Edexcel IGCSE Biology · Spec 3.6
Germination
The conditions needed for germination — water, oxygen and a suitable temperature.
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Germination
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Explained
What a seed needs, and what it lives on until it can feed itself
A seed is a plant waiting. It contains an embryo and a food store, wrapped in a protective coat, and it can stay dormant for a long time until conditions are right.
The parts of a seed
- Testa, the tough outer coat that protects the seed.
- Micropyle, a tiny hole in the testa that lets water in.
- Cotyledon, the food store, usually starch.
- Radicle, which grows into the root.
- Plumule, which grows into the shoot.
The radicle emerges first, before the plumule. That order makes sense: the seedling needs to anchor itself and take up water before it puts anything above ground.
The three conditions
Germination needs water, oxygen and a suitable temperature. Each one has a reason, and the reason is what the marks are for.
Water is needed to activate the enzymes and to transport the dissolved food around the seed. It also swells the seed and splits the testa.
Oxygen is needed for aerobic respiration, which releases the energy for growth. A seed buried too deep or in waterlogged soil cannot get enough.
A suitable temperature is needed because the enzymes work best within a range. Too cold and they work too slowly; too hot and they denature.
Light is not required, which is the point of the standard experiment. A seed germinates in the dark perfectly well, because it is living on its own food store at that stage.
From stored food to photosynthesis
The cotyledon stores starch, which is insoluble and so cannot be moved. Amylase converts it to maltose and then to glucose, which is soluble and can be transported to the growing radicle and plumule.
Some of that glucose is respired for energy and some is used to build new cells. Either way the store is being spent, so the seedling actually loses mass at first.
Once the plumule reaches the light and the first green leaves open, the seedling can photosynthesise and make its own glucose. From that moment it gains mass. The switch from losing to gaining is the point at which it stops depending on the cotyledon.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme for a plan to investigate whether light affects germination shows exactly how these questions are built. Separate marks are available for the variable being changed, for using seeds of the same species, age or mass, for repeating and taking a mean, for the measurement, for a stated time period, and for keeping temperature, water, oxygen and soil the same.
The measurement mark is for counting the number of seeds that have germinated, and the mark scheme adds that measuring the roots or shoots is ignored. Germination is a yes or no event, so it is counted, not measured.
Two notes show where plans fall down. Different light exposure, left unexplained, does not earn the first mark, and repeating at different light periods does not count as repeating. A repeat means doing the same condition again, not doing another condition.
The structure is worth copying into any plan question: what you change, what you keep the same, what you measure, when you measure it, and how many times you repeat.
The classic experiment
Four tubes of seeds test the three conditions one at a time. One has water, air and warmth and germinates. One has no water and does not. One has water but no air, achieved by boiling the water to drive out oxygen and covering it with oil, and does not. One has water and air but sits in a refrigerator and does not.
Each tube differs from the control in exactly one way, which is what allows you to say the missing condition was the cause.
Spec 3.6
What you need to know
- Recall the conditions a seed needs to germinate
- Describe how a seed uses its stored food
- Explain why the young plant later makes its own food
Active recall
Quick check
Answer each question before opening the answer.
Why do germinating seeds need water and oxygen?
Water activates enzymes and mobilises the food store; oxygen is needed for aerobic respiration to release energy for growth.
What is the role of the seed's food store?
It supplies energy and nutrients until the seedling grows leaves and can photosynthesise.
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