What is the primary difference between the plumule and the radicle during seed germination?

The plumule is the embryonic shoot that grows upward to become the stem and leaves, while the radicle is the embryonic root that grows downward to establish water and nutrient uptake from the soil.

How does the source of glucose change as a seedling develops from a seed into a mature plant?

Initially, glucose is produced by breaking down stored starch in the seed's food store via enzymes. Once the seedling grows leaves and emerges into the light, it produces its own glucose through the process of photosynthesis.

Compare the function of water and oxygen in the germination process.

Water is used to swell the seed, break the seed coat, and activate metabolic enzymes, whereas oxygen is specifically required as a reactant in aerobic respiration to release energy for growth.

Why is it a common mistake to assume that seeds require light to germinate?

Most seeds germinate underground in darkness because they rely on internal food stores (starch) for energy via respiration. Light is only required once the seedling emerges and needs to begin photosynthesis.

What happens if seeds are placed in water that has been boiled and covered with a layer of oil?

The seeds will fail to germinate because boiling removes dissolved oxygen and the oil layer prevents more oxygen from diffusing into the water, thereby stopping the aerobic respiration required for growth.

Why do seeds fail to germinate in freezing temperatures even if water and oxygen are present?

Enzymes are temperature-sensitive; at low temperatures, they lack sufficient kinetic energy to catalyze the breakdown of starch into glucose, effectively halting the metabolism needed for germination.

What is the definition of a cotyledon in the context of a seed?

A cotyledon is an embryonic leaf within the seed that often contains or absorbs the food store, providing the necessary nutrients for the embryo until the first true leaves can perform photosynthesis.

Define the term 'imbibition' and its role in plant growth.

Imbibition is the physical absorption of water by the dry seed, which causes it to swell, ruptures the protective testa (seed coat), and triggers the biochemical activation of the embryo.

What specific chemical transformation occurs to starch during germination?

Insoluble starch is hydrolyzed by enzymes into soluble maltose and then into glucose, which can then be easily transported and used by the embryo for respiration.

Why does the dry mass of a seed decrease during the first few days of germination?

The seed is using up its stored food (starch) during respiration to produce energy, and since it cannot yet photosynthesize, it loses carbon as $CO_2$ gas, leading to a net loss in biomass.

Germinating Seeds | Pearson Edexcel IGCSE Biology

3. Reproduction & Inheritance

Germinating Seeds

Summary

Germination is the physiological process by which a plant grows from a seed into a seedling. It involves the activation of metabolic pathways, the breakdown of stored nutrients, and the emergence of the embryo to establish an independent plant capable of photosynthesis.

1. Seed Anatomy & Composition

Embryo Structure: A seed contains the rudimentary plant, consisting of an embryo shoot (plumule) and an embryo root (radicle), which are the first structures to emerge during growth.
Nutrient Reservoirs: Seeds possess a food store primarily composed of starch, along with cotyledons (seed leaves) that provide the initial biomass required for development.
Seed Coat: This protective outer layer maintains the seed in a dormant state until environmental conditions trigger the germination process.

Diagram showing the internal anatomy of a germinating seed with plumule, radicle, and food store.

2. Biochemical Mobilization of Nutrients

Enzymatic Hydrolysis: Upon water uptake, enzymes within the seed are activated to break down complex starch molecules into maltose and subsequently into glucose.
Energy Release: The resulting glucose serves as the primary substrate for aerobic respiration, releasing the chemical energy (ATP) needed for rapid cell division and expansion.
Solubility Advantage: Converting insoluble starch into soluble sugars allows for the easy transport of nutrients from the food store to the growing tips of the root and shoot.

3. Environmental Requirements for Growth

Water (Hydration): Water is essential for swelling the seed to break the coat and acts as a medium for metabolic reactions and enzyme activation.
Oxygen (Aeration): Oxygen is required for respiration; seeds cannot germinate in anaerobic conditions (such as waterlogged soil) as they lack the energy for growth.
Warmth (Thermal Energy): Moderate temperatures optimize enzyme kinetic energy, accelerating the biochemical reactions required for germination up to a specific denaturing point.

4. Growth Dynamics & Self-Sufficiency

5. Key Distinctions in Seed Development

6. Exam Strategy & Tips