0610/23 May/June 2025 Worked Answers (IGCSE Biology 0610 Extended)
40 questions · 40 marks · 45 minutes
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Worked answers for 25 questions
- Step 1: Water moves by osmosis from a higher water potential to a lower water potential. Step 2: The solution has a lower water potential than the cytoplasm, so water moves out of the cells into the solution. Step 3: Losing water makes the cells lose pressure and become flaccid; gaining water and becoming turgid only happens in a solution of higher water potential.Method:Decide the direction of water movement, then state the effect on the cells.Examiner tips
- Lower water potential outside means water leaves the cell and it becomes flaccid.
- Step 1: Because bases pair A with T and C with G, 20% T in molecule 1 means 20% A, so 40% A and T together and 60% C and G. Step 2: In molecule 2, 20% C means 20% G, so 40% C and G together and 60% A and T, meaning molecule 2 has more A bases. Step 3: More A-T pairs means weaker bonding overall, so molecule 2 needs the lowest temperature to separate.Method:Use complementary base pairing to find the proportion of A-T pairs, then compare the strength.Examiner tips
- A pairs with T (weak) and C pairs with G (strong); more A-T means easier to separate.
- Step 1: Photosynthesis uses carbon dioxide and water to make glucose and oxygen. Step 2: To balance the atoms, six molecules of carbon dioxide and six of water give one glucose and six of oxygen. Step 3: So the equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂; the other versions are not balanced or use the wrong reactants.Method:Recall the reactants and products of photosynthesis and pick the balanced equation.Examiner tips
- Remember: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
- Step 1: Distilled water has a higher water potential than the potato cells. Step 2: Water therefore moves into the potato cells by osmosis. Step 3: Taking in water increases the mass of the cube, so it rises from 5 g to about 7 g; a fall to 2 g or 4 g would mean water left, and 5 g would mean no change.Method:Compare the water potential of distilled water with the cells and decide if mass rises or falls.Examiner tips
- In distilled water, plant tissue gains water by osmosis and increases in mass.
- Step 1: Vitamin C is needed to make connective tissue and to heal wounds. Step 2: Without enough vitamin C, the gums become swollen and bleed and wounds heal slowly, a condition called scurvy. Step 3: Lack of calcium weakens bones and teeth, lack of fibre causes constipation, and lack of iron causes anaemia, none of which give these symptoms.Method:Match the symptoms to the deficiency disease and name the missing nutrient.Examiner tips
- Vitamin C deficiency causes scurvy: bleeding gums and slow healing.
- Step 1: Assimilation is when digested food molecules are taken into cells and used to become part of them. Step 2: This is exactly what the statement describes. Step 3: Absorption is moving molecules into the blood, digestion is breaking large molecules into small ones, and ingestion is taking food into the body, so none of these fit.Method:Match the definition to the correct stage of nutrition.Examiner tips
- Assimilation = using absorbed molecules so they become part of the cells.
- Step 1: Blood plasma in arteries carries dissolved amino acids around the body. Step 2: Phloem carries dissolved food, including sucrose and amino acids, by translocation. Step 3: So both arteries and phloem normally carry amino acids.Method:Decide separately whether each structure transports amino acids, then combine.Examiner tips
- Amino acids travel in blood plasma in animals and in phloem in plants.
- Step 1: The ventricles are the lower chambers that pump blood out of the heart. Step 2: Blood leaving the heart travels in arteries, which carry blood away from the heart. Step 3: So blood leaves through arteries from the ventricles; the atria receive blood, and veins bring blood toward the heart.Method:Match the pumping chamber with the vessel that carries blood away.Examiner tips
- Blood leaves the heart in arteries, pumped out by the ventricles.
- Step 1: Cholera toxin makes chloride ions move into the small intestine. Step 2: The extra ions lower the water potential inside the intestine. Step 3: Water then moves out of the blood into the intestine by osmosis, so more water leaves the body through the anus as watery diarrhoea.Method:Follow the chloride ions, the change in water potential, and the resulting water movement.Examiner tips
- Cholera: chloride into the gut, water potential falls, water floods out (diarrhoea).
- Step 1: Respiration releases energy that cells use for active life processes. Step 2: Cell division is an active process that needs this energy. Step 3: Osmosis and transpiration are passive and need no respiratory energy, and photosynthesis takes in light energy rather than using energy from respiration.Method:Pick the active, energy-requiring process from the list.Examiner tips
- Active processes like cell division use energy from respiration; passive ones do not.
- Step 1: During hard exercise the muscles respire anaerobically (1), converting glucose to lactic acid (3). Step 2: Lactic acid then builds up and causes an oxygen debt (4). Step 3: After exercise, breathing stays fast and deep (2) so that oxygen can be used to respire the lactic acid aerobically (5), giving the order 1, 3, 4, 2, 5.Method:Order the statements from cause (exercise) to effect (removing the debt).Examiner tips
- Anaerobic respiration → lactic acid → oxygen debt → deep breathing → aerobic removal.
- Step 1: Filtration in the glomerulus lets small molecules pass from the blood into the nephron. Step 2: Both glucose and urea are small molecules, so both are filtered out. Step 3: Glucose is later reabsorbed further along the nephron, but the question is about filtration, where both pass through.Method:Decide for each molecule whether it is small enough to be filtered.Examiner tips
- Small molecules (glucose, urea, water, ions) are filtered; large proteins and cells are not.
- Step 1: Excess amino acids cannot be stored, so they are broken down in the liver. Step 2: This breakdown, called deamination, removes the nitrogen part and forms urea. Step 3: Fatty acids and sugars are broken down in respiration to release energy, and vitamins are not broken down to make urea.Method:Recall which nutrient contains nitrogen and is broken down to urea.Examiner tips
- Urea is made from excess amino acids by deamination in the liver.
- Step 1: Glucagon raises blood glucose by making the liver release glucose from stored glycogen. Step 2: Adrenaline also raises blood glucose, preparing the body for action. Step 3: Insulin lowers blood glucose, the opposite effect, so the two hormones that increase it are adrenaline and glucagon.Method:Recall the effect of each hormone and pick the two that raise glucose.Examiner tips
- Glucagon and adrenaline raise blood glucose; insulin lowers it.
- Step 1: A potato tuber grows from a stem by mitosis, with no gametes involved. Step 2: Reproduction without gametes from a single parent is asexual reproduction. Step 3: Because it is asexual, potato Y is a genetically identical clone of the parent, so there is no genetic difference and no fusion of gametes.Method:Note that no gametes are involved and conclude the type of reproduction.Examiner tips
- Tubers form by asexual reproduction, giving genetically identical clones.
- Step 1: Self-pollination uses pollen and eggs from the same plant, so the offspring are very similar to the parent. Step 2: This gives little genetic variation in the population. Step 3: With less variation, the population is less able to adapt if the environment changes, so self-pollination reduces this ability by reducing variation.Method:Link self-pollination to reduced variation, then to a reduced ability to adapt.Examiner tips
- Self-pollination lowers variation, making a population less able to adapt.
- Step 1: A gene is a length of DNA that codes for a protein. Step 2: Different versions of the same gene are called alleles. Step 3: A chromosome is a long thread of DNA carrying many genes, a length of DNA is a gene itself, and a protein is the product made from a gene, so none of these names a version of a gene.Method:Recall the vocabulary of inheritance and name the different forms of a gene.Examiner tips
- Alleles are the different versions of the same gene.
- Step 1: Early embryo cells are unspecialised and can become any cell type, so they are stem cells. Step 2: An embryo grows by ordinary cell division, which is mitosis, producing identical cells. Step 3: Meiosis only makes gametes, gametes are single sex cells not a ball of cells, and a zygote is one cell, so the correct description is stem cells undergoing mitosis.Method:Identify the cells as unspecialised stem cells and the division as mitosis.Examiner tips
- Early embryo cells are stem cells that divide by mitosis.
- Step 1: Continuous variation, such as body mass, shows a range of values and is caused by both genes and the environment. Step 2: Discontinuous variation falls into distinct groups and is caused by genes only, with examples like seed shape in peas and ABO blood groups. Step 3: So the correct row pairs continuous variation with genes and the environment, and gives seed shape in peas as discontinuous.Method:Check each cell: continuous needs genes and environment; discontinuous needs genes only.Examiner tips
- Continuous = genes + environment (a range); discontinuous = genes only (distinct groups).
- Step 1: An adaptive feature is an inherited feature that helps an organism survive and reproduce, so statements 1 and 3 are correct. Step 2: Adaptive features do not always show continuous variation; many show discontinuous variation, so statement 2 is wrong. Step 3: Therefore only statements 1 and 3 are correct.Method:Judge each statement against the definition of an adaptive feature.Examiner tips
- An adaptive feature is inherited and aids survival and reproduction; it need not be continuous.
- Step 1: When antibiotic X is used, resistant bacteria survive while non-resistant ones die. Step 2: The survivors reproduce and pass on the resistance, so the proportion of resistant bacteria rises. Step 3: This survival of the better-adapted bacteria is natural selection; it is not chosen by humans (artificial selection), and bacteria do not use meiosis or fertilisation.Method:Explain why resistant bacteria increase and name the type of selection.Examiner tips
- Rising antibiotic resistance is a classic example of natural selection.
- Step 1: The producers have 1000 units and the secondary consumers have 10 units, so the energy lost is 1000 minus 10, which is 990 units. Step 2: As a percentage of the starting 1000 units, that is 990 divided by 1000, times 100. Step 3: This gives 99%, so 99% of the energy is lost between the producers and the secondary consumers.Method:Subtract the secondary consumer energy from the producer energy and find the percentage.Examiner tips
- Energy lost = (start minus end) / start × 100, using the right two levels.
- Step 1: Nitrogen fixation changes nitrogen gas into nitrogen compounds that organisms can use. Step 2: Both lightning and nitrogen-fixing bacteria can carry out this fixing of nitrogen. Step 3: Deamination happens in the liver, while denitrification and nitrification are done by bacteria but not by lightning, so the answer is nitrogen fixation.Method:Identify the process done by both lightning and bacteria in the nitrogen cycle.Examiner tips
- Lightning and nitrogen-fixing bacteria both carry out nitrogen fixation.
- Step 1: A monoculture is a single crop species grown over a large area, so all the plants are alike. Step 2: Because the plants are so similar, a single disease can spread quickly and destroy the whole crop, which is a disadvantage. Step 3: Less insecticide use, lower emissions and more biodiversity would all be advantages, and monocultures actually reduce biodiversity.Method:Pick the option that is genuinely a drawback of growing one identical crop.Examiner tips
- Identical crops let a single disease wipe out the whole field.
- Step 1: First the insulin gene is isolated using restriction enzymes (3) and then inserted into a cut plasmid (1). Step 2: The recombinant plasmids are then placed into bacteria (4). Step 3: Finally the bacteria multiply and make human insulin (2), giving the order 3, 1, 4, 2.Method:Order the steps from isolating the gene to bacteria producing the insulin.Examiner tips
- Isolate the gene, insert it into a plasmid, put the plasmid into bacteria, then grow them.
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