The need to transport substancesSpec 8.1
In short
Substances must be transported into and out of an organism and its cells. Oxygen, dissolved food molecules, water and mineral ions are needed by cells, while carbon dioxide from respiration and urea are waste products that must be removed. In a large animal many cells are far from the outside, so blood transports these substances.
Every cell in an organism needs a supply of some substances and must get rid of others. These substances have to be transported into and out of the organism and its cells.
| Substance | Direction | Why it is needed |
|---|---|---|
| Oxygen | Into cells | Used in aerobic respiration to release energy |
| Carbon dioxide | Out of cells | Waste product of respiration. It must be removed from the body |
| Water | Into and out of cells | Needed for cell reactions and to keep cells working. Water is also lost from the body, for example in urine and sweat |
| Dissolved food molecules | Into cells | Provide a source of energy and the materials for growth and repair, for example glucose and amino acids |
| Mineral ions | Into cells | Needed for many cell processes |
| Urea | Out of cells | A waste product that is made in the body. It is carried in the blood to the kidneys and removed in urine |
In a single-celled organism these substances can simply diffuse across the cell surface. In a large animal many cells are far from the outside world, so substances have to be moved over long distances. Blood is the transport medium in humans.
When asked to describe the need for transport, name the substance, say whether it goes in or out, and give the reason. For example: carbon dioxide is a waste product of respiration, so it must be removed.
Written and checked against the Edexcel GCSE Combined Science (1SC0) specification · Updated October 2026