3.3.1 Surface area to volume ratio

Many cells are too far from exchange surfaces to exchange materials by diffusion/active transport alone. To solve this problem, cells of multicellular organisms bathe in tissue fluid (tissue fluid is the environment around the cells). When absorbed, materials are distributed throughout the tissue fluid so cells can absorb them. Example materials include nutrients (e.g fatty acids, amino acids, glucose, minerals, vitamins), gases (respiratory gases, oxygen and carbon dioxide), heat, and urea (and other excretory products).

Along with the metabolic rate of the organism, it’s size also affects the amount of each exchanged material. E.g organisms with a high metabolic rate require a large surface area to volume ratio. This is because lots of the material can be absorbed at once.

Small organisms have a large surface area to volume ratio. Large organisms have a small area to volume ratio (because they have more volume). For effective exchange organisms should have a large surface area to volume ratio as the surface is where the exchange of materials takes place (so small organisms have effective exchange of substances by means such as diffusion and active transport). However, as I said earlier, larger organisms have smaller surface area to volume ratios. This means diffusion/active transport is not sufficient to sustain the organism.

To overcome this, multicellular organisms have developed a range of adaptations including perhaps a flattened shape so no cell is far from the surface, and/or specialised gas exchange surfaces with a large surface area to volume ratio (e.g human alveoli).

Size and surface area: As the size of an organism increases, its surface area to volume ratio decreases. Think of it like this...

Take a small block (say, 1cm by 1cm), its surface area is 6cm² (6 lots of 1x1 cm), its volume is 1cm³ (1x1x1 cm).

Now take a large block (say, 10cm by 10cm),  its surface area is 600cm² (6 lots of 10x10 cm), its volume is 1000cm³ (10 x 10 x 10 cm).

Now look at the ratios...

The smaller block has a SA:VOL of 6:1, whereas the larger block has a smaller SA:VOL of 600:1000 (which cancels down to 0.6:1). This is because as an organisms size increases its surface area to volume ratio decreases.

To investigate this, you could use agar blocks with phenolphthalein. Take cubes of various sizes (e.g 1cm³, 2cm³, 3cm³, 4cm³...) and add them to the same concentration solution of acid. Time how long it takes each to go colourless. This can be used to determine the effect of surface area to volume ratio on the diffusion of an acid.