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Given below is the diagram… (Class 12 Biology 2025, Q32B)

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BiologyCBSE Class 12 · Board exam 2025, set 57/1/1 · 5 marks · Long answer

CBSE Class 12 Biology 2025: question 32(B)

Set in the CBSE Class 12 board exam of 2025, paper 57/1/1, section E, question 32(B), for 5 marks. Try it first; the hints and our step-by-step solution are below.

Q32(B) · 5 marks · Long answerCompetency-basedHuman Reproduction

Given below is the diagram of human ovum surrounded by a few sperms. Observe the diagram and answer the questions that follows :

  1. (i)

    Compare the fate of sperms 'P', 'Q' and 'R' shown in the diagram.

    0 marks
  2. (ii)

    Write the role of Zona pellucida in this process.

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  3. (iii)

    Analyse the changes occurring in the ovum after the entry of sperm.

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  4. (iv)

    How acrosome and middle piece of a human sperm are able to play an important role in human fertilization ?

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Figure in the paper: Human ovum at centre surrounded by zona pellucida and cells of the corona radiata, with perivitelline space labelled. Three sperms P, Q and R approach the ovum: P touching the zona pellucida, Q near the corona radiata, R farther away. See it in the official paper.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2025 board paper, set 57/1/1.

Hints

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

Hint 1

Look at where each sperm sits relative to the zona pellucida. How many sperms can fertilise one ovum?

Hint 2

Recall the stage at which the human ovum is released, and what completes only after sperm entry.

Hint 3

Link each sperm part to its content: the acrosome carries enzymes, and the middle piece carries mitochondria.

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) Fate of P, Q and R: Sperm P has reached the zona pellucida and its acrosome reacts. It is the one that penetrates and fertilises the ovum. Sperms Q and R are still in or beyond the corona radiata. They cannot enter the ovum and degenerate, because only one sperm is allowed in.

    1 mark

  2. (ii) Role of zona pellucida: When the first sperm contacts the zona pellucida, it changes the membrane and zona of the ovum (zona reaction). These changes block the entry of additional sperms and prevent polyspermy, so only one sperm fertilises the ovum.

    1 mark

  3. (iii) Changes in the ovum after sperm entry: Entry of the sperm stimulates the secondary oocyte to complete meiosis II. This produces a large haploid ovum (ootid) and a small second polar body. The haploid nucleus of the sperm then fuses with that of the ovum, forming a diploid zygote.

    1 mark

  4. (iv) Acrosome: The acrosome is a cap-like structure over the head of the sperm. It is filled with hydrolytic (lytic) enzymes that dissolve the corona radiata and zona pellucida, which lets the sperm enter the cytoplasm of the ovum.

    1 mark

  5. (iv) Middle piece: The middle piece contains numerous mitochondria. They produce energy (ATP) for the movement of the tail, giving the sperm the motility it needs to reach the ovum and fertilise it.

    1 mark

Answer: (i) P fertilises the ovum; Q and R cannot enter. (ii) Zona pellucida changes on contact with the first sperm and blocks entry of other sperms. (iii) The secondary oocyte completes meiosis II, forming the ootid and second polar body; the nuclei fuse into a zygote. (iv) Acrosome enzymes help penetration; middle-piece mitochondria give energy for motility.

Where the marks are

  • States that P fertilises the ovum while Q and R cannot penetrate (1 mark)
  • Zona pellucida changes on sperm contact and blocks entry of additional sperms (1 mark)
  • Completion of meiosis II, forming the second polar body and haploid ovum/ootid (1 mark)
  • Acrosome has enzymes that help the sperm enter the ovum (1 mark)
  • Middle piece has mitochondria that give energy for tail movement/motility (1 mark)

Revise the chapter

  • Chapter: Human Reproduction (notes, formulas, where marks are lost and every past question)
Practise this chapterBoard-style questions from Human Reproduction, one at a time, marked against the answer key and marking points.Back to the full paper2025 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.