membrane structure

• Functions of membranes

  • Divides cell into compartments
  • Partially permeable
  • Site for biochemical reactions(may hold enzymes / ribosomes /electron carriers)
  • Cell recognition/signalling
  • Allows cell to change shape
• Compartmentalisation

  • the formation of separate membrane bound areas in a cell
  • Vital to cell as metabolism involves many different and often incompatible reactions
  • different parts of cell can have different

  optimum conditions (e.g. chemical gradients)
• Fluid mosaic model

  • Fluid- phospholipids are free to move within the layer relative to each other (flexibility)
  • Mosaic- proteins embedded throughout vary in size, shape and position
• Phospholipid bilayer

  • Hydrophilic polar phosphate heads interact with water
  • Hydrophobic non-polar fatty acid tails form the core
• Intrinsic/integral proteins

  • Transmembrane proteins that are embedded in both layers
  • Have amino acids with hydrophobic R-groups on their external surfaces which interact with hydrophobic core to keep them in place
  • Channel proteins = provide a hydrophilic channel that allows passive movement of polar molecules/ions down a concentration gradient
  • Carrier proteins = role in passive and active transport into cells often involves shape of protein changing
• Extrinsic/peripheral proteins

  • Present on one side of bilayer
  • Normally have hydrophilic R-groups on their outer surfaces and interact with polar heads of phospholipids or with intrinsic proteins
  • act as receptors for hormones/neurotransmitters or involved in cell recognition
  • Extrinsic proteins on the inner side can dissociate from the membrane and take part in cell signalling or reactions
• Glycoproteins

  • Type of intrinsic proteins
  • Embedded in plasma membrane with a carbohydrate (sugar) attached
  • Antigens
  • Recognition as self
  • Cell signalling
  • Receptor for hormone/drug
  • Cell adhesion
• Glycolipid

  • Lipids with carbohydrate (sugar) attached
  • Called cell markers or antigens and can be recognised by immune system as ‘self’ or ‘non-self’
• Cholesterol

  • Lipid with a hydrophilic end and hydrophobic end
  • Regulates plasma membrane’s fluidity
  • Cholesterol’s hydrophilic end interacts with phospholipid’s head and cholesterol’s hydrophobic end interacts with phospholipid’s tails, pulling them together
  • prevent membranes becoming solid, by stopping the phospholipid molecules from getting too close and crystallising
• membrane fluidity

  • unsaturated fatty acids are compressed, the ‘kinks’ in their tails maintain some space between the phospholipid molecules.
  • This “elbow room” of the double bonds help to maintain fluidity
• Factors affecting membrane structure

  1. Temperature

  • Increase in temp= increases phospholipid’s kinetic energy
  • phospholipids move more and membrane loses structure
  • eventually membrane willcompletely break down
  • Loss of structure= increases permeability
  • carrier/channel proteins will be denatured
• Factors affecting membrane structure

  2. Solvents

  • Water (polar solvent) keeps bilayer intact- phosphate heads interact with water
  • Many organic solvents are less polar (e.g. alcohols) or not polar (e.g.benzene)- organic solvents will dissolve membranes
  • Lower concentrations of alcohols cause damage by disrupting membrane (disruption makes membrane more permeable)
  • Detergents also act in the same way by binding to lipids