The need for transport systems

In single-celled organisms such as protists, and small multicellular organisms, essential molecules will move to where they're needed by diffusion.

Once an organism – for instance a very early embryo – or species of organism gets beyond a certain size, it needs a transport system to distribute essential molecules efficiently.

A diagram illustrating the circulatory system in the human body

Features of transport systems

Common features of transport systems:

  • they are made up of tubes or 'vessels' that carry materials from one part of the organism to another
  • they make close contact with cells, such as those of exchange surfaces

What substances are transported in the blood?

Substance transportedFromTo
OxygenLungsAll cells in the body
Carbon dioxideAll cells in the bodyLungs
GlucoseDigestive systemLiver, then the rest of the cells in the body

Other substances transported in the blood include:

Oxygen is carried within red blood cells. Dissolved substances are carried in the plasma.

The need for exchange surfaces

Organisms must take in food, oxygen and water, and other essential substances, from the environment. Plants also need carbon dioxide for photosynthesis. Organisms also need to remove waste substances.

Small organisms exchange these essential and waste substances between themselves and the environment. They do this over their body surface. Simple chemical substances can diffuse in and out of their bodies.

Inside their bodies, in small organisms, substances don't have to move far.

The size of their surface, or surface area, defines how quickly they can absorb substances.

The size of their volume defines how much of these substances they need.

Modelling cells

If we represent the cell of an organism by a cube:

It’s straightforward to model cells using cubes.

This is what happens when the cube increases in size:

A table showing the volume of ratios

As the volume increases, surface area does not increase at the same rate.

As multicellular organisms increase in size, they therefore face two problems

Their surface area does not increase as fast as the volumeInsufficient surface area to meet their needsBody systems that add additional absorbing area to exchange surfaces
Their volume increasesDiffusion is not quick enough to move substances to where they're needed in the organism's bodyA transport system