Seeds and Germination

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 3.5 — Practical: investigate the conditions needed for seed germination (required practical)
• 3.6 — How germinating seeds use food reserves until photosynthesis begins

Specification section 3(a) Reproduction — also taught in Sexual and Asexual Reproduction, Flowers, Pollination and Fertilisation, The Human Reproductive Systems, The Menstrual Cycle.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

A dry seed can sit in a drawer for years doing almost nothing, and then produce a plant a week after you put it in damp compost. Nothing was added except water and a warm place — no food, no light, nothing green anywhere near it. So the first question is not what makes a seed grow. It is what a seed is living on while it does.

The answer is that the seed brought supplies. Inside every seed, alongside the embryo, there is a FOOD STORE — mostly starch, and in some seeds oil. The embryo cannot photosynthesise: it has no leaves, it is underground, and there is no light down there anyway. So it lives entirely on that store until the shoot breaks the surface and the first green leaves open. From that moment the plant makes its own food by photosynthesis and the store is finished with, which is exactly why the store is only big enough for the journey.

How a seed uses its store

1. Water is absorbed and the seed swells. This activates the ENZYMES inside it, which cannot work in a dry seed because enzymes work in solution.
2. Enzymes break the insoluble starch of the store down into soluble sugars, such as maltose and glucose. Insoluble store, soluble product — the same reason digestion exists in an animal.
3. Because they are soluble, those sugars can be transported to the growing parts of the embryo.
4. Some of that glucose is used in RESPIRATION, releasing the energy needed for growth — which is why the seed needs oxygen.
5. The rest supplies the raw material for building new cells: cellulose for walls, and, with mineral ions, amino acids and proteins.
6. Once the shoot is above ground and the leaves are green and open, PHOTOSYNTHESIS begins and the plant is self-supporting. The food store is exhausted at about the same time.

Think of it like a rocket carrying its own fuel to orbit

A rocket cannot pick up fuel on the way. It carries everything it needs for the one part of the journey it cannot get help with, burns through it steadily, and jettisons the empty stage the moment it reaches the point where something else can take over. A seed is doing exactly that. The store is heavy, so it is no bigger than it must be — which is why a seed planted too deep dies before it surfaces. It did not run out of luck; it ran out of fuel a few centimetres short.

The three conditions a seed needs — and the reason for each

1. WATER — to activate the enzymes that break down the food store, since enzymes work in solution; and to transport the soluble products to the embryo. It also swells the seed and splits the seed coat.
2. OXYGEN — for aerobic respiration, which releases the energy the embryo needs for growth. Without it the seed cannot use its store.
3. A SUITABLE TEMPERATURE — because the reactions are controlled by enzymes. Too cold and they are far too slow; too hot and the enzymes are denatured and germination stops permanently.

And now the one that catches people out: LIGHT IS NOT REQUIRED. A seed germinating underground has no light and does not need any, because it is not photosynthesising — it is living on its store. Light matters afterwards, once the leaves are open, and not before. If you are asked why a seed germinates in a dark cupboard, the answer is not that it is unusual; it is that light was never on the list.

There is a neat consequence you can measure. If you dig up germinating seeds day by day, dry them and weigh them, the total DRY MASS falls at first. That looks wrong — the seedling is visibly getting bigger. But mass is being lost as carbon dioxide during respiration, and nothing is coming in yet, since there are no leaves. The dry mass keeps falling until the shoot emerges and the leaves start photosynthesising, and only then does it start to rise. The turning point on that graph is the moment the plant stops spending and starts earning.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Germination

The beginning of growth of a seed, in which the embryo uses the seed's food store to develop into a seedling, until photosynthesis can begin.

The three conditions required for germination — each with the reason that earns the mark

• WATER — activates the enzymes, which can only work in solution; allows soluble products to be transported to the embryo; swells the seed and splits the seed coat
• OXYGEN — for aerobic respiration, which releases the energy needed for growth
• A SUITABLE TEMPERATURE — the reactions are enzyme-controlled: too cold and they are too slow, too hot and the enzymes are denatured
• LIGHT IS NOT REQUIRED — the seedling lives on its food store and cannot photosynthesise until its leaves are above ground and green

Statement 3.6 — how the food store is used, in order

1. Water is absorbed and the seed swells, activating the enzymes.
2. Enzymes break down the insoluble starch store into soluble sugars such as maltose and glucose.
3. The soluble sugars are transported to the growing parts of the embryo.
4. Some glucose is used in respiration, releasing energy for growth.
5. The rest supplies raw material for making new cells — cellulose for cell walls, and, with mineral ions, amino acids and proteins.
6. When the leaves are above ground and green, photosynthesis begins and the seedling becomes self-supporting.

Required practical 3.5 — the conditions needed for seed germination

1. Set up four boiling tubes or dishes, each containing 10 cress or mung bean seeds on cotton wool.
2. Tube A — the full set of conditions: damp cotton wool, air, at room temperature (about 20 °C).
3. Tube B — no water: dry cotton wool, air, room temperature.
4. Tube C — no oxygen: damp cotton wool covered with a layer of boiled, cooled water and a film of oil to seal out air, room temperature.
5. Tube D — low temperature: damp cotton wool, air, in a refrigerator at about 4 °C.
6. Leave for one week and count how many seeds in each tube have germinated.
7. Result: only tube A shows substantial germination. Each of B, C and D lacks one condition and shows little or none.

Variables

Independent (changed) — which single condition is removed — water, oxygen, or a suitable temperature
Dependent (measured) — number of seeds that germinate (or the percentage germination)

Sources of error

Reading the dry-mass graph — a favourite six-mark question

• y-axis: total dry mass of the seedlings. x-axis: time since planting
• The curve FALLS at first: the food store is being respired, and mass is lost as carbon dioxide with nothing coming in
• The MINIMUM is the point at which the leaves emerge, open and become green
• After the minimum the curve RISES: photosynthesis now adds more mass than respiration removes
• Dry mass is used rather than fresh mass because a seedling takes up a great deal of water, and water gained would hide the food store being used up

What a seed needs, against what a green plant needs

A. A GERMINATING SEED needs water, oxygen and a suitable temperature. It does NOT need light, because it is living on its own food store.

B. A GREEN SEEDLING needs light as well, because its store is exhausted and it must now make its own food by photosynthesis.

Not this: Seeds need light to germinate, since plants need light to grow.

This: Germination happens underground, in the dark, on the seed's own food store. Light becomes necessary only once the leaves are above the surface and photosynthesis has to take over.

Model answer [3 marks]

Explain why a germinating seed requires water. [3]

Water is absorbed by the seed and activates its enzymes, which can only work in solution. The enzymes break down the insoluble starch of the food store into soluble sugars. Water also allows these soluble products to be transported to the growing parts of the embryo, and it swells the seed so that the seed coat splits.

Model answer [4 marks]

Explain why the dry mass of a batch of germinating seeds falls before it rises. [4]

At first the seedling has no leaves above ground, so it cannot photosynthesise and no new material is being made. It respires the glucose from its food store to release energy for growth, and mass is lost as carbon dioxide, so the total dry mass falls. Once the leaves are above ground and green, photosynthesis begins. When the mass gained by photosynthesis exceeds the mass lost by respiration, the dry mass starts to rise.

Model answer [2 marks]

In an investigation into germination, explain why a tube kept in a refrigerator should be placed in a dark cupboard rather than on a windowsill. [2]

The refrigerated tube is in the dark as well as being cold, so a tube on a windowsill would differ from it in two ways at once. If two variables differ between the tubes, any difference in germination cannot be attributed to temperature alone, and the comparison proves nothing.

Mark-losing trap. Light is NOT needed for germination. The seedling is living on its food store, underground.

Mark-losing trap. Give the REASON for each condition. 'Water, oxygen and warmth' alone is one mark, not three.

Mark-losing trap. Say the enzymes break the store into SOLUBLE sugars. Insoluble starch cannot be transported anywhere.

Mark-losing trap. Dry mass, not fresh mass — a germinating seed takes up water, which would hide the store being used up.

Mark-losing trap. 'Too hot' denatures the enzymes and stops germination permanently. It does not merely slow it down.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — Which three conditions are needed for a seed to germinate?
2. Grade 7 · Explain [2 marks] — In the required practical, one tube of seeds is covered with boiled, cooled water and sealed with a layer of oil. Which condition is this tube testing, and why must the water be boiled first?
3. Grade 8 · Explain [5 marks] — Explain how a germinating seed obtains and uses the materials it needs before its leaves are above ground. Select every statement that belongs in a full-mark explanation.
4. Grade 9 · Calculate [3 marks] — In a germination investigation, 40 seeds are placed in each of four tubes. After one week, tube A (all conditions) has 34 germinated, tube B (no water) has 0, tube C (no oxygen) has 2 and tube D (4 °C) has 3. Calculate the percentage germination in tube A.
5. 9+ · Analyse [5 marks] — A student plants identical bean seeds at four depths — 1 cm, 3 cm, 6 cm and 12 cm — in the same damp compost at the same temperature. Nearly all the seeds at 1 cm and 3 cm produce healthy plants. At 6 cm fewer plants appear and they are weak. At 12 cm no plants appear at all, but digging up the seeds shows that they did germinate and grew long pale shoots that never reached the surface. Select every statement that belongs in a full-mark explanation.

The people behind this science

Two ways into the same idea — the one who proved when a plant needs light and when it does not, and the one who found a plant can spoil air as well as restore it. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Jan Ingenhousz — the one who proved when a plant needs light and when it does not

This page's most counter-intuitive claim is that light is not required for germination, and Ingenhousz is the person who established exactly where the boundary between light and darkness falls in a plant's life. In 1779 he showed that a plant purifies the air only in sunlight, only from its green parts, and that in darkness it spoils the air like an animal. A germinating seed is precisely that case — no green parts, no light, respiring a food store, giving out carbon dioxide. He would recognise the falling dry mass immediately, because it is the same plant doing the same thing his darkened jars measured.

• “How did you show that light, not warmth, is what a plant needs?”
• “What does a plant do in complete darkness?”
• “Which parts of a plant purify the air, and which never do?”
• “Why did Priestley's plant experiments sometimes fail?”
• “What would you expect from a seed germinating in the soil?”

Joseph Priestley — the one who found a plant can spoil air as well as restore it

The claim on this page that a germinating seed consumes oxygen and releases carbon dioxide can be demonstrated with Priestley's own apparatus and nothing else. He was the first to show that a living thing in a sealed jar changes the air in it — a mouse or a candle spoils it, a growing plant in the right conditions restores it. He also never managed to make the plant experiment work reliably, and the reason is on this page: he had not separated light from darkness, and a plant in the dark behaves exactly like the mouse.

• “What happened to the air in a jar with a mouse in it?”
• “How did you show that a sprig of mint could restore that air?”
• “Why did the experiment fail on some occasions and work on others?”
• “What would you expect if you sealed germinating seeds in a jar?”
• “Why did you never accept Lavoisier's explanation of your own discovery?”

Carry on through the course

• Previous lesson: Flowers, Pollination and Fertilisation
• Next lesson: The Human Reproductive Systems
• Sexual and Asexual Reproduction — Two ways to make a new organism — one produces copies, the other produces something that has never existed before. Why any species would want both.
• Flowers, Pollination and Fertilisation — Two completely different solutions to one problem — a plant that cannot move has to get its male gamete to another plant's female gamete.
• The Human Reproductive Systems — Two systems built around the same problem — making gametes and bringing them together — and the hormones that switch them on.
• The Menstrual Cycle — Four hormones, two organs and one repeating question — should the uterus lining be built up, held, or let go?
• The Placenta and the Amniotic Fluid — An exchange surface with a constraint no other one has: two bloodstreams that must trade materials without ever mixing.
• All of Biology · Biology key terms · required practicals · Incandio Science · The 4BI1 specification