Gas Exchange and Cell Membranes

• Diffusion


  The passive movement of molecules from an area of high concentration to an area of low concentration (i.e. down a concentration gradient)
• When an equilibrium is reached on both sides of a membrane, diffusion is still taking place but there is no more net movement from one side to another
• Exchange surfaces for respiration

  • They need to be extremely thin (sometimes just one cell thick), ensuring a short diffusion pathway
  • They have a large surface area to volume ratio which provides more space for the diffusion of gases
  • The organism itself will have features which maximise the concentration gradient of gases across the exchange surface
• Adaptations of the lungs


  • Large surface area: there are approximately 700 million alveoli in our lungs with a combined surface area of 70 square meters
  • Good blood supply: lots of capillaries surround each alveolus
  • Short diffusion distance: the walls of both the alveoli and capillaries are just one cell thick
  • Moist surfaces: the liquid on the surface of alveoli dissolves gases and facilitates diffusion
  • Inhalation and exhalation: breathing in and out replaces the air in the alveoli, maintaining a steep concentration gradient
  • Location in the centre of the body: the lungs are right in the middle of our body, inside our thorax. The core of our body is the warmest, giving the gas molecules more heat energy and making them move around faster (so quicker diffusion)
• Fick's Law


  Describes how the rate of diffusion is affected by the surface area, concentration gradient of gases and the thickness of the exchange surface
• If the rate of diffusion doubles, it is because: Surface area doubles, Concentration gradient doubles, Diffusion distance halves
• Fluid mosaic model


  The structure of the plasma membrane is made up of a bilayer of phospholipids with proteins and cholesterol interspersed throughout the structure. It is 'fluid' because the phospholipids are constantly moving around and 'mosaic' because protein molecules are scattered throughout the phospholipids like tiles in a mosaic
• 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 - antigens are like little 'flags' on the surface of our cells which allows our body to detect which cells are our own and which cells are foreign
• Glycolipids


  Phospholipids with sugar molecules attached. They have a similar function to glycoproteins - they also act as recognition sites and antigens. They also increase membrane stability by forming hydrogen bonds with water molecules
• Cholesterol


  A lipid which 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
• Osmosis


  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.
• Osmosis


  • Movement of water molecules into the root hair cells of plants
• Simple diffusion


  Movement of molecules down their concentration gradients. It is a passive process which means that no energy is required.
• Simple diffusion


  • Oxygen and carbon dioxide move by simple diffusion when they pass from the alveoli into the bloodstream during gas exchange
• Facilitated diffusion


  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


  Movement of molecules against their concentration gradients (from a region of low concentration to a region of high concentration). It involves a carrier protein and uses ATP to release energy.
• Active transport


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


  The cell surrounds a substance and folds its membrane around it, causing a vesicle to form inside the cell containing the ingested substance. It is an active process so will require energy in the form of ATP.
• Endocytosis


  • Phagocytes carrying out phagocytosis, in which the phagocyte engulfs a whole bacterium in order to destroy it
• Exocytosis


  Large substances contained inside vesicles move towards the plasma membrane and fuse with it, causing the substances to either be released outside of the cell or be inserted straight into the membrane. It is an active process which requires ATP.
• Molecules can make their way across the plasma membrane in one of three ways: osmosis, diffusion or active transport. For larger substances, they will rely on endocytosis and exocytosis.
• Cell membrane structure


  Permeability of cell membranes is affected by things like temperature, pH and ethanol
• Experiment to determine effect of a factor on membrane permeability


  1. Prepare five cylinders of beetroot of equal size

  2. Rinse each piece to remove any pigment released during cutting

  3. Prepare a series of test tubes containing the same volume of water

  4. Place a single sample of beetroot into each of the five test tubes

  5. Leave for 15 minutes

  6. Use forceps to remove the pieces of beetroot from each tube

  7. Transfer coloured liquid into a cuvette

  8. Use a colorimeter to measure how much light is absorbed by each liquid
• Temperature and membrane permeability


  • At temperatures below freezing, the permeability of cell membranes increases
  • Between temperatures of 0oC and 45oC, membranes are partially permeable
  • As temperatures exceed 45oC, permeability increases rapidly
• Reason for changes in membrane permeability with temperature


  • At low temperatures, the proteins in the membrane unfold and become deformed, the phospholipids become closely packed together which makes the membrane rigid
  • As temperature increases, the components in the membrane gain kinetic energy and move around more, making the membrane more fluid
  • At high temperatures, proteins in the membrane become denatured and start to unravel, water inside the cell cytoplasm expands, putting pressure on the cell membrane and creating gaps within the bilayer
• Ethanol and membrane permeability


  • Ethanol is a non-polar solvent so it is able to dissolve non-polar substances such as lipids
  • As the ethanol concentration increases, membrane permeability will increase
  • If the ethanol concentration is high enough, enough phospholipids will dissolve to cause the plasma membrane to disintegrate completely which will kill the cell
• Formula for Fick's law:

  
• Cell membrane: