CBSE Important Questions Class 12 Biology Chapter 7 – Human Health and Disease

Class 12 Biology Chapter 7 Human Health and Disease is the seventh chapter of the NCERT Class 12 Biology textbook. It explains common infectious diseases of humans such as malaria, typhoid, pneumonia and the common cold, the life cycle of Plasmodium, the immune system (innate and acquired immunity, antigens, antibodies, B and T lymphocytes), vaccination, allergies, AIDS, cancer, and drug and alcohol abuse.

These CBSE important questions for Class 12 Biology Chapter 7 cover the female Anopheles mosquito as vector and host, the most dangerous Plasmodium species, antigens and the structure of an antibody, how HIV enters and multiplies in macrophages and helper T cells, the routes of HIV transmission, the mechanism by which AIDS weakens immunity, and the role of hygiene in preventing infectious diseases. The set has multiple-choice, short-answer and long-answer questions with answers for board exam revision.

Q1. The female Anopheles mosquito is described as both a vector and a host for the Plasmodium parasite. Justify this statement by explaining the specific events of the parasite's life cycle that occur within the mosquito, differentiating its role as a vector from its role as a host.

Answer: The female Anopheles mosquito acts as a vector because it physically transmits the Plasmodium parasite (in the form of sporozoites) from an infected person to a healthy person through its bite. When it bites a healthy person, it injects sporozoites from its salivary glands into the human blood.
It is also a host because a critical part of the parasite's life cycle occurs within its body:
1. The mosquito ingests blood from a malaria-infected person, which contains the sexual stages of Plasmodium, the male and female gametocytes.
2. Inside the mosquito's gut, the gametocytes mature and fertilisation occurs between the male and female gametes to form a zygote.
3. The zygote develops into a motile ookinete, which penetrates the gut wall and forms an oocyst. Within the oocyst, thousands of sporozoites are produced.
4. The sporozoites migrate to the mosquito's salivary glands, ready to be injected into the next human.
Since sexual reproduction and maturation of the parasite take place inside the mosquito, it is the definitive host and not just a passive carrier.

Q2. Which of the following Plasmodium species is most fatal?
(a) P. vivax
(b) P. malariae
(c) P. falciparum
(d) P. ovale

Answer: (c) P. falciparum
Of all the species of Plasmodium, malignant malaria caused by Plasmodium falciparum is the most serious. If not treated carefully, it can be fatal.

Q3. Antigens are present
(a) in cytoplasm.
(b) on cell surface.
(c) in nucleus.
(d) on nuclear envelope.

Answer: (b) on cell surface.
Antigens are molecules, usually proteins or polysaccharides, that can start an immune response. To be recognised by antibodies and lymphocyte receptors, they must be exposed, so they are present as glycoproteins or glycolipids on the outer surface of the cell membrane.

Q4. Study the diagram given below.

Y-shaped antibody molecule with parts labelled a and b

(i) What does the above diagram illustrate?
(ii) Name the parts labelled ‘a’ and ‘b’.
(iii) Name the type of cells that produce this molecule.

Answer: (i) The diagram illustrates an antibody molecule (immunoglobulin), a Y-shaped protein that plays a key role in the immune response.
(ii) a) Antigen binding site b) Heavy chain
(iii) B-lymphocytes (plasma cells) produce antibodies.

Q5. Name the type of cells the AIDS virus first enters after getting inside the human body. Explain the sequence of events that the virus undergoes within these cells to increase its progeny.

Answer: The AIDS virus (HIV) first enters macrophages and helper T cells.
Sequence of events within macrophages:
1. The viral RNA produces viral DNA with the help of the enzyme reverse transcriptase.
2. The viral DNA gets incorporated into the host cell's DNA.
3. The viral DNA directs the synthesis of new virus particles using the machinery of the host cell.
4. The new viruses are released and infect other cells.

Q6. What are the various routes by which transmission of human immunodeficiency virus takes place?

Answer: (a) Sexual contact with an infected person.
(b) Transfusion of contaminated blood and blood products.
(c) Sharing infected needles, as in the case of intravenous drug abusers.
(d) From an infected mother to her child through the placenta.

Q7. AIDS is caused by HIV that principally infects
(a) all the lymphocytes.
(b) the activator B cells.
(c) the cytotoxic T cells.
(d) the T4 lymphocytes.

Answer: (d) the T4 lymphocytes.
HIV binds to the CD4 receptor present on helper T cells (T4 lymphocytes), enters them and multiplies inside them. The continuous destruction of T4 lymphocytes causes a severe fall in immunity, which results in AIDS.

Q8. Name the two types of cells in which HIV multiplies after gaining entry into the human body.

Answer: Macrophages and helper T-lymphocytes are the two types of cells in which HIV multiplies after gaining entry into the human body. Both carry the CD4 receptor to which the virus binds.

Q9. What is the mechanism by which the AIDS virus causes deficiency of the immune system of the infected person?

Answer: AIDS is caused by the Human Immunodeficiency Virus (HIV), which spreads through sexual or blood-to-blood contact. After getting into the body of the person, the virus enters macrophages, where the RNA genome of the virus replicates to form viral DNA with the help of the enzyme reverse transcriptase. This viral DNA gets incorporated into the host cell's DNA and directs the infected cells to produce virus particles. The macrophages continue to produce virus and in this way act as an HIV factory. HIV then enters helper T-lymphocytes (TH), replicates and produces progeny viruses. The progeny viruses released in the blood attack other helper T-lymphocytes. This repeated process leads to a progressive decrease in the number of helper T-lymphocytes in the body of the infected person, thereby decreasing the immunity of the person.

Replication of a retrovirus (HIV) inside a host cell

Q10. Disease characterised by an extremely weak immune system:
(a) Influenza
(b) AIDS
(c) Chikungunya
(d) Dengue

Answer: (b) AIDS
AIDS (Acquired Immunodeficiency Syndrome) is caused by HIV, which destroys the helper T cells (CD4+ cells) of the immune system. This makes the immune system extremely weak, so the person becomes prone to many other infections. Influenza, chikungunya and dengue are acute infections that the immune system normally overcomes.

Q11. For the prevention and control of many infectious diseases,
(a) maintenance of personal and public hygiene is very important.
(b) taking a balanced diet is very important.
(c) regular check-up by a medical practitioner is very important.
(d) taking proper care of the sleep pattern is most important.

Answer: (a) maintenance of personal and public hygiene is very important.
Infectious diseases spread through contact, droplets, vectors and contaminated food and water. Personal hygiene (hand washing, bathing, clean food) and public hygiene (safe drinking water, proper disposal of sewage and waste, vector control) break the chain of transmission, so they are the most important measures for prevention and control.

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