May/June 2025 Paper 41 Worked Answers (IGCSE Biology 0610 Extended)
34 questions · 80 marks · 75 minutes
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Worked answers for 25 questions
- Step 1: Trophic level 1 is made up of producers, which are usually green plants. Step 2: Producers transfer energy from sunlight into chemical energy in food during photosynthesis. Step 3: So the energy that enters a food chain at trophic level 1 comes from the Sun, not from feeding, minerals or decomposers.Method:Identify trophic level 1 as the producers and recall that they capture energy from sunlight.Examiner tips
- Energy enters almost every food chain as sunlight captured by producers.
- Step 1: Organisms use much of their energy in respiration, releasing it as heat to the surroundings, and in movement, so it is not passed on. Step 2: Not all of an organism is eaten, and some eaten material is not digested and leaves in faeces, so that energy is lost too. Step 3: Energy cannot be created or destroyed, so the other options are wrong.Method:Recall the routes by which energy is lost between trophic levels and pick the valid pair.Examiner tips
- Energy is mostly lost through respiration (heat), movement, and uneaten or undigested material.
- Step 1: A pyramid of energy shows the energy passing through each level over a period of time, so it represents energy transfer and loss more accurately than a single mass measurement. Step 2: Because it uses energy, it also takes account of differences in life span, reproduction rate and body size, allowing fairer comparisons between ecosystems. Step 3: The other options describe biomass measurements or untrue statements, so they are not advantages of a pyramid of energy.Method:Recall what a pyramid of energy measures over time and pick the two genuine advantages.Examiner tips
- A pyramid of energy is always upright and shows energy transfer over time, so comparisons are fairer.
- Step 1: A population is all the organisms of one species living together in the same place. Step 2: They must also be living there during the same period, so the two words are area and time. Step 3: Cell, speed and depth do not describe where and when a population lives.Method:Recall the full definition of a population and choose the words for place and time.Examiner tips
- Population = same species, same area, same time.
- Step 1: Rod cells respond to low light intensities and give black-and-white vision, while cones need bright light and give colour vision. Step 2: A nocturnal animal must see in dim light, so it benefits from having more rods. Step 3: So a nocturnal retina has more rods because rods are sensitive in dim light, making the correct option the only one that matches both the difference and the reason.Method:Match the receptor that works in low light to the needs of a night-active animal.Examiner tips
- Rods = dim light, no colour; cones = bright light, colour. Night animals need more rods.
- Step 1: When light intensity changes, the iris muscles automatically change the size of the pupil. Step 2: This automatic, protective response is called the pupil reflex. Step 3: The knee-jerk reflex involves a leg muscle, phototropism is a plant growth response, and accommodation is focusing on near or far objects.Method:Recall the name of the reflex that controls the pupil in response to light.Examiner tips
- Iris adjusting the pupil to light intensity = the pupil reflex.
- Step 1: In bright light the pupil must get smaller to protect the retina. Step 2: The circular muscles of the iris contract to narrow the pupil, while the radial muscles relax. Step 3: The ciliary muscle changes the lens shape, and there is no suspensory muscle (the suspensory ligaments hold the lens), so the circular muscle is the effector that contracts.Method:Recall which iris muscle contracts to make the pupil smaller in bright light.Examiner tips
- Bright light: circular iris muscles contract, pupil narrows; dim light: radial muscles contract.
- Step 1: When the circular muscles contract, the radial muscles relax, producing opposite effects on the pupil. Step 2: A pair of muscles that work against each other in this way are described as antagonistic. Step 3: Synergistic muscles work together, voluntary actions are under conscious control, and rhythmic refers to a regular repeating action, so antagonistic is correct.Method:Recall the term for two muscles that produce opposite effects on a structure.Examiner tips
- Muscles producing opposite effects on the same structure act antagonistically.
- Step 1: The neurotransmitter is released on the first neurone side, so it is at a high concentration there and a low concentration on the far side. Step 2: Particles move from a high to a low concentration by diffusion, so the neurotransmitter diffuses across the gap. Step 3: It does not need energy (so not active transport), it is not water movement (so not osmosis), and an electrical impulse cannot cross the gap directly.Method:Identify the high-to-low movement of the chemical as diffusion across the synaptic gap.Examiner tips
- Neurotransmitter crosses the synaptic gap by diffusion, high to low concentration.
- Step 1: The pollen tube grows down through the style until it reaches the ovule in the ovary. Step 2: The male nucleus from the pollen grain travels down the inside of the pollen tube. Step 3: The male nucleus then fuses with the female nucleus inside the ovule, and this fusion of nuclei is fertilisation.Method:Trace the pollen tube and male nucleus to the ovule and identify the fusion of nuclei.Examiner tips
- Pollen tube grows to the ovule, male nucleus travels down, then nuclei fuse = fertilisation.
- Step 1: Self-pollination does not depend on another plant or on a pollinator, so it works even when pollinators are scarce or the plant is isolated. Step 2: Because pollen only has to reach the same flower, less pollen and energy are wasted and the chance of successful pollination is higher. Step 3: Self-pollination reduces genetic variation rather than increasing it, and it does not make all offspring male, so the other options are wrong.Method:Pick the reasons that follow from self-pollination not needing a pollinator or a second plant.Examiner tips
- Self-pollination is reliable when pollinators are scarce and it saves pollen and energy.
- Step 1: Self-pollination uses gametes from the same plant, so it produces offspring with very little genetic variation. Step 2: With little variation, fewer individuals are likely to have alleles that help them survive a new disease or a change in conditions. Step 3: This makes the population less able to adapt and more vulnerable to disease, so its risk of extinction rises.Method:Link the low genetic variation of self-pollination to reduced adaptability and survival.Examiner tips
- Only self-pollinating means low variation, so less ability to adapt and higher extinction risk.
- Step 1: A mutation is a random change in a gene or chromosome that can create a new allele, which is a source of variation. Step 2: Random fertilisation means any male gamete can fuse with any female gamete, producing new combinations of alleles. Step 3: Respiration, digestion, diffusion, osmosis, transpiration and translocation are not sources of genetic variation, so only mutation and random fertilisation are correct.Method:Recall the sources of genetic variation and pick the pair that change or recombine genes.Examiner tips
- Genetic variation comes from meiosis, mutation and random fertilisation.
- Step 1: Mitosis is the division of a nucleus that produces two genetically identical cells, each with the same chromosome number as the parent cell. Step 2: This happens because the chromosomes are copied and the copies separate into the two new cells. Step 3: Its roles are growth, the repair and replacement of damaged cells, and asexual reproduction, so the first option is correct while the others describe meiosis, respiration or fertilisation.Method:Define mitosis as identical-cell nuclear division and list growth, repair and asexual reproduction.Examiner tips
- Mitosis = identical cells, same chromosome number; used in growth, repair and asexual reproduction.
- Step 1: Unspecialised cells that divide repeatedly by mitosis and can develop into different specialised cell types are called stem cells. Step 2: Red blood cells and nerve cells are already specialised and do not divide. Step 3: Gametes are specialised sex cells made by meiosis, so the correct answer is stem cells.Method:Recall that unspecialised, repeatedly dividing cells are called stem cells.Examiner tips
- Unspecialised cells that keep dividing are stem cells.
- Step 1: Excretion is the removal of substances the body makes during its chemical reactions (metabolism) that it does not need. Step 2: It also includes getting rid of useful substances when they are present in excess of what the body requires. Step 3: Removing undigested food is egestion, taking in oxygen and glucose is absorption, and breaking down food is digestion, so only the first option defines excretion.Method:Define excretion as the removal of metabolic wastes and excess substances, not undigested food.Examiner tips
- Excretion = removal of metabolic waste and substances in excess; egestion = removal of undigested food.
- Step 1: Respiration in cells produces carbon dioxide as a waste product, which is carried in the blood to the lungs. Step 2: The carbon dioxide diffuses into the alveoli and is breathed out, so it is excreted by the lungs. Step 3: Urea is excreted by the kidneys, while glucose and amino acids are useful substances that are not normally excreted.Method:Recall the waste gas of respiration that the lungs remove from the body.Examiner tips
- Lungs excrete carbon dioxide (and some water vapour); kidneys excrete urea.
- Step 1: Excess amino acids cannot be stored, so the liver removes their nitrogen-containing part in a process called deamination, forming urea. Step 2: The liver also builds amino acids up into proteins, which is part of assimilation. Step 3: The nitrogen is not breathed out as carbon dioxide, amino acids do not form starch, and this processing happens in the liver, not the kidneys, so the other options are wrong.Method:Recall deamination (forming urea) and assimilation (forming proteins) as the two liver processes.Examiner tips
- Liver deaminates excess amino acids to form urea and assimilates others into proteins.
- Step 1: The most recent common ancestor is found where two branches separate highest up the tree, meaning least time has passed since they split. Step 2: The apes and rodents branches separate higher than any other pair, so they share the most recent common ancestor. Step 3: Sharks, bony fish, crocodiles and birds split off lower down, so their common ancestors are older.Method:Find the branch point closest to the present and read off the two groups it joins.Examiner tips
- Branches that join nearest the top share the most recent common ancestor.
- Step 1: The more recently two groups shared a common ancestor, the more similar their DNA base sequences are. Step 2: Birds share the most recent common ancestor with crocodiles, so birds have the most similar DNA to crocodiles. Step 3: Sharks branched off earliest, so they are the most distantly related and have the least similar DNA to crocodiles.Method:Use closeness on the tree to rank DNA similarity: nearest branch most similar, earliest branch least.Examiner tips
- Closer relatives share more similar DNA; the earliest-branching group is least similar.
- Step 1: DNA is made of two strands that twist around each other to form a double helix. Step 2: It contains four bases, A, T, C and G, and these pair in a fixed way, with A always pairing with T and C always pairing with G. Step 3: The paired bases are held together by bonds, so the first option is correct; the others describe a sugar, a protein, or impossible pairing.Method:Recall the double helix of two strands and the fixed base-pairing rules of DNA.Examiner tips
- DNA = two strands, double helix, bases A-T and C-G paired by bonds.
- Step 1: The order of bases in a gene determines the order in which amino acids are joined together. Step 2: The amino acids join to form a protein, such as an enzyme, and the order of amino acids gives the protein its particular shape. Step 3: Because proteins like enzymes control the cell’s reactions, the base sequence controls cell function, so only the first option is correct.Method:Trace the base sequence to amino acid order to protein shape to control of cell reactions.Examiner tips
- DNA base order to amino acid order to protein (enzyme) to controlled cell function.
- Step 1: When the board stops rotating, gravity now acts on one side of each root only. Step 2: Auxin gathers on the lower side, and in roots this slows growth on that side so the root bends downwards. Step 3: Roots are positively geotropic, so they grow downwards in the direction of gravity once the clinostat stops.Method:Recall that roots are positively geotropic and predict downward growth when rotation stops.Examiner tips
- Roots are positively geotropic, so they grow downwards towards gravity.
- Step 1: A tropism is named after the stimulus that causes the growth response. Step 2: The stimulus here is light, and the prefix for light is photo, so the response is phototropism. Step 3: Geotropism is a response to gravity, hydrotropism to water, and thermotropism to temperature, so phototropism is the response to light.Method:Match the light stimulus to the prefix photo to name the response phototropism.Examiner tips
- Photo means light, so the response to light is phototropism.
- Step 1: A wilted plant has lost water, so water leaves the cells by osmosis. Step 2: As water leaves, the vacuole and cytoplasm shrink, the cells lose turgor pressure and become flaccid. Step 3: If a lot of water is lost, the cell membrane pulls away from the cell wall, which is plasmolysis, so the first option correctly states and explains the appearance.Method:Link water loss by osmosis to a shrunken vacuole, low turgor and possible plasmolysis.Examiner tips
- Water loss makes plant cells flaccid and then plasmolysed, with low turgor pressure.
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