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Supplying the cell - OCR GatewayCell models

For an organism to function, substances must move into and out of cells. Three processes contribute to this movement - diffusion, osmosis and active transport.

Part of Combined ScienceScaling up

Cell models

It's straightforward to model the cells of organisms using cubes. By doing this we can easily see how the surface area to volume ratio changes as organisms increase in size.

It’s straightforward to model cells using cubes.

We can investigate the effect of increasing size on surface area to volume ratios using models based on cubes:

A table showing the volume of ratios

So, as the volume increases, the surface area does not increase at the same rate.

If a graph is drawn:

So, as the volume increases, the surface area does not increase at the same rate. If a graph is drawn:

Question

What is the surface area to volume ratio of the highlighted mark?

In the below table scientists have estimated the surface area:volume ratios of various organisms.

OrganismSurface area in square metresVolume in cube metresSurface area:volume
Bacterium6 × 10−121 × 10−186,000,000:1
Blow fly6 × 10−41 × 10−6600:1
Whale6 × 1041 × 1060.06
OrganismBacterium
Surface area in square metres6 × 10−12
Volume in cube metres1 × 10−18
Surface area:volume6,000,000:1
OrganismBlow fly
Surface area in square metres6 × 10−4
Volume in cube metres1 × 10−6
Surface area:volume600:1
OrganismWhale
Surface area in square metres6 × 104
Volume in cube metres1 × 106
Surface area:volume0.06

Large organisms have:

  • mechanisms to increase surface area proportionately, such as additional absorbing areas or adaptations of shape
  • transport systems and keep distances for diffusion to a minimum

Organisms living in harsh environmental conditions may reduce their surface area, for example cacti have adapted to have less surface area to lose water through.

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Osmosis
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Mixing particles

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