Respiration and ATP
What respiration actually is
Respiration is the chemical process in all living cells that releases energy from glucose (and other food molecules) to make ATP.
Two things students routinely get wrong here:
- Respiration does not "produce energy" or "create energy" — energy is released or transferred from glucose.
- Respiration is not breathing. Breathing (ventilation) moves air; respiration happens inside cells, in every organism, all the time.
Where ATP comes in
Glucose holds a lot of energy, but cells cannot use it directly in one lump. Respiration transfers the energy released from glucose into a molecule called ATP.
- ATP is made continuously in respiring cells.
- ATP provides the energy for cells — it is the immediate energy supply that cells use.
- ATP is not stored in large amounts, so cells must respire continuously to keep making it.
What cells use ATP energy for
| Use of ATP energy | Concrete example |
|---|---|
| Muscle contraction | Movement of your legs when running |
| Active transport | Root hair cells taking up nitrate ions against a concentration gradient |
| Making large molecules (synthesis) | Joining amino acids into protein |
| Cell division | Growth and repair of tissue |
| Keeping warm | Mammals and birds maintaining a constant body temperature |
| Nerve impulses | Transmission of impulses along neurones |
Aerobic vs anaerobic — how much ATP?
Aerobic respiration breaks glucose down completely, so it releases much more energy per glucose molecule and makes much more ATP.
Anaerobic respiration breaks glucose down only partially — the products (lactic acid, or ethanol) still contain energy — so far less ATP is made per glucose.
Common mistake: writing that glucose "gives the cell energy" as your whole answer. Say that glucose is respired to release energy, which is used to make ATP, and ATP then supplies energy for the named process. Examiners report candidates incorrectly claiming things like glucose and salt in a solution "provide the cells with energy" — salt provides no energy at all, and glucose only does so once it is respired.
Examiner tip. Never write "respiration produces energy". Energy cannot be produced — it is released from glucose and used to make ATP. Also keep respiration and breathing separate: if a question asks why an organism respires, the answer involves ATP, not oxygen intake.