine. But growth factors are distinct
 Chapter 1 t Cell 17

from cytokines. Growth factors act on the cells of the Functional Significance of Apoptosis
growing tissues. But cytokines are concerned with the
The purpose of apoptosis is to remove unwanted cells
cells of immune system and hemopoietic cells.
without causing any stress or damage to the neighboring
Many growth factors are identified. The known growth
cells. The functional significance of apoptosis:
factors are:
1. Plays a vital role in cellular homeostasis. About
1. Platelet­derived growth factor – PDGF (Chapter 18)
10 million cells are produced everyday in human
2. Colony stimulating factors – CSF (Chapter 16)
body by mitosis. An equal number of cells die by
3. Nerve growth factors – NGF (Chapter 134)
apoptosis. This helps in cellular homeostasis
4. Neurotropins (Chapter 134)
2. Useful for removal of a cell that is damaged beyond
5. Erythropoietin (Chapter 10)
repair by a virus or a toxin
6. Thrombopoietin (Chapter 18)
3. An essential event during the development and in
7. Insulin­like growth factors – IGF (Chapter 66)
adult stage.
8. Epidermal growth factor – present in keratinocytes
and fibroblasts. It inhibits growth of hair follicles and Examples:
cancer cells
i. A large number of neurons are produced during
9. Basic fibroblast growth factor – present in blood
the development of central nervous system.
vessels. It is concerned with the formation of new
But up to 50% of the neurons are removed by
blood vessels
10. Myostatin – present in skeletal muscle fibers. It apoptosis during the formation of synapses
controls skeletal muscle growth between neurons
11. Transforming growth factors (TGF) – present in ii. Apoptosis is responsible for the removal of
transforming cells (cells undergoing differentiation) tissues of webs between fingers and toes during
and in large quantities in tumors and cancerous developmental stage in fetus
tissue. TGF is of two types: iii. It is necessary for regression and disappearance
i. TGF­α secreted in brain, keratinocytes and of duct systems during sex differentiation in fetus
macrophages. It is concerned with growth of (Chapter 74)
epithelial cells and wound healing iv. The cell that looses the contact with neighboring
ii. TGF­β secreted by hepatic cells, T lymphocytes, cells or basal lamina in the epithelial tissue dies
B lymphocytes, macrophages and mast cells. by apoptosis. This is essential for the death of old
When the liver attains the maximum size in enterocytes that shed into the lumen of intestinal
adults, it controls liver growth by inhibiting pro- glands (Chapter 41)
liferation of hepatic cells. TGF­β also causes v. It plays an important role in the cyclic sloughing
immunosuppression. of the inner layer of endometrium, resulting in
menstruation (Chapter 80)
„ CELL DEATH vi. Apoptosis removes the autoaggressive T cells
and prevents autoimmune diseases.
Cell death occurs by two distinct processes:
1. Apoptosis
2. Necrosis. Activation of Apoptosis
Apoptosis is activated by either withdrawal of positive
„ APOPTOSIS signals (survival factors) or arrival of negative signals.
Apoptosis is defined as the natural or programed death Withdrawal of positive signals
of the cell under genetic control. Originally, apoptosis
refers to the process by which the leaves fall from trees Positive signals are the signals which are necessary for
in autumn (In Greek, apoptosis means ‘falling leaves’). the long-time survival of most of the cells. The positive
It is also called ‘cell suicide’ since the genes of the cell signals are continuously produced by other cells or
play a major role in the death. some chemical stimulants. Best examples of chemical
This type of programmed cell death is a normal stimulants are:
phenomenon and it is essential for normal development i. Nerve growth factors (for neurons)
of the body. In contrast to necrosis, apoptosis usually ii. Interleukin-2 (for cells like lymphocytes).
does not produce inflammatory reactions in the The absence or withdrawal of the positive signals
neighboring tissues. activates apoptosis.
 18 Section 1 t General Physiology

Arrival of negative signals 4. Nuclear membrane becomes discontinuous and the

DNA inside nucleus is cleaved into small fragments
Negative signals are the external or internal stimuli which
initiate apoptosis. The negative signals are produced 5. Following the degradation of DNA, the nucleus
during various events like: breaks into many discrete nucleosomal units, which
1. Normal developmental procedures are also called chromatin bodies
2. Cellular stress 6. Cell membrane breaks and shows bubbled
3. Increase in the concentration of intracellular oxidants appearance
4. Viral infection 7. Finally, the cell breaks into several fragments
5. Damage of DNA containing intracellular materials including chromatin
6. Exposure to agents like chemotherapeutic drugs, bodies and organelles of the cell. Such cellular
X-rays, ultraviolet rays and the death-receptor fragments are called vesicles or apoptotic bodies
ligands. 8. Apoptotic bodies are engulfed by phagocytes and
Death-receptor ligands and death receptors dendritic cells.
Death­receptor ligands are the substances which bind Abnormal Apoptosis
with specific cell membrane receptors and initiate the
process of apoptosis. The common death-receptor Apoptosis within normal limits is beneficial for the body.
ligands are tumor necrosis factors (TNF­ α, TNF­ β) and However, too much or too little apoptosis leads to
Fas ligand (which binds to the receptor called Fas). abnormal conditions.
Death­receptors are the cell membrane receptors
which receive the death-receptor ligands. Well-charact- Common abnormalities due to too much apoptosis:
erized death receptors are TNF receptor-1 (TNFR1) and 1. Ischemic­related injuries
TNF-related apoptosis inducing ligand (TRAIL) receptors 2. Autoimmune diseases like:
called DR4 and DR5. i. Hemolytic anemia
Role of mitochondria in apoptosis ii. Thrombocytopenia
iii. Acquired immunodeficiency syndrome (AIDS)
External or internal stimuli initiate apoptosis by activating
the proteases called caspases (cysteinyl-dependent 3. Neurodegenerative diseases like Alzheimer’s
aspartate­specific proteases). Normally, caspases are disease.
suppressed by the inhibitor protein called apoptosis Common abnormalities due to too little apoptosis:
inhibiting factor (AIF).
When the cells receive the apoptotic stimulus, 1. Cancer
mitochondria releases two protein materials. First one is 2. Autoimmune lymphoproliferative syndrome (ALPS).
Cytochrome C and the second protein is called second
mitochondria-derived activator of caspases (SMAC) or „ NECROSIS
its homologudiablo.
Necrosis (means ‘dead’ in Greek) is the uncontrolled
SMAC/diablo inactivates AIF so that the inhibitor is
and unprogramed death of cells due to unexpected and
inhibited. During this process, SMAC/diablo and AIF
aggregate to form apoptosome which activates caspases. accidental damage. It is also called ‘cell murder’ because
Cytochrome C also facilitates caspase activation. the cell is killed by extracellular or external events. After
necrosis, the harmful chemical substances released
Apoptotic Process from the dead cells cause damage and inflammation of
neighboring tissues.
Cell shows sequence of characteristic morphological
changes during apoptosis, viz.: Causes for Necrosis
1. Activated caspases digest the proteins of cyto-
skeleton and the cell shrinks and becomes round Common causes of necrosis are injury, infection,
2. Because of shrinkage, the cell losses the contact inflammation, infarction and cancer. Necrosis is induced
with neighboring cells or surrounding matrix by both physical and chemical events such as heat,
3. Chromatin in the nucleus undergoes degradation radiation, trauma, hypoxia due to lack of blood flow and
and condensation exposure to toxins.
 Chapter 1 t Cell 19

Necrotic Process „ CELL ADAPTATION

Necrosis results in lethal disruption of cell structure and Cell adaptation refers to the changes taking place in a
activity. The cell undergoes a series of characteristic cell in response to environmental changes.
changes during necrotic process, viz. Normal functioning of the cell is always threatened by
1. Cell swells causing damage of the cell membrane various factors such as stress, chemical agents, diseases
and appearance of many holes in the membrane and environmental hazards. Yet, the cell survives and
2. Intracellular contents leak out into the surrounding continues the function by means of adaptation. Only
environment during extreme conditions, the cell fails to withstand the
3. Intracellular environment is altered hazardous factors which results in destruction and death
4. Simultaneously, large amount of calcium ions are of the cell.
released by the damaged mitochondria and other Cellular adaptation occurs by any of the following
organelles mechanisms.
1. Atrophy
5. Presence of calcium ions drastically affects the
2. Hypertrophy
organization and activities of proteins in the intra-
3. Hyperplasia
cellular components
4. Dysplasia
6. Calcium ions also induce release of toxic materials 5. Metaplasia.
that activate the lysosomal enzymes
7. Lysosomal enzymes cause degradation of cellular „ ATROPHY
components and the cell is totally disassembled
resulting in death Atrophy means decrease in size of a cell. Atrophy of more
8. Products broken down from the disassembled cell number of cells results in decreased size or wasting of
are ingested by neighboring cells. the concerned tissue, organ or part of the body.

Reaction of Neighboring Tissues Causes of Atrophy

after Necrosis Atrophy is due to one or more number of causes such
Tissues surrounding the necrotic cells react to the as:
breakdown products of the dead cells, particularly i. Poor nourishment
the derivatives of membrane phospholipids like the ii. Decreased blood supply
arachidonic acid. Along with other materials, arachidonic iii. Lack of workload or exercise
acid causes the following inflammatory reactions in the iv. Loss of control by nerves or hormones
surrounding tissues: v. Intrinsic disease of the tissue or organ.
1. Dilatation of capillaries in the region and thereby
increasing local blood flow Types of Atrophy
2. Increase in the temperature leading to reddening of Atrophy is of two types, physiological atrophy and
the tissues pathological atrophy. Examples of physiological atrophy
3. Release of histamine from these tissues which are the atrophy of thymus in childhood and tonsils in
induces pain in the affected area adolescence. The pathological atrophy is common in
4. Migration of leukocytes and macrophages from skeletal muscle, cardiac muscle, sex organs and brain.
blood to the affected area because of increased
capillary permeability „ HYPERTROPHY
5. Movement of water from blood into the tissues
causing local edema Hypertrophy is the increase in the size of a cell.
6. Engulfing and digestion of cellular debris and Hypertrophy of many cells results in enlargement or
foreign materials like bacteria by the leukocytes and overgrowth of an organ or a part of the body. Hypertrophy
macrophages is of three types.
7. Activation of immune system resulting in the removal
of foreign materials 1. Physiological Hypertrophy
8. Formation of pus by the dead leukocytes during this Physiological hypertrophy is the increase in size due
process to increased workload or exercise. The common
9. Finally, tissue growth in the area and wound healing. physiological hypertrophy includes:
 20 Section 1 t General Physiology

i. Muscular hypertrophy: Increase in bulk of 3. Pathological Hyperplasia

skeletal muscles that occurs in response to
strength training exercise Pathological hyperplasia is the increase in number of
cells due to abnormal increase in hormone secretion.
ii. Ventricular hypertrophy: Increase in size of
It is also called hormonal hyperplasia. For example, in
ventricular muscles of the heart which is advan-
gigantism, hypersecretion of growth hormone induces
tageous only if it occurs in response to exercise.
hyperplasia that results in overgrowth of the body.
2. Pathological Hypertrophy
„ DYSPLASIA
Increase in cell size in response to pathological changes
Dysplasia is the condition characterized by the abnormal
is called pathological hypertrophy. Example is the
change in size, shape and organization of the cell.
ventricular hypertrophy that occurs due to pathological
Dysplasia is not considered as true adaptation and it
conditions such as high blood pressure, where the
is suggested as related to hyperplasia. It is common in
workload of ventricles increases.
epithelial cells of cervix and respiratory tract.
3. Compensatory Hypertrophy
„ METAPLASIA
Compensatory hypertrophy is the increase in size of the
Metaplasia is the condition that involves replacement
cells of an organ that occurs in order to compensate
of one type of cell with another type of cell. It is of two
the loss or dysfunction of another organ of same type.
types.
Examples are the hypertrophy of one kidney when
the other kidney stops functioning; and the increase
1. Physiological Metaplasia
in muscular strength of an arm when the other arm is
dysfunctional or lost. Replacement of cells in normal conditions is called
physiological metaplasia. Examples are transformation
„ HYPERPLASIA of cartilage into bone and transformation of monocytes
into macrophages.
Hyperplasia is the increase in number of cells due to
increased cell division (mitosis). It is also defined as 2. Pathological Metaplasia
abnormal or unusual proliferation (multiplication) of cells
due to constant cell division. Hyperplasia results in gross Pathological metaplasia is the irreversible replacement
enlargement of the organ. Hyperplasia involves constant of cells due to constant exposure to harmful stimuli. For
cell division of the normal cells only. Hyperplasia is of example, chronic smoking results in transformation of
three types. normal mucus secreting ciliated columnar epithelial cells
into non-ciliated squamous epithelial cells, which are
1. Physiological Hyperplasia incapable of secreting mucus. These transformed cells
may become cancerous cells if the stimulus (smoking)
Physiological hyperplasia is the momentary adaptive is prolonged.
response to routine physiological changes in the body.
For example, during the proliferative phase of each
„ CELL DEGENERATION
menstrual cycle, the endometrial cells in uterus increase
in number. Cell degeneration is a process characterized by damage
of the cells at cytoplasmic level, without affecting
2. Compensatory Hyperplasia the nucleus. Degeneration may result in functional
impairment or deterioration of a tissue or an organ. It
Compensatory hyperplasia is the increase in number of
is common in metabolically active organ like liver, heart
cells in order to replace the damaged cells of an organ
and kidney. Degenerative changes are reversible in
or the cells removed from the organ.
most of the cells.
Compensatory hyperplasia helps the tissues and
organs in regeneration. It is common in liver. After
Causes for Cell Degeneration
the surgical removal of the damaged part of liver,
there is increase in the number of liver cells resulting Common causes for cell degeneration:
in regeneration. Compensatory hyperplasia is also 1. Atrophy, hypertrophy, hyperplasia and/or dysplasia
common in epithelial cells of intestine and epidermis. of cell
 Chapter 1 t Cell 21

2. Fluid accumulation in the cell mellitus by cell replacement technique. But, ethical
3. Fat infiltration into the cell issues arise because the embryo has to be destroyed to
4. Calcification of cellular organelles. collect the stem cells.
Stem cells from umbilical cord blood
„ CELL AGING
Stem cells in umbilical cord blood are collected from
Cell aging is the gradual structural and functional the placenta or umbilical cord. Use of these stem cells
changes in the cells that occur over the passage of time. for research and therapeutic purposes does not create
It is now suggested that cell aging is due to damage any ethical issue because it does not endanger the life
of cellular substances like DNA, RNA, proteins and of the fetus or newborn. Because of vitality and easy
lipids, etc. when the cell becomes old. When more availability, the umbilical cord blood stem cells are
cellular substances are damaged, the cellular function becoming a potent resource for transplant therapies.
decreases. This causes deterioration of tissues, organs Nowadays, these stem cells are used to treat about 70
or parts of the body. Finally, the health of the body starts diseases and are used in many transplants worldwide.
declining and this leads to death. So, the cell aging
determines the health and life span of the body. 2. Adult Stem Cells
Embryonic stem cells do not disappear after birth. But
„ STEM CELLS remain in the body as adult stem cells and play a role
in repair of damaged tissues. However, their number
Stem cells are the primary cells capable of reforming
becomes less. Adult stem cells are the undifferentiated
themselves through mitotic division and differentiating
multipotent progenitor cells found in growing children
into specialized cells. These cells serve as repair and adults. These are also known as somatic stem cells
system of the body and are present in all multicellular and are found everywhere in the body. These cells are
organisms. capable of dividing and reforming the dying cells and
regenerating the damaged tissues. So, these stem cells
„ TYPES OF STEM CELLS can also be used for research and therapeutic purposes.
Adult stem cells are collected from bone marrow.
Stem cells are of two types:
Two types of stem cells are present in bone marrow:
1. Embryonic stem cells derived from embryo i. Hemopoietic stem cells, which give rise to blood
2. Adult stem cells derived from adults. cells (Chapter 10)
ii. Bone marrow stromal cells, which can differ-
1. Embryonic Stem Cells entiate into cardiac and skeletal muscle cells.
Embryonic stem cells are derived from the inner cell
mass of a blastocyst which is an early stage of embryo. „ ADVANTAGES OF STEM CELLS
It takes about 4 to 5 days after fertilization to reach Adult stem cells from bone marrow are used in bone
the blastocyst stage and it has about 30 to 50 cells. marrow transplant to treat leukemia and other blood
Embryonic stem cells have two important qualities: disorders since 30 years. Recently, it is known that
i. Self-renewal capacity these stem cells can develop into nerve cells, liver cells,
ii. Pluripotent nature, i.e. these cells are capable of skeletal muscle cells and cardiac muscle cells.
differentiating into all types of cells in ectodermal, Recent discoveries also reveal that the stem cells
are present in several tissues which include blood, blood
endodermal and mesodermal layers.
vessels, skeletal muscle, liver, skin and brain. It is also
Because of these two qualities, the embryonic stem found that these cells are capable of differentiating into
cells can be used therapeutically for regeneration or multiple cell types. So, the cell-based therapy using
replacement of diseased or destroyed tissues. In fact, stem cells may be possible to treat many diseases
embryonic pluripotent stem cells are now cultured and such as heart diseases, diabetes, Parkinson’s disease,
lot of research is going on to explore the possibility of Alzheimer’s disease, spinal cord injury, stroke and
using these cells in curing the disorders like diabetes rheumatoid arthritis.