structure and function of membranes

• compartmentalisation 


  = formation of separate membrane-bound areas in a cell.

  • vital as metabolism contains many different and incompatible reactions.
  • containing reactions in separate parts of the cell allows the specific conditions required for cellular reactions, such as chemical gradients need to be maintained, and protects vital cell components.
• plasma membrane

  = cell surface membrane which separates the cell from it's external environment.
• phospholipid bilayer

  = hydrophilic phosphate heads of the phospholipids form both the inner and outer surface of a membrane, sandwiching the fatty acid tails of the phospholipids to form a hydrophobic core inside the membrane.

  • cells normally exist in aqueous environments. Inside the cell and organelles are usually aqueous environment.
  • phospholipid bilayers are perfectly suited as membranes because the outer surfaces of the hydrophilic phosphate heads can interact with water.
• fluid- mosaic model

  = because the phospholipids are free to move within the layer relative to each other, giving the membrane flexibility, and because the proteins embedded in the bilayer vary in shape, size and position.
• membrane proteins

  = have important roles in the various functions of membranes.

  • there are 2 types of membrane proteins; intrinsic and extrinsic.
• intrinsic proteins

  = are transmembrane proteins that are embedded through both layers of a membrane. They have amino acids with hydrophobic R-group on their external surface, which interact with the hydrophobic core of the membrane, keeping them in place.
• channel proteins

  = intrinsic proteins, involved in transport across the membrane.

  • provide a hydrophilic channel that allows passive movement of polar molecules and ions down a concentration gradient through membranes.
  • held in position by interactions between the hydrophobic core of the membrane and the hydrophobic R-group on the outside of the proteins.
• carrier proteins

  = intrinsic proteins

  • have important role in both passive transport and active transport into cells. This often involves the shape of the protein changing.
• glycoproteins

  = are intrinsic proteins.

  • they are embedded in the cell-surface membrane with attached carbohydrate chains of varying lengths and shapes. Glycoproteins play a role in cell adhesion and as receptors for chemical signals.
  • when chemicals bind to the receptor, it elicits a response from the cell. May cause a direct response or set off a cascade of events inside the cell. Known as cell communication or cell signalling:
• examples of cell signalling:

  -receptors for neurotransmitters such as acetylcholine at nerve cell synapses. The binding of the neurotransmitters triggers or prevents an impulse in the next neurone.

  -receptors for peptide hormones, including insulin+ glucagon which affect the uptake and storage of glucose by cells.
• Glycolipid

  = similar to glycoproteins. They are lipids with attached carbohydrate chains. These molecules are called cell markers or antigens and can be recognised by the cells of the immune system as self or non-self.
• extrinsic proteins

  = are present in one side of the bilayer. They normally have hydrophilic R-groups on their outer surfaces and interact with the polar heads of the phospholipids or with intrinsic proteins.

  -They can be present in either layer and some move between layers.
• cholesterol

  = a lipid with a hydrophilic end and a hydrophobic end, like a phospholipid. It regulates the fluidity of membranes.

  • cholesterol molecules are positioned between phospholipid in the membrane bilayer, with the hydrophilic end interacting with the heads and the hydrophobic with the tails, pulling them together.
  • cholesterol adds stability to membranes without making them too rigid.
  • cholesterol molecules prevent the membranes becoming too solid by stopping the phospholipid molecules from grouping too closely and crystallising.