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Sexual reproduction in plantsEdexcel A-Level Biology B: Revision notes

Section 1

Forming a pollen grain

Pollen grains are made in the anthers.

  1. A diploid pollen mother cell divides by meiosis to give four haploid microspores
  2. The nucleus of each microspore divides by mitosis into a generative nucleus and a tube nucleus
  3. A tough outer wall develops, giving the mature pollen grain

The generative nucleus later divides again to form two male nuclei, which are the male gametes.

Key termspollen mother cellmicrosporegenerative nucleustube nucleus

Section 2

Forming an embryo sac

The embryo sac forms inside the ovule.

  1. A diploid megaspore mother cell divides by meiosis to give four haploid megaspores
  2. Three degenerate; one survives
  3. Its nucleus divides by mitosis three times, giving eight haploid nuclei
  4. The egg cell lies near the micropyle; two polar nuclei lie in the centre; the other nuclei form synergids and antipodal cells
Key termsmegasporeembryo sacmicropylepolar nuclei
Common mistake

Do not mix up meiosis and mitosis. Meiosis happens once (in the mother cell); the later divisions that make the nuclei are mitosis.

Section 3

Getting the male nuclei to the embryo sac

After pollination the pollen grain germinates on the stigma.

  • The tube nucleus controls growth of the pollen tube
  • The tube secretes enzymes that digest the tissues of the style, making a path and releasing nutrients
  • The generative nucleus divides by mitosis to give two male nuclei
  • The tube grows through the micropyle into the ovule and its tip breaks down, releasing the two male nuclei into the embryo sac
Key termspollinationpollen tube

Section 4

Double fertilisation

Two fusions occur in the embryo sac:

  1. One male nucleus fuses with the egg nucleus to form a diploid zygote, which develops into the embryo
  2. The other male nucleus fuses with the two polar nuclei to form a triploid endosperm nucleus (3n), which develops into the endosperm, a food store

Because the endosperm forms only after fertilisation, the plant does not waste resources on unfertilised ovules.

Key termsdouble fertilisationendospermtriploid

Section 5

Core Practical 4: sucrose and pollen germination

  • Prepare sucrose solutions (for example 0, 5, 10, 15 and 20%)
  • Place a drop of each on a cavity slide, add pollen from the same flower, and incubate for the same time at the same temperature
  • Using a microscope, count at least 50 to 100 grains per slide and record the percentage germinated (tube longer than the grain's diameter), or measure tube lengths with a calibrated eyepiece graticule
  • Control: temperature, time, species and age of pollen, volume of solution
  • Repeat and calculate a mean

Interpretation: sucrose supplies energy and wall material, so germination rises. At high concentration the water potential is lower than in the grain, so water leaves by osmosis and germination falls. Pollen can cause allergic reactions, so avoid inhaling it.

Key termseyepiece graticuleosmosispercentage germination

Must know

  • Pollen: mother cell, meiosis, four microspores, mitosis, generative and tube nuclei
  • Embryo sac: megaspore mother cell, meiosis, three degenerate, three mitoses, eight nuclei
  • Pollen tube: tube nucleus controls growth, enzymes digest style, enters through the micropyle
  • Double fertilisation: zygote (2n) and triploid endosperm (3n)
  • Core Practical 4: sucrose concentration affects germination; control variables, repeats

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Sexual reproduction in plants

  1. A student investigated the effect of sucrose concentration on pollen germination. She placed pollen grains from one flower in sucrose solutions of 0%, 5%, 10% and 20% on cavity slides and kept them at 25 °C for 30 minutes. She then counted 100 grains in each solution and recorded the percentage that had produced a pollen tube: 8%, 52%, 81% and 23% respectively.
    Suggest why only 8% of the pollen grains germinated in the 0% sucrose solution.2 marks
  2. In a lily flower, pollen grains form in the anthers from pollen mother cells. In the ovules, the embryo sac forms from a megaspore mother cell. Both types of mother cell are diploid.
    Explain why the pollen mother cell divides by meiosis.2 marks
  3. In flowering plants the male gametes cannot swim to the egg, which lies deep inside the ovule. Each pollen grain contains a generative nucleus and a tube nucleus. After a pollen grain lands on the stigma of a compatible flower, it germinates.
    Explain how the pollen tube grows through the style to reach the ovule.3 marks
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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).