0610/41 Oct/Nov 2025 Worked Answers (IGCSE Biology 0610 Extended)
36 questions · 80 marks · 75 minutes
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Worked answers for 30 questions
- Step 1: The radius is half the diameter, so r = 0.5 ÷ 2 = 0.25 μm. Step 2: Volume = π × r² × h = π × 0.25² × 2 = π × 0.0625 × 2. Step 3: This gives 0.39 μm³ (to two significant figures), with the volume in cubic micrometres.Method:Find the radius from the diameter, substitute into π × r² × h, and give the volume in μm³.Examiner tips
- Always convert diameter to radius first, and remember volume units are cubic.
- Step 1: Bacteria have no membrane-bound nucleus and their DNA is a circular loop free in the cytoplasm. Step 2: Organisms with this type of cell are placed in the prokaryote kingdom. Step 3: Animals, fungi and plants are all eukaryotes whose cells do have a true nucleus.Method:Link the absence of a true nucleus to the prokaryote kingdom.Examiner tips
- No true nucleus = prokaryote kingdom.
- 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.Method:Choose cell structures that a living bacterium has but a non-cellular virus lacks.Examiner tips
- Bacteria are cells (membrane, cytoplasm, ribosomes, wall, DNA loop); viruses are not.
- 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.Method:Identify the one-way flaps as valves and give their backflow-prevention function.Examiner tips
- Valves keep blood moving in one direction by preventing backflow.
- 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.Method:Contrast the two-chambered fish heart with the four-chambered, septum-divided human heart.Examiner tips
- Single circulation = a simpler two-chambered heart with no septum.
- 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.Method:Link the second pass through the heart to higher pressure and separation of the two blood types.Examiner tips
- Double circulation = separate blood + high body pressure = fast oxygen delivery.
- 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.Method:Rearrange the magnification equation to make actual size the subject.Examiner tips
- Rearrange magnification = image ÷ actual to get actual = image ÷ magnification.
- 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.Method:Multiply the length in millimetres by 1000 to convert it to micrometres.Examiner tips
- Convert mm to μm by multiplying by 1000.
- 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.Method:Recall the natural ways nitrogen gas is converted into usable compounds.Examiner tips
- Nitrogen is fixed by lightning and by nitrogen-fixing bacteria (and industrially by the Haber process).
- 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.Method:Recall the nitrogen compound released when decomposers break down waste.Examiner tips
- Decomposition of protein releases ammonia, later changed into nitrate.
- 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.Method:Recall that the liver handles both the building and the breakdown of amino acids.Examiner tips
- The liver both assimilates and deaminates amino acids.
- 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.Method:Describe deamination as removing the amino group and converting it into urea.Examiner tips
- Deamination: remove the nitrogen part of amino acids → form urea.
- 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.Method:Match each gap to the correct gas or process using the carbon and greenhouse-gas story.Examiner tips
- Greenhouse gases trap heat; photosynthesis absorbs carbon dioxide; burning fuels releases it.
- 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.Method:Sequence the events from nutrient enrichment to oxygen depletion and the death of fish.Examiner tips
- Eutrophication: nutrients → algal bloom → light blocked → plants die → bacteria deplete oxygen → fish die.
- 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.Method:Pick the features that make a flower attractive and rewarding to insects.Examiner tips
- Insect-pollinated flowers: scent, nectar, large colourful petals.
- 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.Method:Choose the adaptation that makes pollen attach to a bee rather than blow in the wind.Examiner tips
- Insect-carried pollen is large and sticky or spiky.
- 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.Method:Contrast the leaf and vein features of monocots and dicots.Examiner tips
- Monocot: narrow leaves, parallel veins, one cotyledon, parts in threes.
- 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.Method:Order the steps from pollen release through pollen-tube growth to the fusion of nuclei.Examiner tips
- Anther → stigma → pollen tube down the style → ovule → nuclei fuse.
- 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.Method:Recall that the male human sex cell is the sperm.Examiner tips
- Male gamete = sperm; female gamete = egg.
- 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.Method:Recall the organ in the male system that produces sperm.Examiner tips
- Sperm are produced in the testes.
- 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.Method:Recall that the reduction division producing haploid cells is meiosis.Examiner tips
- Meiosis = reduction division that makes haploid gametes.
- 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.Method:Link the prostate gland to producing the fluid component of semen.Examiner tips
- Prostate gland makes fluid that forms semen so sperm can swim.
- 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.Method:Name the process that loses water vapour from leaves as transpiration.Examiner tips
- Loss of water vapour from leaves = transpiration.
- 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.Method:State which group lost more water, then explain using the water potential gradient.Examiner tips
- High humidity → smaller water potential gradient → slower transpiration.
- 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.Method:Predict that a higher temperature raises transpiration, so the new line is above the old one.Examiner tips
- Higher temperature → faster transpiration → line above the original.
- 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.Method:Explain cooling as the evaporation of sweat taking heat energy from the body.Examiner tips
- Evaporating sweat removes heat energy, cooling the body.
- 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.Method:Recall that exhaled air is moist, so breathing out loses water vapour.Examiner tips
- Expired (breathed-out) air carries water vapour out of the body.
- 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.Method:Recall that the nucleus holds the cell’s DNA as chromosomes.Examiner tips
- DNA is found as chromosomes in the nucleus.
- 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.Method:Pick the molecule made from amino acids, such as an enzyme.Examiner tips
- Enzymes, antibodies and haemoglobin are all proteins.
- 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.Method:Describe mRNA as a mobile copy of the gene that directs amino acid order at the ribosome.Examiner tips
- mRNA = a copy of the gene that takes the code to the ribosome.
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