cell membranes

• All cells and organelles are surrounded by a partially permeable membrane composed of phospholipids with protein molecules between them
• The main function of the cell membrane is controlling the movement of substances in and out of the cell/organelle
• The fluidity of the membrane and the mosaic arrangement of the protein give the structure of the membrane its name – fluid mosaic model
• Structure and functions of the cell membrane:

  • Glycoproteins: recognition sites, act as antigens
  • Phospholipids: form a bilayer, make membrane fluid, have non-polar tails and hydrophilic heads
  • Cholesterol: waterproof the membrane, control stability, have hydrophilic heads and hydrophobic tails
  • Intrinsic proteins: pass through membranes, some form channels or carriers for water soluble molecules
  • Extrinsic proteins: found on the surface only, some act as enzymes
  • Glycolipids: short carbohydrate chains that help make membranes stable by forming hydrogen bonds with H2O, help cells attach to one another
• Cell signalling:

  • Specific ligands are released from the cell and transported to the target cell where they bind to specific receptors on the cell surface membrane
  • This produces a response which may cause a cascade of more reactions
• Three factors affecting the permeability of a cell membrane: heat, ethanol, pH
• Movement across membranes:

  • Diffusion: passive movement of small, non-polar lipid soluble molecules from high to low concentration through the phospholipid bilayer
  • Facilitated diffusion: requires a channel protein to transport polar molecules, charged and water soluble molecules across the membrane
  • Osmosis: net diffusion of water molecules from low to high solute concentration through a partially permeable membrane
  • Active transport: transports molecules through carrier proteins from low to high concentration, requires energy in the form of ATP
  • Cytosis: a form of active transport where parts of the plasma membrane form infoldings or outfoldings, two types are exocytosis and endocytosis
• The rate of gas exchange by diffusion becomes more rapid as:

  • Surface area of the surface increases
  • Diffusion distance decreases
  • Diffusion gradient becomes steeper
• Water potential is the pressure exerted by water molecules that are free to move in a system, measured in kPa