October/November 2025 Paper 43 Worked Answers (IGCSE Biology 0610 Extended)
31 questions · 80 marks · 75 minutes
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Worked answers for 29 questions
- Step 1: In the binomial system the first part of the name is the genus and the second part is the species. Step 2: The genus is written first with a capital letter and the species second. Step 3: Kingdom, phylum, family and variety are other classification terms, not the two parts of a binomial name.Method:Recall that a scientific name is made of a genus followed by a species.Examiner tips
- A binomial name is written as genus then species.
- Step 1: Cell walls that are not made of cellulose, together with feeding by releasing enzymes and absorbing nutrients (saprophytic nutrition), are features of fungi. Step 2: Plants have cellulose cell walls and make their own food by photosynthesis. Step 3: Animals have no cell walls, and prokaryotes are single-celled, so the species is a fungus.Method:Match the non-cellulose walls and absorptive feeding to the fungus kingdom.Examiner tips
- Fungi: multicellular, non-cellulose walls, feed by external digestion and absorption.
- Step 1: Plant cells have a cellulose cell wall, chloroplasts and a large permanent vacuole, which animal cells do not have. Step 2: A nucleus, mitochondria, a cell membrane and ribosomes are found in both plant and animal cells. Step 3: So the cellulose cell wall and chloroplasts are the structures found only in plant cells.Method:Pick the two structures that plant cells have but animal cells lack.Examiner tips
- Plant-only structures: cellulose cell wall, chloroplasts, large permanent vacuole.
- Step 1: The base sequences in the DNA of organisms can be compared to study how closely they are related. Step 2: Species that share a more recent common ancestor have more similar base sequences. Step 3: Height, flower colour and soil type are affected by the environment and do not reliably show evolutionary relationships, so comparing DNA base sequences is the best method.Method:Choose DNA base-sequence comparison and link greater similarity to closer relationship.Examiner tips
- Compare DNA base sequences; more similar sequences mean a more recent common ancestor.
- Step 1: The change in mass is 4090 − 775 = 3315 g. Step 2: Percentage change = (change ÷ original) × 100 = (3315 ÷ 775) × 100 = 427.742…%. Step 3: Rounded to three significant figures this is 428%.Method:Calculate the change in mass, divide by the original mass and convert to a percentage.Examiner tips
- Percentage change = (change ÷ original value) × 100.
- Step 1: Chickens that mature faster reach a saleable size and begin producing eggs at a younger age. Step 2: This means the farmer can sell meat or collect eggs sooner. Step 3: Selling sooner, and over more cycles, increases the farmer's profit, so the first option gives the genuine advantages.Method:Link earlier maturity to selling meat and eggs sooner and to greater profit.Examiner tips
- Faster maturity = eggs and meat sooner = more profit for the farmer.
- Step 1: The farmer chooses the chickens with the largest body mass and breeds them together. Step 2: From their offspring, the farmer again selects those with the largest body mass to breed. Step 3: Repeating this selection over many generations gradually increases the body mass of the chickens.Method:Describe choosing the largest birds, breeding them and repeating the selection.Examiner tips
- Selective breeding: choose the best, breed them, repeat over generations.
- Step 1: The chicken faeces wash into the lake and add nitrate ions, a process called eutrophication. Step 2: The extra nitrate causes rapid growth of producers such as algae; when these die in large numbers, decomposers break them down. Step 3: The decomposers respire aerobically and use up the dissolved oxygen in the water, so aquatic organisms that need oxygen, such as fish, die and the habitat is destroyed.Method:Trace nitrate run-off through algal growth, decomposition and oxygen depletion to organism death.Examiner tips
- Eutrophication: nitrate in → algae grow → algae die → decomposers use up oxygen → organisms die.
- Step 1: Excess amino acids cannot be stored, so they are broken down in the liver by deamination. Step 2: Deamination removes the nitrogen-containing part of each amino acid. Step 3: This nitrogen-containing part is converted into urea, which is then carried in the blood to the kidneys to be excreted.Method:Describe deamination of excess amino acids in the liver to form urea.Examiner tips
- Urea forms by deamination of excess amino acids in the liver.
- Step 1: Water from the soil first enters a root hair cell by osmosis. Step 2: It then moves across the cells of the root cortex. Step 3: Finally it reaches the xylem, which carries it up the plant, so the first pathway is correct.Method:Trace water from the root hair cell across the cortex to the xylem.Examiner tips
- Soil → root hair cell → cortex cells → xylem.
- Step 1: The mass lost is 135.7 − 118.3 = 17.4 g. Step 2: This loss happened over three hours, so the rate is 17.4 ÷ 3. Step 3: 17.4 ÷ 3 = 5.8 g per hour.Method:Subtract the masses to find the loss, then divide by three hours.Examiner tips
- Rate = mass lost ÷ time = 17.4 ÷ 3 = 5.8 g per hour.
- Step 1: The plant at 25 °C had a higher transpiration rate (3.1 g per hour) than the plant at 15 °C (0.9 g per hour), so it lost water faster. Step 2: At the higher temperature the water molecules have more kinetic energy, so more water evaporates from the surfaces of the mesophyll cells inside the leaf. Step 3: This increases the diffusion of water vapour out through the stomata, so the transpiration rate is higher.Method:State the comparison of rates, then explain using kinetic energy, evaporation and diffusion.Examiner tips
- Higher temperature → more kinetic energy → faster evaporation and diffusion → faster transpiration.
- Step 1: Lower humidity means the air around the leaf contains less water vapour. Step 2: This makes a steeper concentration gradient of water vapour between the inside of the leaf and the air outside. Step 3: A steeper gradient means water vapour diffuses out through the stomata faster, so the transpiration rate increases.Method:Predict an increase and explain it using a steeper water-vapour concentration gradient.Examiner tips
- Lower humidity → steeper water-vapour gradient → faster diffusion → more transpiration.
- Step 1: The xylem transports water and mineral ions, and its cells form a long continuous tube with walls strengthened by lignin. Step 2: Water moves upwards because of transpiration pull. Step 3: This pull draws up a continuous column of water molecules, which are held together by forces of attraction (cohesion) between the water molecules.Method:Fill each gap using xylem structure and the transpiration-stream mechanism.Examiner tips
- Xylem: water + mineral ions; tube; lignin; transpiration pull; column; cohesive forces.
- Step 1: On a hot dry day water is lost by transpiration faster than it is taken up by the roots. Step 2: Water then moves out of the cells by osmosis, so the cells become flaccid and lose their turgor pressure. Step 3: Plants rely on this turgor pressure pushing on the cell walls for support, so without it the leaves and stem droop and the plant wilts.Method:Explain wilting as loss of turgor when transpiration exceeds water uptake.Examiner tips
- Wilting: water loss > uptake → cells lose turgor → no support → drooping.
- Step 1: The septum is the muscular wall down the middle of the heart that keeps the oxygenated blood on the left separate from the deoxygenated blood on the right. Step 2: The valves between each atrium and ventricle are the atrioventricular valves, which stop blood flowing back into the atria. Step 3: The aorta, vena cava and coronary artery are blood vessels, not the dividing wall, so only the first option is correct.Method:Name the central dividing wall as the septum and the atrium-to-ventricle valve as atrioventricular.Examiner tips
- The septum divides the heart; atrioventricular valves sit between atria and ventricles.
- Step 1: The left ventricle wall contains more muscle than the right ventricle wall. Step 2: More muscle lets it contract with greater force, creating a higher pressure. Step 3: This higher pressure is needed to pump blood all the way around the whole body, whereas the right ventricle only pumps blood the short distance to the lungs.Method:Link the thicker wall to greater force and the high pressure needed to reach the whole body.Examiner tips
- Left ventricle = more muscle = more force = high pressure to reach the whole body.
- Step 1: Blood from the vena cava enters the right atrium, then the right ventricle. Step 2: The right ventricle pumps it through the pulmonary artery to the lungs, and it returns through the pulmonary vein to the left atrium. Step 3: From the left atrium it passes to the left ventricle, which pumps it out through the aorta, so the first order is correct.Method:Trace blood through the right side, to the lungs, back to the left side, and out to the aorta.Examiner tips
- Vena cava → right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta.
- Step 1: During exercise the muscles respire faster, so the heart beats faster (helped by the hormone adrenaline) to deliver more oxygen and glucose to the muscles and to remove the extra carbon dioxide. Step 2: When the muscles respire anaerobically they produce lactic acid, creating an oxygen debt. Step 3: The heart rate stays high after exercise to keep supplying oxygen so the lactic acid can be broken down in the liver, which is why the rate only returns to normal slowly.Method:Explain the rise using faster respiration and adrenaline, and the slow return using the oxygen debt.Examiner tips
- Exercise: muscles respire more → faster heart (adrenaline) → deliver oxygen/glucose; stays high to repay oxygen debt.
- Step 1: A diet high in saturated fat or salt raises the risk of coronary heart disease. Step 2: Smoking tobacco and high levels of stress also increase the risk. Step 3: Eating fruit, exercising and keeping a healthy body mass lower the risk, so only the first option lists risk factors.Method:Choose the three lifestyle factors that raise the risk of coronary heart disease.Examiner tips
- Risk factors: high-fat/high-salt diet, smoking, stress, genetics, age.
- Step 1: Photosynthesis uses carbon dioxide and water as its reactants. Step 2: It produces glucose and oxygen, giving 6CO2 + 6H2O → C6H12O6 + 6O2, which is balanced. Step 3: The reverse equation is aerobic respiration, the third equation uses oxygen as a reactant wrongly, and the last is anaerobic respiration in yeast.Method:Choose the balanced equation with carbon dioxide and water forming glucose and oxygen.Examiner tips
- Photosynthesis: 6CO2 + 6H2O → C6H12O6 + 6O2.
- Step 1: Plants need magnesium ions to make chlorophyll. Step 2: Without enough magnesium the leaves cannot make chlorophyll and turn yellow. Step 3: Nitrate ions are needed to make amino acids and proteins, while calcium and chloride are not used to make chlorophyll.Method:Recall that magnesium ions are needed to make chlorophyll.Examiner tips
- Magnesium ions are needed to make chlorophyll.
- Step 1: Plants store carbohydrate as starch. Step 2: Starch is insoluble, so it does not affect osmosis and is a good storage molecule. Step 3: Glucose is the soluble sugar made in photosynthesis, cellulose makes cell walls, and glycogen is the storage carbohydrate of animals, not plants.Method:Recall that plants store energy as the insoluble carbohydrate starch.Examiner tips
- Plants store carbohydrate as starch; animals store it as glycogen.
- Step 1: A tropism is a growth response towards or away from a stimulus. Step 2: The shoot grows towards the light, so the stimulus is light and the response is towards it. Step 3: Growth towards light is positive phototropism, not a response to gravity.Method:Identify the stimulus as light and the towards-direction as positive phototropism.Examiner tips
- Growth towards light = positive phototropism.
- Step 1: Auxin is made in the shoot tip and diffuses down the shoot. Step 2: It becomes more concentrated on the shaded side of the shoot. Step 3: The auxin stimulates the cells on the shaded side to elongate more than those on the light side, so the shoot bends towards the light.Method:Describe auxin moving to the shaded side and causing greater cell elongation there.Examiner tips
- Auxin: made in tip → diffuses to shaded side → cells elongate more → shoot bends towards light.
- Step 1: By growing towards the light, the shoot and its leaves receive more light. Step 2: More light means the plant can carry out more photosynthesis. Step 3: More photosynthesis produces more glucose for growth and for respiration, so growing towards light is an advantage.Method:Link growing towards light to more photosynthesis and more glucose for the plant.Examiner tips
- Towards light → more light on leaves → more photosynthesis → more glucose.
- Step 1: The scrotum is the sac that holds and protects the testes. Step 2: The prostate gland produces seminal fluid, and the urethra carries both urine and sperm out of the body. Step 3: The sperm duct carries sperm from the testis to the urethra, so the first option names all four structures correctly in order.Method:Match each description to the scrotum, prostate gland, urethra and sperm duct in order.Examiner tips
- Scrotum holds testes; prostate makes seminal fluid; urethra carries urine and sperm; sperm duct carries sperm.
- Step 1: Testosterone is the male sex hormone produced in the testes. Step 2: At puberty testosterone causes the secondary sexual characteristics, including the deepening of the voice. Step 3: Oestrogen is the female sex hormone, insulin controls blood glucose, and adrenaline prepares the body for action, so testosterone is the correct hormone.Method:Recall that testosterone causes the secondary sexual characteristics in boys.Examiner tips
- Testosterone is the male sex hormone responsible for puberty changes in boys.
- Step 1: A sperm has a tail (flagellum) that lets it swim towards the egg. Step 2: It contains many mitochondria, which release the energy needed for this swimming. Step 3: It has an acrosome containing enzymes that break down the jelly coat of the egg so the sperm can fertilise it, so the first option gives three correct adaptations.Method:Give three sperm adaptations and link each to swimming, energy or fertilising the egg.Examiner tips
- Sperm adaptations: tail to swim, mitochondria for energy, acrosome enzymes to enter the egg.
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