Cells, Organisation & Health

What is the main function of the nucleus in an animal cell?

• AIt produces energy through aerobic respiration
• BIt carries out photosynthesis to make glucose
• CIt contains genetic material that controls the cell's activities✓
• DIt controls the movement of substances into and out of the cell

Answer: The nucleus contains the cell's DNA, organised into chromosomes, which controls the cell's activities. Mitochondria, not the nucleus, release energy through aerobic respiration. The cell membrane, not the nucleus, controls which substances enter and leave the cell.

What are plasmids in a bacterial cell?

• AMembrane-bound structures that release energy by respiration
• BFolds of membrane that control substance movement
• CSmall circular loops of DNA separate from the chromosome✓
• DMembrane-bound structures that carry out photosynthesis

Answer: Plasmids are small circular loops of DNA found in bacterial cells, separate from the main chromosome, and can carry useful genes such as antibiotic resistance. Mitochondria are membrane-bound organelles that release energy by aerobic respiration, not DNA loops. Chloroplasts are membrane-bound organelles that carry out photosynthesis and are only found in plant cells, not bacteria.

Which organelle releases energy for the cell through aerobic respiration?

• ARibosomes
• BMitochondria✓
• CGolgi apparatus
• DCell membrane

Answer: Mitochondria are the organelles where aerobic respiration takes place, releasing energy for the cell to use. The Golgi apparatus processes and packages proteins rather than releasing energy from respiration. Ribosomes build proteins and the cell membrane controls what enters and leaves the cell, so neither releases energy through respiration.

What is the function of chloroplasts in plant cells?

• AControlling the entry and exit of substances across the membrane
• BReleasing energy for the cell through aerobic respiration
• CAbsorbing light energy to carry out photosynthesis✓
• DStoring genetic material that controls the cell's activities

Answer: Chloroplasts contain chlorophyll, which absorbs light energy so the plant can carry out photosynthesis and make glucose. Mitochondria, not chloroplasts, release energy through aerobic respiration. The cell membrane controls which substances enter and leave the cell, and the nucleus stores the genetic material.

Why did the development of electron microscopy improve scientists' understanding of cells?

• AIt magnifies specimens strongly, without needing any special sample preparation
• BIt allows living cells to be observed moving and dividing in real time
• CIt offers much higher magnification and resolution than light microscopes, revealing tiny sub-cellular structures✓
• DIt uses coloured stains to show which chemicals are present in a cell

Answer: Electron microscopes use beams of electrons rather than light, giving far higher magnification and resolution than light microscopes. This let scientists see tiny sub-cellular structures, such as ribosomes and the internal structure of mitochondria, that light microscopes cannot resolve. Electron microscopy requires the sample to be specially prepared and cannot be used to observe living cells.

What is the role of mitosis in the cell cycle?

• AIt produces four genetically different cells for sexual reproduction
• BIt produces two genetically identical cells for growth and repair✓
• CIt breaks down glucose to release energy for the cell
• DIt halves the number of chromosomes to form gametes

Answer: Mitosis is a type of cell division that produces two new cells with identical genetic material to the parent cell, allowing the body to grow and to repair damaged tissue. Meiosis, not mitosis, produces four genetically different gametes with half the chromosome number. Respiration, not mitosis, breaks down glucose to release energy.

What happens to a cell during differentiation?

• AIt halves its chromosome number to form a gamete
• BIt becomes specialised to carry out a particular function✓
• CIt loses its nucleus and all of its genetic material entirely
• DIt divides over and over again without ever specialising for a role

Answer: Differentiation is the process by which a cell changes to become specialised for a particular job, such as a nerve cell or a muscle cell, usually by developing a specific structure. Most animal cells lose the ability to differentiate once specialised, so differentiation does not simply mean repeated division. Halving the chromosome number happens in meiosis, not differentiation.

What causes cancer to develop in the body?

• AA shortage of insulin that causes blood glucose levels to rise sharply
• BA virus that gradually destroys the body's white blood cells over time
• CA blockage that prevents enough oxygen from reaching the body's tissues
• DChanges in cells that lead to uncontrolled growth and division✓

Answer: Cancer results from changes in cells that cause them to grow and divide in an uncontrolled way, which can form a tumour. HIV, not cancer, is a virus that damages white blood cells and weakens the immune system. A shortage of insulin causing high blood glucose describes type 1 diabetes, not cancer.

How do embryonic stem cells differ from adult stem cells in the human body?

• AEmbryonic stem cells can differentiate into any cell type, while adult stem cells can only form a limited range of cells✓
• BEmbryonic stem cells cannot divide, while adult stem cells divide continuously throughout life
• CEmbryonic stem cells can only form blood cells, while adult stem cells can form any cell type
• DEmbryonic stem cells are found only in bone marrow, while adult stem cells are found in embryos

Answer: Embryonic stem cells are unspecialised and can differentiate into almost any type of cell, whereas adult stem cells, such as those in bone marrow, can only differentiate into a limited number of related cell types. It is embryonic stem cells that have this wide potential, not adult stem cells, which is why they are so valuable in medical research. Adult stem cells continue to divide throughout life to replace damaged cells, but they do not have unlimited potential like embryonic stem cells.

What is the main outcome of meiosis in humans?

• AOne large cell containing double the normal chromosome number
• BFour identical gametes, each with the normal chromosome number
• CFour genetically different gametes, each with half the normal chromosome number✓
• DTwo genetically identical body cells, each with the full normal chromosome number

Answer: Meiosis is the type of cell division that produces gametes; it creates four cells that are genetically different from one another and from the parent cell, each with half the normal chromosome number. Mitosis, not meiosis, produces two genetically identical body cells with the full chromosome number. Fertilisation, not meiosis, combines two gametes to restore the full chromosome number.

Why does an enzyme only catalyse reactions involving a specific substrate?

• AIts structure is identical to every other enzyme in the body
• BIts active site can freely change shape to fit any substrate molecule
• CIts active site absorbs light energy to help activate the substrate molecule
• DIts active site has a shape that fits only that particular substrate✓

Answer: Each enzyme has an active site with a shape that fits only a particular substrate molecule, like a lock and key, which is why enzymes are specific to particular reactions. If the active site changed shape to fit anything, the enzyme would lose this specificity. Enzymes are proteins with distinct three-dimensional shapes, so no two types are structurally identical.

What is the key difference between aerobic and anaerobic respiration in animal cells?

• AAnaerobic respiration always uses oxygen while aerobic respiration does not use any oxygen at all
• BAnaerobic respiration produces only carbon dioxide and water as its final waste products overall
• CAerobic respiration uses oxygen and releases much less energy overall than anaerobic respiration
• DAerobic respiration uses oxygen and releases much more energy than anaerobic respiration✓

Answer: Aerobic respiration uses oxygen to break down glucose fully, releasing much more energy than anaerobic respiration, which happens without oxygen. Anaerobic respiration in animal cells produces lactic acid rather than fully breaking down glucose as aerobic respiration does. Carbon dioxide and water are the waste products of aerobic respiration, not anaerobic respiration in animals.

What is diffusion, as it applies to substances moving in and out of cells?

• AThe movement of particles against a concentration gradient using energy from respiration
• BThe net movement of particles from a lower to a higher concentration using energy
• CThe net movement of particles from an area of higher concentration to an area of lower concentration✓
• DThe net movement of water from a dilute to a concentrated solution across a membrane

Answer: Diffusion is the net movement of particles, such as oxygen or carbon dioxide, from a region of higher concentration to a region of lower concentration, and it does not require energy from the cell. Osmosis specifically describes the movement of water across a partially permeable membrane, not general particle movement. Active transport, not diffusion, moves particles against a concentration gradient and requires energy from respiration.

What is osmosis?

• AThe net movement of dissolved sugar molecules down a concentration gradient without using energy
• BThe net movement of water molecules from a concentrated to a more dilute solution through a partially permeable membrane
• CThe movement of water molecules against a concentration gradient using energy from the cell
• DThe net movement of water molecules from a dilute to a concentrated solution through a partially permeable membrane✓

Answer: Osmosis is the net movement of water molecules from a dilute solution to a concentrated solution through a partially permeable membrane. Water actually moves from a dilute region, where there is a higher concentration of water molecules, towards a more concentrated solution, not the other way round. Osmosis refers specifically to water movement, not the diffusion of dissolved substances such as sugars, and it does not require energy from the cell.

Why do root hair cells sometimes need active transport, rather than diffusion, to take up mineral ions from the soil?

• ABecause water is more concentrated inside the root hair cell than in the surrounding soil
• BBecause mineral ions are always able to move freely across cell membranes without needing any specific transport process
• CBecause mineral ions are often more concentrated inside the root hair cell than in the soil, so uptake needs energy✓
• DBecause mineral ions are always less concentrated inside the root hair cell than in the soil, so no energy is needed

Answer: Mineral ions are often at a lower concentration in the soil than inside root hair cells, so they cannot move in simply by diffusion down a concentration gradient; active transport uses energy from respiration to move them in against the gradient. If ions were less concentrated inside the cell than in the soil, diffusion alone could bring them in without extra energy. Water potential, not mineral ion movement, is what osmosis describes.

Why do multicellular organisms need specialised exchange surfaces and transport systems?

• ATheir much larger surface area to volume ratio means diffusion alone is too slow to supply every single cell
• BTheir smaller surface area to volume ratio actually makes diffusion much faster than in small organisms
• CTheir individual cells no longer need any oxygen or nutrients once the organism becomes multicellular
• DTheir smaller surface area to volume ratio means diffusion alone is too slow to supply every cell✓

Answer: As organisms become larger, their surface area to volume ratio decreases, so diffusion across the outer surface alone becomes too slow to supply oxygen and nutrients to every cell, making specialised exchange surfaces and transport systems necessary. A larger surface area to volume ratio, as found in small organisms, actually favours diffusion rather than working against it. All cells, however large the organism, continue to need a steady supply of oxygen and nutrients.

Which substance is carried by the blood plasma to the kidneys for excretion?

• AGlucose, used as fuel for cellular respiration
• BFibrinogen, a protein involved in blood clotting
• CUrea, a waste product of breaking down amino acids✓
• DOxygen, needed for aerobic respiration in cells

Answer: Urea is a waste product formed when excess amino acids are broken down in the liver; it is carried in the blood plasma to the kidneys, where it is removed from the body in urine. Oxygen and glucose are useful substances the blood delivers to cells, not waste products for excretion. Fibrinogen is a clotting protein that stays in the blood rather than being excreted.

Why do arteries have thick, muscular, elastic walls?

• ATo withstand the naturally low pressure of blood as it slowly returns to the heart
• BTo withstand the high pressure of blood as it is pumped away from the heart✓
• CTo allow substances to diffuse easily between the blood and body cells
• DTo store large volumes of blood at low pressure around the body

Answer: Arteries carry blood away from the heart at high pressure, so their thick, muscular and elastic walls withstand this pressure and help push blood along. Veins, not arteries, carry blood back to the heart at low pressure and have thinner walls with valves. Capillaries, not arteries, have very thin walls that allow substances to diffuse between blood and body cells.

How are red blood cells adapted to carry oxygen efficiently?

• AThey contain haemoglobin but keep a nucleus to control oxygen binding
• BThey contain haemoglobin and have no nucleus, leaving more space for oxygen✓
• CThey contain antibodies and have no nucleus at all, leaving much more space for oxygen
• DThey contain no haemoglobin but have a biconcave shape to increase surface area

Answer: Red blood cells contain haemoglobin, which binds to oxygen, and they lack a nucleus, which leaves more internal space to carry oxygen and gives them their biconcave shape. Red blood cells do not have a nucleus at all, so they cannot use one to control anything. Antibodies are made by white blood cells, not carried by red blood cells, which rely on haemoglobin to transport oxygen.

What is the main role of platelets in the blood?

• ACarrying oxygen around the whole body bound tightly to red haemoglobin molecules
• BCarrying digested food molecules that are dissolved in the blood plasma
• CProducing antibodies that help destroy pathogens circulating in the blood
• DHelping blood to clot at a wound to prevent blood loss and infection✓

Answer: Platelets are small cell fragments that help blood to clot at the site of a wound, sealing the wound and helping to prevent both blood loss and the entry of pathogens. Carrying oxygen bound to haemoglobin is the role of red blood cells, not platelets. Producing antibodies against pathogens is the role of white blood cells, not platelets.

What is the function of xylem tissue in a plant?

• ATransporting water and dissolved mineral ions from the roots to the leaves✓
• BControlling gas exchange through pores in the leaf surface
• CTransporting dissolved sugars from the leaves to the rest of the plant
• DAbsorbing light energy for photosynthesis in the leaf cells

Answer: Xylem is made of dead, hollow cells that form tubes carrying water and dissolved mineral ions upward from the roots to the leaves, in a process driven partly by transpiration. Phloem, not xylem, transports dissolved sugars made by photosynthesis from the leaves to the rest of the plant. Xylem tissue does not absorb light or control gas exchange; those are functions of chloroplasts and stomata.

What is translocation in a plant?

• AThe movement of dissolved sugars through phloem tissue from leaves to other parts of the plant✓
• BThe evaporation of water vapour from the surface of the leaf
• CThe movement of water and dissolved mineral ions through xylem tissue from the roots to the leaves
• DThe diffusion of carbon dioxide into the leaf through open stomata

Answer: Translocation is the movement of dissolved sugars, made during photosynthesis, through phloem tissue from the leaves to other parts of the plant, such as the roots and growing tissues. The movement of water and mineral ions through xylem from roots to leaves is a separate process, driven by transpiration, not translocation. Diffusion of carbon dioxide into the leaf and evaporation of water vapour are separate processes involved in photosynthesis and transpiration.

Which environmental change would most increase the rate of transpiration from a leaf?

• AAn increase in air movement around the leaf✓
• BA decrease in air movement around the leaf
• CAn increase in humidity of the air around the leaf
• DA decrease in light intensity falling on the leaf

Answer: Increasing air movement around a leaf removes water vapour from just outside the stomata more quickly, steepening the concentration gradient and increasing the rate of transpiration. Decreasing air movement or increasing humidity would slow the loss of water vapour, reducing transpiration rather than increasing it. Lower light intensity causes stomata to close more, which would decrease transpiration, not increase it.

What is the difference between a communicable and a non-communicable disease?

• AA non-communicable disease is always caused only by a virus, while a communicable disease is not
• BA non-communicable disease can also spread between organisms, while a communicable disease cannot
• CA communicable disease is always caused by a poor diet, while a non-communicable disease is not
• DA communicable disease can spread between organisms, while a non-communicable disease cannot✓

Answer: A communicable disease is caused by a pathogen and can be passed from one organism to another, whereas a non-communicable disease, such as cardiovascular disease, cannot be spread between organisms. It is the communicable disease that spreads, not the non-communicable one, since non-communicable diseases are not caused by pathogens. Non-communicable diseases can have many causes, including diet, but are not defined simply by being diet-related, while communicable diseases are caused by pathogens rather than by viruses alone.

Which type of pathogen causes measles?

• AA protist, which is usually spread by an animal vector
• BA fungus, which often spreads through infectious spores
• CA bacterium, which can reproduce outside living cells
• DA virus, which infects and reproduces inside living cells✓

Answer: Measles is caused by a virus, and viruses can only reproduce inside living host cells, taking over the cell's machinery to make copies of themselves. Bacteria are living cells that can reproduce independently, unlike viruses, and typically do not need a host cell to divide. Protists, such as the malaria parasite, and fungi, such as those causing athlete's foot, cause different diseases and spread in different ways from measles.

How does the bacterium that causes salmonella food poisoning make a person feel ill?

• AIt hijacks body cells and forces them to make many copies of the bacterium
• BIt blocks light from reaching the retina, causing symptoms
• CIt reproduces inside the body and releases toxins that cause symptoms✓
• DIt removes red blood cells directly from the person's bloodstream

Answer: Salmonella bacteria reproduce rapidly inside the body and release toxins, which cause the fever, cramps, vomiting and diarrhoea typical of food poisoning. Hijacking body cells to make copies of itself describes how a virus reproduces, not a bacterium, since bacteria can reproduce independently. Bacteria causing food poisoning do not remove red blood cells or affect the retina.

What causes the plant disease known as rose black spot?

• AA virus, which is typically spread mainly by a specific insect vector
• BA fungus, which is spread in the environment by water or wind✓
• CA protist, which is spread by an infected mosquito
• DA bacterium, which is spread through contaminated soil and water

Answer: Rose black spot is caused by a fungus, and its spores are spread in the environment by water and by wind, causing purple or black spots on leaves and reducing photosynthesis. Tobacco mosaic virus, a different plant pathogen, is instead spread mainly through direct contact — via touch, contaminated tools or hands — rather than by water or wind like rose black spot. Malaria, caused by a protist and spread by mosquitoes, is a human disease, not the cause of rose black spot.

How is HIV mainly spread between people?

• AThrough sexual contact or exchange of bodily fluids such as blood✓
• BThrough close contact with water that contains the malaria protist
• CThrough breathing in tiny droplets from an infected person's cough
• DThrough eating food that has become contaminated with harmful bacteria

Answer: HIV is a sexually transmitted infection, spread mainly through sexual contact or the exchange of bodily fluids such as blood, and it can eventually lead to AIDS by damaging the immune system. Diseases spread by coughed droplets, such as measles, are transmitted very differently from HIV. HIV is a virus, not a bacterium or a protist, so it is not spread through contaminated food or through water containing the malaria parasite.

Which of these is a non-specific defence that helps stop pathogens entering the human body?

• AWhite cells making antibodies against a pathogen
• BVaccination triggering a specific immune response
• CMemory cells recognising a previous infection
• DThe skin acting as a physical barrier against pathogens✓

Answer: The skin acts as a non-specific barrier, physically stopping most pathogens from entering the body, along with defences such as mucus and stomach acid. Producing antibodies against a specific pathogen and memory cells recognising a previous infection are both part of the specific immune response, not a non-specific defence. Vaccination triggers this specific immune response too, rather than being a non-specific barrier.

What is the role of antibodies produced by white blood cells during an infection?

• AReleasing toxins that damage healthy body cells
• BBinding to specific pathogens to help destroy them✓
• CBinding to any pathogen in the body without any specificity
• DAbsorbing oxygen to carry to infected tissue

Answer: White blood cells produce antibodies that bind specifically to antigens on a particular pathogen, marking it for destruction or preventing it from functioning. Antibodies are highly specific to one type of pathogen, so they do not bind to any pathogen without distinction. Carrying oxygen is the role of red blood cells, and releasing toxins that damage healthy cells is not a normal function of antibodies.

How does a vaccine protect a person against a disease?

• AIt contains painkillers that reduce the symptoms of an existing infection
• BIt contains a large, fully active dose of the actual pathogen that causes a mild version of the disease
• CIt contains a small, safe amount of a dead or inactive pathogen that triggers an immune response✓
• DIt contains a full course of strong antibiotics that kill any bacteria already present in the body

Answer: A vaccine usually contains a small, safe quantity of a dead or inactivated pathogen, or a fragment of one, which triggers the immune system to produce antibodies and memory cells without causing the disease itself. It does not contain a large active dose of a pathogen, since that could cause the illness it is meant to prevent. Vaccines work by stimulating immunity rather than containing antibiotics or painkillers, which treat existing infections rather than preventing future ones.

Why must new medicines go through clinical trials on human volunteers before wide use?

• ATo test that the medicine has already successfully passed all of its earlier stages of preclinical testing
• BTo test that the medicine tastes pleasant enough for most patients to take it regularly every single day
• CTo test that the medicine can be manufactured as cheaply and quickly as possible for general public sale
• DTo test that the medicine is both safe and effective at treating the intended condition in humans✓

Answer: Clinical trials test a new medicine on human volunteers to check that it is safe, at what dose, and that it is effective at treating the condition it is designed for, after earlier laboratory and animal testing. Taste and manufacturing cost are not the main safety and effectiveness questions that clinical trials are designed to answer. Clinical trials happen after preclinical testing on cells and tissues, rather than testing whether preclinical testing itself has already happened.

Which lifestyle factor is most strongly linked to an increased risk of type 2 diabetes?

• ADrinking alcohol far above recommended weekly limits
• BBeing underweight as a result of a very restricted diet
• CBeing overweight as a result of a poor diet and low activity levels✓
• DSmoking cigarettes containing nicotine and tar

Answer: Being overweight, often caused by a diet high in sugar and fat combined with low activity levels, is strongly linked to an increased risk of developing type 2 diabetes. Being underweight is not the lifestyle risk factor linked to type 2 diabetes in the way that being overweight is. Smoking and excess alcohol are linked more strongly to other non-communicable diseases, such as lung disease and liver disease, than to type 2 diabetes specifically.

Why might a doctor recommend statins rather than a stent to help a patient with cardiovascular disease?

• AStatins are a surgical procedure that physically widens a narrowed coronary artery
• BStatins are a non-surgical option that lowers blood cholesterol to reduce the risk of further disease✓
• CStents are a non-surgical option that also lowers blood cholesterol to reduce the risk of further disease
• DStatins remove the need for any change to the patient's diet or exercise levels

Answer: Statins are drugs, not a surgical procedure, that reduce the amount of cholesterol in the blood, lowering the risk of fatty deposits building up in arteries and reducing the risk of further cardiovascular disease. Stents, not statins, are the option that involves a device inserted during a procedure to physically hold a narrowed artery open. Taking statins does not remove the benefit of a healthy diet and exercise, since lifestyle changes remain an important part of treatment.

An electron micrograph shows a mitochondrion. The image of the mitochondrion is 45 mm long and the magnification is ×15 000.

What is the real length of the mitochondrion?

• A3 µm✓
• B0.3 µm
• C30 µm
• D300 µm

Answer: Real size = image size ÷ magnification = 45 mm ÷ 15 000 = 0.003 mm. There are 1000 µm in 1 mm, so 0.003 mm = 3 µm. The other values come from slipping a power of ten when converting millimetres to micrometres: 30 µm and 300 µm multiply by too much, and 0.3 µm by too little.

Two single-celled organisms are modelled as cubes. Organism P is a cube of side 20 µm and organism Q is a cube of side 60 µm.

How many times larger is the surface area to volume ratio of P than that of Q?

• A9 times larger
• B27 times larger
• C3 times larger✓
• DThe two ratios are equal

Answer: For a cube of side L, surface area = 6L² and volume = L³, so the ratio is 6 ÷ L. P: 6 ÷ 20 = 0.3 per µm; Q: 6 ÷ 60 = 0.1 per µm. P's ratio is 0.3 ÷ 0.1 = 3 times larger. The tempting 9 is how many times larger Q's surface area is, and 27 is how many times larger Q's volume is; the ratio falls as the cell gets bigger, which is why large organisms need exchange surfaces and transport systems.

A student investigated the effect of temperature on a protease. The protease digests the protein in a cloudy milk suspension, and the student timed how long each mixture took to turn clear. Temperature / °C: 20, 30, 40, 50, 60 Time to turn clear / s: 240, 130, 75, 110, no change after 600

How many times faster is the reaction at 40 °C than at 20 °C?

• A2.0 times faster
• B0.31 times faster
• C165 times faster
• D3.2 times faster✓

Answer: Rate is inversely proportional to the time taken, so the rate ratio is 240 s ÷ 75 s = 3.2. The tempting 2.0 compares the temperatures (40 ÷ 20) rather than the rates; 0.31 is 75 ÷ 240, which gives the ratio the wrong way round, and 165 is the difference in seconds, not a ratio. At 60 °C the protease is denatured: its active site changes shape, so the protein no longer fits.

Three blood vessels were examined. Vessel P: thick wall with much muscle and elastic tissue; blood pressure rises and falls with each heartbeat Vessel Q: wall one cell thick; blood pressure low and steady Vessel R: thin wall and wide lumen; contains valves; blood pressure lowest of the three

Which identifies vessels P, Q and R?

• AP vein, Q capillary, R artery
• BP artery, Q capillary, R vein✓
• CP artery, Q vein, R capillary
• DP capillary, Q artery, R vein

Answer: Arteries carry blood at high pressure straight from the heart, so they have thick muscular, elastic walls and their pressure pulses with each beat. Capillaries have walls one cell thick, giving a short diffusion distance to the tissues. Veins carry low-pressure blood back to the heart, so they have thin walls, a wide lumen and valves to stop backflow. Swapping arteries and veins ignores both the pressure data and the valves.

In a five-year trial, 2000 people at risk of coronary heart disease took a statin and 2000 similar people took a placebo. Heart attacks in the statin group: 56 Heart attacks in the placebo group: 92

By what percentage did taking the statin reduce the number of heart attacks?

• A39%✓
• B61%
• C64%
• D1.8%

Answer: Reduction = (92 − 56) ÷ 92 × 100 = 36 ÷ 92 × 100 = 39%, measured against the placebo group, which shows what would have happened without the drug. The tempting 64% divides the 36 fewer heart attacks by the statin group's 56 instead; 61% is the statin group's count as a percentage of the placebo group's, and 1.8% expresses the difference as a share of all 2000 people in a group.

Measles vaccination uptake and measles cases in a country: Year 1: 90% of children vaccinated; 1200 cases Year 2: 95% of children vaccinated; 300 cases Year 3: 85% of children vaccinated; 2400 cases

Which conclusion is best supported by these data?

• ACases were lowest when uptake was highest, consistent with herd immunity protecting unvaccinated people✓
• BCases were lowest when uptake was highest, showing that the vaccine works by killing the measles virus directly
• CCases rose when uptake fell, showing that the measles virus had mutated into a new strain in that year
• DCases fell by the same number each time uptake rose by five percentage points across the three years

Answer: The more children vaccinated, the fewer cases there were, so a high uptake means the virus finds few people to spread to and even unvaccinated people are protected: herd immunity. A vaccine contains dead or inactive pathogens that stimulate white blood cells to make antibodies; it does not kill the virus itself. The data give no evidence of a new strain, and the changes are not equal: from 85% to 90% cases fell by 1200, but from 90% to 95% they fell by 900.