The Need for a Transport System (Edexcel IGCSE Biology (Modular))

To function properly, organisms must exchange substances, like food molecules and waste, with their environment

• This exchange happens via diffusion, osmosis, and active transport across the cell membrane

Unicellular organisms, like amoebas, have large surface areas relative to their volume, meaning the distance from the surface to the centre is small

Consequently, they don't need specialised exchange surfaces or transport systems, as diffusion, osmosis, and active transport through the cell membrane are sufficient for their needs.

Multicellular organisms, like humans, have bodies composed of many cells

These organisms have multiple cell layers, making the distance from the surface to the centre too long for diffusion alone

Diffusion to all cells would be too slow to meet the organism's needs, so larger organisms require transport systems

• In animals, the circulatory system carries essential substances in the blood

• In plants, the vascular system transports substances:

  • the xylem moves water and minerals from roots to shoots

  • the phloem distributes sugars and amino acids throughout the plant.