all about cell membrane

2. STRUCTURE OF CELL MEMBRANE
Fluid mosaic model
In the year 1972, S. J. Singer and G. Nicolson proposed the fluid
mosaic model of plasma membrane.
This is the most widely accepted model of plasma membrane
structure that explains almost all the properties of plasma membrane.
According to this model the membrane is a viscous structure
rather than a solid where phospholipids and protein molecules are arranged
as a mosaic. Here the phopholipids are found in two layers forming the
base of the membrane while the proteins are scattered in the lipid bilayer.

4. Phospholipids
The phospholipid molecules are amphipathic, having a
polar or hydrophilic head and non polar hydrophobic tail. The polar
head part of each phospholipid molecule faces out side while the
hydrophobic tail faces inward.
This arrangement forms a water resistant barrier which
allows only the lipid molecules to pass through it. The
phospholipid bilayer is 35 Aº thick. Both the layers have different
kinds of lipids. The lipid molecules may move within the
membrane providing the fluidity of the membrane. The membrane
fluidity allows the protein molecules to move along the membrane.

5.  Membrane proteins may be divided operationally into two groups.
 Integral proteins are very firmly associated with the membrane,
removable only by agents that interfere with hydrophobic interactions, such as
detergents, organic solvents, or denaturants. These are often called transmembrane
proteins. They include glycophorins.
 Peripheral proteins associate with the membrane through electrostatic
interactions and hydrogen bonding with the hydrophilic domains of integral
proteins and with the polar head groups of membrane lipids. They can be released
by relatively mild treatments that interfere with electrostatic interactions or break
hydrogen bonds; a commonly used agent is carbonate at high pH. Peripheral
proteins may serve as regulators of membrane-bound enzymes or may limit the
mobility of integral proteins by tethering them to intracellular structures. They
include spectrins.
Membrane Proteins

7. A phospholipid
bilayer acts as a
selectively permeable
barrier

8. A pure phospholipid membrane is permeable to
 Gases: oxygen, carbon dioxide
Hydrophobic molecules: benzene, glycerol
Small uncharged polar molecules: EtOH
All above molecules translocate in the process of passive
diffusion

9. Passive diffusion
Migration of molecules from the area of high
concentration to low concentration (down the gradient)
Reversible
The molecule moves through the imperfections in the
bilayer
Diffusion rate is proportional to the concentration
gradient and hydrophobicity

10. A pure phospholipid bilayer is impermeable to
Water soluble molecules
Charged molecules such as ions
Sugars
Amino acids
How do these molecules enter a cell?
Membrane proteins form special “passages” through
the membrane
 Pumps
Carriers
Channels

16. Diversity of membrane pumps
Light driven
Proton pumping by bacteriorhodopsin
ATP driven
F0F1 ATPases or F and V type pumps
E1E2 or P type pumps
ABC transporters

27. Mechanism of P-Type ATPase Action. The binding of Ca2+ and the phosphorylation of the
ATPase (steps 1 and 2), illustrated here for the Ca2+ ATPase, lead to the eversion of the
binding sites (step 3) and the release of Ca2+ to the luminal side of the membrane (step 4).
Hydrolysis of phosphoaspartate (step 5) and eversion (step 6) reset the enzyme to its initial
state.