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Read the following passage… (Class 12 Biology 2024, Q30)

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BiologyCBSE Class 12 · Board exam 2024, set 57/5/1 · 4 marks · Case-based

CBSE Class 12 Biology 2024: question 30

Set in the CBSE Class 12 board exam of 2024, paper 57/5/1, section D, question 30, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q30 · 4 marks · Case-basedCompetency-basedHuman Health and Disease

Read the following passage and answer the questions that follow :
"Mosquitoes are drastically affecting the human health in almost all the developing tropical countries. Different species of mosquitoes cause very fatal diseases so much so that many humans loose their life and if they survive, are unable to put in productive hours to sustain their life. With the result the health index of the country goes down."

  1. (a)

    Name the form in which Plasmodium gains entry into (i) human body (ii) the female Anopheles body.

    1 mark
  2. (b)

    Why do the symptoms of malaria not appear in a person immediately after being bitten by an infected female Anopheles ? Give one reason. Explain when and how do the symptoms of the disease would appear.

    2 marks

  3. OR

    (b)

    Explain the events which occur within a female Anopheles mosquito after it has sucked blood from a malaria patient.

    2 marks
  4. (c)

    Name a species of mosquito other than female Anopheles and the disease, for which it carries the pathogen.

    1 mark

Source: official question paper from cbse.gov.in (PDF). See it in the full 2024 board paper, set 57/5/1.

Hints

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

Hint 1

Recall the life cycle of Plasmodium and its two hosts.

Hint 2

Before fever appears, the parasite must multiply in two kinds of human cells. Which are they?

Hint 3

Which mosquito spreads dengue?

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) (i) Plasmodium enters the human body as sporozoites, injected with the saliva of an infected female Anopheles.

    ½ mark

  2. (a) (ii) It enters the female Anopheles as gametocytes, taken in with the blood meal.

    ½ mark

  3. (b) [First alternative] Symptoms do not appear immediately because the sporozoites first multiply asexually in the liver cells and then in the RBCs. This takes time (the incubation period).

    1 mark

  4. (b) [First alternative] When the RBCs rupture, they release a toxic substance, haemozoin.

    ½ mark

  5. (b) [First alternative] Haemozoin is responsible for the chill and high fever recurring every 3–4 days.

    ½ mark

  6. (b) [OR alternative] The male and female gametocytes in the blood meal enter the mosquito.

    ½ mark

  7. (b) [OR alternative] Fertilisation takes place in the gut (stomach) of the mosquito.

    ½ mark

  8. (b) [OR alternative] The parasites develop further, and sporozoites are formed that escape from the gut.

    ½ mark

  9. (b) [OR alternative] The sporozoites migrate to the salivary glands, ready to infect the next human bitten.

    ½ mark

  10. (c) Mosquito species: Aedes (or Culex).

    ½ mark

  11. (c) Disease: dengue or chikungunya (or filariasis/elephantiasis for Culex).

    ½ mark

Answer: (a) (i) Sporozoites; (ii) gametocytes. (b) Asexual multiplication in the liver and RBCs takes time; RBC rupture releases haemozoin, causing chill and high fever every 3–4 days. OR: gametocytes fertilise in the mosquito gut, sporozoites form and migrate to the salivary glands. (c) Aedes – dengue/chikungunya (or Culex – filariasis).

Where the marks are

  • (a) Sporozoites (½) and gametocytes (½)
  • (b) Reason: asexual multiplication in liver and RBCs first (1); RBC rupture releases haemozoin (½); this causes chill and high fever every 3–4 days (½)
  • (b) OR: gametocytes taken in, fertilisation in gut, sporozoites formed and escape, migrate to salivary glands (½ × 4)
  • (c) Correct mosquito (½) with its disease (½)

Revise the chapter

Practise this chapterBoard-style questions from Human Health and Disease, one at a time, marked against the answer key and marking points.Back to the full paper2024 board paper, set 57/5/1: 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.