Transport across membranes

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Diffusion is the net passive movement of particles (atoms, ions or

molecules) from a region in which they are in higher concentration to

regions of lower concentration. It continues until the concentration of

substances is uniform throughout.

Osmosis is a special example of diffusion. It is the diffusion of

water through a partially permeable membrane from a more

dilute solution to a more concentrated solution – down the

water potential gradient)

Note: diffusion and osmosis are both passive, i.e. energy from

ATP is not used.

A partially permeable membrane is a barrier that permits the

passage of some substances but not others; it allows the

passage of the solvent molecules but not some of the larger

solute molecules.

Cell membranes are described as selectively permeable

because not only do they allow the passage of water but also allow the passage of certain solutes.

The presence of particular solutes stimulates the membrane to open specific channels or trigger

active transport mechanisms to allow the passage of those chemicals across the membrane.

Active transport is the energy-demanding transfer of a substance across a cell membrane against its

concentration gradient, i.e., from lower concentration to higher concentration.

Special proteins within the cell membrane act as specific protein ‘carriers’.

The energy for active transport comes from ATP generated by respiration (in mitochondria).