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(i) The human male… (Class 12 Biology SQP 2024-25, Q17B)

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BiologyCBSE Class 12 · Sample paper, session 2024-25 · 2 marks · Very short answer

CBSE Class 12 Biology sample paper 2024-25: question 17(B)

From the official CBSE sample question paper for 2024-25, section B, question 17(B), for 2 marks. Try it first; the hints and our step-by-step solution are below.

Q17(B) · 2 marks · Very short answerHuman Reproduction

(i) The human male ejaculates about 200 to 300 million sperm during a coitus, however the ovum is fertilized by only one sperm. How does the ovum block the entry of additional sperms?
(ii) All copulations will not lead to fertilization. Why?

Source: official sample paper from cbseacademic.nic.in (PDF). See it in the full Sample paper 2024-25.

Hints

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

Hint 1

Think about the layer around the ovum that the sperm meets first.

Hint 2

For (ii), recall where in the oviduct fertilisation takes place and when the ovum is available there.

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) When the first sperm contacts the zona pellucida, it induces changes in that layer. The cortical reaction hardens the zona pellucida, and this blocks the entry of any additional sperms, so only one sperm fertilises the ovum.

    1 mark

  2. (ii) Fertilisation happens only in the ampullary region of the fallopian tube. It succeeds only if the ovum and the sperms reach the ampulla at the same time, which happens when coitus occurs close to ovulation. Copulation at other times will not lead to fertilisation.

    1 mark

Answer: (i) Sperm contact changes the zona pellucida, which then blocks further sperm entry. (ii) Fertilisation needs the ovum and sperms to reach the ampullary region simultaneously.

Where the marks are

  • Explains that sperm contact changes the zona pellucida and blocks entry of other sperms (1 mark)
  • States that ovum and sperms must be transported simultaneously to the ampullary region (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 paperSample paper 2024-25: 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.