RowQ
The Vault
RowQ
The Vault
CBSE Class 12 Biology · 11 questions · 26 marks
Health is not simply the absence of illness but a state of complete physical, mental, and social well-being — and this chapter examines what threatens it. You will study the pathogens behind malaria, typhoid, pneumonia, amoebiasis and others, how the innate and acquired arms of the immune system respond, what makes HIV and cancer so difficult to treat, and why drug and alcohol abuse damages adolescents in particular. Learning each disease alongside its exact causative organism is the single highest-yield habit here.
The infective stage of the malarial parasite that enters the human body through the bite of a female Anopheles mosquito is the:
Answer
Sporozoite is correct — sporozoites are stored in the salivary glands of the infected female Anopheles and are injected into the human bloodstream when she bites, after which they invade liver cells. Merozoites are the stage that multiplies in and ruptures red blood cells within the human host, while gametocytes are the stage taken up by the mosquito to continue the cycle in her gut.
A newborn baby receives protective antibodies through the mother's colostrum. This is an example of:
Answer
Passive immunity is correct — the baby receives ready-made antibodies, chiefly IgA, without its own immune system having encountered any antigen. Protection is therefore immediate but temporary, since no memory cells are formed. Active immunity would require the baby's own lymphocytes to respond to an antigen or vaccine and produce antibodies itself.
Which enzyme carried by HIV allows it to convert its RNA genome into DNA inside the host cell?
Answer
Reverse transcriptase is correct — HIV is a retrovirus, and this enzyme copies the viral RNA into complementary DNA, which is then integrated into the host cell's DNA and directs the production of new virus particles. RNA polymerase performs the opposite, normal transcription of DNA to RNA, ligase joins DNA fragments, and restriction endonucleases cut DNA at specific sites.
The property that keeps normal cells from piling over one another in a culture, and which cancer cells lose, is called:
Answer
Contact inhibition is correct — normal cells stop dividing once they touch neighbouring cells, which keeps tissue growth orderly. Cancer cells lose this property and continue to divide, forming a mass of tissue called a tumour. Metastasis is the later spread of malignant cells to distant sites, and apoptosis is programmed cell death.
Assertion (A): A person who has recovered from a particular infection usually resists the same infection far more effectively on a second exposure. Reason (R): The first encounter generates memory B and T lymphocytes that recognise the antigen and mount a rapid, intense secondary response.
Answer
Both A and R are true and R is the correct explanation of A — memory is the defining feature of acquired immunity. The slow, low-intensity primary response leaves behind a population of memory cells, so on re-exposure the antigen is recognised at once and antibody production is far faster and much greater in magnitude, often clearing the pathogen before symptoms appear. This is also the principle on which vaccination works.
Name the causative organism of amoebiasis and typhoid, and state one common route by which both are transmitted.
Answer
Amoebiasis, also called amoebic dysentery, is caused by the protozoan Entamoeba histolytica, which lives in the human large intestine and causes constipation, abdominal pain, cramps, and stools with excess mucus and blood clots. Typhoid is caused by the bacterium Salmonella typhi, which reaches the small intestine and spreads through the blood to other organs, producing sustained high fever, weakness, stomach pain, and loss of appetite. Both are transmitted by the faecal-oral route — through food and water contaminated with the faeces of an infected person, often carried onto food by houseflies acting as mechanical vectors.
Differentiate between active and passive immunity, and state one situation in which passive immunity is deliberately used in medicine.
Answer
Active immunity develops when a person's own immune system encounters an antigen — through a natural infection or through vaccination — and responds by producing antibodies and memory cells. It takes time to develop, but the protection it gives is long-lasting because of immunological memory. Passive immunity is produced by giving a person ready-made antibodies from another source, so their own immune system is not involved. Protection begins immediately but wears off within weeks, and no memory cells are formed. Passive immunity is used deliberately when protection is needed faster than the body can generate it — for example, preformed antibodies as an anti-tetanus injection after a deep wound, or antivenom after a snake bite. Natural passive immunity is seen in the antibodies passed from mother to foetus and in the IgA of colostrum.
Why are AIDS patients susceptible to infections that rarely trouble healthy people?
Answer
HIV specifically infects and destroys helper T lymphocytes, which are central to coordinating the immune response — they are needed to activate B cells for antibody production and to stimulate the cell-mediated response. As the virus multiplies, the number of these cells falls progressively. With the coordinating cells depleted, the body can no longer mount an effective defence, so organisms of low virulence that a healthy immune system would clear without difficulty — such as Mycobacterium, Toxoplasma, and various fungi — establish serious infections. These are called opportunistic infections, and they, rather than the virus itself, are usually what kills the patient.
Describe the life cycle of the malarial parasite in the human host and in the mosquito, and explain why the fever recurs periodically.
Answer
In the human host: an infected female Anopheles mosquito injects sporozoites of Plasmodium into the human bloodstream while taking a blood meal. These sporozoites reach the liver, where they multiply within liver cells. The parasites then leave the liver, enter the bloodstream, and invade red blood cells, where they multiply further and produce merozoites. The infected red cells rupture, releasing merozoites that invade fresh red cells, so the cycle repeats. Some merozoites instead develop into male and female gametocytes, which circulate in the blood without causing further destruction. In the mosquito: when a female Anopheles bites an infected person, she takes up the gametocytes with the blood. In her gut these mature into gametes, fertilisation takes place, and a zygote is formed. The zygote develops further in the wall of the gut and eventually produces large numbers of sporozoites, which migrate to the salivary glands. When this mosquito bites another person, the sporozoites are injected and the cycle begins again. Because the parasite requires two hosts to complete its life cycle, it is described as digenetic; the human is the intermediate host and the female Anopheles is the definitive host and the vector. Why the fever recurs: the rupture of red blood cells releases a toxic substance called haemozoin along with the merozoites. Haemozoin is responsible for the chill and high fever. Because the parasites within red cells develop in a fairly synchronised manner, a large number of red cells burst together at intervals of roughly 48 or 72 hours depending on the species, so a fresh release of haemozoin — and therefore a fresh bout of chills and fever — occurs every three or four days rather than continuously.
Explain what cancer is, distinguish benign from malignant tumours, list the categories of carcinogens, and outline the main approaches to detection and treatment.
Answer
What cancer is: in a normal tissue, cell growth and division are strictly regulated, and cells stop dividing when they come into contact with neighbouring cells — a property called contact inhibition. Cancer cells lose this regulation and continue to divide indefinitely, producing a mass of cells called a tumour. The change usually involves cellular proto-oncogenes, which are normal genes regulating growth that, when activated by carcinogens or oncogenic viruses, become oncogenes and drive uncontrolled proliferation. Benign versus malignant: a benign tumour stays confined to its site of origin, is usually enclosed, grows slowly, and generally causes little damage. A malignant tumour is a mass of rapidly proliferating cells that invades and damages surrounding normal tissue, competes with normal cells for nutrients and starves them, and, most seriously, can shed cells into the blood or lymph that travel to distant sites and start new tumours there. This spread is called metastasis and is the most dangerous property of cancer. Carcinogens fall into three categories. Physical agents include ionising radiation such as X-rays and gamma rays and non-ionising ultraviolet radiation, which damage DNA. Chemical agents include the tobacco-smoke chemicals associated with cancers of the lung, mouth, and throat. Biological agents include oncogenic viruses carrying viral oncogenes. Detection: cancers are detected by biopsy and histopathological study of the tissue, by blood and bone-marrow tests for leukaemias, by imaging techniques such as radiography, CT scan, and MRI, and by using monoclonal antibodies against cancer-specific antigens. Techniques of molecular biology can identify individuals with inherited susceptibility to certain cancers. Treatment: the main approaches are surgical removal of the tumour, radiotherapy in which tumour cells are irradiated while sparing surrounding normal tissue as far as possible, and chemotherapy using drugs that kill dividing cells, most of which have side effects such as hair loss and anaemia. Most patients are treated with a combination of these. In addition, immunotherapy — for example the use of substances such as alpha-interferon to activate the patient's own immune system — is used to help the body destroy the tumour, since tumour cells are otherwise able to escape immune detection.
A public health team visits a settlement beside a stagnant water channel after several residents fall ill. One group of patients has fever that rises with shivering every third day, and blood smears show parasites inside their red blood cells. A second group has persistent high fever, weakness, and abdominal pain; a Widal test on these patients is positive. The team also finds that the settlement draws drinking water from an open well close to leaking drains, and that discarded tyres and containers around the houses hold standing rainwater. (a) Name the disease and causative organism in each group. (b) Explain the link between the stagnant water in the tyres and the first group's illness. (c) Explain how the second group is likely to have become infected. (d) Suggest two preventive measures the team should recommend, one for each disease.
Answer
(a) The first group has malaria, caused by the protozoan Plasmodium — the three-day periodicity points to Plasmodium vivax. The second group has typhoid, caused by the bacterium Salmonella typhi, which the positive Widal test confirms. (b) Water collected in discarded tyres and containers is ideal breeding ground for the female Anopheles mosquito, which lays its eggs in stagnant water. A larger mosquito population means more bites, and each infected female injects Plasmodium sporozoites into the bloodstream when she feeds. Eliminating the breeding sites therefore breaks the transmission cycle at the vector stage. (c) Salmonella typhi is spread by the faecal-oral route. Drainage leaking into an open well contaminates the drinking water with faecal matter from infected persons, and anyone drinking it ingests the bacteria, which then reach the small intestine and spread through the blood to other organs. Contaminated food and houseflies carrying the organism onto food contribute in the same way. (d) For malaria: remove or cover all standing water around the houses, introduce larvivorous fish such as Gambusia into permanent water bodies, spray insecticide, and use mosquito nets and wire-meshed windows to prevent bites. For typhoid: provide a protected, chlorinated drinking water supply, repair the leaking drains so sewage cannot reach the well, insist on hand-washing and safe food handling, and offer typhoid vaccination to residents.
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