Cell membrane composition and functions

1. CELL MEMBRANE

3. • Each cell is surrounded by membrane called Cell membrane
or Plasma membrane
• Selectively permeable
• Thin, pliable, elastic structure
• Thickness - average 7.5 to 10 nm thick

4. • Composition:
It is composed of
Protein-55%
Phospholipid-25%
Cholesterol-13%
Other lipids -4%
Carbohydrate-3%

5. CELL MEMBRANE
• The current model of membrane structure:
-Fluid mosaic model
-By Singer and Nicholson in 1972
• It consists of lipid bilayer and interspersed protein molecules

7. 1. LIPID BILAYER
• It is a double layered film made up of mainly phospholipid
molecules.
• Each phospholipid molecule has 2 ends
- head end is charged, hydrophilic or
polar(phosphate portion)and
- the tail end is uncharged, hydrophobic or
- nonpolar (the fatty acid portion)

8. • Hydrophilic ends face the intracellular water on the inner
surface and the extracellular water on the outer surface.
• Hydrophobic end meet in the interior of the membrane.

9. • Phospholipids are phosphatidylcholine, sphingomyelin and
phosphatidyl ethanolamine
• Glycolipids are usually found in outer lipid layer
• Cholesterol is found incorporated in the hydrophobic region
of the cell membrane

10. 2. CELL MEMBRANE PROTEINS
• Globular masses embedded in the membrane.
• Most are glycoproteins.
There are 2 types of cell membrane proteins :
a)Integral proteins -that pass through the
membrane
b)Peripheral proteins -attached only to one
surface of the membrane

11. 3. MEMBRANE CARBOHYDRATES
• The entire external surface of the cell membrane is loosely
covered by a carbohydrate coat called glycocalyx.
• The membrane carbohydrate occurs mostly in combination
with proteins (glycoproteins) or in combination with lipids
(glycolipids).

13. FUNCTION OF CELL MEMBRANE
– BILIPID LAYER
• Amphipathic
• Lipid layer is impermeable to usual charged water soluble
substances like ions-[ Na+, K+, Cl- & Ca2+],glucose, and urea.
• Fat soluble substances like O2, CO2 ,alcohol & NH3-easily
permeable through the membrane.
Thus, lipid bilayer acts as a semipermeable membrane
• Cholesterol controls much of the fluidity of the membrane.

14. FUNCTIONS OF CELL MEMBRANE PROTEINS
Integral proteins serves the following functions
1. As ion channels – allows passage of ions across cell
membrane
2. Carrier proteins – transport substances along their
electrochemical gradient without expenditure of energy
3. Pumps – transport substances against their electrochemical
gradient with expenditure of energy
4. Aquaporins –act as water channels

16. Peripheral proteins serves the following functions
4. Enzymes - catalyzing reactions at the surface of the membrane
5. Receptors - for specific ligands (neurotransmitters & hormones)
6. Structural proteins - in forming the cell membrane
7. Cell adhesion molecules - that anchors cells to their neighbors
or to basal lamina
8. Glycoproteins and glycolipids - function as identity markers
eg- ABO antigens, MHC proteins

18. • Peripheral proteins are often attached to integral proteins
and function mainly as enzymes or controllers of transport
of substance through cell membrane.

19. FUNCTIONS OF CARBOHYDRATE
1. Many have negative charge and is responsible for negative
charge of cell surface.
2. The glycocalyx of some cells attaches to glycocalyx of other
cells – thus attaching cells to one another.
3. Many act as receptor substance for binding hormones and
then activates enzymes.
4. Some acts as antigens and take part in immune reactions.

20. • Maintains the cell volume
• In muscle and nerve cells, maintains the potential difference
between intracellular and extracellular surfaces.