Quick questions on Transport across cell membranes: diffusion, osmosis and active transport - AQA A-Level Biology

Active transport. The movement of molecules against their concentration gradient (low to high) using carrier proteins and ATP from respiration. The carrier binds the molecule, ATP is hydrolysed, and the protein changes shape to move it across.

The net movement of particles from a high to a low concentration, down a concentration gradient. Small, non-polar molecules (oxygen, carbon dioxide) cross the bilayer directly.

Diffusion of larger or charged molecules (glucose, ions) down their gradient through membrane proteins, because they cannot cross the lipid bilayer:

The net movement of water molecules from a region of higher water potential to a region of lower water potential, across a partially permeable membrane, through the bilayer and through aquaporin channel proteins.

The movement of molecules against their concentration gradient (low to high) using carrier proteins and ATP from respiration. The carrier binds the molecule, ATP is hydrolysed, and the protein changes shape to move it across.

Two substances are moved across together by one carrier protein, with the gradient of one driving the other. The classic example is glucose absorption in the ileum:

Distinguish between facilitated diffusion and active transport. [3 marks]

Explain why the rate of facilitated diffusion reaches a plateau as concentration gradient increases. [2 marks]

A cell of water potential -500 kPa is placed in a solution of water potential -800 kPa. Predict and explain the net movement of water. [2 marks]