Exchange surfaces

Single-celled organisms have a very large surface area to volume ratio, which allows them to exchange materials directly across their cell membrane without the need for specialised structures.

In contrast, large multicellular organisms have a much smaller surface area to volume ratio. 

This means that essential molecules, such as oxygen, nutrients, and waste products, cannot move directly into or out of their cells by diffusion alone, as the diffusion distance is too great. 

Therefore, they rely on specialised surfaces and organ systems to efficiently exchange materials and meet the needs of their cells.

The effectiveness of an exchange surface is increased by:

• A large surface area for more molecules to pass through.

• Thin membranes to provide a short diffusion path.

• An efficient blood supply (in animals) to maintain a steep concentration gradient.

• Ventilation (in animals, for gaseous exchange) to continually refresh the air or water, keeping the concentration gradient steep.

Mammals

Small Intestine: 

• The small intestine is adapted for the absorption of nutrients. 

• Its inner surface is lined with villi, tiny finger-like projections that increase the surface area. 

• Each villus contains microvilli, further increasing the surface area for absorption. 

• The villi are well supplied with capillaries, ensuring a short diffusion path and a good blood supply, allowing for efficient absorption of nutrients.

Lungs: 

• The lungs are specialised for gas exchange. 

• They contain millions of tiny air sacs called alveoli, which provide a large surface area for oxygen to diffuse into the blood and carbon dioxide to diffuse out. 

• The alveoli have thin walls and are surrounded by capillaries, creating a short distance for diffusion. 

• The lungs are ventilated with each breath, which maintains a steep concentration gradient for efficient gas exchange.

One illus in the small intestine (left) and one alveolus in the lung with surrounding capillary (left)

Fish

Gills: 

• Fish use gills for gas exchange in water. 

• Gills are made up of gill filaments, which are covered with tiny structures called lamellae that increase the surface area. 

• The lamellae have a rich blood supply, and the flow of water over the gills helps maintain a steep concentration gradient. 

• Fish use counter-current flow, where water and blood flow in opposite directions, ensuring efficient oxygen uptake and carbon dioxide removal. 

Plants

Roots: 

• Plants absorb water and minerals through their roots. 

• The roots have root hair cells, which are tiny extensions that increase the surface area available for absorption. 

• The thin cell walls of the root hairs allow water and minerals to move into the plant efficiently from the surrounding soil.

Leaves: 

• Leaves are adapted for gas exchange. 

• They have a large surface area and are thin, which helps gases to diffuse efficiently. 

• Stomata (small openings on the leaf surface) control the exchange of gases. Carbon dioxide diffuses in for photosynthesis, and oxygen and water vapour diffuse out.

Cross section of a leaf

Root hair cell in plant roots

Additional maths skills

The surface area to volume ratio is an important factor in the efficiency of exchange surfaces. 

It measures how much of an organism’s surface is in contact with the environment relative to its volume. 

Smaller organisms or structures tend to have a larger surface area to volume ratio, making diffusion more efficient. 

In contrast, larger organisms or structures have a smaller surface area to volume ratio, which requires adaptations like specialised exchange surfaces to meet their needs. 

In the blocks below, the following surface areas (SA), volumes (Vol) and surface area to volume ratios (SA:Vol) apply:

• 1 x 1 x 1 cm: SA = 6 cm2, Vol = 1 cm3, SA:Vol = 6:1

• 1 x 1 x 2 cm: SA = 10 cm2, Vol = 2 cm3, SA:Vol = 5:1

• 2 x 2 x 2 cm: SA = 24 cm2, Vol = 8 cm3, SA:Vol = 3:1

• 1 x 1 x 8 cm: SA = 34 cm2, Vol = 8 cm3, SA:Vol = 4.25:1