0610/21 Oct/Nov 2025 Worked Answers (IGCSE Biology 0610 Extended)
40 questions · 40 marks · 45 minutes
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Worked answers for 31 questions
- Step 1: Organisms that are closely related have more similar base sequences in their DNA. Step 2: Comparing the sequence of bases in DNA shows how alike two species are and so is used to classify them. Step 3: All organisms use the same four types of base, and proteins are made of amino acids not bases, so those options cannot be used.Method:Recall that DNA base sequences are compared to classify organisms by relatedness.Examiner tips
- Use the order of the DNA bases, not just how many there are.
- Step 1: Magnification = image size divided by actual size, using the same units. Step 2: Convert 5 cm to micrometres: 5 cm = 50 000 micrometres, and the actual length is 5 micrometres. Step 3: Magnification = 50 000 divided by 5 = 10 000.Method:Convert 5 cm to 50 000 micrometres and divide by the actual 5 micrometres.Examiner tips
- Always convert to the same unit before dividing; 1 cm = 10 000 micrometres.
- Step 1: Active transport moves substances from a lower to a higher concentration, that is against the concentration gradient, using energy from respiration. Step 2: The substances are moved across the cell membrane, which is a partially permeable membrane. Step 3: So active transport is against the gradient and does require a partially permeable membrane.Method:Recall that active transport is uphill movement across the cell membrane using energy.Examiner tips
- Active transport: against the gradient, across the membrane, using energy.
- Step 1: The spongy mesophyll lies in the lower part of the leaf near the stomata, so statement 1 is correct. Step 2: Palisade cells are tall and closely packed while the spongy mesophyll has air spaces, so statement 3 is correct. Step 3: Both palisade and spongy mesophyll cells contain chloroplasts, so statement 2 is wrong, leaving 1 and 3 only.Method:Test each statement; the only false one claims that only palisade cells have chloroplasts.Examiner tips
- Both mesophyll layers have chloroplasts; the palisade layer simply has more.
- Step 1: Bile is made in the liver and contains no enzymes. Step 2: Bile emulsifies fats, breaking large fat droplets into many small ones and so increasing their surface area for lipase to act on. Step 3: Bile is alkaline and neutralises stomach acid to give the optimum pH for enzymes in the small intestine, not for pepsin in the acidic stomach.Method:Recall that bile physically breaks up fats rather than chemically digesting them.Examiner tips
- Bile emulsifies fats and neutralises acid; it is not an enzyme.
- Step 1: Trypsin is a protease made in the pancreas and released into the duodenum. Step 2: As a protease, trypsin breaks down proteins, eventually giving amino acids. Step 3: So trypsin acts in the duodenum and produces amino acids; fatty acids come from lipase, and proteins are the substrate, not a product.Method:Identify trypsin as a pancreatic protease acting in the duodenum to give amino acids.Examiner tips
- Trypsin = pancreatic protease acting in the duodenum, giving amino acids.
- Step 1: Xylem vessels are formed from dead cells joined end to end to make continuous hollow tubes. Step 2: The end walls break down so there are no cross walls, letting water flow freely. Step 3: Being dead and empty they have no mitochondria, and their walls are thickened with lignin, so they are not thin.Method:Recall the structure of a xylem vessel as a continuous tube of dead cells.Examiner tips
- Xylem = dead, hollow, lignified tubes with no cross walls.
- Step 1: A source is a part that makes or releases sucrose, and a sink is a part that uses or stores it. Step 2: Leaves make sucrose by photosynthesis, so they are sources. Step 3: Growing flowers and developing fruits use and store sucrose, so they are sinks.Method:Label the photosynthesising leaves as sources and the growing fruits and flowers as sinks.Examiner tips
- Leaves are sources; growing or storing parts such as fruits and flowers are sinks.
- Step 1: Clotting seals wounds to keep pathogens out, and platelets help blood to clot, so statement 3 is correct. Step 2: In clotting, fibrinogen is converted to fibrin (not the reverse), so statement 1 is wrong. Step 3: Lymphocytes make antibodies and phagocytes engulf pathogens, so statements 2 and 4 swap the cells over and are wrong, leaving 3 only.Method:Eliminate the statements that reverse clotting or swap the white blood cell roles.Examiner tips
- Fibrinogen turns into fibrin; lymphocytes make antibodies; phagocytes engulf.
- Step 1: In a double circulation the blood travels through the heart twice on each full circuit of the body. Step 2: It passes once on the way to the lungs (pulmonary circuit) and once on the way to the rest of the body (systemic circuit). Step 3: The blood passes through the lungs, kidneys or brain only once per circuit, so only the heart statement is correct.Method:Recall that double circulation means two passes through the heart per complete circuit.Examiner tips
- Double circulation = blood through the heart twice (lungs once, body once).
- Step 1: Each pathogen carries its own specific antigens, so statement 2 is correct. Step 2: Lymphocytes make antibodies with a shape that fits one specific antigen, so statement 1 is correct. Step 3: A vaccine contains weakened or harmless antigens that trigger an immune response, so statement 3 is correct, making all three correct.Method:Check each statement; all three correctly describe antigens, antibodies and vaccination.Examiner tips
- Specific antigen, specific antibody; vaccines present antigens to the body.
- Step 1: During inspiration the diaphragm contracts and flattens, increasing the volume of the chest. Step 2: The external intercostal muscles contract to pull the ribs up and out, also increasing the chest volume. Step 3: The internal intercostal muscles relax during inspiration, so only the external intercostals and the diaphragm contract.Method:Recall which muscles increase chest volume during breathing in.Examiner tips
- Breathing in: diaphragm and external intercostals contract; internal intercostals relax.
- Step 1: Respiration releases energy that cells use for energy-requiring processes. Step 2: Building proteins from amino acids is an energy-requiring process, so protein synthesis needs energy from respiration. Step 3: Diffusion, osmosis and transpiration are passive and driven by concentration or water-potential gradients, so they do not need energy from respiration.Method:Pick the process that builds molecules, which is the one needing energy.Examiner tips
- Building molecules (like proteins) costs energy; passive movements do not.
- Step 1: Anaerobic respiration in human muscle breaks glucose down without oxygen. Step 2: The only product is lactic acid; no carbon dioxide or water is formed. Step 3: Carbon dioxide and water are products of aerobic respiration, not of anaerobic respiration in muscle.Method:Recall the single product of anaerobic respiration in human muscle.Examiner tips
- Anaerobic respiration in muscle gives lactic acid only.
- Step 1: In a healthy person all the glucose filtered into the nephron is reabsorbed, so the urine leaving the nephron contains no glucose, making statement 2 correct. Step 2: Glucose is filtered out at the glomerulus, so blood leaving has less glucose than blood entering, making statement 1 wrong. Step 3: The nephron reabsorbs most but not all of the water (some leaves as urine), so statement 3 is wrong, leaving 2 only.Method:Use the facts that glucose is fully reabsorbed and water is only partly reabsorbed.Examiner tips
- All glucose is reabsorbed; not all water is, since urine is watery.
- Step 1: In bright light the pupil must get smaller to protect the retina. Step 2: The circular muscles of the iris contract to make the pupil smaller. Step 3: At the same time the radial muscles relax, allowing the pupil to narrow, so circular contract and radial relax.Method:Work out that the pupil narrows, so the circular muscles contract and the radial muscles relax.Examiner tips
- Bright light: circular muscles contract, radial muscles relax, pupil small.
- Step 1: Offspring that are all identical to the parent are needed so they all make large, sweet fruits. Step 2: Asexual reproduction produces genetically identical clones with no genetic variation, so it gives offspring like the parent. Step 3: Seeds are made by sexual reproduction, and sexual reproduction introduces variation, so it would not guarantee identical fruits.Method:Choose asexual reproduction because it produces clones identical to the desired parent.Examiner tips
- Asexual reproduction makes identical clones; sexual reproduction creates variation.
- Step 1: Fertilisation is the fusion of a male gamete nucleus with a female gamete (egg cell) nucleus. Step 2: The female gamete is inside the ovule, so the male nucleus travels down the pollen tube to reach it there. Step 3: The pollen grain and pollen tube only carry the male nucleus, and the stigma is where pollen lands, so fertilisation happens in the ovule.Method:Recall that the egg cell is in the ovule, so fertilisation occurs there.Examiner tips
- Pollination is on the stigma; fertilisation is inside the ovule.
- Step 1: The uterus lining is shed during menstruation around days 1 to 5, so it is thin early in the cycle. Step 2: After ovulation at about day 14 the lining continues to thicken, ready for a possible pregnancy. Step 3: By about day 21 the lining is at its thickest, so day 21 is the answer.Method:Track the lining across the cycle: thin during the period, thickest after ovulation.Examiner tips
- Lining: shed at the start, rebuilt to thickest after ovulation (around day 21).
- Step 1: A female is XX, so every egg cell carries an X chromosome, making statement 2 correct. Step 2: A male is XY, so half the sperm carry an X and half carry a Y, making statement 3 correct. Step 3: Eggs never carry a Y, and sperm are not all Y, so statements 1 and 4 are wrong, leaving 2 and 3.Method:Use XX for females and XY for males to decide what each gamete can carry.Examiner tips
- Eggs are always X; sperm are half X and half Y.
- Step 1: Meiosis is a reduction division that halves the chromosome number to make gametes. Step 2: It starts with a diploid cell (the full chromosome number) and produces haploid daughter cells (half the number). Step 3: So a diploid parent cell produces haploid daughter cells.Method:Recall that meiosis is the reduction division making haploid gametes from a diploid cell.Examiner tips
- Meiosis: diploid to haploid, halving the chromosome number.
- Step 1: A colour-blind female carries the colour-blindness allele on both of her X chromosomes. Step 2: A son receives his single X chromosome from his mother and his Y chromosome from his father, so his X must carry the colour-blindness allele. Step 3: With no second X to mask it, the recessive allele is expressed, so the son is certain (100%) to be colour blind.Method:Track the son X from the colour-blind mother; he must inherit and express the allele.Examiner tips
- A son inherits his X from his mother; a colour-blind mother passes the allele to every son.
- Step 1: A change in the base sequence of DNA is the definition of a gene mutation. Step 2: So a gene mutation is the source of variation that alters the DNA base sequence. Step 3: Mitosis copies DNA unchanged, while random fertilisation and random mating shuffle existing alleles without changing the base sequence.Method:Pick the process defined as a change in the DNA base sequence.Examiner tips
- A gene mutation is a change in the DNA base sequence.
- Step 1: Seed banks store the seeds of plants, so they conserve plant species, not animals. Step 2: Artificial insemination, captive breeding programmes and in vitro fertilisation are all used to increase numbers of endangered animals. Step 3: So the technique not used for animals is seed banks.Method:Find the method that stores plant material rather than helping animals breed.Examiner tips
- Seed banks conserve plants; the other methods are animal breeding techniques.
- Step 1: Sunlight is first absorbed (trapped) by chlorophyll in the plant. Step 2: Photosynthesis then converts this into chemical energy held in carbohydrates such as glucose. Step 3: These carbohydrates are then stored in the plant, giving the order: trapped by chlorophyll, chemical energy in carbohydrates, stored in plants.Method:Start with light capture by chlorophyll and follow the energy into stored carbohydrates.Examiner tips
- Order: chlorophyll traps light, then chemical energy in carbohydrates, then stored.
- Step 1: Trophic level 1 is the producer and trophic level 2 is the primary consumer (herbivore), so energy passes from a producer to a herbivore, not herbivore to carnivore, making statement 1 wrong. Step 2: Trophic level 2 to 3 is primary consumer to secondary consumer, so statement 2 is correct. Step 3: Trophic level 3 to 4 is secondary consumer to tertiary consumer, so statement 3 is correct, leaving 2 and 3.Method:Number the trophic levels correctly, starting with the producer at level 1.Examiner tips
- Level 1 = producer, level 2 = primary consumer, level 3 = secondary consumer.
- Step 1: Excess amino acids cannot be stored, so the body removes their nitrogen-containing part. Step 2: This removal of the nitrogen-containing part of amino acids, which happens in the liver, is called deamination. Step 3: Decomposition and denitrification are carried out by microorganisms in the soil, and digestion breaks down food in the gut, so they do not fit.Method:Pick the process defined as removing the nitrogen part of amino acids.Examiner tips
- Deamination removes the nitrogen-containing part of amino acids in the liver.
- Step 1: Extra nutrients cause rapid growth of algae and plants, which then die in large numbers. Step 2: Decomposers feed on the dead material and multiply, increasing their aerobic respiration. Step 3: This increased respiration by decomposers uses up the dissolved oxygen, so oxygen levels in the water fall.Method:Trace eutrophication to decomposers respiring more and removing dissolved oxygen.Examiner tips
- Dead algae feed decomposers, whose respiration removes the oxygen.
- Step 1: Chemical fertilisers add mineral ions such as nitrate that help crops grow, so they do increase crop yields. Step 2: Fertilisers do not kill weeds, so they do not reduce the use of herbicides and do not reduce competition between crops and weeds. Step 3: So only increasing crop yields is a reason, giving no, yes, no.Method:Recall that fertilisers supply nutrients to raise yields but do nothing to weeds.Examiner tips
- Fertilisers raise yields by feeding crops; weed control is done by herbicides.
- Step 1: Pectinase breaks down pectin, the substance that holds plant cell walls together in fruit. Step 2: Breaking down pectin releases more juice from the fruit pulp, so more juice can be extracted. Step 3: Pectinase also makes the juice clearer, not cloudier, and it does not digest proteins or starch.Method:Recall that pectinase digests pectin so more juice can be extracted.Examiner tips
- Pectinase digests pectin, releasing more and clearer juice.
- Step 1: DNA ligase is the enzyme that joins pieces of DNA together. Step 2: Joining happens when the insulin gene is sealed into the cut plasmid, so this is the stage that uses DNA ligase. Step 3: Identifying and cutting out DNA use restriction enzymes, and inserting the plasmid into bacteria does not need ligase.Method:Find the joining step, where the gene is sealed into the plasmid, because that needs ligase.Examiner tips
- Restriction enzymes cut DNA; DNA ligase joins it.
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