Edexcel GCSE Biology (1BI0) — Exchange and Transport in Animals
You can sprint at top speed for about 15 seconds before your legs feel like lead, but you can walk for hours without stopping. This difference comes down to how your cells release energy from food. Both aerobic respiration and anaerobic respiration break down nutrient molecules like glucose to release energy. This energy is stored in the "energy currency" molecule called ATP (Adenosine Triphosphate). Both processes are exothermic reactions, meaning they transfer energy to their surroundings, often as heat. However, the way they break down glucose and the amount of ATP they yield are very different.
When asked to compare these two processes in an exam, you must provide both similarities and differences. Aerobic respiration uses oxygen to fully break down glucose, while anaerobic respiration happens when oxygen is absent, leading to an incomplete oxidation of glucose. Because the breakdown is incomplete, energy remains "locked" in the chemical bonds of the products.
| Feature | Aerobic Respiration | Anaerobic Respiration |
|---|---|---|
| Similarities | Uses glucose as a reactant. Exothermic (releases energy). | Uses glucose as a reactant. Exothermic (releases energy). |
| Oxygen | Requires oxygen. | Does not require oxygen. |
| Oxidation of glucose | Complete oxidation. | Incomplete oxidation. |
| ATP Yield | Very large (roughly 32–38 ATP molecules per glucose molecule). |
| Very small (exactly 2 ATP molecules per glucose molecule). |
| Efficiency | Highly efficient (roughly 16–19 times more efficient). | Much less efficient. |
| Products (Animals) | Carbon dioxide () and water (). | Lactic acid only. |
| Products (Plants & Yeast) | Carbon dioxide () and water (). | Ethanol and carbon dioxide (). |
You must know both the word and balanced symbol equations for these processes. In yeast cells, anaerobic respiration is known as fermentation and is economically important for making bread rise and brewing alcoholic drinks.
1. Aerobic Respiration (All Organisms) Glucose + Oxygen Carbon dioxide + Water
2. Anaerobic Respiration (Animals) Glucose Lactic acid
3. Anaerobic Respiration (Plants and Yeast) Glucose Ethanol + Carbon dioxide
The two types of respiration occur in different parts of the cell. Anaerobic respiration takes place entirely within the cytoplasm, which is the jelly-like substance where many chemical reactions happen.
In contrast, while the very first stage of aerobic respiration starts in the cytoplasm, the bulk of the oxygen-dependent process and energy release occurs inside the mitochondria. Mitochondria contain the specific enzymes needed for aerobic respiration. Cells with particularly high energy demands, such as muscle cells for contraction or sperm cells for swimming, contain a very high number of mitochondria.
During vigorous exercise, your body may not be able to supply enough oxygen to your muscle cells, forcing them to rely on anaerobic respiration. This causes lactic acid to build up. Lactic acid is a mild toxin that lowers the tissue pH and causes muscle fatigue, a condition where muscles lose their ability to contract efficiently and begin to cramp.
To clear this mild toxin, lactic acid is transported out of the muscles via the bloodstream and taken to the liver. In the liver, the lactic acid is either oxidised into carbon dioxide and water, or converted back into glucose (which can be stored as glycogen).
Because oxidising the lactic acid requires oxygen, you continue to breathe heavily and maintain a high heart rate even after you stop exercising. This extra oxygen required by the body to react with and remove the accumulated lactic acid is called the oxygen debt.
Never use the words 'produce' or 'create' when talking about energy in exams. You will lose marks! Always say respiration 'releases' or 'transfers' energy.
Students often incorrectly state that anaerobic respiration in humans produces carbon dioxide. In human muscle cells, the ONLY product is lactic acid.
In 6-mark 'Compare' questions, ensure you explicitly state the similarities (e.g., both use glucose, both are exothermic) as well as the differences, using comparative words like 'whereas' or 'more efficient than'.
To easily remember the balancing for the aerobic respiration symbol equation, remember the '666' rule: there are six O2, six CO2, and six H2O molecules.
Aerobic respiration
The chemical process in cells that uses oxygen to fully break down nutrient molecules to release a large amount of energy (ATP).
Anaerobic respiration
The chemical process in cells that partially breaks down nutrient molecules without oxygen, releasing a small amount of energy.
ATP (Adenosine Triphosphate)
The 'energy currency' of the cell, used to store and transfer energy for cellular processes like muscle contraction.
Exothermic
A chemical reaction that transfers energy to the surroundings, often in the form of heat.
Complete oxidation
The full breakdown of glucose in the presence of oxygen, leaving no energy locked in the products.
Incomplete oxidation
The partial breakdown of glucose in the absence of oxygen, resulting in products that still contain locked chemical energy.
Fermentation
A type of anaerobic respiration in plant and yeast cells that produces ethanol and carbon dioxide.
Cytoplasm
The jelly-like substance inside a cell where anaerobic respiration takes place.
Mitochondria
Sub-cellular organelles where the majority of aerobic respiration occurs to release energy.
Lactic acid
A mild toxic product of anaerobic respiration in animal cells that lowers tissue pH.
Muscle fatigue
A condition where muscles lose their ability to contract efficiently due to long periods of activity and lactic acid buildup.
Glycogen
A storage carbohydrate in animals, which the liver can create by converting lactic acid back into glucose.
Oxygen debt
The volume of extra oxygen required by the body after exercise to react with and remove accumulated lactic acid.
Put your knowledge into practice — try past paper questions for Biology
Aerobic respiration
The chemical process in cells that uses oxygen to fully break down nutrient molecules to release a large amount of energy (ATP).
Anaerobic respiration
The chemical process in cells that partially breaks down nutrient molecules without oxygen, releasing a small amount of energy.
ATP (Adenosine Triphosphate)
The 'energy currency' of the cell, used to store and transfer energy for cellular processes like muscle contraction.
Exothermic
A chemical reaction that transfers energy to the surroundings, often in the form of heat.
Complete oxidation
The full breakdown of glucose in the presence of oxygen, leaving no energy locked in the products.
Incomplete oxidation
The partial breakdown of glucose in the absence of oxygen, resulting in products that still contain locked chemical energy.
Fermentation
A type of anaerobic respiration in plant and yeast cells that produces ethanol and carbon dioxide.
Cytoplasm
The jelly-like substance inside a cell where anaerobic respiration takes place.
Mitochondria
Sub-cellular organelles where the majority of aerobic respiration occurs to release energy.
Lactic acid
A mild toxic product of anaerobic respiration in animal cells that lowers tissue pH.
Muscle fatigue
A condition where muscles lose their ability to contract efficiently due to long periods of activity and lactic acid buildup.
Glycogen
A storage carbohydrate in animals, which the liver can create by converting lactic acid back into glucose.
Oxygen debt
The volume of extra oxygen required by the body after exercise to react with and remove accumulated lactic acid.
