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Cells and Microscopy

Movement of molecules across membranes

  • 3 things to know
  • Components 01, 02 and 03

Overview

Recall questions on simple and facilitated diffusion, channel and carrier proteins, active transport and respiratory inhibitors, endocytosis, phagocytosis, pinocytosis and exocytosis, surface area to volume ratio in agar cubes, and investigating membrane permeability with beetroot and a colorimeter.

What you need to know

Answer each one before you reveal it.

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Key term

Why does the rate of facilitated diffusion level off at high concentrations when simple diffusion does not?

Key term

Outline how beetroot can be used to investigate the effect of temperature on membrane permeability.

Key term

What is the difference between a channel protein and a carrier protein?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Define diffusion.

    Model answer

    Diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of their random motion.

  2. 2

    Name two molecules that cross the plasma membrane by simple diffusion and explain why they can.

    Model answer

    Oxygen and carbon dioxide cross by simple diffusion because they are small and non-polar, so they dissolve in the hydrophobic core of the bilayer and pass straight through.

  3. 3

    What is facilitated diffusion?

    Model answer

    It is the passive movement of a substance down its concentration gradient through a specific channel or carrier protein in the membrane, without the use of ATP.

  4. 4

    What is the difference between a channel protein and a carrier protein?

    Model answer

    A channel protein forms a water-filled pore that is selective for a particular ion and may be gated, whereas a carrier protein binds the solute and changes shape to release it on the other side.

  5. 5

    Why does the rate of facilitated diffusion level off at high concentrations when simple diffusion does not?

    Model answer

    Facilitated diffusion depends on a limited number of transport proteins, so once every one of them is occupied the rate cannot increase further, whereas simple diffusion is limited only by the concentration gradient and the area of bilayer available.

  6. 6

    Define active transport.

    Model answer

    Active transport is the movement of a substance across a membrane against its concentration gradient by a carrier protein, using ATP as the immediate source of energy.

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