0610/23 Oct/Nov 2025 Worked Answers (IGCSE Biology 0610 Extended)
40 questions · 40 marks · 45 minutes
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Worked answers for 28 questions
- Step 1: The seven characteristics of all living organisms are movement, respiration, sensitivity, growth, reproduction, excretion and nutrition. Step 2: Reproduction, making new organisms, is one of these seven and is shown by every living thing. Step 3: Breathing is only carried out by some animals, while photosynthesis and transpiration occur only in plants, so none of these is shown by all organisms.Method:Compare each option with the seven characteristics of living organisms and pick the universal one.Examiner tips
- Use the MRS GREN list of the seven life processes shared by all organisms.
- Step 1: In the binomial system each organism has a two-part Latin name. Step 2: The first part is the genus (written with a capital letter) and the second part is the species. Step 3: The kingdom is a much larger group that is not part of the two-part scientific name, so the correct order is genus then species.Method:Recall the structure of a binomial name and choose genus followed by species.Examiner tips
- Remember: genus first (capital letter), then species (lower case).
- Step 1: A palisade cell is a plant cell and a neurone is an animal cell. Step 2: Plant cells have an extra outer cellulose cell wall that animal cells do not have. Step 3: Both cells have a cell membrane, cytoplasm and a nucleus, so the only component unique to the palisade cell is the cell wall.Method:Identify which listed structure is present in plant cells but absent from animal cells.Examiner tips
- Plant cells have a cell wall, large vacuole and chloroplasts; animal cells do not.
- Step 1: The wall surrounds the lumen on both sides, so the lumen diameter is the external diameter minus twice the wall thickness: 36 minus (2 times 9) = 36 minus 18 = 18 mm. Step 2: Convert millimetres to micrometres by multiplying by 1000: 18 mm = 18 000 micrometres. Step 3: So the diameter of the lumen is 18 000 micrometres.Method:Subtract twice the wall thickness to find the lumen in mm, then convert to micrometres.Examiner tips
- Lumen diameter = external diameter − (2 × wall thickness); 1 mm = 1000 micrometres.
- Step 1: In diffusion, particles spread out from where they are crowded together to where there are fewer of them. Step 2: This means net movement is from a higher concentration to a lower concentration. Step 3: Moving from high to low concentration is moving down the concentration gradient, so the correct words are higher, lower and down.Method:Recall that diffusion moves particles from high to low concentration, i.e. down the gradient.Examiner tips
- Diffusion is always high → low concentration, which is down the concentration gradient.
- Step 1: Plants need magnesium ions to make chlorophyll, the green pigment used in photosynthesis. Step 2: Chlorophyll therefore contains magnesium. Step 3: Cellulose, glucose and glycerol are carbohydrates or lipid building blocks that contain only carbon, hydrogen and oxygen, so they do not contain magnesium.Method:Recall which substance needs magnesium and rule out the carbon-based molecules.Examiner tips
- Link magnesium ions to chlorophyll; a shortage causes yellow leaves (chlorosis).
- Step 1: Chemical digestion uses enzymes to break large insoluble molecules into smaller soluble ones. Step 2: Breaking starch down into reducing sugars is done by the enzyme amylase, so it is chemical digestion. Step 3: Churning by the stomach, grinding by teeth and mixing with bile are all physical (mechanical) processes that do not break chemical bonds within food molecules.Method:Identify the process that changes molecules with enzymes rather than just the size of food pieces.Examiner tips
- Chemical digestion = enzymes breaking molecules; physical digestion = breaking food into smaller pieces.
- Step 1: Transpiration is the loss of water vapour from the leaves, and it speeds up when the air can take up water quickly. Step 2: Low humidity steepens the water-vapour gradient, wind carries the vapour away, and high temperature increases evaporation. Step 3: So the fastest transpiration occurs with low humidity, windy conditions and a high temperature.Method:Pick the combination that increases evaporation and keeps the water-vapour gradient steep.Examiner tips
- Dry + windy + warm = fast transpiration; humid + still + cool = slow.
- Step 1: A source makes or releases sucrose and a sink uses or stores it. Step 2: Roots store and use sugars, so they act as sinks (statement 2 correct), and spongy mesophyll cells photosynthesise, so they can act as sources (statement 4 correct). Step 3: Palisade cells photosynthesise and so act as sources, not only sinks (statement 1 wrong), and phloem transports sucrose and amino acids, not just glucose (statement 3 wrong); so only 2 and 4 are correct.Method:Check each statement against the definitions of source and sink and the contents of phloem.Examiner tips
- Photosynthesising leaf cells are sources; roots and growing or storage tissues are sinks.
- Step 1: Veins carry blood back to the heart at low pressure, so they contain valves to stop the blood flowing backwards. Step 2: A one-cell-thick wall is a feature of capillaries, not veins. Step 3: A narrow lumen and thick muscular walls are features of arteries, so the correct feature of veins is that valves are present.Method:Recall the structure of veins and identify the valve as their distinctive feature.Examiner tips
- Veins: wide lumen, thin walls and valves; arteries: narrow lumen and thick muscular walls.
- Step 1: Active immunity is when the body makes its own antibodies in response to an antigen. Step 2: Producing antibodies after an infection means the person has made the antibodies themselves, so this is active immunity. Step 3: Antibodies passed in breast milk, across the placenta, or given by injection are made by someone else, so these are examples of passive immunity.Method:Identify the case where the person produces antibodies themselves.Examiner tips
- Active = you make your own antibodies; passive = you receive ready-made antibodies.
- Step 1: During expiration the chest volume must decrease. Step 2: The external intercostal muscles relax so the ribs move down and in, and the diaphragm relaxes and moves up into a dome shape. Step 3: Both changes reduce the volume of the chest, raising the pressure and pushing air out, so expiration involves the intercostals relaxing and the diaphragm relaxing and moving up.Method:Choose the changes that reduce chest volume to force air out.Examiner tips
- Expiration: intercostals and diaphragm relax, ribs drop, diaphragm domes up, chest gets smaller.
- Step 1: Urea is made in the liver, so the pathway must start at the liver, and it is excreted at the kidney, so it must end there. Step 2: Blood leaves the liver in the hepatic vein, travels to the heart and lungs and back to the heart. Step 3: It then leaves the heart and reaches the kidney through the renal artery, so the route is liver, hepatic vein, heart, lungs, heart, renal artery, kidney.Method:Start at the liver and trace the blood through the heart and lungs to the kidney.Examiner tips
- Liver makes urea → hepatic vein → heart → lungs → heart → renal artery → kidney.
- Step 1: To focus on a near object the lens must become thicker and more curved. Step 2: The ciliary muscles contract, which makes the ring of muscle smaller and slackens the suspensory ligaments. Step 3: With the ligaments slack the lens is no longer pulled thin and becomes fatter, so for a near object the ciliary muscles are contracted and the suspensory ligaments are slack.Method:Decide the lens must thicken for a near object, then state the muscle and ligament changes.Examiner tips
- Near object: ciliary muscles contract, ligaments slacken, lens fattens.
- Step 1: An arriving impulse first triggers the release of neurotransmitter into the gap (event 2). Step 2: The neurotransmitter then diffuses across the gap (event 4) and binds with receptor proteins on the next neurone (event 3). Step 3: This binding starts a new impulse in the next neurone (event 1), giving the order 2, 4, 3, 1.Method:Order the events from release of neurotransmitter to the new impulse in the next neurone.Examiner tips
- Release → diffuse → bind to receptors → new impulse.
- Step 1: To avoid getting too cold the body must make more heat and lose less heat. Step 2: Shivering makes the muscles contract rapidly, releasing heat, while vasoconstriction reduces blood flow to the skin surface so less heat is lost. Step 3: Sweating and vasodilation both increase heat loss, so they would cool the body further; the correct pair is shivering and vasoconstriction.Method:Choose the actions that increase heat production and reduce heat loss.Examiner tips
- Cold response: shiver to make heat, vasoconstrict to keep heat in.
- Step 1: Hormonal control uses chemicals carried in the blood, so it acts slowly but its effects last a long time. Step 2: Nervous control uses electrical impulses, so it acts very fast but its effects are short-lived. Step 3: The only fully correct row is therefore hormonal control being slow to act with a long-lasting effect.Method:Match the type of control with the correct speed and duration, recalling that hormones are slow and long-lasting.Examiner tips
- Hormonal = slow + long; nervous = fast + short.
- Step 1: Auxin is made in the shoot tip (statement 3 correct) and moves to the shaded or lower side of a shoot or stem. Step 2: In a horizontal stem it gathers on the lower side (statement 1 correct), and it gathers on the darker side, not the side with more light (statement 2 wrong). Step 3: Auxin promotes cell elongation rather than preventing it (statement 4 wrong), so only statements 1 and 3 are correct.Method:Check each statement against how auxin is made and where it accumulates.Examiner tips
- Auxin: made in tip, collects on shaded/lower side, increases cell elongation.
- Step 1: Antibiotics kill or stop the growth of bacteria but have no effect on viruses. Step 2: Cholera is caused by a bacterium, so it can be treated with antibiotics. Step 3: HIV is a virus and so cannot be treated with antibiotics, rickets is caused by a lack of vitamin D rather than a pathogen, and not all pathogen diseases are bacterial, so "all diseases caused by pathogens" is wrong.Method:Pick the disease that is bacterial, since antibiotics only treat bacteria.Examiner tips
- Antibiotics treat bacterial infections only; they do not work on viruses.
- Step 1: During pregnancy the placenta takes over the production of progesterone. Step 2: Progesterone maintains the lining of the uterus so that the pregnancy can continue. Step 3: FSH and LH are produced by the pituitary gland and testosterone by the testes, so the hormone made by the placenta is progesterone.Method:Recall which hormone the placenta makes to support pregnancy.Examiner tips
- The placenta makes progesterone to maintain the uterus lining during pregnancy.
- Step 1: A gene is a length of DNA that codes for a particular feature. Step 2: Alleles are different forms, or versions, of the same gene that can produce different versions of a characteristic. Step 3: A pair of chromosomes, the number of genes on a chromosome, and two genes side by side are all different ideas, so alleles are different versions of the same gene.Method:Recall the definition of an allele as a version of a gene.Examiner tips
- Allele = an alternative version of a gene (e.g. the allele for brown vs blue eyes).
- Step 1: Polydactyly is dominant, so a cat with no extra toes must have two recessive alleles. Step 2: Crossing two heterozygous cats gives offspring in the ratio 1 homozygous dominant : 2 heterozygous : 1 homozygous recessive. Step 3: Only the homozygous recessive cat has no extra toes, which is 1 out of 4, so the probability is 0.25.Method:Draw the cross, count the homozygous recessive offspring, and express it as a probability.Examiner tips
- Heterozygous × heterozygous gives a 1:2:1 ratio; the recessive type is 1 in 4.
- Step 1: Red-green colour blindness is a sex-linked condition controlled by a recessive allele carried on the X chromosome. Step 2: Because males have only one X chromosome, a single recessive allele makes them colour blind, so it is more common in males than females. Step 3: It is not codominance, it can still occur in females who inherit two recessive alleles, and it is more common in males, so the correct statement is that the recessive allele is on an X chromosome.Method:Recall that colour blindness is X-linked recessive and test each statement against that.Examiner tips
- Red-green colour blindness = X-linked recessive, more common in males.
- Step 1: Meiosis produces haploid cells, the gametes (feature 1 correct), and halves the chromosome number, so it is a reduction division (feature 3 correct). Step 2: Meiosis shuffles the genetic material, so the cells produced are genetically different and show variation (feature 4 correct). Step 3: Feature 2 is wrong because producing genetically identical cells is a feature of mitosis, so meiosis is associated with features 1, 3 and 4.Method:Match each feature to meiosis, remembering that identical cells come from mitosis.Examiner tips
- Meiosis: haploid cells, reduction division, variation. Mitosis: identical cells.
- Step 1: A characteristic that increases an organism’s chances of surviving and reproducing in its environment is called an adaptive feature. Step 2: Competition is the struggle between organisms for limited resources, not a characteristic. Step 3: An observable feature is any visible characteristic and a phenotype is the set of an organism’s characteristics, so the correct term is adaptive feature.Method:Recall the definition of an adaptive feature and rule out the other terms.Examiner tips
- Adaptive feature = an inherited characteristic that helps survival and reproduction.
- Step 1: In the carbon cycle, carbon dioxide is taken out of the air by green plants. Step 2: Plants use carbon dioxide in photosynthesis to make glucose, so photosynthesis removes carbon dioxide from the atmosphere. Step 3: Combustion, decomposition and respiration all release carbon dioxide into the air, so they add it rather than remove it.Method:Identify the process that uses carbon dioxide rather than producing it.Examiner tips
- Photosynthesis takes in carbon dioxide; respiration, combustion and decomposition give it out.
- Step 1: Herbivores eat producers (plants), so they are the first animals in a food chain. Step 2: The trophic level of organisms that eat producers is called the primary consumer level. Step 3: Secondary, tertiary and quaternary consumers are higher levels that eat other animals, so herbivores are primary consumers.Method:Place herbivores in the food chain just after the producers as primary consumers.Examiner tips
- Producer → primary consumer (herbivore) → secondary consumer → tertiary consumer.
- Step 1: First the insulin gene must be cut out of a human chromosome using an enzyme (step 2), then placed into a bacterial plasmid (step 3). Step 2: The modified bacteria then reproduce rapidly to make many copies (step 4). Step 3: Finally the insulin they make is removed from the culture (step 1), so the order is 2, 3, 4, 1.Method:Order the steps so the gene is moved into bacteria before they grow and produce insulin.Examiner tips
- Cut out the gene → insert into plasmid → bacteria reproduce → collect insulin.
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