Respiration
What Is Respiration?
Respiration is the process by which living cells release energy from organic molecules (usually glucose) in the form of ATP (adenosine triphosphate). It occurs in every living cell, continuously.
- Energy is released and transferred to ATP.
- ATP is the universal energy currency of cells: it powers all cellular processes.
- Occurs in the mitochondria (aerobic) and cytoplasm (anaerobic).
Respiration vs Breathing
Respiration is a cellular chemical process (releasing energy from glucose). Breathing (ventilation) is the mechanical process of moving air in and out of lungs. These are not the same thing.
Aerobic Respiration
Aerobic respiration uses oxygen to fully break down glucose, releasing the maximum amount of energy.
Word equation:
glucose + oxygen → carbon dioxide + water (+ energy released as ATP)
Balanced symbol equation:
- Occurs in the mitochondria
- Releases large amounts of ATP
- Requires a continuous supply of oxygen and glucose
Anaerobic Respiration
Anaerobic respiration occurs without oxygen. It releases far less energy than aerobic respiration and produces different by-products depending on the organism.
In Animals (and humans during intense exercise):
Word equation:
glucose → lactic acid (+ small amount of ATP)
- Lactic acid builds up in muscles → muscle fatigue and pain
- After exercise, lactic acid is oxidised (broken down) in the liver: this requires extra oxygen (the oxygen debt)
In Plants and Yeast (fermentation):
Word equation:
glucose → ethanol + carbon dioxide (+ small amount of ATP)
- Used industrially in brewing (alcohol) and baking (CO₂ makes bread rise)
Aerobic vs Anaerobic Respiration
| Feature | Aerobic | Anaerobic |
|---|---|---|
| Oxygen required? | Yes | No |
| ATP yield | High (38 ATP per glucose) | Low (2 ATP per glucose) |
| End products | CO₂ + H₂O | Animals: lactic acid; Plants/yeast: ethanol + CO₂ |
| Location | Mitochondria | Cytoplasm |
| Duration | Sustained activity | Short bursts only |
Practical: Investigate Respiration (Spec 2.39)
Investigating aerobic respiration using germinating seeds:
Method:
- Place germinating seeds (actively respiring) in a sealed flask with a CO₂ indicator (hydrogencarbonate indicator: red at neutral pH).
- As seeds respire, CO₂ is produced → indicator turns yellow (acidic).
- Compare to a control flask with boiled (dead) seeds: no colour change.
Alternative: measuring oxygen consumption:
- Use a respirometer: sealed apparatus with a manometer (fluid level indicator).
- Soda lime absorbs CO₂ produced.
- As O₂ is consumed, the manometer fluid moves: rate of movement indicates rate of respiration.
Variables to investigate: temperature, type of substrate, germinating vs. non-germinating seeds.
Exam Questions
Q: Write the word equation for aerobic respiration.
Glucose + oxygen → carbon dioxide + water
Q: Write the word equation for anaerobic respiration in animals.
Glucose → lactic acid
Q: State two differences between aerobic and anaerobic respiration.
- Aerobic respiration requires oxygen; anaerobic respiration does not. 2. Aerobic respiration releases much more energy (ATP) per glucose molecule than anaerobic respiration.
Q: Explain why lactic acid builds up in muscles during vigorous exercise.
During intense exercise, muscles cannot receive oxygen fast enough for aerobic respiration. They switch to anaerobic respiration, which produces lactic acid as a by-product. Lactic acid accumulates faster than it can be removed, causing fatigue and pain.
Q: Explain what is meant by oxygen debt.
After vigorous exercise, extra oxygen is needed to oxidise (break down) the lactic acid that accumulated during anaerobic respiration. This extra oxygen requirement above the resting level is called the oxygen debt.
Exam Tip
The equation for aerobic respiration is the reverse of photosynthesis. Both involve the same molecules (glucose, O₂, CO₂, H₂O) but in opposite directions. In an exam, clearly state which process you are referring to and which direction the molecules move.