Biology · 16 Reproduction

16.3 Sexual reproduction in plants

Before you start

Sexual reproduction is the fusion of the nuclei of two gametes to form a zygote, which grows into a new individual (16.1).
A gamete is a sex cell, and it has half the number of chromosomes that an ordinary body cell has.
The nucleus of a cell holds the chromosomes, so 'nucleus' in this topic means 'where the genetic information is'.
Aerobic respiration uses oxygen to release energy from glucose (Topic 12).
Enzymes need water to work in, are slowed by low temperature, and are denatured by high temperature (Topic 4).

Syllabus objectives covered (12)

Identify in diagrams and images and draw the following parts of an insect-pollinated flower: sepals, petals, stamens, filaments, anthers, carpels, style, stigma, ovary and ovules
State the functions of the structures listed in 16.3.1
Identify in diagrams and images and describe the anthers and stigmas of a wind-pollinated flower
Distinguish between the pollen grains of insect-pollinated and wind-pollinated flowers
Describe pollination as the transfer of pollen grains from an anther to a stigma
State that fertilisation occurs when a pollen nucleus fuses with a nucleus in an ovule
Describe the structural adaptations of insect-pollinated and wind-pollinated flowers
Investigate and describe the environmental conditions that affect germination of seeds, limited to the requirement for: water, oxygen and a suitable temperature
Describe self-pollination as the transfer of pollen grains from the anther of a flower to the stigma of the same flower or a different flower on the same plant
Describe cross-pollination as the transfer of pollen grains from the anther of a flower to the stigma of a flower on a different plant of the same species
Discuss the potential effects of self-pollination and cross-pollination on a population, in terms of variation, capacity to respond to changes in the environment and reliance on pollinators
Describe the growth of the pollen tube and its entry into the ovule followed by fertilisation (details of production of endosperm and development are not required)

Part 1 of 9

The flower is four rings, outside in

A flower looks complicated in a photograph and simple in a plan. The plan is this: four rings of parts, one inside the other, all attached to the swollen top of the flower stalk. Learn the order of the rings and you can label a flower you have never seen before.
From the outside in the rings are: sepals, then petals, then stamens, then the carpel at the very centre. Outermost is the part that protects, innermost is the part that becomes the seed.
This is also the drawing to produce if you are asked for one: half the flower, carpel cut open so the ovules show.
Sepals are the outermost ring. In most insect-pollinated flowers they are small and green, and before the flower opened they were the outside of the bud.
Petals are the next ring in. In an insect-pollinated flower they are the large, brightly coloured parts that you would call 'the flower' in ordinary speech.
Stamens come next, and this is the male part. One stamen is two structures, not one: a filament, which is the thin stalk, and an anther, which is the head sitting on top of the filament. Stamen equals filament plus anther.
That matters on a labelling question. If you write 'stamen' with the line pointing only at the head, an examiner reads it as the wrong label, because the head has its own name — anther.
The carpel sits at the centre and is the female part. It is one continuous structure with three named regions. Top: the stigma, the surface that pollen lands on. Middle: the style, the stalk that holds the stigma up above the ovary. Bottom: the ovary, the swollen base.
Ovules are inside the ovary, attached to its inner wall. This is the pair people swap most often, so fix it now: the ovary is the container, the ovules are the small round structures inside it. One ovary, several ovules.
Say the names of the female parts downwards in order — stigma, style, ovary, ovules — and you have described a path from the outside of the flower to the deepest point inside it. That path is exactly the route the pollen tube will take later, which is why the order is worth having by heart.
If a question asks you to draw the flower, draw a half-flower cut down the middle: one sepal and one petal each side, one whole stamen each side, and the carpel in the centre cut open so the ovules show. Use single clean lines with no shading, and label with straight lines that touch the structure they name.

Part 2 of 9

What each part is for

A function is a job, not a position. 'The anther is at the top of the filament' describes where it is and earns nothing. 'The anther produces pollen grains' is a function and earns the mark. Check every answer you write against that test.
Sepals enclose and protect the flower while it is still a bud. Say 'in bud' — protection is only their job before the flower opens.
Petals attract insects. In the full answer, say how: they are large, brightly coloured and scented, and there is often nectar at their base, so insects come to the flower and brush against the reproductive parts.
Anther produces pollen grains. The pollen grains contain the male gametes — more precisely, the male nuclei. Do not write 'the anther contains pollen'; producing it is the job.
Filament holds the anther up, in a position where an insect (or the wind) can reach the pollen and carry it away.
Stigma receives the pollen grains. In an insect-pollinated flower it is sticky, so a grain brushed onto it stays there.
Style connects the stigma to the ovary, and the pollen tube grows down through it. So the style is a route as well as a support.
Ovary contains the ovules, and after fertilisation it develops into the fruit.
Ovule contains the female gamete — the female nucleus — and after fertilisation it develops into the seed. Notice the ovule is not itself the gamete; it is a structure with a gamete inside it.
Two pairs to keep straight, because the marks depend on it. Ovary becomes fruit; ovule becomes seed. Anther makes pollen; stigma catches pollen. Anther and stigma are not partners in one structure — the anther belongs to the stamen and the stigma belongs to the carpel.

Part 3 of 9

Pollination is a transfer. Fertilisation is a fusion.

This is the single place in the topic where careful candidates lose marks, so read it slowly. Two different events, two different definitions, separated in time and in place.
Pollination is the transfer of pollen grains from an anther to a stigma. That is the whole definition. It is a journey, made by a whole pollen grain, ending on the outside surface of a stigma.
Notice what is not in that definition. No nuclei. No fusion. No egg. No seed. Nothing has joined together and nothing new has been made. Pollen has simply moved from one place to another.
Fertilisation occurs when a pollen nucleus fuses with a nucleus in an ovule. That is a different event. It happens later, and it happens deep inside the ovary, not on the stigma.
So the two words are separated by time — hours or days can pass between them — and by place: the stigma is at the top of the carpel, the ovule is at the bottom, inside the ovary.
Hours or days apart, and the whole length of the carpel apart.
Here is the proof that they must be separate words. A grain of pollen from the wrong species can land on a stigma. Pollination has happened. No pollen tube grows, no nuclei fuse, no seed is made. Fertilisation has not happened. One event occurred without the other.
What a swap costs. Writing 'pollination is when the pollen fertilises the egg' scores zero on a pollination question, because the required word transfer is absent, and it is treated as a contradiction on a fertilisation question, because fertilisation does not happen on the stigma.
Two words are load-bearing here and vaguer words will not be accepted in their place. In a pollination answer the mark hangs on transfer — 'moved' or 'carried' will normally be accepted, 'joins' will not. In a fertilisation answer the mark hangs on fuse or fusion of nuclei — 'the sperm meets the egg' does not say that anything joined.
One more piece of vocabulary. The single cell made by that fusion is a zygote. Writing 'a new cell is formed' when the question asks what fertilisation produces will not score; the examiner is looking for the word zygote.

Part 4 of 9

How the pollen nucleus reaches the ovule

You have a pollen grain sitting on a stigma and a female nucleus buried inside an ovule at the base of the carpel. Something has to cross that distance. Here is what does.
First the pollen grain germinates on the stigma. A tube begins to grow out of it — the pollen tube.
Get this the right way round, because it is a frequent error: the tube grows down the style. The pollen grain itself stays where it landed, on the stigma. Nothing swims and nothing is pushed.
The tube grows down through the tissue of the style towards the ovary. It secretes enzymes that digest a path through the tissue ahead of it, which is how it makes room to grow.
The grain never moves. Only the tube travels, carrying the nucleus inside it.
The pollen nucleus — the male gamete — travels down inside the tube. The tube is the delivery route; the nucleus is what is being delivered. Plant male gametes cannot move on their own, and this is the whole reason the tube exists.
The tube reaches the ovary and then enters an ovule through a small opening in the ovule wall. (That opening is called the micropyle. The syllabus asks you for 'entry into the ovule', so the name is a bonus, not a requirement.)
The tip of the tube breaks open and the pollen nucleus passes into the ovule, where it fuses with the female nucleus. That fusion is fertilisation, and it produces a zygote.
Write the answer as a sequence, because the marks are for the order as much as the content: pollen grain germinates on the stigma, pollen tube grows down the style, tube enters the ovule, pollen nucleus travels down the tube, pollen nucleus fuses with the nucleus in the ovule, a zygote is formed.
Two things you can safely leave out: endosperm, and the details of how the seed then develops. The syllabus explicitly excludes them, so writing about them costs you time and earns nothing.

Part 5 of 9

Wind-pollinated flowers, and why they look so different

Every difference between these two kinds of flower comes from one question: what is carrying the pollen? An insect has to be attracted, landed on and rubbed against. The wind cannot be attracted at all — it can only be caught.
Insect-pollinated flowers, the parts that attract. Petals are large, brightly coloured and scented, and there are nectaries producing nectar at the base of the flower. All of this brings the insect in, and the nectar gives it a reason to push right into the flower.
Insect-pollinated flowers, the reproductive parts. The anthers are inside the flower and firmly attached, so the insect must brush past them and pick up pollen on its body. The stigma is also inside the flower, small, and sticky, so pollen rubs off the insect onto it as the insect passes.
Wind-pollinated flowers, the parts that would attract. They are reduced or gone. Petals are small and green, or absent altogether; there is no scent and no nectar. There is no point spending resources attracting an animal that is not part of the plan.
Wind-pollinated anthers. Large, and hanging outside the flower on long filaments, attached loosely so that they swing freely in the wind. That is what shakes the pollen out into the moving air.
Wind-pollinated stigmas. Feathery or branched, and also hanging outside the flower. Feathery means a very large surface area, which is what makes it likely to catch a grain drifting past on the air. A small sticky pad would catch almost nothing.
The two objectives here are different sizes, so notice which one is being asked. 'Describe the anthers and stigmas of a wind-pollinated flower' wants only those two structures — outside the flower, anthers on long loose filaments, stigmas feathery. 'Describe the structural adaptations' is broader and wants petals, scent and nectar as well.
How to write a comparison so it scores. Two rules. First, compare like with like: petals against petals, stigmas against stigmas. 'Insect-pollinated flowers have large petals; wind-pollinated flowers have feathery stigmas' compares nothing, because there is no shared feature in the sentence.
Second, every adaptation needs its reason attached. 'The stigma is feathery' is a description. 'The stigma is feathery, so it has a large surface area to catch pollen carried in the air' is an adaptation, and only the second version earns an Explain mark.

Part 6 of 9

Telling the pollen grains apart

The same logic runs through the pollen itself. Ask what has to happen to a grain and the differences follow.
Insect-pollinated pollen has to stick to a hairy insect. So the grains are larger, and their outer surface is sticky or spiky, which catches on the insect's body hairs.
Wind-pollinated pollen has to stay airborne. So the grains are smaller, lighter and smooth, with nothing to weigh them down or catch on anything.
Quantity is the difference examiners ask about most. Wind-pollinated plants produce far more pollen than insect-pollinated plants.
The reason is worth understanding rather than memorising. The wind blows wherever it blows; the chance that any one grain lands on a stigma of the same species is tiny, so the vast majority is wasted and huge numbers are needed. An insect flies from one flower to another flower of the same species, delivering pollen almost to the right address, so very little is wasted and far less needs to be made.

Part 7 of 9

Self-pollination and cross-pollination

Both of these are still pollination — still the transfer of pollen grains from an anther to a stigma. The only thing that changes is whose anther and whose stigma.
Self-pollination is the transfer of pollen grains from the anther of a flower to the stigma of the same flower, or to the stigma of a different flower on the same plant.
That second half is the part people leave out, and it is often a separate marking point. Pollen moving between two flowers is still self-pollination as long as both flowers are on one plant.
Cross-pollination is the transfer of pollen grains from the anther of a flower to the stigma of a flower on a different plant of the same species.
'Of the same species' is load-bearing. Pollen landing on a different species is not cross-pollination in any useful sense, because no pollen tube grows and no fertilisation follows.
So the dividing line is the plant, not the flower. Ask yourself one question: did the pollen come from a different plant? Yes means cross. No means self, however far it travelled within that one plant.
Important precision point: self-pollination is not asexual reproduction and does not produce clones. Two gametes still fuse, so it is still sexual reproduction, and because gametes are made by meiosis the offspring are still genetically different from each other — just less different than they would be after cross-pollination.

Part 8 of 9

What each kind of pollination does to a population

The syllabus names three things to talk about here, and a full-mark answer usually touches more than one of them: variation, the ability to cope with a change in the environment, and reliance on pollinators. Work through them in that order.
Variation. In self-pollination both gametes come from the same plant, so both carry alleles drawn from one parent's genome. The offspring have less genetic variation. In cross-pollination the gametes come from two different plants, so alleles are combined from two sources and the offspring have more genetic variation.
Capacity to respond to a change in the environment. This follows straight from variation, and the chain of reasoning is what earns the marks. More variation means it is more likely that some individuals happen to carry an allele that lets them survive a new disease, a drought or a rise in temperature.
Those individuals survive and reproduce, so the population continues. In a population with little variation, if the change kills one plant it is likely to kill nearly all of them, because they are so similar. The population can be wiped out.
Reliance on pollinators. Cross-pollination needs pollen to arrive from another plant, which usually means an insect must visit, or the wind must carry it far enough. If pollinating insects become scarce, or the plant is growing on its own with no others of its species nearby, very few seeds are produced.
Self-pollination has no such dependency. A single isolated plant can pollinate itself and set seed reliably, without any pollinator and without any other plant. That is a real advantage, not a consolation prize.
So neither is simply 'better', and a question asking you to discuss this wants both sides. Self-pollination is reliable but leaves a population fragile. Cross-pollination produces a population that can adapt, but depends on something outside the plant to happen.

Part 9 of 9

Germination: three conditions and no more

Germination is the beginning of growth of the seed into a seedling. For it to start, a seed needs three environmental conditions: water, oxygen and a suitable temperature.
Three. Not four. Light is not required for germination, and adding it to a list of three is one of the easiest marks to throw away in this topic — on a 'State three' question an incorrect item counts toward your three, so it takes the place of a right answer.
The reason light is not needed is that the seed carries its own food store, so it does not have to photosynthesise until the shoot reaches the light. Many seeds germinate perfectly well buried in soil, in total darkness.
Why water. Enzymes only work in solution, so the seed must take up water before the enzymes that break down its stored food can become active. The water also makes the seed swell, which splits the seed coat and lets the young root out.
Why oxygen. Growth needs energy, and that energy is released by aerobic respiration, which requires oxygen. Say both halves — 'for respiration' on its own is thin; 'for aerobic respiration, to release energy for growth' is the full answer.
Why a suitable temperature. The reactions of germination are controlled by enzymes. Too cold and the reactions are far too slow because there is too little kinetic energy. Too hot and the enzymes are denatured and stop working altogether. Notice the syllabus word is suitable, not warm — hot is just as wrong as cold.
How to investigate it. Set up four boiling tubes, each containing the same number of the same kind of seed on cotton wool. Each tube removes exactly one condition, so that anything that happens can be blamed on that one thing.
Tube A: damp cotton wool, air above it, left at room temperature. Every condition is present. This is the control, and it is the tube the other three are compared against.
Tube B: dry cotton wool, air, room temperature. Water removed.
Tube C: seeds under water that has been boiled and then allowed to cool, with a layer of oil poured on the surface, at room temperature. Oxygen removed. Boiling drives dissolved oxygen out of the water and the oil stops oxygen from the air dissolving back in — you need both, and a question about this tube wants both reasons.
Tube D: damp cotton wool, air, placed in a refrigerator. A suitable temperature removed.
Leave all four for several days, then count how many seeds have germinated in each, or measure the length of the young root. The result is that seeds germinate only in tube A, where all three conditions were present.
The thing that makes this a fair test is that only one variable changes between the control and each other tube. If a question asks why tube A is needed, the answer is that it is the control — without it you have nothing to compare the other results against.

Worked examples

Example 1. Describe what happens from the moment a pollen grain lands on a stigma of the same species until fertilisation is complete.

Show the working

Start at the stigma, because that is where the question puts you. The grain germinates there and stays there — the first sentence must not have the grain moving.
A pollen tube grows out of the pollen grain and grows down through the style. Name the style; 'grows downwards' without naming the tissue is vaguer than the mark scheme wants.
The tube grows towards the ovary and enters an ovule through a small opening in its wall.
The pollen nucleus, which is the male gamete, travels down inside the tube. Say this explicitly — it is the step that explains why the tube is there at all.
The pollen nucleus fuses with a nucleus in the ovule. Use the word fuses. This is the moment of fertilisation and it is the only sentence in the answer that describes fertilisation.
Finish with the product: a zygote is formed.

Answer: The pollen grain germinates on the stigma and a pollen tube grows out of it. The tube grows down through the style to the ovary and enters an ovule through an opening in the ovule wall. The pollen nucleus travels down inside the tube and fuses with a nucleus in the ovule. This fusion is fertilisation, and it produces a zygote.

Example 2. A gardener removes every stamen from a flower on a cherry tree before the flower opens, and leaves the flower on the tree. Several weeks later that flower has produced a fruit containing a seed. Explain how this is possible.

Show the working

Work out what was actually removed. The stamens carry the anthers, and the anthers produce pollen grains — so this flower could not make any pollen of its own.
Now ask what pollination requires. It requires pollen grains to be transferred from an anther to a stigma. It does not require the anther and the stigma to be on the same flower, or even on the same plant.
So pollen must have arrived from somewhere else. It came from an anther on a different cherry tree, carried by an insect, and landed on this flower's stigma. That is cross-pollination.
Then the normal sequence followed: a pollen tube grew down the style, the pollen nucleus fused with a nucleus in an ovule, and fertilisation took place.
Finally connect fertilisation to what the gardener actually saw. The fertilised ovule developed into the seed and the ovary developed into the fruit.

Answer: Removing the stamens removed the anthers, so this flower produced no pollen. Pollen was carried by an insect from an anther on a different cherry tree and transferred to this flower's stigma, which is cross-pollination. A pollen tube then grew down the style and the pollen nucleus fused with a nucleus in an ovule. The fertilised ovule became the seed and the ovary became the fruit.

Example 3. Maize is pollinated by the wind. Compare a maize flower with an insect-pollinated flower such as a buttercup, in terms of petals, anthers, stigmas and pollen grains.

Show the working

Set the answer out feature by feature, so that every sentence contains both flowers. That is what makes it a comparison rather than two descriptions.
Petals. Buttercup: large, brightly coloured, scented, with nectar, to attract insects. Maize: small and green or absent, with no scent or nectar, because there is nothing to attract.
Anthers. Buttercup: inside the flower and firmly attached, so an insect must brush past them. Maize: large, hanging outside the flower on long filaments and loosely attached, so they swing in the wind and release pollen into the air.
Stigmas. Buttercup: inside the flower, small and sticky, so pollen rubs off the insect's body. Maize: feathery and hanging outside the flower, giving a large surface area to catch pollen from the air.
Pollen grains. Buttercup: larger, sticky or spiky, produced in smaller amounts. Maize: smaller, lighter and smooth, produced in very large amounts because most grains are lost.
Check the finished answer for reasons. Every one of those four pairs has a 'so that' or a 'because' attached — without them these are descriptions, not adaptations.

Answer: Petals: the buttercup's are large, coloured and scented with nectar to attract insects, whereas maize has small green petals or none, with no scent or nectar. Anthers: the buttercup's are inside the flower and firmly attached so insects brush against them, whereas maize anthers hang outside on long loose filaments so the wind shakes pollen out. Stigmas: the buttercup's is inside the flower, small and sticky so pollen rubs off an insect, whereas the maize stigma is feathery and hangs outside the flower, giving a large surface area to catch pollen from the air. Pollen: the buttercup's grains are larger, sticky or spiky and fewer, whereas maize grains are small, light, smooth and produced in very large numbers because most are lost in the wind.

Example 4. A student sets up four tubes of cress seeds on cotton wool. Tube A: damp cotton wool, open to the air, at 20 degrees Celsius. Tube B: dry cotton wool, open to the air, at 20 degrees Celsius. Tube C: seeds under boiled and cooled water covered with a layer of oil, at 20 degrees Celsius. Tube D: damp cotton wool, open to the air, at 4 degrees Celsius. Predict the result in each tube and explain each prediction.

Show the working

First identify what each tube is testing, by finding the one thing that differs from tube A. B has no water, C has no oxygen, D has no suitable temperature. A has all three.
Tube A: the seeds germinate. All three required conditions are present. A is the control, and it is the comparison for the other three.
Tube B: no germination. Without water the enzymes in the seed cannot work, because enzymes need to be in solution, and the seed cannot swell and split its coat.
Tube C: no germination. Boiling removed the dissolved oxygen and the oil layer stopped more dissolving in, so the seeds cannot carry out aerobic respiration and cannot release the energy needed for growth.
Tube D: no germination, or germination so slow that nothing is visible. At 4 degrees Celsius the enzyme-controlled reactions are far too slow, because the molecules have too little kinetic energy.
Note what tube D does not show. The seeds are not dead — moved to 20 degrees Celsius they would begin to germinate. Low temperature slows enzymes; it does not denature them.

Answer: Only tube A germinates, because water, oxygen and a suitable temperature are all present. B fails for lack of water, so the enzymes cannot work and the seed cannot swell. C fails for lack of oxygen, so no aerobic respiration takes place and no energy is released for growth. D fails, or is extremely slow, because at 4 degrees Celsius the enzyme-controlled reactions of germination are far too slow.

Where people go wrong

Mistake: Using 'pollination' and 'fertilisation' as if they were the same word.

Instead: Pollination is the transfer of pollen grains from an anther to a stigma. Fertilisation is the fusion of a pollen nucleus with a nucleus in an ovule.

They happen at different times and in different places — one on the outside of the stigma, one deep inside the ovary. Pollen can be transferred and never fertilise anything, which is exactly why two words exist.

Mistake: Writing that the pollen grain travels down the style to the ovule.

Instead: The pollen grain stays on the stigma. A pollen tube grows out of it and down the style, and the pollen nucleus travels inside the tube.

Plant male gametes cannot move by themselves. The tube is the whole mechanism of delivery, so saying the grain moves removes the only part of the process the question is testing.

Mistake: Treating 'ovary' and 'ovule' as interchangeable, or calling the ovule the female gamete.

Instead: The ovary is the container at the base of the carpel; the ovules are the structures inside it; the female gamete is the nucleus inside an ovule. After fertilisation the ovule becomes the seed and the ovary becomes the fruit.

These are three nested things, and a question asking where fertilisation happens, or what becomes the seed, is testing precisely whether you have them nested in the right order.

Mistake: Including light in the conditions needed for germination.

Instead: Water, oxygen and a suitable temperature. Light is not required.

The seed has its own food store and does not photosynthesise until the shoot is above ground. On a 'State three' question an incorrect item still counts as one of your three, so light pushes out a correct answer.

Mistake: Saying self-pollination produces clones, or calling it asexual reproduction.

Instead: Self-pollination is still sexual reproduction — two gametes fuse — and the offspring are still genetically varied, just less varied than after cross-pollination.

Gametes are produced by meiosis, which shuffles the alleles even within one plant. The correct comparison word is 'less variation', not 'no variation'.

Mistake: Answering a comparison question with a list of features and no reasons, or comparing unmatched features.

Instead: Compare like with like — petals against petals, stigmas against stigmas — and attach a reason to each: 'feathery, so it has a large surface area to catch pollen from the air'.

The command word Compare asks for similarities and differences, which needs a shared feature in each sentence, and an Explain mark is only released by the reason, never by the description alone.

Check yourself

Name the four rings of a flower from the outside inwards, then name the two parts of a stamen and the three regions of a carpel.
Answer
Sepals, petals, stamens, carpel. A stamen is a filament and an anther. A carpel has a stigma at the top, a style below it and an ovary at the base, with ovules inside the ovary.
State the function of the sepals, the filament, the style and the ovule.
Answer
Sepals enclose and protect the flower while it is in bud. The filament holds the anther in a position where its pollen can be removed. The style connects the stigma to the ovary and the pollen tube grows down through it. The ovule contains the female gamete and becomes the seed after fertilisation.
Define pollination and define fertilisation, in one sentence each, and state where in the flower each one happens.
Answer
Pollination is the transfer of pollen grains from an anther to a stigma, and it happens on the surface of the stigma. Fertilisation occurs when a pollen nucleus fuses with a nucleus in an ovule, and it happens inside an ovule, in the ovary.
Put these in order: the nucleus fuses with a nucleus in the ovule; the tube grows down the style; the pollen grain germinates; the tube enters the ovule.
Answer
The pollen grain germinates; the tube grows down the style; the tube enters the ovule; the pollen nucleus fuses with a nucleus in the ovule.
Describe the anthers and stigmas of a wind-pollinated flower, and state two ways its pollen grains differ from those of an insect-pollinated flower.
Answer
The anthers are large and hang outside the flower on long filaments, loosely attached so they swing in the wind. The stigmas are feathery and also hang outside the flower, giving a large surface area to catch pollen from the air. Its pollen grains are smaller, lighter and smooth (rather than sticky or spiky), and are produced in far greater numbers.
Give two structural adaptations of an insect-pollinated flower, each with the reason it helps.
Answer
The petals are large, brightly coloured and scented, with nectar, so that insects are attracted to the flower. The stigma is sticky and lies inside the flower, so that pollen rubs off the body of an insect pushing past it. (Anthers inside the flower and firmly attached, so the insect must brush against them, is also acceptable.)
Pollen is carried from a flower on an apple tree to another flower on the same apple tree. Is that self-pollination or cross-pollination, and why?
Answer
Self-pollination, because the pollen has been transferred to the stigma of a different flower on the same plant. Cross-pollination requires the pollen to reach a flower on a different plant of the same species.
Explain why a population of plants produced by cross-pollination is more likely to survive a new disease than one produced by self-pollination.
Answer
Cross-pollination combines gametes from two different plants, so the offspring have more genetic variation. It is therefore more likely that some individuals carry an allele that makes them resistant to the disease, and those individuals survive and reproduce. In a self-pollinated population there is much less variation, so if the disease kills one plant it is likely to kill nearly all of them.
State the three conditions needed for germination, and explain why oxygen is one of them.
Answer
Water, oxygen and a suitable temperature. Oxygen is needed for aerobic respiration, which releases the energy required for growth.

Exam questions

Try each part first. Open the mark scheme after you have written your answer.

Question 1. A student is studying a buttercup, which is an insect-pollinated flower.

(a) Give the names of the two structures that together make up a stamen. 1 mark No calculator

Mark scheme

B1 filament AND anther — both required for the mark

(b) State the function of the anther and the function of the stigma. 2 marks No calculator

Mark scheme

B1 anther produces pollen grains (accept produces pollen containing the male gametes / male nuclei; ignore 'holds' or 'contains' alone)
B1 stigma receives / catches the pollen grains, owtte

(c) Describe what is meant by pollination. 1 mark No calculator

Mark scheme

B1 the transfer of pollen grains from an anther to a stigma. CON if the answer states that nuclei fuse or that a seed is formed

(d) A pollen grain from another buttercup lands on the stigma. Describe what happens next, up to and including fertilisation. 4 marks No calculator

Mark scheme

B1 the pollen grain germinates and a pollen tube grows out of it
B1 the pollen tube grows down through the style (towards the ovary)
B1 the pollen tube enters the ovule (accept: through the micropyle / an opening in the ovule wall)
B1 the pollen nucleus travels down the tube and fuses with a nucleus in the ovule, forming a zygote. CON if the pollen grain itself is said to travel down the style

Question 2. Grasses are pollinated by the wind.

(a) Describe the anthers and the stigmas of a wind-pollinated flower. 3 marks No calculator

Mark scheme

B1 anthers hang outside the flower, on long filaments / loosely attached so they move in the wind
B1 stigmas are feathery / branched
B1 stigmas hang outside the flower / have a large surface area to catch pollen from the air

(b) Compare the pollen grains of a wind-pollinated flower with those of an insect-pollinated flower. 2 marks No calculator

Mark scheme

B1 wind-pollinated pollen is smaller / lighter / smooth, whereas insect-pollinated pollen is larger / sticky / spiky, ORA
B1 wind-pollinated flowers produce much larger numbers of pollen grains, ORA

(c) Explain how the petals and the stigma of an insect-pollinated flower are adapted for pollination by insects. 2 marks No calculator

Mark scheme

B1 petals are large / brightly coloured / scented (or nectar is present), so that insects are attracted to the flower
B1 stigma is sticky and inside the flower, so pollen rubs off the insect's body onto it and stays there

Question 3. A species of wild flower can reproduce by self-pollination and by cross-pollination. In one valley the plants grow in dense patches; on an isolated hillside a few plants grow far apart from one another.

(a) Describe the difference between self-pollination and cross-pollination. 2 marks No calculator

Mark scheme

B1 self-pollination: pollen is transferred from an anther to the stigma of the same flower, or of a different flower on the same plant
B1 cross-pollination: pollen is transferred to the stigma of a flower on a different plant of the same species

(b) A new disease reaches the valley. Suggest why a population produced mainly by self-pollination is less likely to survive the disease than one produced mainly by cross-pollination. 3 marks No calculator

Mark scheme

B1 self-pollination produces offspring with less genetic variation (both gametes come from the same plant), ORA for cross-pollination
B1 so it is less likely that any individual carries an allele giving resistance to the disease
B1 so few or no plants survive to reproduce and the population may die out, ORA

(c) Suggest one advantage of self-pollination to the plants growing far apart on the hillside. 1 mark No calculator

Mark scheme

B1 seed can still be produced without pollen arriving from another plant / without relying on a pollinator reaching it, owtte

Question 4. A student investigates germination using four boiling tubes, each with 20 cress seeds. Tube A: damp cotton wool, open to the air, 20 degrees Celsius. Tube B: dry cotton wool, open to the air, 20 degrees Celsius. Tube C: seeds under boiled and cooled water with a layer of oil on the surface, 20 degrees Celsius. Tube D: damp cotton wool, open to the air, 4 degrees Celsius. After five days the student counts the seeds that have germinated in each tube.

(a) Explain why the water in tube C was boiled and then covered with a layer of oil. 2 marks No calculator

Mark scheme

B1 boiling removes / drives out the dissolved oxygen from the water
B1 the oil layer stops oxygen from the air dissolving back into the water

(b) No seeds germinated in tube C. Explain this result. 2 marks No calculator

Mark scheme

B1 there is no oxygen, so the seeds cannot carry out aerobic respiration
B1 so no energy is released for growth / for cell division

(c) State why tube A was included in the investigation. 1 mark No calculator

Mark scheme

B1 it is the control, in which all three conditions are present, giving a comparison for the other tubes, owtte

How this is examined

Pollination is a transfer of pollen grains; fertilisation is a fusion of nuclei. Two events, not one.: A one-mark Describe or Define of pollination near the start of a flower question, with fertilisation asked separately later — and often a distractor answer offered in a multiple-choice or a 'complete the sentence' part. For pollination the mark needs 'transfer' (or moved/carried) plus 'pollen grains', 'anther' and 'stigma' — mentioning nuclei or seeds in a pollination definition is treated as a contradiction. For fertilisation the mark needs 'a pollen nucleus fuses with a nucleus in an ovule'; 'the pollen meets the egg' does not say anything joined and does not score.
Why a wind-pollinated flower is built completely differently from an insect-pollinated one.: A three- or four-mark Describe or Compare, often with a partly filled table to complete, or attached to a photograph of a grass flower with the anthers hanging out of it. Matched pairs with a reason attached to each: 'anthers hang outside on long loose filaments so the wind shakes the pollen out', 'the stigma is feathery, giving a large surface area to catch pollen from the air', 'petals are small and green with no nectar because no insect needs to be attracted'. A bare list of features with no 'so that' scores the description marks only.
The route from stigma to ovule — what grows, what travels, and in what order.: A three- or four-mark Describe, usually the last part of the flower question and Extended-only, phrased as 'describe what happens after the pollen grain lands on the stigma'. Four sequenced points: the pollen grain germinates and a pollen tube grows out of it; the tube grows down through the style; the tube enters the ovule; the pollen nucleus passes down the tube and fuses with a nucleus in the ovule to form a zygote. Saying the pollen grain travels down the style loses the delivery mark. Endosperm and seed development are excluded — do not spend time on them.
What each flower part does, said as a job rather than a position.: A labelled flower diagram to complete, followed by 'State the function of X' for two or three of the labels, one mark each. A verb that names the job: 'the anther produces pollen grains', 'the stigma receives pollen grains', 'the sepals protect the flower while it is in bud', 'the ovule contains the female gamete and becomes the seed', 'the ovary contains the ovules and becomes the fruit'. Describing where a part sits earns nothing.
The price a population pays for self-pollination, and the price it pays for cross-pollination.: A three-mark Suggest or Explain set in a scenario — a new disease, a drought, a fall in the number of pollinating insects, or an isolated plant with no neighbours. A chain, not a single statement: less genetic variation, so fewer or no individuals carry an allele that lets them survive the change, so the population may die out. For the other side: cross-pollination relies on a pollinator or on the wind, whereas a self-pollinating plant can set seed alone. Saying self-pollination produces 'clones' or 'no variation' is wrong and can cancel a correct point.
The three conditions for germination, and the tube in an experiment that proves each one.: A 'State three' worth three marks, usually followed by an Explain about how one condition was removed, or about why the control tube is needed. Water, oxygen and a suitable temperature — exactly those three, with light left out, because a wrong item counts toward your three. For the oxygen tube both halves are needed: boiled to drive out dissolved oxygen, and a layer of oil so that oxygen cannot dissolve back in. For oxygen's role, say 'aerobic respiration, to release energy for growth', not just 'respiration'.

Words to know

Pollination: The transfer of pollen grains from an anther to a stigma.
Fertilisation: The fusion of a pollen nucleus with a nucleus in an ovule, producing a zygote.
Gamete: A sex cell. In a flowering plant the male gamete is the nucleus carried in the pollen grain and the female gamete is the nucleus inside the ovule.
Zygote: The single cell formed when two gamete nuclei fuse.
Stamen: The male part of a flower: a filament with an anther on top.
Carpel: The female part of a flower: stigma, style and ovary, with ovules inside the ovary.
Ovule: A structure inside the ovary containing the female gamete; it becomes the seed after fertilisation.
Pollen tube: The tube that grows out of a germinating pollen grain, down the style and into the ovule, carrying the pollen nucleus.
Self-pollination: Transfer of pollen from the anther of a flower to the stigma of the same flower, or of a different flower on the same plant.
Cross-pollination: Transfer of pollen from the anther of a flower to the stigma of a flower on a different plant of the same species.
Germination: The beginning of growth of a seed into a seedling, requiring water, oxygen and a suitable temperature.

All Biology lessons Next: 16.4 Sexual reproduction in humans