Use the fluid-mosaic model to explain appropriate properties of plasma membranes.

Receptors enable the cell to detect hormonal messages;

Enzymes aid chemical reactions outside the cell;

Protein channels facilitate diffusion across the membrane;

Carrier proteins are involved in facilitated diffusion and active transport;

The phospholipid bilayer is semi-permeable so lets water and other substances diffuse in and out.

The role of carrier proteins and protein channels in facilitated diffusion.

Facilitated diffusion is similar to normal diffusion in that molecules are moving from an area of high concentration to an area of low concentration and they are both passive, this means they require no energy. Facilitated diffusion is faster and for molecules that are too big to fit through the plasma membrane, or can't get through because of their charge. Glucose and amino acids both diffuse through the cell membrane in this way.

Intrinsic proteins in the fluid mosaic transport molecules from one side to the other:

Carrier proteins chemically bond to a specific molecule, when they do this they change shape, the new shape enables the molecule to be released on the other side of the membrane;

Protein channels are passages through the membrane which molecules can pass through if they are the right size to fit through or if they have the right charge to be attracted. Some have gate like mechanisms, by which the protein will allow molecules through as a reaction to stimuli specific of molecule it wants to carry.

Diffusion is the passive movement of substances down a concentration gradient. Surface area, difference in concentration and the thickness of the exchange surface affect the rate of diffusion.

Substances will move from an area of high concentration to an area of low concentration (diffusion.) This often happens when two different environments are connected by a membrane e.g. inside a cell and outside a cell.

A bigger surface area increases the rate of diffusion because there are more opportunities for substances to move across a membrane.

The bigger the difference in concentration the faster the diffusion.

Thickness of the exchange surface affects how far substances have to travel to diffuse into a space: the thicker the surface the longer it takes.