Membrane Transport Mechanism



1)Cross membrane movement of small molecules:

• Diffusion(passive and facilitated)
• Active transport

2)Cross membrane movement of large  molecules:

• endocytosis 
• exocytosis

3)Signal transmission across the membrane:

cell surface receptors

• signal transduction
• signal internalization 

movement of intracellular receptors

4)intercellular contact and communication

Simple Diffusion:

• it's a passive movement across the membrane that does not  require any metabolic energy
• it's not carrier mediated
• it occurs down the electrochemical gradient(high conc. to low conc.)

Factors affecting the rate of Diffusion:

1) Its concentration gradient across the membrane.Solutes move from high to low concentration.

(2) The electrical potential across the membrane.Solutes move toward the solution that has the opposite charge. The inside of the cell usually has a negative charge.

(3) The permeability coefficient of the substance for the membrane. 

(4) The hydrostatic pressure gradient across the membrane. Increased pressure will increase the rate and force of the collision between the molecules and the membrane. 

(5) Temperature. Increased temperature will increase particle motion and thus increase the frequency of collisions between external particles and the membrane. In addition, a multitudeof channels exist in membranes that route the entry of ions into cells.

(6)Membrane thickness decreases the diffusion thickness.

Facilitated diffusion:

• it occurs down the electrochemical gradient(high conc. to low conc.) similar to simple diffusion.
• does not require metabolic energy so it is a passive transport.
• it's more rapid than simple diffusion
• it is a carrier mediated that require a carrier protein to tranport a molecule.

example:glucose transport in muscle and adipose tissues.

Active transport:

It is a kind of transport in which energy is required in the form of ATP.

a)Primary active transport:when ATP is used directly at the site of transportation then it's called primary active transport.e.g Na+-K+ ATPase and Ca+-ATPase present in endoplasmic reticulum.

b)Secondary active transport: 

The transport of two or more solutes is coupled.

One of the solutes (usually Na⁺) is transported “downhill” and provides energy for the “uphill” transport of the other solute(s).

 Metabolic energy is not provided directly, but indirectly from the Na⁺ gradient that is maintained across cell membranes.

If the solutes move in the same direction across the cell membrane, it is called cotransport, or symport.

• Examples are Na⁺-glucose cotransport in the small intestine and Na⁺-K⁺-2Cl^- cotransport in the renal thick ascending limb.

If the solutes move in opposite directions across the cell membranes, it is called countertransport, exchange, or antiport.

• Examples are Na⁺-Ca²⁺ exchange and Na⁺-H⁺ exchange.

Vesicle transport mechanism:

a)Endocytosis:it is the cell intake of extracellular vesicle.it may be

• Phagocytosis: it is the process of engulfing by cell.
• Pinocytosis:the substances ingested are in solution.it is the drinking if extracellular fluid.
• Clathrin-mediated endocytosis: occurs at membrane indentations where the protein clathrin accumulates.

b)Exocytosis:Vesicles containing material for export are targeted to the cell membrane.it is usually Ca+ dependent.