Carbohydrates

Carbohydrates are found on the outer surface and are mainly attached to membrane proteins although sometimes they can also be attached to phospholipids.

• Proteins attached to carbohydrates are known as glycoproteins.
• Phospholipids attached to carbohydrates are known as glycolipids.

Cell transport

Certain substances are able to move across the semi-permeable membrane in either direction. This is achieved either by:

• passive transport (which relies on kinetic energy)
• active transport (which requires ATP).

Diffusion

Simple diffusionor lipid diffusion is a form of passive transport. Substances move down their concentration gradient until the concentrations are the same on each side of the membrane or in equilibrium.

Very small hydrophilic molecules, such as oxygen, hydrogen and carbon dioxide, can move across the cell membrane by diffusion. The cell cannot switch diffusion on or off.

Surface area can affect the rate at which diffusion takes place. Microvilli, found in the small intestine, are an extension of the plasma membrane. By increasing the surface area they increase the rate at which diffusion can take place.

The difference in substance concentration also has an effect. If there’s a much higher concentration of a substance on one side of the membrane then diffusion will start off rapidly and then begin to subside as the concentrations begin to reach equilibrium on both sides.

Thickness makes a difference too. A capillary, for example, has a very thin membrane (only one cell thick) so that substance can diffuse quickly across it.

The rate at which a substance can diffuse can be given by Fick’s law:

rate of diffusion