Adaptations of Exchange Surfaces (SQA National 5 Biology): Revision Note

Increasing the efficiency of absorption

• Efficient exchange and absorption of materials in the body depend on certain key structural adaptations

• These include having a large surface area, thin walls, and an extensive blood supply, all of which increase the rate of diffusion and transport of substances

Large surface area

• A large surface area provides plenty of space for exchange to occur, so allowing many molecules to cross a membrane at the same time; this leads to faster exchange

• Structures that create a large surface area include:

  • villi in the small intestine

  • microvilli on the surface of cells, e.g. in the small intestine and inside the kidneys

  • alveoli in the lungs

  • root hair cells on plant roots

Thin walls

• Thin walls mean there is only one or a few cell layers for materials to cross

• This gives a shorter distance for diffusion, leading to faster exchange

• For example, structures with walls that are only one cell thick include:

  • alveoli

  • capillaries

  • lining of the small intestine

Extensive blood supply

• A dense capillary networks right next to the surface of tissues provides an extensive blood supply for diffusion

• This helps to maintain steep concentration gradients by quickly removing absorbed substances, and bringing more to be absorbed

• For example:

  • in the lungs blood carries oxygen away from, and carbon dioxide to, the alveoli

  • in the intestine blood carries nutrients, such as glucose and amino acids, away from the site of absorption