2. Structure of cell membrane
• Trilayer (7-10 nm) which separate living cell
from nonliving surroundings
• Two electron dense layer separated by
electron lucent layer
• Composition:
– Lipids 40%
– Proteins 55%
– Carbohydrates 05%
4. Fluid mosaic model
• Singer and Nicolson 1972
• Membrane is an aggregation of proteins & other molecules
embedded in the fluid matrix of the lipid bilayer
• Phospholipid bilayer :-
– Continuous structure
– Fluid in nature
• Change in shape without disruption
– barrier to entry or exit of polar substances
• Proteins :-
– Mosaic of globular proteins inserted into the Phospholipid
bilayer.
– “iceberg of proteins in the ocean of lipid”
– Gatekeepers -- regulate traffic
• 50 lipid molecules for each protein molecule
8. Fluidity of cell
membrane
• Has a consistency of olive oil
• Lipid molecule exchange places
about 10 million times per second
(flip flop )
• % of unsaturated fatty acids in
phospholipids
– Double bound – kink in the tail
• Cholesterol
– More stronger at normal
temperature
– Increased fluidity at low
temperature
• Invitro fertilization is possible
9. Proteins in the cell membrane
• Proteins determine membrane’s specific
functions
• Classification of membrane proteins:
1. Depending upon their location
• Integral proteins
• Peripheral proteins
– Extrinsic proteins
– Intrinsic proteins
2. Depending upon their composition
• Lipoprotein
• glycoprotein
10. Integral proteins
• Extend across the entire lipid bilayer
• Not easily removable (without disrupting the
membrane)
• Proteins are amphipathic in nature
– Hydrophilic amino acids
• Serine, threonine,cycteine,tyrosine, asparagines etc
– Hydrophobic amino acids
• Glycine, alanine, valine, leucine, isoleucine, methionine etc
• Hydrophilic amino acid faces ECF and ICF
• Hydrophobic amino acids interact with the fatty
acid tail
• Single pass or multiple pass
12. Peripheral proteins
• Peripheral proteins are found at the inner
or outer surface of the membrane
• Can be stripped away from the membrane
without disturbing membrane integrity
• Types:
– Intrinsic proteins- enzymes
– Extrinsic proteins- Glycoproteins
• Cell surface identity marker & Cell adhesion
molecules
13. Functions of membrane proteins
1. Formation of Channel
– Passageway to allow specific substance to pass through
2. Transporter Proteins
– Bind a specific substance, change their shape & move it across
membrane
3. Act as Enzyme
– speed up reactions
4. Receptor Proteins
– Cellular recognition site
– Bind to substance—hormones
5. Cell Identity Marker
– Allow cell to recognize other similar cells
6. Linker
– Anchor proteins in cell membrane or to other cells
– Allow cell movement
– cell shape & structure
14. Functions of cell membrane proteins
Outside
Plasma
membrane
Inside
Transporter Cell surface
receptor
Enzyme
activity
Cell surface
identity marker
Attachment to the
cytoskeleton
Cell adhesion
15. Carbohydrate in cell membrane
• Two forms
– Glycoproteins
– Glycolipids
• Glycocalyx layer covers the entire cell
membrane
• Functions:
– Do not allow the negatively charged molecules to
pass
– Tight fixation of the cells with one another
– Also serve as receptor
– Make the cell surface slippery
16.
17. Intercellular junctions
• Junctions present between two cells.
• Cell adhesion molecules:
– Integrins-heterodimers, binds to receptors
– IgG subfamily -binding with antigens
– Cadherins-
• Ca 2+ dependent and homophilic
– Selectins- binds to carbohydrates
• Mechanism of adhesions:
– Anchorage with cytoskeleton
– Homophilic binding
– Heterophilic binding
– Binding to laminins present in the ECF
18. Types of junctions
Tight junctions / Zona occludens
Adherens junctions / Zona adherens
Desmosomes
Hemidesmosomes
Gap junctions
19. Tight junctions
– Zona occludens
– Cell membrane of the cells fuse with each other
– No space in between the cells
– Act like barrier for the movement of substances
– Examples:
• Apical margins of epithelial cells
• Renal tubular epithelial cells
• Blood brain barrier
• Blood aqueous barrier, blood testes barrier etc
20. Adherens junctions
• Cell membranes of adjacent cells are separated by a
15-20 nm wide space
• Focal obliteration by dense accumulation of
proteins
• Radiation of intermediate filaments into the cell
from the site of junction
• Types :
– Desmosomes –focal thickening on both the cells
surfaces
– Hemidesmosomes- focal thickening on one cell surface
• Examples :
– Epidermis
– Cells attachment to basal lamina and actin filament
21. Gap junctions
• Channels on the lateral surfaces of the cells
• Exchange of molecules between the cells
• Channel have six (6) sub units- connexin
• Intercellular space 2-3 nm
• Example :
– Heart
– Basal part of the epithelial cells of intestinal
mucosa
• Functions:
– Intercellular exchange of substances with a
molecular weight < 1000
– Rapid propagation of electrical impulse
– Exchange of chemical messengers between cells
