Specification H420. Explore a topic, practise the skill, and see where to focus next.
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A transmission electron microscope uses electrons with a much shorter wavelength than visible light, so it has a higher resolving power and can distinguish structures that are closer together.
DNA consists of two nucleotide polymer strands coiled into a double helix. Complementary bases pair A with T and C with G.
A valid design varies nitrate concentration alone, controls other growth factors, starts with comparable material and uses replicates. This isolates the proposed cause and gives an estimate of biological variation.
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Correct answer: Electrons have a much shorter wavelength, giving greater resolving power
A transmission electron microscope uses electrons with a much shorter wavelength than visible light, so it has a higher resolving power and can distinguish structures that are closer together.
Correct answer: Two polymer strands form a double helix with complementary base pairs
DNA consists of two nucleotide polymer strands coiled into a double helix. Complementary bases pair A with T and C with G.
Correct answer: Use several nitrate concentrations, replicate each treatment and keep light, temperature and starting frond number constant
A valid design varies nitrate concentration alone, controls other growth factors, starts with comparable material and uses replicates. This isolates the proposed cause and gives an estimate of biological variation.
For parents: look at the explanation together. A correct answer is encouraging; a missed answer gives you something specific to work on. Broader practice over time is needed to understand progress.
Unknown potentially pathogenic material is unsuitable for routine school work. An approved non-pathogenic strain, risk-assessed aseptic technique and appropriate incubation conditions achieve the learning aim safely.
Watch for: Do not name a biological process without explaining the mechanism. For Planning, link the relevant structure or condition to the resulting effect.
The colorimeter infers absorbance from transmitted light. Marks, liquid on the outside or optical differences between cuvette faces introduce unwanted variation, so clean handling and consistent orientation improve comparability.
Watch for: Do not name a biological process without explaining the mechanism. For Implementing, link the relevant structure or condition to the resulting effect.
A best-fit line represents the overall relationship rather than measurement noise. Its form follows the pattern, and it should not be forced through the origin without a scientific or data-based reason.
Watch for: Do not name a biological process without explaining the mechanism. For Analysis, link the relevant structure or condition to the resulting effect.
Finer resolution reduces reading uncertainty, while a larger displacement makes the same absolute uncertainty a smaller fraction of the measurement. Calibration checks the scale against known values.
Watch for: Do not name a biological process without explaining the mechanism. For Evaluation, link the relevant structure or condition to the resulting effect.
Scientific communication links a claim to quantitative or qualitative evidence and distinguishes interpretation from evaluation. Relevant limitations and improvements show how strongly the evidence supports the claim.
Watch for: Do not name a biological process without explaining the mechanism. For Practical skills, link the relevant structure or condition to the resulting effect.
The criteria include electrochemical or biosensor measurements, data logging and computer modelling. Competence requires calibration, suitable acquisition settings, transparent processing and evaluation of assumptions or uncertainty.
Watch for: Do not name a biological process without explaining the mechanism. For Use of apparatus and techniques, link the relevant structure or condition to the resulting effect.
A suitable slide preparation avoids folds and trapped air, uses an appropriate mounting medium and preserves the structures being examined. Thin material improves visibility but does not change the microscope's theoretical resolving power.
Watch for: Do not name a biological process without explaining the mechanism. For Cell structure, link the relevant structure or condition to the resulting effect.
Carbohydrates contain carbon, hydrogen and oxygen, whereas proteins also contain nitrogen and may contain sulfur through amino acids such as cysteine and methionine. Elemental composition does not by itself describe the full structure.
Watch for: Do not name a biological process without explaining the mechanism. For Biological molecules, link the relevant structure or condition to the resulting effect.
ATP is rapidly hydrolysed and regenerated. Phosphate transfer can raise a reactant's free energy or alter its reactivity, so energy released by respiration can be coupled to cellular work without ATP serving as a permanent energy store.
Watch for: Do not name a biological process without explaining the mechanism. For Nucleotides and nucleic acids, link the relevant structure or condition to the resulting effect.
The spread suggests either natural variation or an anomalous result. Repeats under identical controlled conditions allow the anomaly to be investigated and a mean of concordant measurements to provide more reliable evidence.
Watch for: Do not name a biological process without explaining the mechanism. For Enzymes, link the relevant structure or condition to the resulting effect.
Rising temperature increases molecular movement and can denature membrane proteins; greater heat disrupts bilayer organisation. More pigment crosses the tonoplast and plasma membrane, increasing supernatant absorbance.
Watch for: Do not name a biological process without explaining the mechanism. For Biological membranes, link the relevant structure or condition to the resulting effect.
Multipotent haematopoietic stem cells self-renew and produce progenitors. Differential gene expression drives erythroid features such as haemoglobin accumulation and neutrophil features such as lobed nuclei and phagocytic machinery.
Watch for: Do not name a biological process without explaining the mechanism. For Cell division, cell diversity and cellular organisation, link the relevant structure or condition to the resulting effect.
Boyle's law links increased thoracic and lung volume to decreased pressure. Air then moves down the pressure gradient from the atmosphere into the lungs; the reverse pressure gradient drives quiet expiration as the muscles relax and elastic tissue recoils.
Watch for: Do not name a biological process without explaining the mechanism. For Exchange surfaces, link the relevant structure or condition to the resulting effect.
Cardiac muscle is myogenic. The AVN delay permits atrial emptying before the conducting system carries excitation rapidly to the ventricular apex, producing coordinated contraction toward the outflow vessels.
Watch for: Do not name a biological process without explaining the mechanism. For Transport in animals, link the relevant structure or condition to the resulting effect.
A thick cuticle reduces evaporation, while sunken stomata and hairs maintain a humid boundary layer and reduce the water-vapour gradient. Floating hydrophyte leaves instead commonly place stomata on the upper surface.
Watch for: Do not name a biological process without explaining the mechanism. For Transport in plants, link the relevant structure or condition to the resulting effect.
Population impact depends on efficacy and coverage as well as duration of immunity and transmission. A balanced judgement weighs benefits, risks, equity, autonomy and evidence rather than asserting that one factor settles the issue.
Watch for: Do not name a biological process without explaining the mechanism. For Communicable diseases, disease prevention and the immune system, link the relevant structure or condition to the resulting effect.
Conservation can maintain ecosystem services, genetic resources and ecotourism income, but countries may bear unequal costs or have competing land-use priorities. Monitoring and enforcing agreed action across borders is also challenging.
Watch for: Do not name a biological process without explaining the mechanism. For Biodiversity, link the relevant structure or condition to the resulting effect.
If populations are isolated, different selection pressures and mutations can change their allele frequencies. Over many generations the differences may become large enough to prevent successful interbreeding, producing separate species.
Watch for: Do not name a biological process without explaining the mechanism. For Classification and evolution, link the relevant structure or condition to the resulting effect.
Large organisms contain specialised, interdependent cells. Nervous and hormonal signalling connect receptors, coordination centres and effectors so responses occur on an appropriate time and spatial scale.
Watch for: Do not name a biological process without explaining the mechanism. For Communication and homeostasis, link the relevant structure or condition to the resulting effect.
Low glomerular filtration rate reduces removal and regulation of solutes. Haemodialysis can exchange substances across a partially permeable membrane, while a successful transplant may restore kidney functions but requires compatibility and immunosuppression.
Watch for: Do not name a biological process without explaining the mechanism. For Excretion as an example of homeostatic control, link the relevant structure or condition to the resulting effect.
Once threshold is reached, sodium-channel opening causes further depolarisation and more channel opening. Inactivation of sodium channels and delayed potassium-channel opening then repolarise the membrane.
Watch for: Do not name a biological process without explaining the mechanism. For Neuronal communication, link the relevant structure or condition to the resulting effect.
Automated feedback can better match changing glucose demand than fixed injections. However, interstitial readings lag blood changes and safe use still depends on accurate sensing, algorithms, infusion function and human response.
Watch for: Do not name a biological process without explaining the mechanism. For Hormonal communication, link the relevant structure or condition to the resulting effect.
A concentration series with a solvent control can reveal a dose response only if starting material, environment, duration and measurement are comparable. Replicates allow biological variation and uncertainty to be assessed.
Watch for: Do not name a biological process without explaining the mechanism. For Plant and animal responses, link the relevant structure or condition to the resulting effect.
Hydrogen carbonate indicator responds to carbon dioxide-related pH change, so comparable starting conditions, exposure and controls are essential. Objective colorimetry can reduce subjective endpoint judgement.
Watch for: Do not name a biological process without explaining the mechanism. For Photosynthesis, link the relevant structure or condition to the resulting effect.
Electron carriers and ATP synthase are embedded in the inner mitochondrial membrane. Folding it into cristae increases the area for oxidative phosphorylation, while the matrix contains enzymes for the link reaction and Krebs cycle.
Watch for: Do not name a biological process without explaining the mechanism. For Respiration, link the relevant structure or condition to the resulting effect.
Hox genes are a conserved subset of homeobox genes that regulate developmental gene networks along body axes. Altering one regulator can therefore change the identity or placement of multiple structures rather than one enzyme alone.
Watch for: Do not name a biological process without explaining the mechanism. For Cellular control, link the relevant structure or condition to the resulting effect.
A physical barrier divides the gene pool. Mutation, drift and different selection pressures change allele frequencies independently; if reproductive barriers later prevent successful interbreeding, the populations constitute separate species.
Watch for: Do not name a biological process without explaining the mechanism. For Patterns of inheritance, link the relevant structure or condition to the resulting effect.
PCR can amplify selected variable regions and electrophoresis separates their fragments or alleles. A profile supports identity or relationship inference probabilistically, so sample quality, contamination and reference-population frequencies still matter.
Watch for: Do not name a biological process without explaining the mechanism. For Manipulating genomes, link the relevant structure or condition to the resulting effect.
Micropropagation rapidly multiplies scarce material and can maintain selected genotypes. Risks include contamination, somaclonal change and genetic uniformity, so it complements rather than replaces in situ conservation.
Watch for: Do not name a biological process without explaining the mechanism. For Cloning and biotechnology, link the relevant structure or condition to the resulting effect.
Systematic sampling along the gradient estimates distribution, while replicate transects reveal spatial variation. Consistent quadrat size, defined abundance measure and recorded distance make results comparable.
Watch for: Do not name a biological process without explaining the mechanism. For Ecosystems, link the relevant structure or condition to the resulting effect.
Both forms can reduce growth, survival or reproduction when organisms need the same limiting resource. Intraspecific competitors often have especially similar requirements, while interspecific competition can affect realised niches and community composition.
Watch for: Do not name a biological process without explaining the mechanism. For Populations and sustainability, link the relevant structure or condition to the resulting effect.
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OCR · Biology A · A Level · Biology · H420
Version 4.1, April 2026
Source checked: 2026-08-28. The current official specification controls assessment requirements and option choices.
Yes. This page offers three original questions from three different skills on this exact course. Each answer has an explanation, followed by a sample heatmap showing what was correct and what to revisit. No account is needed, and taster answers are not saved.
It shows the outcome of these three answers and gives a specific skill to discuss or practise next. It is not a full assessment, a mastery score or a grade prediction. Broader practice over time is needed to understand progress.
LearningP currently maps 31 assessed areas for specification H420. The visible topic map below is derived from the verified route; the current official specification remains controlling.
The verified source bank contains 369 original LearningP question records for this route. Every mapped area meets the current publication minimum and passed the latest blocker and review audit.
No. LearningP uses official specifications and assessment materials to map content and demand, while its practice questions and explanations are independently authored.
No. LearningP reports practice evidence, coverage, strengths and gaps. It does not guarantee or automatically predict examination outcomes.
Use the official OCR · Biology A specification and assessment-resource pages linked on this page, together with information supplied by the learner’s school.