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Why we need transport systems: xylem and phloem

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Tiny organisms get everything they need by diffusion. Large ones need transport systems, such as xylem and phloem in plants.

Small organisms

A unicellular organism has a large surface area to volume ratio and a very short diffusion distance to every part of its cell.
So it can rely on diffusion across its surface for oxygen, carbon dioxide, food and waste.

Large organisms

A multicellular organism has a smaller surface area to volume ratio, and many cells are far from the surface.
Diffusion alone would be far too slow, so it needs a transport system to carry substances around the body: blood vessels in animals, xylem and phloem in plants.

Xylem and phloem

Worked example

Why can an amoeba rely on diffusion but a tree cannot?

  1. An amoeba is one cell with a large surface area to volume ratio, so every part is close to the surface.
  2. A tree is multicellular with a small surface area to volume ratio; many cells are far from the surface.

Answer: The amoeba has a large surface area to volume ratio and short distances; the tree needs a transport system.

Key idea

Unicellular organisms rely on diffusion because of their large surface area to volume ratio. Multicellular organisms need transport systems. Xylem carries water and mineral ions up from the roots; phloem carries sucrose and amino acids around the plant.

The interactive lesson includes the diagrams for this topic.

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