Transport across membranes

• Substances can move across the cell membrane by:

  Simple diffusion

  Facilitated diffusion

  Osmosis

  Active transport
• Diffusion

  Movement of molecules from a region of higher concentration to a region of lower concentration

  Doesn’t require an external source of energy.

  Occurs in lungs, kidneys and intestines.
• Facilitated Diffusion

  Involves the movement of molecules from a region of higher concentration to a region of lower concentration.

  Requires carrier proteins or channel proteins.

  Carrier proteins change shape to move substances across a membrane.

  Channel proteins provide a space so large substances can move across the membrane.

  Doesn’t require an external source of energy.

  Occurs across plasma membranes to transport glucose and amino acids from the bloodstream into cells.
• Osmosis

  Movement of water molecules from a region of low concentration to high concentration across a semi-permeable membrane.

  It doesn’t require an external energy source.

  Occurs in the small intestine.
• Active Transport

  Low to high concentration (against the gradient)

  Requires carrier proteins and energy provided by the hydrolysis (break down) of ATP.

  Occurs in the internal lining of the small intestine.
• Co-Transport

  Involves the movement of one substance down it’s concentration gradient, bringing another substance with it. This second substance moves against its own concentration gradient.

  This process requires energy in the form of ATP in order to set up the concentration gradient for the first substance by moving molecules.
• At best, diffusion only results in the concentrations on either side of the intestine becoming equal. This means that not all the available glucose and amino acids from a person’s diet can be absorbed using diffusion. In order to maximise the amount of glucose and amino acids that can be absorbed, active transport is also used, making this an example of co-transport
• Cells can be adapted for rapid and efficient transport across their internal or external membrane by having a large surface area (many folds or microvilli). They may also have large numbers of protein channels and carrier proteins.