Form 5 · Worked answers

Sexual Reproduction in Flowering Plants, worked answers

Fully worked answers for Sexual Reproduction in Flowering Plants, original structured and essay questions with mark-scheme keywords highlighted.

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How this topic is examined

Labelling a flower and giving the function of each part.
Comparing wind and insect pollination.
Describing fertilisation and the fate of the ovule and ovary.
Explaining seed dispersal mechanisms and linking each fruit or seed feature to its dispersal method.
Stating and explaining the three conditions needed for a seed to germinate.
Comparing the advantages and disadvantages of self-pollination and cross-pollination.

Model answer structure

Read the command word and answer to the marks, one clear point per mark.
Define the key biological term precisely before you explain it.
Explain the process or reason in the correct sequence, using the right terms.
Where useful, add a labelled diagram or a worked example.
End with the link the question asks for (cause → effect, structure → function).

Fully worked answers

Explain how a pollen grain forms inside an anther, from the pollen mother cell to a grain ready for pollination.

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Inside the anther, a **pollen mother cell** divides by **meiosis** to produce four haploid **microspores**, each of which develops into a **pollen grain** with a tough, patterned wall. Within the grain, one nucleus divides by **mitosis** into a **tube nucleus** and a **generative nucleus**; the generative nucleus later divides again to form **two male gametes**.

pollen mother cellmeiosismicrosporestube nucleusgenerative nucleustwo male gametes

Describe how the embryo sac forms, and state how many nuclei and cells it normally contains at maturity.

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An **embryo sac mother cell** in the ovule divides by **meiosis** to produce four haploid **megaspores**, of which **only one survives** while the others degenerate. This surviving megaspore divides by **mitosis three times** without the cell dividing, producing **eight nuclei**. At maturity the embryo sac contains an **egg cell**, **two synergid cells**, **three antipodal cells**, and **two polar nuclei**, giving a structure usually described as **seven cells, eight nuclei**.

embryo sac mother cellmeiosismegasporesmitosis three timeseight nucleiegg cellsynergidantipodalpolar nuclei

Distinguish between self-pollination and cross-pollination, and explain one structural feature that favours cross-pollination.

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**Self-pollination** transfers pollen within the **same flower or plant**, while **cross-pollination** transfers pollen between **different plants of the same species**, carried by an external agent such as **wind or an insect**. Some flowers favour cross-pollination because the **anther and stigma of the same flower mature at different times**, so a flower's own pollen is rarely ready when its own stigma can receive pollen, making it more likely to **receive pollen from another plant**.

self-pollinationcross-pollinationwind or insectanther and stigma mature at different timesreceive pollen from another plant

Explain what is meant by double fertilisation in a flowering plant, and name the two structures it produces.

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After the **pollen tube** enters the ovule through the **micropyle**, it releases **two male gametes**. In **double fertilisation**, one male gamete fuses with the **egg cell** to form a diploid **zygote**, which develops into the **embryo**, while the other fuses with the **two polar nuclei** to form a triploid nucleus that develops into the **endosperm**, a food store for the embryo. Because two fusions occur from one pollen tube, this process is **unique to flowering plants**.

pollen tubemicropyletwo male gameteszygoteembryopolar nucleiendospermunique to flowering plants

Compare a monocotyledonous seed with a dicotyledonous seed in terms of cotyledon number and food storage.

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A **monocotyledonous** seed, such as a maize grain, has **one cotyledon** and stores its food mainly in the **endosperm**, which persists until germination. A **dicotyledonous** seed, such as a bean, has **two cotyledons**, which **absorb the endosperm's food** before germination, so the cotyledons themselves become the main food store.

monocotyledonousone cotyledonendosperm persistsdicotyledonoustwo cotyledonsabsorb endosperm food

State the three conditions needed for a seed to germinate, and explain the role of each.

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A seed needs **water**, to soften the **testa** and **activate stored enzymes**; **oxygen**, to allow **aerobic respiration** that releases the energy the embryo needs to grow; and a **suitable temperature**, at which these enzymes work **efficiently**. **Light is not required** for germination itself, although the seedling needs it soon afterwards for photosynthesis.

watertestaactivate enzymesoxygenaerobic respirationsuitable temperaturelight not required

Explain how seed dormancy and seed dispersal each help a flowering plant species to survive.

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**Dormancy** lets a seed survive a period unfavourable for growth, such as **drought or cold**, by slowing its metabolism almost to a stop until conditions improve. **Dispersal** carries the seed away from the **parent plant**, reducing **competition for light, water and soil nutrients** between parent and offspring and allowing the species to **colonise new areas**.

dormancydrought or coldmetabolism sloweddispersalparent plantreduced competitioncolonise new areas

Phrasing that earns marks

Structure of a flower: The stamen, made up of the anther and filament, is the male reproductive part of a flower, while the carpel, made up of the stigma, style and ovary, is the female part; a single flower may contain one or more of each, and the number and arrangement of these parts is often used to identify a species. Some flowers, described as incomplete, lack one or more of these parts, such as petals or stamens, yet can still reproduce successfully.
Development of gametes: Pollen grains, each containing a male gamete, form inside the anther, while the embryo sac, containing the female gamete, forms inside the ovule within the ovary; both are produced by meiosis, which creates genetic variation among the gametes even before fertilisation combines two parents' genes.
Pollination: Pollination is the transfer of pollen grains from the anther to the stigma, either of the same flower (self-pollination) or of a different flower on a different plant of the same species (cross-pollination); the transfer is carried out by an external agent such as wind or an insect, and floral features often reveal which agent a flower relies on. A flower can receive its own pollen and pollen from another plant at the same time, so both types of pollination are not always mutually exclusive events.
Fertilisation: After pollination, a pollen tube grows down through the style towards the ovule, carrying two male gametes; in double fertilisation, one male gamete fuses with the female gamete to form a diploid zygote and the other fuses with two nuclei in the embryo sac to form a triploid tissue called the endosperm, which stores food for the developing embryo. Because two separate fusions occur from the same pollen tube, double fertilisation is unique to flowering plants and is not seen in animals or in non-flowering plants such as ferns.
Seeds and fruits: After fertilisation, the ovule develops into a seed containing the embryo, its food store and a protective seed coat, while the ovary wall develops into the fruit, which protects the seed or seeds inside and, in many species, also assists with dispersal.
Importance of seeds: Seeds allow a plant species to survive conditions unfavourable for growth, such as drought or cold, by remaining dormant until conditions improve, and to be carried away from the parent plant to colonise new areas with less competition for resources.
Seed dispersal: Seeds and fruits are dispersed by wind, using light weight and structures such as wings or feathery plumes; by water, using a buoyant, waterproof fruit wall; by animals, either through hooks and spines that cling to fur or through a fleshy, edible fruit that is eaten and the seeds later deposited elsewhere; or explosively, when a dry pod twists and splits suddenly, flinging seeds away from the parent plant. A single fruit sometimes combines more than one feature, such as being both lightweight and having a hook, so a real specimen may not fit neatly into only one dispersal category.
Germination requirements: A seed generally needs three conditions to germinate: water, to soften the seed coat and activate enzymes; oxygen, to allow aerobic respiration that releases the energy needed for growth; and a suitable temperature, at which these enzymes work efficiently; light is not usually required for germination itself, although the seedling needs it soon afterwards for photosynthesis. These three conditions are often tested through a controlled experiment comparing seeds given different combinations of water, air and temperature to show which factor is limiting.

Frequently asked questions

What is the difference between pollination and fertilisation?

Pollination is the transfer of pollen grains from the anther to the stigma, either within the same flower (self-pollination) or between different plants of the same species (cross-pollination). Fertilisation happens afterwards: a pollen tube grows down to the ovule and the male gamete fuses with the female gamete to form a zygote. Pollination brings the gametes close; fertilisation is their fusion.

What happens to the flower after fertilisation?

After fertilisation, the fertilised ovule develops into a seed, which contains the embryo and a food store. The ovary wall develops into the fruit, which protects the seeds and often helps to disperse them. The other flower parts, such as the petals and stamens, usually wither and fall off. Recognising which structure has become which part after fertilisation is often tested directly by name in a labelling question.

How do wind-pollinated and insect-pollinated flowers differ?

Insect-pollinated flowers are usually large and colourful with scent and nectar to attract insects, and have sticky stigmas and pollen. Wind-pollinated flowers are usually small and dull with no scent, and have feathery stigmas and light, smooth pollen produced in large amounts, so the wind can carry it easily.