May/June 2025 Paper 21 Worked Answers (IGCSE Biology 0610 Extended)
40 questions · 40 marks · 45 minutes
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Worked answers for 31 questions
- Step 1: A leaf takes in carbon dioxide to make food by photosynthesis, which is the plant form of nutrition. Step 2: So the characteristic that requires carbon dioxide to diffuse into a leaf is nutrition. Step 3: Excretion, movement and respiration do not need carbon dioxide to enter the leaf; respiration in fact releases carbon dioxide.Method:Match the use of carbon dioxide to the life process that needs it as a raw material.Examiner tips
- Nutrition in plants is photosynthesis, which uses carbon dioxide.
- Step 1: Birds are the only group with feathers, and they lay hard-shelled eggs. Step 2: So hard-shelled eggs and feathers together are characteristic only of birds. Step 3: Hair is a mammal feature, wings are also found in insects and bats, and soft-shelled eggs are laid by reptiles.Method:Pick the pair of features that both belong only to birds.Examiner tips
- Feathers are unique to birds; mammals have hair and reptiles have scales.
- Step 1: Both plant and animal cells have a cell membrane and a nucleus. Step 2: So cell membranes and nuclei are found in both types of cell. Step 3: Cell walls and chloroplasts are found only in plant cells, so any pair including them is not common to both.Method:Choose the pair of structures present in animal cells as well as plant cells.Examiner tips
- Animal cells lack a cell wall, chloroplasts and a large permanent vacuole.
- Step 1: Active transport moves particles from a low to a high concentration, so statement 1 is correct and statement 2 is wrong. Step 2: It needs energy from respiration, so statement 3 is correct, and because it needs respiration it can only happen in living cells, so statement 4 is correct. Step 3: Therefore statements 1, 3 and 4 are correct.Method:Judge each statement against the definition of active transport and combine the correct ones.Examiner tips
- Active transport: low to high concentration, using energy, in living cells only.
- Step 1: In DNA the bases pair in a fixed way: adenine pairs with thymine, and cytosine pairs with guanine. Step 2: So thymine and adenine form a correct base pair. Step 3: The other combinations mix bases that do not pair together.Method:Recall the complementary base pairing rules and find the matching pair.Examiner tips
- A pairs with T; C pairs with G.
- Step 1: Protein is detected with the biuret test, which does not need heating. Step 2: A positive biuret test changes the colour from blue to purple. Step 3: The Benedict’s test is for reducing sugars, needs heating and gives a brick-red colour, so it is not the protein test.Method:Match the protein test to its reagent, heating requirement and colour change.Examiner tips
- Biuret + no heat + blue to purple identifies protein.
- Step 1: Spongy mesophyll has many air spaces that allow efficient gas exchange. Step 2: The long, rectangular cells packed with chloroplasts to absorb light are the palisade mesophyll, not the spongy mesophyll. Step 3: So spongy mesophyll has air spaces but not long rectangular light-absorbing cells.Method:Separate the air-space adaptation (spongy) from the tall-cell adaptation (palisade).Examiner tips
- Spongy mesophyll = air spaces for gas exchange; palisade = tall cells for light.
- Step 1: The child eats more vitamin C (54 mg) than the RDA (50 mg) and exactly the RDA of iron (11 mg), so scurvy and anaemia are not risks. Step 2: The child eats only 5 units of vitamin D against an RDA of 10, so vitamin D is short, which causes rickets. Step 3: Therefore only condition 2, rickets, is a risk.Method:Find which nutrient is eaten below its RDA, then name the matching deficiency disease.Examiner tips
- Vitamin D shortage causes rickets; iron shortage causes anaemia; vitamin C shortage causes scurvy.
- Step 1: Assimilation is the taking in and use of absorbed nutrients by the cells of the body. Step 2: So the uptake and use of nutrients by cells is assimilation. Step 3: Absorption is the movement of nutrients into the blood, ingestion is taking food into the body, and egestion is removing undigested waste.Method:Recall the definitions of the nutrition terms and match the one about cells using nutrients.Examiner tips
- Assimilation = cells using nutrients; absorption = nutrients entering the blood.
- Step 1: Trypsin is a protease, so it breaks down protein. Step 2: Trypsin is made in the pancreas and works in the small intestine, so it breaks down protein in the small intestine. Step 3: Maltose is broken down by maltase, and the protease in the stomach is pepsin, not trypsin.Method:Identify trypsin as a protease and recall where in the gut it acts.Examiner tips
- Trypsin = a protease acting in the small intestine; pepsin acts in the stomach.
- Step 1: A source makes or releases sugars, and a sink uses or stores them. Step 2: In spring the green leaves photosynthesise and export sugars, so they are sources. Step 3: The growing flowers use up sugars, so they are sinks.Method:Decide which parts make sugars (sources) and which use them (sinks) in spring.Examiner tips
- Sources release sugars (leaves); sinks use or store them (flowers, roots).
- Step 1: In a single circulation the blood passes through the heart only once per circuit. Step 2: Blood leaving the heart goes first to the gills to be oxygenated, then on to the body, including the muscles. Step 3: From the muscles it returns straight to the heart, so the order is gills, muscles, heart.Method:Trace blood from the heart through the gills and body and back to the heart.Examiner tips
- Fish single circulation: heart to gills to body to heart.
- Step 1: The cholera toxin acts on the wall of the gut. Step 2: It causes water to move out of the blood and into the gut. Step 3: This extra water in the gut produces severe watery diarrhoea, which can cause dehydration.Method:Connect the watery diarrhoea of cholera to water moving into the gut.Examiner tips
- Cholera toxin draws water into the gut, causing diarrhoea.
- Step 1: Cell division and muscle contraction both need energy released by respiration. Step 2: Diffusion and osmosis are passive processes that do not require energy from respiration. Step 3: So only processes 1 and 3 require energy from respiration.Method:Separate the energy-requiring processes from the passive ones.Examiner tips
- Cell division and muscle contraction use energy; diffusion and osmosis do not.
- Step 1: The glomerulus is a knot of capillaries where blood is filtered under pressure. Step 2: Small molecules such as water, glucose, urea and ions are filtered out of the blood into the tubule. Step 3: Reabsorption happens later along the tubule, protein synthesis happens at ribosomes, and deamination happens in the liver.Method:Identify the glomerulus as the filtering structure of the nephron.Examiner tips
- Glomerulus = filtration; tubule = reabsorption.
- Step 1: Rod cells are very sensitive to dim light, so they are used for night vision. Step 2: Rod cells cannot tell colours apart; colour vision is the job of cone cells. Step 3: So rod cells are used for night vision but not for colour vision.Method:Recall what rod cells detect and rule out colour vision.Examiner tips
- Rods = dim-light (night) vision; cones = colour vision.
- Step 1: The glands on top of the kidneys are the adrenal glands, which release adrenaline. Step 2: Adrenaline prepares the body for action, widening the pupils to let in more light. Step 3: Adrenaline also increases heart rate and blood glucose, so the decreases listed are wrong, and sexual characteristics develop in response to sex hormones.Method:Identify the adrenal glands and recall the effects of adrenaline.Examiner tips
- Adrenal glands release adrenaline, which widens the pupils and speeds the heart.
- Step 1: When the body overheats, the arterioles supplying the skin capillaries widen, which is vasodilation. Step 2: More blood then flows near the skin surface, so more heat is lost to the surroundings. Step 3: Vasoconstriction would reduce heat loss (a response to cold), and it is the arterioles, not the veins, that control this flow.Method:Choose the change that increases blood flow to the skin to lose heat.Examiner tips
- Overheating to vasodilation of skin arterioles to more heat loss.
- Step 1: A tropism is a growth response to a stimulus, and phototropism is a response to light. Step 2: A shoot growing towards light is phototropism. Step 3: Growth in response to gravity is gravitropism, and reactions using light such as photosynthesis are not tropisms.Method:Pick the growth response to light, not a response to gravity or a chemical reaction.Examiner tips
- Phototropism = growth towards or away from light.
- Step 1: Antibiotics work on structures and processes found in bacteria, so they kill bacteria. Step 2: Viruses do not have these structures, so antibiotics have no effect on viruses. Step 3: So antibiotics kill bacteria but do not affect viruses.Method:Recall that antibiotics act on bacteria only.Examiner tips
- Use antibiotics for bacterial infections, not viral ones.
- Step 1: Producing offspring from one parent by budding is asexual reproduction. Step 2: Asexual reproduction involves no gametes, so the offspring is genetically identical to the parent. Step 3: So the parent and offspring are genetically identical and this is asexual reproduction.Method:Recognise budding as asexual reproduction and recall that it gives identical offspring.Examiner tips
- Budding = asexual reproduction = genetically identical offspring.
- Step 1: Mitosis copies every chromosome and shares the copies equally between two daughter cells. Step 2: Each daughter cell therefore has the same number of chromosomes as the parent and an identical set. Step 3: Halving the chromosome number happens in meiosis, and the number is never doubled in the daughter cells.Method:Recall that mitosis gives identical daughter cells with the same chromosome number.Examiner tips
- Mitosis: same number of chromosomes, genetically identical.
- Step 1: The first cross gives a mixture of red and white hair (roan), so neither allele is fully dominant; both are expressed together, which is codominance. Step 2: Crossing two roan (heterozygous) cattle then gives 1 red : 2 roan : 1 white, the ratio expected from codominance. Step 3: If one colour were dominant the offspring would not show the blended roan phenotype.Method:Recognise the blended roan phenotype and 1:2:1 ratio as codominance.Examiner tips
- A blended heterozygous phenotype and a 1:2:1 ratio signal codominance.
- Step 1: All body cells from the same organism contain the same genes, so statement 2 is correct. Step 2: A specialised cell only switches on (expresses) the genes for the proteins it needs, so statement 3 is correct. Step 3: Different specialised cells express different genes, so statement 1 is wrong.Method:Judge each statement against how specialised cells use a shared set of genes.Examiner tips
- Same genes in every cell, but each cell type expresses a different set.
- Step 1: In a hot, dry habitat a plant must reduce water loss. Step 2: Few stomata, a thick waxy cuticle and a small surface area all reduce the amount of water lost. Step 3: Many stomata, a thin cuticle or a large surface area would all increase water loss, so they are not suited to dry habitats.Method:Choose the features that all reduce water loss in a hot, dry habitat.Examiner tips
- Dry-habitat leaves: few stomata, thick cuticle, small surface area.
- Step 1: Natural selection happens without human involvement, driven by the environment. Step 2: Wild animals evolving long necks to reach high leaves is natural selection with no humans involved. Step 3: Artificial selection is carried out by humans, antibiotic resistance arises by natural selection, and making insulin is genetic engineering, so the other rows are mismatched.Method:Check that the type of selection, human involvement and example are all consistent.Examiner tips
- Natural selection = no humans; artificial selection = chosen by humans.
- Step 1: A pyramid of energy has a bar for each trophic level whose size shows the energy at that level. Step 2: So it shows the energy stored at each trophic level. Step 3: It does not directly show the energy lost between levels, the input from the Sun, or one total for the whole ecosystem.Method:Recall that a pyramid of energy displays energy level by level.Examiner tips
- A pyramid of energy shows the energy at each trophic level.
- Step 1: Nitrate appeared only in the unheated soil, so something in that soil turned the added ammonium into nitrate. Step 2: Nitrifying bacteria convert ammonium ions into nitrate ions; heating to 100 degrees Celsius killed them in the heated sample, so no nitrate formed. Step 3: All nitrate had already been removed, so it was not broken down, and ammonium was added directly, so nitrogen fixation was not needed.Method:Identify the step that converts ammonium to nitrate and the bacteria that heating destroyed.Examiner tips
- Nitrifying bacteria turn ammonium ions into nitrate ions.
- Step 1: Biodiversity is the number of different species found in an area. Step 2: So the term for the number of different species living in an area is biodiversity. Step 3: Conservation is protecting species, an ecosystem is a community plus its environment, and a population is all the members of one species.Method:Match the definition about the variety of species to the term biodiversity.Examiner tips
- Biodiversity = the variety of different species in an area.
- Step 1: Eutrophication is caused by extra nutrients, such as nitrates, entering water. Step 2: Sewage adds these nutrients to water, so it can cause eutrophication. Step 3: Carbon dioxide and methane are greenhouse gases, and non-biodegradable plastic pollutes physically, so none of these cause eutrophication.Method:Pick the pollutant that adds nutrients to water.Examiner tips
- Sewage and fertiliser run-off add nutrients that cause eutrophication.
- Step 1: First the plasmid is cut open with restriction enzymes (process 1) so the human gene can be added. Step 2: The recombinant plasmids are then inserted into bacteria (process 3), and those bacteria are allowed to multiply (process 4). Step 3: Finally the bacteria express the human gene to make insulin (process 2), giving the order 1, 3, 4, 2.Method:Order the steps logically from cutting the plasmid to expressing the insulin gene.Examiner tips
- Cut the plasmid, insert the gene, multiply the bacteria, then make insulin.
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