IGCSE Biology 0610/33 October/November 2025 Past Paper with Answers

Question 1

Three correct statements can be made about enzymes. Which option lists three correct statements about enzymes?

1. they are biological catalysts, they are found in plant cells, and they increase the rate of reactions
2. they all have an optimum temperature of 20 °C, they are carbohydrates, and they are changed by reactions
3. they are biological catalysts, they are carbohydrates, and they are used up in reactions
4. they are found only in animal cells, they slow down reactions, and they have an optimum temperature of 20 °C

Question 2

Protease enzymes act on a particular type of food molecule. Which option correctly names the substrate that proteases act on and the products formed?

1. the substrate is protein and the products are amino acids
2. the substrate is starch and the products are glucose molecules
3. the substrate is fat and the products are fatty acids and glycerol
4. the substrate is protein and the products are simple sugars

Question 3

The activity of a protease enzyme was measured at different pH values. The activity rose from a low value to a maximum at pH 9, then fell back to zero by pH 12. Which option best describes the activity of this enzyme?

1. the activity increases to a maximum at the optimum pH of 9 and then decreases to zero at pH 12
2. the activity stays constant at every pH value from pH 8 to pH 12
3. the activity is highest at pH 12 and lowest at pH 9
4. the activity decreases steadily as the pH increases from pH 8 to pH 12

Question 4 · Answer A

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.

Question 5 · Answer A

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.

Question 6 · Answer A

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.

Question 7 · Answer A

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.

Question 8 · Answer A

Step 1: The thin layer that surrounds the cell and controls what passes in and out is the cell membrane. Step 2: The jelly-like material that fills the cell and where reactions occur is the cytoplasm. Step 3: Animal cells have no cell wall, vacuole-store or chloroplasts of this kind, so the boundary is the cell membrane and the filling is the cytoplasm.

Question 9 · Answer A

Step 1: The large round central structure is the nucleus, which contains the genetic information and controls the cell’s activities. Step 2: The small oval structures are mitochondria. Step 3: Mitochondria are the site of aerobic respiration, which releases energy, so the nucleus controls the cell and the mitochondria release energy.

Question 10 · Answer A

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.

Question 11 · Answer A

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.

Question 12 · Answer A

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.

Question 13 · Answer A

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.

Question 14 · Answer A

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.

Question 15 · Answer A

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.

Question 16 · Answer A

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.

Question 17 · Answer A

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.

Question 18 · Answer A

Step 1: The cap at the tip of the sperm head is the acrosome, which holds enzymes. Step 2: These enzymes are released to digest a path through the egg membrane so the sperm can enter. Step 3: The long thread is the flagellum (tail), which beats to swim the sperm to the egg, so the cap is the acrosome (enzymes) and the thread is the flagellum (movement).

Question 19 · Answer A

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.

Question 20 · Answer A

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.

Question 21 · Answer A

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.

Question 22 · Answer A

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.

Question 23 · Answer A

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%.

Question 24 · Answer A

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.

Question 25 · Answer A

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.

Question 26 · Answer A

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.

Question 27 · Answer A

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.

Question 28 · Answer A

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.

Question 29 · Answer A

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.

Question 30 · Answer A

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.

Question 31 · Answer A

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.

Question 32 · Answer A

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.

Question 33 · Answer A

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.

Question 34 · Answer A

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.

Question 35 · Answer A

Step 1: With no light, the plant responds only to gravity. Step 2: The shoot grows away from gravity, so its tip curves upwards. Step 3: The roots grow towards gravity, so they curve downwards, giving an upward-curving shoot and downward-curving roots.

Question 36 · Answer A

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.

Question 37 · Answer A

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.

Question 38 · Answer A

Step 1: The dark circle at the centre of the eye is an opening, not a solid part. Step 2: Light passes into the eye through this opening. Step 3: This central opening is the pupil, while the iris is the coloured ring around it, so the structure is the pupil.

Question 39 · Answer A

Step 1: The coloured ring around the pupil is the iris, which is made of muscle. Step 2: The iris muscles change the size of the pupil. Step 3: By widening or narrowing the pupil, the iris controls how much light enters the eye, which is its function.

Question 40 · Answer A

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.

Question 41 · Answer A

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.

Question 42 · Answer A

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.

Question 43 · Answer A

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.