L1 - specialised exchange surfaces

• Fick’s Law: the rate at which a substance can diffuse
• Fick’s Law formula: $\frac{surface\ area\ x\ concentration\ difference}{dis\tan ce}$
• Things that affect gas exchange rate:

  -area available for diffusion

  -distance over which diffusion occurs

  -concentration gradient

  -speed at which molecules diffuse through membranes
• The surface area to volume ratio is very important in biology, because the larger the ratio is, the more surface there is for exchanges
• Single-celled organisms have a high SA:V ratio which allows for be exchange of substances to occur via simple diffusion:

  -the large surface area allows for maximum absorption of nutrients and gases and secretion of waste products

  -the small volume means the diffusion distance to all organelles is short
• As organisms increase in size their SA:V ratio decreases:

  -there is less surface area for the absorption of nutrients and gases and secretion of waste products

  -the greater volume results in a longer diffusion distance to the cells and tissues of the organism
• Formulae to remember:

  -circumference is $2\pi r$

  -area of a circle is $\pi r^2$

  -surface area of a cuboid is $2\left(bh+bl+hl\right)$

  -volume of a cuboid is $hbl$
• Formulae given in exam:

  -surface area of a cylinder is $2\pi r\left(r+l\right)$

  -volume of a cylinder is $\pi r^2l$

  -surface area of a sphere is $4\pi r^2$

  -volume of a sphere is $\frac{4}{3}\pi r^3$
• Surface area to volume ratio is calculated by dividing the surface area by the volume
• Multicellular organisms have special exchange surfaces to increase the rate of diffusion

  -they are too large to rely upon difusión for gas exchange. they must also have specialised tissues to perform this function
• Multicellular cannot solely rely on diffusion for survival and so usually require a transport system. this due to a few different factors:

  -their size means only outer cells will receive nutrients

  -their SA:V ratio is low because tissues and structural support are required to give strength, adding to the size and decreasing the ratio

  -their levels of activity; active animals need energy from food to all cells which is not possible via diffusion
• Features of a good exchange surface:

  -large surface area

  -thin walls

  -moist surfaces

  -good blood supply
• Large surface area: the larger the area over which diffusion can occur, the faster it will occur
• Thin walls: the shorter the distance for diffusion to occur the faster it will occur
• Moist surfaces: gases can dissolve before diffusing
• Good blood supply: as soon as the required substance has diffused across it is removed. this maintains the concentration gradient, the higher the concentration gradient, the faster the rate of diffusion
• A flat worm has a flattened shape so no cell is ever far from the surface
• A fish has a specialised gas exchange surface with large areas to increase the surface area to volume ratio. it has gills and a transport system to move substances around the body
• A human has a specialised gas exchange surface with large areas to increase the surface area to volume ratio. the lungs have alveoli and a transport system to move substances around the body
• Alveoli feature present to enable efficient exchange:

  -walls are one cell thick

  -walls are made of flattened squamous epithelium cells reducing diffusion distance

  -capillaries are in close contact with the walls

  -breathing movements of lungs ventilate the lungs replacing used air with fresh air (diffusion gradient)
• How alveoli features are important in the function of the exchange surface:

  -ventilation maintains a high oxygen concentration and low carbon dioxide

  -a good blood supply ensures carbon dioxide levels are higher than in the alveoli

  -thin walls ensure fast diffusion

  -wall is permeable to oxygen and carbon dioxide, allowing gas exchange
• Root hair cell feature present to enable efficient exchange:

  -a root hair that increase surface area
• How root hair cell features are important in the function of the surface:

  -rate of water uptake by osmosis is greater because of larger surface area
• Fill features present to enable efficient exchange:

  -have a large surface area

  -have a rich blood supply

  -have a thin outer layer of cells

  -water moves over the gills continuously
• How fill features are importing in the function of the exchange surface:

  -water flow over gills maintains high concentration gradient

  -good blood supply ensures carbon dioxide concentration is higher than in the water

  -thin gill wall ensures quick oxygen diffusion

  -wall is permeable to oxygen and carbon dioxide, allowing gas exchange