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

• 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

Unicellular organisms such as amoeba do not require transport systems due to their large surface area to volume ratio

Multicellular organisms

• 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

Some examples of transport systems in plants and animals

• 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

Some examples of exchange surfaces in plants and animals

Properties of exchange surfaces

• 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:

Calculating the surface area to volume ratio