Edexcel IGCSE Biology · Spec 3.5
Germination - Required Practical
Required practical: investigating the conditions that seeds need in order to germinate.
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Germination - Required Practical
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Explained
Four tubes, three conditions, one control
Seeds need water, oxygen and a suitable temperature to germinate. To show that each one is necessary, you set up four tubes: one with everything, and three that each lack exactly one condition.
The four tubes
- Tube A has water, air and a warm room. This is the control, and the seeds germinate.
- Tube B has dry cotton wool, air and warmth. No water, so no germination.
- Tube C has water and warmth but no oxygen. No germination.
- Tube D has water and air but stands in a refrigerator. Too cold, so no germination.
The logic depends on each tube differing from the control in only one way. If tube D were both cold and dry, you could not say which caused the failure.
Removing the oxygen
Tube C is the one that needs explaining, because you cannot simply take the air out.
The water is boiled first, which drives out the dissolved oxygen, then cooled before use so that it does not kill the seeds. A layer of oil is poured on top, which stops oxygen from the air redissolving.
Both steps are needed and both are marked. Boiling alone leaves the water free to reabsorb oxygen as it stands; oil alone traps the oxygen that was already dissolved.
The variables
The independent variable is the condition being removed. The dependent variable is the number of seeds that germinate. The control variables are everything else: the same species, number, age and size of seeds, the same volume of water where water is present, and the same time allowed.
Germination is counted, not measured. A seed either has germinated or it has not, so the result is a number of seeds rather than a length of root.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme for an evaluation of a seed experiment lists what can be criticised, and the list is worth having in mind before you are asked. It credits that the experiment was not repeated so is not reliable, that it was only done for a short period, that the temperature was not controlled or not stated, and that the results could be explained by either aerobic or anaerobic respiration.
Elsewhere in the same question it credits the idea that dry seeds are not respiring, or respiring very slowly, because the enzymes are not active and there is no water for the metabolic reactions. That is the explanation behind tube B, and it is worth more than saying the seeds needed water.
On a planning question about seeds, another mark scheme awards separate marks for stating how the condition is changed, for using seeds of the same species, age or mass, for repeating and taking a mean, for counting the number germinated, for a stated time period, and for keeping temperature, water, oxygen and soil the same. It adds that repeating at a different setting does not count as repeating.
That structure is the template for any plan: what you change, what you keep the same, what you count, when you count it, and how many times you repeat.
Improving reliability
Use many seeds in each tube rather than one or two, because an individual seed may be dead or damaged for reasons unconnected with the experiment. Ten or twenty per tube lets you report a percentage.
Repeat the whole experiment and take a mean. Leave it long enough, since germination takes days rather than hours, and record how long you left it.
And check the seeds are viable to begin with, which is exactly what the control tube does. If nothing germinates in tube A, the experiment has told you nothing about water, oxygen or temperature.
Spec 3.5
What you need to know
- Describe how to test the conditions seeds need
- Explain how cooled boiled water is used to remove oxygen
- Identify the controls and ways to improve reliability
Active recall
Quick check
Answer each question before opening the answer.
What three conditions do seeds need to germinate?
Water, oxygen and a suitable (warm) temperature.
How could you test whether seeds need oxygen to germinate?
Set up identical seeds but remove oxygen from one set, keeping water and temperature the same, and compare germination.
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