May/June 2025 Paper 33 Worked Answers (IGCSE Biology 0610 Core)
38 questions · 80 marks · 75 minutes
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Worked answers for 32 questions
- Step 1: The seven characteristics of living organisms are movement, respiration, sensitivity, growth, reproduction, excretion and nutrition. Step 2: Both respiration (releasing energy from food) and reproduction (making new bacteria) are on this list. Step 3: Evaporation, condensation, dissolving and freezing are physical changes, not characteristics of living organisms.Method:Recall the seven characteristics of living organisms and pick the pair that are both on the list.Examiner tips
- Learn MRS GREN: movement, respiration, sensitivity, growth, reproduction, excretion, nutrition.
- Step 1: The number rose from a low value in 2010 to its highest value in 2020, so overall it increased. Step 2: The steepest two-year rise was between 2014 and 2016. Step 3: The highest value, reached in 2020, was 205 per 100 000 people, so the correct set is increased, 2014 to 2016, 205.Method:Interpret the trend, the steepest change and the final data value in turn.Examiner tips
- Read the overall trend, the steepest section and the final value separately.
- Step 1: A population of 800 000 contains 800 000 ÷ 100 000 = 8 groups of 100 000 people. Step 2: Each group of 100 000 has 100 infected people. Step 3: So the number infected is 100 × 8 = 800 people.Method:Scale the rate of 100 per 100 000 up to the population of 800 000.Examiner tips
- Number infected = rate per 100 000 × (population ÷ 100 000).
- Step 1: A mutation can make some bacteria resistant, so the antibiotic no longer kills them. Step 2: These resistant bacteria survive and reproduce, so the resistant type becomes more common each year. Step 3: As more bacteria become resistant, fewer are killed by the antibiotic.Method:Link the falling effectiveness of the antibiotic to the spread of resistant bacteria.Examiner tips
- Bacteria that survive an antibiotic are described as resistant.
- Step 1: Barrier methods such as condoms stop the pathogen passing between people during sex. Step 2: Testing identifies infected people so they can be treated, and treatment removes the source of infection. Step 3: Contact tracing finds and treats other people who may be infected, so all three methods in the correct option help control the spread.Method:Pick the option whose methods all reduce transmission of a sexually transmitted infection.Examiner tips
- Control of STIs uses barrier methods, testing, treatment, contact tracing and education.
- Step 1: The wide band of rounded packing cells between the surface of a root and its central tissues is the cortex. Step 2: So the cells of region D are cortex cells. Step 3: Palisade and guard cells are found in leaves and red blood cells are found in animals, so none of these is correct.Method:Name the packing tissue of the root by its position.Examiner tips
- The packing layer of a root is the cortex.
- Step 1: Xylem vessels carry water and dissolved mineral ions up from the roots through the plant. Step 2: The thick walls of xylem also help to support the plant. Step 3: Transporting sugars and amino acids is the job of phloem, and photosynthesis happens in leaf cells, so only the first option gives two functions of xylem.Method:Recall that xylem moves water and minerals upwards and helps support the plant.Examiner tips
- Xylem carries water and mineral ions and gives support.
- Step 1: A long, thin outgrowth from a surface cell of a root that reaches into the soil is a root hair cell. Step 2: It increases the surface area for absorbing water and mineral ions. Step 3: Guard cells are in leaves, and nerve cells and sperm cells are animal cells, so none of these is correct.Method:Name the absorbing outgrowth on the surface of a root.Examiner tips
- The thin outgrowths on a root that absorb water are root hair cells.
- Step 1: Osmosis is the movement of water through a partially permeable membrane from a dilute solution to a more concentrated solution. Step 2: Water enters the root hair cell in exactly this way, so the process is osmosis. Step 3: Active transport moves ions using energy, respiration releases energy, and transpiration is water loss from leaves, so none of these describes water entering the root.Method:Identify the movement of water down a water gradient as osmosis.Examiner tips
- Water entering a cell across a partially permeable membrane is osmosis.
- Step 1: The long, thin shape of a root hair cell gives it a large surface area in contact with the soil. Step 2: A larger surface area allows more water to be absorbed by osmosis. Step 3: A waxy cuticle would reduce uptake, chloroplasts are for photosynthesis, and root cells have no nerve endings, so the key feature is the large surface area.Method:Link the long thin shape to a large surface area for absorbing water.Examiner tips
- A large surface area speeds up the absorption of water.
- Step 1: Active transport moves substances across a cell membrane against the concentration gradient, so it needs a membrane and energy from respiration. Step 2: Diffusion is the net movement of a substance from a higher to a lower concentration, so a substance moves down a concentration gradient. Step 3: Diffusion does not need energy from respiration, so only the first option matches both processes.Method:Match each feature to the correct process by recalling how each one works.Examiner tips
- Active transport: energy + membrane, against the gradient. Diffusion: no energy, down the gradient.
- Step 1: Aerobic respiration releases energy from glucose using oxygen inside the mitochondria. Step 2: So the structures where aerobic respiration occurs are the mitochondria. Step 3: Chloroplasts carry out photosynthesis, ribosomes make proteins, and the vacuole stores cell sap, so none of these is correct.Method:Recall the cell structure that releases energy by aerobic respiration.Examiner tips
- Aerobic respiration happens in the mitochondria.
- Step 1: A hormone is a chemical substance made by an endocrine gland. Step 2: It is carried around the body dissolved in the blood. Step 3: It then alters the activity of specific target organs, so the first option gives the full meaning of a hormone.Method:Define a hormone by its source, its transport and its effect on target organs.Examiner tips
- Hormone = chemical from a gland, carried in the blood, acting on target organs.
- Step 1: Hormones are carried slowly in the blood, so they act more slowly than the fast electrical impulses of the nervous system. Step 2: Hormones may be involved in homeostasis, for example controlling blood glucose. Step 3: Hormones are chemicals, not impulses, and their effects usually last longer, not shorter, than nervous effects.Method:Contrast the slow chemical action of hormones with the fast nervous system.Examiner tips
- Hormones: slower, longer-lasting, carried in blood, and can help control homeostasis.
- Step 1: Adrenaline prepares the body for action (the fight-or-flight response). Step 2: The heart pumps more blood carrying oxygen to the muscles, and faster breathing removes more carbon dioxide. Step 3: The pupils widen to let in more light, so the first option correctly completes all four effects.Method:Use the fight-or-flight role of adrenaline to choose each correct word.Examiner tips
- Adrenaline = action: more oxygen to muscles, more carbon dioxide breathed out, wider pupils.
- Step 1: Photosynthesis uses carbon dioxide and water as its raw materials. Step 2: Using light energy, these are converted into glucose and oxygen. Step 3: So the word equation is carbon dioxide + water → glucose + oxygen; the reverse equation describes respiration.Method:Write the raw materials on the left and the products on the right.Examiner tips
- Photosynthesis: carbon dioxide + water → glucose + oxygen (using light).
- Step 1: Photosynthesis needs the green pigment chlorophyll, which is held inside the chloroplasts. Step 2: So photosynthesis takes place in the chloroplasts. Step 3: The nucleus controls the cell, the mitochondrion is for respiration, and the cell wall gives support, so none of these is the site of photosynthesis.Method:Recall the green cell structure that holds chlorophyll.Examiner tips
- Chloroplasts contain chlorophyll and are the site of photosynthesis.
- Step 1: The green colour of a leaf comes from the pigment chlorophyll. Step 2: The white parts of a variegated leaf have no chlorophyll, which is why they are not green. Step 3: Haemoglobin is in blood, glucose is a sugar, and cellulose makes the cell wall, so the green pigment is chlorophyll.Method:Name the green pigment that is absent from the white parts of the leaf.Examiner tips
- Chlorophyll is the green pigment; white leaf areas lack it.
- Step 1: The decrease in bubbles is 13 − 3 = 10 bubbles per minute. Step 2: Percentage decrease = (decrease ÷ original) × 100 = (10 ÷ 13) × 100 = 76.9. Step 3: Rounded to a whole number this is 77 %.Method:Find the fall in bubbles, divide by the starting value and multiply by 100.Examiner tips
- Divide the fall by the original (15 cm) value, then multiply by 100.
- Step 1: The further the lamp is from the plant, the lower the light intensity reaching it. Step 2: Light is needed for photosynthesis, so less light means a slower rate of photosynthesis. Step 3: The bubbles are the oxygen released by photosynthesis, so a slower rate produces fewer bubbles each minute.Method:Connect distance to light intensity, then to the rate of photosynthesis shown by the bubbles.Examiner tips
- Greater distance means lower light intensity, so slower photosynthesis and fewer oxygen bubbles.
- Step 1: The wide muscular organ where a baby develops is the uterus, so L is the uterus. Step 2: The canal leading to the outside of the body is the vagina, so M is the vagina. Step 3: The narrow ring of muscle between the uterus and the vagina is the cervix, so N is the cervix.Method:Use the position and description of each structure to name the uterus, cervix and vagina.Examiner tips
- Uterus = where a baby grows; cervix = ring of muscle; vagina = canal to the outside.
- Step 1: An egg cell contains energy stores in its cytoplasm to nourish the early embryo. Step 2: It has a jelly coat that changes after fertilisation to stop more than one sperm entering. Step 3: A tail and many mitochondria are features of a sperm cell, a waxy cuticle is a plant feature, and a biconcave shape is a red blood cell feature, so only the first option describes egg cells.Method:Pick the two features that suit a large food-storing gamete, not a sperm or other cell.Examiner tips
- Egg cells: energy stores for the embryo and a jelly coat that changes after fertilisation.
- Step 1: Discontinuous variation has a limited number of distinct phenotypes with no intermediates, so that statement is correct. Step 2: Variation may be produced by mutation, which is also correct. Step 3: Variation is the differences between individuals, not the similarities between different species, and it is caused by both genes and the environment, not genes only.Method:Judge each statement against the definitions of variation and discontinuous variation.Examiner tips
- Discontinuous variation = distinct groups; mutation is a source of variation.
- Step 1: After choosing the most drought-tolerant parents, the farmer breeds these chosen plants together. Step 2: From their offspring, the farmer again chooses those with the highest drought tolerance. Step 3: Repeating this selection and breeding over many generations gradually produces plants that tolerate drought well.Method:Follow the selective breeding cycle of breeding and re-selecting the best individuals.Examiner tips
- Breed the best, select the best offspring, and repeat over many generations.
- Step 1: When humans choose which organisms breed together to improve a feature, the process is selective breeding. Step 2: The farmer is choosing the most drought-tolerant plants to breed, so this is selective breeding. Step 3: Natural selection is driven by the environment, genetic engineering changes genes directly, and cloning makes identical copies, so none of these matches.Method:Name the human-directed breeding process as selective breeding.Examiner tips
- Humans choosing the parents to breed = selective breeding (artificial selection).
- Step 1: In a binomial name the first word is the genus and the second word is the species. Step 2: In Vicia faba, the first word is Vicia, so Vicia is the genus. Step 3: "faba" is the species, "Vicia faba" is the whole name, and "bean" is a common name, so only Vicia is the genus.Method:Take the first word of the binomial name as the genus.Examiner tips
- Genus first (capital letter), then species; the genus of Vicia faba is Vicia.
- Step 1: A yellow-spotted plant must be bb, so all its gametes carry b. Step 2: Crossing Bb with bb gives offspring Bb, bb, Bb, bb. Step 3: Bb plants have black spots and bb plants have yellow spots, so the phenotypic ratio is 1 black-spotted : 1 yellow-spotted.Method:Work out the genotypes, fill the Punnett square and count the phenotypes.Examiner tips
- Yellow (recessive) = bb, so its gametes are both b; Bb × bb gives a 1:1 ratio.
- Step 1: A trophic level is a feeding stage in the web. Step 2: Counting up the longest chain gives reed grasses (1), snails or beetles (2), cichlid fish (3), chambo fish (4), and kampango catfish or fish eagles (5). Step 3: So there are five trophic levels.Method:Count the feeding stages in the longest chain from producer to top predator.Examiner tips
- Count each feeding stage, starting at the producer as level 1.
- Step 1: The second trophic level is the primary consumers that eat the producer, which are the snails and beetles, so the snail fits. Step 2: An organism feeding at more than one trophic level eats prey from different levels; the fish eagle eats both chambo fish and kampango catfish. Step 3: So the snail is at the second trophic level and the fish eagle feeds at more than one level.Method:Place the plant-eaters at level two and find a predator that eats prey from several levels.Examiner tips
- Second level = primary consumers (plant-eaters); top predators often feed at more than one level.
- Step 1: Overfishing removes chambo fish faster than they can reproduce, lowering the population. Step 2: A fall in the number of cichlid fish reduces the food available to the chambo fish, so fewer survive. Step 3: Fewer predators, more food, less pollution and a fishing ban would all tend to increase the population, not decrease it.Method:Choose factors that reduce survival or food supply, not ones that help the population grow.Examiner tips
- Populations fall when more individuals are removed or food becomes scarce.
- Step 1: Protecting habitats keeps the places where endangered species live, so they can survive and breed. Step 2: Captive breeding programmes increase the numbers of a species in safe conditions. Step 3: Seed banks and zoos store species so they can be bred again in future, so all three methods in the correct option conserve species.Method:Pick the option whose three methods all protect or increase endangered species.Examiner tips
- Conserve species by protecting habitats, captive breeding, and using seed banks and zoos.
- Step 1: A sustainable resource is replaced at the same rate as it is used. Step 2: Because it is produced as fast as it is removed, the supply does not run out. Step 3: Using a resource up quickly, never using it, or removing it faster than it is replaced are all the opposite of being sustainable.Method:Define a sustainable resource by balancing the rate of removal with the rate of replacement.Examiner tips
- Sustainable = replaced as fast as it is used, so it does not run out.
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