The need to transport substancesSpec 8.1
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.