The Seven Life Processes Defined
Each characteristic describes a process, not a structure. Examiners often give a scenario and ask you to identify which process is occurring.
- Movement: an action by an organism or part of an organism causing a change of position or place. Animals move their whole body; plants show movement such as roots growing towards water (hydrotropism).
- Respiration: the chemical reactions in cells that break down nutrient molecules and release energy for metabolism. This is not breathing: it happens in every living cell.
- Sensitivity: the ability to detect and respond to changes in the environment (stimuli). A Venus flytrap closing on an insect demonstrates sensitivity in plants.
- Growth: a permanent increase in size and dry mass through an increase in cell number or cell size. Swelling by absorbing water alone is not true growth.
- Reproduction: the process that makes more of the same kind of organism, either sexually or asexually.
- Excretion: the removal of toxic waste products of metabolism from the body. Carbon dioxide from respiration and urea from amino-acid deamination are excretory products. Egestion (removal of undigested food) is not excretion.
- Nutrition: taking in materials for energy, growth and development. Autotrophs (plants) make their own food; heterotrophs (animals) consume organic molecules made by other organisms.
Applying MRS GREN to Different Organisms
The examiner may describe an organism you have never studied and ask whether it is living. Check all seven characteristics:
| Characteristic | Amoeba (unicellular) | Oak tree (plant) | Human (animal) |
|---|---|---|---|
| Movement | Pseudopodia extend | Roots grow towards water | Muscles contract |
| Respiration | Aerobic in cytoplasm & mitochondria | In every cell, day and night | Aerobic; anaerobic during intense exercise |
| Sensitivity | Moves away from strong light | Shoots grow towards light | Nervous and hormonal responses |
| Growth | Increases in size before binary fission | Grows throughout its life at meristems | Grows until adulthood |
| Reproduction | Binary fission (asexual) | Seeds (sexual); runners possible | Sexual reproduction |
| Excretion | CO₂ diffuses out; contractile vacuole removes excess water | O₂ is a waste product of photosynthesis | CO₂ via lungs; urea via kidneys |
| Nutrition | Engulfs food (heterotroph) | Photosynthesis (autotroph) | Ingests food (heterotroph) |
Key point: Viruses do not carry out all seven processes independently: they cannot respire or grow outside a host cell. This is why they are not classified as living organisms in the Cambridge syllabus.
Common Exam Traps and How to Avoid Them
Respiration ≠ Breathing. Breathing (ventilation) is a mechanical process that moves air in and out of the lungs. Respiration is the chemical reaction inside cells. If a question asks "Do plants respire?", the answer is yes: all living cells respire, 24 hours a day.
Excretion ≠ Egestion. Faeces have never been inside body cells: the food simply passed through the gut without being absorbed. Excretion specifically removes waste products of metabolism. CO₂ (from respiration) and urea (from deamination in the liver) are the classic examples.
Growth must be permanent. A balloon filled with water gets bigger, but it hasn't grown. Growth involves an increase in cell number (mitosis) or permanent cell enlargement, increasing dry mass.