May/June 2025 Paper 31 Worked Answers (IGCSE Biology 0610 Core)
39 questions · 80 marks · 75 minutes
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Worked answers for 34 questions
- Step 1: The nucleus holds the chromosomes, which carry the genetic material (DNA). Step 2: Using these instructions, the nucleus controls the activities of the cell. Step 3: Releasing energy is the job of mitochondria, controlling entry and exit is the membrane, and photosynthesis happens in chloroplasts.Method:Recall that the nucleus stores DNA and directs the cell.Examiner tips
- Nucleus = genetic material + control centre of the cell.
- Step 1: Every cell, animal or plant, is surrounded by a cell membrane and filled with cytoplasm. Step 2: Ribosomes are also found in all cells, so cell membrane and cytoplasm are a correct pair. Step 3: Cell walls, chloroplasts and large permanent vacuoles are found only in plant cells, not in all cells.Method:Pick structures common to both animal and plant cells.Examiner tips
- Cell membrane, cytoplasm and ribosomes are in every cell; wall, chloroplasts and large vacuole are plant-only.
- Step 1: The long projection gives the root hair cell a large surface area. Step 2: This large surface area, together with a large permanent vacuole, speeds up the uptake of water and mineral ions from the soil. Step 3: Chloroplasts are for photosynthesis, a waxy layer reduces water loss in leaves, and cilia move mucus, so none of these fit a root hair cell.Method:Link the shape of the cell to faster absorption of water and minerals.Examiner tips
- Root hair cell = large surface area + large vacuole for absorbing water and mineral ions.
- Step 1: Actual size = image size ÷ magnification, so both the image size and the magnification are needed. Step 2: Measuring the image and knowing the magnification lets you calculate the real length. Step 3: The number of cells, the stain colour and the cell width do not appear in this calculation.Method:Recall the magnification equation and pick the two quantities it uses.Examiner tips
- Actual size = image size ÷ magnification, so you need image size and magnification.
- Step 1: The bubble count rises (5→21) as carbon dioxide increases, and carbon dioxide is a raw material used in photosynthesis. Step 2: The count is 21 at both 0.4 and 0.5, so it stays the same between 0.4 and 0.5 arbitrary units, and the gas released as bubbles is oxygen. Step 3: Keeping the lamp at a fixed distance keeps the light intensity constant, so the correct set is increases; raw; 0.4 and 0.5; oxygen; intensity.Method:Read the trend from the data, then recall the raw materials and products of photosynthesis.Examiner tips
- Carbon dioxide is a raw material; oxygen is the product; lamp distance controls light intensity.
- Step 1: The green pigment is chlorophyll, found in the chloroplasts. Step 2: Chlorophyll absorbs light energy and transfers it to chemical reactions. Step 3: This energy is used to make carbohydrates such as glucose in photosynthesis, so chlorophyll is essential for the plant’s nutrition.Method:Name the pigment, then state how it captures light energy for photosynthesis.Examiner tips
- Chlorophyll absorbs light energy and transfers it to make glucose.
- Step 1: Respiration breaks down glucose to release energy and produces carbon dioxide as a waste gas. Step 2: Plants respire all the time, so respiration is the process that produces carbon dioxide. Step 3: Photosynthesis uses carbon dioxide, while transpiration is water loss and translocation moves sugars, so none of these produce carbon dioxide.Method:Recall which process releases carbon dioxide as a waste gas.Examiner tips
- Respiration produces carbon dioxide; photosynthesis uses it.
- Step 1: Physical digestion (churning) occurs in the stomach, which also releases hydrochloric acid. Step 2: The pancreas secretes the enzymes protease and amylase into the small intestine. Step 3: The pancreas also secretes the hormone insulin, so the order is stomach; stomach; pancreas; pancreas.Method:Match each function to the organ that performs it in the digestive system.Examiner tips
- Stomach: churning + acid. Pancreas: protease, amylase, lipase + insulin.
- Step 1: Hydrochloric acid kills many of the microorganisms taken in with food. Step 2: It also makes the stomach contents acidic, giving the optimum (low) pH for the protease enzymes in the stomach to work. Step 3: Bile emulsifies fats, the small intestine absorbs food, and blood carries gases, so these are not functions of stomach acid.Method:Recall the two roles of stomach acid and reject jobs done by bile or blood.Examiner tips
- Stomach acid: kills microorganisms + gives the optimum pH for protease.
- Step 1: Protease is an enzyme that digests large protein molecules. Step 2: It breaks proteins down into amino acids, which are small and soluble. Step 3: Being small and soluble, the amino acids can be absorbed into the blood, so protease is important for making proteins absorbable.Method:State that protease is an enzyme, name its product, and explain why that helps absorption.Examiner tips
- Protease digests protein into amino acids that are small and soluble enough to absorb.
- Step 1: Iodine solution turns blue-black when starch is present and stays yellow-brown when it is not. Step 2: Tube X has starch only, so it turns blue-black. Step 3: In tube Y the amylase digests the starch into sugars, so no starch remains and the iodine stays yellow-brown.Method:Decide which tube still has starch, then apply the iodine colour change.Examiner tips
- Starch present → blue-black; starch digested → yellow-brown.
- Step 1: Iodine only turns blue-black if starch is present. Step 2: In Y the amylase has broken the starch down into sugars, so no starch is left and the colour is different. Step 3: In X there is no amylase, so the starch remains and gives the blue-black colour, which is why the two tubes differ.Method:Explain the colour difference by where the starch has been digested.Examiner tips
- Different colours = starch digested in one tube but not the other.
- Step 1: Sense organs are made of receptor cells that detect changes. Step 2: The changes they detect are called stimuli. Step 3: Common stimuli include light, sound, touch, temperature and chemicals, so the missing stimulus in the list is sound, giving receptor; stimuli; sound.Method:Recall the definition of a sense organ and the named list of stimuli.Examiner tips
- Receptor cells detect stimuli such as light, sound, touch, temperature and chemicals.
- Step 1: In dim light the circular muscles of the iris relax and the radial muscles contract, so the pupil widens (its diameter increases). Step 2: A wider pupil lets more light into the eye. Step 3: This extra light helps the person to see in the dim conditions, so the pupil widens to allow more light in.Method:State how the pupil changes in dim light, then explain why more light is needed.Examiner tips
- Dim light → pupil widens → more light enters. Bright light → pupil narrows.
- Step 1: The blind spot is the part of the retina that contains no light receptor cells. Step 2: This is the point where the optic nerve leaves the back of the eye. Step 3: The cornea, iris and lens are at the front of the eye and are not the blind spot, which is at the back where the nerve exits.Method:Locate the blind spot at the exit point of the optic nerve from the retina.Examiner tips
- Blind spot = where the optic nerve leaves the retina, so no receptors are present.
- Step 1: The biggest percentage change is +7.55 at 0.10 mol per dm³, so that cylinder showed the largest change. Step 2: At 1.00 mol per dm³ the mass falls (−5.28), because water leaves the cylinder by osmosis into the more concentrated solution. Step 3: All the mass changes happen because water crosses the partially permeable cell membranes, so those three conclusions are correct.Method:Compare the percentage values and apply the idea that water moves by osmosis.Examiner tips
- Osmosis moves water across the membrane; the largest percentage value marks the largest change.
- Step 1: Water moves across a partially permeable membrane from a dilute to a more concentrated solution. Step 2: This movement of water is called osmosis. Step 3: Active transport moves ions against a gradient using energy, respiration releases energy, and transpiration is water loss from leaves, so the process here is osmosis.Method:Name the process by which water crosses the cell membrane.Examiner tips
- Osmosis = movement of water across a partially permeable membrane.
- Step 1: Higher temperature gives water molecules more energy, speeding up osmosis. Step 2: A larger surface area of the cylinders lets water cross the membranes faster. Step 3: Colour, time of day, beaker mass and other irrelevant factors do not affect the rate, so temperature and surface area are the influencing factors.Method:Pick the variables that change how fast water crosses the membranes.Examiner tips
- Rate of osmosis rises with higher temperature and larger surface area.
- Step 1: As water enters, the plant cell swells and presses outwards on its surroundings. Step 2: The strong cellulose cell wall is rigid and resists this pressure, stopping the cell from bursting. Step 3: The membrane, cytoplasm and nucleus cannot provide this support, so it is the cell wall that prevents bursting.Method:Identify the rigid plant-cell structure that resists the pressure of incoming water.Examiner tips
- The cellulose cell wall is rigid and prevents a plant cell from bursting.
- Step 1: Water dissolves many substances, so it acts as a solvent in the body. Step 2: Blood, which is mostly water, transports dissolved substances around the body, so water is used for transport. Step 3: Enzymes are proteins, genetic material is DNA, and phagocytosis is carried out by white blood cells, so those are not uses of water.Method:Choose the two roles that describe water dissolving and carrying substances.Examiner tips
- Water acts as a solvent and as the transport medium of the blood.
- Step 1: A balanced diet provides carbohydrates, proteins, fats, vitamins, minerals, fibre and water. Step 2: Carbohydrate and protein are two of these named components. Step 3: Oxygen and carbon dioxide are gases, urea and bile are body wastes/secretions, and haemoglobin and insulin are made in the body, so none of these are dietary components.Method:Recall the seven components of a balanced diet and pick two.Examiner tips
- Balanced diet = carbohydrate, protein, fat, vitamins, minerals, fibre and water.
- Step 1: Arteries have thick muscular walls and a narrow lumen to withstand high pressure, so vessel Z is an artery. Step 2: Veins have thinner walls and a large lumen to carry blood at low pressure, so vessel X is a vein. Step 3: Capillaries have very thin walls (one cell thick) and a very narrow lumen, so vessel Y is a capillary.Method:Match each set of wall and lumen features to the correct vessel type.Examiner tips
- Artery: thick wall, narrow lumen. Vein: thin wall, large lumen. Capillary: one cell thick.
- Step 1: Veins contain valves that arteries and capillaries do not have. Step 2: Because blood in veins is at low pressure, these valves stop the blood flowing backwards. Step 3: They ensure a one-way flow of blood back to the heart; valves do not pump, and cilia and villi are found elsewhere.Method:Name the structure unique to veins and state how it controls blood flow.Examiner tips
- Valves in veins ensure one-way flow of blood back to the heart.
- Step 1: The kidneys filter the blood and remove urea, a waste made in the liver. Step 2: They also remove excess water and excess mineral ions to keep the body balanced. Step 3: Glucose and amino acids are reabsorbed, gases are exchanged in the lungs, and cells and proteins are too large to be filtered, so the kidneys excrete urea and excess water.Method:Recall what the kidney removes from the blood and reject reabsorbed substances.Examiner tips
- Kidneys excrete urea, excess water and excess mineral ions.
- Step 1: An organ system is a group of organs working together, such as the nervous system and the respiratory (gas exchange) system. Step 2: Other human organ systems include the reproductive, excretory and endocrine systems. Step 3: Chloroplasts and vacuoles are cell parts, haemoglobin and enzymes are molecules, and xylem and phloem are plant tissues, so they are not human organ systems.Method:Pick two groups of organs that form recognised human systems.Examiner tips
- Human organ systems include nervous, respiratory, reproductive, excretory and endocrine.
- Step 1: There are 10 millimetres in 1 centimetre. Step 2: So 0.5 cm = 0.5 × 10 = 5 mm. Step 3: The smallest macroplastic is therefore 5 mm in diameter.Method:Convert centimetres to millimetres by multiplying by ten.Examiner tips
- 1 cm = 10 mm, so 0.5 cm = 5 mm.
- Step 1: The macroplastic mass is zero until 1970 and then increases. Step 2: The microplastic mass increases to a maximum in 2000 and then decreases. Step 3: So macroplastics rise from 1970 onwards while microplastics rise then fall, peaking in 2000.Method:Take each plastic type separately and describe how its mass changes with time.Examiner tips
- Describe each curve in turn, quoting the years where they start, peak or change.
- Step 1: Animals may mistake floating plastic for food and eat it, which can block or damage their gut. Step 2: Animals can also become trapped or tangled in larger pieces of plastic, which can injure or suffocate them. Step 3: Plastic is not a food or a benefit, so the harmful effects are being eaten and trapping animals.Method:Pick two genuine harmful effects of plastic on marine animals.Examiner tips
- Plastics harm animals by being eaten or by trapping and choking them.
- Step 1: Pollution adds harmful substances to water. Step 2: Fertiliser washed off farmland (or untreated sewage) adds nutrients that pollute the water and can cause eutrophication. Step 3: Oxygen from plants, rainwater and sunlight are natural and do not pollute, so the source of pollution is fertiliser.Method:Choose a man-made input that adds harmful nutrients to the water.Examiner tips
- Fertiliser run-off and untreated sewage are common water pollutants.
- Step 1: The characteristics of living organisms are movement, respiration, sensitivity, growth, reproduction, excretion and nutrition. Step 2: Respiration and growth are two of these characteristics. Step 3: Evaporation, condensation, magnification, dilution, filtration and distillation are physical or laboratory processes, not life processes, so the answer is respiration and growth.Method:List the seven life processes and pick two that are not reproduction.Examiner tips
- MRS GREN: Movement, Respiration, Sensitivity, Growth, Reproduction, Excretion, Nutrition.
- Step 1: The female gamete (sex cell) is the egg cell, made in the ovary. Step 2: It is released ready to be fertilised by a sperm. Step 3: The sperm is the male gamete, a zygote forms after fertilisation, and an embryo is a later stage, so the female sex cell is the egg cell.Method:Name the female gamete released before fertilisation.Examiner tips
- The female gamete is the egg cell; the male gamete is the sperm.
- Step 1: Fertilisation is the joining (fusion) of a male gamete nucleus with a female gamete nucleus. Step 2: When a sperm nucleus fuses with an egg nucleus, this is fertilisation. Step 3: Pollination is in flowering plants, germination is a seed growing, and menstruation is loss of the uterus lining, so the process is fertilisation.Method:Name the fusion of the male and female gamete nuclei.Examiner tips
- Fertilisation = fusion of a sperm nucleus and an egg nucleus.
- Step 1: When the sperm and egg nuclei fuse, a single cell called the zygote is formed. Step 2: The zygote then divides to form an embryo. Step 3: A gamete is a sex cell before fertilisation, and an embryo and foetus are later stages, so the cell formed straight after fertilisation is the zygote.Method:Name the single cell produced when the gametes fuse.Examiner tips
- Zygote = the single cell formed at fertilisation, before it divides.
- Step 1: After fertilisation the zygote divides repeatedly as it moves down the oviduct. Step 2: It becomes a hollow ball of cells (stage 4) by the time it reaches the uterus. Step 3: This hollow ball implants (embeds) into the thick uterus lining, so stage 4 is the stage that implants.Method:Follow the dividing cells to the uterus and pick the stage that embeds in the lining.Examiner tips
- Implantation = the ball of cells embedding into the uterus lining.
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