Exam Insights

• Natural selection: use the context and say "allele"Watch out: when explaining natural selection, use the context in the question (name the selection pressure, e.g. malaria or antibiotics), say the mutation produced an allele, and say allele, not "gene". Stock phrases with no context don't get the marks. 2023 P1 Q3.5 · 2025 P1 Q8.1
• Explain selection with alleles and frequencyWhen explaining directional or stabilising selection, don't just describe the outcome. Explain it: individuals with the favoured phenotype survive and reproduce, pass on their alleles, and the allele frequency changes over generations. 2019 P1 Q4.1 · 2018 P1 Q6.4
• Aseptic technique: what scoresCredited aseptic steps include: disinfect surfaces, flame the neck of the bottle, use a sterile pipette and spreader, lift the lid at an angle, and work near a Bunsen flame (upward air movement). "Flame the plate", "wear gloves" or "work quickly with the lid off" don't score. 2023 P1 Q5.1 · 2022 P1 Q2.3

• Energy is released, not producedWatch out: ATP hydrolysis releases energy. Never write that energy is "produced" or "made": answers that did lost the mark. 2025 P1 Q5.1 · 2025 P3 Q1.3 · 2017 P3 Q1.3
• Describing ATP's structureWhen describing ATP's structure, say ribose, adenine and three phosphate groups. Adenine and ribose together are called adenosine, so "adenosine and three phosphates" is also correct. But "adenine and three phosphates" (no ribose), "ATP is made of ADP and Pi" and "pentose sugar" weren't credited. 2024 P1 Q10.3
• ATP hydrolase vs ATP synthaseWatch out: ATP hydrolase breaks ATP down by hydrolysis, which uses water. ATP synthase makes ATP by condensation, which releases water. 2024 P1 Q10.3

• Low index of diversity: one species dominatesWatch out: a low d with high species richness means one or a few species are present in very large numbers (they dominate). It doesn't mean every species has only a few individuals. 2023 P3 Q5.2
• Representative samples: random and manyWatch out: to make samples representative, take random samples and a large number of them (or keep going until the running mean is stable). "Using the same equipment" makes it fair, not representative. 2019 P1 Q3.2
• Farming questions: link it to the farmerWatch out: when asked for an advantage to a farmer, don't stop at "more biodiversity". Link it to the farm, e.g. more predators of pests, so fewer pesticides are needed or yield increases. 2019 P1 Q3.4
• Why biomass instead of counting?Watch out: don't just say counting is "difficult". Say why: the individual plants are too small or too numerous to count. 2022 P1 Q1.3

• Glucagon activates enzymes; it isn't an enzymeWatch out: glucagon is a hormone, so it doesn't convert anything itself. It binds to receptors on liver cells and activates enzymes that convert glycerol and amino acids into glucose. 2019 P2 Q5.1
• Glycogenesis, glycogenolysis, gluconeogenesisWatch out: the names tell you what happens. Glycogenesis makes glycogen from glucose; glycogenolysis breaks glycogen down to glucose; gluconeogenesis makes new glucose from glycerol and amino acids. Glycolysis is the first stage of respiration, and glucagon is the hormone. 2019 P2 Q5.3 · 2019 P2 Q5.4 · 2024 P2 Q2.3
• How insulin increases glucose uptakeWatch out: insulin doesn't open existing channels. It causes vesicles containing channel (carrier) proteins to fuse with the cell-surface membrane, so there are more of these proteins and more glucose enters by facilitated diffusion. The vesicles contain the channel proteins, not glucose. 2018 P3 Q1.3 · 2021 P2 Q5.2
• cAMP is made from ATPWatch out: adenylate cyclase converts ATP into cAMP. cAMP then activates protein kinase, which activates the enzymes that convert glycogen to glucose. 2019 P2 Q5.4
• Insulin is complementary to its receptorWatch out: insulin is a hormone, not an enzyme or substrate. A change in its tertiary structure means it's no longer complementary to its receptor, so it can't bind. 2021 P2 Q5.1
• Type II: the cells are less sensitive to insulinWatch out: in type II diabetes, insulin is still made, but the cells' receptors are less sensitive (responsive) to it. Link it to specific risk factors (weight, diet, lack of exercise) rather than "lifestyle". 2024 P2 Q9.1

• Protein in urine means the glomerulus is damagedWatch out: proteins are normally too large to pass through the basement membrane of the glomerulus, so they stay in the blood. If protein is found in urine, the basement membrane has been damaged and proteins have been filtered out. It isn't because protein wasn't reabsorbed. 2025 P3 Q2.2 · 2020 P2 Q8.1
• Ultrafiltration: high hydrostatic pressure, through gapsWatch out: small molecules are forced out of the glomerulus by high hydrostatic (blood) pressure, through gaps in the capillary endothelium and the basement membrane, not through the cells. 2019 P3 Q1.1
• Proximal convoluted tubule: features of its cellsWatch out: if a question asks about the cells of the proximal convoluted tubule, give features of each cell and what each one does: microvilli for a large surface area, many mitochondria to make ATP for active transport, and many carrier proteins for facilitated diffusion, active transport and co-transport. Include "many". 2023 P2 Q6.1
• Water moves out of the loop and collecting duct, not into themWatch out: water moves out of the loop of Henle and collecting duct, not into them. The filtrate is becoming urine, so water is removed from it by osmosis and reabsorbed into the blood. 2019 P3 Q1.4 · 2022 P2 Q9.3
• Made in the hypothalamus, released from the posterior pituitaryWatch out: the hypothalamus makes ADH, but it's released into the blood from the posterior pituitary. "Pituitary" alone isn't enough. 2022 P2 Q9.1
• Longer loop of Henle, more sodium ionsWatch out: a thicker medulla means a longer loop of Henle, so more sodium ions are moved into the medulla. This lowers its water potential, so more water is reabsorbed by osmosis. 2019 P3 Q1.4

• Cellulose: microfibrils and flipped monomersWatch out: cellulose is made of microfibrils, not "myofibrils" (those are in muscle). And it's alternate monomers (β-glucose) that are flipped: say "monomer" or "β-glucose", not just "molecule". 2022 P1 Q9.2 · 2023 P1 Q2.1
• Mono, di or poly: maltose is a disaccharideWatch out: maltose is a disaccharide (two glucose molecules), not a monosaccharide or a polysaccharide, and glucose is a monosaccharide. A disaccharide is not a polymer: it's only two monomers. 2023 P3 Q2.3 · 2019 P1 Q10.3
• Glycogen: the animal store, released by hydrolysisWatch out: animals store glucose as glycogen, not starch. When glucose is needed, say glycogen is hydrolysed to glucose, not just "converted" to glucose. 2023 P2 Q10.3 · 2024 P2 Q2.3
• Benedict's test: say heat, and explain the resultWatch out: write heat with Benedict's; "use a water bath" on its own doesn't say it's heated. To explain a positive result, name the reducing sugars present (e.g. glucose, fructose). Sucrose is non-reducing, so it isn't what's detected. 2024 P1 Q4.2
• Non-reducing sugars: show a negative test firstWatch out: to show a non-reducing sugar (e.g. sucrose) is present, first do a normal Benedict's test and say it stays blue (no reducing sugar there). Then boil with acid to hydrolyse it into reducing sugars, neutralise, and heat with Benedict's again. Without the negative first test, a red result could just come from reducing sugars that were already there. 2019 P1 Q10.2

• Prophase and anaphaseWatch out: spindle fibres start attaching to the centromeres in prophase, not only in metaphase. In anaphase, say the centromeres split before the chromatids are pulled to opposite poles. 2024 P1 Q5.1
• Mitotic index is a proportionWatch out: the mitotic index is a proportion (e.g. 0.13), so don't convert it to a percentage unless asked. Because it's a proportion, having more or fewer cells in the field of view doesn't change it. 2022 P3 Q3.4 · 2022 P3 Q3.5
• Describing binary fissionWhen describing binary fission, say the circular DNA and the plasmids both replicate, then the cytoplasm divides, giving each cell one copy of the circular DNA and a variable number of plasmids. "The genetic material replicates" isn't specific enough. 2025 P3 Q1.1 · 2017 P1 Q4.1

• Emulsification is not micelle formationWatch out: emulsification and micelles are different. Emulsification by bile salts breaks fat into small droplets, giving a larger surface area for lipase; it isn't hydrolysis. Micelles form after digestion, when fatty acids and monoglycerides associate with bile salts. They carry them to the epithelial cells, where they're absorbed by diffusion. 2024 P1 Q2.3 · 2020 P1 Q2.3 · 2025 P1 Q1.1
• Microvilli vs villiWatch out: microvilli are folds of an epithelial cell's membrane. Villi are folds of the whole gut wall, made of many cells. "Increased surface area" is what microvilli do, not what they are. 2024 P1 Q2.2 · 2020 P1 Q1.3
• Starch digestion has two stagesWhen describing starch digestion, give both stages: amylase hydrolyses starch to maltose (say maltose, not just "disaccharide"), then membrane-bound maltase (a disaccharidase) hydrolyses maltose to glucose. Don't go straight to glucose. 2022 P1 Q9.3
• Endopeptidases vs exopeptidasesWatch out: endopeptidases hydrolyse peptide bonds inside the chain, and exopeptidases hydrolyse bonds at the ends. Together they're faster because endopeptidases create more ends for exopeptidases to act on. 2023 P1 Q4.3 · 2021 P3 Q5.1

• Prokaryotic DNA is still double-strandedWatch out: prokaryotic DNA is double-stranded (a double helix), just like eukaryotic DNA. It's not single-stranded, and it isn't all in plasmids. When asked how prokaryotic and eukaryotic DNA differ, describe the DNA itself (circular vs linear, histones or not, short vs long), not the cell (e.g. "prokaryotes have no nucleus"). 2018 P1 Q1.6 · 2020 P1 Q8.1
• Non-coding DNA isn't only intronsWatch out: non-coding DNA isn't just introns. There are also non-coding multiple repeats between genes. 2020 P1 Q8.2
• "Degenerate": triplets code for amino acidsWhen explaining "degenerate", say more than one triplet (codon) codes for the same amino acid. Talk about triplets or codons, not "bases", and say they code for amino acids, not "produce" them. 2018 P1 Q5.2

• Helicase breaks hydrogen bonds, but it isn't hydrolysisWatch out: DNA helicase breaks the hydrogen bonds between the two strands, but this isn't a hydrolysis reaction. Hydrolysis uses water to break covalent bonds, such as phosphodiester or glycosidic bonds. 2018 P1 Q9.1 · 2019 P1 Q6.2
• What DNA polymerase doesWatch out: DNA polymerase joins adjacent nucleotides by forming phosphodiester bonds in condensation reactions. It doesn't form the hydrogen bonds or the complementary base pairs, and it joins nucleotides, not bases. 2018 P1 Q9.1 · 2019 P1 Q6.2 · 2020 P1 Q5.1
• Replication happens in interphaseWatch out: DNA replication happens during interphase (the S phase), before the cell divides. It isn't part of mitosis. 2019 P1 Q6.1

• Name the bases in fullWatch out: write the bases' names in full: adenine, thymine, cytosine, guanine and uracil. Answers that gave only the letters (A, T, C, G, U) weren't credited. 2022 P1 Q7.1 · 2023 P1 Q10.2 · 2019 P1 Q6.2
• Phosphodiester bonds vs hydrogen bondsWatch out: phosphodiester bonds join the sugar of one nucleotide to the phosphate of the next, forming the sugar-phosphate backbone. They don't form between bases: the bases are paired by hydrogen bonds. 2025 P1 Q2.2 · 2023 P1 Q10.2 · 2020 P1 Q10.1
• DNA is a double helix; RNA is single-strandedWatch out: say DNA is a double helix, not just a "helix". RNA, including mRNA and tRNA, is single-stranded and contains uracil, not thymine. 2022 P1 Q7.1 · 2024 P1 Q6.1

• Faeces aren't excretionWatch out: faeces are undigested food that was never absorbed, so they're not excretion. Urine is excretion. 2023 P2 Q7.3
• Link heat loss to respirationWatch out: respiration releases some energy as heat, which is lost to the surroundings, so it isn't passed to the next trophic level. "Energy is used in respiration" or "energy is produced" doesn't explain the loss. 2023 P2 Q7.3 · 2022 P2 Q2.4
• Say energy is lost between trophic levelsWatch out: when explaining why eating animals is less efficient than eating crops, first say energy is lost at each trophic level, then say how (respiration, faeces, urine). 2022 P2 Q2.4
• Dry mass: heat until the mass is constantWatch out: heat the sample (e.g. in an oven) and reweigh it until the mass is constant. Don't burn it: burning is calorimetry, not drying. 2025 P2 Q3.1
• Calorimetry: kJ and per gramWatch out: in calorimetry calculations, convert J to kJ (÷ 1000), and divide by the sample's dry mass when the answer is per gram. 2025 P2 Q3.2 · 2020 P2 Q3.2
• Productivity units: mass, per area, per timeWatch out: productivity units need mass (or energy), per area, per time, e.g. kg m⁻² h⁻¹ or kJ ha⁻¹ year⁻¹. Start the units with the mass or energy unit. 2022 P3 Q5.4
• GPP and NPP are for plantsWatch out: GPP and NPP describe producers (plants). For animals (consumers), use N = I − (F + R). 2021 P3 Q4.1

• Explain how the enzyme lowers activation energyWhen explaining how an enzyme speeds up a reaction, don't stop at "lowers the activation energy". Say the active site changes shape around the substrate (not just "the tertiary structure changes"), which bends or strains the bonds in the substrate so they break more easily. 2025 P1 Q2.4 · 2018 P1 Q4.1
• Temperature doesn't lower the activation energyWatch out: a higher temperature gives molecules more kinetic energy, so more collisions have enough energy to reach the activation energy. It doesn't lower the activation energy; only enzymes do that. 2024 P1 Q7.3
• Denaturation: name the right bondsWhen explaining denaturation, say the hydrogen and ionic bonds between R groups in the tertiary structure break. Don't just say "bonds break", and don't say peptide bonds break: they join the amino acids in the primary structure, which stays the same. 2017 P1 Q4.2
• Inhibitors: say exactly where they bindA competitive inhibitor binds to the active site, so say "active site", not just "a binding site". A non-competitive inhibitor binds to a site other than the active site, not "at the side of the active site". 2025 P1 Q3.3 · 2019 P1 Q1.1

• The nucleolusWatch out: the nucleolus does not contain all the cell's DNA. Its job is to make rRNA and assemble ribosomes. 2022 P1 Q1.1
• Lysosome vs lysozymeWatch out: a lysosome is an organelle, and lysozymes are the enzymes inside it, so don't mix up the two words. A lysosome is also not a cell, so it can't engulf anything: instead, a vesicle fuses with the lysosome and its hydrolytic enzymes digest the contents. 2023 P3 Q4.1
• How vesicles release a proteinWhen describing how a cell secretes an enzyme, say the vesicles fuse with the cell-surface membrane and release it by exocytosis. "Takes it out of the cell" or "to where it's needed" isn't enough. 2024 P1 Q10.2

• Rate of diffusion is slower, not lessWatch out: damage (e.g. a thicker diffusion barrier or a smaller surface area) means gases diffuse more slowly, so the rate of diffusion is lower. "Less diffusion" doesn't describe what has changed. 2020 P1 Q6.1 · 2022 P1 Q8.3
• Insects: the exchange surface is the tracheolesWatch out: for adaptations of the insect gas exchange surface, write about the tracheoles (highly branched, thin walls, supply tissues directly), not the spiracles. Give the feature and explain it, e.g. highly branched tracheoles give a large surface area for rapid diffusion. 2023 P1 Q2.2 · 2017 P1 Q9.1
• Breathing in: muscles contract and move the ribsWhen describing breathing in, say the external intercostal muscles contract and move the ribcage up and out, and the diaphragm contracts and flattens. Don't say muscles "expand" or push the lungs, and keep the two intercostal muscles separate. 2024 P1 Q3.1 · 2025 P3 Q6.2 · 2018 P1 Q10.1

• Homologous chromosomes carry the same genesWatch out: homologous chromosomes are a pair that carry the same genes (at the same loci), which is why they can pair up and swap alleles in meiosis. Saying "one from each parent" isn't enough. 2018 P1 Q1.5
• Say "homologous pairs" for crossing over and independent segregationWhen describing crossing over or independent segregation, say it happens between homologous chromosomes (pairs). "Chiasma" alone doesn't get the mark. 2023 P1 Q10.3
• Non-disjunction: what happens and the resultWatch out: in non-disjunction, one pair of homologous chromosomes (or sister chromatids) fails to separate, so both move to the same pole. One gamete gets an extra copy of that chromosome (n + 1), and the other gets none (n − 1). If one of these gametes is fertilised, the zygote has an abnormal chromosome number, e.g. trisomy 21 (Down syndrome). 2025 P1 Q7.4 · 2022 P1 Q4.3
• Every mutation makes a new alleleWhen explaining mutations, say a mutation forms a new allele: a new version of the gene with a different base sequence. This is true even if the amino acid sequence doesn't change (the genetic code is degenerate). For a positive effect, say the new allele increases the chance of survival and reproductive success, so it's more likely to be passed on. 2020 P1 Q10.3

• Why high blood glucose dehydrates cellsWatch out: glucose lowers the water potential of the blood, so it's lower than inside the cells. Water moves out of the cells by osmosis, down the water potential gradient. "Diffusion" or "concentration gradient" doesn't explain it. 2025 P2 Q2.2
• How negative feedback worksWatch out: a change is first detected by receptors. The response then opposes the change, returning the level to the set point. Don't skip the detection step. 2025 P2 Q2.4
• What makes it positive feedbackWatch out: in positive feedback, each step increases the next. A causes more B, and more B causes more A, so the change keeps growing. Show the idea of "more" at each step. 2018 P3 Q1.1
• Why exercise lowers blood pHWatch out: more respiration during exercise produces more carbon dioxide, which lowers blood pH (makes it more acidic). Anaerobic respiration in animals produces lactate, not carbon dioxide. 2025 P2 Q2.5

Antigens

• Cells that stimulate an immune responseWatch out: when asked which types of cell can stimulate an immune response, don't name B cells and T cells: those are the cells that respond. The cells that stimulate a response are ones with antigens the body recognises as foreign: pathogens, cells from other people or other species, abnormal body cells and antigen-presenting cells. 2023 P3 Q6.1

Antibodies

• Antibodies have a binding site, not an active siteWatch out: antibodies have a binding site, not an active site. Don't describe an antigen-antibody complex as an "enzyme-substrate" or "antibody-substrate" complex. 2020 P1 Q7.3 · 2022 P1 Q3.6

B cells and the humoral response

• Plasma cells make antibodiesWatch out: antibodies are made by plasma cells, which develop from B cells, not T cells. B cells don't become macrophages. 2018 P1 Q7.5 · 2020 P3 Q3.4

Active and passive immunity

• Antivenom is passive immunityWatch out: antivenom gives passive immunity: the antibodies are given ready-made. It isn't a vaccine, so it doesn't trigger the person's own immune response, and active immunity would be too slow to help after a bite. 2018 P1 Q7.1

Monoclonal antibodies and ELISA

• ELISA: the colour comes from an enzymeWatch out: when explaining the colour change in an ELISA test, say an enzyme (attached to the antibody) reacts with its substrate to make the coloured product. "A coloured dye", or the antigen-antibody complex reacting with the substrate, doesn't get the mark. 2023 P1 Q9.1
• What a control line showsWatch out: a line at the control (C) shows the sample has moved past the test line (T), so a negative result at T is real. Don't just explain why a colour appears at C. 2023 P1 Q9.2

HIV and viruses

• HIV's genetic material is RNAWatch out: HIV's genetic material is RNA, not DNA. 2019 P1 Q5.1 · 2024 P3 Q1.3
• HIV replication: be specificWhen describing how HIV replicates inside helper T cells, give the specific steps, e.g. reverse transcriptase makes DNA from the viral RNA, the DNA is inserted into the host DNA, then transcribed into mRNA, which is translated into viral proteins. "New viruses are made using the cell's own machinery" is too vague. 2021 P3 Q2.1
• Why there's no HIV vaccineHIV mutates quickly, so its antigens keep changing (antigenic variability). Memory cells made after a vaccine wouldn't recognise the new antigens. Watch out: "HIV has no cell wall" explains why antibiotics don't work, not vaccines. 2024 P3 Q1.2

• Due to chance: talk about the difference, not the resultsWatch out: don't say the results are due to chance or not significant. Say the difference between the means is (or isn't) likely to be due to chance, or is (or isn't) significant. 2023 P1 Q5.5
• Standard deviations: use 2 × SD and compare rangesWatch out: if a table gives one standard deviation but the question says mean ± 2 SD covers 95% of the data, use 2 × SD. Give both ranges (four numbers), then say whether they overlap and what that means for the difference in means. 2023 P1 Q5.5

• Ester bond: name it, and know which atoms are removedWatch out: the bond between glycerol and a fatty acid is an ester bond, not glycosidic (that's between sugars). The water removed is the H from an OH group on glycerol and the OH from the COOH of the fatty acid: not an H from a C–H, and not an OH from both. 2024 P1 Q1.1
• Triglycerides: three water moleculesWatch out: one glycerol joins to three fatty acids, so three condensation reactions happen and three molecules of water are removed. Saying just "water is removed" misses the mark. 2021 P1 Q8.1
• Saturation: double bonds and hydrogen atomsWatch out: describe saturation in terms of C=C double bonds (e.g. "as the number of double bonds increases, melting point decreases"). If you talk about hydrogen, say hydrogen atoms, not hydrogen molecules. 2024 P1 Q1.3
• Bilayer: heads like water, tails don'tWatch out: the phosphate heads are hydrophilic and the fatty acid tails are hydrophobic; don't swap them. To explain the bilayer, link it to water: heads face the water on both sides, tails point away from it. If you draw it, label the heads, tails and where the water is. 2025 P1 Q1.2
• Membrane fluidity: unsaturated fatty acidsWatch out: link the cause to the effect. More unsaturated fatty acids in the phospholipids make the cell-surface membrane more fluid, because the kinks at the C=C double bonds stop the tails packing closely together, so the phospholipids can move more freely. 2024 P1 Q1.4
• Emulsion test: milky white, not cloudyWatch out: a positive emulsion test is a white (milky) emulsion. "Cloudy" isn't accepted. 2019 P1 Q10.2

• Water moves by osmosis, not tissue fluidWatch out: when explaining the return of tissue fluid, say water moves back into the capillary by osmosis, not "tissue fluid". Be precise, e.g. "the blood has a lower water potential than the tissue fluid, so water moves from the tissue fluid into the capillary by osmosis." 2023 P1 Q9.6 · 2017 P1 Q3.3
• Arteries don't pump bloodWatch out: the muscle in artery walls doesn't pump blood, and the aorta isn't a second pump. Elastic tissue stretches when blood surges in and recoils, which smooths blood flow and maintains pressure. Vasoconstriction happens in arterioles, not the aorta. 2024 P1 Q8.1 · 2020 P1 Q3.2
• Valves: explain the causeWhen explaining how a valve keeps blood flowing one way, give the pressure cause for both: it opens when pressure is higher behind it, and closes when pressure is higher in front of it. Saying when it opens or closes isn't enough. 2019 P3 Q3.3
• Haemoglobin affinity: loading and unloading
  • If haemoglobin has a lower affinity for oxygen, this means oxygen is unloaded more easily, but loaded less easily.
  • If haemoglobin has a higher affinity for oxygen, this means oxygen is loaded more easily, but unloaded less easily.
  2019 P1 Q9.3 · 2020 P3 Q6.1

Xylem and transpiration

• Xylem isn't translocationWatch out: water moves up the xylem by cohesion-tension, not by "source to sink", which is translocation in the phloem. Say how the tension is created: water evaporates from the leaves (transpiration), pulling the water column up. 2023 P1 Q2.3
• Stomata open for gas exchangeWatch out: stomata open to let carbon dioxide in for photosynthesis. Water loss is a consequence, not the reason they open. 2022 P1 Q6.1

Phloem and translocation

• Sucrose, by mass flow, not diffusionWhen describing translocation, name sucrose as the carbohydrate, say it's co-transported into the phloem with H⁺ by companion cells, and say it moves by mass flow: the whole solution (sap) flows down a hydrostatic pressure gradient. Don't call this diffusion, which is individual molecules moving down a concentration gradient. 2022 P1 Q9.1
• Tracer experiments: explain, don't describeWhen interpreting a radioactive tracer experiment, explain the results: the radioactive carbon is used in photosynthesis to make sugars, which are then translocated in the phloem. Just saying where the radioactivity appears isn't enough. 2018 P1 Q8.1

• Dimers aren't polymersWatch out: a dimer (e.g. a disaccharide or dipeptide) is not a polymer. When asked for examples of polymers, give molecules made of many monomers, e.g. polysaccharides, polypeptides or polynucleotides. 2019 P1 Q10.3
• Where the water goesWatch out: don't swap the two reactions. Condensation releases water as a bond forms; hydrolysis uses water to break a bond. 2024 P1 Q10.3
• Name the bondWhen describing a condensation or hydrolysis reaction, say that a bond is formed or broken and name it (e.g. glycosidic, peptide, phosphodiester), not just what happens to the water. 2019 P1 Q10.3 · 2018 P1 Q10.3
• Not every reaction with water is hydrolysisWatch out: not every reaction involving water is condensation or hydrolysis. In photosynthesis, water is split by photolysis, not hydrolysed. 2022 P3 Q6.1

• Know which filaments are in each bandWatch out: explain the bands by the filaments they contain. I band = actin only, H zone = myosin only, and the darkest region = where actin and myosin overlap (part of the A band). 2020 P2 Q4.1
• Binding site, not active siteWatch out: tropomyosin blocks the myosin binding sites on actin. Don't call them "active sites"; that's enzyme terminology. 2019 P2 Q4.2
• Say "myosin head", not "myofibril"Watch out: ATP breaks the actinomyosin bridge and moves the myosin head. Write "myosin", not "myofibril", and mention the head. 2023 P2 Q10.1
• Compare both fibre typesWatch out: when a question asks about fast and slow fibres, compare both, e.g. fast fibres contract quickly, whereas slow fibres contract slowly. 2024 P2 Q2.3
• Muscles contract and relaxWatch out: describe muscles as contracting and relaxing, not "constricting", "expanding" or "lengthening". 2020 P2 Q1.1

• Sodium-potassium pump: 3 Na⁺ out, 2 K⁺ inWatch out: the sodium-potassium pump moves 3 sodium ions out and 2 potassium ions in by active transport (i.e. against their concentration gradients), which needs ATP. 2021 P2 Q2.1
• Respiratory inhibitor: the resting potential isn't maintainedWatch out: a respiratory inhibitor means less ATP is made (not "no energy"), so the sodium-potassium pump can't actively transport ions. The ion concentration gradients aren't maintained, so the resting potential isn't maintained. 2021 P2 Q2.3
• Myelin: electrical insulation, so depolarisation only at the nodesWatch out: myelin is electrical insulation (not thermal), so depolarisation only happens at the nodes of Ranvier. This is saltatory conduction. Use the word "depolarisation" when you describe it. 2021 P2 Q2.2

• Name the nitrogen-containing compoundWatch out: saprobionts break down proteins, amino acids, DNA or urea into ammonia (ammonium ions). Name the compound, not just "nitrogen compounds" or "dead matter". 2025 P2 Q10.3 · 2019 P3 Q2.1
• Nitrification: ammonium, then nitrite, then nitrateWatch out: nitrifying bacteria oxidise ammonium to nitrite, then nitrite to nitrate. Get the order right, and don't call it reduction. 2025 P2 Q10.3
• Nitrifying isn't nitrogen-fixingWatch out: nitrogen-fixing bacteria turn N₂ gas into ammonia. Nitrifying bacteria turn ammonium into nitrates. Don't confuse either with ammonification, where saprobionts turn dead matter and waste into ammonium. 2025 P2 Q10.3 · 2019 P3 Q2.1
• Saprobionts are aerobic; denitrification makes N2Watch out: most saprobionts respire aerobically. Denitrification turns nitrate into nitrogen gas, not ammonia. 2021 P3 Q4.2
• Eutrophication: photosynthesis, saprobionts, respirationWatch out: say plants die because they can't photosynthesise. Say saprobionts (not just "bacteria" or "decomposers") use oxygen in respiration, so fish can't respire. The nutrients and decomposition products aren't toxic. 2021 P2 Q10.3

Photosynthesis

• It's reduced NADP, not reduced NADWatch out: the light-dependent reaction makes ATP and reduced NADP. Reduced NAD belongs to respiration, so writing it here loses the mark. 2020 P2 Q3.4 · 2024 P2 Q3.3
• What ATP and reduced NADP actually doWatch out: in the light-independent reaction, ATP provides energy (not "phosphate") and reduced NADP reduces GP to triose phosphate. It's a reduction, not an oxidation. 2024 P2 Q3.3
• Write triose phosphate in fullWatch out: write triose phosphate in full. "TP" isn't an abbreviation the spec uses, so answers using only TP have lost marks. (GP is fine, because the spec abbreviates it.) If you're unsure whether an abbreviation is allowed, check the spec, or write the name in full. 2022 P3 Q5.3 · 2022 P2 Q1.2
• Photoionisation: say chlorophyllWatch out: in photoionisation, chlorophyll absorbs light and its electrons are excited and lost. Be specific: say chlorophyll, not just "chloroplast" or "photosystem". And don't confuse it with photolysis (the splitting of water): describing photolysis instead scored nothing. 2019 P2 Q7.2

Chromatography (RP7)

• Don't mix up the pigment solution and the solventWatch out: the pigment (chlorophyll) solution goes on the pencil line. The solvent goes in the container, with its level below the line. Use an organic solvent, never water. 2023 P2 Q1.1 · 2019 P2 Q7.4
• Pencil, because ink would mix with the pigmentsWatch out: don't just say ink would "run" or "smudge". Say the ink would dissolve and mix with the leaf pigments, or that a pencil line stays visible in the same place. 2019 P2 Q7.3

• Features of ALL prokaryotesWatch out: plasmids, a capsule and flagella are only found in some prokaryotes. Features of all prokaryotes are: no membrane-bound organelles, circular DNA not associated with proteins, and murein in the cell wall. Don't give 70S ribosomes as unique to prokaryotes: eukaryotic cells contain them too, in their mitochondria and chloroplasts. 2022 P1 Q3.1 · 2025 P1 Q4.1
• Capsule vs capsidWatch out: a capsule is a slimy layer around some bacteria. A capsid is the protein coat of a virus. Don't mix them up. 2021 P1 Q10.3 · 2025 P1 Q4.1
• What all viruses haveAll virus particles have genetic material (DNA or RNA, not always the same one), a capsid and attachment proteins, which bind to receptors on host cells. Don't include features only some viruses have (e.g. reverse transcriptase, a lipid envelope), or bacterial structures. 2023 P1 Q1.1
• What "acellular" meansWatch out: acellular means viruses aren't made of cells: they have no cytoplasm and no organelles. Don't confuse it with non-living: "they need a host cell to reproduce" explains why they're non-living, not why they're acellular. 2023 P1 Q1.2

• RNA polymerase forms phosphodiester bondsWatch out: RNA polymerase joins adjacent RNA nucleotides by phosphodiester bonds (between the sugar and phosphate). It doesn't form hydrogen bonds, pair complementary bases, or join the bases together. 2020 P1 Q10.1 · 2025 P1 Q2.2
• mRNA vs tRNA: both single-stranded, both have uracilWatch out: both mRNA and tRNA contain uracil, not thymine, and tRNA is single-stranded: it folds into a clover leaf held by hydrogen bonds, but it isn't double-stranded or a double helix. 2024 P1 Q6.1
• Translation: the amino acid joins, using ATPIn translation, say each tRNA carries a specific amino acid, and that it's the amino acid (not the tRNA) that joins the polypeptide by a peptide bond, using ATP. 2020 P1 Q10.2 · 2021 P1 Q3.1
• Genome isn't genotypeWatch out: the genome is the complete set of genes in a cell, not all the genes in a species or population, and not the genotype. The proteome is the full range of proteins the cell is able to produce: include "able to produce", and don't confuse it with the phenotype. 2022 P3 Q4.1

• Quaternary structureWatch out: quaternary structure means more than one polypeptide, not four (haemoglobin just happens to have four). A protein made of a single polypeptide has no quaternary structure: its folded shape is its tertiary structure. 2023 P1 Q10.1 · 2019 P1 Q4.2
• How a quaternary protein formsWhen asked to describe how a quaternary protein is formed from its monomers, give each level and mention which bonds are formed: primary, amino acids joined by peptide bonds in condensation reactions; secondary, hydrogen bonds; tertiary, interactions between R groups; quaternary, interactions between two or more polypeptides. Students most often left out the peptide bonds or condensation. 2023 P1 Q10.1
• DenaturationWatch out: denaturation (by heat or a change in pH) changes a protein's tertiary structure, not its primary structure. So say that the hydrogen and ionic bonds between R groups break. The peptide bonds of the primary structure aren't broken, and just writing "bonds" isn't enough. 2017 P1 Q4.2 · 2018 P3 Q1.2
• Drawing an amino acidWatch out: when drawing an amino acid, the hydrogens of the amine group (NH₂) and carboxyl group (COOH) belong to those groups, not the central carbon. The central carbon carries the amine group, the carboxyl group, one hydrogen and the R group. (See the amino acid diagram) 2025 P3 Q4.1 · 2020 P1 Q1.5
• Not all proteins are enzymesWatch out: not all proteins are enzymes. Only enzymes have an active site. For other proteins, such as antibodies, receptors or hormones, don't mention an active site, enzyme or substrate. Say the molecule is complementary to its binding site, or that the protein's tertiary structure changes. 2018 P3 Q1.2 · 2021 P2 Q5.1 · 2020 P2 Q2.2 · 2020 P1 Q7.3 · 2022 P1 Q3.6
• Say "tertiary structure"Watch out: write tertiary structure in full. "3D shape", "3-D structure" and abbreviations weren't credited. 2018 P1 Q5.5 · 2021 P2 Q5.1

• Pyruvate is reduced, not oxidisedWatch out: in anaerobic respiration, pyruvate is reduced (it gains hydrogen from reduced NAD) to lactate or ethanol. This regenerates NAD so glycolysis can continue. 2019 P2 Q4.3 · 2025 P2 Q1.3
• Why less oxygen is used: explain the chainWatch out: if something blocks the Krebs cycle or the electron transfer chain, fewer electrons reach the end of the chain. Oxygen is the final electron acceptor, so less oxygen is needed. 2021 P2 Q1.2 · 2025 P2 Q1.3
• Keep respiration and photosynthesis terms apartWatch out: respiration uses NAD and FAD, not NADP. RuBP, GP, photolysis and the Calvin cycle belong to photosynthesis. 2021 P2 Q1.1 · 2025 P2 Q1.2 · 2025 P2 Q1.3
• Glycolysis: net gain, and triose phosphate is oxidisedWatch out: say there is a net gain of ATP (4 made, 2 used). Say that triose phosphate is oxidised to pyruvate. 2021 P2 Q1.1
• Bacteria have no mitochondriaWatch out: bacteria don't have mitochondria. In prokaryotes, the electron transfer chain and ATP synthase are in the cell-surface membrane. 2025 P3 Q1.3
• Decarboxylation, not carboxylationWatch out: CO₂ is removed in the link reaction and the Krebs cycle. That's decarboxylation, not carboxylation. 2025 P2 Q1.2

• Heat is released, not producedWhen linking SA:V to metabolic rate, explain fully: a larger SA:V means more heat lost, so a higher metabolic rate (more respiration) is needed to release heat and keep body temperature constant. Watch out: say heat or energy is released, never "produced". 2024 P3 Q3.1 · 2019 P1 Q9.4
• SA:V = surface area ÷ volumeWatch out: the SA:V ratio is surface area ÷ volume, not volume ÷ surface area. 2022 P1 Q8.2
• Why larger animals need specialised systemsDon't just say larger animals have a smaller SA:V ratio. Explain the advantage: a specialised exchange system overcomes the long diffusion pathway, giving faster diffusion. 2021 P1 Q2.1

• The fractionation solutionWatch out: when explaining the solution used in cell fractionation, be specific. Isotonic prevents osmotic damage to the organelles (not the cells). Buffered keeps the pH constant so enzymes and proteins aren't denatured: "prevents damage" isn't enough. 2024 P1 Q10.1
• Centrifuge speedsWatch out: to remove or collect large organelles like nuclei, spin at a low speed first, then increase the speed for smaller organelles. Don't jump straight to high speeds. 2022 P1 Q7.2
• What "higher resolution" meansWatch out: higher resolution means the microscope can distinguish points that are closer together. Electron microscopes have higher resolution than optical ones. 2025 P1 Q4.2
• TEM vs SEMWatch out: TEM gives 2D images showing internal structure. SEM gives 3D images of the surface. Don't mix them up. 2025 P1 Q4.2 · 2017 P1 Q10.1

• Vesicles fuse; acetylcholine diffusesWatch out: vesicles fuse with the presynaptic membrane and release acetylcholine. It's the acetylcholine that diffuses across the synaptic cleft, not the vesicles. 2024 P2 Q1.2 · 2020 P2 Q5
• Calcium ions enter the synaptic knobWatch out: say calcium ions diffuse into the synaptic knob (or presynaptic neurone). "Into the presynaptic membrane" or "into the synapse" isn't precise enough. 2023 P2 Q2.1
• Say "receptors", and say where they areWatch out: acetylcholine binds to receptors on the postsynaptic membrane (at a neuromuscular junction, the sarcolemma). Not "the muscle", and not an "active site". 2022 P2 Q7.3 · 2024 P2 Q1.2 · 2020 P2 Q5
• Sodium ions enter, causing depolarisationWatch out: don't stop at "sodium ion channels open". Say sodium ions enter the postsynaptic neurone, causing depolarisation. If threshold is reached, an action potential is produced. 2024 P2 Q1.2 · 2023 P2 Q2.1
• Fewer action potentials, not weaker onesWatch out: if less acetylcholine binds, there are fewer (or no) action potentials. Action potentials are all-or-nothing, so they're never "weaker". 2022 P2 Q7.3
• Summation: say what reaches thresholdWatch out: name the type (temporal or spatial), then explain that enough neurotransmitter is released, or enough sodium ions enter, to reach threshold. "More impulses" on its own isn't enough. 2022 P2 Q7.2

• Renamed species: new evidence, not new evolutionWatch out: species get renamed because of new evidence, e.g. comparing DNA base sequences, not because they have evolved or mutated. Include the word base: "DNA base sequencing", not just "DNA sequencing". 2023 P1 Q6.3
• Counting genera: look at the first nameWatch out: the first part of a binomial is the genus, so count the different first names, not the number of species. E.g. Chrysolina graminis, Chrysolina herbacea and Gastrophysa viridula are 3 species but only 2 genera (Chrysolina and Gastrophysa). 2025 P3 Q3.1
• Different species: explain the offspringWatch out: don't just say two organisms are different species. Explain why: they can't breed (e.g. their gametes can't fuse), or if they do, the offspring are not fertile. 2019 P1 Q7.3

• Active transport needs a carrier proteinWatch out: active transport uses carrier proteins and ATP, not channel proteins. 2024 P1 Q2.1 · 2021 P1 Q10.2
• Water moves by osmosis, down a water potential gradientWatch out: explain water movement using water potential, not "water concentration", and call it osmosis, not diffusion. 2024 P1 Q4.4 · 2023 P1 Q4.1
• How co-transport worksWhen explaining co-transport in the ileum, start with the Na⁺/K⁺ pump: it actively transports Na⁺ out of the cell into the blood, keeping the Na⁺ concentration low inside the cell. Na⁺ then moves in from the lumen by facilitated diffusion through a co-transporter, carrying glucose with it. 2020 P1 Q1.2 · 2021 P1 Q10.2

• Give the property and why it mattersWatch out: for each property of water, give the property and why it's important to organisms, e.g. "high heat capacity, so it buffers changes in temperature". And don't forget water is a metabolite (used in hydrolysis, released in condensation). 2019 P1 Q10.1
• Water is the solvent, not the soluteWatch out: water is the solvent (the substances dissolved in it are the solutes). Then say why it matters: metabolic reactions happen in solution, and dissolved substances can be transported. 2019 P1 Q10.1
• Heat capacity vs latent heatWatch out: these are different. High heat capacity: a high amount of energy is needed to raise the temperature, so it buffers temperature changes. Large latent heat of vaporisation: a high amount of energy is needed to evaporate it, so it gives a cooling effect (e.g. sweating) with little water lost. 2019 P1 Q10.1
• Cohesion: say what it doesWatch out: don't just write that cohesion "aids transpiration". Explain that it supports columns of water in the xylem (the transpiration stream), or produces surface tension. 2019 P1 Q10.1
• Sodium ions: explain the gradientWatch out: explain why moving Na⁺ out of the cell matters. Active transport of Na⁺ out keeps the Na⁺ concentration low inside the cell, so Na⁺ diffuses in from the ileum through the co-transporter, bringing glucose with it. 2020 P1 Q1.2