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Questions on sexual reproduction in flowering plants
The parts of a flower and what each does, how a pollen grain forms in the anther by meiosis and then mitosis, how the embryo sac forms in the ovule, pollination and the growth of the pollen tube with the roles of the tube nucleus, the generative nucleus and the enzymes the tube secretes, double fertilisation giving a diploid zygote and a triploid endosperm, what the ovule and ovary become, and the core practical on sucrose concentration and pollen tube growth.
14 original questions · 42 marks · the sexual reproduction in flowering plants notes · Cell cycles, reproduction and development
To print: the written questions (PDF, 47 KB) · the mark schemes (PDF, 51 KB)
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Describe how a pollen grain containing two male nuclei develops from a pollen mother cell in the anther.
Mark scheme
- B1 the pollen mother cell is diploid and divides by meiosis
- B1 giving a tetrad of four haploid cells, or microspores, each of which becomes a pollen grain
- B1 the microspore nucleus divides by mitosis into a tube nucleus and a generative nucleus, and a wall with an outer exine forms around the grain
- B1 the generative nucleus divides by mitosis to give two haploid male nuclei
Describe how the embryo sac forms inside an ovule.
Mark scheme
- B1 a diploid megaspore mother cell in the nucellus divides by meiosis
- B1 this gives four haploid megaspores, three of which degenerate
- B1 the nucleus of the surviving megaspore divides by mitosis three times, giving eight haploid nuclei
- B1 these form an egg cell and two synergids at the micropyle end, two polar nuclei in the centre and three antipodal cells at the other end
After pollination, the male nuclei have to travel from the stigma to the embryo sac. Explain the roles of the tube nucleus, the pollen tube and the enzymes it secretes in getting them there.
Mark scheme
- B1 the pollen tube grows from the grain through the stigma and down the style, carrying the male nuclei inside it
- B1 the tube nucleus, near the tip, controls the growth of the tube, for example by its genes being transcribed to make the enzymes
- B1 the tip secretes hydrolytic enzymes that digest a path through the tissues of the style, and the products are absorbed and used by the growing tube
- B1 the tube grows towards chemicals from the ovule, enters through the micropyle and releases the male nuclei into the embryo sac
A student investigated the effect of sucrose concentration on the germination of pollen grains. The table shows the results, which are constructed for this question. Describe and explain the results.
Sucrose concentration / g per 100 cm³ Mean percentage of grains germinated after one hour 0 4 5 18 10 46 15 62 20 35 25 8 Mark scheme
- B1 germination rises from 4% at 0 g to a peak of 62% at 15 g per 100 cm³, then falls to 8% at 25 g
- B1 at low concentrations the solution has a higher water potential than the grain, so water enters by osmosis and grains or tube tips burst
- B1 at high concentrations the solution has a lower water potential, so water leaves by osmosis and the tube cannot take up the water it needs to grow
- B1 sucrose is also a respiratory substrate, providing ATP for tube growth, which helps explain the rise from 0 g
Describe the process of double fertilisation in a flowering plant.
Mark scheme
- B1 the pollen tube releases two male nuclei into the embryo sac
- B1 one male nucleus fuses with the egg nucleus to form a diploid zygote
- B1 the other male nucleus fuses with the two polar nuclei to form a triploid endosperm nucleus
Maize has a diploid number of 20. Complete the table by giving the number of chromosomes in each nucleus.
Nucleus Number of chromosomes pollen mother cell ? generative nucleus ? male nucleus ? polar nucleus ? zygote ? endosperm nucleus ? Mark scheme
- B1 pollen mother cell 20 and generative nucleus 10
- B1 male nucleus 10 and polar nucleus 10
- B1 zygote 20 and endosperm nucleus 30
State the function of each of these parts of a flower: the anther, the stigma and the ovary.
Mark scheme
- B1 anther: where pollen grains are made, and from which they are released
- B1 stigma: the surface that receives pollen, on which the pollen grain germinates
- B1 ovary: contains and protects the ovules, and becomes the fruit after fertilisation
Describe how a student could measure the mean length of pollen tubes growing on a microscope slide.
Mark scheme
- B1 calibrate an eyepiece graticule against a stage micrometer at the magnification used
- B1 measure the length of each tube in eyepiece units and convert to micrometres using the calibration
- B1 measure a set number of tubes, chosen at random or in several fields of view, at a fixed time, and calculate the mean
Compare the formation of pollen grains in the anther with the formation of the embryo sac in the ovule.
Mark scheme
- B1 both begin with a diploid mother cell dividing by meiosis to give four haploid cells
- B1 in the anther all four haploid cells become pollen grains, whereas in the ovule three of the four megaspores degenerate and only one develops
- B1 in the pollen grain the nucleus divides by mitosis to give two nuclei, then the generative nucleus divides again, whereas the megaspore nucleus divides by mitosis three times to give eight nuclei
A pea flower is pollinated and later produces a pod containing seeds. Explain which parts of the flower become the seeds, the seed coats and the pod.
Mark scheme
- B1 each ovule becomes a seed, after the zygote inside it has divided by mitosis to form an embryo
- B1 the integuments of each ovule become its seed coat, the testa
- B1 the ovary wall becomes the pod, which is the fruit
Explain why the endosperm of a seed is triploid.
Mark scheme
- B1 the endosperm nucleus forms when one male nucleus fuses with two polar nuclei, each of which is haploid
- B1 so it has three sets of chromosomes, and the endosperm grows from it by mitosis, which keeps all three sets
In one field of view a student counts 48 pollen grains, 30 of which have a pollen tube longer than the diameter of the grain. Calculate the percentage of grains that have germinated.
Mark scheme
- M1 30 divided by 48, multiplied by 100
- A1 62.5%, or 63% to two significant figures
Explain why the two male nuclei delivered by one pollen tube are genetically identical to each other.
Mark scheme
- B1 both are formed from the generative nucleus by mitosis
- B1 mitosis produces nuclei with identical copies of the parent nucleus's DNA, so there is no crossing over or independent assortment to make them differ
The endosperm develops only after a male nucleus has fused with the polar nuclei. Suggest an advantage to the plant of the endosperm forming in this way rather than before fertilisation.
Mark scheme
- B1 the endosperm forms only in ovules where an egg is being fertilised at the same time
- B1 so the plant does not spend starch and other stored food on ovules that will never contain an embryo
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