Biology: Cells, DNA and the human body

Statements 1 and 2 are correct. NCERT gives the share of each leucocyte: neutrophils 60-65 per cent (the most abundant), lymphocytes 20-25 per cent, monocytes 6-8 per cent, eosinophils 2-3 per cent and basophils 0.5-1 per cent (the least). Neutrophils and monocytes are the phagocytic cells; basophils release histamine, serotonin and heparin and take part in inflammatory reactions. Statement 3 is wrong on both counts: lymphocytes are about a quarter of the leucocytes, not half, and they are not phagocytes; B and T lymphocytes are responsible for specific immune responses.

All three statements are correct. In the embryo, blood cells first form in the yolk sac, then mainly in the liver (and to some extent the spleen and lymph nodes) during mid- gestation, and in the bone marrow towards the end of pregnancy. After childhood, marrow in the shafts of long bones turns fatty, and adult haematopoiesis takes place chiefly in the red marrow of flat bones and vertebrae and the ends of long bones. All lineages come from self- renewing haematopoietic stem cells, which split into myeloid (red cells, platelets, granulocytes, monocytes) and lymphoid (B, T and NK cells) branches. This is why a bone marrow transplant can rebuild a patient's whole blood and immune system.

Statements 2 and 3 are correct. NCERT describes the spleen as a large bean- shaped secondary lymphoid organ containing mainly lymphocytes and phagocytes; it acts as a filter of the blood by trapping blood-borne micro-organisms and holds a large reservoir of erythrocytes. Its red pulp removes worn-out red cells (about 120 days old), earning it the name 'graveyard of RBCs'. Its white pulp, especially its macrophages and marginal-zone B cells, is vital against encapsulated bacteria, so people without a spleen are advised pneumococcal, Hib and meningococcal vaccines. Statement 1 is wrong: T lymphocytes mature in the thymus, a primary lymphoid organ.

Pairs 1, 2 and 3 are correct. Monocytes (6-8 per cent of leucocytes) migrate into tissues and become macrophages, the long-lived phagocytes of each organ: Kupffer cells line the liver sinusoids and clear bacteria and old red cells arriving from the gut through the portal vein; microglia are the resident immune cells of the central nervous system; alveolar macrophages ('dust cells') clean the air sacs of the lungs. Osteoclasts, also from the monocyte- macrophage lineage, are found in bone, where they resorb bone matrix during remodelling, not in the kidneys.

Option (b) is the incorrect statement. Humans have no lymph heart; lymph moves slowly in one direction because of the squeezing of surrounding skeletal muscles, breathing movements, the contraction of smooth muscle in lymph vessel walls and one-way valves. (Some amphibians and reptiles do have lymph hearts.) The other statements are correct: the thoracic duct returns most lymph to the venous blood at the left subclavian vein; lymph nodes are secondary lymphoid organs where lymphocytes meet trapped antigens, so they enlarge during infection; and lacteals absorb fats, as NCERT notes.

Statements 1 and 2 are correct. Celsus (1st century AD) listed rubor, calor, tumor and dolor (redness, heat, swelling, pain). These signs come from vasodilation and increased permeability of capillaries caused by mediators such as histamine from mast cells, which let plasma and leucocytes (first neutrophils, then monocytes) reach the site; pus is mostly dead neutrophils, tissue debris and microbes. Statement 3 is wrong: inflammation is an innate response that starts within minutes of injury or infection, long before any specific antibody can be made.

All four statements are correct. The complement system is a cascade of more than 30 plasma and membrane proteins, mostly synthesised by hepatocytes, that 'complements' antibodies. It is switched on by three routes: the classical pathway (antibody bound to antigen), the lectin pathway (mannose-binding lectin recognising microbial sugars) and the alternative pathway (direct contact with microbial surfaces). Its end products C5b to C9 form the membrane attack complex that lyses Gram-negative bacteria such as Neisseria; C3b coats microbes (opsonisation) for phagocytes; and C3a and C5a attract leucocytes and promote inflammation.

Statements 1 and 2 are correct. The Bombay phenotype was reported by Bhende, Deshpande and Bhatia in The Lancet in 1952. Because the H antigen is the precursor of A and B, people who inherit two inactive H (FUT1) genes have neither H nor A nor B on their red cells; they naturally make anti-H as well as anti-A and anti-B. Statement 3 is false: ordinary group O red cells carry plenty of H antigen, so they are destroyed by the anti-H of a Bombay recipient. Such a patient can receive blood only from another Bombay-phenotype donor.

B cells and T cells protect the body from pathogens (d). B and T lymphocytes are white blood cells that carry out adaptive immunity, which targets particular germs. B cells make antibodies that tag germs. Cytotoxic T cells kill infected cells, and helper T cells organise the whole response. Both keep a memory of past germs, which is why vaccines work. Option (a) is wrong: allergies come from an overreaction of the immune system, not protection from it. Option (b) is wrong: pain and inflammation are eased by drugs like painkillers. Option (c) is wrong: these cells drive immunity; they do not suppress it.

Notice that HIV attacks helper T cells (CD4). CAR-T therapy, now made in India, reprograms a patient's T cells to fight cancer.

Answer as per the official UPSC answer key.

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Nitric oxide dilates blood vessels (a). The inner lining of blood vessels makes nitric oxide (NO) from the amino acid arginine. NO relaxes the smooth muscle in vessel walls, which widens them and increases blood flow, lowering blood pressure. Nitroglycerine for angina and drugs like sildenafil work through this pathway. Option (b) is wrong: nitrous oxide is laughing gas, used in anaesthesia and a greenhouse gas. Option (c) is wrong: nitrogen dioxide is a toxic air pollutant from vehicles. Option (d) is wrong: nitrogen pentoxide is an industrial chemical, not made by the body.

Notice that the discovery of NO as a body signal won the 1998 Nobel Prize in Medicine. Beetroot and leafy greens supply nitrates that the body turns into NO.

Answer as per the official UPSC answer key.

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Statements 1, 3 and 4 are correct (c). Carbon nanotubes are tiny tubes of carbon atoms, like rolled sheets of graphene. They are very strong, light and good conductors. Statement 1 is right because they are studied as carriers of drugs and antigens (for vaccines) into cells. Statement 3 is right because they are used in biochemical sensors, for example to detect glucose or disease markers. Statement 4 is right because research has shown that enzymes such as myeloperoxidase, found in white blood cells, can break them down. Statement 2 is wrong in UPSC's key, as artificial capillaries made from them are not established.

Notice the concerns: some long nanotubes behave like asbestos in the lungs. Sumio Iijima described them in 1991.

Answer as per the official UPSC answer key.

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All three per UPSC's key (d), but iron's place is doubtful. Sodium ions rush into nerve and muscle cells to create the action potential that triggers contraction (3). Calcium released inside the muscle cell binds troponin and lets actin and myosin slide (1). Iron is not directly part of the contraction mechanism; it sits in myoglobin and haemoglobin, supplying the oxygen muscles need to keep working, so many argue for (c). UPSC counted iron as required. (a) and (b) fail on any reading.

Notice the others: potassium restores the resting state, and magnesium helps muscles relax; low calcium causes tetany and cramps. Anaemia from iron deficiency, common in India, causes muscle fatigue. Trap: 'required for contraction' can be read narrowly or broadly.

Answer as per the official UPSC answer key.

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Option (d) is correct. Aspartame, made of two amino acids, gives about 4 kcal per gram, the same as sugar, but it is about 200 times sweeter. So only a tiny amount is needed, and the food ends up with very few calories. (a) is wrong: it is not as sweet as sugar, and the body does digest and oxidise it. (b) is wrong: processing does not make it resistant to oxidation; in fact it breaks down with heat. (c) is wrong: its metabolites do yield calories.

Notice the debate: the WHO's cancer agency (IARC) classed aspartame as 'possibly carcinogenic' in 2023, but JECFA kept the safe daily intake at 40 mg/kg body weight. People with phenylketonuria must avoid it.

Answer as per the official UPSC answer key.

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