Topic 8: Exchange and Transport

• All organisms must take in substances they need from the environment and get rid of any waste products:

  • Cells need O2 for aerobic respiration which produces CO2 as a waste product (gases move by diffusion between cells/environment)
  • Water is taken up by cells through osmosis and dissolved food molecules/mineral ions (product of digestion - glucose/amino acids) diffuse along with it
  • Urea (waste product of mineral ions/amino acids) diffuses from cells to the blood plasma for removal from the body via kidneys
• The smaller the substance, the larger the surface area to volume ratio is meaning its easier to exchange substances (SA:V = SA/V)
• Unicellular Organisms - gases/dissolved substances diffuse directly in and out of the cell across the membrane (large surface area means enough substances supply the volume)
• Multicellular organisms - smaller surface area (not enough substances to supply entire volume using only outside surfaces) so exchange surface and mass transport system adapted to maximise effectiveness
• Gases are exchanged in the lungs by diffusion in the alveoli:

  • transfers O2 to the blood and removes CO2
  • millions of alveoli to maximise gas exchange efficiency
  • blood arriving contains lots of CO2 and little O2 which maximised the concentration gradient of diffusion for both
  • O2 diffuses out the air in alveoli (high concentration) into blood (low concentration) and CO2 diffuses in the opposite direction
• Alveoli Adaptations:

  • moist lining to dissolve gases
  • good blood supply to maintain concentration gradients (many capillaries)
  • very thin walls to minimise distance (one cell thick)
  • enormous surface area (75m2)
• The rate of diffusion of any substance is affected by 3 factors:

  • Distance - diffuse more quickly when they haven't as far to move
  • Concentration Gradient (difference) - diffuse faster if there is a big difference in concentration between the areas they're diffusing from and to
  • Surface Area - the more surface area available, the faster molecules move across
• Fick's Law shows how the rate of diffusion is proportional meaning it doubles when surface area/concentration gradient doubles or distance halves.
• The heart, blood and blood vessels work together to deliver oxygen and remove carbon dioxide to/from all the cells in the body as the circulatory system
• Arteries - carry blood away from the heart

  • heart pumps blood out at high pressure so the artery walls are strong and elastic
  • walls are thick compared to the size of the hole down the middle (lumen)
  • contain thick layers of strong muscle and elastic fibres that can stretch/spring back
• Capillaries - involved in the exchange of materials at tissues

  • arteries branch into capillaries
  • very tiny and narrow so they can carry blood really close to every cell to exchange substances
  • permeable walls are one cell thick to increase diffusion by decreasing distance
  • supplies food/oxygen and takes away waste like CO2
• Veins - carry blood to the heart

  • capillaries join up to form veins
  • low pressure blood so walls don't need to be thick
  • bigger lumen helps blood flow despite lower pressure
  • valves help keep blood flowing in one direction
• Blood is made of multiple cells:

  • erythrocytes (red blood cells)
  • phagocytes (white blood cell type)
  • lymphocytes (white blood cell type)
  • platelets
  • plasma
• Red Blood Cells (Erythrocytes) - carry oxygen from the lungs to all the cells in the body

  • biconcave disc shape allows large surface area to absorb O2
  • no nucleus to maximise space for O2
  • red pigment called haemoglobin contains iron
  • haemoglobin binds to O2 to become oxyhaemoglobin in the lungs the reverses to release O2 into cells
• White Blood Cells - part of the immune system and has 2 types

  • phagocytes can change shape to engulf unwelcome microorganisms (phagocytosis)
  • lymphocytes produce antibodies against microorganisms
  • lymphocytes produce antitoxins to neutralise any toxins released
  • multiply to fight infection so a blood test will show a high concentration
• Platelets - clot blood to prevent blood loss and microorganisms entering

  • small fragments of cells with no nucleus
  • lack of platelets can cause excessive bleeding and bruising
• Plasma - a pale straw coloured liquid that carries everything in the blood

  • nutrients like glucose and amino acids
  • hormones and urea
  • proteins and CO2
  • antibodies and antitoxins
  • red/white blood cells and platelets
• Mammals have a double circulatory system meaning the heart pumps blood around the body in two circuits:

  1. deoxygenated blood is pumped to the lungs to take in O2 then returns as oxygenated blood
  2. oxygenated blood is pumped to all the other organs to deliver O2 then returns as deoxygenated blood
• Fish have a single circulatory system so deoxygenated blood is pumped right around the body where O2 is picked up in the gills
• The mammalian heart has 4 chambers and 4 major blood vessels:

  1. Vena cava sends deoxygenated blood from the body to the right atrium
  2. Deoxygenated blood moved to the right ventricle which pumps it through the pulmonary artery to the lungs
  3. Oxygenated blood then moves through the pulmonary vein from the lungs to the left atrium
  4. Oxygenated blood then moves to the left ventricle where it is pumped to the entire body through the aorta
• The left ventricle has a much thicker wall than the right ventricle because it needs more muscle to pump blood at high pressure to the whole body not just to the lungs.

  Valves prevent backflow in the heart
• Respiration - the process of transferring and releasing energy from the breakdown of organic compounds like glucose
• Energy is used for things like:

  • Metabolic Processes - making larger molecules from smaller ones (proteins from amino acids)
  • Contracting Muscles - in animals
  • Maintaining a steady Body Temperature - in mammals and birds
• As energy is transferred to the environment, respiration is an exothermic reaction and some is transferred by heat.
• Aerobic Respiration - happens when plenty of O2 is available

  • most efficient way to transfer energy from glucose
  • happens constantly in plants and animals

  glucose + oxygen ----> carbon dioxide + water
• Anaerobic Respiration (animals) - without oxygen and transfers much less energy so is less efficient

  • vigorous exercise means there's a lack of O2 despite heart/breathing rate increasing
  • glucose partially breaks down, producing lactic acid
  • lactic acid builds up in muscles, leading to pain and cramps

  glucose ----> lactic acid
• Anaerobic Respiration (plants) - similar to animals but produce ethanol and CO2 instead of lactic acid

  • fungi like yeast does this

  glucose ----> ethanol + carbon dioxide
• By measuring the amount of oxygen consumed by small organisms in a given time, you can calculate their rate of respiration. A respirometer measures the effect of temperature on the rate
• Respiration Practical: (1)

  1. Add soda lime granules to 2 test tubes to absorb CO2 (corrosive so wear goggles and gloves)
  2. Place cotton wool on top of the granules then place woodlice in the test tube and glass beads with the same mass in the control tube (cotton separates woodlice from soda lime granules to be ethical)
  3. Set up the respirometer then use the syringe to set manometer fluid to a known level
  4. Place apparatus in a 15'C water bath for a set period of time
• Respiration Practical: (2)

  5. Decreases in air volume reduces the pressure in the tube, causing the coloured liquid in the manometer to move towards the test tube

  6. Distance moved by liquid in a given time can help calculate the volume of oxygen taken in by the woodlice per minute

  7. Repeat experiment with the water bath at different temperatures to see how changing temperature effects the rate

  8. Don't leave the woodlice in the respirometer for long so they don't die as it's unethical