Respiration
Summary: Respiration is the chemical process by which cells release energy (ATP) from glucose. Aerobic respiration uses oxygen: C6H12O6 + 6O2 → 6CO2 + 6H2O + energy (ATP). Anaerobic respiration in animals produces lactic acid: glucose → lactic acid + energy. Oxygen debt occurs after vigorous exercise when lactic acid builds up and must be oxidised. Anaerobic respiration in yeast (fermentation) produces ethanol and CO2: glucose → ethanol + CO2 + energy. Fermentation is used in bread making (CO2 makes dough rise) and brewing (ethanol). Aerobic respiration releases much more energy per glucose molecule than anaerobic respiration. Tags: igcse biology respiration aerobic anaerobic fermentation ATP oxygen-debt Created: 2026-07-16 Last Updated: 2026-07-16
1. What Is Respiration?
Respiration is the chemical process by which living cells release energy from organic molecules (primarily glucose). The energy is stored in molecules of ATP (adenosine triphosphate).
Key points:
- Respiration is NOT the same as breathing (ventilation). Breathing is the physical movement of air into and out of the lungs. Respiration is a chemical reaction inside cells
- Respiration occurs in all living cells, 24 hours a day
- The energy released is used for: muscle contraction, active transport, cell division, maintaining body temperature, synthesising large molecules (proteins, DNA), nerve impulse transmission
Uses of energy (ATP) in the body:
| Use | Description |
|---|---|
| Muscle contraction | ATP provides energy for actin and myosin filaments to slide past each other |
| Active transport | Moving substances against their concentration gradient across cell membranes (e.g. mineral ions in root hair cells) |
| Cell division | Mitosis and meiosis require energy for DNA replication and chromosome movement |
| Maintaining body temperature | Heat released during respiration helps maintain 37 degrees C in mammals and birds |
| Protein synthesis | Linking amino acids together to make proteins requires ATP |
| Nerve impulses | Active transport of Na+ and K+ ions across neurone membranes to reset the resting potential |
2. Aerobic Respiration
Aerobic respiration is the breakdown of glucose using oxygen to release a large amount of energy.
Word equation:
Glucose + Oxygen → Carbon dioxide + Water + Energy (ATP)
Balanced chemical equation:
C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy (ATP)
Key features of aerobic respiration:
- Requires oxygen
- Produces carbon dioxide and water as waste products
- Releases a large amount of energy (~36-38 ATP molecules per glucose molecule)
- Occurs mainly in the mitochondria of cells
- This is the most efficient form of respiration — most organisms use it as their primary energy source
3. Anaerobic Respiration in Animals
Anaerobic respiration is the breakdown of glucose without oxygen to release a relatively small amount of energy.
Word equation (in animals):
Glucose → Lactic acid + Energy (ATP)
Key features:
- Does NOT require oxygen
- Produces lactic acid (not CO2 or water)
- Releases much less energy per glucose molecule (~2 ATP molecules) than aerobic respiration
- Occurs in the cytoplasm (not in mitochondria)
- Only used as a short-term emergency measure — when oxygen supply to muscles is insufficient during vigorous exercise
When does anaerobic respiration occur? During vigorous exercise (sprinting, heavy lifting), muscle cells use up oxygen faster than the blood can deliver it. The cells switch to anaerobic respiration to continue producing ATP, but this is inefficient and produces lactic acid.
Lactic acid build-up:
- Lactic acid accumulation lowers the pH in muscle cells
- This affects enzyme activity and causes muscle fatigue, cramps, and pain
- Lactic acid must be removed — it is transported in the blood to the liver
4. Oxygen Debt
Oxygen debt is the extra oxygen needed after vigorous exercise to oxidise the accumulated lactic acid and remove it from the body.
What happens during recovery (repaying the oxygen debt):
- After exercise stops, breathing rate and heart rate remain elevated
- Extra oxygen is taken in (heavy breathing)
- In the liver:
- Some lactic acid is oxidised (using the extra O2) to CO2 and H2O, releasing energy
- Some lactic acid is converted back to glucose (using energy)
- Some lactic acid is converted to glycogen for storage in the liver and muscles
- Once lactic acid levels return to normal, breathing and heart rate return to resting levels
Breathing heavily after exercise takes in the extra oxygen needed to oxidise the lactic acid built up during anaerobic respiration.
5. Anaerobic Respiration in Yeast (Fermentation)
Yeast and some bacteria can carry out anaerobic respiration known as fermentation.
Word equation (fermentation):
Glucose → Ethanol + Carbon dioxide + Energy (ATP)
Key features:
- Does NOT require oxygen
- Produces ethanol (alcohol) and carbon dioxide (no lactic acid)
- Releases a small amount of energy
- Ethanol is toxic to yeast at high concentrations (~15%)
Industrial uses of fermentation:
| Application | Useful Product | How It Works |
|---|---|---|
| Bread making | Carbon dioxide (CO2) | Yeast is mixed with flour (contains starch → broken down to glucose) and water. The yeast ferments the glucose, producing CO2 — the gas bubbles get trapped in the dough, making it rise (dough expands). When baked, the ethanol evaporates and the heat kills the yeast |
| Brewing (beer, wine) | Ethanol | Yeast ferments sugars in grains (beer — barley) or fruit juice (wine — grapes). The ethanol accumulates and is the desired product. Beer uses hops for flavouring. The CO2 produced gives beer its fizz (or CO2 is added artificially) |
| Biofuel (bioethanol) | Ethanol | Yeast ferments sugars from crops (sugar cane, maize) to produce ethanol. This can be used as a renewable fuel for vehicles |
Conditions for fermentation:
- Anaerobic conditions (no oxygen — otherwise yeast would respire aerobically and produce CO2 + H2O, not ethanol)
- Warm temperature (~25-30 degrees C) — optimum for yeast enzymes. Too hot → enzymes denature. Too cold → enzymes work too slowly
- Glucose (sugar source) — substrate for fermentation
- Slightly acidic pH (yeast enzymes work best at pH ~5-6)
6. Comparison: Aerobic vs Anaerobic Respiration
| Feature | Aerobic Respiration | Anaerobic Respiration (Animals) | Anaerobic Respiration (Yeast/Fermentation) |
|---|---|---|---|
| Oxygen required? | Yes | No | No |
| Where in cell? | Mitochondria | Cytoplasm | Cytoplasm |
| Glucose breakdown | Complete | Incomplete | Incomplete |
| Products | CO2 + H2O | Lactic acid | Ethanol + CO2 |
| Energy released per glucose | Large (~36-38 ATP) | Small (~2 ATP) | Small |
| Duration | Sustainable long-term | Short-term only | Sustained until ethanol becomes toxic (~15%) |
| Waste product fate | CO2 exhaled, H2O used or excreted | Lactic acid oxidised later using extra O2 (oxygen debt) | Ethanol is the desired product in brewing |
Related Notes
- Gas Exchange in Humans — The respiratory system delivers O2 and removes CO2; breathing is NOT respiration
- Plant Nutrition — Glucose produced in photosynthesis is the substrate for respiration
- Transport in Animals — Blood transports O2 and glucose to cells; removes CO2 and lactic acid
- Human Nutrition — Glucose comes from carbohydrate digestion
- Enzymes — Respiration is catalysed by many enzymes; denaturation at high temperatures
- Biotechnology and Genetic Modification — Industrial fermentation uses
- IGCSE-Bio-Index — Full IGCSE Biology index
Sources
- BBC Bitesize GCSE Biology — Respiration / Aerobic and anaerobic respiration, BBC (free educational resource)
- OpenStax Biology 2e — Chapter 7: Cellular Respiration, Rice University (free, CC BY 4.0)
- Cambridge IGCSE Biology 0610 — Topic 12: Respiration, Cambridge Assessment International Education
- CK-12 Biology for High School — Cellular Respiration chapter, CK-12 Foundation (free, CC BY-NC 3.0)
Common Misconceptions
| Misconception | Reality |
|---|---|
| ”Respiration is the same as breathing” | Breathing (ventilation) is the physical movement of air into and out of the lungs. Respiration is the chemical process of releasing energy from glucose inside cells |
| ”Plants only photosynthesise, they don’t respire” | Plants respire 24 hours a day in all their living cells. During the day, photosynthesis masks respiration (net O2 production), but respiration still occurs |
| ”Anaerobic respiration produces CO2 in humans” | In humans/animals, anaerobic respiration produces lactic acid, not CO2. CO2 is only produced in aerobic respiration (and in yeast fermentation) |
| “Lactic acid is a waste product with no further use” | Lactic acid is oxidised (using extra O2) after exercise — it is not simply excreted. Some is converted back to glucose/glycogen in the liver |
| ”More oxygen means more anaerobic respiration” | Anaerobic respiration occurs when there is insufficient oxygen. If there is plenty of oxygen, cells use aerobic respiration |
| ”Fermentation and anaerobic respiration are exactly the same thing” | Fermentation is one type of anaerobic respiration, carried out by yeast and some bacteria, producing ethanol + CO2. Animals do anaerobic respiration producing lactic acid — this is not called fermentation |