Lecture 2 - Ion transport across membranes

Question 1

What is the nernst potential and what are the gradients found across biological membranes?

Answer 1

The Nernst potential is the transmembrane voltage (electrical potential) at which transmembrane ion movement is at equilibrium for a given ion concentration gradient.

Membrane voltage (electrical gradient) exists across biological membranes due to the selective permeability of the membrane and ion concentration gradients.

Both ion concentration (chemical) and electrical gradients influence the electrochemical gradient which is quantified and used to establish whether an ion is moving passively or actively across the membrane.

Question 2

What are primary pumps?

Answer 2

Primary pumps use a source of energy such as ATP to pump ion against their electrochemical gradients. They also are responsible for establishing the electrochemical gradients for driver ions.

Question 3

What are carriers?

Answer 3

There are two main types of carrier protein. Those that carry out secondary active transport and others that simply allow facilitated diffusion:

1. Some carriers can carry out secondary active transport where a driver ion is transported down its electrochemical gradient which allows a second ion to be transported up its respective electrochemical gradient. 
	○ Symporters or co-transporters are carriers where both ions are moved in the same direction
	○ Antiporters or counter-transporters are carriers where the ions are transported in opposite directions across the membrane
 Secondary active transport can be electrogenic or electroneutral

2. Some carriers are facilitators (uniporters) which allow facilitated diffusion passively down electro chemical gradients or chemical gradients if the molecule has no charge.

Question 4

What are channels?

Answer 4

Channel protein transport is always passive down electrochemical gradients. It is usually highly regulated by defined open and shut kinetics referred to as gating. It is also usually selective for specific ions.

Pumps generate drier ion gradients which are dissipated by carriers and channels. Clearly the rate at which he driver ion gradient can be dissipated by carriers and channels is greater than their generation with pumps. This is why the abundance of pumps is much greater