SA:VOL

• Exchange:

  examples of things that need to be interchanged between an organism and its environment include;

  • respiratory gases (oxygen and carbon dioxide)
  • nutrients (glucose, fatty acids, amino acids, vitamins, minerals)
  • excretory products (urea and carbon dioxide)
  • heat

  exchange takes place at the surface of an organism, but the materials absorbed are used by the cells that mostly make up its volume. For exchange to be effective, the exchange surface(s) of the organism must be large compared with its volume
• Surface area to volume ratio:

  • small organisms have a surface area that is large enough, compared with their volume, to allow efficient exchange across their body surface (larger SA:VOL)
  • however, as organisms become larger, their volume increases at a faster rate than their surface area
• Surface area to volume ratio:

  • simple diffusion of substances across the outer surface can only meet the needs to relatively inactive organisms (if they have a small SA:VOL)
  • even if the outer surface could supply enough of a substance, it would still take too long to reach the middle of the organism if diffusion alone was the method of transport
  • the rate at which substances can enter or leave a cell/organism by diffusion depends on the surface area. The rate at which substances are used/produced depends on the volume
• SA:VOL calculations:

  • to calculate the SA:VOL assume shapes have a uniform shape

  surface area calculations

  • squares/rectangles - length x width = area
  • triangles - 0.5 x base x height =area
  • circles - π x radius$^2$ = area (π x diameter = circumference)
  • spheres - 4 x π x radius$^2$
  • calculate the area of each side and add them together
• Volume calculations:

  • cubes/cuboids - length x width x height = volume
  • prisms - area of cross section x length = volume
  • spheres - 4/3 x π x radius$^3$ = volume
• Ratios:

  • way of comparing quantities
  • always written in simplest form
  • x:1
  • to simplify, divide each side by the smallest number
• Uncertainty:

  the amount of error that a reading/measurement might have. This can be turned into percentage error

  • readings - making a judgement of one value (reading a number from a balance)
  • measurement - judge 2 values and find the difference (measuring length with a ruler). Uncertainties need to be combined for this
• Exchange in single-celled organisms:

  • single-celled organisms, such as amoeba, are very small, giving them a large SA:VOL, giving a short diffusion pathway, so substance can diffuse quickly in/out across the cell-surface membrane
• Exchange in multi-cellular organisms:

  • in multicellular organisms, diffusion across the cell-surface membrane is too slow as some cells are deep in the body (long diffusion pathway) and, especially large animals, have a small SA:VOL
  • in order to absorb and excrete substances at a fast enough rate to stay alive, these organisms develop specialised exchange surfaces. These substances need to be transported to all cells in the organisms by mass transport (like the circulatory system in animals)
• Size, metabolic rate and heat exchange:

  • the metabolic rate of a small animal is relatively much greater than that of a large animal
  • heat is released by metabolic processes (respiration). The release of heat will be related to the mass or volume of the organism, but the rate of heat loss is determined by the surface area
  • small animals have a larger SA:VOL so lose heat faster. They need a higher metabolic rate, so they respire faster and so replace the lost heat
• Animals and metabolic rates:

  • animals with the fastest metabolic rate tend to be those that are warm blooded and move around a lot. In order to maintain this metabolic rate, they have specific exchange systems
  • animals living in extreme climates often have evolved modified body shapes to minimise or maximise heat exchange. For example, animals living in cold climates will have a large body size which is compact in shape, giving a small SA:VOL, reducing the rate of heat loss
• Animal adaptations:

  • animals have physiological or behavioural adaptations to aid exchange (kidney structure adaptations to prevent water loss in hot climates, hibernation in cold climates or spending time during the heat of the day in water)