4 Cell-surface membrane

• Hydrophilic heads - point to the outside and are attracted to water
• Hydrophobic tails - point to the inside and are hydrophobic
• Lipid soluble material moves through the membrane via this phospholipid bilayer
• Prevents water-soluble substances from entering and leaving the cell
• Makes the membrane flexible and self-healing

• Some proteins do not span the entire bilayer. They either act as mechanical support to the membrane or in conjunction with glycolipids they act as cell receptors for molecules such as hormones
• Other proteins span the entire bilayer
• They can be channel proteins which are water-filled tubes to allow water-soluble ions to diffuse across the membrane
• They can be carrier proteins that bind to ions or molecules such as glucose and amino acids which changes the shape of the carrier protein allowing the molecule to pass across the membrane
• Provide structural support
• Act as channels
• Allow active transport
• Form cell-surface receptors to identify other cells and for hormones to attach
• Adhesion

• They add strength to membranes
• Very hydrophobic
• Prevents water loss and dissolved ions
• Pull together the fatty acid tails limiting their movement without making the membrane too rigid
• reduces lateral movement of phospholipids
• MAkes the membrane less fluid at high temperatures

Glycolipids

• Made up of a carbohydrate covalently bonded to a lipid
• The carbohydrate extends out away from the bilayer into the watery environment around the cells
• Acts as a cell-surface receptor for specific chemicals e.g. the ABO blood system is a result of glycolipids
• Acts as a recognition site
• Maintains the stability of the membrane
• Helps cells to attach to one another to be able to form tissues

Glycoproteins

• Carbohydrate chains are attached to extrinsic proteins
• Act as cell-surface receptors for hormones and neurotransmitters
• Helps cells to form tissues
• Allows cells to recognise other cells such as lymphocytes being able to recognise self-cells

• Controls the movement of substances in and out of the cell
• Most molecules do not freely move through the membrane because
  • Not soluble in lipids to pass through the bilayer
  • Too large to pass through the channels
  • Of the same charge as the protein which will repel the molecule
  • Electrically charged (polar) and therefore cannot pass through the non-polar hydrophobic tails in the bilayer