The Need for Transport (Edexcel GCSE Combined Science: Biology): Revision Note

In order for any organism to function properly, it needs to exchange substances between itself and the environment such as:

• Oxygen

• Carbon dioxide

• Water

• Dissolved food molecules

• Mineral ions

• Urea

This exchange of substances occurs across the cell membrane

There are three transport processes that living organisms use for exchange: diffusion, osmosis and active transport

Unicellular (single-celled) organisms like amoeba have very large surface areas (SA) in comparison to their volumes

• This means that the distance between the surface of the organism to its centre is very small

As a result, unicellular organisms do not need to have specialist exchange surfaces or transport systems; as diffusion, osmosis and active transport through the cell membrane occur at a sufficient rate to meet the needs of the organism

For larger, multicellular organisms the distance between the surface of the organism to its centre is relatively long

This is why larger organisms usually have exchange surfaces and transport systems; as diffusion, osmosis and active transport cannot happen sufficiently to meet a larger organism’s needs otherwise

Transport systems in animals include:

• The blood and circulatory system - carries the necessary substances around the body

Transport systems in plants include:

• The xylem - moves water and mineral ions from roots to shoots

• The phloem - moves sugars and amino acids to where they are needed in the plant

Large, multicellular organisms like humans have relatively small surface areas (SA) in comparison to their volumes

This is why larger organisms need exchange surfaces within their transport systems to carry out diffusion, osmosis and active transport at a sufficient rate

Exchange surfaces in animals include:

• The lungs and alveoli for gas exchange

• The small intestines and villi for absorption of digested food

Exchange surfaces in plants include:

• Roots and root hairs where mineral ions and water are absorbed

• The leaves for gas exchange

Multicellular organisms have surfaces and organ systems that maximise the exchange of materials by increasing the efficiency of exchange in a number of ways:

• Having a large surface area to increase the rate of transport

• A barrier that is as thin as possible to separate two regions, to provide as short a diffusion path as possible for substances to move across

In addition, animals have:

• A large network of blood vessels throughout the body:

  • To reduce the distance of exchange of materials between cells and the bloodstream

  • To move substances towards or away from exchange surfaces to maintain concentration gradients

• Gas exchange surfaces that are well ventilated to maintain concentration gradients

You should be able to calculate and compare surface area to volume ratios

You can model the effect of how increasing size affects surface area to volume ratio using simple cubes: