Explain how surface area to volume ratio affects the exchange of substances in organisms

What this dot point is asking

SEAB wants you to explain how the surface area to volume ratio of an organism affects how easily it can exchange substances with its surroundings. You should be able to calculate the ratio for simple shapes, explain why it falls as an organism gets bigger, and use this to explain why large organisms need specialised exchange surfaces such as lungs, gills and a leaf.

The answer

What the ratio means

Every organism must take in substances (oxygen, food) and remove waste (carbon dioxide) across its surface. The surface area to volume ratio compares the amount of surface available for exchange with the amount of living material (volume) that needs supplying.

For a cube of side $L$:

So the ratio of surface area to volume is $\dfrac{6L^2}{L^3} = \dfrac{6}{L}$, which gets smaller as $L$ gets larger.

Why the ratio falls as size rises

As an object gets bigger, its volume increases faster than its surface area. A small object has a large surface area for each unit of volume; a large object has a small surface area for each unit of volume. This is why the ratio falls with size.

Small organisms

A single-celled organism is tiny and has a large surface area to volume ratio. Diffusion across its cell surface membrane alone is fast enough to supply every part of it, so it needs no special exchange surface, and the distance to its centre is short.

Large organisms

A large organism has a small surface area to volume ratio. Diffusion across its outer surface cannot supply all its inner cells, and substances would have too far to travel. It therefore needs:

• Specialised exchange surfaces with a very large surface area (lungs, gills, the lining of the small intestine, leaves).
• A transport system (such as blood) to carry substances quickly to and from every cell.

The features of a good exchange surface

A good exchange surface has a large surface area, a thin barrier (short diffusion distance), and a good blood supply or ventilation to keep the concentration gradient steep. These are exactly the factors that speed diffusion.

Examples in context

Example 1. The alveoli of the lungs. Millions of tiny air sacs give the human lungs a surface area of around the size of a tennis court, packed into the chest. This enormous surface compensates for the body's small surface area to volume ratio and allows enough oxygen to diffuse in.

Example 2. A flat, thin leaf. A leaf is broad and thin, giving a large surface area for absorbing light and exchanging gases while keeping the diffusion distance short. Its shape is a direct response to the need for efficient exchange.

Try this

Q1. State the surface area and volume of a cube with sides of $2\ \text{cm}$, and give the ratio. [2 marks]

• Cue. Surface area $= 6 \times 2^2 = 24\ \text{cm}^2$; volume $= 2^3 = 8\ \text{cm}^3$; ratio $= 24 : 8 = 3 : 1$.

Q2. Explain why the surface area to volume ratio falls as an organism gets bigger. [2 marks]

• Cue. Volume increases faster than surface area as size rises, so there is less surface area for each unit of volume, lowering the ratio.

Q3. State two features of an efficient gas exchange surface. [2 marks]

• Cue. Any two of: a large surface area, a thin barrier (short diffusion distance), a good blood supply or ventilation to keep the gradient steep.