IGCSE Biology | Answers | 25 Questions
State the definition of respiration.
In which cells does respiration take place?
This includes cells in animals, plants, fungi, and microorganisms — not just human cells, and not just cells that are active or moving.
State three uses of energy in living organisms.
State three more uses of energy in living organisms, different from those you gave in Question 3.
Describe aerobic respiration. Include: what substance is broken down, what gas is required, and how much energy is released compared to anaerobic respiration.
State the word equation for aerobic respiration.
State the balanced chemical equation for aerobic respiration.
Describe anaerobic respiration. Include: what substance is broken down, what is not needed, and how much energy is released compared to aerobic respiration.
State the word equation for anaerobic respiration in humans.
State the word equation for anaerobic respiration in yeast.
A student says: “Humans only use energy when they are exercising.” Give two uses of energy that show the body needs energy even when completely at rest.
Any two of the following (must be uses that clearly apply at rest):
- Maintaining a constant body temperature — the body continuously generates heat to stay at 37°C, even when not moving.
- Protein synthesis — cells are always building proteins for growth and repair, which requires energy.
- Passage of electrical impulses along neurones — the nervous system is constantly active, sending signals around the body.
- Active transport — cells are always moving substances across membranes against concentration gradients, which requires energy.
A student mixes yeast with glucose solution and places the flask in a water bath at 35°C. She then repeats at 20°C. The yeast produces more CO₂ per minute at 35°C. Explain why higher temperature increases the rate of respiration in yeast.
Respiration in yeast is controlled by enzymes. At higher temperatures, the enzyme molecules and glucose molecules move faster and collide more often. This means the enzymes carry out the reactions of respiration more quickly, so more CO₂ is produced per minute.
The same student repeats the yeast experiment at 65°C. She finds the rate of CO₂ production drops sharply. Explain why the rate decreases sharply at 65°C.
At 65°C, the temperature is too high. The enzymes that control respiration in yeast become denatured — their shape changes permanently, so they can no longer bind to glucose and carry out respiration. Without working enzymes, respiration slows or stops completely, and very little CO₂ is produced.
During a 400 m race, a runner’s muscles start to ache and feel very tired. Name the substance that builds up in the muscles, and explain why it accumulates during intense exercise.
During intense exercise, the muscles need energy faster than aerobic respiration can supply it (there is not enough oxygen delivered quickly enough). The muscles switch to anaerobic respiration, which produces lactic acid. Lactic acid builds up in the muscles and passes into the blood because it is produced faster than it can be removed.
Explain what is meant by oxygen debt (also called EPOC — Excess Post-exercise Oxygen Consumption).
After exercise, the body continues to take in more oxygen than normal until all the lactic acid has been broken down. The amount of extra oxygen used for this is called the oxygen debt (or EPOC).
After a sprint race, a runner’s heart continues to beat rapidly for several minutes. Explain how a fast heart rate helps to remove the oxygen debt.
A fast heart rate keeps the blood flowing quickly around the body. This rapid blood flow transports lactic acid from the muscles to the liver. In the liver, the lactic acid is broken down using oxygen. Without the fast heart rate, lactic acid would move slowly and take much longer to be removed.
After the same sprint race, the runner also continues to breathe fast and deeply. Explain how this helps to remove the oxygen debt.
Fast and deep breathing brings extra oxygen into the lungs, which then passes into the blood. This oxygen is carried to the liver, where it is used to break down the lactic acid. Without the extra oxygen supply from continued deep breathing, the liver could not break down the lactic acid quickly enough.
A student says: “Anaerobic respiration is better than aerobic respiration because it doesn’t need oxygen.” Explain why this statement is misleading.
The statement is misleading because aerobic respiration releases a much larger amount of energy from the same amount of glucose. Anaerobic respiration only partially breaks down glucose and releases a small amount of energy.
The body uses aerobic respiration whenever possible because it is far more efficient. Anaerobic respiration is only used when oxygen supply is insufficient — it is not a preferred or “better” option; it is a short-term solution that also causes lactic acid to build up, which is harmful.
A student records the following results for yeast respiration:
| Temperature (°C) | CO₂ bubbles per minute |
|---|---|
| 10 | 4 |
| 25 | 15 |
| 40 | 28 |
| 55 | 6 |
Describe the pattern shown by these results and explain it.
Pattern: The rate of respiration increases from 10°C to 40°C, reaching a peak at 40°C. Above 40°C the rate falls sharply — at 55°C the rate is lower than even at 10°C.
Explanation:
- 10°C to 40°C: As temperature increases, enzyme molecules have more energy and move faster. They collide with glucose molecules more frequently, so respiration speeds up and more CO₂ is produced.
- Above 40°C (at 55°C): The temperature exceeds the optimum for the yeast’s enzymes. The enzymes become denatured — their shape changes and they can no longer carry out respiration. The rate drops sharply as a result.
Aerobic and anaerobic respiration in humans both begin with glucose. State one product that is the same in both types of respiration in humans, and one product that is different.
Aerobic respiration produces carbon dioxide and water. Anaerobic respiration in humans produces lactic acid (no carbon dioxide or water).
A student says: “Plants make their own food through photosynthesis, so they do not need to respire.”
(a) State the definition of respiration. (b) State four uses of energy in living organisms. (c) Explain why the student’s statement is incorrect.
Any four of: muscle contraction • protein synthesis • cell division • active transport • growth • passage of electrical impulses along neurones • maintenance of a constant body temperature.
Photosynthesis makes glucose, but it does not release energy for the plant to use. To use the energy stored in glucose, the plant must carry out respiration. Plants need energy for the same reasons as other organisms: to grow, to build proteins, to carry out active transport, and for cell division. Without respiration, plants cannot power any of these processes — even though they can make their own glucose through photosynthesis.
A student is comparing aerobic and anaerobic respiration.
(a) Word equation for aerobic respiration. (b) Balanced chemical equation for aerobic respiration. (c) Word equations for anaerobic respiration in (i) humans and (ii) yeast. (d) Two differences between aerobic and anaerobic respiration.
- Oxygen: Aerobic respiration requires oxygen; anaerobic respiration does not.
- Energy released: Aerobic respiration releases a relatively large amount of energy; anaerobic respiration releases a relatively small amount of energy from the same amount of glucose.
Also acceptable: Different products (aerobic produces CO₂ + water; anaerobic in humans produces lactic acid).
A student investigates the effect of temperature on the rate of respiration in yeast.
(a) Describe how to carry out the investigation, including equipment and what is measured. (b) Predict results at 20°C, 37°C and 60°C with explanations. (c) Identify one variable to control and explain why.
- Mix a fixed amount of yeast with a fixed volume of glucose solution in a flask.
- Place the flask in a water bath set to a specific temperature and allow it to equilibrate.
- Attach a delivery tube from the flask into a container of water (or limewater), so that CO₂ bubbles can be counted.
- Count the number of CO₂ bubbles produced per minute — this is the measure of the rate of respiration.
- Repeat the whole procedure at several different temperatures, keeping all other variables the same.
- 20°C: A moderate rate of CO₂ production. Enzymes are active but working slowly because the temperature is below the optimum. Molecules have less energy and collide less frequently.
- 37°C: A higher rate of CO₂ production. This temperature is closer to the optimum for yeast enzymes, so they work faster and more glucose is broken down per minute.
- 60°C: A very low or zero rate of CO₂ production. At this temperature the yeast enzymes are denatured — their shape changes permanently and they can no longer catalyse respiration.
Amount (mass) of yeast or volume/concentration of glucose solution.
If the amount of yeast or glucose changes between experiments, then the rate of CO₂ production will change for a reason other than temperature. This would make the results unreliable and it would be impossible to draw a valid conclusion about the effect of temperature alone.
During vigorous exercise, the body builds up an oxygen debt.
(a) Explain why lactic acid builds up in muscles and blood during vigorous exercise. (b) Explain what is meant by oxygen debt. (c) Describe how the oxygen debt is removed, including the role of (i) heart rate and (ii) breathing.
During vigorous exercise, the muscles need energy faster than aerobic respiration can supply it — the oxygen delivery to muscles is not fast enough. The muscles switch to anaerobic respiration, which does not require oxygen but produces lactic acid. Lactic acid accumulates in the muscles and passes into the blood because it is being produced faster than it can be removed and broken down.
Oxygen debt (EPOC) is the extra amount of oxygen the body needs after exercise to break down the lactic acid that built up during anaerobic respiration. After exercise stops, the body must continue to take in more oxygen than normal to repay this debt.
After exercise, the heart rate stays elevated, keeping blood circulating quickly. This transports lactic acid from the muscles through the blood to the liver, where it can be broken down.
Breathing stays fast and deep after exercise, supplying extra oxygen to the lungs and then to the blood. This oxygen is delivered to the liver, where it is used to break down the lactic acid. Once all the lactic acid has been removed, the oxygen debt is repaid and breathing and heart rate return to normal.
A long-distance runner uses aerobic respiration for most of a race. In the final sprint, her muscles switch to anaerobic respiration. After crossing the finish line, she continues to breathe heavily and her heart rate stays high for several minutes.
(a) Word equation for aerobic respiration. (b) Name the product that builds up during the sprint and explain why. (c) Explain fully how the continued fast heart rate and deep breathing remove the oxygen debt, and state what happens once it is repaid.
During the final sprint, the runner’s muscles need energy much faster than aerobic respiration can provide it — the oxygen being delivered is not sufficient for the demand. The muscles switch to anaerobic respiration, which produces lactic acid. Lactic acid builds up in the muscles and passes into the blood because it is being produced faster than it can be removed.
After the race, the runner’s heart continues to beat rapidly. This keeps blood flowing quickly, transporting lactic acid from the muscles to the liver.
At the same time, fast and deep breathing brings extra oxygen into the blood. This oxygen is also carried to the liver, where it is used to break down the lactic acid.
Once all the lactic acid has been broken down and the oxygen debt is fully repaid, the runner’s heart rate and breathing rate return to their normal resting levels.
