KS3 Biology › Reproduction in plants
KS3 Biology › Reproduction in plants
These are the points the worksheet questions are built around.
A flower cannot walk anywhere, yet flowering plants reproduce sexually, so genetic material still has to travel between individuals. Two linked stages make that happen: the first is pollination, which is the delivery of pollen, and the second is fertilisation, which is the joining of sex cells. How much rests on the first stage is clear from food production: roughly seventy out of the hundred-odd crops supplying most of the world's diet are pollinated by bees.
In pollination, grains of pollen move from an anther to a stigma. The anther is the male organ, held up on a slender filament, and the anther and filament together form the stamen. The stigma is the receiving surface of the female organ, and it is sticky so that arriving grains stay in place. Inside each grain is the male gamete, a sex cell carrying half of the DNA needed for a new plant. The remaining half belongs to the female gamete, the ovum, which waits within the ovary at the base of the flower.
Pollen is often mistaken for a tiny seed, but the two are quite different. Pollen is the vehicle that carries the male sex cell, doing the job that sperm does in animals. A seed is produced only after fertilisation has taken place, and it represents the next generation, ready to grow.
Pollen may land on the stigma of the flower that made it, which is self-pollination, or it may be moved to a flower on a different plant, which is cross-pollination. The second route only works when both plants belong to the same species. Apple pollen deposited on a rose achieves nothing, because the gametes cannot fuse. The benefit of cross-pollination is variation. A self-pollinated seed inherits all of its DNA from a single parent, whereas a cross-pollinated seed inherits from two, which creates many more combinations. Reshuffling one deck of cards gives few new orders, while mixing two decks gives many. One kind of flower appearing in a range of petal colours shows the effect, and a species with more variation has better prospects if its environment changes.
A pollen grain is tiny, about seven hundredths of a millimetre wide, close to the width of one human hair, and it is normally noticed only as a yellow dust when vast numbers gather. Being so small and light, pollen can be transported by wind, by water or by animals. Grasses shed huge clouds of it into the air, and this airborne pollen is what causes hay fever. A few aquatic plants release their pollen onto the water surface. Most flowering plants, however, depend on animals: close to nine in ten kinds do. Insects such as beetles, wasps, flies and butterflies all do the work, and bees carry out more of it than any other group.
When a bee lands to sip sugary nectar, pollen clings to its furry body, and on reaching another flower a little of it is brushed onto that flower's stigma, which is cross-pollination in action. Bright petals and strong scents advertise the food on offer to passing insects. This is why the food supply is tied to bees, with apples and strawberries given as examples, and why a fall in bee numbers would mean less food.
After landing, the male gamete is still at the top of the flower while the ovum is at the bottom, so the two need a route between them. A pollen grain that lands grows a thin pollen tube down through the style, the stalk linking the stigma to the ovary, and the male gamete moves along inside it. The distances can be large: in maize the tube may extend tens of centimetres, even though it starts from a grain no wider than a hair.
On reaching the ovary, the tube meets an ovule, which holds the ovum. Here the nucleus of the male gamete merges with the nucleus of the ovum, and that merger is fertilisation. Chromosomes are bundles of DNA held in the nucleus, and because each gamete supplies only half of a set, the merged cell ends up with a complete one. This cell is the zygote. Human fertilisation follows the same pattern, with two gametes fusing to give a single zygote.
From the zygote, growth and division produce an embryo, which develops inside a seed. Once the seed has been released and carried away from the parent, it can germinate when conditions suit, sending out a root, a shoot and leaves. The seedling then grows on to become a new plant that takes half of its DNA from each parent and so resembles both of them.
Three terms are often confused, and they always come in the same order. Pollination is pollen arriving on the stigma, fertilisation is the fusing of nuclei in the ovary, and germination is the point at which the seed begins to grow.
In short: pollen carrying the male gamete is moved from anther to stigma by wind, water or, most often, animals such as bees, and cross-pollination between plants of one species adds variation by mixing DNA from two parents. A pollen tube then carries the male gamete down the style to the ovule in the ovary, where the nuclei fuse in fertilisation to form a zygote. That zygote becomes an embryo within a seed, and the seed germinates into a seedling and finally a new plant resembling both parents.
These are the words that appear as key term cards in the video — worth knowing cold before the test.
Revising for a test? Key terms, objectives and quiz questions on one printable page.
The 3 questions from the video, one at a time. See if you caught them.
1. Where must pollen land for pollination to happen?
Why: The stigma is the sticky tip of the female part, and it is where pollen has to land.
2. Which of these does NOT carry pollen between flowers?
Why: Earthworms live in the soil, well away from flowers, so they never pick up pollen.
3. Pollen has just landed on a stigma. Has fertilisation happened yet?
Why: Landing on the stigma is only pollination, and fertilisation comes later when the nuclei fuse in the ovary.
Score: 0 out of 3 —
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Reproduction in plants
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