Respiration
Structures and Functions in Living Organisms · 7 question types
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 18% of your exam marks.
Aerobic and anaerobic respiration equations and comparisons are consistently tested.
Showing that respiration produces carbon dioxide
Principle: living cells release carbon dioxide during respiration. Limewater (calcium hydroxide solution) turns cloudy white when carbon dioxide is bubbled through it, so it can be used to detect the CO₂ given off by a respiring organism.
Apparatus: three sets of boiling tubes, each containing a delivery tube that bubbles into a second boiling tube of limewater. The three test tubes hold:
- Tube A: 10 germinating peas on damp cotton wool (the test)
- Tube B: 10 boiled (dead) peas on damp cotton wool (control for whether the organism is alive)
- Tube C: 10 glass beads on damp cotton wool (control for whether the seed material itself matters)
Method:
- Set up the three identical test tubes as above, each sealed with a bung connected by a delivery tube to a fresh tube of limewater. Make sure each delivery tube dips below the limewater so any gas bubbles through it.
- Leave the whole apparatus in a warm place (25–30 °C) for several hours or overnight.
- Check each limewater tube for cloudiness.
Expected results:
| Tube | What's inside | Limewater result | Why |
|---|---|---|---|
| A | Germinating peas | Cloudy | The seeds are alive and respiring, releasing CO₂ |
| B | Boiled peas | Stays clear | Boiling killed the cells, so they cannot respire |
| C | Glass beads | Stays clear | No living material, no respiration |
The boiled-seed control rules out the possibility that the seed material (rather than living respiration) is releasing CO₂. The glass-bead control rules out the apparatus or air itself being responsible.
Describe what limewater shows in a respiration experiment
What comes up: a 3-mark "describe" question asks you to explain how limewater (or hydrogen carbonate indicator) is used to compare carbon dioxide in inhaled and exhaled air, or to detect CO₂ from a respiring organism. Sometimes framed as "what results would you expect and why?"
Write (three marks): (1) use limewater (or hydrogen carbonate indicator, or sodium hydrogencarbonate solution) as the indicator; (2) the limewater in the tube connected to the respiring sample turns cloudy/milky because carbon dioxide is released; (3) the control tube (with dead/boiled material or inhaled air) stays clear because no carbon dioxide is produced, confirming the change is due to living respiration.
Watch out: the mark scheme requires you to say the limewater "turns cloudy" — writing "changes colour" on its own scores no credit. Also, in exhaled-air experiments the mark scheme specifically rejects the claim that inhaled air contains no CO₂; it accepts "less CO₂" in inhaled air but not "no CO₂".
Showing that respiration releases heat
Principle: aerobic respiration releases energy, and not all of that energy is captured as ATP. A noticeable fraction is released as heat, which can be detected with a thermometer in a well-insulated container.
Apparatus: two vacuum flasks, each containing a thermometer and damp cotton wool. The flasks are inverted (mouth-down) so warm air does not simply float out. One flask holds germinating seeds, the other holds boiled (dead) seeds as a control.
Both the seeds and the flasks are first sterilised with weak bleach to kill any microbes. This step is crucial: without it, microbes growing on the seeds will respire themselves and produce heat, giving misleading results.
Method:
- Sterilise both flasks and both sets of seeds with weak bleach.
- Place boiled seeds in flask A and germinating seeds in flask B.
- Insert a thermometer into each flask with the bulb buried in the seeds.
- Plug the mouth of each flask with cotton wool (holds the thermometer in place and reduces heat loss).
- Invert the flasks and leave them in a place with constant room temperature for 2–3 days.
- Record the temperature inside each flask at the start and at the end.
Expected results: the temperature in the flask with germinating seeds rises by several degrees (e.g. 20 °C → 25 °C). The temperature in the flask with dead seeds stays the same as the room.
