By the end of this lesson
- Locate seed food reserves.
- Explain reserve mobilisation during germination.
- Relate respiration to early seedling growth.
BIG QUESTION
How does a seedling grow before it has functional leaves?
Enzymes digest food reserves; soluble products move to the embryo for respiration and biosynthesis.
Seeds contain stored chemical resources
Food reserves are stored in cotyledons or endosperm, depending on the species. They may include starchA storage carbohydrate in plants made of many glucose molecules; it turns iodine solution blue-black., oils and proteins.
The embryo cannot initially rely on photosynthesis because it has little exposed green leaf area.
Enzymes mobilise reserves
Water uptake activates enzymes. Amylase hydrolyses starch to soluble sugars; proteases and lipases mobilise other reserves.
Soluble products move to growing regions such as the radicle and plumule.
Respiration and biosynthesis support growth
Sugars are respired to release energy for mitosis, active transport and biosynthesis. Other products provide carbon skeletons and amino acids for new cells.
As leaves expand and photosynthesis exceeds respiratory demand, dependence on stored reserves falls.
Reserve mobilisation
Storage molecules must be converted before the embryo can use them.
| Reserve | Mobilised product | Use |
|---|---|---|
| Starch | Soluble sugars. | Respiration and biosynthesis. |
| Oil | Fatty acids and glycerol. | Respiration and membranes. |
| Protein | Amino acids. | New proteins and other molecules. |
| Mineral reserve | Mineral ions. | Enzymes, nucleic acids and cells. |
Language that earns precision
- cotyledon
- Seed leaf that may store or absorb food for an embryo.
- endosperm
- Nutritive tissue that stores food in many seeds.
- mobilisation
- Conversion and movement of stored reserves into usable forms.
- plumule
- Embryonic shoot.
Common misconceptions
Claim: A germinating seed immediately gets food from soil.
Correction: It initially uses reserves already stored inside the seed.
Claim: Stored starch is respired directly.
Correction: It is first hydrolysed to soluble sugars.
Claim: Respiration creates energy.
Correction: It releases energy from chemical stores for use in cellular processes.
Match the answer to the command word
Water → enzyme activation → soluble products → respiration/growth.
Food reserves supply both energy-releasing substrates and building materials.
Recall, apply, then justify
Explain how starch reserves support germination.
Hint
Enzyme, product, transport and respiration.
Suggested marking points
- Water activates enzymes including amylase.
- Starch is hydrolysed into soluble sugars.
- Sugars move to the growing embryo.
- Sugars are respired to supply energy for growth.
Explain why reserve use decreases after leaves expand.
Hint
What new process supplies sugars?
Suggested marking points
- Expanded green leaves carry out more photosynthesis.
- Photosynthesis supplies sugars, reducing dependence on stored food.
Predict changes in dry mass of the seed reserve and whole seedling before and after photosynthesis becomes established.
Planning hint
Distinguish reserve loss from carbon gain.
What a strong design includes
- The stored reserve dry mass decreases during germination.
- Reserve molecules are transported and respired.
- Carbon dioxide loss during respiration reduces total dry mass initially.
- Some reserve material becomes new seedling tissue.
- After leaves photosynthesise effectively, carbon dioxide is fixed.
- Whole-seedling dry mass can then begin to increase net of respiration.