Original open focus revision; complete specification coverage remains unfinished.
1 · Cell measurements and scale
magnification · resolution
edexcel_igcse_biology:cells:open:1 · 2 marks · points
Which comparison of two cell images is reliable?
- Compare screen lengths only
- Convert both lengths using their own scale bars
- Assume both images have the same zoom
Answer and reasoning · Réponse et raisonnement
Convert both lengths using their own scale bars
- A scale bar provides a known real distance in the same image. Convert the image length and real length to the same unit before dividing. Magnification is a ratio and has no unit.
edexcel_igcse_biology:cells:open:2 · 4 marks · points
An image is 12 mm long; the cell is 40 micrometres long. Calculate magnification.
Answer and reasoning · Réponse et raisonnement
300
- Appropriate relation; consistent substitution; correct result; meaningful unit and interpretation.
A nucleus is not the only cell structure. Do not claim bacteria have no DNA, or that more magnification always means better resolution.
2 · Osmosis and a fair potato experiment
osmosis · control variable
edexcel_igcse_biology:osmosis:open:1 · 2 marks · points
Why blot all samples before the final weighing?
- To remove surface solution consistently
- To stop all molecular movement
- To make every final mass equal
Answer and reasoning · Réponse et raisonnement
To remove surface solution consistently
- Use percentage change to compare samples with different initial masses. A zero percentage change estimates a solution concentration with no net water movement. This is an estimate from a trend, not proof that water molecules stop moving.
edexcel_igcse_biology:osmosis:open:2 · 4 marks · points
A 4.00 g cylinder ends at 3.60 g. Find percentage mass change.
Answer and reasoning · Réponse et raisonnement
-10 %
- Appropriate relation; consistent substitution; correct result; meaningful unit and interpretation.
Blotting removes surface solution. Weighing a wet cylinder without blotting adds liquid that did not enter the cells. Repeats reduce random variation but do not correct a miscalibrated balance.
3 · Enzyme rate and controlled measurements
enzyme · denaturation
edexcel_igcse_biology:enzymes:open:1 · 2 marks · points
Which change best tests the effect of pH?
- Change pH and temperature together
- Use buffers of different pH at one temperature
- Change enzyme concentration at each pH
Answer and reasoning · Réponse et raisonnement
Use buffers of different pH at one temperature
- Measure rate using product formed per unit time or a fixed endpoint. For an endpoint test, 1/time is a rate proxy if the same amount of product or substrate change defines the endpoint each time.
edexcel_igcse_biology:enzymes:open:2 · 4 marks · points
A fixed endpoint takes 50 s. Calculate the rate proxy 1/time.
Answer and reasoning · Réponse et raisonnement
0.02 1/s
- Appropriate relation; consistent substitution; correct result; meaningful unit and interpretation.
An optimum is specific to the enzyme and conditions. Low temperature usually slows the reaction; it does not necessarily denature the enzyme. Endpoint intervals limit precision.
4 · Digestion, transport and health evidence
digestion · absorption
edexcel_igcse_biology:nutrition:open:1 · 2 marks · points
Which statement about bile is correct?
- It is a protease
- It emulsifies lipids
- It breaks every nutrient into monomers
Answer and reasoning · Réponse et raisonnement
It emulsifies lipids
- Absorption moves soluble products into blood or lymph. Thin exchange surfaces and a large surface area shorten diffusion paths and increase transfer. Enzyme activity and transport are different processes.
edexcel_igcse_biology:nutrition:open:2 · 4 marks · points
9 of 36 participants report an outcome. Calculate the percentage.
Answer and reasoning · Réponse et raisonnement
25 %
- Appropriate relation; consistent substitution; correct result; meaningful unit and interpretation.
Bile is not an enzyme. A positive food test identifies a component under the test conditions; it does not show that a food is healthy or unhealthy in every diet.
5 · Photosynthesis and limiting factors
limiting factor · photosynthesis
edexcel_igcse_biology:photosynthesis:open:1 · 2 marks · points
A rate-light graph reaches a plateau. What follows?
- Light is still certainly the only limit
- Another factor may now limit the rate
- Photosynthesis has stopped
Answer and reasoning · Réponse et raisonnement
Another factor may now limit the rate
- Change only one factor when testing a limiting factor. At low light, extra light may increase rate. At a plateau, the changed factor is no longer the main limit in that range; the graph alone does not identify which other factor is limiting.
edexcel_igcse_biology:photosynthesis:open:2 · 4 marks · points
9.0 cubic centimetres of gas are collected in 3.0 minutes. Find the rate.
Answer and reasoning · Réponse et raisonnement
3 cm³/min
- Appropriate relation; consistent substitution; correct result; meaningful unit and interpretation.
Moving a lamp changes light and may also change temperature. A photosynthesis experiment needs control of heating, not just a ruler.
6 · Respiration, ATP and energy transfers
respiration · ATP
edexcel_igcse_biology:respiration:open:1 · 2 marks · points
Why use a temperature-controlled water bath for respirometry?
- To make every organism the same mass
- To reduce gas-volume changes unrelated to respiration
- To stop oxygen uptake
Answer and reasoning · Réponse et raisonnement
To reduce gas-volume changes unrelated to respiration
- Anaerobic respiration releases less energy per glucose than aerobic respiration. In human muscles it produces lactic acid; in yeast it produces ethanol and carbon dioxide. Breathing supplies oxygen but is not itself respiration.
edexcel_igcse_biology:respiration:open:2 · 4 marks · points
0.60 cubic centimetres are used in 3.0 minutes by 2.0 g of tissue. Find the mass-specific rate.
Answer and reasoning · Réponse et raisonnement
0.1 cm³/min/g
- Appropriate relation; consistent substitution; correct result; meaningful unit and interpretation.
Breathing ventilates the lungs; respiration consists of chemical reactions in cells. A moving respirometer marker can also reflect temperature or pressure changes.
7 · Feedback and internal conditions
homeostasis · negative feedback
edexcel_igcse_biology:homeostasis:open:1 · 2 marks · points
Which response opposes a rise in blood glucose?
- Increased insulin action
- Increased release of glucose from glycogen
- Stopping all cellular respiration
Answer and reasoning · Réponse et raisonnement
Increased insulin action
- When blood glucose is high, insulin helps increase glucose uptake and storage as glycogen. When it is low, glucagon supports release of glucose from stores. These responses are coordinated, not identical effects of two hormones.
edexcel_igcse_biology:homeostasis:open:2 · 4 marks · points
A value rises from 4.0 to 5.0. Calculate the percentage increase.
Answer and reasoning · Réponse et raisonnement
25 %
- Appropriate relation; consistent substitution; correct result; meaningful unit and interpretation.
Negative feedback does not mean the response is harmful or the measured value becomes negative. Diabetes has different mechanisms; do not treat every case as a failure to make insulin.
8 · Inheritance, variation and probability
allele · genotype
edexcel_igcse_biology:inheritance:open:1 · 2 marks · points
What does a 25% probability in Aa × Aa mean?
- Exactly one of every four siblings must be aa
- Each fertilization has probability one quarter of aa under this model
- The fourth child must be aa
Answer and reasoning · Réponse et raisonnement
Each fertilization has probability one quarter of aa under this model
- In a simple monohybrid cross Aa × Aa, gametes carry A or a. Combining independent gametes gives AA, Aa, Aa and aa. The predicted probabilities describe many possible fertilizations, not a fixed order of children.
edexcel_igcse_biology:inheritance:open:2 · 4 marks · points
In Aa × aa, what percentage of offspring are predicted to be aa?
Answer and reasoning · Réponse et raisonnement
50 %
- Appropriate relation; consistent substitution; correct result; meaningful unit and interpretation.
A dominant allele can be rare. Mutation is a source of new variation; selection changes the relative success of existing variants rather than directing mutations toward a goal.
9 · Sampling populations without choosing the answer
quadrat · population
edexcel_igcse_biology:ecology:open:1 · 2 marks · points
Which method investigates change from a path into woodland?
- Choose only large plants
- Use a transect with fixed sampling distances
- Take one convenient sample
Answer and reasoning · Réponse et raisonnement
Use a transect with fixed sampling distances
- Estimate total abundance by multiplying mean density by area, with consistent units. This assumes sampled areas represent the habitat. Patchiness and too few samples widen uncertainty.
edexcel_igcse_biology:ecology:open:2 · 4 marks · points
Mean count is 6 in a 0.5 square metre quadrat. Find density per square metre.
Answer and reasoning · Réponse et raisonnement
12
- Appropriate relation; consistent substitution; correct result; meaningful unit and interpretation.
Quadrats suit organisms that do not move quickly. A food chain arrow shows the direction of energy transfer, not the direction a predator travels.
10 · Disease transmission and immune response
pathogen · antigen
edexcel_igcse_biology:infection:open:1 · 2 marks · points
What explains the spread of antibiotic resistance?
- Bacteria deliberately change all their genes
- Resistant variants survive and reproduce more successfully
- Antibiotics turn viruses into bacteria
Answer and reasoning · Réponse et raisonnement
Resistant variants survive and reproduce more successfully
- After vaccination, memory cells can support a faster secondary response. Antibiotic resistance arises through heritable variation and selection; an individual bacterium does not choose to become resistant because it needs to survive.
edexcel_igcse_biology:infection:open:2 · 4 marks · points
12 cases occur among 600 people. Find cases per 1,000 people.
Answer and reasoning · Réponse et raisonnement
20
- Appropriate relation; consistent substitution; correct result; meaningful unit and interpretation.
Antibiotics do not act on viruses in the same way they act on bacteria. A vaccine is not an immediate cure for an established infection.