2f Respiration¶
Part of 2 Structure and Functions in Living Organisms.
Respiration is how cells release usable energy from food. The main distinctions here are between aerobic and anaerobic pathways and between energy release in living tissue and what you can measure in practical work.
What You Need to Learn¶
Further detail: Pearson Edexcel International GCSE Biology specification.
On this page you'll learn about ATP and aerobic respiration, anaerobic respiration, and respiration in practical work. The notes bring these ideas together into one clear overview of respiration.
ATP and Aerobic Respiration¶
Respiration takes place continuously in living cells, releasing energy from nutrient molecules. This energy is stored in a molecule called ATP, which is then used to drive processes such as active transport, muscle contraction and protein synthesis.
Aerobic respiration uses oxygen and oxidises glucose fully, releasing the most energy:
Word equation: glucose + oxygen → carbon dioxide + water
Symbol equation: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
Most of the reactions of aerobic respiration occur in the mitochondria, which is why cells with high energy demand — such as muscle cells and sperm cells — contain many mitochondria.
Anaerobic Respiration¶
Anaerobic respiration occurs when oxygen supply is insufficient, such as during a sprint. In humans, most respiration is aerobic, but muscles can respire anaerobically for short periods when oxygen cannot be supplied quickly enough. Because glucose is not fully oxidised, less energy is released than in aerobic respiration.
In animals (including humans), the equation is:
glucose → lactic acid
Lactic acid accumulates in muscles during intense exercise, contributing to fatigue, muscle cramps and sometimes the sensation of a stitch. An oxygen debt builds up because lactic acid must be broken down and removed after exercise. This is why breathing and heart rate can stay elevated for a time after exercise: extra oxygen is needed to remove the lactic acid.
In plant cells and yeast (called fermentation), the equation is:
glucose → ethanol + carbon dioxide
The carbon dioxide released by yeast causes bread to rise. The ethanol produced is the basis of alcoholic drinks.
Comparing Aerobic and Anaerobic Respiration¶
| Feature | Aerobic | Anaerobic |
|---|---|---|
| Oxygen required? | Yes | No |
| Energy released | More | Less |
| End products (animals) | CO₂ + water | Lactic acid |
| End products (yeast/plants) | CO₂ + water | Ethanol + CO₂ |
| Location in cell | Mainly mitochondria | Cytoplasm |
Respiration in Practical Work¶
Investigating carbon dioxide production from respiring organisms: Germinating seeds or small organisms are placed in a sealed flask connected by a capillary tube to hydrogen-carbonate indicator solution. A separate flask containing sodium hydroxide (to absorb CO₂ from the air) and a control flask (with dead or boiled seeds) ensure that colour changes are due to respiration. Hydrogen-carbonate indicator is red at normal atmospheric CO₂ concentration, purple when CO₂ falls and yellow when CO₂ rises. The indicator near respiring seeds turns yellow as CO₂ from respiration dissolves in it.
Investigating heat production from respiring organisms: Living germinating seeds and dead (boiled) seeds as a control are placed in separate vacuum flasks with thermometers insulated in cotton wool. After a few days, the flask with living seeds shows a greater temperature rise because respiration releases energy, some of which appears as heat.
Explore a respirometer model¶
Use the interactive below to load organisms into a sealed chamber, absorb carbon dioxide with potassium hydroxide, and watch capillary fluid rise as oxygen is used. It helps visualise one way aerobic respiration can be measured in a closed apparatus, but it complements the carbon-dioxide and heat practicals described above rather than replacing them. Open full interactive.
In this model, the organisms use oxygen and release carbon dioxide. The potassium hydroxide absorbs the carbon dioxide, so the total gas volume in the sealed tube falls. That pressure drop allows atmospheric pressure on the open side to push the capillary fluid upwards, so the fluid movement is being used here as a model of oxygen uptake during aerobic respiration.
What this simulation does not show¶
- The fluid movement is generated from a simplified rate model rather than from measured gas-volume changes from real organisms.
- Potassium hydroxide is treated as absorbing carbon dioxide completely, so the remaining volume change reflects oxygen uptake only.
- The named course practical focuses on carbon dioxide release and heat production. This model is mainly useful for visualising oxygen uptake during aerobic respiration in a sealed chamber.
Key Terms¶
- Respiration: the chemical process in cells that releases energy from food molecules.
- ATP: an energy-carrying molecule used to power cell activities.
- Aerobic respiration: respiration that uses oxygen and releases a large amount of energy.
- Anaerobic respiration: respiration that occurs without oxygen and releases less energy.
- Lactic acid: the product of anaerobic respiration in animal cells.
- Fermentation: anaerobic respiration in yeast or plant cells, producing ethanol and carbon dioxide.
- Oxygen debt: the extra oxygen needed after anaerobic exercise to remove lactic acid from the body.
- Hydrogen-carbonate indicator: a solution that changes colour with CO₂ concentration, used to detect respiration.