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When a microorganism invades… (Class 12 Biology 2023, Q29)

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BiologyCBSE Class 12 · Board exam 2023, set 57/5/1 · 4 marks · Case-based

CBSE Class 12 Biology 2023: question 29

Set in the CBSE Class 12 board exam of 2023, paper 57/5/1, section D, question 29, for 4 marks. Try it first; the hints and our step-by-step solution are below.

Q29 · 4 marks · Case-basedCompetency-basedHuman Health and Disease

When a microorganism invades a host, a definite sequence of events usually occur leading to infection and disease, causing suffering to the host. This process is called pathogenesis. Once a microorganism overcomes the defense system of the host, development of the disease follows a certain sequence of events as shown in the graph. Study the graph given below for the sequence of events leading to appearance of a disease and answer the questions that follow :

  1. (a)

    In which period, according to the graph there are maximum chances of a person transmitting a disease / infection and why ?

    1 mark
  2. (b)

    Study the graph and write what is an incubation period. Name a sexually transmitted disease that can be easily transmitted during this period. Name the specific type of lymphocytes that are attacked by the pathogen of this disease.

    2 marks

  3. OR

    (b)

    Draw a schematic labelled diagram of an antibody.

    2 marks
  4. (c)

    In which period, the number of immune cells forming antibodies will be the highest in a person suffering from pneumonia ?
    Name the immune cells that produce antibodies.

    1 mark

Figure in the paper: Graph: x-axis Time, y-axis Number of microorganisms. A bell-shaped curve rises and falls. Labelled regions: Incubation period (initial left part), Period of illness (rising part, dark shaded), Period of decline (falling part, light shaded), 'Death if immune responses and/or medical treatments fail' pointing to the peak. Two horizontal bands at the bottom: 'Mild signs or symptoms' and 'No signs or symptoms'. See it in the official paper.

Source: official question paper from cbse.gov.in (PDF). See it in the full 2023 board paper, set 57/5/1.

Hints

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

Hint 1

On the graph, find where the number of microorganisms is highest.

Hint 2

The incubation period lies before symptoms appear. Think of an STD with a very long symptomless phase.

Hint 3

Recall which lymphocytes give humoral immunity and which ones HIV destroys.

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) The period of illness gives the maximum chance of transmission.

    ½ mark

  2. In this period the number of microorganisms is at its highest, and they are released in large numbers through coughing and sneezing (droplet infection). (The incubation period, when there are no symptoms so the person takes no precautions, is also accepted.)

    ½ mark

  3. (b) The incubation period is the time between the entry of the pathogen (infection) and the appearance of the first signs or symptoms of the disease.

    1 mark

  4. AIDS can be easily transmitted during this long symptomless period, which may last 5–10 years.

    ½ mark

  5. HIV attacks helper T-lymphocytes (T cells).

    ½ mark

  6. (b) OR – Antibody diagram, described: a Y-shaped molecule (H₂L₂) with two heavy chains forming the stem and arms, and two light chains on the outer side of the arms. Disulphide (S–S) bonds join the chains. The tips of the arms are the antigen-binding sites (variable regions), and the rest is the constant region.

  7. (c) Antibody-forming immune cells are highest during the period of illness.

    ½ mark

  8. Antibodies are produced by B-lymphocytes (B cells, which become plasma cells).

    ½ mark

Answer: (a) Period of illness (highest microbe load, droplet spread); (b) Incubation period = time from infection to appearance of symptoms; AIDS; helper T-lymphocytes [OR antibody diagram: 2 heavy + 2 light chains, S–S bonds, antigen-binding sites]; (c) Period of illness; B-lymphocytes

Where the marks are

  • Names the period of illness (½ mark)
  • Reason: maximum microorganisms, spread by droplets (½ mark); incubation period with a valid reason is also accepted
  • Defines incubation period as the time from infection to appearance of symptoms (1 mark)
  • Names AIDS (½ mark)
  • Names T-lymphocytes (helper T cells) (½ mark)
  • OR (b): antibody diagram with any four correct labels, e.g. heavy chain, light chain, antigen-binding site, S–S bond (½ × 4 = 2 marks)
  • (c) Names the period of illness (½ mark) and B-lymphocytes (½ mark)

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 paper2023 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.