Coordination, Ecosystems & Genetics

Which pathway correctly describes the route a nerve impulse takes during a simple reflex action?

• AReceptor to sensory neurone to relay neurone to motor neurone to effector✓
• BReceptor to motor neurone directly to the effector, completely bypassing the spinal cord
• CEffector to motor neurone to sensory neurone to receptor
• DReceptor to relay neurone to sensory neurone to effector

Answer: A reflex is a fast, automatic response that does not require conscious thought from the brain. The impulse passes from a receptor through a sensory neurone, then a relay neurone in the spinal cord, then a motor neurone to the effector. Any pathway missing the relay neurone or running in the opposite direction does not match how a reflex arc actually works.

What are the two organs that make up the central nervous system in humans?

• AThe brain and the spinal cord✓
• BThe spinal cord and the motor neurones
• CThe brain and the peripheral nerves
• DThe brain and the sensory receptors

Answer: The central nervous system coordinates the body's response to information from receptors. It is made up of the brain, which processes information, and the spinal cord, which carries impulses to and from the brain and controls reflexes. Sensory receptors, motor neurones and peripheral nerves are part of the wider nervous system but are not part of the central nervous system itself.

How does hormonal coordination in the human body differ from nervous coordination in terms of speed and duration of effect?

• AHormones travel in the blood and act just as fast as nerve impulses but do not last as long
• BHormones travel in the blood and act more slowly but for longer than nerve impulses✓
• CHormones travel along neurones and act faster but for a shorter time than nerve impulses
• DHormones are released directly onto muscles and act instantly like a reflex

Answer: Nerve impulses travel rapidly along neurones and produce fast, short-lived effects, while hormones are released into the blood and travel more slowly to their target organs. Because hormones must be carried around the body in the bloodstream, their effects tend to last longer than a nerve impulse's effect. Hormones are not released directly onto muscles like a reflex, and they do not act as quickly as nerve impulses.

Thyroxine is regulated by negative feedback. What does this mean for thyroxine levels in the blood?

• AThyroxine level stays fixed regardless of how much is released into the blood
• BA rise in thyroxine level reduces the signal that stimulates further thyroxine release, bringing the level back down✓
• CA fall in thyroxine level switches off the gland completely until the next day
• DA rise in thyroxine level increases the signal that stimulates further thyroxine release, raising the level even higher than before

Answer: Negative feedback keeps a hormone level within a normal range by reversing any change that occurs. When thyroxine levels rise, this reduces the stimulus that triggers further thyroxine release, so the level falls back towards normal. A system where rising thyroxine caused even more thyroxine to be released would not be stable and is not how negative feedback works.

Which hormone from the pituitary gland stimulates an egg to mature in the ovary during the menstrual cycle?

• AProgesterone, a pregnancy hormone
• BOestrogen, a female sex hormone
• CLH (luteinising hormone)
• DFSH (follicle-stimulating hormone)✓

Answer: FSH, or follicle-stimulating hormone, is released by the pituitary gland and causes an egg to mature in the ovary during the menstrual cycle. Luteinising hormone triggers the release of the mature egg, while oestrogen and progesterone control the build-up and maintenance of the uterus lining. Only FSH has the specific role of stimulating egg maturation.

Which of these is a non-hormonal method of contraception?

• AA condom, which acts as a barrier to stop sperm reaching an egg✓
• BA contraceptive injection, which releases hormones to prevent ovulation
• CThe contraceptive pill, which contains hormones that prevent ovulation
• DA contraceptive implant, which releases progesterone over several years

Answer: A condom works as a physical barrier that prevents sperm from reaching an egg, so it does not rely on hormones. The contraceptive pill, implant and injection all release hormones into the body to prevent ovulation or otherwise stop pregnancy. Because these three methods all use hormones, only the condom is a non-hormonal method.

What is meant by homeostasis in the human body?

• AThe chemical breakdown of glucose within cells in order to release usable energy for the body to use
• BThe transfer of genetic information from parents to offspring
• CThe continuous biological process by which the body produces entirely new cells to replace tissue that has been damaged
• DThe regulation of the internal environment to maintain optimal conditions for cell function✓

Answer: Homeostasis is the maintenance of a stable internal environment despite changes outside or inside the body, keeping conditions such as temperature and blood glucose within narrow limits. This regulation allows cells to function normally and enzymes to work at their optimum conditions. Growth, respiration and inheritance are all separate biological processes rather than definitions of homeostasis.

What is the key difference between type 1 and type 2 diabetes?

• AType 1 diabetes only affects blood glucose after meals, whereas type 2 diabetes affects blood glucose at all times
• BIn type 1 diabetes the pancreas fails to produce enough insulin, whereas in type 2 diabetes the body's cells stop responding properly to insulin✓
• CType 1 diabetes is caused entirely by diet, whereas type 2 diabetes is caused entirely by genetics
• DIn type 1 diabetes the body's cells gradually stop responding to insulin properly, whereas in type 2 diabetes the pancreas fails to produce enough insulin in the first place

Answer: Type 1 diabetes develops when the pancreas produces little or no insulin, usually because the immune system has destroyed the insulin-producing cells. Type 2 diabetes develops when the body's cells no longer respond properly to insulin, even though the pancreas may still produce it. Diabetes is not caused solely by diet or solely by genetics, and both types affect blood glucose control throughout the day, not just after meals.

Which word equation correctly summarises photosynthesis?

• ACarbon dioxide + oxygen, in light, form glucose + water
• BGlucose + oxygen, using light energy overall, form carbon dioxide + water
• CGlucose + water, using light energy overall, form carbon dioxide + oxygen
• DCarbon dioxide + water, using light energy, form glucose + oxygen✓

Answer: Photosynthesis uses light energy to convert carbon dioxide and water into glucose, releasing oxygen as a by-product. This means carbon dioxide and water are the reactants and glucose and oxygen are the products, not the other way round. Any equation that reverses these reactants and products describes respiration rather than photosynthesis.

Why is photosynthesis classed as an endothermic reaction?

• AIt transfers energy from the chemical bonds of glucose, out into the environment as heat
• BIt releases energy to the surroundings as the plant respires continuously
• CIt occurs only in cold conditions where the environment loses energy to the plant
• DIt transfers energy from the environment, as light, into the chemical bonds of glucose✓

Answer: An endothermic reaction takes in energy from its surroundings rather than releasing it. Photosynthesis absorbs light energy from the environment and stores it as chemical energy in the bonds of glucose molecules. A reaction that released energy to the surroundings, such as respiration, would be exothermic rather than endothermic.

Why are photosynthetic organisms described as primary producers in most food chains?

• AThey convert light energy into chemical energy stored in biomass, which other organisms then feed on✓
• BThey convert chemical energy stored in biomass back into light energy, which other organisms somehow absorb directly
• CThey consume other organisms first before passing energy along the food chain
• DThey produce oxygen only, which forms the entire diet of herbivores

Answer: Photosynthetic organisms such as plants and algae make their own food using light energy, converting it into chemical energy stored in biomass. Because nearly all other organisms in a food chain rely directly or indirectly on this stored energy, photosynthesisers are described as primary producers. They do not consume other organisms first, and oxygen is only a by-product of photosynthesis, not the sole food source for herbivores.

A plant kept in dim light is moved to bright light while temperature and carbon dioxide concentration stay constant. What happens to its rate of photosynthesis?

• AIt increases, because light intensity was limiting the rate of photosynthesis✓
• BIt increases briefly then drops to zero as the plant uses up its glucose store
• CIt stays the same, because light intensity has no effect on photosynthesis rate
• DIt decreases, because bright light damages the chlorophyll in the leaves

Answer: The rate of photosynthesis is controlled by whichever factor is in shortest supply relative to the plant's needs, known as a limiting factor. If temperature and carbon dioxide remain constant, increasing the light intensity removes light as the limiting factor and the rate of photosynthesis rises. Light does not damage chlorophyll at typical growing intensities, and light intensity is well established to affect the rate of photosynthesis.

Increasing the carbon dioxide concentration around a plant increases its rate of photosynthesis up to a point, after which the rate levels off. What does this levelling off show?

• ACarbon dioxide concentration will always limit the rate of photosynthesis no matter how much extra is added to the environment
• BCarbon dioxide has become toxic to the plant at this concentration, which is significantly slowing down its rate of photosynthesis
• CThe plant has stopped photosynthesising completely now that carbon dioxide has become available in such an unusually large excess
• DCarbon dioxide is no longer the limiting factor, so some other factor such as light or temperature is now limiting the rate✓

Answer: As carbon dioxide concentration increases, the rate of photosynthesis rises until it levels off at a plateau. This levelling off shows that carbon dioxide is no longer in short supply and that another factor, such as light intensity or temperature, is now limiting the rate. Carbon dioxide does not become toxic to plants at the concentrations used in these investigations, and photosynthesis does not stop once carbon dioxide is plentiful.

On a graph of photosynthesis rate against light intensity, the curve becomes a horizontal line at high light intensities. Which statement best explains this?

• AAt high light intensity, the rate of photosynthesis has become equal to the rate of respiration
• BAt high light intensity, light is still the limiting factor, since the leaf absorbs it more slowly than before
• CAt high light intensity, a different factor such as temperature or carbon dioxide concentration is limiting the rate instead✓
• DAt high light intensity, the plant's chlorophyll is broken down by the light, so the rate of photosynthesis starts to fall

Answer: A graph of photosynthesis rate against light intensity that flattens at high light levels shows that light is no longer the limiting factor at that point. Instead, a different factor such as temperature or carbon dioxide concentration must be limiting the rate, since increasing light further makes no difference. A flat line shows the rate holding steady rather than falling, as it would if chlorophyll were being broken down, and at high light the rate is well above the point where photosynthesis just balances respiration.

Which sequence correctly shows the levels of organisation in an ecosystem, from smallest to largest?

• AEcosystem, community, population, organism
• BPopulation, organism, ecosystem, community
• CCommunity, organism, population, ecosystem
• DOrganism, population, community, ecosystem✓

Answer: An ecosystem is built up from smaller units, starting with a single organism. A group of organisms of the same species in one area forms a population, several populations living together form a community, and a community together with its physical environment forms an ecosystem. Any order that reverses or mixes up this sequence does not reflect how ecosystems are structured.

Which of these is an abiotic factor that could affect the organisms living in a pond?

• AThe spread of disease among the fish
• BThe number of predators present
• CThe temperature of the water✓
• DCompetition for food between two fish

Answer: Abiotic factors are non-living physical or chemical conditions in an environment, such as temperature, light, pH and moisture. Water temperature is abiotic because it does not depend on other living organisms. Predator numbers, competition and disease spread are all biotic factors, since they arise from interactions between living organisms.

Two species of bird in the same habitat both feed on the same type of insect. What term describes this relationship?

• APredation, because one bird species hunts the other directly
• BMutualism, because both species benefit from sharing the insects equally
• CInterspecific competition, because they are competing for the same resource✓
• DInterspecific competition, because they are helping each other find food

Answer: Interspecific competition occurs when two different species compete for the same limited resource, such as food, space or light. Two bird species relying on the same type of insect are competing for that shared food resource, not cooperating or preying on one another. Mutualism describes a relationship where both species actively benefit from cooperating, which is not the case here.

Which process in the carbon cycle returns carbon dioxide to the atmosphere from living organisms?

• ARespiration✓
• BCombustion
• CPhotosynthesis
• DFossilisation

Answer: Living organisms release carbon dioxide back into the atmosphere through respiration, as they break down glucose to release energy. Photosynthesis does the opposite, removing carbon dioxide from the atmosphere to build biomass. Combustion of fossil fuels also releases carbon dioxide, but this comes from long-dead organic matter rather than directly from living organisms, and fossilisation locks carbon away rather than releasing it.

What process describes water vapour turning into liquid water droplets to form clouds in the water cycle?

• AEvaporation
• BCondensation✓
• CPrecipitation
• DTranspiration

Answer: Condensation is the process by which water vapour cools and changes into liquid water droplets, which is how clouds form in the water cycle. Evaporation is the reverse process, turning liquid water into vapour, while precipitation is the falling of that water back to Earth as rain or snow. Transpiration refers specifically to water vapour loss from plants, not cloud formation.

What role do decomposer microorganisms play in cycling materials through an ecosystem?

• AThey build up dead organisms and waste material into much larger organic structures, locking nutrients away permanently
• BThey convert carbon dioxide directly into glucose using sunlight, in exactly the same way that ordinary green plants do
• CThey absorb nutrients directly from the soil and pass them straight into the bloodstreams of nearby grazing animals
• DThey break down dead organisms and waste, releasing nutrients such as carbon and nitrogen back into the environment✓

Answer: Decomposer microorganisms such as bacteria and fungi break down dead plants, animals and waste material. This releases nutrients such as carbon and nitrogen back into the soil and atmosphere, where they can be used again by other organisms. Decomposers do not build up dead material into larger structures, and they do not photosynthesise or feed nutrients directly into animal bloodstreams.

A student wants to estimate the population size of daisies in a field using quadrats. What should they do to get a reliable estimate?

• APlace a single quadrat in the area with the most daisies and multiply by the field's area
• BEstimate the number of daisies by eye without using a quadrat at all
• CPlace quadrats randomly at several locations across the field and take an average count✓
• DPlace quadrats only along the field's edge, where daisies are easiest to reach

Answer: A reliable estimate of population size requires sampling that represents the whole area being studied, not just one part of it. Placing quadrats randomly at several locations across the field and averaging the counts reduces bias and gives a fair estimate of the daisy population. Sampling only the densest patch, only the edges, or estimating by eye would all give inaccurate and unreliable results.

Which of these is a negative human impact on biodiversity?

• ABreeding programmes in zoos to increase numbers of an endangered species
• BClearing a forest to create farmland, destroying the habitat of many species✓
• CPlanting a variety of native trees to restore a damaged woodland
• DCreating a nature reserve, which protects an endangered species' habitat

Answer: Human activities can reduce biodiversity by destroying the habitats that species depend on. Clearing forest for farmland removes the trees and other plants that many species rely on for food and shelter, directly reducing biodiversity in that area. Creating nature reserves, running zoo breeding programmes and planting native trees are all actions that support and restore biodiversity rather than damage it.

Why is maintaining a high level of biodiversity generally considered important for humans?

• AA high level of biodiversity guarantees completely that no species will ever become extinct at any point in future
• BA wide range of species provides food security, medicines and stable ecosystems that support human life✓
• CA wide range of species increases competition between organisms, which actually reduces the food available to humans
• DA wide range of species removes the need for humans to farm crops or raise livestock

Answer: A wide range of species supports ecosystems that provide humans with food, clean water, medicines and other resources. High biodiversity also makes ecosystems more stable and resilient to change, which benefits human populations that depend on them. Biodiversity does not guarantee species will never go extinct, and it does not remove the need for farming, since most food still comes from managed crops and livestock.

What is the correct description of a genome?

• AOne strand of a DNA double helix only
• BThe entire genetic material of an organism✓
• CThe physical characteristics an organism displays
• DA single gene that codes for one protein

Answer: A genome is the entire set of genetic material found in an organism, including all of its genes. This is different from a single gene, which codes for only one protein, and different from a phenotype, which describes an organism's observable characteristics. A genome includes both strands of the DNA double helix, not just one.

In genetics, what is an allele?

• AA different version of the same gene✓
• BThe observable characteristic produced by a gene
• CAn organism carrying two identical gene alleles
• DThe physical position of a gene on a chromosome

Answer: A gene can exist in different forms, and each of these different forms is called an allele. For example, a gene for eye colour might have a brown allele and a blue allele, both coding for the same characteristic but producing different versions of it. An allele is not a chromosome location, a homozygous organism, or a phenotype itself.

An organism with the genotype Bb for a gene is described as what?

• AHomozygous, because it carries two identical alleles for that gene
• BHomozygous, because it carries two different alleles for that gene
• CHeterozygous, because it carries two different alleles for that gene✓
• DHeterozygous, because it carries two identical alleles for that gene

Answer: An organism is heterozygous for a gene when it carries two different alleles, such as B and b. A homozygous organism carries two identical alleles, either both dominant or both recessive. Because Bb contains one of each allele, it is correctly described as heterozygous rather than homozygous.

A plant's genotype gives it the potential to grow tall, but poor soil conditions mean it only grows to a medium height. What does this show?

• APhenotype is determined entirely by the environment, and genotype has no influence at all
• BPhenotype is influenced by both genotype and the environment, not by genotype alone✓
• CGenotype changes permanently in response to environmental conditions during the plant's lifetime
• DPhenotype is determined entirely by genotype, and the environment has no influence at all

Answer: Phenotype, an organism's observable characteristics, is the result of both its genotype and the environment it experiences. A plant with the genetic potential for tall growth may still end up shorter if it grows in poor soil, showing that environment can limit what genotype alone would otherwise produce. Phenotype is therefore never determined entirely by genotype or entirely by environment on its own, and genotype itself does not change permanently in response to environmental conditions.

In pea plants, the allele for tall stems (T) is dominant over the allele for short stems (t). If two heterozygous (Tt) plants are crossed, what proportion of the offspring is expected to be short?

• AOne quarter, because only offspring with two recessive alleles (tt) are short✓
• BThree quarters, because most offspring will show the recessive characteristic
• COne half, because half the offspring will inherit the recessive allele from at least one parent
• DNone, because the dominant tall allele always masks the recessive allele completely

Answer: Crossing two heterozygous (Tt) parents produces offspring in a 1:2:1 ratio of TT, Tt and tt genotypes. Only the tt offspring, which make up one quarter of the total, lack a dominant tall allele and so show the recessive short phenotype. The dominant allele does not always eliminate the recessive characteristic completely, since it can still be passed on and expressed when paired with another recessive allele.

Human height varies continuously across a population, producing a smooth range from short to tall. What does this suggest about the inheritance of height?

• AHeight is controlled by multiple genes interacting together, not by a single gene✓
• BHeight is controlled by a single recessive gene inherited only from the mother
• CHeight is controlled by a single dominant gene with two clear alleles
• DHeight is controlled entirely by environmental factors, with no genetic contribution

Answer: Characteristics controlled by a single gene with two alleles typically produce a small number of distinct categories, not a smooth range of values. Human height instead varies continuously across a population, which is a strong indication that many genes act together to influence it. Height is not controlled by a single dominant or recessive gene, and although environment such as diet plays a role, genetics also makes a substantial contribution.

How is the sex of a human offspring determined?

• ABoth parents always contribute a Y chromosome if the offspring is to be male
• BThe mother's egg carries either an X or a Y chromosome, which combines with the father's X chromosome
• CThe father's sperm carries either an X or a Y chromosome, which combines with the mother's X chromosome✓
• DSex is determined by hormone levels in the mother during pregnancy, not by chromosomes

Answer: In humans, egg cells always carry an X chromosome, while sperm cells carry either an X or a Y chromosome. If a sperm carrying a Y chromosome fertilises the egg, the offspring develops as male; if it carries an X chromosome, the offspring develops as female. Hormone levels in the mother do not determine the offspring's sex, since this is fixed at fertilisation by which chromosome the sperm contributes.

What is a mutation in genetics?

• AThe process of a cell dividing to form two identical daughter cells
• BA change in an organism's phenotype caused only by its environment
• CThe combination of alleles inherited from two parents
• DA change in the DNA sequence of an organism's genome✓

Answer: A mutation is a random change in the sequence of bases within an organism's DNA. This is different from the combination of alleles inherited from two parents, and different from mitosis, which produces genetically identical daughter cells. A mutation affects the genome itself, not simply the phenotype through environmental influence alone.

According to the theory of evolution by natural selection, why do certain characteristics become more common in a population over many generations?

• ACharacteristics become common purely by random chance, with no link to survival or reproduction
• BIndividuals with characteristics better suited to their environment are more likely to survive and reproduce, passing those characteristics on✓
• CIndividuals deliberately change their characteristics during their lifetime to suit their environment better
• DAll individuals in a population reproduce at exactly the same rate regardless of their characteristics

Answer: Natural selection favours individuals whose characteristics make them better suited to survive and reproduce in their environment. Because these individuals are more likely to pass their genes to the next generation, useful characteristics become more common in the population over time. Individuals cannot deliberately change their own characteristics to adapt, and if all individuals reproduced at the same rate regardless of their traits, natural selection would not occur.

Bacteria in a population develop resistance to an antibiotic after repeated exposure. Which explanation for this is best supported by natural selection?

• AThe bacteria built up a tolerance to the antibiotic during their lifetimes
• BResistant bacteria already present survived the antibiotic and reproduced, passing on the resistance allele✓
• CThe patient became resistant to the antibiotic, so it stopped working on the bacteria
• DThe antibiotic itself caused mutations, creating resistance in the bacteria it reached

Answer: Antibiotic resistance in bacteria is strong evidence for evolution by natural selection. A small number of bacteria already carry a resistance allele before exposure to the antibiotic; these survive treatment and reproduce, passing the resistance allele on to future generations. The antibiotic itself does not cause the resistance mutation, bacteria do not build up a tolerance during their lifetimes, and it is the bacteria rather than the patient that become resistant.

How does selective breeding differ from genetic engineering as a way of producing organisms with desired characteristics?

• ASelective breeding produces results within a single generation, whereas genetic engineering takes many generations
• BSelective breeding chooses parents with desired characteristics to breed over generations, whereas genetic engineering directly changes an organism's genome✓
• CSelective breeding directly changes an organism's genome, whereas genetic engineering chooses parents with desired characteristics to breed
• DSelective breeding only ever works on plants, whereas genetic engineering only ever works on animals

Answer: Selective breeding involves choosing organisms with desirable characteristics to breed together, gradually increasing the frequency of those characteristics over many generations. Genetic engineering instead directly alters an organism's genome in a laboratory, transferring or editing genes rather than relying on breeding over time. Both techniques can be applied to plants and animals, and selective breeding typically takes many generations rather than producing results within just one.

In a reflex arc, the sensory neurone and the motor neurone are each 0.90 m long, and impulses travel along both at 60 m/s. The impulse also crosses two synapses, each of which delays it by 0.002 s.

What is the total time from the receptor being stimulated to the impulse reaching the effector?

• A30 ms
• B34 ms✓
• C32 ms
• D19 ms

Answer: Travel time along both neurones = 1.80 m ÷ 60 m/s = 0.030 s. Add the two synaptic delays: 0.030 + 2 × 0.002 = 0.034 s = 34 ms. The tempting 30 ms leaves out the synapses, 32 ms includes only one synapse, and 19 ms counts only one neurone (0.015 s) plus both delays. Synapses slow the impulse because a chemical must diffuse across the gap.

Two people each drank a glucose solution after fasting, and their blood glucose was measured. Time / min: 0, 30, 60, 90, 120 Person X / mmol/dm³: 5.0, 8.0, 6.5, 5.5, 5.0 Person Y / mmol/dm³: 7.0, 14.0, 16.5, 15.5, 14.0

Which explanation best fits person Y's results?

• AY's pancreas makes little or no insulin, so glucose is not moved into the cells✓
• BY's pancreas makes too much glucagon, so the glucose cannot be stored as glycogen
• CY's liver stores glucose as glycogen faster, so the glucose is released from it later
• DY absorbs glucose from the gut slowly, so the glucose takes longer to reach the blood

Answer: In X, blood glucose rises then returns to normal within two hours because the pancreas releases insulin, which makes cells take up glucose and the liver store it as glycogen. Y starts higher, rises further and stays high: the pattern of type 1 diabetes, where the pancreas makes little or no insulin. Glucagon is released when glucose is LOW, storing glucose faster would bring the level down, and Y's glucose rises faster than X's, so it is not being absorbed slowly.

In IVF treatment, a woman is first given injections of FSH and LH.

What is the purpose of these hormones?

• ATo thicken the uterus lining ready for the embryo to implant
• BTo stop the eggs maturing until the doctors are ready to collect them
• CTo make several eggs mature so that they can be collected✓
• DTo help the sperm swim faster towards the eggs in the laboratory

Answer: FSH stimulates eggs to mature in the ovaries, and LH triggers their release, so the hormones give several mature eggs to collect and fertilise in the laboratory. The uterus lining is built up and maintained by oestrogen and progesterone, not by FSH and LH; stopping eggs from maturing is how hormonal contraception works, and the hormones act on the woman, not on the sperm.

Light intensity is inversely proportional to the square of the distance from a lamp. A leaf disc produces oxygen at a rate of 48 units when the lamp is 10 cm away.

Assuming the rate is proportional to light intensity, what is the predicted rate when the lamp is moved to 20 cm?

• A12 units✓
• B24 units
• C6 units
• D96 units

Answer: Doubling the distance makes the light intensity 1 ÷ 2² = ¼ as great, so the predicted rate is 48 ÷ 4 = 12 units. The tempting 24 treats intensity as simply inversely proportional to distance, 6 divides by 2³, and 96 assumes moving the lamp away increases the light. In practice the rate is only proportional to light intensity while light is the limiting factor.

The rate of photosynthesis of a plant was measured at different light intensities and two carbon dioxide concentrations, at a constant temperature. Light intensity (arbitrary units): 1, 2, 3, 4 Rate at 0.04% CO₂: 10, 20, 25, 25 Rate at 0.10% CO₂: 10, 21, 40, 52

What is limiting the rate at light intensity 4 and 0.04% CO₂?

• ALight intensity, as the rate at 0.04% CO₂ keeps rising as the light gets brighter
• BCarbon dioxide concentration, as raising it increases the rate at that light intensity✓
• CTemperature, as the rate stops rising at both of the carbon dioxide concentrations
• DChlorophyll, as the rate is the same at both of the carbon dioxide concentrations

Answer: At 0.04% CO₂ the rate levels off at 25 between light intensities 3 and 4, so more light makes no difference there. At the same light intensity, raising the CO₂ to 0.10% raises the rate to 52, so CO₂ concentration was the limiting factor. The rate at 0.04% does not keep rising, at 0.10% it is still rising, and the two concentrations give very different rates at light intensity 4.

A couple already have three sons. They are expecting a fourth child.

What is the probability that this child will be a girl?

• A25%
• B50%✓
• C75%
• D12.5%

Answer: Sex is decided at fertilisation: the mother's egg always carries an X chromosome and half of the father's sperm carry X and half carry Y, so every child has a 50% chance of being XX (a girl). Each fertilisation is independent, so earlier children do not change the odds. The other values wrongly treat the earlier births as affecting this one, for example 12.5% = ½ × ½ × ½.