IGCSE Biology 0610/41 October/November 2025 Past Paper with Answers

Question 1

A dichotomous key identifies seven species of bacteria from their shape and any hairlike structures (flagella). The key reads: if hairlike structures are visible go to step 2, otherwise go to step 4. At step 2, more than one hairlike structure goes to step 3 and a single hairlike structure means Vibrio cholerae. At step 3, hairlike structures attached at one end only means Helicobacter pylori and hairlike structures attached all over means Salmonella typhi. At step 4, a spiral shape means Treponema pallidum, otherwise go to step 5. At step 5, cells grouped into a chain go to step 6 and cells not in a chain mean Staphylococcus aureus. At step 6, cylindrical cells in the chain mean Streptobacillus moniliformis and circular cells in the chain mean Streptococcus pyogenes. A bacterium has no hairlike structures, is not spiral-shaped, and is grouped into a chain of circular cells. Which species is it?

1. Streptococcus pyogenes
2. Streptobacillus moniliformis
3. Staphylococcus aureus
4. Treponema pallidum

Question 2

A cylinder-shaped bacterium has a length of 2 μm and a diameter of 0.5 μm. The volume of a cylinder is calculated using the formula π × r² × h, where r is the radius and h is the length. What are the radius and the volume of this bacterium, with their correct units?

1. radius = 0.25 μm and volume = 0.39 μm³
2. radius = 0.5 μm and volume = 1.57 μm³
3. radius = 0.25 μm and volume = 0.13 μm³
4. radius = 0.25 μm and volume = 0.20 μm³

Question 3

Bacteria are single-celled organisms with no true nucleus. To which kingdom do bacteria belong?

1. the prokaryote kingdom
2. the animal kingdom
3. the fungus kingdom
4. the plant kingdom

Question 4 · Answer A

Step 1: A virus has no cells, no cytoplasm and no organelles. Step 2: Its outer layer is a coat made of protein, which surrounds and protects the core. Step 3: The inner core is the genetic material (the nucleic acid) that the virus uses to take over a host cell.

Question 5 · Answer A

Step 1: A bacterium is a complete cell, so it has a cell membrane, cytoplasm, ribosomes, a cell wall and a loop of DNA. Step 2: A virus is not a cell, so it has none of these; it has only a protein coat around genetic material. Step 3: So a cell membrane and cytoplasm are both found in bacteria but not in viruses.

Question 6 · Answer A

Step 1: The vessel from the heart to the lungs is the pulmonary artery, carrying deoxygenated blood. Step 2: The vessel from the heart to the body is the aorta, carrying oxygenated blood, while the vessel from the intestines to the liver is the hepatic portal vein, carrying deoxygenated blood. Step 3: The vessel leaving a kidney towards the heart is the renal vein, carrying deoxygenated blood, so only the first option is correct throughout.

Question 7 · Answer A

Step 1: Flaps inside a vessel that keep blood flowing one way are valves. Step 2: When blood tries to flow backwards, the valve flaps close and stop it. Step 3: So the structure is a valve and its function is to prevent the backflow of blood.

Question 8 · Answer A

Step 1: A human heart has four chambers and a septum that fully separates the left and right sides. Step 2: A fish heart has only two chambers and no septum, because oxygenated and deoxygenated blood do not need to be kept apart in a single circulation. Step 3: A fish heart also has fewer valves and walls of more similar thickness, so the first option is correct.

Question 9 · Answer A

Step 1: In a double circulation the blood is re-pumped by the heart after leaving the lungs, so it reaches the body at high pressure. Step 2: Oxygenated and deoxygenated blood are kept separate, so fully oxygenated blood is delivered quickly to respiring tissues. Step 3: This supports a high metabolic rate, while the lungs receive blood at lower pressure to avoid damage, so the first option is correct.

Question 10 · Answer A

Step 1: The cells are inside the lumen of a blood vessel, so they are blood cells. Step 2: They are small, rounded, have no nucleus and a pale centre (a biconcave dip), which are the features of red blood cells. Step 3: Nerve, root hair and sperm cells are not found flowing inside a blood vessel.

Question 11 · Answer A

Step 1: Magnification = image size ÷ actual size. Step 2: Rearranging this formula to find the actual size gives actual size = image size ÷ magnification. Step 3: So the measured image size is divided by the magnification to give the real diameter.

Question 12 · Answer A

Step 1: There are 1000 micrometres in one millimetre. Step 2: To convert from millimetres to micrometres, multiply by 1000. Step 3: 0.0315 × 1000 = 31.5 μm.

Question 13 · Answer A

Step 1: Bacteria that turn nitrate ions back into nitrogen gas carry out denitrification, so the process releasing nitrogen to the atmosphere is denitrification. Step 2: Bacteria that turn ammonia into nitrate ions carry out nitrification, so forming nitrate from the decomposition compound is nitrification. Step 3: Nitrogen fixation makes nitrogen compounds from atmospheric nitrogen and assimilation builds proteins, so they do not fit here.

Question 14 · Answer A

Step 1: Lightning provides enough energy to combine nitrogen and oxygen in the air, fixing nitrogen. Step 2: Nitrogen-fixing bacteria in the soil and in root nodules also convert nitrogen gas into compounds. Step 3: Respiration, photosynthesis, evaporation and the other options do not fix nitrogen from the air.

Question 15 · Answer A

Step 1: When decomposers break down the proteins in waste, they release nitrogen as ammonia (ammonium ions). Step 2: Nitrifying bacteria then convert this ammonia into nitrate ions. Step 3: Glucose, carbon dioxide and oxygen contain no usable nitrogen in this step, so they are not compound Y.

Question 16 · Answer A

Step 1: Assimilation is the building of absorbed amino acids into the body’s own proteins, which happens in the liver. Step 2: Deamination, the removal of the nitrogen part of excess amino acids, also happens in the liver. Step 3: The lungs, bladder and skin do not carry out deamination of amino acids.

Question 17 · Answer A

Step 1: Deamination removes the nitrogen-containing part (the amino group) from excess amino acids. Step 2: This nitrogen part is converted into urea, which is then excreted. Step 3: Building proteins is assimilation, breaking down glucose is respiration, and reabsorbing water happens in the kidney, so they do not describe deamination.

Question 18 · Answer A

Step 1: Methane and carbon dioxide add to the enhanced greenhouse effect, and methane comes from cattle and from flooded rice crops. Step 2: Carbon dioxide is the gas absorbed by trees during photosynthesis. Step 3: Carbon dioxide is released when fossil fuels are burned, so gap 5 is burning, making the first option correct throughout.

Question 19 · Answer A

Step 1: The extra nitrate ions from sewage and fertiliser make algae grow rapidly, forming an algal bloom on the surface. Step 2: The bloom blocks light, so the water plants below cannot photosynthesise and they die. Step 3: Bacteria decompose the dead plants and algae, multiply, and respire aerobically, using up the dissolved oxygen so that the fish die from lack of oxygen.

Question 20 · Answer A

Step 1: Insect-pollinated flowers attract insects with a strong scent and sugary nectar as a reward. Step 2: They also have large, brightly coloured petals that insects can see easily. Step 3: The other options describe wind-pollinated flowers, which are dull, scentless and have small petals.

Question 21 · Answer A

Step 1: Insect-pollinated pollen must stick to a visiting insect. Step 2: The grains are therefore large with a sticky or spiky surface so they cling to the hairs on a bee’s body. Step 3: Small, smooth, light or feathery pollen is an adaptation for wind pollination instead.

Question 22 · Answer A

Step 1: Monocotyledons have narrow, strap-like leaves with parallel veins. Step 2: Dicotyledons have broader leaves with branching, net-like veins. Step 3: Monocots also have one cotyledon and flower parts in threes, so the options that swap these features around are wrong.

Question 23 · Answer A

Step 1: The stalk that holds up an anther is the filament. Step 2: The outer leaf-like part that protects the bud is the sepal. Step 3: The small body inside the ovary that becomes a seed is the ovule, so the first option names all three correctly.

Question 24 · Answer A

Step 1: Pollen is released from the anther and is transferred to the stigma. Step 2: A pollen tube then grows from the pollen grain down through the style to reach an ovule in the ovary. Step 3: The male nucleus travels down the pollen tube and fuses with the female nucleus in the ovule, which is fertilisation and forms a zygote.

Question 25 · Answer A

Step 1: A gamete is a sex cell, and the male gamete in humans is the sperm cell. Step 2: The egg cell is the female gamete. Step 3: A zygote is the cell formed after fertilisation, and a red blood cell is not a gamete at all.

Question 26 · Answer A

Step 1: Male gametes (sperm) are made in the testes. Step 2: The ovaries make female gametes, so they are not part of the male system. Step 3: The bladder stores urine and the prostate gland makes fluid, so neither produces sperm.

Question 27 · Answer A

Step 1: Gametes must be haploid so that the chromosome number is restored at fertilisation. Step 2: Meiosis is the reduction division that halves the chromosome number, producing haploid cells. Step 3: Mitosis makes identical diploid cells, fertilisation joins gametes, and digestion breaks down food, so none of these makes haploid gametes.

Question 28 · Answer A

Step 1: The prostate gland adds a fluid to the sperm. Step 2: This fluid mixes with the sperm to make semen and provides a medium in which the sperm can swim (and some nutrients). Step 3: Sperm are made in the testes, urine is stored in the bladder, and oxygen is carried by red blood cells, so the other options are wrong.

Question 29 · Answer A

Step 1: Water vapour evaporates from the surfaces of cells inside the leaf and diffuses out through the stomata. Step 2: This loss of water vapour from the leaves is called transpiration. Step 3: Translocation moves sugars, respiration releases energy, and pollination transfers pollen, so they are not this process.

Question 30 · Answer A

Step 1: Both groups lose mass as water vapour, but the leaves in low humidity lose more than those in high humidity. Step 2: In high humidity there is already a lot of water vapour in the air, so the water potential gradient between the leaf and the air is smaller. Step 3: A smaller gradient means less water vapour diffuses out through the stomata, so transpiration is slower in high humidity.

Question 31 · Answer A

Step 1: Raising the temperature gives water molecules more energy, so evaporation inside the leaf is faster. Step 2: This increases the rate of transpiration, so more water vapour is lost at each time. Step 3: On a graph the new line would lie above the original 20 °C low-humidity line.

Question 32 · Answer A

Step 1: Sweat glands in the skin release sweat onto the skin surface. Step 2: As the sweat evaporates, it takes heat energy from the body. Step 3: Losing this heat lowers the body temperature back towards the normal set point, which is part of homeostasis.

Question 33 · Answer A

Step 1: Air inside the lungs becomes saturated with water vapour from the moist lung surfaces. Step 2: When a person breathes out, this water vapour leaves the body in the expired air. Step 3: So breathing out (expiration) is another way the body loses water vapour.

Question 34 · Answer A

Step 1: DNA is stored as chromosomes inside the nucleus of the cell. Step 2: The nucleus controls the cell’s activities through this DNA. Step 3: The cell membrane controls what enters and leaves, ribosomes build proteins, and a vacuole stores cell sap, so none of these holds the DNA.

Question 35 · Answer A

Step 1: Proteins include enzymes, antibodies, haemoglobin, keratin and membrane carriers. Step 2: Amylase is an enzyme, so it is a protein. Step 3: Glucose and starch are carbohydrates and a fatty acid is part of a fat, so none of these is a protein.

Question 36 · Answer A

Step 1: mRNA is made as a copy of the base sequence of a gene, while the DNA itself stays in the nucleus. Step 2: The mRNA then carries this coded information out of the nucleus to a ribosome in the cytoplasm. Step 3: At the ribosome the order of bases in the mRNA determines the order in which amino acids are joined to build the protein.