The need for exchange surfaces

• what is the equation for the surface area of a cube where the length is x?

  $6x2$
• what is the equation for the volume of a cube where the length is x?

  $x3$
• what is the equation for the surface area of a sphere with the radius of r?

  $4pir2$
• what is the equation for the volume of a sphere?

  $4/3 pi r 3$
• what is the equation for the surface area of a cylinder?

  $2pirh +$$2pir2$
• what is the equation for the volume of a cylinder?

  $pir2h$
• how does SA to V ratio change as a shape gets bigger?

  • as the size of the object increases, the volume will increase more rapidly than the surface area, and the ratio will decrease
• what are biological implications of the SA to V ratio changing depending on the size of the organism?

  • a high ratio between the surface area and volume will allow efficient substance exchange
  • increasing the ratio will result in more rapid heating and cooling of an organism
• how does diffusion distance affect the rate of diffusion?

  • longer distance - slower rate
  • shorter distance - faster rate
• how does surface area affect rate of diffusion?

  • larger surface area - faster rate
  • smaller surface area - slower rate
• how does movement affect rate of diffusion?

  • more vibrations (more kinetic energy) of molecules - faster rate
• how does concentration difference affect rate of diffusion?

  • steeper concentration gradient - faster rate
• how does temperature affect rate of diffusion?

  • high temp (more kinetic energy) - faster rate
  • lower temp (less kinetic energy) - slower rate
• how does size of molecule affect rate of diffusion?

  • larger molecule - slower rate
  • smaller molecule - faster rate (most don't need assistance from transport proteins)
• how does increased surface make exchange efficient?

  • provides area needed for exchange and overcomes the limitations of the SA:V ratio of larger organisms e.g. root hair cells, villi in small intestine
• how does thin layers make exchange efficient?

  • distance that substances have to diffuse are shirt, making process fast and efficient e.g. alveoli in lungs, villi in small intestine
• how does a good blood supply make exchange efficient?

  • stepper the concentration gradient, the faster the diffusion
  • ensures substances are constantly delivered to and removed from exchange surface e.g. alveoli, villi, fish gills
• how does ventilation to maintain diffusion gradient make exchange efficient?

  • for gases - helps maintain concentration gradients
  • increases rate of exchange
  • e.g. alveoli, fish gills where ventilation means a flow of water carrying dissolved gases
• how are villi adapted for efficient exchange?

  • short diffusion path (thin)
  • larger surface area (to volume ratio)
  • good blood supply
  • good transport system to maintain a steep diffusion gradient
• for the experiment using agar stained cubes in hydrochloric acid what is the IV, DV and CV?

  • IV - size of cubes (SA/V ratio)
  • DV - time taken to decolourise (rate 100/t)
  • CV - concentration of hydrochloric acid, type of agar, temperature, concentration of indicator in the agar
• what are limitations of the phenolphthalein experiment?

  • colour change is subjective to the person carrying out experiment
  • if multiple tests aren't done, an anomaly could happen
• how do these limitations affect the results?

  • if same person doesn't carry out test, results wouldn't be repeatable
  • difficult to compare results between different groups, reproducibility is low
  • don't know if results are anomalous or not (if no repeats are done)
• how would you improve the method to overcome these limitation?

  • use same person to time when colour change has occurred for each repeat
  • use a colour chart for comparisons (use colorimeter)
  • repeat, disregard anomalies, calculate mean
• how do you convert the measurements of time to rate?

  • rate = mm/s
  • = distance (mm) / time (s)
  • time (s) = distance / rate
• a smaller surface area to volume ratio of agar cube means there is a greater distance for the acid to penetrate to the centre, so rate will be slower
• what are the implications of the results (SA:V ratio) to living organisms?

  larger organisms (small SA:V) will need an exchange surface and/or transport system to overcome the limitation of a small SA:V
• how would you modify the method of investigating rate of diffusion in agar cubes, to investigate temperature?

  • keep cube dimensions the same, and put the acid/cubes in a thermostatically controlled water bath at a range of temperatures (e.g. 0 to 80 celcius)
• what is counter-current flow?

  • substances flowing side by side in different directions, taking substances from one another (a concentration gradient allows this)
  • increases efficiency of exchange - don't reach same concentration, maintaining gradient