Membrane Transport- Pumps and Carriers

Second last! w00t!

This lecture doesn't have a "lecture objectives" slide, so I'll just list the dot points in the summary and expand on each one as much as I can.

1. Pores, channels and transporters increase permeability.

As I mentioned in my previous post, some substances do not diffuse through the membrane readily and thus require pores/channels or transporters to carry them through.

2. Transporters can be passive carriers or active pumps. 4. Primary active transport use an ATPase. 5. Secondary active transport uses a co-transporter to move one thing down its concentration gradient and another up its gradient.

Transporters, or carrier proteins, work by binding a substance to one side, then changing conformation to release the substance on the other side. If the substance is being carried down its concentration gradient, this process requires no energy (as the energy is provided by the concentration gradient).

Some carriers carry multiple substances. These are called symports, if they carry substances in the same direction, or antiports, if they carry substances in opposite directions. Carriers can carry substances against their concentration gradient if they are also carrying another substance down its concentration gradient. In this way the energetically unfavourable movement of a molecule up its concentration gradient is coupled to the "favourable" movement of a molecule down its concentration gradient. This is called secondary active transport.

Active pumps carry substances up their concentration gradients, but unlike the other passive carriers mentioned above, they require hydrolysis of ATP to do so. This is called primary active transport.

3. Transporters show saturation.

Transporters, unlike channels, can only transport a certain number of molecules at a time (as they must bind and change shape). There are also only a certain number of transporters in a cell's membrane. Hence, once all of the transporters are occupied, no more molecules can be transported across at the same time. When all of the transporters are occupied and the rate of transport ceases to increase, the transporters are said to be saturated.

6. Active transport keeps the contents of cells far from equilibrium.

As active transport can transport molecules up their concentration gradients, they can create a situation in which there is a large concentration of a molecule on one side of the membrane and a low concentration on the other. This is the case in the Na+/K+ pump: active transport keeps most of the Na+ outside the cell and most of the K+ inside the cell. This creates concentration gradients which can be used to drive other processes in the cell, such as the transport of glucose into the cell via the Na+/Glucose symport.