Students should be able to: explain the adaptations of specialised cells…

The idea

The rate of movement across membranes depends on three variables. Surface area: more membrane allows more particles to cross per unit time. External membranes may have microvilli (e.g. ileum), while internal membranes like mitochondrial cristae are folded mainly to increase surface area for the electron transport chain and ATP synthase, though they also increase membrane area generally. Number of channel or carrier proteins: channel proteins provide routes for facilitated diffusion of charged or polar solutes, while carrier proteins mediate facilitated diffusion and active transport (pumps). Rate rises with protein number until all are occupied (saturation). Steepness of the gradient: a greater concentration or water potential difference increases net movement rate. A blood supply carrying away absorbed glucose maintains a steep gradient.

What earns the marks

• A greater surface area of internal or external membranes increases the rate of transport, because more membrane is available for movement.
• More channel proteins increase the rate of facilitated diffusion, and more carrier proteins increase the rate of facilitated diffusion or active transport.
• The rate plateaus once all carrier proteins are saturated, because they are occupied and cannot work faster.
• A steeper concentration or water potential gradient increases the rate of net movement.
• Folded internal membranes, such as cristae, increase membrane surface area; their main role is to support the electron transport chain and ATP synthase, so use them as a transport example only with that qualification.

Common mistakes and how to put them right

• Answering a contrast question with facts about only one process.

  Put it right: Both sides of the difference are needed.

• Saying the rate rises indefinitely with concentration in facilitated diffusion.

  Put it right: It plateaus once carrier proteins are saturated.

• Describing an adaptation without connecting it to a rate.

  Put it right: State the mechanism by which the adaptation changes the rate.

• Ignoring internal membranes when asked about cellular adaptations for transport.

  Put it right: Include cristae or other folded internal membranes, qualified by their main role.

• Stating that channel proteins carry out active transport.

  Put it right: Active transport requires carrier proteins (pumps); channel proteins only allow facilitated diffusion.

Exam tips

• If a graph plateaus for carrier-mediated transport, name the limiting factor: all carrier proteins are occupied.
• Check what the stem has already ruled out, since questions often say 'other than surface area'.