Surface area to volume ratio and gas exchange

• Explain why large, multicellular organisms need specialised exchange surfaces and transport systems?

  • large organisms have smaller surface area to volume ratios.
  • they cannot rely on diffusion across their surface alone to supply sufficient amounts of oxygen to all of their cells
• explain how exchange surfaces are specially adapted to facilitate gas exchange

  • exchange surfaces have large surface areas, large concentration gradients and a short diffusion distance for example smaller mammals have larger surface area : volume rations than larger mammals therefore lose heat more rapidly. to maintain their body temperature they have a higher metabolic rate which releases heat energy
• describe gas exchange in single celled organisms

  • oxygen is required to produce ATP during aerobic respiration, and carbon dioxide is produced as a waste product
  • this produces a concentration gradient of these gases
  • single celled organisms rely on diffusion to exchange of oxygen and CO2 down their concentration gradient
• define how insects limit water loss

  • the most effective surface area for gas exchange is a large, thin and permeable area. unfortunately this may cause fatal dehydration from evaporation
  • therefore insects must balance water loss and gas exchange, to limit water loss insects have:
  • waterproof covering over their body surfaces. this is usually a rigid outer skeleton covered with a waterproof cuticle
  • small surface area to volume ratio to minimise the area over which water is lost
• define gas exchange in fish

  -like the lungs, the gills have a large surface are for gas exchange, water moves in through the mouth and out through the gills

  • the mouth opens, increasing the volume and so decreases the pressure inside the mouth
  • water moves into the mouth down a pressure gradient
  • the mouth then shuts, decreasing the volume and so increasing the pressure
  • water is forced over the gills down a pressure gradient
  • oxygen diffuses into the blood in the gills
• define gas exchange in dicotylendonous plants

  -Diffusion of CO2

  1. mesophyll cells photosynthesise, using CO2, which reduces the concentration of CO2 in cells
  2. CO2 diffuses from the air spaces into the cells, down a concentration gradient
  3. this reduces the CO2 concentration in the air space causing CO2 to diffuse into the air spaces from the outside of the leaf, through the stomata, down a concentration gradient
• describe gas exchange in humans

  • oxygen enters the mouth down a pressure gradient, travels down the trachea into the lungs
  • oxygen diffuses across the alveolar epithelium
  • then through the capillary endothelium
  • and then combines with haemoglobin in red blood cell
• define gas exchange in insects

  -

  1. oxygen enters the insect through spiracles and into the trachea. spiracles close
  2. oxygen diffuses through the trachea into the tracheoles down a concentration gradient
  3. oxygen is delivered directly to the tissue to be used for aerobic respiration

  • note that carbon dioxide produced by respiring tissues moves in the opposite direction and leaves the insect when spiracles open
• describe the adaptations of xerophytic plants to limit water loss

  1. reduced numbers of stomata
  2. stomata in pits
  3. hairs to trap water
  4. rolled leaves
  5. leaves reduced to spines - reduced surface area for evaporation
  6. thick waxy cuticles - waterproof

  points 2,3 and 4 reduce water loss by increasing humidity, this reduces the water potential gradient between inside the leaf and outside the environment
• describe how lung function can be measured

  tidal volume - volume of air in one breath

  ventilation rate - number of breaths per minute (15-18 pm for healthy)

  forced expiratory volume - maximum volume of air that can be breathed out in 1 second

  forced vital capacity - maximum volume of air it is possible to breathe forcefully out of the lungs after a really deep breath
• describe ficks law

  ficks law state that:

  diffusion rate is proportional to -

  $\frac{surface\ area\times difference\ in\ concentration}{diffusion\ dis\tan ce\ }$
• what is countercurrent flow in fish gas exchange

  • the position of the filament and lamellae means that blood and water flow in opposite directions - countercurrent flow
  • water and blood flow in opposite directions
  • this maintains the concentration gradient of oxygen so the oxygen concentration is always higher in the water than the blood along the whole length of the gill lamellae
  • this allows diffusion of oxygen to occur along the whole length of lamellae
• why diffusion happens in the gas exchange system of insects

  -oxygen

  1. tissues aerobically respire using oxygen, which reduces the concentration of oxygen at tissue
  2. oxygen diffuses from an area of high concentration to an area of low concentration at the tissue
  3. this lowers the oxygen concentration in the trachea so oxygen diffuses into the trachea from outside the insect via the spiracles

  the opposite for co2
• what are the adaptations of a leaf for gaseous exchange

  
  

  1. flat - gives large surface area
  2. many stomata - pores to allow air to move in and out of leaf
  3. air spaces in leaf so short diffusion distance between mesophyll cells and air
• define gas exchange in dicotylendonous plants


  -Diffusion of O2

  1. mesophyll cells produce O2 in photosynthesis and this increases the concentration of O2 in cells
  2. O2 diffuses into the air spaces from cells, down a concentration gradient
  3. this increases the concentration of O2 in the air spaces, causing O2 to diffuse from the air spaces to outside the leaf via the stomata, down a concentration gradient
• rate of diffusion equation

  $rate\ of\ diffusion\ =$$\ \frac{surface\ area\ \times concentration\ gradient}{diffusion\ dist}$