This document provides an overview of cell structure and function. It discusses the structure and functions of cellular organelles like the cell membrane, mitochondria, endoplasmic reticulum, lysosomes, peroxisomes, and cytoskeleton. It also describes intercellular junctions and cell adhesion molecules. A case study example is provided to illustrate glycogen storage disorder.

1. General physiology
Dr Bhakti R Kharate
M.D Physiology
Associate professor

2. Cell structure and function

3. Objectives
At the end of the session students should be able to :
 State the functions of cellular organelles .
 Describe the structure of plasma membrane and explain its functional
significance
 Name various cytoskeletal elements of cell and give their functions.
 Classify intercellular junctions and give their functions.
 Classify cell adhesion molecules and give their functions.

4. Cell structure

5. Cell membrane
• Cell Membrane
 protective sheath of the cell.
exchanges materials between the
cytoplasm and the external
environment
 facilitates interaction between cells,
and helps in anchoring the cells to
each other.

6. • Widely accepted one is fluid-mosaic model described by Singer and
Nicolson in 1972.
1. Cell membrane consists of a double-layer of phospholipid that
contains protein molecules.
2. The membrane lipids are present in the fluid form that allows the
flexibility of the membrane without disturbing the structural
integrity.
3. The membrane proteins are loosely attached and float in the fluid
phospholipid bilayer.
FLUID-MOSAIC MODEL OF THE MEMBRANE

7. • The major lipids in the cell
membrane are phospholipids,
glycolipids, and cholesterol.
1. The phospholipids are
phosphatidylcholine,
sphingomyelin,
phosphatidylserine, and
phosphatidyl ethanolamine

9. Factors Contributing to Fluidity of Membrane
The fluidity of the membrane is mainly dependent on the temperature and lipid
composition of the membrane.
• Effect of temperature:
• With rise in temperature, the unsaturated bond among fatty acid side chains lose
their orderly arrangement.
• Lipid Composition of Membrane- increase in degree of unsaturation, increase in
length of fatty acid and increase in phospholipid content membrane fluidity
increases.
• Role of Cholesterol: Though cholesterol profoundly influences membrane fluidity,
it behaves differently at different temperature. It is rather a bulky molecule
placed among phospholipid bilayer
• Ratio of cholesterol to phospholipid important for membrane integrity

10. Functions of the Lipid Bilayer:
The main function of the lipid bilayer of a cell membrane is to create a
permeability barrier between the interstitial fluid and the cytoplasm.
• Lipid soluble substances like oxygen and alcohol can pass easily through
the cell membrane.
• Water soluble substances like urea and glucose cannot pass easily. Thus,
lipid bilayer makes the membrane semipermeable.

11. • Membrane proteins are of two
types:
• Integral proteins and
• Peripheral proteins .
Membrane proteins

12. • Integral proteins
• Serve as channel proteins
• Act as carriers
• Serve as ion pumps
• Serve as receptor and enzyme
proteins
• Antigenic functions

13. Peripheral Proteins
1. Intrinsic proteins:
 enzymes or anchor proteins fo
maintain cell shape.
2. Extrinsic proteins:
 cell adhesion molecules (CAM s)
for anchoring with neighboring
cells.
 They can be removed without
disrupting the membrane.

14. • The external surface of the cell membrane is loosely covered by a carbohydrate
layer known as the cell coat or glycocalyx.
• Functions of the Glycocalyx:
 Glycocalyx serves as a protective coat. Carbohydrate is negatively charged and
therefore prevents negative particles like protein molecules to interact between
cells.
 Some carbohydrate molecules serve as receptors.
MEMBRANE CARBOHYDRATES

15. Functions of Cell Membrane
1. Cell membrane maintains a constant and distinctive intracellular environment,
2. Cell membrane maintains cell volume by actively transferring ions across it,
especially by pumping sodium ions out of the cell.
3. In neurons and muscle cells, it maintains a potential difference between
intracellular and extracellular surfaces, which enables cells to respond to various
stimuli.
4. Cell membrane helps in recognizing foreign cells or antigens so that they can be
destroyed by phagocytes.

16. RBC membrane
• Defects in membrane
proteins-
• hereditary spherocytosis,
• elliptocytosis .

17. Mitochondria
• “power houses”
• cigar-shaped organelles
• shape, size and number vary in
different tissues of the body
• abundant in cells that have high
rate of metabolism, as in liver,
cardiac muscle, etc
 Cellular oxidation
 Formation of ATP
 Self replication
• .

19. • The endoplasmic reticulum (ER)
consists of a network of membranous
tubules, vesicles, and flattened
cisternae.
• synthesis of proteins and lipids for the
membrane of cell and organelles, and
secretory vesicles of the cytoplasm.

20. Functions of Smooth ER
• Synthesis of lipids. Hence, it is abundant in cells that synthesize cholesterol,
steroid hormones, and phospholipids.
• In muscles, it is called sarcoplasmic reticulum, which is specialized for the
storage of calcium ions that is released during excitation-contraction coupling.
• site for the detoxification or neutralization of hormones and toxic substances

22.  Formation of secretory vesicles: GA is the site for the packaging of secretory
products into the secretory granules.
 Formation of primary lysosomes
 Transport of materials through vesicles
 Replenishes cell membrane
 Synthesis of chondroitin sulphate and hyaluronic acid – major component of
bone matrix and cartilage, major constituent in the mucus and glandular
secretions, part of extracellular matrix of in the interstitial space.

24. Functions of Lysosome
1. Digestive organ of the cell:
2. Bactericidal activity: (i) Lysozymes that dissolve cell membrane of bacteria;
(ii) Lysoferrin present in lysosome binds iron and other substances before
they can promote bacterial growth; and (iii) Acidic pH of about 5.0 in
lysosomes activates lysosomal hydrolases and inactivates bacterial metabolic
systems.
3. Autolysis: When a cell is damaged or ruptured, the enzymes liberated within
the cytoplasm digest the cytoplasmic contents, a process known as
autolysis.
4. Recycling of cell organelles: The worn-out cell organelles are transported to
the lysosomes . Lysosomal enzymes digest these organelles and the process
is known as autophagy. Nutrients liberated are reused.

25. • Lysosomal storage disorders
• Fabry disease- alpha galactosidase A deficiency
• Gaucher disease- acid beta glucosidase deficiency
• Tay Sachs disease- beta hexosaminidase A
• Pompe disease- acid alpha glucosidase deficiency

26. Peroxisomes
• Inherited disorders absence of
peroxisomes or defective
function
• Zellweger syndrome –profound
neurological impairment , child
dies within a year.

27. Case study
• An eighteen year old boy , Rohan appears to have liver disease.
• A liver biopsy is performed, and different microscopic techniques are
employed for viewing the samples.
• The biopsy reveals an unusually extensive smooth endoplasmic
reticulum.
• An abnormally large amount of glycogen granules are found, and
many intact granules are seen within secondary lysosomes.
• h/o drug abuse
• Lab analysis reveals abnormally low amount of the enzyme that
hydrolyses glycogen.
• What is the relationship between these observations?

28. • Functions of ER and lysosomes .
• Why is the endoplasmic reticulum so well developed, what beneficial
function might this serve?
• Could his apparent liver disease be caused by another disorder ?
• What condition may he have that would explain the presence of
intact glycogen granules in his lysosomes?

29. • Extensive ER- enzymes to metabolise drugs
• Liver disease – because of drug abuse
• Low enzymes for breakdown of glycogen- storage disorder- genetic
lysosomal enzyme lacking.

30. Cytoskeleton
• Maintain structural integrity of the cell
• Allows appropriate change in cell shape for mobility & participation of
cell in various activities.
• Microfilaments, microtubules, and intermediate filaments.

32. • Microfilaments are the smallest component of the cytoskeleton.
These thin filaments are made of actin proteins strung together in a
spiralling chain.
• Function : contractile function, cell shape and movement, transport
and secretion of cellular materials, movement of chromosomes and
cell division

33. • Intermediate filaments are larger ,thicker and stronger.
• The main function is provide structure and support in the cell by
forming the skeletal network for the cell.
• Absence of the filaments leads to easy rupture of cell- formation of
blister
• Some parts of the body are made up entirely of intermediate
filaments, like hair and nails, made from a protein called keratin.

34. • Microtubules are the largest filaments in the cytoskeleton, Made of
the protein tubulin. They are dynamic - constantly remodelling in
response to the cell's needs.
• They have two main jobs in the cell:
• Transporting materials around the cell and cell division.
• Contribute to cell motility and structure and function of cilia
• Cytotoxic drugs act by inhibiting microtubular functions
• Anticancer drugs – vincristine promotes disassembly of microtubules.
• Colchicine inhibits microtubule assembly

35. Intercellular junctions
• Junctions formed between the cells
• 2 types:
1. junctions that tie cells together- tight junctions,
2. junctions that allow transfer of ions and small molecules

36. Tight junctions
• Zona occludens- stability , strength & tie the cells together
• Found in epithelium of GI tract, urinary tract, hepatobiliary tract
• Located towards the apical region of the cells
• Obliterate the intercellular space
• Selectively leaky due to ions and water channels. -selective
permeability barrier
• Tight junctions between endothelial cells of cerebral blood vessels
contribute to the effectiveness of blood-brain barrier.

38. Anchoring Junctions
• Cell-to-cell anchoring junctions
are desmosome and zonula
adherens
• Desmosomes are the junctions
characterized by focal thickening
of two adjacent cell membranes
• Zonula Adherens- major site of
attachment for intracellular
microfilaments

39. Cell to basal lamina anchoring junctions
• Hemidesmosomes : In
appearance, they look like half
of desmosomes, and therefore
are called hemidesmosome.
• Microfilaments are attached to it
intracellularly.
• Focal Adhesions- Intracellularly,
they are associated with actin
filaments. Therefore, they assist
in cell movement.

40. Gap Junctions
• Gap junctions are called nexus
between the cells at which the
intercellular space is narrowed
from its 25 nm diameter to 3nm
• Made up of special
transmembrane proteins known
as connexons

42. • Mutation of connexon gene –
• 20 genes code for connexon
protein –
• diseases specific to the tissue
• Charcot Marie Tooth Disease
specifically affects the peripheral
neurons causing peripheral
neuropathy

43. Functions of gap junctions
• Serve as Electrical synapses- behave as physiological syncytium
• permit passage of organic solutes such as sugars and amino acids with
molecular weight up to 1000 from cell-to-cell.
• Chemical messengers and hormones are also exchanged between cells
through gap junctions.
• Their activities are regulated by intracellular calcium concentration and
cytosolic pH.
• Increase in any of these parameters can prevent transport through gap
junctions by closing their channels.

44. Cell adhesion molecules
• Integrins, IgG superfamily proteins, cadherins and selectins.

45. Functions of CAMs
They zip cell-to-cell. Cell adherence is strengthened by CAMs.
For their attachment with cytoskeleton, they play role in cell
movement.
Cellular signals are transmitted out of the cell or extracellular
signals are transmitted into the cell via CAMs.
They play significant role in inflammation and wound
healing.
CAMs prevent apoptosis. Loss of cellular contact from
extracellular matrix due to defects in CAMs hastens the process
of apoptosis.

46. •Thank u

47. Apoptosis
• Programmed cell death
• Cell suicide
• Genes play an active role

48. Mechanism
• Common pathway – activation of caspases
• Caspases –group of cysteine proteases
• DNA fragmentation, cytoplasmic and chromatin condensation
• Cell breaks up ,phagocytosis of debris

49. Significance of apoptosis
• Important in fetal life- removal of webs during fetal life, regression of
duct systems for sexual differentiation.
• Degeneration and regeneration of neurons and synapse formation
• Gets rid of inappropriate clones of immunocytes.
• Cyclic breakdown of endometrium leading to menstruation.

50. • Abnormal apoptosis – autoimmune diseases, degenerative disorders
and cancer.

52. • Ultracentrifugation
• Cells are homogenized
• Resulting suspension is centrifuged
• Nuclei sediment first
• Mitochondria
• High speed centrifugation generates forces of 100,000 times gravity
causes granules (microsomes) to sediment.(ribosomes,peroxisomes)

53. Tools for examining cellular components
• Light microscope – resolution 0.2 micrometre
• Electron microscope – 0.002 micrometre
• Phase contrast ,Fluorescent, Confocal microscopy techniques