Characteristics of living organisms
Section: Biology | Syllabus: Cambridge IGCSE Combined Science (0653)
The Seven Characteristics of Living Organisms (MRS GREN)
All living organisms carry out seven life processes. These are often remembered using the mnemonic MRS GREN: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, Nutrition.
| Characteristic | Syllabus definition | Plant example | Animal example |
|---|---|---|---|
| Movement | An action by an organism or part of an organism causing a change of position or place | Root tip growing towards water; shoot bending towards light | A cheetah running; a heart muscle contracting |
| Respiration | The chemical reactions in cells that break down nutrient molecules and release energy for metabolism | Glucose broken down in root cells to release energy for active transport of mineral ions | Glucose broken down in muscle cells to release energy for contraction |
| Sensitivity | The ability to detect and respond to changes in the internal or external environment | Shoot growing towards light (phototropism) | Pupil constricting in bright light; withdrawing a hand from a hot object |
| Growth | A permanent increase in size and dry mass | A seedling developing into a mature tree | A baby increasing in height and mass over several years |
| Reproduction | The processes that make more of the same kind of organism | A strawberry plant producing runners; a flower producing seeds | A bacterium dividing by binary fission; a mammal giving birth |
| Excretion | The removal of the waste products of metabolism and substances in excess of requirements | Oxygen and carbon dioxide diffusing out through the stomata | Carbon dioxide exhaled via the lungs; urea excreted in urine by the kidneys |
| Nutrition | The taking in of materials for energy, growth and development | Absorbing carbon dioxide, water and mineral ions; producing glucose by photosynthesis | Ingesting and digesting food; absorbing nutrients in the small intestine |
The seven characteristics of living organisms are commonly remembered using the mnemonic MRS GREN
Worked Example: Applying MRS GREN
- Question: a gardener notices that a sunflower's shoot has bent to grow towards a nearby window. Which characteristic of living organisms does this demonstrate, and why?
- Answer: this demonstrates sensitivity - the plant has detected the direction of the light (a change in its external environment) and responded by growing towards it
Common Mistakes
- ❌ Confusing excretion with egestion
- ✓ excretion is the removal of waste products made inside cells by metabolism (e.g. CO₂ from respiration, urea from the breakdown of excess amino acids). Egestion is the passing out of undigested food (faeces) through the anus - this food was never absorbed into body cells, so it is not a product of metabolism and is not excretion
- ❌ Measuring growth using wet mass
- ✓ growth must be measured as a permanent increase in dry mass. Wet mass can rise or fall simply due to water uptake or loss, so a temporary increase in wet mass does not count as growth
- ❌ Thinking plants cannot move because they cannot walk or run
- ✓ the definition of movement does not require locomotion of the whole organism - movement of a part of an organism counts. Roots growing downwards and shoots bending towards light are both examples of movement in plants
Interactive revision notes, videos and practice questions load below.