Germinating Seeds

A seed is more than just a packet to be planted — it is a tiny, dormant (resting) plant embryo packaged together with its own store of food. That food store is called a food reserve.

Germination is the word for when a seed starts to grow into a young plant (a seedling). Given the right conditions — water, oxygen and a suitable temperature — the embryo inside the seed switches on and begins to grow a tiny root and shoot.

Here is the problem the seed has to solve. At the start of germination the seedling has no green leaves. Leaves are where photosynthesis happens, so with no leaves the seedling cannot make its own food yet. Yet growing still costs energy and raw materials.

The seed's solution is the food reserve it carried from the start. The seedling lives off this stored food until it has grown enough to make its own. This whole idea — surviving on stored food until photosynthesis can take over — is exactly what spec point 3.6 is testing.

Exam tip. The key phrase to learn is that the seedling uses stored food reserves until it can photosynthesise. The word "until" matters — it shows you understand this is only a temporary, early stage.

The food reserve inside a seed is mostly starch — a large carbohydrate that is excellent for storage. Starch is stored in part of the seed called the cotyledon(s) or, in some seeds, the endosperm. (For Double Award you simply need to know the seed stores food such as starch; the exact storage tissue is helpful background.)

Why store food as starch rather than as sugar?

• Starch is insoluble in water, so it does not dissolve and leak away, and it does not affect the water balance (osmosis) of the seed.
• Starch is compact — a lot of energy can be packed into a small space.

But this storage form has one catch. Because starch is a big, insoluble molecule, it cannot be transported around the seedling or used directly by cells. Before it can be useful, the starch must be broken down into something small and soluble — and that is the job of an enzyme, which we look at next.

Exam tip. A neat two-part fact to remember: starch is good for storage because it is insoluble, but it must be broken down (digested) before it can be used. Examiners reward students who know storage food is starch and that it has to be converted first.

For the stored starch to be useful, it must be turned into a small, soluble sugar that the seedling's cells can actually use and move around. This breakdown is carried out by an enzyme called amylase.

Amylase breaks down starch into glucose (a soluble sugar):

starch  --(amylase)-->  glucose

Glucose is soluble, so it can now:

• dissolve and be transported to the growing parts of the seedling (the root and shoot), and
• be used directly by cells for respiration and growth.

So the food reserve goes through a simple two-stage logic: store it as insoluble starch, then digest it with amylase into soluble glucose when it is needed.

[ stored starch ]  --amylase-->  [ glucose ]  -->  used for respiration + growth

Exam tip. Name the enzyme precisely: amylase, and the product precisely: glucose. A common marking point is the full sentence "amylase breaks down starch into glucose". Avoid vague answers like "the seed digests itself" — the examiner wants the enzyme and the sugar named.

Once the starch has been broken down into glucose, the seedling puts this glucose to work in two main ways.

1. Respiration (releasing energy). The glucose is used in aerobic respiration, which releases the energy the seedling needs to grow. This is why a germinating seed needs oxygen — respiration uses oxygen to release energy from glucose:

glucose + oxygen → carbon dioxide + water (+ energy released)

2. Growth (building the seedling). Glucose also provides the raw material to build new substances and new cells. As the seedling makes new cells, it grows a longer root (to anchor it and take up water) and a shoot that pushes up towards the light.

This continues until the seedling has grown its first green leaves. Once leaves appear, the seedling can finally do something the seed never could on its own — photosynthesise.

A useful flow chart of the whole process:

Exam tip. State both uses of the glucose: respiration (to release energy) and growth. Many students give only one. The energy/growth pair is a reliable double marking point.

The food reserve in a seed is limited — it is only meant to last for the early part of the seedling's life. So there is a race: the seedling must grow leaves before the stored food runs out.

Once the seedling has green leaves, it can photosynthesise, using light energy to make its own glucose from carbon dioxide and water. From this point on, the seedling is self-sufficient — it no longer depends on the seed's food reserve.

You can summarise the whole life of the food reserve as a switchover from one food source to another:

Stage	Food source	What is happening
Before germination	None used (seed is dormant)	Food stored as starch in the seed
Germinating seedling (no leaves)	Stored food reserve	Amylase breaks starch → glucose; glucose used for respiration + growth
Seedling with leaves	Photosynthesis	The plant makes its own glucose and no longer needs the seed store

This is why a seedling kept in the dark will eventually die once its food store is used up — without light it can never photosynthesise to replace the food, no matter how much starch the seed held.

Exam tip. The complete spec-3.6 answer joins everything up: the seed stores food (starch) → on germination amylase breaks it down to glucose → glucose is used for respiration and growth → this lasts until the seedling grows leaves and can photosynthesise to make its own food.