Mock Paper 3 — 40 MCQ

Answer: B — Mitochondrion. Mitochondria are the sites of aerobic respiration — they produce the majority of ATP via the Krebs cycle and electron transport chain. They have a double membrane; the inner membrane is folded into cristae (increasing surface area for ATP production). Ribosomes make proteins. Chloroplasts carry out photosynthesis (in plant cells). The vacuole stores water, minerals, and waste products.

Answer: C. The cell membrane is selectively (partially) permeable — it allows some molecules through more easily than others. Small, non-polar molecules (O₂, CO₂) diffuse freely. Water passes through by osmosis (via aquaporins). Glucose, amino acids, and ions require specific protein channels or active transport. Large molecules (starch, proteins) generally cannot cross. This selective permeability is essential for maintaining the cell's internal environment.

Answer: B — Shrink (crenate). In a hypertonic solution (more solute, lower water potential than cell), water moves out of the cell by osmosis down the water potential gradient. The red blood cell loses water, shrinks, and becomes crenated (spiky/star-shaped). In hypotonic solution (less solute than cell): water enters → cell swells → may burst (haemolysis in RBCs). Isotonic: no net water movement → cell unchanged. Plants: plasmolysis in hypertonic, turgidity in hypotonic.

Answer: C. Active transport moves substances AGAINST their concentration gradient (from low to high concentration) using energy (ATP) and carrier proteins (pumps). Examples: absorption of glucose and amino acids from the gut into blood; uptake of mineral ions (nitrates, phosphates) into root hair cells from the soil. Diffusion and osmosis are passive (no energy needed) and move substances down concentration gradients.

Answer: A — Salivary glands and pancreas. Salivary amylase (in mouth) begins starch digestion → maltose. Pancreatic amylase continues in the small intestine. Maltose is then broken down to glucose by maltase in the small intestine. Protease (pepsin in stomach, trypsin from pancreas) digests proteins. Lipase (from pancreas) digests lipids. The liver produces bile (no enzymes) which emulsifies fats, increasing surface area for lipase.

Answer: B. Villi adaptations for absorption: (1) Large surface area (villi + microvilli = brush border); (2) Single-cell-thick epithelium = short diffusion distance; (3) Dense capillary network — maintains concentration gradient, carries away glucose and amino acids; (4) Lacteals (lymphatic vessels) — absorb fatty acids and glycerol (reformed to triglycerides, then chylomicrons into lymph). Goblet cells secrete mucus to lubricate and protect the wall.

Answer: B — Thylakoid membranes. The light-dependent stage occurs in the thylakoid membranes (grana): light energy is absorbed by chlorophyll → excites electrons → ATP and NADPH produced → water is split (photolysis) → O₂ released. The light-independent stage (Calvin cycle) occurs in the stroma: CO₂ is fixed using ATP and NADPH → glucose produced. Chloroplast has two main regions: grana (stacks of thylakoids) and stroma (fluid between grana).

Answer: A. Aerobic respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP (energy). Glucose is completely oxidised; up to 38 ATP produced per glucose. Occurs in cytoplasm (glycolysis) and mitochondria (Krebs cycle + electron transport chain). Option B is photosynthesis. Option C is anaerobic respiration (fermentation in yeast). Aerobic produces much more ATP than anaerobic (38 vs 2 ATP per glucose).

Answer: C — Lactic acid. In animals during vigorous exercise: glucose → lactic acid + ATP (2 ATP). Lactic acid builds up in muscles, causing the burning sensation and fatigue. After exercise, extra oxygen (oxygen debt) is used to convert lactic acid back to glucose (in liver) or oxidise it to CO₂ and H₂O. In yeast and plants (anaerobic): glucose → ethanol + CO₂ + ATP. This is why yeast is used in brewing and bread-making.

Answer: B. Gas exchange in alveoli occurs by diffusion. O₂ concentration is higher in alveolar air than in deoxygenated blood → O₂ diffuses into blood. CO₂ is higher in blood → diffuses into alveoli. Adaptations: alveoli have large surface area, one-cell-thick walls, moist surface, and dense capillary network (maintains steep concentration gradient). Diffusion is entirely passive — no energy or active transport needed.

Answer: D — Red blood cells (erythrocytes). RBCs contain haemoglobin — a protein that reversibly binds O₂ (oxyhaemoglobin in lungs, releases O₂ at tissues). RBCs are biconcave discs (large surface area), small (short diffusion distance), lack nucleus (more space for Hb), and are flexible (pass through capillaries). Plasma transports CO₂ (as HCO₃⁻), nutrients, hormones, urea. WBCs fight infection. Platelets are for clotting.

Answer: C — Right atrium. Circulation path: body → vena cava → RIGHT ATRIUM → right ventricle → pulmonary artery → lungs (oxygenated) → pulmonary vein → LEFT ATRIUM → left ventricle → aorta → body. The right side pumps deoxygenated blood to lungs; the left side pumps oxygenated blood to the body. The left ventricle has thicker walls (higher pressure needed to pump blood around the whole body vs. just to the lungs).

Answer: B. Transpiration = evaporation of water from leaves (mainly through stomata). Rate increases with: higher temperature (more water evaporates), lower humidity (steeper water vapour gradient from leaf to air), higher wind speed (removes water vapour from around leaf, maintaining gradient), more light (stomata open wider). Rate decreases in dark (stomata close), high humidity, or drought conditions (stomata close via ABA hormone).

Answer: C — Glomerulus / Bowman's capsule. Ultrafiltration: high blood pressure forces small molecules (water, glucose, urea, ions, amino acids) from glomerular capillaries into Bowman's capsule → forms filtrate. Large molecules (proteins, blood cells) stay in blood. Then: selective reabsorption in proximal tubule (glucose, amino acids, most water reabsorbed). Loop of Henle: concentrates urine. Collecting duct: final water reabsorption controlled by ADH. Urine = water + urea + excess salts.

Answer: B — Insulin. After a meal: blood glucose rises → beta cells of islets of Langerhans in pancreas secrete insulin → cells (liver, muscle) take up glucose → liver converts glucose to glycogen (glycogenesis) → blood glucose returns to normal. When blood glucose falls (e.g. fasting/exercise): alpha cells secrete glucagon → liver converts glycogen back to glucose (glycogenolysis) → blood glucose rises. Diabetes: Type 1 = no insulin produced; Type 2 = cells resistant to insulin.

Answer: C — Sweating and vasodilation. Too hot: (1) Sweat glands produce sweat → evaporation of sweat removes latent heat from skin → cooling; (2) Arterioles near skin dilate (vasodilation) → more blood flows to skin surface → more heat radiated to environment. Too cold: vasoconstriction (less blood to skin surface → less heat lost); shivering (muscle contractions → generate heat); erection of hairs (traps insulating air layer — more effective in hairy animals). All controlled by hypothalamus (thermostat of body).

Answer: B. Reflex arc: stimulus → RECEPTOR → SENSORY neurone → (spinal cord) RELAY neurone → MOTOR neurone → EFFECTOR (muscle/gland) → response. Reflexes are rapid, automatic, and involuntary — they bypass the brain (though the brain is aware after). Example: touching something hot → hand pulls away before you consciously feel pain. The synapse between neurons uses neurotransmitters to transmit the impulse.

Answer: B — Chemical neurotransmitters. At a synapse: (1) Impulse arrives at pre-synaptic membrane → (2) vesicles containing neurotransmitter (e.g. acetylcholine) fuse with membrane → (3) neurotransmitter released into synaptic cleft → (4) diffuses across gap → (5) binds to receptors on post-synaptic membrane → (6) new impulse generated. Synapses ensure impulses travel in one direction only (neurotransmitter only released from pre-synaptic side). Some drugs work by mimicking or blocking neurotransmitters.

Answer: B. Nervous system: fast (milliseconds), short-lived, specific target (one effector), electrical impulse along neurones. Endocrine (hormonal) system: slower (seconds to minutes), longer-lasting, more widespread (hormone travels in blood to target organs with matching receptors). Example: adrenaline (stress hormone) affects heart rate, lung airways, liver, blood vessels simultaneously — widespread effect. Insulin affects liver, muscle, and fat tissue.

Answer: B — Four genetically different haploid cells. Meiosis: 2 divisions → 4 haploid gametes (n) that are genetically DIFFERENT (due to crossing over and independent assortment). Used in sexual reproduction (gamete formation). Mitosis: 1 division → 2 genetically IDENTICAL diploid cells (2n). Used for growth, repair, and asexual reproduction. Haploid (n) means half the number of chromosomes: humans n=23 (gametes) vs 2n=46 (body cells).

Answer: B — 25%. Cross: Tt × Tt. Using a Punnett square: TT (25%), Tt (50%), tt (25%). Genotype ratio 1:2:1; phenotype ratio 3 tall : 1 dwarf. Only tt plants are dwarf (homozygous recessive) = 25%. This is Mendel's Law of Segregation. Probability of dwarf = 1/4 = 25%. Each offspring has an independent 25% chance regardless of previous offspring (each is a separate random event).

Answer: B — Father's sperm (X or Y). Females: XX (all eggs carry X). Males: XY (sperm carry either X or Y). At fertilisation: egg (X) + X sperm → XX (female); egg (X) + Y sperm → XY (male). The sex of the offspring is determined by which sperm fertilises the egg — entirely random (50:50). The father determines sex, not the mother (all eggs carry X). This is why sex-linked conditions are inherited via the X chromosome.

Answer: C. Removing foxes (predator) → rabbits no longer preyed upon → rabbit population initially increases (less predation). However, as rabbit numbers increase, they consume more grass → grass is overgrazed → food becomes scarce → rabbits starve → population eventually decreases. This demonstrates how removing one species disrupts the ecosystem balance. Predators regulate prey populations and prevent overgrazing — they are important for ecosystem stability.

Answer: B. At each trophic level, energy is lost because: (1) Respiration — organisms use energy for movement, growth, maintaining body temperature (heat lost to environment); (2) Faeces — undigested materials not absorbed; (3) Uneaten parts — bones, fur, roots not consumed; (4) Excretion. Only about 10% of energy is transferred to the next level. This limits food chain length to ~4-5 levels. Eating plants (fewer trophic levels) is more energy-efficient than eating meat.

Answer: C — Decomposition. Decomposers (bacteria and fungi) break down dead organic matter and waste. As they respire, they release CO₂ back into the atmosphere. Other ways carbon returns to atmosphere: respiration by all living organisms; combustion of fossil fuels and wood; volcanic eruptions. Carbon is removed from atmosphere by: photosynthesis (plants); dissolved in oceans (forms carbonates); fossilisation (carbon locked in coal/oil over millions of years).

Answer: B — B-lymphocytes (plasma cells). When antigens enter the body, specific B-lymphocytes are activated → multiply → differentiate into plasma cells → secrete antibodies (specific to that antigen). Antibodies: proteins that bind to and neutralise antigens, agglutinate pathogens, or mark them for phagocytosis. Some B-cells become memory cells → rapid response if same antigen reappears (immunological memory = basis of vaccination). T-lymphocytes: destroy infected cells (cell-mediated immunity). Phagocytes engulf pathogens by phagocytosis (non-specific).

Answer: B. Vaccines contain dead/weakened pathogens, antigens, or antigen fragments. These are harmless but trigger an immune response → B-lymphocytes produce antibodies AND memory cells. If the real pathogen is later encountered, memory cells enable rapid, large-scale antibody production → pathogen destroyed before disease develops. This is active immunity (body makes its own antibodies). Passive immunity: ready-made antibodies given (e.g. antivenom) — rapid but short-lived (no memory cells). Herd immunity: enough vaccinated → disease cannot spread easily.

Answer: B. Antibiotics target structures unique to bacteria: cell walls (penicillin inhibits cell wall synthesis), bacterial ribosomes (different from eukaryotic ribosomes), or bacterial enzymes. Viruses have no cell wall, no ribosomes, no independent metabolism — they use the HOST cell's machinery to replicate. There are no equivalent structures for antibiotics to target. Antivirals (e.g. oseltamivir for flu) work differently, targeting virus-specific proteins. Overuse of antibiotics leads to antibiotic-resistant bacteria (MRSA).

Answer: C. Denaturation = irreversible change in enzyme's 3D shape. The active site is distorted so the substrate can no longer fit → enzyme inactive. Caused by: high temperature (breaks hydrogen bonds holding the tertiary structure) or extreme pH (alters charges on amino acids). At 0°C, enzyme activity is very low (low kinetic energy) but NOT denatured — warm it up and activity resumes. Denaturation is permanent. Each enzyme has an optimum temperature and pH for maximum activity.

Answer: B — Specificity. The lock-and-key model: the active site of an enzyme has a specific 3D shape, complementary ONLY to one substrate (like a key fitting only its lock). Only the correct substrate binds → enzyme-substrate complex forms → reaction occurs → products released → enzyme unchanged (can be reused). This explains enzyme specificity. The induced-fit model (more accurate) suggests the active site slightly changes shape when the substrate binds. Enzymes are biological catalysts — unchanged after the reaction.

Answer: C — Levels off (plateau). At low substrate concentration: rate increases with concentration (more enzyme-substrate collisions). At high concentration: all enzyme active sites are occupied (saturated) — adding more substrate makes no difference. Rate is now limited by enzyme concentration. To increase rate further: add more enzyme. The plateau on a rate-vs-substrate-concentration graph shows enzyme saturation. Increasing temperature or enzyme concentration shifts the plateau higher.

Answer: B. Biological detergents contain proteases (digest protein stains: blood, sweat, grass) and lipases (digest fat/oil stains: food, cooking oil). They work effectively at lower temperatures (30–40°C) — energy-saving compared to hot washes. High temperatures denature the enzymes. Other industrial enzyme uses: amylase (convert starch to sugar in food industry), isomerase (convert glucose to fructose — sweeter, used in diet drinks), pectinase (clarify fruit juices).

Answer: C. Palisade mesophyll: (1) Located at the upper surface — nearest to incoming sunlight; (2) Cells are tall, column-shaped, and tightly packed — maximises light absorption; (3) Rich in chloroplasts — main site of photosynthesis. Spongy mesophyll (below): loosely packed cells with large air spaces — for gas diffusion. Guard cells control stomata (lower surface): allow CO₂ in, O₂ and water vapour out. Waxy cuticle (upper surface): waterproof, reduces water loss.

Answer: B — Chlorosis (yellowing). Magnesium (Mg²⁺) is the central atom of the chlorophyll molecule. Without Mg²⁺, chlorophyll cannot be made → leaves turn yellow (chlorosis) → less photosynthesis. Other deficiency symptoms: Nitrate deficiency → stunted growth, yellow-green leaves (nitrogen needed for amino acids/proteins/chlorophyll). Phosphate deficiency → poor root growth, purple leaves (phosphorus for ATP and DNA). Iron deficiency → chlorosis (Fe needed for chlorophyll synthesis).

Answer: B. Auxin (IAA — indole acetic acid) is produced at the shoot tip. When light comes from one side, auxin migrates to the shaded side. Higher auxin concentration on shaded side → faster cell elongation on that side → shoot bends towards light. Roots: auxin inhibits root growth at high concentrations. So in geotropism: auxin accumulates on the lower side of a horizontal root → inhibits root growth on that side → root bends down (positive gravitropism).

Answer: B — Dissolved sugars and amino acids (translocation). Phloem: transports sucrose (from photosynthesis in leaves) and amino acids to all parts of the plant — up to growing regions, down to roots (storage). Movement can be in both directions. Xylem: transports water and dissolved mineral ions from roots upward — one direction only (transpiration stream). Xylem cells are dead, hollow, with lignified walls. Phloem cells are living (sieve tube elements + companion cells).

Answer: B. Somatic (body) cell mutations affect the organism but are NOT inherited by offspring (gametes are unaffected). The mutated cell passes the mutation to its daughter cells via mitosis — this can lead to cancer if cell division becomes uncontrolled. Germline mutations (in gametes or cells that produce gametes) CAN be inherited. Mutations are random changes in DNA sequence. They can be caused by mutagens: UV radiation, X-rays, gamma rays, certain chemicals (carcinogens), and some viruses.

Answer: B. Natural selection: (1) Variation exists within a population (due to mutation and sexual reproduction); (2) Organisms produce more offspring than can survive (overproduction); (3) Competition for resources (food, mates, territory); (4) Individuals with advantageous characteristics (better adapted) survive and reproduce more (survival of the fittest); (5) Advantageous alleles passed to offspring → become more frequent over generations → population adapts. Over long periods → new species (speciation).

Answer: B — Identical to human insulin. Genetic engineering: the human insulin gene is isolated and inserted into a plasmid → plasmid inserted into bacteria (E. coli) → bacteria multiply in fermenters → produce human insulin in large amounts. The insulin is IDENTICAL to human insulin (same DNA sequence) → no immune rejection. Advantages over animal insulin (pig/cow): no ethical concerns, no immune reactions, large-scale production, relatively cheap. Used by all Type 1 diabetics today.

Answer: B — Ammonia. Nitrogen cycle: N₂ (atmosphere) → NH₃ (ammonia) by nitrogen-fixing bacteria (Rhizobium in root nodules, or free-living Azotobacter). NH₃ → NO₂⁻ → NO₃⁻ (nitrates) by nitrifying bacteria (Nitrosomonas, Nitrobacter). Plants absorb nitrates → make amino acids and proteins. Animals eat plants. Dead organisms/waste → decomposers release NH₃ (ammonification). Denitrifying bacteria: NO₃⁻ → N₂ (back to atmosphere). Nitrates used as fertilisers can cause eutrophication if they leach into waterways.