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M.Prasad Naidu

MSc Medical Biochemistry, Ph.D,.

Plasma Membrane

Separates different parts of cell , So that cellular activities are compartmentalised.

Highly selective permeability

Fluid Mosaic Model – Plasma membrane

Fluid Mosaic Model

  • Phospholipids are arranged in bilayers with polar head groups oriented towards extra cellular or cytoplasmic side with a hydrophobic core .

  • Made up of carbohydrates - glycoproteins and glycolipids

  • Phospholipids are most common lipids

    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.

Membrane proteins

1.Integral proteins :

Rhodopsin of retinal cell

Receptors for hormones,growth factors

Immunoglobulins on cell membrane of leukocytes

2.Peripheral proteins:

Secretory proteins

Peptide hormones

Transfer of molecules across membrane

Small molecules

Passive Transport:

It is the movement of molecules across the cell that doesn’t require expenditure of energy.

They are of 3 types.

Simple diffusion

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

  • Steepness of concentration gradient

  • Temperature – Increased temperature will increase particle motion and increased diffusion.

  • Charge – presence of oppositely charged particles on the other side will increase diffusion. More toward the solution that has opposite charge. The inside of the cell usually has a negative charge.

  • Diameter of diffusing molecule – smaller the molecule faster the diffusion

  • Rate of diffusion of lipophilic substances is directly proportional to its lipid solubility and diffusion coefficient.

  • Fatty acids , steroids diffuse rapidly but water soluble substances sugars, inorganic ions diffuse very slowly.

  • Simple diffusion not saturable.

    eg: Diffusion of Pentoses , gases like O2, N2, CO2, NO.(Nitric oxide)

Facilitated diffusion

Facilitated diffusion / Transport

It is the passive movement of molecules down the concentration gradient with the aid of a transport protein ( carrier protein)

  • Also called passive mediated transport

  • Present only in eukaryotes

  • No need of energy.

  • Transport proteins undergo a conformational change (shape change) so that the molecule can pass through the membrane by Ping pong mechanism.

Facilitated diffusion is saturable

Factors determining facilitated diffusion

  • Concentration gradient across membrane

  • Amount of carrier protein available

  • Rapidity of solute carrier interaction.

    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.

Active transport

Transport of moleculesagainst concentration gradient with expenditure of energy.

Requires transporters

Operates Unidirectionally

Saturable at higher concentration of solute.25 to 30% of total energy requirements in a cell is used for active transport system.

Active transport


Primary Active transport – Requires energy directlyeg: Na+ pump.

Secondary active transport – Requires energy


eg: glucose transport into intestinal mucosal cell

Sodium Potassium Pump

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.

  • Extracellular side

    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

Functions of Sodium Potassium Pump

  • Control cell volume

  • Renders nerve and muscle cells electrically excitable.

  • Active transport of aminoacids and sugar

Clinical Aspect

Sodium potassium ATPase is inhibited by digitalis (digoxin) which increases force of contraction of heart muscle by altering the excitability.

OUABAIN is another inhibitor

Transport systems

Symport :

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 .

Glucose permeases:

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.

1.Eg:polysaccharides,proteins,polynucleotides etc.

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 .

eg: collagen

3. They can enter extracellular fluid and signal other cells . Eg: Insulin , parathormone etc

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