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.
Learning Objectives¶
| ID | Official specification wording | Main teaching sections |
|---|---|---|
2f-lo-1 |
2.34 understand how the process of respiration produces ATP in living organisms 2.35 know that ATP provides energy for cells 2.36 describe the differences between aerobic and anaerobic respiration 2.37 know the word equation and the balanced chemical symbol equation for aerobic respiration in living organisms 2.38 know the word equation for anaerobic respiration in plants and in animals |
ATP and Aerobic Respiration, Anaerobic Respiration |
2f-lo-2 |
2.39 practical: investigate the evolution of carbon dioxide and heat from respiring seeds or other suitable living organisms | Respiration in Practical Work |
ATP and Aerobic Respiration¶
Respiration is an exothermic reaction that 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. 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 and muscle cramps. An oxygen debt builds up: after exercise, the body continues to take up extra oxygen to oxidise or 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 aerobic respiration is exothermic.
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.
- Exothermic: describing a process that releases energy to the surroundings.
- Hydrogen-carbonate indicator: a solution that changes colour with CO₂ concentration, used to detect respiration.