October/November 2025 Paper 33 Worked Answers (IGCSE Biology 0610 Core)
43 questions · 80 marks · 75 minutes
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Worked answers for 37 questions
- Step 1: Enzymes are biological catalysts: they speed up (increase the rate of) reactions without being used up or changed. Step 2: They are made of protein, not carbohydrate, and they are found in all living cells, including plant cells. Step 3: Enzymes do not all share an optimum of 20 °C, so the three correct statements are biological catalysts, found in plant cells and increase the rate of reactions.Method:Pick the three statements that match the true properties of enzymes.Examiner tips
- Enzymes = biological catalysts, made of protein, not used up, found in all living cells.
- Step 1: A protease is an enzyme that digests protein. Step 2: It breaks the large protein molecule down into its building blocks. Step 3: The building blocks of protein are amino acids, so the substrate is protein and the products are amino acids.Method:Match the protein-digesting enzyme to its substrate and product.Examiner tips
- Protease: protein → amino acids.
- Step 1: As the pH rises towards 9 the activity increases, reaching a maximum at the optimum pH of 9. Step 2: Above pH 9 the activity decreases as the pH moves away from the optimum. Step 3: By pH 12 the activity has fallen to zero, so the curve increases to a peak at pH 9 then decreases to zero at pH 12.Method:Trace the curve from low pH up to the peak at pH 9 and down to zero at pH 12.Examiner tips
- Describe an enzyme–pH graph using the optimum (peak) and the points of zero activity.
- Step 1: Protease enzymes are made by the pancreas and released into the small intestine. Step 2: The gall bladder stores bile (not an enzyme) and the salivary glands secrete amylase, not protease. Step 3: The rectum stores faeces, so the organ that secretes protease is the pancreas.Method:Identify the gland that makes the gut proteases.Examiner tips
- The pancreas secretes protease, lipase and amylase into the small intestine.
- Step 1: The pancreas releases protease into the small intestine. Step 2: Here the protease completes the digestion of proteins into amino acids. Step 3: The mouth digests starch, the oesophagus carries food, and the large intestine absorbs water, so the protease acts in the small intestine.Method:Locate where pancreatic enzymes are released and act.Examiner tips
- Pancreatic protease acts in the small intestine.
- Step 1: Hydrochloric acid kills many of the harmful microorganisms (bacteria) taken in with food. Step 2: It also makes the stomach contents strongly acidic. Step 3: This low pH is the optimum for the stomach protease, so the two functions are killing microorganisms and providing the optimum pH for the enzyme.Method:Choose the defensive and the enzyme-related function of the acid.Examiner tips
- Stomach acid: kills microbes and gives the low optimum pH for protease.
- Step 1: Physical digestion breaks food into smaller pieces without changing the molecules. Step 2: The muscular wall of the stomach contracts and churns the food. Step 3: This mixing breaks the food into smaller pieces and mixes it with the gastric juice, which is the stomach’s role in physical digestion.Method:Pick the muscular, size-reducing action as physical digestion.Examiner tips
- Stomach physical digestion = muscular churning to mix and break up food.
- Step 1: Gas exchange happens where the air is closest to the blood. Step 2: The lungs end in millions of tiny air sacs called alveoli. Step 3: The thin, moist walls of the alveoli are the gas exchange surface, so the answer is the alveoli.Method:Name the air sacs where gases pass between air and blood.Examiner tips
- Gas exchange surface in the lungs = the alveoli.
- Step 1: Respiration in the body cells produces the waste gas carbon dioxide. Step 2: This carbon dioxide is carried in the blood to the lungs. Step 3: It diffuses into the alveoli and is breathed out, so the gas excreted by the lungs is carbon dioxide.Method:Recall the waste gas that the lungs remove from the blood.Examiner tips
- The lungs excrete carbon dioxide, the waste gas of respiration.
- Step 1: Urea is carried in the blood to the kidneys, which filter it out. Step 2: From each kidney the urine flows down a ureter to the bladder, where it is stored. Step 3: It then leaves the body through the urethra, so the order is blood, kidney, ureter, bladder, urethra.Method:Order the structures by the route urine takes out of the body.Examiner tips
- Excretion path: blood → kidney → ureter → bladder → urethra.
- Step 1: The kidneys remove waste and excess substances from the blood to form urine. Step 2: As well as urea, they remove excess water and excess ions (salts). Step 3: Useful glucose, amino acids, blood cells and proteins are kept in the blood, so the kidneys excrete excess water and excess ions.Method:Pick the excess substances the kidneys remove besides urea.Examiner tips
- Kidneys excrete urea, excess water and excess ions.
- Step 1: Inside the leaf, water evaporates from the moist surfaces of the mesophyll cells into the air spaces. Step 2: The water vapour then diffuses out of the leaf through the stomata. Step 3: This movement out is by diffusion, so the words are mesophyll, air, stomata and diffusion.Method:Fill each blank with the correct leaf structure or process for transpiration.Examiner tips
- Transpiration: evaporation from mesophyll → air spaces → out through stomata by diffusion.
- Step 1: The rate is the distance moved divided by the time taken. Step 2: The bubble moved 45 mm in 3 minutes, so the rate is 45 ÷ 3. Step 3: 45 ÷ 3 = 15, so the rate of transpiration is 15 mm per minute.Method:Divide the 45 mm distance by the 3 minutes to get the rate.Examiner tips
- Rate = distance ÷ time = 45 ÷ 3 = 15 mm per minute.
- Step 1: The rate rises with temperature up to 40 °C and then stays at 23 mm per minute, so it increases then remains constant. Step 2: From 20 °C to 30 °C the rate goes from 6 to 18 mm per minute, which is three times as large (it triples). Step 3: From 20 °C to 45 °C the rate rises from 6 to 23 mm per minute, an increase of 17 mm per minute, so all three conclusions are correct.Method:Test each statement against the data and choose the set that is all correct.Examiner tips
- Check each conclusion against the actual values (6, 18, 23) before ticking it.
- Step 1: Gametes are the sex cells, such as a sperm and an egg. Step 2: Fertilisation is the joining together of these two gametes. Step 3: More precisely it is the fusion of the nucleus of the male gamete with the nucleus of the female gamete, so fertilisation is the fusion of the nuclei of two gametes.Method:Define fertilisation as the joining of the two gamete nuclei.Examiner tips
- Fertilisation = fusion of the nuclei of two gametes.
- Step 1: Sperm are the male gametes. Step 2: They are made in the male reproductive organs, the testes. Step 3: The ovary makes egg cells, the prostate adds fluid, and the bladder stores urine, so sperm are produced in the testis.Method:Name the male organ that makes the gametes.Examiner tips
- Sperm are produced in the testes.
- Step 1: Humans have a pair of sex chromosomes that determine sex. Step 2: A female has two X chromosomes (XX). Step 3: A male has one X and one Y chromosome, so the sex chromosomes in a male are XY.Method:Recall the male sex-chromosome pair.Examiner tips
- Male sex chromosomes = XY; female = XX.
- Step 1: The mother is XX, so every egg carries an X. Step 2: The father is XY, so half of the sperm carry X and half carry Y. Step 3: An X sperm gives a girl (XX) and a Y sperm gives a boy (XY), so there is a 1 in 2, or 0.5, chance of a boy.Method:Use the equal numbers of X and Y sperm to give a probability of 0.5.Examiner tips
- Boy or girl is a 1 : 1 chance, so the probability of a boy is 0.5.
- Step 1: The greatest increase is shown by the steepest part of the line, where production climbs the most in five years. Step 2: This steepest section of the graph occurs between 1980 and 1985. Step 3: The other periods show smaller rises, so the greatest increase is from 1980 to 1985.Method:Identify the steepest five-year interval on the line.Examiner tips
- Greatest increase = steepest five-year section of the line.
- Step 1: The increase is 350 − 150 = 200 million tonnes. Step 2: Divide the increase by the starting value: 200 ÷ 150 = 1.333. Step 3: Multiply by 100 to get 133.3%, which rounds to 133%.Method:Find the increase, divide by the 1960 value, and multiply by 100.Examiner tips
- (350 − 150) ÷ 150 × 100 = 133%.
- Step 1: Fertilisers add the mineral ions that crops need to grow well, increasing yield. Step 2: Herbicides kill weeds that would otherwise compete with the crop. Step 3: Selective breeding produces higher-yielding varieties, so all three methods increase crop production.Method:Pick the three genuine yield-raising methods.Examiner tips
- Increase crop yield: fertilisers, herbicides, insecticides, selective breeding, machinery, monoculture, GM.
- Step 1: In intensive farming the animals are kept warm and have little room to move. Step 2: They therefore use less energy on movement and on keeping warm. Step 3: More of the food energy goes into growth instead, giving a higher yield more cheaply, which are the advantages.Method:Link reduced energy loss to greater growth and yield.Examiner tips
- Intensive livestock: less energy lost to movement/heat → more growth → higher yield, cheaper.
- Step 1: Sewage is rich in nutrients and is broken down by bacteria, which multiply rapidly. Step 2: These bacteria use up the oxygen dissolved in the water. Step 3: With less oxygen, fish and other aquatic organisms die and biodiversity falls, so sewage lowers oxygen and kills aquatic life.Method:Link bacterial breakdown of sewage to falling oxygen and death of aquatic life.Examiner tips
- Sewage → bacteria multiply → oxygen falls → fish die, biodiversity drops.
- Step 1: A binomial scientific name has two parts: the genus name first, then the species name. Step 2: In Lactobacillus bulgaricus, Lactobacillus is written first. Step 3: So Lactobacillus is the genus name, while bulgaricus is the species name and the full name together is the binomial.Method:Take the first word of the two-part name as the genus.Examiner tips
- Binomial name = genus (first) + species (second).
- Step 1: Bacteria reproduce very quickly, so large numbers are produced in a short time. Step 2: They can also make useful complex molecules, such as the substances that turn milk into yoghurt. Step 3: They are also easy to grow with few ethical concerns, so two reasons are rapid reproduction and making complex molecules.Method:Pick the two genuine advantages of using bacteria.Examiner tips
- Bacteria in biotech: fast reproduction + make complex molecules + few ethical concerns.
- Step 1: 100 g of yoghurt has 3.5 g of protein, so 200 g has 2 × 3.5 = 7 g. Step 2: The person needs 45 g of protein in total. Step 3: Subtract the protein already eaten: 45 − 7 = 38 g, so there is 7 g in the yoghurt and 38 g more is needed.Method:Scale the protein to 200 g, then subtract from the 45 g target.Examiner tips
- 200 g → 2 × 3.5 = 7 g; then 45 − 7 = 38 g.
- Step 1: Fats are an energy-rich food, providing more energy per gram than other nutrients. Step 2: They act as a long-term energy store. Step 3: A layer of fat under the skin also insulates the body against heat loss, so fats are an energy store/source and provide insulation.Method:Recall the energy and insulation roles of dietary fat.Examiner tips
- Fats: energy store/source, insulation, protection of organs.
- Step 1: Dietary fibre comes from plant material that we cannot digest. Step 2: Wholegrain cereals, fruit and vegetables are rich in this plant fibre. Step 3: Butter, egg white and sugar contain little or no fibre, so a good source of fibre is wholegrain bread, fruit and vegetables.Method:Pick the plant-based food rich in indigestible fibre.Examiner tips
- Fibre sources: wholegrains, fruit, vegetables, beans, nuts.
- Step 1: Anaerobic respiration takes place without oxygen. Step 2: In yeast, glucose is broken down without oxygen. Step 3: The products are alcohol (ethanol) and carbon dioxide, so the equation is glucose → alcohol + carbon dioxide.Method:Recall the yeast fermentation equation with its two products.Examiner tips
- Yeast anaerobic respiration: glucose → ethanol + carbon dioxide.
- Step 1: As yeast respires anaerobically in the dough it produces carbon dioxide gas. Step 2: This gas is trapped as bubbles inside the stretchy dough. Step 3: The bubbles make the dough expand, so the carbon dioxide makes the bread rise.Method:Link the carbon dioxide gas to the rising of the dough.Examiner tips
- Yeast → carbon dioxide → bubbles → bread rises.
- Step 1: Yeast respires anaerobically to produce alcohol and carbon dioxide. Step 2: This is used in brewing to make alcoholic drinks such as beer and wine. Step 3: The alcohol (ethanol) can also be used as a biofuel, so another use of yeast is brewing or biofuel production.Method:Pick an alcohol-based use of yeast.Examiner tips
- Other uses of yeast: brewing alcohol, making biofuel.
- Step 1: A tropism is a growth response to a stimulus from one direction. Step 2: Here the stimulus is gravity, since no light reaches the plant. Step 3: A growth response to gravity is called gravitropism (geotropism), so that is the name of this response.Method:Match the gravity stimulus to its tropism name.Examiner tips
- Response to gravity = gravitropism (geotropism).
- Step 1: The function of a root hair cell is to absorb water and mineral ions from the soil. Step 2: Each cell has a long, narrow hair-like extension. Step 3: This extension gives a large surface area, which increases the rate of absorption, so the cell absorbs water and ions and is adapted by its large surface area.Method:State the absorption role and link the long shape to a large surface area.Examiner tips
- Root hair cell: absorbs water and ions; long extension → large surface area.
- Step 1: Genes are found on chromosomes, which are made of DNA. Step 2: A gene is a particular length, or section, of this DNA. Step 3: Each gene carries the code for making one protein, so a gene is a length of DNA that codes for a protein.Method:Define a gene as a coding section of DNA.Examiner tips
- Gene = a length of DNA that codes for a protein.
- Step 1: Discontinuous variation puts individuals into a few distinct groups with no in-between values. Step 2: Here people are either short-eyelashed or long-eyelashed, which is just two categories. Step 3: There are no intermediate lengths, so eyelash length shows discontinuous variation.Method:Identify the two clear-cut categories with no in-between as discontinuous variation.Examiner tips
- Discontinuous variation = a few distinct categories, no intermediates.
- Step 1: A genotype with two identical alleles is homozygous; with two different alleles it is heterozygous. Step 2: ee has two identical recessive alleles, so it is homozygous recessive. Step 3: Ee has one of each allele, so it is heterozygous, giving ee homozygous recessive and Ee heterozygous.Method:Classify each genotype by whether its two alleles are the same or different.Examiner tips
- ee = homozygous recessive; Ee = heterozygous.
- Step 1: The male ee produces only e gametes, and the female Ee produces E and e gametes. Step 2: Combining these gives the offspring genotypes Ee and ee in equal numbers. Step 3: Ee shows long eyelashes (E is dominant) and ee shows short, so the ratio is 1 long : 1 short.Method:Combine the gametes in a Punnett square and read off the phenotype ratio.Examiner tips
- ee × Ee → Ee and ee → 1 long : 1 short.
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