Respiration — energy release in cells
Respiration is the chemical release of energy from glucose inside living cells — it happens in every cell, all the time, and it powers the other life processes (movement, active transport, growth, staying warm). Keep three terms apart: respiration = releasing energy chemically in cells; breathing (ventilation) = the physical movement of air in and out of the lungs; gas exchange = swapping gases across a surface. Plants, fungi and bacteria all respire but do not breathe.
Excretion, growth and homeostasis defined
Excretion is the removal of the waste products of metabolism — mainly carbon dioxide from respiration and urea from excess protein — not undigested food (that is egestion). Growth is a permanent increase in size and dry mass from more or larger cells; dry mass is specified because water uptake can make an organism temporarily bigger without real growth. Control of internal conditions (homeostasis) keeps the internal environment (e.g. temperature, water) constant.
The eight life processes (MRS GREN + control)
Every living organism carries out eight life processes: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion and Nutrition (the mnemonic MRS GREN), plus the eighth — control of internal conditions (homeostasis). Learn the two-element mark-scheme definition of each. An object must show ALL eight to count as living — one or two (a car that moves, a candle that "uses oxygen") is never enough, so don't stop at MRS GREN's seven.
Drawn from real examiner reports.
Respiration ≠ breathing
Writing that respiration "means breathing" or "taking in oxygen" loses the mark. Respiration is the chemical release of energy from glucose in living cells; breathing (ventilation) is only the physical movement of air in and out of the lungs, and gas exchange is swapping gases across a surface. A plant respires in every cell but has no lungs and does not breathe.
Excretion ≠ egestion
A frequent error is defining excretion as "getting rid of waste" and giving faeces as the example. Faeces is undigested food that was never absorbed into cells, so removing it is egestion, not excretion. Excretion is specifically the removal of the waste products of metabolism — carbon dioxide from respiration and urea from breaking down excess amino acids.
Naming a process isn't defining it
When a question says define or describe a characteristic, writing the word alone — "respiration", "excretion" — is not a definition and earns nothing. The mark scheme wants the two elements: e.g. respiration is the chemical release of energy AND it happens in living cells. Always give the full statement, not an isolated key word, and match the depth the command word asks for.
Sensitivity ≠ movement
Sensitivity and movement are separate characteristics that often get merged. Sensitivity is detecting a stimulus AND responding to it; movement is only one possible response. A plant shoot bending towards light shows sensitivity though its movement is slow — it detected the light and responded. Do not call a response "just movement" when the question is about sensitivity.
Growth ≠ taking in water
Growth means a permanent increase in dry mass, brought about by cells dividing or enlarging. A wilted plant that swells after watering, or a person heavier after a drink, has gained water, not grown — remove the water and the dry mass is unchanged. This is why growth is measured as an increase in dry mass, not ordinary mass: it strips out the temporary effect of water.
Homeostasis ≠ sensitivity
Control of internal conditions (homeostasis) and sensitivity are easy to confuse. Homeostasis keeps the INSIDE of the body constant — e.g. temperature, blood glucose and water content. Sensitivity is detecting and responding to changes in the OUTSIDE surroundings. A lizard sensing that the air is hot shows sensitivity; keeping its own body temperature steady is homeostasis.
Run the MRS GREN + control checklist
Two question shapes dominate. For "state or define the characteristics", run down MRS GREN and add the eighth — control of internal conditions — so you never miss one. For "why is X not alive", run the same checklist and name the processes the object does NOT carry out.
Describe vs explain — link your points
Describe means say what happens ("the shoot moves towards light"); explain means give the reason or mechanism ("it detects the light and responds by growing towards it"). Read the command word and give full, linked statements, not isolated key words — a bare term never explains.
Don't over-answer numbered lines
If a question asks for a set number of characteristics, only the first answers on the numbered lines are marked. Writing extra ones hoping one is right cannot gain marks — decide on your best answers and give exactly the number asked for, no more.
All living organisms share eight characteristics (life processes). A useful mnemonic is MRS GREN for seven of them, plus the eighth (control of internal conditions).
| Characteristic | Two-element mark-scheme definition |
|---|---|
| Nutrition | Taking in of nutrients (food) that provide the raw materials and energy for growth, repair and other life processes |
| Respiration | The chemical release of energy from glucose in living cells (not breathing) |
| Excretion | The removal of the waste products of metabolism (e.g. carbon dioxide, , and urea) |
| Sensitivity | The ability to detect and respond to changes (stimuli) in the surroundings |
| Movement | An action that changes the position or place of an organism or part of it |
| Control of internal conditions | The maintenance of a constant internal environment (homeostasis) |
| Reproduction | The process that makes more of the same kind of organism |
| Growth and development | A permanent increase in size and dry mass (by an increase in cell number and/or cell size) |
Why "dry mass" for growth? Taking in water can make an organism temporarily bigger or heavier without real growth, so growth is measured by the increase in dry mass (mass with the water removed).
Define respiration (as a characteristic of living organisms).
State four of the characteristics shared by all living organisms. (4 marks)
