0610/43 May/June 2025 Worked Answers (IGCSE Biology 0610 Extended)
35 questions · 80 marks · 75 minutes
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Worked answers for 30 questions
- Step 1: Chemical digestion uses enzymes to break large molecules into small molecules. Step 2: Large molecules are often insoluble, so breaking them into small, soluble molecules is essential. Step 3: Only these small, soluble molecules can be absorbed through the wall of the small intestine into the blood, so the first option is correct.Method:Link enzymes breaking down large insoluble molecules to small soluble ones that can be absorbed.Examiner tips
- Chemical digestion: large insoluble molecules to small soluble molecules for absorption.
- Step 1: Maltase is the enzyme that digests the sugar maltose. Step 2: Maltose is a disaccharide made of two glucose units joined together. Step 3: Maltase breaks maltose into glucose, so the product is glucose; starch, amino acids and fatty acids are not products of this reaction.Method:Recall that maltase breaks maltose into its glucose units.Examiner tips
- Maltase digests maltose into glucose.
- Step 1: Maltase is bound to the membranes of the epithelial cells that line the small intestine, including those covering the villi. Step 2: Maltose is therefore digested as it comes into contact with these cell membranes. Step 3: It is not digested in the stomach, inside red blood cells or in the bile, so the first option is correct.Method:Locate maltase on the epithelial cell membranes lining the small intestine.Examiner tips
- Maltase is on the membranes of the epithelial cells lining the small intestine.
- Step 1: The stomach produces acidic gastric juice (hydrochloric acid) giving a very low pH that denatures amylase, changing the shape of its active site so that starch can no longer fit and form enzyme-substrate complexes. Step 2: In the small intestine, bile is released which is alkaline and neutralises the acidic mixture from the stomach. Step 3: This raises the pH to a suitable level for amylase, so amylase can digest starch in the small intestine but not in the acidic stomach.Method:Contrast the acidic stomach denaturing amylase with bile neutralising acid in the small intestine.Examiner tips
- Stomach acid denatures amylase; bile neutralises the acid so amylase works in the small intestine.
- Step 1: A large surface area provides plenty of space for gases to diffuse across. Step 2: A thin wall, often only one cell thick, gives a short diffusion distance for oxygen and carbon dioxide. Step 3: A good blood supply from a network of capillaries keeps a steep concentration gradient, so the first option lists three correct features.Method:Select the three features that increase the rate of diffusion across the alveoli.Examiner tips
- Alveoli: large surface area, thin walls, good blood supply, good ventilation.
- Step 1: During inspiration the diaphragm contracts and flattens downwards. Step 2: At the same time the external intercostal muscles contract to pull the ribs up and out. Step 3: The internal intercostal muscles are used in forced breathing out, so during inspiration it is the external intercostals and the diaphragm that contract.Method:Recall which muscles contract to enlarge the chest when breathing in.Examiner tips
- Inspiration: external intercostal muscles and diaphragm contract.
- Step 1: The cartilage rings are firm and form a support in the wall of the trachea and bronchi. Step 2: This support holds the airway open so air can pass freely, even when pressure inside falls. Step 3: Mucus is made by goblet cells, gas exchange happens in the alveoli, and breathing muscles move air, so keeping the airway open is the role of cartilage.Method:Recall that firm cartilage rings hold the airways open during breathing.Examiner tips
- Cartilage keeps the airway open so it does not collapse.
- Step 1: Aerobic respiration uses glucose (C₆H₁₂O₆) and oxygen (O₂) as the reactants. Step 2: The products are carbon dioxide (CO₂) and water (H₂O). Step 3: Balancing the atoms gives C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O, with six oxygen molecules, six carbon dioxide and six water molecules.Method:Write glucose plus oxygen to carbon dioxide and water, then balance with sixes.Examiner tips
- Aerobic respiration: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O.
- Step 1: Physical activity increases respiration in the muscles, raising the carbon dioxide concentration in the blood. Step 2: The brain detects this high carbon dioxide and sends more nerve impulses to the muscles used in breathing. Step 3: These muscles contract more strongly and more often, and adrenaline may also be released, so the rate and depth of breathing increase.Method:Trace the chain from activity to high carbon dioxide, brain detection, and faster deeper breathing.Examiner tips
- Activity raises blood CO₂; the brain detects it and increases the rate and depth of breathing.
- Step 1: When blood glucose rises, the pancreas detects this and releases the hormone insulin. Step 2: Insulin causes the liver to remove glucose from the blood and convert it into glycogen for storage. Step 3: Because the rise in glucose triggers a response that lowers it back to normal, this is an example of negative feedback (homeostasis).Method:Identify the gland, the storage organ, the storage molecule and the type of control in turn.Examiner tips
- Pancreas releases insulin, liver stores glucose as glycogen, controlled by negative feedback.
- Step 1: In type 1 diabetes the pancreas does not make enough insulin, so insulin must be injected to control blood glucose. Step 2: The person also monitors their blood glucose concentration regularly to keep it within safe limits. Step 3: Counting carbohydrates and matching food to activity also help, but antibiotics, extra sugar and painkillers do not treat diabetes, so the first pair is correct.Method:Choose the treatments that replace the missing insulin and track blood glucose.Examiner tips
- Type 1 diabetes: inject insulin and monitor blood glucose.
- Step 1: Adrenaline causes stored glycogen to be broken down into glucose, raising the blood glucose concentration. Step 2: This extra glucose is delivered to the muscles, where it is used in respiration to release energy. Step 3: The energy released is used for muscle contraction, allowing the body to fight or run away quickly, so the first option is the advantage.Method:Link raised glucose to more respiration and more energy for the muscles in fight or flight.Examiner tips
- Adrenaline breaks glycogen to glucose, raising blood glucose for extra respiration and energy.
- Step 1: The large air spaces allow gases such as carbon dioxide and oxygen to diffuse through the leaf for photosynthesis and respiration. Step 2: They also make the leaf less dense, providing buoyancy so it floats on the water surface and receives plenty of light. Step 3: Starch storage, water transport, trapping insects and making pigment are not functions of the air spaces, so the first option is correct.Method:Pick the two roles linked to gas movement and floating on the water surface.Examiner tips
- Leaf air spaces allow gas diffusion and give buoyancy so the leaf floats.
- Step 1: The green pigment is chlorophyll, found in the chloroplasts of the palisade mesophyll. Step 2: Chlorophyll absorbs (traps) light energy from sunlight. Step 3: This light energy is transferred into chemical energy that is used to synthesise carbohydrates in photosynthesis, so the first option is correct.Method:Link the green pigment to absorbing light and converting it to chemical energy in photosynthesis.Examiner tips
- Chlorophyll traps light energy and transfers it to chemical energy for making carbohydrates.
- Step 1: A thick waxy cuticle on a land plant reduces water loss, but a water lily is surrounded by water and does not need to prevent water loss. Step 2: A thin cuticle therefore allows carbon dioxide to enter the leaf more easily and lets more light reach the palisade mesophyll. Step 3: This makes photosynthesis more efficient and saves the plant resources, so the first option explains the adaptation.Method:Explain that water is abundant, so a thin cuticle improves carbon dioxide and light entry.Examiner tips
- Floating leaves do not need a thick cuticle, so a thin cuticle aids carbon dioxide and light entry.
- Step 1: There are 100 mm² in 1 cm², so to change from per mm² to per cm² you multiply by 100. Step 2: 573 × 100 = 57 300. Step 3: So the stomatal density is 57 300 stomata per cm².Method:Multiply the per-mm² density by 100 to convert to per cm².Examiner tips
- Per mm² to per cm²: multiply by 100.
- Step 1: Pondweed is fully submerged, so oxygen and carbon dioxide diffuse straight into the leaf from the surrounding water and no stomata are needed. Step 2: A water lily floats, so only its upper surface is in contact with the air, and that is where its stomata are, allowing gas exchange with the air. Step 3: The lower surface rests on the water, so stomata there would be useless, which is why the water lily has none on its lower surface.Method:Relate stomatal position to which leaf surface is in contact with air rather than water.Examiner tips
- Submerged leaves need no stomata; floating leaves keep stomata on the upper, air-facing surface.
- Step 1: Monocotyledonous plants typically have long, narrow leaves. Step 2: The veins in these leaves run parallel to one another rather than forming a network. Step 3: Broad leaves with branching veins and flower parts in fours and fives are features of dicotyledons, so the strap-shaped leaves with parallel veins identify rice as a monocotyledon.Method:Pick the leaf feature, parallel veins, that distinguishes monocotyledons.Examiner tips
- Monocotyledons have narrow leaves with parallel veins.
- Step 1: Plant roots take in nitrate ions by active transport, which needs energy from aerobic respiration. Step 2: Low oxygen in waterlogged soil reduces aerobic respiration, so less energy is released for active transport and fewer nitrate ions are absorbed. Step 3: Nitrate ions are needed to make amino acids, so with fewer nitrate ions the plant makes fewer amino acids and less protein.Method:Trace the chain from low oxygen to reduced active transport of nitrate and reduced protein synthesis.Examiner tips
- Less oxygen → less respiration → less energy for active transport of nitrate → less protein.
- Step 1: Antibiotics are medicines that act against bacteria. Step 2: They kill bacteria or stop them reproducing, which is why they treat bacterial infections. Step 3: Antibiotics do not work against viruses, and they are not used to kill mammals or insects, so bacteria is the correct answer.Method:Recall that antibiotics act against bacteria, not viruses.Examiner tips
- Antibiotics kill bacteria but have no effect on viruses.
- Step 1: Using antibiotics only when they are truly necessary, such as for serious bacterial infections, reduces how often bacteria are exposed to them. Step 2: Completing the full course ensures all the bacteria are killed, leaving none to develop resistance. Step 3: Overusing antibiotics, stopping early, sharing them or using them for viral illnesses all increase resistance, so the first pair is correct.Method:Choose the responsible-use practices that reduce bacterial exposure to antibiotics.Examiner tips
- Limit resistance: only use antibiotics when needed and complete the course.
- Step 1: In the binomial system every organism has a two-part scientific name. Step 2: The first word, written with a capital letter, is the genus. Step 3: The second word is the species, so in Penicillium chrysogenum, Penicillium is the genus.Method:Recall that the first word of a scientific name is the genus.Examiner tips
- Binomial name: first word = genus, second word = species.
- Step 1: Thread-like structures called hyphae are characteristic of fungi. Step 2: Cell walls made of chitin, rather than cellulose, are also a feature of fungi. Step 3: Together with reproduction by spores, these features show the organism belongs to the fungi kingdom, not the animals, plants or bacteria.Method:Match the hyphae and chitin cell wall features to the fungi kingdom.Examiner tips
- Hyphae + chitin cell walls + spores = fungi.
- Step 1: Fermenters are used to grow microorganisms that make useful products on a large scale. Step 2: Insulin can be made by genetically modified bacteria grown in a fermenter, as can mycoprotein and ethanol. Step 3: Haemoglobin, starch and cellulose are made inside animals or plants, not produced in a fermenter, so insulin is the correct answer.Method:Pick the product that microorganisms can make on a large scale in a fermenter.Examiner tips
- Fermenters make products such as insulin, mycoprotein and ethanol.
- Step 1: The fungus that produces penicillin respires aerobically and needs a supply of oxygen. Step 2: Oxygen is therefore bubbled into the fermenter to keep the fungus respiring and growing. Step 3: Carbon dioxide is a waste product, while nitrogen and hydrogen are not used for respiration, so the gas needed is oxygen.Method:Identify the gas needed for aerobic respiration of the fungus, which is oxygen.Examiner tips
- Aerobic respiration in the fermenter needs a supply of oxygen.
- Step 1: The temperature is kept at a suitable value using a water jacket so the enzymes of the fungus work efficiently without being denatured. Step 2: The pH is kept suitable with a buffer, and nutrients are supplied for the fungus to grow and make penicillin. Step 3: A stirrer keeps the temperature, nutrients and oxygen evenly distributed, and the fermenter is kept sterile, so the first option correctly describes the controlled conditions.Method:List the controlled conditions (temperature, pH, nutrients, mixing, sterility) and how each is maintained.Examiner tips
- Fermenter conditions: suitable temperature (water jacket), suitable pH (buffer), nutrients, stirring, sterile.
- Step 1: Yeast is a single-celled fungus that respires anaerobically to produce carbon dioxide and ethanol. Step 2: In bread-making the carbon dioxide makes the dough rise. Step 3: In biofuel production the ethanol made by yeast is collected and used as fuel, so the organism is yeast.Method:Recall the fungus that ferments sugars to carbon dioxide and ethanol.Examiner tips
- Yeast is used in bread-making and to produce ethanol biofuel.
- Step 1: Forests are often cleared to make space for agriculture, such as growing crops or grazing animals. Step 2: Trees are also logged (cut down) for timber and for fuel, reducing the forest. Step 3: Planting forests, protecting trees, more rainfall and fewer people would not cause forest loss, so the first pair gives the correct causes.Method:Pick the two human activities that remove trees: clearing for farming and logging.Examiner tips
- Deforestation causes include clearing for agriculture and logging for timber and fuel.
- Step 1: Trees remove carbon dioxide from the atmosphere by photosynthesis, so cutting them down means less carbon dioxide is removed. Step 2: Burning the felled trees or letting them decompose adds even more carbon dioxide to the atmosphere. Step 3: This extra carbon dioxide traps more infrared (heat) radiation from the ground, increasing the greenhouse effect and raising global temperatures.Method:Link reduced photosynthesis and burning of trees to higher carbon dioxide and a stronger greenhouse effect.Examiner tips
- Fewer trees + burning them = more carbon dioxide = stronger greenhouse effect = warmer.
- Step 1: Removing forest destroys the habitats of many organisms, which can lead to the extinction of species and a reduction in biodiversity. Step 2: Without tree roots to hold the soil, soil erosion increases, which can cause flooding. Step 3: Increasing habitats, richer soil, cooler temperatures or larger animal populations are not effects of deforestation, so the first option is correct.Method:Select the genuine harmful effects on wildlife and soil that follow from removing forest.Examiner tips
- Deforestation causes habitat loss, extinction, reduced biodiversity, soil erosion and flooding.
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