IGCSE Biology
A student wrote · Characteristics and Classification of Living OrganismsA species is a group of organisms that look the same as each other.
Why it loses marksLooking alike is not the definition. Two unrelated species can look almost identical, and the male and female of one species can look completely different. The examiner wants the test of breeding.
The answer that scoresA species is a group of organisms that can reproduce to produce fertile offspring. The offspring must be fertile: a horse and a donkey can breed, but the mule they produce is sterile, so horses and donkeys are separate species.
Revise this topicCharacteristics and Classification of Living Organisms
A student wrote · Characteristics and Classification of Living OrganismsThe scientific name of a human is Homo Sapiens, and the genus is sapiens.
Why it loses marksThere are two errors. The species part never takes a capital letter, and the genus is the first name of the two, not the second.
The answer that scoresThe scientific name is Homo sapiens: genus Homo, with a capital H, followed by species sapiens, with a lower case s, with the whole name in italics or underlined. The genus always comes first and the species second.
Revise this topicCharacteristics and Classification of Living Organisms
A student wrote · Characteristics and Classification of Living OrganismsViruses belong to the prokaryote kingdom because they are very small and have no nucleus.
Why it loses marksViruses are not cells at all, so they cannot be placed in any of the five kingdoms. Being small is not one of the features used to classify organisms.
The answer that scoresViruses are not placed in any kingdom because they are not cells. A virus consists of genetic material surrounded by a protein coat, and it can only reproduce inside a living host cell, so it cannot carry out the life processes on its own.
Revise this topicCharacteristics and Classification of Living Organisms
A student wrote · Characteristics and Classification of Living OrganismsIn my key, couplet 1 reads: 1a the organism is small and lives in water, 1b the organism has wings.
Why it loses marksThe two statements in a couplet must be a pair of opposites describing the same observable feature, so that exactly one is true for each specimen. Mixing size and habitat with wings means both statements could be true at once, and the key breaks down.
The answer that scoresEach couplet must offer two opposite statements about the same observable feature, for example '1a has wings ... go to 2' and '1b has no wings ... go to 3'. Every specimen must fit exactly one statement at each step, so it reaches one name only.
Revise this topicCharacteristics and Classification of Living Organisms
A student wrote · Organisation of the OrganismA plant cell has a cell wall and an animal cell has a cell membrane instead.
Why it loses marksThis treats the two structures as alternatives, when a plant cell has both. Every cell has a cell membrane, because the membrane controls what enters and leaves the cell - without it the cell could not survive. The plant cell wall is an extra layer on the outside.
The answer that scoresAll cells have a cell membrane, a thin partially permeable layer that controls what enters and leaves the cell. A plant cell also has a cell wall, made of cellulose, outside the cell membrane. The cell wall strengthens the cell and gives it a fixed shape, but it is fully permeable, so it does not control what enters the cell - that is the membrane's job.
Revise this topicOrganisation of the Organism
A student wrote · Organisation of the OrganismBacteria have no nucleus, so they have no DNA and no genetic information.
Why it loses marksHaving no nucleus does not mean having no DNA. A bacterial cell keeps its genetic information as a single loop of circular DNA lying free in the cytoplasm, and it may also carry plasmids. The examiner is testing whether you know where the DNA is, not whether it exists.
The answer that scoresA bacterial cell has no nucleus, so its DNA is not enclosed in a membrane. It carries one loop of circular DNA free in the cytoplasm, which holds the genes that control the cell's activities, and it may also contain plasmids - small rings of extra DNA that often carry genes such as those for antibiotic resistance.
Revise this topicOrganisation of the Organism
A student wrote · Organisation of the OrganismA red blood cell has a large nucleus so that it can control how much oxygen it carries.
Why it loses marksA mature red blood cell has no nucleus at all. The nucleus is lost as the cell matures, and this is precisely what makes the cell good at its job: without it there is more room inside the cell for haemoglobin, the pigment that carries oxygen.
The answer that scoresA mature red blood cell has no nucleus, which leaves more space for haemoglobin, so each cell can carry more oxygen. It is also shaped like a biconcave disc, which gives it a large surface area for oxygen to diffuse in and out quickly. The trade-off is that it cannot divide or repair itself, so it survives only about four months before being replaced.
Revise this topicOrganisation of the Organism
A student wrote · Organisation of the OrganismMagnification = actual size / image size.
Why it loses marksThe formula is inverted. Magnification is how many times larger the image is than the specimen, so the image size must be divided by the actual size. Getting this the wrong way round also produces a magnification below 1, which is an easy warning sign.
The answer that scoresmagnification = image size / actual size. Rearranged, actual size = image size / magnification and image size = actual size x magnification. If a cell measures 30 mm in an image magnified 1500 times, the actual size is 30 mm / 1500 = 0.02 mm, which is 20 micrometres.
Revise this topicOrganisation of the Organism
A student wrote · Movement Into and Out of CellsOsmosis is the movement of water and salt from a high concentration to a low concentration through a partially permeable membrane.
Why it loses marksTwo errors for the price of one. Osmosis moves water only, and the direction is given in water potential, not in concentration of the solution. Salt does not cross a partially permeable membrane by osmosis.
The answer that scoresOsmosis is the net movement of water molecules from a region of higher water potential (a dilute solution) to a region of lower water potential (a more concentrated solution), through a partially permeable membrane. Only water moves, and it moves towards the more concentrated solution.
Revise this topicMovement Into and Out of Cells
A student wrote · Movement Into and Out of CellsActive transport moves particles from a region of high concentration to a region of low concentration, using energy from respiration.
Why it loses marksThe direction is the wrong way round, and the direction is the mark. The energy detail is correct but cannot compensate, because a high-to-low movement down a gradient is diffusion and needs no energy at all.
The answer that scoresActive transport moves particles through a cell membrane from a region of lower concentration to a region of higher concentration - against the concentration gradient - using energy from respiration and protein carriers in the membrane.
Revise this topicMovement Into and Out of Cells
A student wrote · Movement Into and Out of CellsA plant cell placed in a concentrated salt solution becomes turgid because water moves into it.
Why it loses marksBoth the direction of water movement and the resulting state are wrong. A concentrated solution outside the cell has a lower water potential than the cell contents, so water leaves the cell.
The answer that scoresWater leaves the cell by osmosis because the solution outside has a lower water potential than the cell contents. The cell contents shrink and stop pressing outwards on the cell wall, so the cell becomes flaccid. If enough water is lost, the cytoplasm and cell membrane pull away from the cell wall and the cell is plasmolysed.
Revise this topicMovement Into and Out of Cells
A student wrote · Movement Into and Out of CellsDiffusion needs energy from respiration because the particles have to be pushed from one place to another.
Why it loses marksNothing pushes particles during diffusion. Confusing diffusion with active transport is one of the most common errors in this topic, and it costs the mark for the source of energy.
The answer that scoresThe energy for diffusion comes from the kinetic energy of the random movement of the molecules and ions themselves. No energy from respiration is needed, so diffusion is a passive process and continues even in a dead cell. Energy from respiration is needed only for active transport, which moves particles against a concentration gradient.
Revise this topicMovement Into and Out of Cells
A student wrote · Biological MoleculesProteins are made of carbon, hydrogen and oxygen, the same as carbohydrates.
Why it loses marksThis omits nitrogen, which is the element that distinguishes proteins from carbohydrates and fats. The whole point of the element question is that proteins contain an extra element.
The answer that scoresProteins contain carbon, hydrogen, oxygen and nitrogen. Carbohydrates and fats contain carbon, hydrogen and oxygen only, so nitrogen is the element that identifies a molecule as a protein.
Revise this topicBiological Molecules
A student wrote · Biological MoleculesCellulose cannot be digested because it is made from a different sugar from starch.
Why it loses marksBoth starch and cellulose are polymers of glucose. The difference is not the sugar but the arrangement of the glucose units and the bonds between them, which human enzymes cannot hydrolyse in cellulose.
The answer that scoresStarch and cellulose are both polymers of glucose, but the glucose units are joined in different arrangements. Human digestive enzymes can hydrolyse the bonds in starch but not the bonds in cellulose, so cellulose passes through the gut undigested as dietary fibre, adding bulk and helping to prevent constipation.
Revise this topicBiological Molecules
A student wrote · Biological MoleculesBenedict's solution turns brick-red as soon as it is added to a reducing sugar.
Why it loses marksBenedict's solution only changes colour when the mixture is heated. Adding it at room temperature to a glucose solution leaves it blue, so this answer loses the mark for the conditions of the test.
The answer that scoresBenedict's solution is added to the sample and the mixture is heated, for example in a water bath. A positive result is a colour change from blue to a brick-red precipitate, and the colour passes through green, yellow and orange as the concentration of reducing sugar increases.
Revise this topicBiological Molecules
A student wrote · Biological MoleculesThe biuret test turns purple when the sample is heated with protein.
Why it loses marksThis mixes up the two tests. Biuret solution gives its purple colour at room temperature and needs no heating; the test that needs heat is Benedict's test for reducing sugars.
The answer that scoresThe biuret test is carried out at room temperature: biuret solution is added to the sample and a positive result for protein is a colour change from blue to purple (violet). No heating is required.
Revise this topicBiological Molecules
A student wrote · EnzymesA student wrote: 'When the temperature gets too high the enzyme is killed.'
Why it loses marksEnzymes are proteins, not living things, so they cannot be killed. The word the examiner wants is denatured, and the mark depends on explaining what happens to the active site.
The answer that scoresAt high temperature the bonds that hold the enzyme's folded shape break. The active site changes shape, so the substrate no longer fits it and no enzyme-substrate complexes can form. The enzyme is denatured, which is a permanent change - cooling it down does not restore the shape.
Revise this topicEnzymes
A student wrote · EnzymesA student wrote: 'The enzyme is used up during the reaction, so more enzyme has to be added each time.'
Why it loses marksThis confuses an enzyme with a reactant. Enzymes are catalysts and are not changed or used up, and saying they are is a direct contradiction of the definition.
The answer that scoresThe enzyme is not changed by the reaction. The substrate binds to the active site, is converted into products, and the products are released, leaving the enzyme exactly as it was. The same enzyme molecule can therefore catalyse the reaction again and again, which is why only a small amount is needed.
Revise this topicEnzymes
A student wrote · EnzymesA student wrote: 'Enzymes are not specific - one enzyme can act on any substrate.'
Why it loses marksThis ignores the lock-and-key model. Specificity is a core idea in this topic and a question on it is almost guaranteed, so a sweeping statement like this cannot score.
The answer that scoresEach enzyme has an active site with a shape complementary to one particular substrate. A molecule with a different shape does not fit the active site, so no enzyme-substrate complex forms and no reaction is catalysed. Amylase digests starch but has no effect on protein because protein does not fit its active site.
Revise this topicEnzymes
A student wrote · EnzymesA student wrote: 'Enzymes stop working at low temperature because the enzyme has no energy.'
Why it loses marksThe enzyme is not the only particle that matters, and 'has no energy' is imprecise. Low temperature slows a reaction by reducing kinetic energy and the frequency of effective collisions - the enzyme itself is undamaged.
The answer that scoresAt low temperature the enzyme and substrate molecules have less kinetic energy, so they move more slowly, collide less often and collide with less energy, so there are fewer effective collisions and the rate is low. The enzyme is not denatured, so warming it restores the rate.
Revise this topicEnzymes
A student wrote · Plant NutritionPhotosynthesis uses carbon dioxide and glucose to make oxygen and water.
Why it loses marksThe reactants and products have been swapped. The examiner tests whether you know which substances go in and which come out, so reversing them loses every mark in the equation.
The answer that scoresPhotosynthesis takes in carbon dioxide and water and uses energy from light, trapped by chlorophyll, to make glucose and oxygen: carbon dioxide + water -> glucose + oxygen. Glucose is the carbohydrate product and oxygen is released as a by-product.
Revise this topicPlant Nutrition
A student wrote · Plant NutritionA variegated leaf turns blue-black all over after a starch test, because the whole leaf photosynthesises.
Why it loses marksOnly the green parts of a variegated leaf contain chlorophyll, so only they can absorb light and make starch. This mistake shows a failure to link chlorophyll to the leaf's colour.
The answer that scoresAfter destarching and testing with iodine, only the green parts of a variegated leaf turn blue-black. The white patches contain no chlorophyll, so they cannot absorb light and cannot make glucose or starch. This is the evidence that chlorophyll is needed for photosynthesis.
Revise this topicPlant Nutrition
A student wrote · Plant NutritionThe rate of photosynthesis keeps increasing as light intensity increases.
Why it loses marksThe graph levels off. Once light is no longer limiting, another factor takes over, and the rate cannot rise further until that factor is increased too.
The answer that scoresAs light intensity increases the rate of photosynthesis rises at first, because more light is absorbed by chlorophyll. Above a certain intensity the rate levels off (plateaus), because light is no longer the limiting factor - carbon dioxide concentration or temperature has become limiting instead.
Revise this topicPlant Nutrition
A student wrote · Plant NutritionMagnesium ions are needed to make proteins, and nitrate ions are needed to make chlorophyll.
Why it loses marksThe two roles have been swapped. Each ion has one specific job in the syllabus, and mixing them up loses the marks for both.
The answer that scoresNitrate ions are needed for making amino acids, and therefore proteins, so a shortage stunts growth. Magnesium ions are needed for making chlorophyll, so a shortage turns the leaves yellow because the plant cannot make enough chlorophyll to absorb light.
Revise this topicPlant Nutrition
A student wrote · Human NutritionA student wrote: 'Digestion is when the teeth break food into smaller pieces, so chewing is chemical digestion.'
Why it loses marksChewing changes only the size of the food, not its molecules, so it is physical (mechanical) digestion. Calling it chemical digestion loses the mark, because the examiner is testing whether you know that chemical digestion is the breakdown of large insoluble molecules into small soluble ones.
The answer that scoresChewing is physical digestion: it breaks food into smaller pieces without any chemical change to the molecules, and its purpose is to increase the surface area so that enzymes can act faster. Chemical digestion is different - it uses enzymes to break large insoluble molecules into small soluble molecules that can be absorbed, for example amylase breaking starch down to maltose.
Revise this topicHuman Nutrition
A student wrote · Human NutritionA student wrote: 'The pancreas and the liver both make digestive enzymes that pass into the small intestine.'
Why it loses marksThe pancreas makes digestive enzymes, but the liver does not - it makes bile. Bile contains no enzymes, so this answer contradicts a core syllabus fact and cannot score.
The answer that scoresThe pancreas produces pancreatic juice, which contains amylase, protease and lipase, and releases it into the duodenum. The liver produces bile, which is stored in the gall bladder and released into the duodenum. Bile contains no enzymes: it is alkaline, so it neutralises the acid from the stomach, and it emulsifies fats, breaking large droplets into smaller ones to increase the surface area for lipase.
Revise this topicHuman Nutrition
A student wrote · Human NutritionA student wrote: 'Villi absorb food, and the lacteal carries the glucose and amino acids away into the blood.'
Why it loses marksThis swaps the two transport routes inside a villus. Glucose and amino acids are water-soluble and travel in the blood; the lacteal carries the products of fat digestion. The examiner awards the mark for the correct route, not just for naming the structures.
The answer that scoresGlucose and amino acids are absorbed into the dense network of capillaries in the villus and are carried away in the blood, which also keeps the concentration gradient steep. The lacteal is the vessel at the centre of the villus and it absorbs the fatty acids and glycerol from fat digestion, which are reassembled into fats before they eventually enter the blood.
Revise this topicHuman Nutrition
A student wrote · Human NutritionA student wrote: 'Scurvy is caused by a lack of calcium and rickets is caused by a lack of vitamin C.'
Why it loses marksThe two deficiency diseases have been swapped. Scurvy is a vitamin C deficiency and rickets is caused by a lack of vitamin D or calcium, so this answer loses both marks and shows the functions have not been learned alongside the diseases.
The answer that scoresScurvy is caused by too little vitamin C. Vitamin C is needed to make collagen, so without it the gums bleed and swell, teeth loosen and wounds heal slowly. Rickets is caused by too little vitamin D, or too little calcium: vitamin D is needed for calcium to be absorbed from the intestine and deposited in bone, so without it the bones stay soft and bend, and the leg bones bow.
Revise this topicHuman Nutrition
A student wrote · Transport in PlantsPhloem transports water and mineral ions from the roots to the leaves.
Why it loses marksThis mixes up the two transport tissues. Water and mineral ions travel in the xylem, and the xylem carries them upwards. Phloem carries sucrose and amino acids, and it carries them in both directions.
The answer that scoresXylem transports water and mineral ions from the roots to the leaves, and also supports the plant. Phloem transports sucrose and amino acids from sources to sinks, in both directions, because these substances are loaded into the phloem wherever they are produced and removed wherever they are needed.
Revise this topicTransport in Plants
A student wrote · Transport in PlantsTranspiration is the movement of water up the xylem from the roots to the leaves.
Why it loses marksThat describes the transpiration stream, not transpiration. Transpiration is specifically the loss of water vapour from the leaves, so this answer loses the mark even though it uses the right words.
The answer that scoresTranspiration is the loss of water vapour from the leaves. Water evaporates from the surfaces of the mesophyll cells into the air spaces and then diffuses out of the leaf through the stomata. The movement of water up the xylem to replace it is called the transpiration stream.
Revise this topicTransport in Plants
A student wrote · Transport in PlantsTranspiration is faster when the air is more humid, because there is more water vapour in the air to move away.
Why it loses marksThis gets the direction of the effect wrong. Humid air already contains a lot of water vapour, so the concentration of water vapour outside the leaf is closer to that inside, the diffusion gradient is shallower, and transpiration slows down.
The answer that scoresTranspiration is slower when the air is more humid. A high humidity means the water vapour concentration outside the leaf is closer to the concentration inside the leaf, so the diffusion gradient between the air spaces and the outside air is shallower and water vapour diffuses out more slowly.
Revise this topicTransport in Plants
A student wrote · Transport in PlantsXylem vessels are living cells, which is why they can carry water up the stem.
Why it loses marksXylem vessels are dead at maturity and have no cell contents. Being alive is not what lets them carry water - in fact the opposite is true: the cells die and their contents disappear so that the tube is hollow and nothing blocks the flow.
The answer that scoresXylem vessels are dead at maturity. Their thick walls contain lignin, which makes them strong and waterproof; they have no cell contents, so the tube is hollow; and their cells are joined end to end with no cross walls, so they form one long continuous tube through which water can flow freely from the roots to the leaves.
Revise this topicTransport in Plants
A student wrote · Transport in AnimalsArteries carry oxygenated blood and veins carry deoxygenated blood.
Why it loses marksThis describes most vessels but not all of them, and it misses the actual definition. The examiner is testing whether you know that arteries and veins are named for the direction the blood travels, not for how much oxygen it carries.
The answer that scoresArteries carry blood away from the heart and veins carry blood back to the heart. Most arteries carry oxygenated blood, but the pulmonary artery carries deoxygenated blood from the heart to the lungs, and the pulmonary vein carries oxygenated blood from the lungs back to the heart.
Revise this topicTransport in Animals
A student wrote · Transport in AnimalsBlood is pumped from the heart to the lungs in the pulmonary vein and returns to the heart in the pulmonary artery.
Why it loses marksThe two vessels have been swapped. Both names are tested together, so getting them the wrong way round loses the mark for both.
The answer that scoresThe pulmonary artery carries deoxygenated blood away from the heart (from the right ventricle) to the lungs. The pulmonary vein carries oxygenated blood from the lungs back to the heart, into the left atrium.
Revise this topicTransport in Animals
A student wrote · Transport in AnimalsRed blood cells have a nucleus that controls the haemoglobin which carries oxygen.
Why it loses marksRed blood cells lose their nucleus as they mature. Naming a structure they do not have loses the mark, and it wastes the point you could be making about haemoglobin.
The answer that scoresRed blood cells have no nucleus, which leaves more room for haemoglobin inside the cell. The haemoglobin combines with oxygen in the lungs to form oxyhaemoglobin and releases it in the tissues. Their biconcave shape gives a large surface area to volume ratio, so oxygen diffuses in and out quickly.
Revise this topicTransport in Animals
A student wrote · Transport in AnimalsThe right ventricle has the thickest wall because it pumps blood all the way to the lungs.
Why it loses marksIt is the left ventricle that has the thicker wall. The error comes from confusing the distance with the pressure: the lungs are close to the heart and need only low pressure, whereas the whole body does not.
The answer that scoresThe left ventricle has the thicker muscular wall because it pumps blood all round the body, so it must generate a high pressure. The right ventricle pumps blood only to the lungs, a much shorter journey, so it needs only a low pressure and has a thinner wall.
Revise this topicTransport in Animals
A student wrote · Diseases and ImmunityThe skin is a chemical barrier because it kills bacteria.
Why it loses marksThe skin does not kill pathogens chemically. It is a mechanical barrier: its dead, dry outer layer physically blocks pathogens from entering, so classifying it as chemical loses the mark.
The answer that scoresThe skin is a mechanical barrier. Its tough outer layer is dead and dry, so pathogens cannot pass through it, and any cut is quickly sealed by a blood clot. The chemical barriers are substances such as the hydrochloric acid in the stomach and the enzymes in mucus, which kill pathogens rather than simply blocking them.
Revise this topicDiseases and Immunity
A student wrote · Diseases and ImmunityAfter a vaccination you are immune because the vaccine contains antibodies.
Why it loses marksA vaccine contains weakened, dead or inactive pathogens or their antigens - not antibodies. This is active immunity: the body makes its own antibodies and memory cells in response to the antigens.
The answer that scoresA vaccine contains weakened, dead or inactive pathogens, or their antigens. These antigens stimulate an immune response by lymphocytes, which produce antibodies, and memory cells are produced that give long-term immunity. Because the body makes its own antibodies, this is active immunity. A vaccine that supplied ready-made antibodies would give only passive, short-term immunity.
Revise this topicDiseases and Immunity
A student wrote · Diseases and ImmunityThe mosquito is the pathogen that causes malaria.
Why it loses marksThe mosquito is the vector, not the pathogen. It carries the pathogen from one host to another but does not cause the disease itself. Naming the wrong organism loses the mark.
The answer that scoresThe mosquito is the vector. The pathogen that causes malaria is the protoctist Plasmodium, which the mosquito picks up when it bites an infected person and injects into the next person it bites. The mosquito transmits the pathogen but does not cause the disease itself.
Revise this topicDiseases and Immunity
A student wrote · Diseases and ImmunityA breast-fed baby is protected for life because breast milk contains memory cells.
Why it loses marksPassive immunity is short-term. Breast milk contains ready-made antibodies, but no memory cells are produced, so once the borrowed antibodies are broken down the protection is gone.
The answer that scoresBreast milk passes ready-made antibodies from the mother to the baby, which is passive immunity. No memory cells are produced, and the borrowed antibodies are gradually broken down, so the protection lasts only a few months. Active immunity, gained by infection or vaccination, is the type that produces memory cells and lasts much longer.
Revise this topicDiseases and Immunity
A student wrote · Gas Exchange in HumansVentilation and respiration are the same thing - breathing in and out releases the energy from glucose.
Why it loses marksVentilation is the mechanical movement of air; respiration is the chemical release of energy inside cells. Using the two words interchangeably loses marks in any question that mentions energy or cells.
The answer that scoresVentilation (breathing) is the movement of air into and out of the lungs, caused by the ribs, intercostal muscles and diaphragm. Respiration is the chemical reactions in cells that break down nutrient molecules such as glucose and release energy for metabolism. Ventilation supplies the oxygen that aerobic respiration needs and removes the carbon dioxide it produces, but it releases no energy itself.
Revise this topicGas Exchange in Humans
A student wrote · Gas Exchange in HumansExhaled air contains much more nitrogen than inhaled air, because the body makes nitrogen during respiration.
Why it loses marksThe body neither uses nor produces nitrogen. Its proportion is the same in inhaled and exhaled air, so this statement is wrong and would lose the mark for composition.
The answer that scoresNitrogen makes up about 78% of both inhaled and exhaled air. The body does not use nitrogen in respiration, so it is breathed in and out unchanged. The gases that change are oxygen (falls from about 21% to about 16%), carbon dioxide (rises from about 0.04% to about 4%) and water vapour (increases).
Revise this topicGas Exchange in Humans
A student wrote · Gas Exchange in HumansWhen you breathe in, the diaphragm relaxes and moves up, and the ribs move down and in.
Why it loses marksThis describes exhalation, not inhalation. Reversing the movements of the diaphragm and ribs loses the marks for the whole ventilation question.
The answer that scoresDuring inhalation the diaphragm contracts, flattens and moves down, and the external intercostal muscles contract so the ribs move up and out. The volume of the thorax increases and the pressure inside falls below atmospheric pressure, so air is drawn in. During exhalation the diaphragm relaxes and becomes domed, moving up, and the ribs move down and in, so the volume decreases and air is pushed out.
Revise this topicGas Exchange in Humans
A student wrote · Gas Exchange in HumansThe carbon monoxide in tobacco smoke causes lung cancer.
Why it loses marksCarbon monoxide and carcinogens do different jobs and must not be mixed up. Carbon monoxide reduces the oxygen-carrying capacity of the blood; it is the carcinogens in tar that cause cancer.
The answer that scoresCarbon monoxide combines with haemoglobin in red blood cells more readily than oxygen does, reducing the amount of oxygen the blood can carry, so less oxygen reaches the tissues. Lung cancer is caused by the carcinogens in tar, which cause mutations in the cells lining the airways and lead to uncontrolled cell division. Nicotine is the addictive stimulant, and emphysema is caused by the destruction of the alveolar walls.
Revise this topicGas Exchange in Humans
A student wrote · RespirationRespiration and breathing are the same thing - you respire when you breathe in and out.
Why it loses marksRespiration is a chemical process that releases energy inside cells; breathing is a mechanical process that moves air. Confusing the two loses marks in any question that mentions cells, energy or metabolism.
The answer that scoresBreathing is the movement of air into and out of the lungs, brought about by the ribs, diaphragm and intercostal muscles. Respiration is the chemical reactions in cells that break down nutrient molecules such as glucose and release energy for metabolism. Breathing supplies the oxygen for aerobic respiration and removes the carbon dioxide that respiration produces.
Revise this topicRespiration
A student wrote · RespirationAnaerobic respiration in yeast produces lactic acid, and in muscles it produces alcohol and carbon dioxide.
Why it loses marksThe two products have been swapped. Each equation is a separate syllabus statement, so getting them the wrong way round loses the mark for both.
The answer that scoresIn yeast, anaerobic respiration produces alcohol (ethanol) and carbon dioxide: glucose -> alcohol + carbon dioxide. In muscles during vigorous exercise it produces lactic acid: glucose -> lactic acid. The two reactions start from the same substrate but end with different products.
Revise this topicRespiration
A student wrote · RespirationAnaerobic respiration releases more energy than aerobic respiration because it is quicker.
Why it loses marksIt is faster, but it releases much less energy per glucose molecule because the glucose is only partly broken down. This comparison is a specific syllabus point the examiner tests.
The answer that scoresAnaerobic respiration releases much less energy per glucose molecule than aerobic respiration, because the nutrient molecule is not broken down completely - glucose is converted only to lactic acid in muscle, or to alcohol and carbon dioxide in yeast. Aerobic respiration breaks glucose down completely to carbon dioxide and water, so it releases a relatively large amount of energy.
Revise this topicRespiration
A student wrote · RespirationThe oxygen debt is repaid by breathing the lactic acid out of the lungs.
Why it loses marksLactic acid is not breathed out. It is carried in the blood to the liver and broken down there. Naming the wrong route loses the marks for the whole removal process.
The answer that scoresLactic acid is carried in the blood from the muscles to the liver. The heart rate stays fast to transport it, and breathing stays deep and fast to supply the oxygen. In the liver the lactic acid is respired aerobically, using that oxygen, and converted back into glucose. The debt is repaid when the lactic acid has been removed - not by breathing it out.
Revise this topicRespiration
A student wrote · Excretion in HumansExcretion and egestion are the same thing - both are ways of getting rid of waste from the body.
Why it loses marksOnly excretion removes the waste products of metabolism. Egestion removes undigested food that was never absorbed into the body's cells, so it is not a product of metabolism at all. This is one of the most commonly lost marks in the topic.
The answer that scoresExcretion is the removal of the waste products of metabolism and of substances present in excess of requirements - for example carbon dioxide from the lungs and urea, excess ions and excess water from the kidneys. Egestion is the removal of undigested food from the alimentary canal as faeces. The material egested was never absorbed into the body's cells, so it was never part of metabolism.
Revise this topicExcretion in Humans
A student wrote · Excretion in HumansUrea is made in the kidney and then excreted in the urine.
Why it loses marksUrea is formed in the liver, from excess amino acids, by deamination. The kidney only removes it from the blood. Naming the wrong organ loses the marks for both the formation and the excretion of urea.
The answer that scoresExcess amino acids are broken down in the liver by deamination. Deamination removes the nitrogen-containing part of the amino acid, which is converted into urea, while the rest of the molecule is respired to release energy. The urea is released into the blood and carried to the kidneys, which excrete it in the urine.
Revise this topicExcretion in Humans
A student wrote · Excretion in HumansIn the nephron, all of the water is reabsorbed back into the blood, so the urine contains only urea.
Why it loses marksOnly most of the water is reabsorbed, not all of it. Urine always contains excess water and excess ions as well as urea, and the volume of water left behind is exactly what ADH controls.
The answer that scoresIn the tubule all of the glucose, most of the water and some of the ions are reabsorbed back into the blood. What is left is urine, which contains urea, excess water and excess ions. Because some water always stays behind, the volume and concentration of the urine can be adjusted by ADH - if all the water were reabsorbed there would be no urine at all.
Revise this topicExcretion in Humans
A student wrote · Excretion in HumansThe dialysis fluid should contain no glucose, because glucose is a waste product that the kidney needs to remove.
Why it loses marksGlucose is not a waste product - it is the body's fuel, and it is normally reabsorbed completely. If the fluid contained no glucose, glucose would diffuse out of the blood and the patient would lose it. This error shows a misunderstanding of both reabsorption and dialysis.
The answer that scoresThe dialysis fluid contains the correct concentration of glucose (and salts) but no urea. Because the glucose concentration matches that of the blood, there is no net diffusion of glucose out of the blood, so the patient keeps the glucose the body needs. Because there is no urea in the fluid, urea diffuses out of the blood down its concentration gradient and is removed.
Revise this topicExcretion in Humans
A student wrote · Coordination and ResponseA reflex action is rapid because the impulse travels up to the brain and straight back down again.
Why it loses marksThe brain is not part of the reflex pathway. Writing that the impulse goes to the brain and back describes a voluntary response, not a reflex, and it loses the mark.
The answer that scoresA reflex action is rapid and automatic because the impulse follows a short pathway: receptor -> sensory neurone -> relay neurone in the spinal cord -> motor neurone -> effector. The brain is not involved, so no conscious thought is needed and the response is very fast. The brain is informed afterwards, which is why you feel the pain after your hand has already moved.
Revise this topicCoordination and Response
A student wrote · Coordination and ResponseThe iris controls how much light enters the eye by changing the size of the lens.
Why it loses marksThe iris changes the diameter of the pupil, not the lens. The lens changes shape during accommodation to focus light onto the retina.
The answer that scoresThe iris controls how much light enters the eye by changing the diameter of the pupil, using the antagonistic circular and radial muscles. In bright light the circular muscles contract and the pupil constricts; in dim light the radial muscles contract and the pupil dilates. The lens does a different job: it changes shape to focus light onto the retina, which is accommodation.
Revise this topicCoordination and Response
A student wrote · Coordination and ResponseInsulin raises the blood glucose concentration and glucagon lowers it.
Why it loses marksThe two hormones have been swapped. Insulin is the one that lowers blood glucose, so this answer is the exact reverse of the truth.
The answer that scoresInsulin decreases the blood glucose concentration: it causes the liver to convert glucose into glycogen for storage and increases the uptake of glucose by cells. Glucagon increases the blood glucose concentration: it makes the liver convert stored glycogen back into glucose. They are antagonistic hormones and together they keep blood glucose close to the set point.
Revise this topicCoordination and Response
A student wrote · Coordination and ResponseAuxin collects on the lit side of a shoot, so that side grows faster and the shoot bends away from the light.
Why it loses marksAuxin moves away from the light to the shaded side. Saying it collects on the lit side reverses the mechanism and the direction of the bend.
The answer that scoresLight causes auxin to move from the lit side to the shaded side of the shoot. The shaded side therefore has more auxin, its cells elongate more, and it grows longer than the lit side, so the shoot bends towards the light. That is positive phototropism, and it gives the leaves the best possible access to light for photosynthesis.
Revise this topicCoordination and Response
A student wrote · DrugsAntibiotics will cure the flu because they kill the virus that causes it.
Why it loses marksAntibiotics act on bacteria, not viruses. The syllabus states that antibiotics do not affect viruses, so this answer cannot score, however confident it sounds.
The answer that scoresAntibiotics kill bacteria but have no effect on viruses. Influenza is caused by a virus, which has no bacterial cell wall or metabolism for the antibiotic to attack, so an antibiotic cannot cure flu. The patient needs rest and symptom relief while the immune system clears the virus.
Revise this topicDrugs
A student wrote · DrugsThe patient became resistant to the antibiotic, so it stopped working in them.
Why it loses marksResistance develops in the bacteria, not in the patient. The examiner wants natural selection in the bacterial population, so saying the person is resistant loses the mark.
The answer that scoresThe bacteria became resistant. A random mutation gave some bacteria resistance to the antibiotic; when the antibiotic was used it killed the non-resistant bacteria, and the resistant survivors reproduced, so the bacterial population is now mostly resistant and the antibiotic is less effective.
Revise this topicDrugs
A student wrote · DrugsAlcohol is a stimulant because it makes people feel confident and talkative.
Why it loses marksThis confuses the user's mood with the drug's action on the nervous system. Alcohol slows nervous impulses, so it is a depressant.
The answer that scoresAlcohol is a depressant: it slows the transmission of nervous impulses at the synapse, which impairs judgement, slows reaction time and reduces coordination. It may make a person feel more confident, but that is a social effect, not evidence that it speeds up the nervous system.
Revise this topicDrugs
A student wrote · DrugsA patient can stop taking antibiotics as soon as the symptoms disappear.
Why it loses marksStopping early leaves some bacteria alive, including the less susceptible ones, which then reproduce and increase resistance.
The answer that scoresThe patient must complete the full course. Stopping when the symptoms improve leaves some bacteria alive; these multiply, and if any are less affected by the antibiotic, resistance becomes more likely and the infection can return.
Revise this topicDrugs
A student wrote · ReproductionPollination and fertilisation are the same thing - the pollen grains fuse with the egg in the stigma.
Why it loses marksPollination is only the transfer of pollen from an anther to a stigma. Fertilisation is the fusion of the nuclei and happens later, in the ovule. Merging the two loses marks in any question that asks about either process.
The answer that scoresPollination is the transfer of pollen grains from an anther to a stigma. Fertilisation happens afterwards: a pollen tube grows down the style to the ovule, and a pollen nucleus fuses with a nucleus in the ovule. Pollination is a transfer; fertilisation is a fusion of nuclei.
Revise this topicReproduction
A student wrote · ReproductionThe mother's blood and the fetus's blood mix in the placenta, so the fetus shares the mother's blood.
Why it loses marksThe two blood supplies are kept separate in the placenta. If they mixed, the mother's immune system and the fetus's could attack each other. This is one of the most commonly lost marks in human reproduction.
The answer that scoresThe mother's blood and the fetus's blood do not mix. They are separated by the placenta, and substances move across it by diffusion: dissolved nutrients such as glucose, oxygen and antibodies pass from the mother to the fetus, while carbon dioxide and other excretory products pass from the fetus to the mother. The umbilical cord carries the fetus's blood to and from the placenta.
Revise this topicReproduction
A student wrote · ReproductionSeeds need light and soil to germinate, and the seedling gets its food from the soil.
Why it loses marksLight and soil are not needed for germination, and the seedling does not feed from the soil at first. The seed has its own food store, and it respires using oxygen rather than photosynthesising.
The answer that scoresGermination requires water, oxygen and a suitable temperature - not light, and not soil. Water softens the seed coat and activates the enzymes that break down the food store; oxygen is needed for aerobic respiration to release energy; and a suitable temperature keeps those enzymes working at the right rate. The seedling uses the food store inside the seed, not nutrients from the soil.
Revise this topicReproduction
A student wrote · ReproductionAfter fertilisation the ovary becomes the seed and the ovule becomes the fruit.
Why it loses marksThe two have been swapped. Getting this the wrong way round loses marks in any question about the formation of seeds and fruits.
The answer that scoresAfter fertilisation the ovule becomes the seed, because the ovule contains the fertilised egg cell, and the ovary becomes the fruit, which develops around the seeds to protect them and help in dispersal.
Revise this topicReproduction
A student wrote · InheritanceIn a cross between two heterozygous parents the offspring are 3 tall to 1 short, so three out of every four plants are homozygous.
Why it loses marksThis confuses the phenotypic ratio with the genotypic ratio. The 3 : 1 ratio describes the features you can see, not the alleles carried.
The answer that scoresThe genotypic ratio is 1 TT : 2 Tt : 1 tt, so only one plant in four is homozygous dominant and two in four are heterozygous. Because TT and Tt both show the dominant feature, the phenotypic ratio is 3 tall : 1 short. Always give the genotype ratio and the phenotype ratio separately.
Revise this topicInheritance
A student wrote · InheritanceA gamete contains two alleles of each gene, one on each chromosome.
Why it loses marksGametes are haploid - they contain a single set of chromosomes. Writing two alleles in a gamete box shows the examiner that you have not understood meiosis, and it loses the mark for the gametes in a genetic diagram.
The answer that scoresA gamete contains one allele of each gene, because it has a single set of chromosomes (23 in humans). A diploid body cell contains two alleles of each gene, one on each chromosome of a pair. This is why the two alleles of a gene separate when gametes are formed.
Revise this topicInheritance
A student wrote · InheritanceIn codominance the two alleles blend together to give an intermediate phenotype, like pink flowers from red and white parents.
Why it loses marksCodominance is not blending. In codominance both alleles are fully expressed at the same time. An intermediate phenotype is a different situation and is not what Cambridge means by codominance.
The answer that scoresIn codominance both alleles in a heterozygous organism contribute to the phenotype, so both are seen at once - for example a person with the genotype I^A I^B has both the A and the B antigens on their red blood cells and has blood group AB. Neither allele is hidden and neither is diluted.
Revise this topicInheritance
A student wrote · InheritanceSickle cell anaemia is caused by a bacterium, and people who carry the allele are immune to malaria.
Why it loses marksSickle cell anaemia is not an infection - it is caused by a mutation in the gene for haemoglobin. Carriers also have only some resistance to malaria, not immunity.
The answer that scoresSickle cell anaemia is caused by a mutation that changes the base sequence of the haemoglobin gene, so the abnormal haemoglobin HbS is made and red blood cells become sickle-shaped. Carriers (HbA HbS) are healthy and have some resistance to malaria, because the malarial parasite develops less well in their red blood cells, but they are not immune to it.
Revise this topicInheritance
A student wrote · Variation and SelectionA student wrote: the antibiotic caused a mutation in the bacteria so that they became resistant.
Why it loses marksThis reverses the process. Mutations are random and happen before the antibiotic is used; the antibiotic does not create them. Writing it the wrong way round loses every mark for the mechanism, even if the rest of the answer is right.
The answer that scoresThe mutation that gives resistance arises at random, so a few bacteria in the population are already resistant before the antibiotic is taken. When the antibiotic is used, it kills the non-resistant bacteria, so the resistant bacteria survive, reproduce and pass on the resistance allele. Over many generations the resistant strain becomes common, and the antibiotic no longer works.
Revise this topicVariation and Selection
A student wrote · Variation and SelectionA student wrote: height is an example of discontinuous variation because people are either tall or short.
Why it loses marksHeight has no distinct categories. Classifying people as 'tall' or 'short' invents a boundary that does not exist, and there is a continuous range of values in between. This is one of the most common confusions in the topic.
The answer that scoresHeight shows continuous variation because it results in a range of phenotypes between two extremes, with no intermediates or separate categories, and it is measured on a scale. It is caused by both genes and the environment and is plotted as a normal distribution curve. Discontinuous variation, such as ABO blood group, has a limited number of distinct phenotypes with no intermediates and is caused by genes only.
Revise this topicVariation and Selection
A student wrote · Variation and SelectionA student wrote: selective breeding is always an advantage because it produces a better crop.
Why it loses marksThis ignores the risks and gives no balanced judgement. Selective breeding can reduce genetic variation and lead to inbreeding, which leaves a population vulnerable to disease. A one-sided answer cannot score the evaluation marks.
The answer that scoresSelective breeding can raise yield, improve quality and give resistance to disease, so it benefits the farmer and the food supply. However, breeding from a small number of individuals reduces genetic variation and can cause inbreeding, so the population may become more susceptible to a new disease, because there may be no resistant individuals left. Genetic engineering is an alternative that introduces a single desired gene more quickly and precisely.
Revise this topicVariation and Selection
A student wrote · Variation and SelectionA student wrote: an individual adapts to its environment during its lifetime, so it becomes better suited.
Why it loses marksAdaptation does not happen to an individual within its own lifetime. Individual organisms cannot change their inherited features to suit the environment; what changes is the population, over many generations.
The answer that scoresAdaptation is the process, resulting from natural selection, by which populations become more suited to their environment over many generations. Individuals that happen to have advantageous inherited features are more likely to survive and reproduce, so they pass those alleles on and the advantageous alleles become more common in the population. The individual does not change; the population does.
Revise this topicVariation and Selection
A student wrote · Organisms and Their EnvironmentA community is all the organisms living in one area, and a population is all the plants in that area.
Why it loses marksBoth definitions are wrong. A population is one species, not all the plants, and a community is all the populations of different species. Mixing the two loses the marks for both definitions.
The answer that scoresA population is a group of organisms of one species, living in the same area, at the same time. A community is all of the populations of different species in an ecosystem. So a pond may contain a population of water lilies and a population of frogs, and together all its populations make up the community.
Revise this topicOrganisms and Their Environment
A student wrote · Organisms and Their EnvironmentEnergy transfer between trophic levels is only 10 per cent because 90 per cent of the energy is used for growth and stored in the organism.
Why it loses marksOnly a small fraction of the energy is used for growth. Most of it is lost to the environment, so this reverses the science and misses the marks for the losses.
The answer that scoresOnly about 10 per cent of the energy is transferred to the next trophic level because most of it is lost: as heat released in respiration, in movement, in excretory products such as urea, and in parts of the organism that are not eaten, such as bones, roots and fur. Very little is stored as growth.
Revise this topicOrganisms and Their Environment
A student wrote · Organisms and Their EnvironmentIn a food chain the arrow points from the eater to the food, because it shows which organism eats which.
Why it loses marksThe arrow shows the direction of energy transfer, not who does the eating. Reversing the arrows loses the mark even if the organisms are named correctly.
The answer that scoresThe arrow shows the direction of energy transfer, so it points from the organism being eaten to the organism that eats it: grass -> rabbit -> fox. Energy passes from the grass into the rabbit when the rabbit feeds, and then into the fox when the fox eats the rabbit.
Revise this topicOrganisms and Their Environment
A student wrote · Organisms and Their EnvironmentA pyramid of numbers always has a wide base and gets narrower towards the top, so it shows energy loss well.
Why it loses marksA pyramid of numbers does not always narrow towards the top: one large producer such as an oak tree can support thousands of insects, giving a top-heavy, inverted pyramid. It also ignores the size of the organisms and the energy lost.
The answer that scoresA pyramid of numbers can be inverted, because a single large producer may support a very large number of small consumers - one oak tree and the insects that feed on it. A pyramid of biomass is better because it compares the mass of living material and takes account of the size of the organisms, and a pyramid of energy is better still because it measures the energy available at each level and is never inverted.
Revise this topicOrganisms and Their Environment
A student wrote · Human Influences on EcosystemsExcess fertiliser in a lake kills the fish because the fertiliser is poisonous to them.
Why it loses marksFertiliser does not poison fish directly. It acts as a nutrient, so the exam is testing whether you can trace the sequence through to the lack of dissolved oxygen. Writing 'poisonous' scores nothing.
The answer that scoresNitrate ions leached from the fields increase the availability of nutrients, so the growth of producers such as algae increases. When the producers die, the decomposers that break them down increase in number and respire aerobically, which reduces the dissolved oxygen in the water. Fish and other organisms that require dissolved oxygen then die.
Revise this topicHuman Influences on Ecosystems
A student wrote · Human Influences on EcosystemsCarbon dioxide causes acid rain, and sulphur dioxide causes global warming.
Why it loses marksThe two gases and their effects have been swapped. Each gas has one cause-and-effect chain in the syllabus, and swapping them loses the marks for both.
The answer that scoresSulphur dioxide, released by burning fossil fuels that contain sulphur, dissolves in rainwater to form acid rain, which damages lakes, soil, trees and buildings. Carbon dioxide is a greenhouse gas; the extra carbon dioxide released by burning fossil fuels and by deforestation increases the greenhouse effect, trapping more heat and causing global warming.
Revise this topicHuman Influences on Ecosystems
A student wrote · Human Influences on EcosystemsA farmer should grow a large monoculture because it increases biodiversity.
Why it loses marksThis is the opposite of the truth. A monoculture is a single crop, so it supports far fewer species than a mixed habitat. Biodiversity falls, not rises.
The answer that scoresA monoculture has advantages of efficiency - machinery can be used over the whole field and planting and harvesting are cheaper - but it reduces biodiversity, because only one plant species is grown, and a single pest or disease can destroy the entire crop. It also usually needs heavy applications of fertiliser and pesticide.
Revise this topicHuman Influences on Ecosystems
A student wrote · Human Influences on EcosystemsCutting down a forest only matters because we lose the trees' wood.
Why it loses marksThis ignores the effects the syllabus actually asks for. Deforestation is examined as habitat destruction, so the marks come from the effects on biodiversity, extinction, soil, flooding and carbon dioxide.
The answer that scoresDeforestation reduces biodiversity because the species living in the forest lose their habitat, food and breeding sites, and species found only there may become extinct. The soil is lost because there are no roots to hold it, flooding increases because rainwater runs off the bare ground, and carbon dioxide in the atmosphere rises because the trees that removed it by photosynthesis have gone and burning the timber releases more.
Revise this topicHuman Influences on Ecosystems
A student wrote · Biotechnology and Genetic ModificationA student wrote: 'The restriction enzyme joins the human gene to the plasmid.'
Why it loses marksThe two enzymes are easily confused, and the examiner is testing exactly this. A restriction enzyme cuts DNA; it cannot join two pieces together.
The answer that scoresThe restriction enzyme cuts the DNA, isolating the gene and leaving sticky ends, and the same enzyme cuts open the plasmid so that its sticky ends are complementary. The enzyme that joins the gene to the plasmid DNA is DNA ligase, and the result is a recombinant plasmid.
Revise this topicBiotechnology and Genetic Modification
A student wrote · Biotechnology and Genetic ModificationA student wrote: 'In bread-making the yeast makes alcohol, and the alcohol makes the dough rise.'
Why it loses marksThe student has named the wrong product. Yeast does make ethanol as well, but ethanol is not what makes bread rise, and this loses the mark.
The answer that scoresIn bread-making the yeast respires anaerobically and produces carbon dioxide as well as ethanol. The carbon dioxide bubbles are trapped in the dough, so it rises and becomes light and spongy. The small amount of ethanol evaporates during baking, so it does not affect the taste.
Revise this topicBiotechnology and Genetic Modification
A student wrote · Biotechnology and Genetic ModificationA student wrote: 'A fermenter does not need to be sterile, because the microorganisms that make the product grow faster than any contaminant.'
Why it loses marksThis misses the whole point of sterility. A contaminant does not have to outgrow the culture to cause damage: it competes for the same nutrients and can spoil the product.
The answer that scoresThe fermenter, the nutrients and the air are all sterilised and the vessel is sealed, so that no unwanted microorganisms can enter. A contaminant would compete with the culture for nutrients, reduce the yield of the product and could spoil the product, making the whole batch worthless.
Revise this topicBiotechnology and Genetic Modification
A student wrote · Biotechnology and Genetic ModificationA student wrote: 'Genetic modification is just a faster form of selective breeding.'
Why it loses marksThe two processes are different in kind, not just in speed. Selective breeding cannot move a gene from one species into another, and that is the key difference the examiner wants.
The answer that scoresSelective breeding chooses organisms that already show a useful characteristic and breeds them together, so the genes come from the same species and it takes many generations. Genetic modification cuts out an individual gene and inserts it into another organism, so it can transfer a gene between different species and produce a new variety in a single generation.
Revise this topicBiotechnology and Genetic Modification