IGCSE Biology 0610/23 May/June 2025 Past Paper with Answers

Question 1

An aquatic plant is placed in water and light is shone onto it. The plant releases bubbles of oxygen gas that rise up through the water. Which characteristic of all living things is being demonstrated?

1. nutrition
2. breathing
3. photosynthesis
4. respiration

Question 2

Four different single-celled organisms are examined. Which one is a prokaryote?

1. a rod-shaped cell surrounded by a wall, with no nucleus and a single loop of genetic material lying free in the cytoplasm
2. an irregular, blob-shaped cell with no cell wall and a single round nucleus
3. an oval cell with one long whip-like flagellum, a nucleus and a chloroplast
4. a rounded cell with a cell wall, a nucleus and a curved chloroplast

Question 3

Plant cells are placed in a sugar solution that has a lower water potential than the cytoplasm of the cells. What happens to the cells?

1. The cells lose water and become flaccid.
2. The cells gain water and become flaccid.
3. The cells gain water and become turgid.
4. The cells lose water and become turgid.

Question 4 · Answer A

Step 1: Because bases pair A with T and C with G, 20% T in molecule 1 means 20% A, so 40% A and T together and 60% C and G. Step 2: In molecule 2, 20% C means 20% G, so 40% C and G together and 60% A and T, meaning molecule 2 has more A bases. Step 3: More A-T pairs means weaker bonding overall, so molecule 2 needs the lowest temperature to separate.

Question 5 · Answer A

Step 1: The substrate is the molecule the enzyme binds and breaks down, which is molecule 1; molecules 2 are the smaller products. Step 2: Process R distorts the active site, which is denaturation. Step 3: The stomach enzyme works best in acidic conditions, so it is alkaline conditions that denature it, making R caused by alkaline conditions.

Question 6 · Answer A

Step 1: Photosynthesis uses carbon dioxide and water to make glucose and oxygen. Step 2: To balance the atoms, six molecules of carbon dioxide and six of water give one glucose and six of oxygen. Step 3: So the equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂; the other versions are not balanced or use the wrong reactants.

Question 7 · Answer A

Step 1: Distilled water has a higher water potential than the potato cells. Step 2: Water therefore moves into the potato cells by osmosis. Step 3: Taking in water increases the mass of the cube, so it rises from 5 g to about 7 g; a fall to 2 g or 4 g would mean water left, and 5 g would mean no change.

Question 8 · Answer A

Step 1: If tissue P is the maternal blood, then substance 1 moves from mother to fetus and substance 2 moves from fetus to mother. Step 2: Useful materials like oxygen and amino acids pass from mother to fetus (substance 1), while wastes like carbon dioxide and urea pass from fetus to mother (substance 2). Step 3: So substance 2 is correctly identified as urea; carbon dioxide cannot be substance 1, amino acids move toward the fetus not the mother, and starch is too large to cross.

Question 9 · Answer A

Step 1: Palisade mesophyll cells are tall, column-shaped cells packed close together just below the upper surface. Step 2: They are full of chloroplasts so they absorb the most light for photosynthesis. Step 3: The thin surface cells are epidermis, the vein is the vascular bundle, and the lower cell next to a pore is part of the lower epidermis.

Question 10 · Answer A

Step 1: Vitamin C is needed to make connective tissue and to heal wounds. Step 2: Without enough vitamin C, the gums become swollen and bleed and wounds heal slowly, a condition called scurvy. Step 3: Lack of calcium weakens bones and teeth, lack of fibre causes constipation, and lack of iron causes anaemia, none of which give these symptoms.

Question 11 · Answer A

Step 1: Assimilation is when digested food molecules are taken into cells and used to become part of them. Step 2: This is exactly what the statement describes. Step 3: Absorption is moving molecules into the blood, digestion is breaking large molecules into small ones, and ingestion is taking food into the body, so none of these fit.

Question 12 · Answer A

Step 1: Protein digestion needs the enzyme protease (pepsin) and acidic conditions. Step 2: These are first found in the stomach, so protein digestion begins there. Step 3: The oesophagus only carries food down, the pancreas makes enzymes that act later, and the small intestine continues protein digestion but does not begin it.

Question 13 · Answer A

Step 1: In a leaf the xylem is on the upper side of the vein, which is number 1. Step 2: In a root the xylem forms the central star, which is number 4. Step 3: In a stem the xylem is on the inner side of each vascular bundle, which is number 6, so the row is leaf 1, root 4, stem 6.

Question 14 · Answer A

Step 1: Blood plasma in arteries carries dissolved amino acids around the body. Step 2: Phloem carries dissolved food, including sucrose and amino acids, by translocation. Step 3: So both arteries and phloem normally carry amino acids.

Question 15 · Answer A

Step 1: The ventricles are the lower chambers that pump blood out of the heart. Step 2: Blood leaving the heart travels in arteries, which carry blood away from the heart. Step 3: So blood leaves through arteries from the ventricles; the atria receive blood, and veins bring blood toward the heart.

Question 16 · Answer A

Step 1: A white blood cell with a lobed nucleus and grainy cytoplasm is a phagocyte. Step 2: Phagocytes defend the body by engulfing and digesting pathogens. Step 3: Lymphocytes, which have a large round nucleus, are the cells that produce antibodies, so the named cell here is a phagocyte that engulfs pathogens.

Question 17 · Answer A

Step 1: Cholera toxin makes chloride ions move into the small intestine. Step 2: The extra ions lower the water potential inside the intestine. Step 3: Water then moves out of the blood into the intestine by osmosis, so more water leaves the body through the anus as watery diarrhoea.

Question 18 · Answer A

Step 1: Large airways near the larynx, such as the trachea and bronchi, have a large lumen and contain rings of cartilage, matching point 1. Step 2: The structures attached to the alveoli are the smallest airways (bronchioles), with a small lumen far from the larynx, matching point 2. Step 3: So the structure attached to the alveoli is point 2 and the one containing cartilage is point 1.

Question 19 · Answer A

Step 1: Respiration releases energy that cells use for active life processes. Step 2: Cell division is an active process that needs this energy. Step 3: Osmosis and transpiration are passive and need no respiratory energy, and photosynthesis takes in light energy rather than using energy from respiration.

Question 20 · Answer A

Step 1: During hard exercise the muscles respire anaerobically (1), converting glucose to lactic acid (3). Step 2: Lactic acid then builds up and causes an oxygen debt (4). Step 3: After exercise, breathing stays fast and deep (2) so that oxygen can be used to respire the lactic acid aerobically (5), giving the order 1, 3, 4, 2, 5.

Question 21 · Answer A

Step 1: Filtration in the glomerulus lets small molecules pass from the blood into the nephron. Step 2: Both glucose and urea are small molecules, so both are filtered out. Step 3: Glucose is later reabsorbed further along the nephron, but the question is about filtration, where both pass through.

Question 22 · Answer A

Step 1: Excess amino acids cannot be stored, so they are broken down in the liver. Step 2: This breakdown, called deamination, removes the nitrogen part and forms urea. Step 3: Fatty acids and sugars are broken down in respiration to release energy, and vitamins are not broken down to make urea.

Question 23 · Answer A

Step 1: P carries the impulse from the receptor into the spinal cord, which is the job of a sensory neurone. Step 2: Q lies within the grey matter and links the two, so it is the relay neurone. Step 3: R carries the impulse out to the effector muscle, which is the motor neurone, giving P sensory, Q relay, R motor.

Question 24 · Answer A

Step 1: To focus on a near object the ciliary muscle (E) contracts. Step 2: This slackens the suspensory ligaments (G), which are pulled less, so the lens (F) becomes thicker and more rounded. Step 3: A thicker lens bends light more, so the refraction is greater, focusing the near object on the retina.

Question 25 · Answer A

Step 1: Glucagon raises blood glucose by making the liver release glucose from stored glycogen. Step 2: Adrenaline also raises blood glucose, preparing the body for action. Step 3: Insulin lowers blood glucose, the opposite effect, so the two hormones that increase it are adrenaline and glucagon.

Question 26 · Answer A

Step 1: Light from the right makes auxin move to the shaded left side of the tip, so block X collects more auxin than block Y. Step 2: When block X is placed on a cut stem, the auxin diffuses down the side under the block and makes that side grow longer, so the shoot bends away from the block. Step 3: Block Y came from the lit side and holds little auxin, so its shoot grows straight; only the X shoot curves.

Question 27 · Answer A

Step 1: A potato tuber grows from a stem by mitosis, with no gametes involved. Step 2: Reproduction without gametes from a single parent is asexual reproduction. Step 3: Because it is asexual, potato Y is a genetically identical clone of the parent, so there is no genetic difference and no fusion of gametes.

Question 28 · Answer A

Step 1: Self-pollination uses pollen and eggs from the same plant, so the offspring are very similar to the parent. Step 2: This gives little genetic variation in the population. Step 3: With less variation, the population is less able to adapt if the environment changes, so self-pollination reduces this ability by reducing variation.

Question 29 · Answer A

Step 1: Menstruation is the breakdown and loss of the uterus lining, shown by the lining getting thinner. Step 2: The last day of menstruation is when the lining stops thinning and reaches its lowest point, which is partway through the first week. Step 3: Of the choices, only day 4 falls before day 7 while the lining is at its thinnest, so day 4 is the last day of menstruation.

Question 30 · Answer A

Step 1: A gene is a length of DNA that codes for a protein. Step 2: Different versions of the same gene are called alleles. Step 3: A chromosome is a long thread of DNA carrying many genes, a length of DNA is a gene itself, and a protein is the product made from a gene, so none of these names a version of a gene.

Question 31 · Answer A

Step 1: Early embryo cells are unspecialised and can become any cell type, so they are stem cells. Step 2: An embryo grows by ordinary cell division, which is mitosis, producing identical cells. Step 3: Meiosis only makes gametes, gametes are single sex cells not a ball of cells, and a zygote is one cell, so the correct description is stem cells undergoing mitosis.

Question 32 · Answer A

Step 1: The disease is X-linked recessive, so an affected male has the allele on his single X chromosome, inherited from his mother. Step 2: Individual 4 is affected, so his unaffected mother (individual 2) must carry one disease allele, making her heterozygous; likewise individual 5 is affected, so his unaffected mother (individual 3) is also heterozygous. Step 3: A male has only one X chromosome so cannot be a heterozygous carrier, and affected males (4 and 5) have the disease rather than being carriers.

Question 33 · Answer A

Step 1: Continuous variation, such as body mass, shows a range of values and is caused by both genes and the environment. Step 2: Discontinuous variation falls into distinct groups and is caused by genes only, with examples like seed shape in peas and ABO blood groups. Step 3: So the correct row pairs continuous variation with genes and the environment, and gives seed shape in peas as discontinuous.

Question 34 · Answer A

Step 1: An adaptive feature is an inherited feature that helps an organism survive and reproduce, so statements 1 and 3 are correct. Step 2: Adaptive features do not always show continuous variation; many show discontinuous variation, so statement 2 is wrong. Step 3: Therefore only statements 1 and 3 are correct.

Question 35 · Answer A

Step 1: When antibiotic X is used, resistant bacteria survive while non-resistant ones die. Step 2: The survivors reproduce and pass on the resistance, so the proportion of resistant bacteria rises. Step 3: This survival of the better-adapted bacteria is natural selection; it is not chosen by humans (artificial selection), and bacteria do not use meiosis or fertilisation.

Question 36 · Answer A

Step 1: The producers have 1000 units and the secondary consumers have 10 units, so the energy lost is 1000 minus 10, which is 990 units. Step 2: As a percentage of the starting 1000 units, that is 990 divided by 1000, times 100. Step 3: This gives 99%, so 99% of the energy is lost between the producers and the secondary consumers.

Question 37 · Answer A

Step 1: Nitrogen fixation changes nitrogen gas into nitrogen compounds that organisms can use. Step 2: Both lightning and nitrogen-fixing bacteria can carry out this fixing of nitrogen. Step 3: Deamination happens in the liver, while denitrification and nitrification are done by bacteria but not by lightning, so the answer is nitrogen fixation.

Question 38 · Answer A

Step 1: The number of organisms falls only when more organisms die than are born. Step 2: This happens in the death phase, where the curve is falling, which is phase D. Step 3: At the slow start "phase D, where the curve is falling at the end" and steep rise "phase A, the slow-starting part at the beginning" births exceed deaths, and on the plateau "phase B, the steeply rising part" births and deaths are roughly equal, so only D shows deaths exceeding births.

Question 39 · Answer A

Step 1: A monoculture is a single crop species grown over a large area, so all the plants are alike. Step 2: Because the plants are so similar, a single disease can spread quickly and destroy the whole crop, which is a disadvantage. Step 3: Less insecticide use, lower emissions and more biodiversity would all be advantages, and monocultures actually reduce biodiversity.

Question 40 · Answer A

Step 1: First the insulin gene is isolated using restriction enzymes (3) and then inserted into a cut plasmid (1). Step 2: The recombinant plasmids are then placed into bacteria (4). Step 3: Finally the bacteria multiply and make human insulin (2), giving the order 3, 1, 4, 2.