Unit 8

• Purpose of exchange


  All living things must have metabolism to survive. To carry out metabolic processes, we need to take in substances from the environment and excrete waste substances to the environment.
• Substances needed by cells

  • Glucose
  • Oxygen
  • Water
• Waste substances that need to be excreted

  • Urea
  • Carbon dioxide
  • Excess water
  • Excess mineral ions
• Unicellular organisms


  • Can simply use diffusion to exchange enough substances to supply their entire volume
• Multicellular organisms


  • Cannot simply use diffusion, require transport systems to move substances
• Main transport systems in humans

  • Respiratory system (lungs)
  • Circulatory system (heart)
• Fick's law


  Equation that combines the rate of diffusion with surface area, concentration gradient, and membrane thickness
• Factors affecting rate of diffusion

  • Surface area
  • Concentration gradient
  • Membrane thickness
• Gas exchange


  Transfer of oxygen from air into blood and carbon dioxide from blood to air
• Lungs


  • Contain millions of air sacs called alveoli
  • Alveoli are surrounded by a network of capillaries
• Adaptations of alveoli and capillaries

  • Epithelial cells are very thin and flattened
  • Alveoli are folded to increase surface area
  • Blood flows constantly in capillaries to maintain steep concentration gradient
• Components of blood


  • Erythrocytes (red blood cells)
  • Leukocytes (white blood cells)
  • Platelets
  • Plasma
• Erythrocytes


  • Transport oxygen around the body
  • Biconcave disc shape increases surface area to volume ratio
  • No nucleus allows more space for haemoglobin
• Leukocytes


  • Important in immune response
  • Phagocytes engulf pathogens
  • Lymphocytes produce antibodies
• Platelets


  Aid blood clotting at site of wound
• Plasma


  Liquid that carries components of blood
• Blood vessels


  • Arteries
  • Capillaries
  • Veins
• Arteries


  • Thick muscular walls with elastic fibres
  • Narrow lumen to maintain high blood pressure
• Veins


  • Thinner muscular walls
  • Larger lumen
  • Contain valves to prevent backflow
• Capillaries


  • Very small lumen so erythrocytes move one-at-a-time
  • Thin walls to reduce diffusion distance
• Single circulatory system


  Deoxygenated blood goes through heart, then to gills to absorb oxygen, then to body
• Double circulatory system


  Heart pumps blood in two circuits: 1) Deoxygenated blood to lungs, 2) Oxygenated blood to body
• Cardiac cycle


  1. Deoxygenated blood flows into right atrium

  2. Atria contract, blood flows into ventricles

  3. Ventricles contract, blood flows into pulmonary artery and aorta

  4. Ventricles relax (diastole)

  5. Blood flows to lungs and body
• Heart


  • Left ventricle wall is thicker than right ventricle
  • Septum separates deoxygenated and oxygenated blood
• Cardiac output


  Total volume of blood pumped by a ventricle to an artery every minute
• Respiration


  Process of releasing energy from breakdown of organic compounds, usually glucose
• Metabolic processes powered by respiration

  • Synthesis of organic polymers
  • Muscle contraction
  • Thermoregulation
  • Active transport
  • Maintenance, repair and division of cells
• Aerobic respiration


  Uses oxygen, most efficient type of respiration
• Anaerobic respiration


  Does not use oxygen, partially breaks down glucose into lactic acid
• Anaerobic respiration in plants and fungi produces ethanol and carbon dioxide instead of lactic acid