Cell Biology
Structure, osmosis, diffusion, active transport, cell organisation
1
Explain why a red blood cell placed in distilled water will burst (lyse), while a plant cell in the same conditions will not. [4]
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Mark scheme [4]
- Distilled water has a higher water potential than both cells; water enters both by osmosis [1]
- Red blood cell has only a cell membrane (no cell wall); as water enters, the membrane stretches and eventually ruptures (lysis) [1]
- Plant cell has a rigid cellulose cell wall that resists further expansion [1]
- When the outward pressure of the cell wall (wall pressure) equals the inward osmotic pressure, net water entry stops — the cell becomes turgid but does not burst [1]
2
Compare diffusion and active transport. Give two similarities and two differences. [4]
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Mark scheme [4]
- Similarity: both move substances across cell membranes [1]
- Similarity: both involve carrier/channel proteins (or: both move specific particles) [1]
- Difference: diffusion moves substances DOWN a concentration gradient (passive, no energy); active transport moves substances AGAINST the concentration gradient (requires ATP) [1]
- Difference: diffusion can occur in any direction; active transport is directional and selective [1]
3
Describe the function of each of the following organelles: (i) mitochondria, (ii) ribosomes, (iii) nucleus. [3]
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Mark scheme [3]
- Mitochondria: site of aerobic respiration; produces ATP (energy) for the cell [1]
- Ribosomes: site of protein synthesis (translation of mRNA into proteins) [1]
- Nucleus: contains DNA (genetic material); controls cell activities and protein synthesis [1]
Biological Molecules & Digestion
Carbohydrates, proteins, lipids, food tests, digestion, absorption
4
Describe the chemical digestion of starch from the mouth to the small intestine, naming the enzymes and products at each stage. [4]
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- Mouth: salivary amylase (in saliva) digests starch → maltose [1]
- Stomach: no starch digestion (amylase denatured by low pH) [1]
- Small intestine: pancreatic amylase (from pancreas) digests remaining starch → maltose [1]
- Maltase (on epithelium of small intestine) digests maltose → glucose; glucose is absorbed into blood by active transport [1]
5
Describe how the small intestine is adapted for efficient absorption of digested food products. Give three adaptations. [3]
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Mark scheme — any 3 [3]
- Long length — provides large surface area and long contact time for absorption [1]
- Villi (finger-like projections) — greatly increase surface area for absorption [1]
- Microvilli on epithelial cells — further increase surface area [1]
- Rich capillary network in each villus — maintains steep concentration gradient; carries away absorbed nutrients quickly [1]
- Lacteals (lymph vessels) in villi — absorb fatty acids and glycerol (as chylomicrons) [1]
- Thin epithelium — short diffusion distance [1]
Enzymes
Lock and key, temperature, pH, enzyme uses, inhibitors
6
Explain why each enzyme catalyses only one specific reaction. Use the terms "active site" and "substrate" in your answer. [3]
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- Each enzyme has a uniquely shaped active site [1]
- Only a specific substrate with a complementary shape can fit into the active site (lock and key model) [1]
- No enzyme-substrate complex can form with any other substrate → no catalysis [1]
7
Pepsin (pH optimum ~2) and trypsin (pH optimum ~8) are both proteases. Predict what would happen if you placed pepsin in the small intestine (pH ~8) and explain why. [3]
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- Pepsin activity would be very low / minimal at pH 8 [1]
- pH 8 is far from pepsin's optimum (pH 2); the changed pH alters the ionic charges on amino acids in the active site [1]
- The active site shape changes; the protein substrate no longer fits → little or no enzyme-substrate complex forms → minimal catalysis [1]
⚠ Trap: pH 8 does not necessarily denature pepsin permanently — it reduces activity. Say "alters active site shape" not "destroys" unless the pH is extreme.
Plant Nutrition
Photosynthesis, limiting factors, mineral nutrition, leaf structure
8
A grower increases the CO₂ concentration in a greenhouse but finds the rate of photosynthesis stops increasing at a certain point. Explain what is likely limiting the rate and how the grower could increase it further. [4]
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- When CO₂ is no longer limiting, another factor is now the limiting factor [1]
- The limiting factor is likely light intensity or temperature [1]
- To increase rate further: increase light intensity — provides more energy for the light-dependent reactions [1]
- Increase temperature (up to the optimum ~25–35°C) — increases enzyme activity in the light-independent reactions (Calvin cycle) [1]
9
A plant shows yellowing of young leaves, stunted shoot growth, and pale green older leaves. Suggest the mineral deficiency and explain the role of the missing mineral. [3]
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- Mineral deficiency: nitrogen (nitrate ions) [1]
- Nitrogen is needed to make amino acids → proteins (structural and enzymatic) [1]
- Without sufficient nitrogen, the plant cannot synthesise enough protein for growth → stunted; cannot make enough chlorophyll → chlorosis (yellowing) [1]
Also accept magnesium for yellowing alone — but nitrate fits all symptoms (stunting + yellowing).
Transport in Humans
Blood vessels, heart, blood components, coronary heart disease
10
Describe what happens at the capillaries to allow exchange of materials between the blood and tissue cells. [4]
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Mark scheme [4]
- Capillaries have walls only one cell thick — very short diffusion distance [1]
- Oxygen and glucose diffuse FROM blood into tissue cells (down concentration gradient) [1]
- Carbon dioxide and waste products (e.g. urea) diffuse FROM tissue cells INTO blood [1]
- Capillaries form dense networks to ensure all cells are close to a blood supply; large total surface area for exchange [1]
Homeostasis & Coordination
Blood glucose, temperature regulation, kidney, nervous system, hormones
11
On a hot day, a person begins to sweat. Explain how sweating and vasodilation help maintain body temperature. [4]
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Mark scheme [4]
- Sweating: sweat glands secrete sweat onto skin surface; water in sweat evaporates [1]
- Evaporation requires latent heat of vaporisation — this heat energy is taken from the skin, cooling the body [1]
- Vasodilation: arterioles near the skin surface dilate (widen) [1]
- More blood flows near the skin surface → more heat is lost by radiation / convection to the environment [1]
⚠ Trap: Do NOT say "blood vessels move to the skin" — they dilate. Vasodilation means widening, not movement.
12
Explain how the kidneys regulate the water content of the blood when a person has drunk a large volume of water. Name the hormone involved. [4]
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- Blood water content rises → detected by osmoreceptors in the hypothalamus [1]
- Less ADH (antidiuretic hormone) is released by the pituitary gland [1]
- Kidney tubule walls become less permeable to water → less water is reabsorbed into the blood [1]
- A large volume of dilute (pale) urine is produced → blood water content returns to normal (negative feedback) [1]
Genetics & Inheritance
Monohybrid crosses, sex linkage, mutations, meiosis vs mitosis
13
Cystic fibrosis (CF) is caused by a recessive allele (f). Two carriers have children. Show the cross and calculate the probability that a child has cystic fibrosis. [4]
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- Parent genotypes: Ff × Ff (both carriers = heterozygous) [1]
- Gametes: F or f from each parent [1]
- Punnett square: FF, Ff, Ff, ff [1]
- Probability of CF (ff): 1 in 4 = 25% [1]
14
State three ways in which meiosis differs from mitosis. [3]
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Mark scheme — any 3 [3]
- Meiosis produces 4 cells; mitosis produces 2 cells [1]
- Meiosis produces haploid (n) cells; mitosis produces diploid (2n) cells [1]
- Meiosis produces genetically different (variable) cells; mitosis produces genetically identical cells [1]
- Meiosis involves two divisions; mitosis involves one [1]
- Meiosis occurs in gonads (testes/ovaries) to produce gametes; mitosis occurs in body cells for growth/repair [1]
Ecology & Health
Food webs, nutrient cycles, population, disease, immunity, evolution
15
Explain how the overuse of antibiotics in medicine and agriculture leads to the development of antibiotic-resistant bacteria. [4]
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- Random mutations occur in bacterial populations, occasionally producing bacteria with resistance to an antibiotic [1]
- When antibiotics are used, non-resistant bacteria are killed; resistant bacteria survive [1]
- Resistant bacteria reproduce rapidly and pass on resistance genes to offspring (natural selection) [1]
- Over many generations, the proportion of resistant bacteria increases until the antibiotic is no longer effective against that population [1]
16
Describe the roles of bacteria in the nitrogen cycle. Include nitrification, denitrification, and nitrogen fixation. [4]
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- Nitrogen fixation: nitrogen-fixing bacteria (e.g. Rhizobium in root nodules) convert atmospheric N₂ into ammonia (NH₃) / ammonium ions — making nitrogen available to plants [1]
- Nitrification: nitrifying bacteria convert ammonia → nitrites → nitrates (NO₃⁻) in soil — making nitrogen accessible to plant roots [1]
- Denitrification: denitrifying bacteria convert nitrates → N₂ gas — returning nitrogen to the atmosphere; reduces soil fertility [1]
- Decomposers (bacteria and fungi) break down dead organisms / protein → ammonia, re-entering the nitrogen cycle [1]
17
Explain the principle of herd immunity. Why is it important for a high percentage of the population to be vaccinated? [3]
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- Herd immunity: when a sufficient proportion of a population is immune to a disease, transmission is interrupted even for unvaccinated individuals [1]
- Infected individuals are unlikely to encounter susceptible (unvaccinated) people → chains of transmission are broken [1]
- This protects vulnerable people who cannot be vaccinated (e.g. newborns, immunocompromised individuals) — so high vaccination coverage is essential to protect the whole community [1]