Biological membranes

Cell membranes are partially permeable barriers

Because cell membranes form a barrier and separate the cell contents from the cells exterior environment or separate organelles from the cytoplasm, they need to allow some molecules through, into or out of the cell. Some organelles also have membranes within them and these form barriers as well.

Permeability refers to the ability to let substances pass through.

• Some very small molecules simply diffuse through the cell membrane in between its structural molecules.
• Some substances dissolve in the lipid layer and pass through
• Other substances pass through the special protein channels or are carried by carrier proteins.

Because the membranes do not let all types of molecule pass through them. They are described as partially permeable barriers. The properties of the component molecules of the cell membrane determine its permeability.

Roles of membranes:

At the surface of cells - the membrane is called the plasma membrane (sometimes the cell surface membrane).

• Separates the cells components from its external environment (in single called organisms the environment is the external surroundings; in multicellular organisms the environment is the tissue or fluid of cells surrounding it)
• Regulates transport of materials into and out of the cell
• May contain enzymes involved in specific metabolic pathways
• Has antigens so that the organisms immune system can recognise the cell as being self and not attack it
• May release chemicals that signal to other cells
• Contains receptors for such chemical signals so is a site for cell communication or signalling - hormone and drugs may bind to membrane bound receptors)
• May be the site of chemical reactions

Within cells

The membranes around many organelles present in eukaryotic cells separate the organelle contents from the cell cytoplasm, so that each organelle is a discrete entity and able to perform its function. In some organelles, metabolic processes occur on membranes.

• Mitochondria have folded inner membranes - cristae. These give a large surface area for some of the reactions of aerobic respiration and localise some of the enzymes needed for respiration to occur.
• The inner membranes of chloroplasts, called thylakoid membranes, house chlorophyll. On these membranes some of the reactions of photosynthesis occur.
• There are some digestive enzymes on the plasma membranes of epithelial cells that line the small intestine, and these enzymes catalyse some of the final stages in the breakdown of certain types of sugars.