Skip to content

The figure given below… (Class 12 Biology SQP 2023-24, Q22)

questionbuddy.in/q/cbse-12-biology-2023-24-sqp-q22-shows-sperms

BiologyCBSE Class 12 · Sample paper, session 2023-24 · 3 marks · Short answer

CBSE Class 12 Biology sample paper 2023-24: question 22

From the official CBSE sample question paper for 2023-24, section C, question 22, for 3 marks. Try it first; the hints and our step-by-step solution are below.

Q22 · 3 marks · Short answerCompetency-basedSexual Reproduction in Flowering Plants

The figure given below shows 3 sperms A, B and C.

  1. a)

    Which one of the three sperms will gain entry into the ovum?

    0 marks
  2. b)

    Describe the associated changes induced by it on P and Q.

    0 marks

Figure in the paper: Figure: Ovum surrounded by few sperms. Ovum surrounded by granulosa cells; sperms A, B, C approach, A is in contact with the layer P (zona pellucida region), Q is the ovum cytoplasm centre, R is another labelled part, perivitelline space labelled. See it in the official paper.

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2023-24.

Hints

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

Hint 1

Which sperm is already touching the zona pellucida?

Hint 2

What prevents a second sperm from entering after the first?

Hint 3

Which meiotic division is still incomplete in the secondary oocyte at ovulation?

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. a) Sperm A, the one in contact with the zona pellucida, will enter the ovum.

    ½ mark

  2. b) Change in P (zona pellucida): Contact of sperm A with the zona pellucida (P) causes changes in the membrane that block the entry of additional sperms (B and C). This ensures monospermy.

    ½ mark

  3. The acrosomal secretions of sperm A help it pass through P and the plasma membrane into the cytoplasm of Q. This entry triggers completion of meiosis II of the secondary oocyte (Q).

    1 mark

  4. Change in Q (secondary oocyte): Meiosis II is unequal and forms a second polar body and a haploid ovum (ootid). The haploid nucleus of sperm A then fuses with the haploid nucleus of the ovum, forming a diploid zygote.

    1 mark

Answer: a) Sperm A. b) The zona pellucida changes to block entry of other sperms; sperm entry completes meiosis II in Q, giving a second polar body and a haploid ovum, which fuses with the sperm nucleus to form a diploid zygote.

Where the marks are

  • Identifies sperm A (0.5 mark)
  • Zona pellucida changes to block polyspermy (0.5 mark)
  • Acrosomal secretions help entry; entry induces completion of meiosis II (1 mark)
  • Second polar body and haploid ovum form; fusion gives diploid zygote (1 mark)

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 paperSample paper 2023-24: 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.