Cell membranes basics

• • Cell surface membranes surround cells.
  • They are a barrier between the cell and its environment, controlling which substances enter and leave the cell
  • They're partially permeable - substances move across the cell surface membrane by diffusion, osmosis or active transport
• • The basic structure of the cell membrane is composed of many lipids, proteins and carbohydrates
  • Fluid mosaic model (describes the arrangement of molecules)- phospholipid molecules form a continuous double layer called a bilayer.
  • The bilayer is fluid because the phospholipids are constantly moving
  • Proteins are scattered through the bilayer like tiles in a mosaic including channel proteins and carrier proteins which allow large molecules and ions to pass through the membrane
• • Receptor proteins on the cell surface membrane allow the cell to detect chemicals released from other cells
  • Some proteins are able to move sideways through the bilayer while others are in fixed positions
  • Some proteins have carbohydrates attached and are called glycoproteins
  • Some lipids also have a carbohydrate attached and are called glycolipids
  • Cholesterol molecules are present within the bilayer
• Phospholipids

  • Phospholipid molecules form a barrier to dissolved substances
  • Phospholipids have a hydrophilic head - attracts water, and a hydrophilic tail - repels water
  • The molecules atomically arrange themselves into a bilayer - the heads face outwards while tails point inwards
  • The centre of the bilayer is hydrophobic so the membrane doesn't allow water-soluble substances to diffuse through it
  • Small, non-polar substances and water can diffuse through the membrane
• Cholesterol

  • Cholesterol gives the membrane stability and fits between phospholipids
  • Cholesterol binds to the hydrophobic tails of the phospholipids, causing them to pack more closely together - this restricts the movement of the phospholipids making the membrane less fluid and more rigid
  • Cholesterol also has hydrophobic regions, so its able to create a further barrier to polar substances moving through the membrane
• Temperatures below 0

  • The phospholipids don't have much energy, so they can't move very much
  • They're packed closely together and the membranes rigid
  • Channel proteins and carrier proteins in the membrane denature increasing the permeability of the membrane
  • Ice crystals may form and pierce the membrane making it highly permeable when it thaws
• Temperatures between 0 and 45

  • The phospholipids can move around and aren't packed tightly together - the membrane is partially permeable
  • As the temperature increases the phospholipids move more because they have more energy - this increases the permeability of the membrane
• Temperatures above 45

  • The phospholipid bilayer starts to melt and the membrane becomes more permeable
  • Water inside the cell expands putting pressure on the membrane
  • Channel proteins and carrier proteins in the membrane denature so they can't control what enters or leaves the cell - this increases the permeability of the membrane