Membrane structure and function

• Compartmentalisation


  Formation of separate membrane-bound areas in a cell.
• Enzymes have to be in particular positions for a chemical reactions to take place.
• Plasma membrane


  Cell surface membrane which separates the cell from its external environment.
• Singer and Nicholson: Fluid mosaic model, membrane is described as fluid and allowing proteins of various sizes and structures to be embedded in the membrane.
• Plasma membrane contains various proteins and lipids- the type and number of which are particular to each cell type.
• Intrinsic proteins(Integral proteins)


  Embedded through both layers of the membrane.

  Hydrophobic R group on the external surface which interact with the hydrophobic core of the membrane, keeping them in place.
• Extrinsic proteins(Peripheral proteins)


  Present on one side of the bilayer.

  Hydrophilic R group on outer surface which interact with the polar heads of the phospholipids.

  Can be present in either layer and some move between layers.
• Channel proteins(IP)


  Provide hydrophilic channel that allows passive movement of polar molecules and ions down a concentration gradient. Held in place by hydrophobic and hydrophilic interactions.
• Carrier proteins (IP)


  Important in passive transport and active transport in cells. Active transport often involves changes in protein shape.
• Glycolipids


  Lipids with attached carbohydrate chain called cell markers or antigens. They can be recognised by cells of the immune system as self (of the organism) or non-self (cell belonging to another organism).
• Cholesterol


  A lipid with a hydrophilic end and a hydrophobic end.

  Regulates the fluidity of membranes and is positioned between phospholipids in a membrane bilayer.

  Hydrophilic end interacts with the phosphate heads and hydrophobic end interacts with tails; pulling them together. Therefore adding stability.

  Cholesterol prevents the membrane from becoming too solid by preventing the phospholipids from grouping too closely and crystallising.
• Glycoproteins


  Intrinsic proteins with attached carbohydrate chains.

  Play a role in cell adhesion (when cells join together to form tight junctions in certain tissues) and receptors for chemical signalling.
• Phospholipid bilayer

  Hydrophilic phosphate heads form both the inner and outer surface of the membrane. They sandwich the hydrophobic fatty acid tails of the phospholipid to form a hydrophobic core.
• Cells normally exist in aqueous environments which allows the hydrophilic phosphate heads to interact with water.
• Glycoproteins: Cell signalling/communication

  When the chemical binds to the receptor, it elicits a response from the cell. This may cause a direct response or set off a cascade of events inside the cell.
• Glycoproteins: Cell signalling examples

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

  Receptors for peptide hormones, like insulin and glucagon, which affect the uptake and storage of glucose by cells.

  Some drugs act by binding to receptors (e.g. beta blockers).
• Site of chemical reactions

  Proteins in the membranes forming organelles or present within organelles have to be in particular positions for a reaction to take place.

  For example: the electron carriers and the enzyme ATP synthase have to be in correct positions within the cristae for ATP production in respiration. The enzymes of photosynthesis are found on the membrane sacks within the chloroplasts.