Structures and Functions in Living Organisms · 7 question types
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.
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:
Method:
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.
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:
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.