Membranes also contain cholesterol. Like unsaturated fatty acids, cholesterol disturbs the close packing of phospholipids and keeps them more fluid. This can be important for organisms living at low temperatures when membranes can solidify. Cholesterol also increases flexibility and stability of membranes.
Channel proteins and carrier proteins are involved in the selective transport of polar molecules and ions across the membrane. Proteins sometimes act as enzymes, for example the microvilli on epithelial cells lining some parts of the gut contain digestive enzymes in their cell surface membranes.
Glycoproteins are proteins with branching carbohydrate side chains like 'antennae'. There is an enormous number of possible shapes to these side chains, so each type of cell can have its own specific markers. This enables cells to recognise other cells, and behave in an organised way, for example during development of tissues and organs in multicellular organisms. It also means that foreign antigens can be recognised and attacked by the immune system.
Glycolipids also have branching carbohydrate side chains and are involved in cell-cell recognition. They may act as receptor sites for chemical signals. With glycoproteins they are also involved in sticking the correct cells together in tissues.
The cell membrane also have several functions which include: osmosis, diffusion, active transport and exocytosis and endocytosis. Besides specialised cell membranes are the sites of important chemical reactions such as photosynthesis and respiration. For example photosynthesis takes place on the thylakoid membranes.
Osmosis is the diffusion of water across a membrane. Water molecules can diffuse freely across a membrane, but always down their concentration gradient, so water therefore diffuses from a dilute to a concentrated solution.
Diffusion is the passage of substances fram a region of high concentration to a region of lower concentration. This occurs along an electrochemical gradient. This mechanism is passive since it does not need energy to occur. A substance might also be allowed through the membrane by a specific molecule. This is known as facilitated diffusion and can occur through a channel protein or a carrier protein.
Active transport is an energy consuming transport of molecules across a membrane against a concentration gradient. An example of active transport is shown by the sodium/potassium pump. The sodium/potassium pump is a protein which serves to maintain the right concentrations of sodium and potassium in and out of the cells. The pump operates to maintain the concentrations of sodium and potassium in such a way that it replaces 2 potassium ions for every 3 sodium ions found in the cell. Secondary active transport does not use ATP directly but takes advantage of a previously existing concentration gradient but still can move materials against the concentration gradient. For example a glucose molecule bind with a sodium ion to cross the cell membrane.
Endocytosis is the transport of materials into a cell. Materials are enclosed by a fold of the cell membrane, which then pinches shut to form a closed vesicle. Exocytosis is the transport of materials out of a cell. It is the exact reverse of endocytosis. Materials to be exported must first be enclosed in a membrane vesicle.
