Transport across membranes

Describe the co-transport of two particles across a membrane against their concentration gradients.

To start, a transport protein binds the first particle and a molecule of ATP.

The transport protein hydrolyses ATP into one molecule of ADP and one phosphate ion. This causes the transport protein to change shape, forcing the first particle across the membrane.

The shape change reveals a second binding site in the protein, to which the second particle binds.

The phosphate ion is released, triggering the protein to revert to its original shape. This forces the second particle across the membrane in the opposite direction.

The phospholipid bilayer has a nonpolar core made of hydrophobic fatty acid tails.

This permits small, non-polar, uncharged and hydrophobic particles to cross the membrane by simple diffusion, but blocks the diffusion of large, polar, charged and hydrophilic particles. (max 2 marks)

Transport proteins allow large, polar, charged and hydrophilic particles to cross the membrane by facilitated diffusion.

Transport proteins control which of these particles diffuse across the membrane by opening and closing in response to signals, such as chemical messengers. (max 2 marks)

The rate of passive transport depends on the surface area to volume ratio, the length of the diffusion pathway, how steep the concentration gradient is, and the number of transport proteins in a given area (facilitated diffusion only). (max 2 marks)

Transport proteins can actively transport particles across the membrane against their concentration gradient, using energy from ATP. (1 mark)

Some transport proteins can co-transport two particles at the same time, using energy from ATP or the passive transport of one particle. (1 mark)

Only particles with a shape that is complementary to the structure of the binding site will be transported by the protein. (1 mark)