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(i) Explain the development… (Class 12 Biology 2024, Q31a)

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BiologyCBSE Class 12 · Board exam 2024, set 57/1/1 · 5 marks · Long answer

CBSE Class 12 Biology 2024: question 31(a)

Set in the CBSE Class 12 board exam of 2024, paper 57/1/1, section E, question 31(a), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q31(a) · 5 marks · Long answerSexual Reproduction in Flowering Plants

(i) Explain the development of male gametophyte in an angiosperm. (2½)
(ii) Draw a labelled diagram of a three-celled male gametophyte. (2½)

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 57/1/1.

Hints

Open one hint at a time and try again before the next.

Hint 1

Start from the sporogenous tissue of the anther and follow the cell divisions.

Hint 2

After meiosis, how many mitotic divisions produce a 3-celled pollen grain?

Hint 3

For the diagram, include the two wall layers, the germ pore, the vegetative cell and the two male gametes.

Step-by-step solution

Each step shows the marks it earns, as an examiner would award them. Checked by an independent re-solve from the question paper.

  1. (i) In a young anther, the sporogenous tissue occupies the centre of each microsporangium. Each cell of this tissue is a potential microspore mother cell (pollen mother cell).

    ½ mark

  2. (i) Each microspore mother cell undergoes meiosis to form four haploid microspores. This process is called microsporogenesis.

    ½ mark

  3. (i) The microspores stay arranged in a cluster of four cells, the microspore tetrad.

    ½ mark

  4. (i) As the anther matures and dehydrates, the microspores dissociate and develop into pollen grains, the male gametophytes.

    ½ mark

  5. (i) Each pollen grain undergoes an asymmetric mitotic division to form a large vegetative cell and a small generative cell. In about 40% of angiosperms, pollen is shed at this 2-celled stage. In the rest, the generative cell divides mitotically to give two male gametes, so the pollen is shed at the 3-celled stage.

    ½ mark

  6. (ii) Diagram (described): a rounded pollen grain with a two-layered wall. Label 1: exine, the thick outer layer of sporopollenin.

    ½ mark

  7. (ii) Label 2: germ pore, a gap in the exine.

    ½ mark

  8. (ii) Label 3: intine, the thin inner layer of cellulose and pectin.

    ½ mark

  9. (ii) Label 4: vegetative cell with its large, irregular vegetative nucleus.

    ½ mark

  10. (ii) Label 5: two male gametes (sperm cells), small spindle-shaped cells with dense cytoplasm, lying in the cytoplasm of the vegetative cell.

    ½ mark

Answer: (i) Microspore mother cell → meiosis → microspore tetrad → pollen grains → asymmetric mitosis forms vegetative and generative cells; generative cell → 2 male gametes; (ii) labelled 3-celled pollen: exine, intine, germ pore, vegetative nucleus/cell, two male gametes

Where the marks are

  • Microspore mother cell in sporogenous tissue (½ mark)
  • Meiosis / microsporogenesis (½ mark)
  • Microspore tetrad (½ mark)
  • Microspores mature into pollen grains (½ mark)
  • Asymmetric division into vegetative and generative cells, generative cell giving two male gametes (½ mark)
  • Correct labels: exine, intine, germ pore, vegetative cell/nucleus, male gametes (½ × 5 marks)

Revise the chapter

Practise this chapterBoard-style questions from Sexual Reproduction in Flowering Plants, one at a time, marked against the answer key and marking points.Back to the full paper2024 board paper, set 57/1/1: every question, by section.

The question is transcribed from the official paper; figures are described in words. The solution, hints and chapter tag are ours. Spotted an error? Write tosupport@questionbuddy.in.