Surface area to volume ratio and gas exchange

In a nutshell

Unicellular organisms exchange substances through their cell membrane. This summary explores why large multicellular organisms need mass transport systems for substance exchange and how size and surface area influence this.

Surface area to volume ratio

The surface area to volume ratio is a way of representing how large an organism's surface area is in comparison to its volume. As an organism gets larger, their volume increases at a faster rate than their surface area. So, larger organisms have a smaller surface area to volume ratio. This is shown in the table below.

Example

Organism
Surface area	$1\times 1\times 6=6\ cm^2$	$2\times 2\times 6=24\ cm^2$	$3\times 3\times 6= 54\ cm^2$
Volume	$1\times 1\times 1=1\ cm^3$	$2\times 2\times 2=8\ cm^3$	$3\times 3\times 3=27\ cm^3$
Surface area to volume ratio	$\frac{6}{1}=6:1$	$\frac{24}{8}=3:1$	$\frac{54}{27}=2:1$

Size and substance exchange

All organisms need to take in nutrients and excrete waste. The surface area to volume ratio of an organism affects how it exchanges substances.

Small organisms have a large surface area to volume ratio and they are one cell thick, therefore the diffusion distance is very short. This means that diffusion is sufficient to supply the cell with the substances they require.

However, large organisms cannot exchange substances directly with their environment because they have smaller surface area to volume ratios. There is a larger diffusion distance between the body surface and many of the cells so diffusion would not meet the demands of cells. To overcome the limitations of diffusion, larger animals have mass transport systems.

Adaptations for exchange

Large organisms have a higher metabolic rate which means they need to transport more nutrients and more waste products. Therefore, they need effective transport and exchange systems.

Example

Adaptation	Description
*Large surface area*	Gas exchange surfaces have a large surface area which increases the rate of diffusion.
*Thin*	Gas exchange surfaces are often one cell thick therefore the diffusion distance is very short. This also increases the rate of diffusion.
*Steep concentration gradient*	The gas that is exchanged is quickly transported away from the exchange surfaces. This helps to maintain a steep concentration gradient and increases the rate of diffusion.

Note: The xylem and phloem form a transport system in plants.

Heat exchange

Despite adaptations, metabolism is an inefficient process that releases heat. The amount of heat released depends on the size, shape, behaviour and physiology of the animal.

Size and shape

Smaller organisms lose heat easily as their relative surface area is larger than a larger organism (their surface area to volume ratio is greater). This means that smaller organisms need a relatively high metabolic rate to generate enough heat to keep them warm.

The shape of an organism will also impact heat exchange. This is shown in the example below.

Examples

	Shape	Effect
Arctic fox	The arctic fox lives in a cold environment. It has small ears and a rounded head.	Its rounded head and small ears reduces the surface area to volume ratio and therefore reduces the heat loss.
Fennec fox	The fennec fox is native to the desert. It has large pointed ears and a pointed face.	Its large ears and pointed face increases the surface area to volume ratio and also the heat loss.

Behaviour and physiology

Organisms also have behavioural and physiological adaptations to alter the amount of exchange that takes place. Some examples are explained in the table below.

Examples

Problem	Adaptation
Organisms with a large surface area to volume ratio lose more water through evaporation from their body surface.	Some animals, particularly those living in hot environments, will have adaptations to their kidneys. This means they produce less urine to compensate for the increased water loss.
Small organisms have a high metabolic rate but food is not always available.	Organisms in cold environments will need to eat foods like nuts that can provide a lot of energy to maintain their metabolism.
Organisms living in cold environments will need to produce a lot of heat to maintain their body temperature.	These organisms may have thick layers of fur to keep them better insulated.