Structure & function of cell

©Goutam Mallik, Assistant Professor, Pharmacology
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CHAPTER – 2
CELLULAR LEVEL OF ORGANISATION
Structure and Functions of cell
The smallest unit of protoplasm, capable of carrying out independent
existence, The word cell (L. cella-a storeroom, a chamber) was first
Introduced in the biology by Robert Hooke.
CELL
 The cell is the structural and functional unit of the living matter and is capable of carrying on the
processes of life independently.
 The cells are not of same size and shape only due to the presence of differentiation of functions.
But there are certain structural characteristic features which are common to them all. The
structure of the cell is consisting of following three important parts.:
1. Cell membrane
2. Cytoplasm
3. Nucleus
CELL MEMBRANE
 Cell membrane is a protective sheath, enveloping the cell body. It is also known as plasma
membrane or plasmalemma.
 Cell membrane is composed of three types of substances:
1. Proteins (55%)
2. Lipids (40%)
3. Carbohydrates (5%)
STRUCTURE OF CELL MEMBRANE
 All membranes are composed of lipids (mainly phospholipids, cholesterol and glycolipids) and
proteins, in approximately equal ratios.
 Plasma membrane lipids form a lipid bilayer, a layer two molecules thick.
 The hydrophobic ends of each lipid molecule face the interior of the membrane and the hydrophilic
ends face outwards.
 Most proteins are embedded within, or float in, the lipid bilayer as a fluid mosaic.
 Some proteins, because of extensive hydrophobic regions of their polypeptide chains, span the
entire width of the membrane (transmembrane proteins), whereas others are only superficially
attached to the bilayer by lipid groups.

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 Both are integral (intrinsic) membrane proteins, as distinct from peripheral (extrinsic) membrane
proteins, which are membrane-bound only through their association with other proteins.
 Carbohydrates in the form of oligosaccharides and polysaccharides are bound either to proteins
(glycoproteins) or to lipids (glycolipids), and project mainly into the extracellular domain.
Fig: Structure of cell membrane as seen under electron microscope (diagrammatic representation)
Fig: The molecular organization of the plasma membrane
 According to the fluid mosaic model of membrane structure. Intrinsic or integral membrane proteins
include diffusion or transport channel complexes, receptor proteins and adhesion molecules.
 These may span the thickness of the membrane (transmembrane proteins) and can have both
extracellular and cytoplasmic domains.
 Transmembrane proteins have hydrophobic zones, which cross the phospholipid bilayer and allow
the protein to ‘float’ in the plane of the membrane.

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 Some proteins are restricted in their freedom of movement where their cytoplasmic domains are
tied to the cytoskeleton.
Properties of Membrane Carbohydrates—The Cell “Glycocalyx.”
 The “glyco” portions of these molecules almost invariably protrude to the outside of the cell,
hanging outward from the cell surface.
 Many other carbohydrate compounds, called proteoglycans—which are mainly carbohydrate
substances bound to small protein cores—are loosely attached to the outer surface of the cell as
well.
 Thus, the entire outside surface of the cell often has a loose carbohydrate coat called the
glycocalyx.
 The carbohydrate segments attached to the outer surface of the cell have several important
functions:
1. Many of them have a negative electrical charge, which gives most cells an overall negative
surface charge that repels other negatively charged objects.
2. The glycocalyx of some cells attaches to the glycocalyx of other cells, thus attaching cells to one
another.
3. Many of the carbohydrates act as receptor substances for binding hormones, such as insulin;
when bound, this combination activates attached internal proteins that, in turn, activate a cascade
of intracellular enzymes.
4. Some carbohydrate fragments enter into immune reactions.
Properties of Lipid Layers of the Cell Membrane
 The characteristic feature of lipid layer is that, it is fluid in nature and not a solid structure. So, the
portions of the membrane move from one point to another point along the surface of the cell.
 Major lipids are: 1. Phospholipids 2. Cholesterol.
 Phospholipids are mainly Amino-phospholipids, sphingomyelins, phosphatidylcholine,
phosphatidyl-ethylamine, phosphatidylglycerol, phosphatidylserine and phosphatidyl-
inositol.
 Cholesterol molecules are arranged in between the phospholipid molecules.
 Phospholipids are soft and oily structures and cholesterol helps to ‘pack’ the phospholipids in the
membrane. So, cholesterol is responsible for the structural integrity of lipid layer of the cell
membrane.
Properties Protein Layers of the Cell Membrane
 Protein layers give protection to the central lipid layer. The protein substances present in these
layers are mostly glycoproteins.
 Protein molecules are classified into two categories:
1. Integral proteins or transmembrane proteins (intrinsic proteins).

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2. Peripheral proteins or peripheral membrane proteins(extrinsic proteins).
 Integral proteins provide the structural integrity of the cell membrane.
 Channel proteins help in the diffusion of water-soluble substances like glucose and electrolytes.
 Carrier or transport proteins help in the transport of substances across the cell membrane by
means of active or passive transport.
 Pump: Some carrier proteins act as pumps, by which ions are transported actively across the cell
membrane.
 Receptor proteins serve as the receptor sites for hormones and neurotransmitters.
 Enzymes: Some of the protein molecules form the enzymes and control chemical (metabolic)
reactions within the cell membrane.
 Antigens: Some proteins act as antigens and induce the process of antibody formation.
 Cell adhesion molecules or the integral proteins are responsible for attachment of cells to their
neighbours or to basal lamina.
Function of Plasma Membrane
 Transport: It facilitates the transport of materials across it. It is selectively permeable to certain
substance and helps transports of substances needed for survival. The various transport
mechanisms are:
A. Diffusion ( oxygen, carbon dioxide, small molecules, etc.) and passive osmosis (water).
B. Transmembrane protein channels and transporters Aquaporins for water transport, ion channels
for sodium and potassium transport, etc.
C. Endocytosis: It is a process by which cell absorbs molecules by engulfing them: Example:
 Pinocytosis: Small vesicles, by a process of pinocytosis (drinking by cells), encircle and
carry fluid within it across the membrane. By the pinocytic process fluid of smaller
molecules (0.01-2.0 µm) can be engulfed in.
 Phagocytosis: The phagocytosis (eating by cells) is similar to engulfing of solid materials by
the amoeba.
D. Exocytosis in which cells removed undigested products brought in by endocytosis, or to
secrete enzymes and hormones or to excrete substances outside the cell.
 Helps in the protection of cell. It surrounds cytoplasm of cell and forms a physical barrier between
intracellular component and extracellular compartment.
 It anchors to the cytoskeleton to the extracellular matrix and thereby provide shape to the cell and
maintains its structural integrity.
 Receives stimuli from the outside. The protein component of cell membrane acts as ligand
receptors. The cell membrane contains receptor site for some hormones, immune proteins and
neurotransmitters thus the cell recognizes and process these signals.
 Takes in food and excretes waste products.

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 They aid in cell recognition (identifiers). Example: Glycoproteins (e.g., major histocompatibility
complex, ABO blood group antigens). The surface protein markers which are embedded in the cell
identify the cells, thus helps neighbouring cell to communicate with each other.
 The proteins in cell membrane act as enzymes and catalyse reactions and thus involved in
metabolic process.
Fig: Different phases of pinocytosis; the particle in solution approaching the cell surface (a and b)
and gradually being drunk (c to f)
Cytoplasm
 The cytoplasm is the protoplasm, which surrounds the nucleus and is bounded peripherally by the
cell membrane.
 Cytoplasm may be homogeneous, vacuolated, granular, reticular or fibrillar.
 The cytoplasm can be classified into two groups.
1. Cytoplasmic organelles
2. Cytoplasmic inclusions

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Fig: Structure of Human cell under Light Microscope
Fig: 3D structure of Human Cell

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Endoplasmic Reticulum
 These consist of network of canals (tubules) and vesicles (cisternae).
 These are three-dimensional and bounded by membrane of about 80 A in thickness.
 The elements of the endoplasmic reticulum may connect intermittently with the plasma membrane
at one hand and on the other hand with the outer nuclear membrane.
 Two types of endoplasmic reticulum have been recognized:
1. Rough-surfaced endoplasmic reticulum: This reticulum is studded with osmiophilic
granules-the ribosomes lying in rows in contact with the membranes of the endoplasmic
reticulum . The roughness of the membrane is due to the presence of these granules-
Palade granules.
2. Smooth-surfaced endoplasmic reticulum: This type of endoplasmic reticulum does not
possess osmiophilic granules-the ribosomes at the outer border of the membrane. This is
why it is smooth.
(A) (B)
Fig: (A) Endoplasmic reticulum (B) Structure of the endoplasmic reticulum. (Modified from DeRobertis
EDP, Saez FA, DeRobertis EMF: Cell Biology, 6th ed. Philadelphia: WB Saunders, 1975.)

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Fig: A typical Golgi apparatus and its relationship to the endoplasmic reticulum (ER)
and the nucleus.
Function of Endoplasmic Reticulum
1. As the smooth-surfaced endoplasmic reticulum is very abundant in the interstitial (Leydig) cells of
the testis and in cells of the corpus luteum, this reticulum is concerned with the synthesis of steroid
hormones.
2. In the parietal cells of the gastric mucosa, it is concerned with secretion of hydrochloric acid.
3. In the skeletal muscle, it (sarcoplasmic reticulum) is concerned in some way with binding of the
Ca++ ions and also plays role in conducting impulses in the substances of muscle cells.
4. In the liver cells both types of reticula are concerned with the synthesis of protein and
carbohydrate.
5. Smooth endoplasmic reticulum is also concerned with catabolism and detoxification of toxic
substances like some drugs and carcinogens (cancer-producing substances) in the liver.
6. Outer surface of smooth endoplasmic reticulum contains many enzymes which are involved in
various metabolic processes of the cell.
Golgi Apparatus
 Golgi apparatus or Golgi body or Golgi complex is a membrane-bound organelle, involved in the
processing of proteins.
 It is present in all the cells except red blood cells.
 It is named after the discoverer Camillo Golgi.
 Usually, each cell has one Golgi apparatus.
 Some of the cells may have more than one Golgi apparatus.
 Each Golgi apparatus consists of 5 to 8 flattened membranous sacs called the cisternae.
 Golgi apparatus is situated near the nucleus. It has two ends or faces, namely cis face and trans
face.
 The cis face is positioned near the endoplasmic reticulum.
 Reticular vesicles from endoplasmic reticulum enter the Golgi apparatus through cis face.
 The trans face is situated near the cell membrane.

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 The processed substances make their exit from Golgi apparatus through trans face.
Fig: Golgi Apparatus
Function of Golgi Apparatus
1. Processing of materials: Vesicles containing glycoproteins and lipids are transported into Golgi
apparatus. Here, the glycoproteins and lipids are modified and processed.
2. Packaging of materials: All the processed materials are packed in the form of secretory
granules, secretory vesicles and lysosomes, which are transported either out of the cell or to
another part of the cell. Because of this, Golgi apparatus is called the ‘post office of the cell’.
3. Labelling and deliveryof materials: Finally, the Golgi apparatus sorts out the processed and
packed materials and labels them (such as phosphate group), depending upon the chemical
content for delivery (distribution) to their proper destinations. Hence, the Golgi apparatus is called
‘shipping department of the cell’.
Mitochondria
 These are relatively solid bodies, granular, rod-shaped or filamentous in form and remain scattered
throughout the cytoplasm (dimensions varying from 0.5 to 5.0 microns).
 In humans, mitochondria mDNA is inherited solely from the mother.
 They are surrounded by a trilaminar double membrane, the inner one of which remains folded and
forms a number of partitions, the cristae mitochondriales. These cristae may be complete, septate
or incomplete.
 Numerous projecting particles known as elementary particles are present on the inner
mitochondrial membrane and cristae. The fluid of the intramitochondrial space is called matrix.
( A )
( B )

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Fig: (A) Mitochondria (B) Electron
micrographic representation of the basal
portion of the renal tubular epithelium (rat)
embedded on basement lamina showing
part of the, Golgi apparatus, numerous
nucleus (C) Mitochondria showing internal
structures.
(C)
Function of Mitochondria
 Production of energy Mitochondrion is called the ‘power house’ or ‘power plant’ of the cell because
it produces the energy required for cellular functions. The energy is produced during the oxidation
of digested food particles like proteins, carbohydrates and lipids by the oxidative enzymes in
cristae.
 During the oxidative process, water and carbon dioxide are produced with release of energy. The
released energy is stored in mitochondria and used later for synthesis of ATP.
 Synthesis of ATP The components of respiratory chain in mitochondrion are responsible for the
synthesis of ATP by utilizing the energy by oxidative phosphorylation.
 ATP molecules diffuse throughout the cell from mitochondrion. Whenever energy is needed for
cellular activity, the ATP molecules are broken down.
 Cytochrome C and second mitochondria-derived activator of caspases (SMAC)/diablo secreted in
mitochondria are involved in apoptosis.
 Other functions Other functions of mitochondria include storage of calcium and detoxification of
ammonia in liver.
Lysosome
 Lysosomes are vesicular organelles that form by breaking off from the Golgi apparatus and then
dispersing throughout the cytoplasm.
 The lysosome is quite different in various cell types, but it is usually 250 to 750 nanometers in
diameter.
 Among the organelles of the cytoplasm, the lysosomes have the thickest covering membrane. The
membrane is formed by a bilayered lipid material.

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Fig: Digestive function of lysosomes
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.
 Lysosomes are often called ‘garbage system’ of the cell because of their degradation activity.it is
also called suicidal sac of cell. About 50 different hydrolytic enzymes, known as acid
hydroxylases are present in the lysosomes, through which lysosomes execute their functions.
 Lysosomal functions involve two mechanisms:
1. Heterophagy: Digestion of extracellular materials engulfed by the cell via endocytosis.
2. Autophagy: Digestion of intracellular materials such as worn-out cytoplasmic organelles.
Function of Lysosome
1. Degradation of macromolecules like protein, lipid and carbohydrate.
2. The rough endoplasmic reticulum wraps itself around the worn-out organelles like mitochondria and
form the vacuoles called autophagosomes.The enzymes in the secondary lysosome are activated.
Now, these enzymes digest the contents of autophagosome.
3. Removal of excess secretory products in the cells Lysosomes in the cells of the secretory glands
remove the excess secretory products by degrading the secretory granules.
4. Secretory function – secretory lysosomes Recently, lysosomes having secretory function called
secretory lysosomes are found in some of the cells, particularly in the cells of immune system.

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Centrosomes &Centrioles
 It consists of another specialised part of clear cytoplasm, the centrosphere, containing in its interior
two or more deeply staining particles-the centriole (generally arranged in pairs, i.e. diplosome)
lying close to the nucleus in the resting cell.
 The centriole is an empty cylinder which is 3 to 5 micron long and the compact walls of centrioles
are made of thin parallel nine tubular structures longitudinally arranged. Each tubule consists, in
turn, of three subunits or triplets.
 The pericentriolar bodies or satellites which are round in shape are attached to centriolar tubules
by a filament of chromatid.
 Centriole is closely related to spindle formation during mitosis (normal cell division) and also in
sustaining other fibrillar structure like cilia and flagella. At the beginning of cell division, the
centriole and centrosome divide.
 A system of radiating lines, made up of microtubules, then grows out from each of the two newly
formed centrioles and the whole structure, due to its star-like shape, is called aster. The two asters
grow in size and repel each other till they occupy the opposite poles of the elongated cell.
 Centrioles control polarisation of spindle fibres and play some part in their formation.
Fig: Arrangement of centriolar tubule, nine in number, each tubule consisting of three
centriolar subunits in right sided inset.
Nucleus
Nucleus is covered by a membrane called nuclear membrane and contains many components.
Major components of nucleus are nucleoplasm, chromatin and nucleolus.

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The nucleus is generally a round body occupying the centre of the cell. Its shape, size, position
and number vary. The nucleus may contain many lobes.
Nucleoplasm
 Nucleoplasm is a highly viscous fluid that forms the ground substance of the nucleus.
 It is similar to cytoplasm present outside the nucleus
 . Nucleoplasm surrounds chromatin and nucleolus.
Chromatin
 Chromatin is a thread-like material made up of large molecules of DNA.
Chromosomes
 Chromosome is the rod-shaped nuclear structure that carries a complete blueprint of all the
hereditary characteristics of that species.
 A chromosome is formed from a single DNA molecule coiled around histone molecules.
 Each DNA contains many genes.
Nucleolus
 Nucleolus is a small, round granular structure of the nucleus. Each nucleus contains one or more
nucleoli. The nucleolus contains RNA and some proteins, which are similar to those found in
ribosomes.
(A) (B)
Fig: (A) Diagram shows chromosomes extending through amorphous matrix and innermost limit
of cytoplasm and porous nuclear membrane (B)Structure of nucleus.
Functions of Nucleus
Major functions of nucleus are the control of cellular activities and storage of hereditary material. Several
processes are involved in the nuclear functions.

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1. Control of all the cell activities that include metabolism, protein synthesis, growth and reproduction
(cell division).
2. Synthesis of RNA.
3. Formation of subunits of ribosomes.
4. Sending genetic instruction to the cytoplasm for protein synthesis through messenger RNA (mRNA).
5. Control of the cell division through genes.
6. Storage of hereditary information (in genes) and transformation of this information from one
generation of the species to the next.
Ribosome
 Ribosomes are the organelles without limiting membrane.
 These organelles are granular and small dot-like structures with a diameter of 15 nm.
 Ribosomes are made up of 35% of proteins and 65% of ribonucleic acid (RNA). RNA present in
ribosomes is called ribosomal RNA (rRNA). Ribosomes are concerned with protein synthesis in the
cell.
 Ribosomes are called ‘protein factories’ because of their role in the synthesis of proteins.
 Messenger RNA (mRNA) carries the genetic code for protein synthesis from nucleus to the
ribosomes.
 The ribosomes, in turn arrange the amino acids into small units of proteins.
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References
1. CC Chaterjee’s Human Physiology 12th Ed Vol.1
2. Essentials of Human Physiology for Pharmacy by Laurie Kelly
3. Essentials of Human Anatomy & Physiology 5th
Ed by Valerie C. Scanlon & Tina Sanders
4. Gray’s Anatomy, The Anatomical Basis of Clinical Practice 41st
Ed by Elsevier
5. Guyton and Hall Textbook of Medical Physiology 13th
Ed by John E. Hall
6. Essentials of Medical Physiology 6th
Ed by K Sembulingam and Prema Sembulingam

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