The need for exchange surfaces and transport systems

• the smaller the cube (organism), the larger the surface area to volume ratio. For example, single-cell organisms, such as bacteria, have a relatively large surface area, compared to their volume. This allows sufficient transport of particles into and out of the cell to meet their needs
• Large, multicellular animals have much smaller surface area to volume ratios. This means that they cannot get all of the substances they need by diffusion alone. They need specialised exchange surfaces in order to get all of the substances that they need
• the effectiveness of the specialised exchange surface is increased when:

  • there is a large surface area
  • the membrane is thin, to provide a short diffusion pathway
  • there is a good blood supply (in animals) to remove the diffused particles, and help to maintain a high concentration gradient
  • it is ventilated (in animals for gas exchange)