IGCSE Biology 0610/21 October/November 2025 Past Paper with Answers

Question 1

A young plant in a pot is left near a window so that light reaches it from one side only. At the start the plant is short with a straight stem. Two weeks later the plant has become taller with more leaves, and its stem has bent so the shoot grows towards the light. Which two characteristics of living organisms are demonstrated by these changes?

1. growth and sensitivity
2. excretion and growth
3. excretion and reproduction
4. sensitivity and reproduction

Question 2

Modern classification groups organisms according to how closely related they are. Which feature can be compared between organisms to classify them?

1. the sequence of bases in DNA
2. the total number of bases in DNA
3. the number of different types of base in DNA
4. the sequence of bases in one protein

Question 3

Spirogyra is a green alga that lives in fresh water. A single Spirogyra cell has a cell wall, a chloroplast, cytoplasm, a nucleus and a large vacuole. Which option correctly states which of these five features Spirogyra shares with plant cells?

1. cell wall yes, chloroplast yes, cytoplasm yes, nucleus yes and vacuole yes
2. cell wall yes, chloroplast yes, cytoplasm no, nucleus no and vacuole yes
3. cell wall yes, chloroplast no, cytoplasm yes, nucleus yes and vacuole no
4. cell wall no, chloroplast yes, cytoplasm yes, nucleus no and vacuole yes

Question 4 · Answer A

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.

Question 5 · Answer A

Step 1: Water moves by osmosis from a solution of higher water potential into one of lower water potential. Step 2: If the solution has a higher water potential than the tissue, water enters the cells, so the stick swells and becomes longer and firmer. Step 3: Sticks that shorten lost water to a lower-water-potential solution, and an unchanged stick was in a solution of equal water potential.

Question 6 · Answer A

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.

Question 7 · Answer A

Step 1: Below the peak, raising the temperature gives molecules more kinetic energy, so up to 25 degrees the reaction speeds up. Step 2: The enzyme does not denature at low temperatures, and above the peak (35 to 45 degrees) the rate falls because the enzyme is denaturing, so effective collisions decrease. Step 3: The peak is at about 30 degrees, so the optimum is 30 not 40 degrees, leaving only the first statement correct.

Question 8 · Answer A

Step 1: A factor is limiting when increasing it increases the rate. Step 2: At 0.02 percent the rate is still rising steeply with carbon dioxide, so adding more carbon dioxide speeds it up, meaning carbon dioxide concentration is the limiting factor. Step 3: Light is plentiful in full sunlight, humidity does not directly limit photosynthesis, and temperature is kept constant, so none of those is limiting here.

Question 9 · Answer A

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.

Question 10 · Answer A

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.

Question 11 · Answer A

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.

Question 12 · Answer A

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.

Question 13 · Answer A

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.

Question 14 · Answer A

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.

Question 15 · Answer A

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.

Question 16 · Answer A

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.

Question 17 · Answer A

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.

Question 18 · Answer A

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.

Question 19 · Answer A

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.

Question 20 · Answer A

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.

Question 21 · Answer A

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.

Question 22 · Answer A

Step 1: Antibiotics kill non-resistant bacteria, so resistant bacteria survive and reproduce, which is natural selection, making statement 1 correct. Step 2: Using antibiotics when they are not essential gives even more chances for resistant bacteria to be selected, making statement 3 correct. Step 3: Antibiotics do not act on viruses, so they cannot reduce viral diseases, making statement 2 wrong, leaving 1 and 3.

Question 23 · Answer A

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.

Question 24 · Answer A

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.

Question 25 · Answer A

Step 1: Hormone X maintains the uterus lining throughout pregnancy and stays high in the late stages. Step 2: Early in pregnancy the ovary makes this progesterone, but by the late stages the placenta takes over and secretes most of it. Step 3: The uterus and vagina do not secrete this hormone, so the placenta is the source in late pregnancy.

Question 26 · Answer A

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.

Question 27 · Answer A

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.

Question 28 · Answer A

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.

Question 29 · Answer A

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.

Question 30 · Answer A

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.

Question 31 · Answer A

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.

Question 32 · Answer A

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.

Question 33 · Answer A

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.

Question 34 · Answer A

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.

Question 35 · Answer A

Step 1: Photosynthesis removes carbon dioxide from the atmosphere and fixes it into plants. Step 2: So the arrow for photosynthesis must point from carbon dioxide in the atmosphere into the plants. Step 3: Arrows pointing up to the atmosphere from plants, animals or combustion show respiration or burning, which release carbon dioxide, not photosynthesis.

Question 36 · Answer A

Step 1: The slow start where few organisms are reproducing is the lag phase, which is region 1. Step 2: The steeply rising section, where numbers increase rapidly, is the exponential (log) phase, region 2. Step 3: The final falling part, where deaths outnumber births, is the death phase, region 4 (region 3 is the stationary phase).

Question 37 · Answer A

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.

Question 38 · Answer A

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.

Question 39 · Answer A

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.

Question 40 · Answer A

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.