Gas exchange

• Plasma membrane structure


  Bilayer of phospholipids with proteins and cholesterol interspersed throughout

  
• Fluid mosaic model


  Describes the arrangement of molecules in the membrane - 'fluid' because the phospholipids are constantly moving around and 'mosaic' because protein molecules are scattered throughout the phospholipids like tiles in a mosaic
• The fluid mosaic model is the best representation of membrane structure based on the evidence which is currently available
• As we learn more about the structure of the plasma membrane, the fluid mosaic model may be updated
• Components of the plasma membrane

  • Phospholipids
  • Glycoproteins
  • Glycolipids
  • Cholesterol
  • Intrinsic proteins
  • Extrinsic proteins

  
• Phospholipids


  Consist of a hydrophilic head group which faces the intracellular / extracellular fluid and two hydrophobic tails which point towards each other, away from water. They are the main component of the plasma membrane and form a barrier to anything which is not lipid-soluble (such as ions and glucose)
• Glycoproteins


  Proteins with sugar molecules attached. They act as recognition sites and antigens
• Glycolipids


  Phospholipids with sugar molecules attached. They also act as recognition sites and antigens, and increase membrane stability by forming hydrogen bonds with water molecules
• Cholesterol


  Slots in between the phospholipid tails, pushing them closer together. It regulates the stability and fluidity of the plasma membrane
• Intrinsic proteins


  Proteins which span both bilayers of the plasma membrane. They act as channels or carrier proteins to transport water-soluble molecules
• Extrinsic proteins


  Proteins which are found on the surface of the plasma membrane. They usually function as enzymes and catalyse chemical reactions inside the cell
• Ways molecules can cross the plasma membrane

  • Osmosis
  • Diffusion
  • Active transport
• Osmosis


  The movement of water molecules down its concentration gradient across a partially permeable membrane. It is a passive process so does not require energy in the form of ATP
• Simple diffusion


  The movement of molecules down their concentration gradients, passing directly through the phospholipid bilayer. It is a passive process which means that no energy is required
• Facilitated diffusion


  The movement of molecules down their concentration gradients, with the help of a carrier protein or a channel protein within the cell membrane
• Facilitated diffusion


  Movement of glucose molecules into liver cells through glucose transporter proteins embedded in the plasma membrane
• Active transport


  When molecules move against their concentration gradients, they do so by active transport using a carrier protein which carries the molecule from one side of the membrane to the other, using ATP
• Active transport


  • Transport of glucose from the villi of the intestine into the bloodstream
• Endocytosis


  Cell surrounds the substance and folds its membrane around it, causing a vesicle to form inside the cell containing the ingested substance, requires ATP
• Endocytosis


  • Phagocytes carrying out phagocytosis, engulfing a whole bacterium to destroy it
• Exocytosis


  Substances contained inside vesicles move towards the plasma membrane and fuse with it, releasing the substances outside the cell or inserting them into the membrane, requires ATP
• Active transport process


  Vesicle fuses with cell surface membrane, releasing contents outside the cell
• The permeability of cell membranes is affected by things like temperature, pH and ethanol
• Experiment to determine effect of temperature on membrane permeability

  Prepare beetroot samples, water baths of varying temperatures, place beetroot in test tubes in water baths, measure light absorption of coloured liquid to determine membrane permeability