Bioenergetics · GCSE Biology
Aerobic respiration
Define aerobic respiration for GCSE Biology: glucose plus oxygen to carbon dioxide and water in mitochondria, exothermic energy transfer, and why every living cell — including plants — respires.
Respiration is not breathing. Aerobic means with oxygen: glucose + oxygen → carbon dioxide + water, mainly in mitochondria, transferring much more energy than anaerobic respiration.
The important bits
What you need to know
- 1
Respiration is an exothermic chemical reaction that transfers energy from glucose to the cell. It is not the same as breathing (ventilation), which only moves air.
- 2
Aerobic respiration word equation: glucose + oxygen → carbon dioxide + water. Symbol: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O. Energy is transferred; do not write that energy is made.
- 3
It takes place mainly in mitochondria. Cells with high energy demand (muscle, sperm, root hair, phloem companion cells) contain many mitochondria.
- 4
Uses of the transferred energy: movement (muscle contraction), active transport, building large molecules (proteins, cellulose), cell division, and keeping a constant body temperature in birds and mammals.
- 5
Aerobic respiration transfers much more energy per glucose molecule than anaerobic respiration, which is why endurance exercise needs a good oxygen supply.
- 6
Plants and algae also respire aerobically in mitochondria, day and night. In the light they may show a net uptake of CO₂ because photosynthesis is faster, but respiration has not stopped.
- 7
Gas exchange supplies the oxygen and removes the carbon dioxide: alveoli in humans, stomata and spongy mesophyll in leaves. Blood transports glucose and oxygen to tissues.
- 8
During exercise, breathing rate, breathing depth and heart rate rise to deliver more oxygen and glucose to muscles and to remove more carbon dioxide.
Quotations worth analysing
Short evidence. Real method.
“glucose + oxygen → carbon dioxide + water”
State that it is exothermic and occurs in mitochondria. This is the reverse skeleton of photosynthesis, not the same process.
“C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O”
If you write this with the arrow the other way, you have photosynthesis. Check products: CO₂ and water, not glucose.
“Mitochondria are the site of aerobic respiration.”
Pair with “transfers energy for…” and a named process such as contraction or active transport.
Go deeper
Why must I never say respiration is breathing?
Breathing (ventilation) moves air in and out of the lungs so that oxygen can diffuse into blood and carbon dioxide can diffuse out. Respiration is the chemical reaction in every living cell that uses that oxygen (if aerobic) to transfer energy from glucose. A plant has no lungs and still respires. A muscle cell respires faster during a sprint; the athlete breathes faster to match supply to demand. If you define respiration as breathing, you lose the definition mark and then mis-locate the process in the lungs instead of mitochondria. Write: exothermic reaction in cells, mainly mitochondria, glucose + oxygen → carbon dioxide + water. Breathing is the delivery system, not the reaction.
Go deeper
How do exercise graphs link to this equation?
Muscles need more energy, so they need more glucose and oxygen, and they produce more carbon dioxide (and heat). Heart rate rises so blood delivers reactants and removes products faster. Breathing rate and tidal volume rise so alveoli maintain a steep diffusion gradient. If the paper shows pulse rate climbing then staying high after the run, the after-phase is partly oxygen debt (next pages) and still aerobic recovery. Describe the pattern first, then explain with the equation. “So they can run” is not an explanation. Glycogen in muscle and liver is a glucose store; it is hydrolysed to keep blood and muscle glucose available. Never write that energy is produced or created; write transferred from glucose.
Go deeper
Do plant cells really respire in the day?
Yes. Chloroplasts run photosynthesis; mitochondria run aerobic respiration at the same time. Some oxygen from photosynthesis is used in mitochondria; some carbon dioxide from respiration is used in chloroplasts. Net gas exchange depends on which process is faster. In bright light, net O₂ out and net CO₂ in. In the dark, only respiration: O₂ in, CO₂ out. Root cells never photosynthesise, so they take oxygen from air spaces in soil — waterlogged soil can suffocate roots and then active transport of ions fails. This is why “plants photosynthesise, animals respire” as a split is false. Both kingdoms respire. Only photosynthetic cells do the other equation as well.
See the idea in action
A root hair cell absorbs nitrate against a concentration gradient. Energy for that active transport is transferred by aerobic respiration in mitochondria: glucose + oxygen → carbon dioxide + water. The oxygen comes from air in the soil. If the soil is waterlogged, oxygen is scarce, aerobic respiration slows, less energy is transferred, and ion uptake falls. The same equation in a muscle cell transfers energy for contraction. In both cells, respiration is chemical, not breathing.
Exam technique
Turn knowledge into marks
Define respiration as a chemical reaction in cells, not breathing. Give the aerobic equation, name mitochondria, and say energy is transferred (not made). Mention that plants respire all the time.
Common mistakes
Do not give these marks away
- 01
Defining respiration as breathing, or saying energy is produced rather than transferred.
- 02
Writing the photosynthesis equation, or locating aerobic respiration in chloroplasts.
- 03
Claiming plants do not respire in the day, or that only animals have mitochondria.
Where does aerobic respiration mainly take place in a eukaryotic cell?
AChloroplasts
BNucleus
CMitochondria
DCell wall
Show the answer
Mitochondria. Mitochondria are the site of aerobic respiration. Chloroplasts are for photosynthesis. The nucleus stores DNA. Cell walls do not respire.
Quick questions
If this is the bit you searched
What is the word equation for aerobic respiration?
Glucose + oxygen → carbon dioxide + water (energy is transferred).
What is the symbol equation for aerobic respiration?
C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O.
Is respiration the same as breathing?
No. Breathing moves air in and out of the lungs. Respiration is the exothermic reaction in cells that transfers energy from glucose.
Do plants respire?
Yes, every living plant cell respires all the time, mainly in mitochondria. Photosynthesis is an extra process in cells that have chloroplasts.