MSc Medical Biochemistry, Ph.D,.
Separates different parts of cell , So that cellular activities are compartmentalised.
Highly selective permeability
Fluid Mosaic Model
which are amphipathic nature.
Lipidbilayer shows free lateral movement of its components, hence it is fluid in nature.
Fluidity depends on the cholesterol content as well as nature of fatty acids.
1.Integral proteins :
Rhodopsin of retinal cell
Receptors for hormones,growth factors
Immunoglobulins on cell membrane of leukocytes
Transfer of molecules across membrane
It is the movement of molecules across the cell that doesn’t require expenditure of energy.
They are of 3 types.
It is the passive movement of molecules from area of high concentration to an area of low concentration.
Factors affecting net diffusion of a substance
eg: Diffusion of Pentoses , gases like O2, N2, CO2, NO.(Nitric oxide)
Facilitated diffusion / Transport
It is the passive movement of molecules down the concentration gradient with the aid of a transport protein ( carrier protein)
Facilitated diffusion is saturable
Factors determining facilitated diffusion
eg: Glucose entry from intestinal cell to blood via GLUT2.
It is the passage of WATER across a membrane from an area of higher to lower concentration.
Osmosis is a diffusion but it specifically refers to the H2O molecules. Water passes from hypotonic to hypertonic solution across membrane.
Transport of moleculesagainst concentration gradient with expenditure of energy.
Saturable at higher concentration of solute.25 to 30% of total energy requirements in a cell is used for active transport system.
Primary Active transport – Requires energy directlyeg: Na+ pump.
Secondary active transport – Requires energy
eg: glucose transport into intestinal mucosal cell
The Sodium Potassium ATPase is an integral protein and requires phospholipids for its activity.
This system is an oligomeric protein containing 2α subunits(larger) and 2βsubunits (smaller).
β subunits are glycoproteins.
Binding sites for Na+ & ATP on the inside (cytoplasmic side) and for K+ on the outside.
βαα Cytosolic side
For each ATP hydrolysed 3 Na+ move out of cell and 2 K+ enter into the cell.
Note: In nerve and muscle 60 – 70% of energy
synthesized is used for this pump.
In resting cells 35% of energy maybe used.
Kidney cortex contain more activity of sodium potassium
Na+ - K+ Pump
Sodium potassium ATPase is inhibited by digitalis (digoxin) which increases force of contraction of heart muscle by altering the excitability.
OUABAIN is another inhibitor
A cotransport system in which transporter carries two solutes in same direction across membrane.
eg: sodium glucose transport into mucosal cell (useful in ORT).
Sodium amino acid transporters in mammalian cells..
A co-transport system in which two solutes are transported in opposite directions, simultaneously.
eg: Na+ - K+ pump.
Cl- - Hco3 exchange in RBC membrane.
Mitochondria contains several antiport mechanisms
1. ATP / ADP
2.Aspartate / Glutamate
3. Phosphate / OH-
4. Malate / Phosphate
Involves transport of a single solute through the membrane bi-directionally .
Present in mammalian cell membrane. six members have been identified, named as GLU T1, GLU T2 etc
These permeases are active with D-glucose
Transport of large molecules
Receptor mediated endocytosis
Molecules are transported into the cell by the in
folding of cell membrane around the substance
& internalising it.
2.Also helps in regulating the a number of hormone receptors
Endocytosis requires ATP, Calcium in ECF, and contractile elements in cell.(microfilaments).
Phagocytosis ---Cell eating
is a process by which bacteria , viruses ,dead tissue etc are engulfed by leukocytes
Pinocytosis - cell drinking
cellular uptake of fluid and fluid contents.
eg: Protein molecules.
Receptor mediated endocytosis
Substance to be transported bind with a receptor which is then engulfed by endocytosis.
eg: Transport of LDL Cholesterol.
Also responsible for some diseases
1. Viral (eg: Hepatic cells)
2. Poliomyelitis (affecting motor neuron)
3. AIDS (affecting T Cells)
Most cells release macro molecules to the exterior by exocytosis.
This process helps in membrane remodelling when the components synthesized in golgi are carried in vesicles to the Plasma membrane.
Calcium triggers exocytosis
Molecules released by exocytosis may
1.Attach to cell surface and become peripheral proteins. eg : antigens
2. Become part of extracellular matrix .
3. They can enter extracellular fluid and signal other cells . Eg: Insulin , parathormone etc