The seven characteristics
All living organisms carry out seven life processes. A useful memory aid is MRS GREN: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion and Nutrition. Something is only classed as living if it shows all seven.
Key idea
Movement – an action by an organism causing a change of position or place. Respiration – chemical reactions in cells that break down nutrient molecules to release energy. Sensitivity – detecting and responding to changes (stimuli) in the environment. Growth – a permanent increase in size and dry mass. Reproduction – making more of the same kind of organism. Excretion – removal of toxic waste products of metabolism. Nutrition – taking in materials for energy, growth and development.
Excretion is not egestion
A common exam error is confusing excretion with egestion. Excretion removes wastes made by the body's own reactions, such as carbon dioxide from respiration and urea from the breakdown of excess amino acids. Egestion is the removal of undigested food (faeces), which was never absorbed into the body's cells.
Remember
- Plants move too – their leaves and shoots grow towards light, though they do not move from place to place.
- Respiration is a cellular chemical process; do not confuse it with breathing (gas exchange).
- Growth means an increase in dry mass, not just taking in water.