Edexcel IGCSE Biology · Spec 2.39
Respiration Practical
Required practical: showing that respiring organisms release carbon dioxide and heat, using a control.
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Respiration Practical
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Explained
Showing that seeds respire
Respiration releases energy from glucose, and it produces carbon dioxide and heat. Both of those are detectable, so two straightforward experiments can demonstrate that something is alive and respiring.
Germinating seeds are used because they respire rapidly while growing, and because a dead version of the same thing is easy to prepare.
Showing carbon dioxide is released
Put germinating seeds in a sealed flask connected to a tube of limewater. Draw air slowly through with a pump, so the air that has passed over the seeds bubbles through the limewater.
Limewater turns milky, or cloudy, when carbon dioxide passes through it. If it turns milky, the seeds have released carbon dioxide.
Air drawn in should pass through soda lime or a first tube of limewater first, to remove the carbon dioxide already in the air. Otherwise you cannot say the gas came from the seeds.
Showing heat is released
Put germinating seeds in a vacuum flask with a thermometer through the stopper, and record the temperature over a day or two.
The temperature rises. The vacuum flask matters because it reduces energy transfer to the surroundings, so the small amount of heat released accumulates enough to measure.
Rinse the seeds in disinfectant first. Without that, microorganisms growing on them would respire too, and the rise could be theirs rather than the seeds'.
Why a control is essential
The control is an identical flask containing seeds that have been boiled and then cooled. Boiling kills them, so they cannot respire, but they are otherwise the same material in the same apparatus.
If the living seeds warm up and turn the limewater milky while the dead seeds do neither, the difference must be caused by the one thing that differs, which is that the seeds are alive.
Without the control, a temperature rise could be the room warming up, and cloudy limewater could be carbon dioxide from the air. The control is what rules out the alternative explanations, and saying so is what the mark is for.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme gives a short instruction that belongs in every answer on this topic: ignore produces energy.
Respiration does not produce or create energy. It releases energy that was already stored in glucose, and transfers it to a form the cell can use. The mark scheme credits releases energy or provides ATP, and passes over produces energy without awarding anything.
The same mark scheme is strict about naming structures too. On a question about which organelle carries out respiration, it says there is no credit if the mitochondrion is called the cytoplasm or a chloroplast, and that naming any of the other labelled structures for respiration earns nothing.
On a related practical it lists the control variables individually: the temperature, the time, and the volume of each solution. Each is a separate thing to hold constant, and a question asking for one wants a specific quantity rather than the phrase keep everything else the same.
What the results show
Aerobic respiration uses glucose and oxygen and produces carbon dioxide and water, releasing energy. The word equation is worth writing out, because both experiments are simply detecting one of its products.
If a question asks why the dead seeds still produce a small effect, the answer is usually microorganisms on their surface, which is exactly why the disinfectant step exists.
Spec 2.39
What you need to know
- Show that respiring organisms release carbon dioxide
- Show that respiration releases heat
- Explain why we use a control
Active recall
Quick check
Answer each question before opening the answer.
How can you show that living organisms produce carbon dioxide?
Use hydrogencarbonate indicator (turns yellow) or limewater (turns milky/cloudy) with the respiring organisms.
Why do you need a control tube with no organisms?
To show the result is caused by the organisms and not by other factors.
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