Question

for each method below describe a specific example of how it was used in the physiology part.

1. active transport

2. facillitated transport

3. exocytosis

Answer 1

1) ACTIVE TRANSPORT:

Active transport refers to the mechanism of transport of substances against the chemical for electrical gradient.active transport involves expenditure of energy which is liberated by The breakdown of ATP. Ionic substances such as Na, K, Ca, Cl and non ionic substances like Glucose, amino acids, urea are transported actively across the cell membrane. It is of two types:-

Primary Active Transport

Secondary Active Transport

Primary Active Transport: The energy is derived directly from the breakdown of ATP. Examples are Sodium Potassium pump, calcium pump, potassium hydrogen pump.

Sodium Potassium pump:- it is present in all cells of the body. It is involved with the active transport of sodium ions outwards through the cell membrane and potassium ions inwards simultaneously. This pump is responsible for maintaining a negative electrical potential inside the cells. Structure: the carrier protein involved in Sodium Potassium pump is a complex consisting of to separate protein units. A larger alpha subunit and a smaller beta subunit. the alpha subunit is mainly concerned with Sodium Potassium transport. It has three intracellular sites, one each for binding sodium ions (3Na) and ATP and phosphorylation site. Two extracellular sites, one each for binding potassium ions 2K and ouabain. Mechanism: the enzyme ATPase is activated when three sodium ions and 1 ATP molecule bind to their respective binding sites. The activated ATPase hydrolyze ATP to ADP and a high energy Phosphate bond of energy is released. The energy liberated is believed to cause a conformational change in the carrier protein molecule extruding sodium into the extracellular fluid. This is followed by binding of two potassium ions to the receptor site on the extracellular surface of carrier protein and dephosphorylation of subunit which returns to its previous conformation release in potassium into cytoplasm. Functions: Control cell volume-- When the pump fails, the cell swells up and burst. Create electrical potential across the cell membrane.

Secondary Active Transport:- The energy is derived secondarily from the energy which has been stored in the form of ionic concentration differences between the the two sides of membrane created in the first place by primary active transport. Example:

Sodium co-transport: the glucose is transported into most cells against large concentration gradient. It is transported with sodium with the help of a carrier protein. The carrier protein undergoes conformational change only when both Sodium and glucose are attached to it. Hence they are transported simultaneously. It occurs during absorption of glucose from the intestine into blood.

Sodium Counter Transport:- the carrier protein involved here acts as an antiport. Example: Sodium Hydrogen Counter Transport in proximal tubules of kidney where Na moves inside and H moves outside the cell.

2) FACILITATED DIFFUSION: the water soluble substances like glucose and amino acids have large size therefore they cannot diffuse through the protein channels by simple diffusion. They are transported with the help of some proteins called carrier proteins. Mechanism: a conformational change occurs in the carrier protein after the molecule to be transported is bound at the receptor site. Repetitive spontaneous configurational changes allow the diffusion of molecule. Types of carrier proteins: Uniport (carrier protein transports only one type of molecules), Symport (transport of one substance is linked with the transfer of another substance, eg facilitated diffusion of Glucose in renal tubules is linked with transport of sodium), Antiport (the carrier proteins exchange one substance for another, eg Na-K exchange in renal tubules).

3) EXOCYTOSIS: it is the process by which substances are expelled from the cell without passing through the cell membrane. The substances which are to be expelled are collected in the form of vesicles which move towards the cell membrane. The vesicular membrane then fuses with the cell membrane. The area of fusion breaks down releasing the contents to the exterior and leaving the cell membrane intact. Release of hormones and enzymes by secretory cells of the body occurs by EXOCYTOSIS. It requires calcium and energy along with docking proteins.