Cell surface membrane structure

• A simplified structure of the cell surface membrane:

  A) Hydrophilic phosphate head

  B) Hydrophobic hydrocarbon tails

  C) Phospholipid bilayer

  
• Proteins are embedded in the cell surface membrane, in the phospholipid bilayer
• Some proteins occur on the surface of the bilayer and never extend completely across it. They act as mechanical support or as cell receptors for molecules such as hormones
• Protein channels and carrier proteins span across the whole phospholipid bilayer
• Protein channels form water filled tubes to allow water soluble ions to diffuse across the membrane
• Carrier proteins bind to ions or molecules like glucose and amino acids, then change shape to move these molecules across the membrane
• Cholesterol adds strength to the membranes. They are also very hydrophobic and so prevent the loss of water and dissolved ions from the cell
• Cholesterol pulls the fatty acid tails of the phospholipid molecules together, limiting their movement but without making the membrane too rigid
• Glycolipids are made of a carbohydrate bonded with a lipid
• Glycolipids have the carbohydrate part of them extend from the phospholipid bilayer into the watery environment outside of the cell, and acts are a receptor for specific chemicals (eg: the human ABO blood system operates as a result of this)
• In glycoproteins, carbohydrate chains are attached to many extrinsic proteins on the outer surface of the cell membrane
• Glycoproteins also act as cell surface receptors for hormones and neurotransmitters