Biological Membranes

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

The plasma membrane ( sometimes called the cell surface membrane ) :

• Separates the cell's contents from its external environment.
• Regulates transport of materials into and out of the cell.
• May contain enzymes involved in specific metabolic pathways.
• Has antigens so that the organism's 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, and so is a site for cell communication or singalling, hormones and drugs may bind to memebrane-bound receptors.
• May be the site of chemical reactions.

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

• Mitochondria have folded inner membranes, called cristae. These provide a large surface area needed 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.
• Digestive Enzymes on the plasma membrane of epithelial cells that line the small intestine catalyse some of the final stages in the breakdown of certain types of sugar.

• The fabric of the membrane is the lipid bilayer made up of layers of phospholipid molecules. 
• Their hydrophillic heads are in contact with the watery environment. The hydrophobic tail regions are in the centre of the membrane away from the water.
• Some of the proteins have pores and act as channels to allow ions, which are surrounded by water molecules, to pass through.
• Some proteins are carriers and, by changing their shape, carry specific molecules across the membrane. 
• Other proteins may be attached to the carrier proteins and function as enzymes, antigens or receptor sites for complementary-shaped signalling chemicals such as hormones.
• Eukaryotic cell membranes contain cholesterol, which is important for helping to regulate the fluidity of the membrane, maintain mechanical stability and resist the effects of temperature changes on the structure of the membrane.
• Outside the membrane is the glycocalyx - formed from the carbohydrate chains attached to either glycolipids or glycoproteins in the membrane.