CSIR Life Sciences Notes
UNIT- 2 [A] – Membrane structure and function
Definition of terms used in syllabus above – “Membrane structure and function”
Cell membrane or Plasma membrane
It is a thin, delicate, elastic and living boundary. Term ‘cell’ was given by C. Nageli and C. Cramer (1885).
This term is used when the cell is observed under a simple microscope. However, when this boundary was observed under an electron microscope, a clear differentiation of two layers was noticed.
Out of these, one layer was the real called plasma membrane or plasmalemma by Plower in 1931. Which was surrounded by a layer of cell cement.
The appropriate term which cannot be used for this membrane is plasma membrane. The ratio of lipid and protein may vary depending upon the cell types.
The detailed structure of the plasma membrane was studied through an electron microscope in 1950.
According to this study, following compounds are present in plasma membrane
These are amphipathic molecules having a hydrophilic head and two hydrophobic fatty acid chains in the tail.
These form the continuous structural framework of the cell membrane. Phospholipids constitute the major category of membrane lipids.
Some of these lipids are rich in unsaturated fatty acids.
The feature provides fluidity to the membrane. Phosphatidylserine, phosphatidylcholine, p-ethanol amine (cephalin) are common phospholipids of the plasmalemma . Sterols, glycolipids, and sphingolipids are also found in membranes.
The proteins of the plasmalemma are called membrane proteins. These are of two types:-
- Integral membrane protein
- Peripheral membrane protein
Integral membrane protein
- These proteins are partially or completely embedded in the plasma membrane.
- They are difficult to extract.
- These constitute about 70% of total protein content of the cell. They are also called as transmembrane or tunnel proteins or intrinsic proteins.
- They are involved in the transfer of a solute or polar molecules across the bilayer, as they act as, channel proteins. e.g. Glycophorin and porins.
Peripheral membrane protein
- These proteins are present mainly on the outer surface of the membrane.
- They are easy to extract.
- These constitute about 30% of total protein content of the membrane.
- They are responsible for cell interaction and cell recognition. e.g. Spectrin.
Oligosaccharides (sialic acid) are the main carbohydrates present in the plasmalemma. These work as key components in cell recognition.
Two types of membrane models :
Micellar or Sub-Unit Models
- Lipid Bilayer Hypothesis
- Protein-lipid-protein Hypothesis (Sandwich Models)
- Danielli and Davidson Model
- Unit Membrane Model
- Kavanau’s Lipid Pillar Model
- Models in which proteins are considered to penetrate lipid layers
- Benson’s Model
- Lanard and Singers Model
- Fluid Mosaic Model
Micellar or Sub-Unit Models
Sub-unit or Micellar model suggested by Hilleir and Hofman (1953).
- Protein Crystal Model
Active transport is the movement of the molecules against the gradient. Molecules move across the membrane from the region of lower concentration to the higher concentration.
Osmosis is a special type of diffusion of a lipid when solvent moves through a semipermeable membrane from a place of higher diffusion pressure to a place of lower diffusion pressure. In other terms, It is the migration of solvent from a hypotonic solution (of lower concentration) to the hypertonic solution (of higher concentration) through a semi-permeable membrane to keep the concentration.
Two type of osmosis:
Membrane Channels & Pumps
They are two families of biological membrane proteins.
Membrane Channels involved in the passive transport of various biological compounds across membrane barriers.
Membrane Pumps involved in active transport of various biological compounds across membrane barriers.
They are constructed from proteins that present in the membranes of cells and these channels function as pores to permit the flux of ions down their electrochemical potential gradient. Ion channels are of many different types, some of the ion channels are selective for specific types of ions such as K+ (Potassium ion), Na+ (Sodium ion), and Ca2+ (Calcium ion).