0610/42 Oct/Nov 2025 Worked Answers (IGCSE Biology 0610 Extended)
32 questions · 80 marks · 75 minutes
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Worked answers for 28 questions
- Step 1: Physical digestion breaks food into smaller pieces, for example by the teeth and the churning of the stomach. Step 2: It increases the surface area of the food but does not change the food molecules chemically. Step 3: Breaking molecules with enzymes is chemical digestion, absorption is the uptake into blood, and egestion is the removal of faeces.Method:Define physical digestion as a size reduction with no chemical change to the molecules.Examiner tips
- Physical digestion = smaller pieces, larger surface area, no chemical change.
- Step 1: Bile emulsifies fat, breaking large drops of fat into many small droplets. Step 2: This greatly increases the surface area of the fat. Step 3: The larger surface area lets the enzyme lipase work faster, so bile aids physical digestion by preparing the fat for chemical digestion.Method:Link emulsification to increased surface area and faster enzyme action.Examiner tips
- Emulsification = big fat drops into small droplets = more surface area for lipase.
- Step 1: Incisors are the chisel-shaped front teeth that cut and bite food. Step 2: Molars are the broad back teeth that grind and chew food. Step 3: Teeth do not absorb nutrients, make saliva or store food, so only the first option pairs each tooth with its correct function.Method:Name two teeth and give the cutting or grinding function that matches each.Examiner tips
- Incisors and canines at the front cut and tear; premolars and molars at the back grind.
- Step 1: Plants are multicellular organisms. Step 2: They are producers, making their own food by photosynthesis using chlorophyll in chloroplasts, and have cellulose cell walls. Step 3: Being single-celled, feeding on others, moving around or having chitin walls describe other kingdoms, not plants.Method:Pick the two statements that match the producer, multicellular nature of plants.Examiner tips
- Plant features: multicellular, photosynthesise, chloroplasts, cellulose cell wall, non-motile.
- Step 1: The five kingdoms are animal, plant, fungus, protoctist and prokaryote. Step 2: The animal kingdom and the fungus kingdom are two of these, other than plants. Step 3: Leaves and flowers are plant parts, vertebrates and insects are groups within the animal kingdom, and viruses are not placed in a kingdom.Method:Name two of the recognised kingdoms that are not the plant kingdom.Examiner tips
- Five kingdoms: animal, plant, fungus, protoctist, prokaryote.
- Step 1: In a monocotyledon stem the vascular bundles are scattered across the whole cross-section. Step 2: In a dicotyledon stem the vascular bundles are instead arranged in a ring towards the outer edge. Step 3: They are not scattered, not all in the centre, and they are certainly present, so only the ring arrangement is correct.Method:Contrast the scattered monocot arrangement with the ring arrangement of a dicot.Examiner tips
- Dicot stem: vascular bundles in a ring; monocot stem: scattered.
- Step 1: In a dicotyledon root the xylem is found in the centre, often in a star shape. Step 2: The phloem lies between the arms of the xylem, around the central xylem. Step 3: Root hairs are extensions of the surface cells that project from the outer edge of the root, so only the first option places all three correctly.Method:Place xylem in the centre, phloem around it, and root hairs on the surface.Examiner tips
- Dicot root: central xylem, surrounding phloem, surface root hairs.
- Step 1: Xylem transports water and dissolved mineral ions from the roots up the plant. Step 2: The thick walls of xylem vessels also help to support the plant. Step 3: Transport of sucrose and amino acids is the job of phloem, and the other options describe processes xylem does not carry out.Method:Give the transport-of-water role and the support role of xylem.Examiner tips
- Xylem: transports water and mineral ions, and supports the plant.
- Step 1: The girdled trees gave 1796 g of fruit compared with 958 g for the ungirdled trees, so girdling roughly doubled the fruit mass. Step 2: Phloem normally translocates sugars (sucrose) made by the leaves away from the branch towards other parts of the plant. Step 3: Removing the phloem traps these sugars in the branch above the cut, so more sugar is available for the growing fruit, giving a greater fruit mass.Method:State the comparison of masses, then explain using translocation of sugars in phloem.Examiner tips
- Girdling removes phloem, trapping sugars above the cut so the fruit gets more.
- Step 1: Pectinase breaks down pectin in the cell walls of the fruit. Step 2: This releases more juice and makes the juice clearer, increasing the yield. Step 3: Protease digests protein, lipase digests fat and catalase breaks down hydrogen peroxide, so none of these is used to make fruit juice.Method:Choose the enzyme that digests pectin in fruit cell walls.Examiner tips
- Pectinase breaks down pectin to release and clear fruit juice.
- Step 1: A catalyst increases the rate of a chemical reaction. Step 2: It is not used up or chemically changed during the reaction, so it can be used again. Step 3: A catalyst speeds up rather than slows down a reaction, and it does not supply energy, so only the first option is correct.Method:Define a catalyst as a substance that speeds up a reaction without being changed.Examiner tips
- Catalyst: speeds up a reaction, not used up or changed.
- Step 1: Iodine turns blue-black with starch, so a faster loss of the blue-black colour means the amylase has broken down the starch faster. Step 2: At 0 °C the particles have little kinetic energy, so there are few successful collisions between amylase and starch and the reaction is very slow. Step 3: As the temperature rises towards 40 °C the particles gain kinetic energy and collide more frequently and successfully, forming more enzyme-substrate complexes and breaking the starch down faster.Method:Explain the faster colour loss using increasing kinetic energy and successful collisions up to 40 °C.Examiner tips
- Higher temperature (up to the optimum) = more kinetic energy = more successful collisions = faster reaction.
- Step 1: Enzymes are specific, meaning each enzyme has an active site that fits only its own substrate. Step 2: Protease has an active site shaped for protein, not for starch, so starch cannot bind and no enzyme-substrate complex forms. Step 3: Because the starch is not broken down, it remains present and continues to turn the iodine blue-black at every temperature.Method:Use enzyme specificity to explain why protease leaves the starch undigested.Examiner tips
- Specific enzymes only fit their own substrate; protease cannot digest starch.
- Step 1: Each enzyme has an optimum pH at which it works fastest. Step 2: Moving away from this pH slows the enzyme and can denature it, so pH affects enzyme action. Step 3: The colour of the container, the time of day and the mass of the test tube have no effect on the enzyme.Method:Name pH as a factor, besides temperature, that affects enzymes.Examiner tips
- Besides temperature, pH affects enzyme activity.
- Step 1: A food chain starts with a producer and the arrows point from food to feeder. Step 2: Grass (producer) is eaten by the grasshopper, which is eaten by the corsac fox, which is eaten by the steppe eagle. Step 3: This gives four trophic levels in the correct order, while the other chains have too few levels or the arrows the wrong way round.Method:Build the chain from grass up to the steppe eagle with four linked organisms.Examiner tips
- Food chain: producer first, arrows from food to feeder, four levels here.
- Step 1: Energy is transferred along a food chain but a large amount is lost at each trophic level, for example as heat from respiration and in waste. Step 2: This means less energy is passed on to the next level. Step 3: With less energy available, less biomass can be supported, so there are fewer organisms at each higher trophic level.Method:Link the loss of energy at each level to fewer organisms being supported.Examiner tips
- Energy lost at each level → less energy → fewer organisms higher up.
- Step 1: A small starting population contains only a little genetic variation. Step 2: Breeding among closely related individuals (inbreeding) increases the chance of harmful recessive alleles pairing up, raising the risk of inherited disease. Step 3: With little variation the species is also less able to adapt to environmental change, so the population is more vulnerable.Method:Explain the risk through reduced genetic variation, inbreeding and poorer adaptability.Examiner tips
- Small population → little genetic variation → inbreeding → inherited disease, poor adaptability.
- Step 1: Protecting the habitat and passing laws against hunting keep the horses and their environment safe. Step 2: Monitoring the population and educating local people help conservationists respond to threats and gain support. Step 3: Removing habitat, allowing hunting, adding predators or causing pollution would all harm the population, so only the first option helps maintain it.Method:Select genuine conservation measures that protect the horses and their habitat.Examiner tips
- Conservation methods: protect habitat, laws against hunting, monitoring, education, veterinary care.
- Step 1: The egg and sperm joining to form a zygote is fertilisation. Step 2: The zygote dividing repeatedly by mitosis to form an embryo is cell division. Step 3: The embryo settling into the uterus lining is implantation, so the order is fertilisation, mitosis, implantation.Method:Name each stage in the order the egg becomes a zygote, an embryo and then implants.Examiner tips
- IVF order: fertilisation → mitosis (cell division) → implantation.
- Step 1: Keeping animals close together uses less land for a given output, so more land is free for other uses. Step 2: It gives a higher yield and a more consistent quality of product, often more cheaply. Step 3: These improve food security and profits, so the first option lists real advantages, while the others describe disadvantages or false claims.Method:Choose the three statements that genuinely benefit food production.Examiner tips
- Intensive farming: less land, higher yield, cheaper and more consistent products.
- Step 1: In intensive systems many animals are kept close together. Step 2: This overcrowding lets bacterial infections spread quickly between animals. Step 3: Antibiotics are used to treat and help prevent these bacterial diseases, so the first option is correct, while antibiotics do not kill viruses or act as vitamins.Method:Link the close housing of animals to the spread of bacterial disease and the use of antibiotics.Examiner tips
- Antibiotics treat and prevent bacterial disease that spreads in crowded conditions.
- Step 1: The increase is 400 − 260 = 140 mg per kg. Step 2: Percentage increase = (increase ÷ original) × 100 = (140 ÷ 260) × 100 = 53.846…%. Step 3: Rounded to two significant figures this is 54%.Method:Calculate the increase, divide by the original 2010 value and convert to a percentage.Examiner tips
- Percentage increase = (increase ÷ original value) × 100.
- Step 1: A chance mutation in the DNA of some bacteria makes them resistant to the antibiotic. Step 2: When the antibiotic is used it kills the non-resistant bacteria, but the resistant ones survive and reproduce, passing on the resistance allele. Step 3: Over many generations the proportion of resistant bacteria increases, which is natural selection acting on the population.Method:Explain resistance as mutation followed by natural selection of surviving resistant bacteria.Examiner tips
- Mutation → antibiotic kills non-resistant → resistant survive and reproduce → natural selection.
- Step 1: The breathing rate returns to its resting value 20 minutes after exercise, and the heart rate falls by 150 − 60 = 90 beats per minute between 20 and 40 minutes. Step 2: During exercise lactic acid builds up in the muscles, creating an oxygen debt, and it is carried in the blood to the liver. Step 3: Breathing stays high to supply oxygen, which is used in aerobic respiration to break down the lactic acid, so the first option is correct throughout.Method:Read the two graph values, then complete the oxygen-debt explanation with muscles, liver, oxygen and aerobic respiration.Examiner tips
- Oxygen debt: lactic acid in muscles → carried to liver → broken down by aerobic respiration using oxygen.
- Step 1: Excess amino acids are deaminated, removing their nitrogen-containing part. Step 2: This happens in the liver, where the nitrogen is converted into urea. Step 3: The kidney removes urea from the blood, the lungs deal with carbon dioxide, and the bladder stores urine, so the liver is the organ that produces urea.Method:Recall that urea is formed in the liver during deamination.Examiner tips
- Urea is produced in the liver by deamination.
- Step 1: Urea is carried in the blood from the liver to the kidneys. Step 2: The kidneys filter the blood and remove urea, which leaves the body in urine. Step 3: The liver makes urea but does not excrete it, and the pancreas and stomach are part of digestion, so the kidney is the excretory organ for urea.Method:Recall that the kidney filters urea from the blood and excretes it in urine.Examiner tips
- The kidney excretes urea in the urine.
- Step 1: Anaerobic respiration in yeast breaks down glucose without oxygen. Step 2: The products are ethanol and carbon dioxide, giving C6H12O6 → 2C2H5OH + 2CO2. Step 3: The equation with oxygen is aerobic respiration, the lactic acid equation (C3H6O3) is anaerobic respiration in muscle, and the reversed equation is not respiration.Method:Choose the equation where glucose forms ethanol and carbon dioxide with no oxygen.Examiner tips
- Anaerobic respiration in yeast: glucose → ethanol + carbon dioxide.
- Step 1: When bread dough is made, yeast respires anaerobically. Step 2: The carbon dioxide it releases forms bubbles that make the dough rise. Step 3: Cheese and butter are dairy products and salt is a mineral, so bread is the product made using yeast.Method:Pick the food whose production relies on carbon dioxide from yeast.Examiner tips
- Yeast makes bread rise by releasing carbon dioxide.
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