Core Practical: Investigating Respiration — Respiration | IGCSE Biology

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 18% of your exam marks.

stable

Very High

Stable18%

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

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.

Exam tip

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 germinating seeds in flask A and boiled 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.