Level:
Correct: 0
Incorrect: 0
Marks: 0/0
Exam Practice
Course:
Topics:
AQA A-Level Biology (Paper 2) Smart practice A mixed round aimed at your weak spots
Unit 5
Energy transfers in and between organisms
Photosynthesis
The light-dependent reaction
0%
The light-independent reaction
0%
Limiting factors and investigating photosynthesis
0%
Respiration
Glycolysis and anaerobic respiration
0%
Aerobic respiration
0%
Energy and ecosystems
Energy and ecosystems
0%
Nutrient cycles
Nutrient cycles
0%
Unit 6
Organisms respond to changes in their internal and external environments
Stimuli, both internal and external, are detected and lead to a response
Survival and response
0%
Receptors
0%
Control of heart rate
0%
Nervous coordination
Nerve impulses
0%
Synaptic transmission
0%
Skeletal muscles are stimulated to contract by nerves and act as effectors
Skeletal muscles and muscle contraction
0%
Homeostasis is the maintenance of a stable internal environment
Principles of homeostasis and negative feedback
0%
Control of blood glucose concentration
0%
Control of blood water potential
0%
Unit 7
Genetics, populations, evolution and ecosystems
Inheritance
Monohybrid, multiple allele and sex-linked crosses
0%
Dihybrid crosses, autosomal linkage and epistasis
0%
The chi-squared test in genetic crosses
0%
Populations
Populations and the Hardy-Weinberg principle
0%
Evolution may lead to speciation
Evolution may lead to speciation
0%
Populations in ecosystems
Populations in ecosystems and population size
0%
Succession and conservation
0%
Unit 8
The control of gene expression
Alteration of the sequence of bases in DNA can alter the structure of proteins
Gene mutations and their effects on proteins
0%
Gene expression is controlled by a number of features
Stem cells: most of a cell's DNA is not translated
0%
Regulation of transcription and translation
0%
Gene expression and cancer
0%
Using genome projects
Using genome projects
0%
Recombinant DNA technology
Producing and amplifying DNA fragments
0%
Vectors, transformation, marker genes and gene therapy
0%
Differences in DNA between individuals of the same species can be exploited
DNA probes, screening and genetic counselling
0%
Genetic fingerprinting
Genetic fingerprinting
0%
Unit 9
Comprehension
Comprehension questions
Comprehension: energy transfer and responses
0%
Comprehension: genetics, ecosystems and gene expression
0%
Avg
score:
–
Topics
done:
0/34

Comprehension: energy transfer and responses

Practise 18 exam-style questions on Comprehension: energy transfer and responses for AQA A-Level Biology (Paper 2). Write your answer as you would in the exam and get it marked in seconds against the mark scheme, point by point, with the Study Helper on hand if you get stuck.

  • 18 questions
  • 1–7 marks each
  • Instant marking
  • No sign-up needed
18 questions, marked as you go

Sample questions with model answers

Here are 4 of the questions in this topic, each with a model answer, an explanation of why it earns the marks and the mark scheme an examiner would use.

Question 1

2 marks

Read the following passage.

Some sea slugs feed on green algae in an unusual way. A slug pierces an algal cell and sucks out its contents. Most of the cell contents are digested, but the chloroplasts are taken up intact by the cells lining the slug's highly branched gut, which spreads out just beneath its thin, flat body surface. In one species, these stolen chloroplasts continue to absorb light and fix carbon dioxide for several months.

Scientists investigated whether the chloroplasts help the slugs to survive when food is scarce. They collected 40 slugs that had just fed and kept them without food in sea water at 15 °C. Twenty slugs had 12 hours of light each day and twenty were kept in continuous darkness. After 60 days, the mean mass of the slugs in the light had fallen from 24.0 mg to 19.2 mg. The mean mass of the slugs in the dark had fallen from 23.8 mg to 13.1 mg.

In an alga, most chloroplast proteins are coded for by genes in the nucleus of the algal cell. Chloroplast proteins are continually damaged, broken down and replaced. In the slug, the stolen chloroplasts gradually lose their ability to photosynthesise.

Use the information in the passage and your own knowledge to answer the following question.

Calculate the percentage decrease in the mean mass of the slugs kept in the dark (paragraph 2). Give your answer to 3 significant figures.

Model answer

Decrease in mean mass = 23.8 - 13.1 = 10.7 mg
Percentage decrease = 10.7 ÷ 23.8 × 100 = 44.96 = 45.0% (3 significant figures)

Explanation

Percentage change = change ÷ starting value × 100, so divide the decrease (10.7 mg) by the starting mass (23.8 mg), not by the final mass. To 3 significant figures the answer is 45.0: the final zero is significant, so 45 is not enough. The slugs kept in the light lost only 20.0% of their mass, which suggests that their stolen chloroplasts supplied some of the organic substances that they respired.

Mark scheme
  1. 1 mark

    Correct method: (23.8 - 13.1) ÷ 23.8 × 100; or a correct final answer of 45.0 (%) with no working

    A correct answer of 45.0% with no working scores 2 marks. Accept for 1 mark: 45 or 44.96 or 44.958 (not given to 3 significant figures) OR 0.450 (not multiplied by 100) OR evidence of 10.7 (the decrease in mass) with an incorrect final answer. Do not award this mark for 81.7 (the decrease divided by the final mass, 13.1) or for 20.0 (the percentage decrease of the slugs kept in the light).

  2. 1 mark

    45.0 (%)

    Do not accept 45 or 44.96 for this mark (3 significant figures are needed). Ignore a minus sign.

Question 2

2 marks

Read the following passage.

Malignant hyperthermia is a rare inherited condition that is usually discovered during an operation. In an affected person, some of the gases used as general anaesthetics cause calcium ions to leak continuously out of the sarcoplasmic reticulum of skeletal muscle fibres into the sarcoplasm.

Within minutes, the patient's muscles become stiff and rigid. The muscle cells use ATP and oxygen very rapidly. The concentration of carbon dioxide in the air the patient breathes out rises, and so does the concentration of lactate in the blood. Body temperature can rise by 1 °C every 5 minutes. Without treatment, the patient may die.

The anaesthetist must stop giving the anaesthetic gas immediately and inject a drug called dantrolene. Dantrolene binds to the calcium ion channel proteins in the membrane of the sarcoplasmic reticulum and reduces the release of calcium ions through them.

The condition is usually caused by a dominant allele of the gene that codes for these channel proteins. Before an operation, patients are asked whether any of their relatives has had a problem with an anaesthetic.

Use the information in the passage and your own knowledge to answer the following question.

Before an operation, patients are asked whether any of their relatives has had a problem with an anaesthetic (paragraph 4).

Explain why.

Model answer

The condition is caused by a dominant allele, so each child of an affected person has a 1 in 2 chance of inheriting it and being affected, and the allele is likely to be found in several members of a family. People with the allele have no symptoms until they are given one of the trigger gases, so a reaction in a relative may be the only sign that the patient could have the allele, and the anaesthetist can then avoid the trigger gases.

Explanation

A dominant allele is expressed in heterozygotes, so a parent who has it passes it to about half of their children. This allele affects the phenotype only in a particular environment, when the trigger gases are given, so people can have it for years without knowing. Asking about relatives is a simple way of identifying patients who may have the allele before they are exposed to the trigger.

Mark scheme
  1. 1 mark

    The condition is (usually) caused by a dominant allele, so a person with only one copy of the allele / a heterozygous person is affected

    Max 2. Reject 'gene' for allele.

  2. 1 mark

    Each child of a person with the allele has a 1 in 2 / 50% chance of inheriting it, so the allele is likely to be present in several members of a family

    Accept 'the allele is inherited / passed on within families'.

  3. 1 mark

    A person with the allele has no symptoms unless they are exposed to the anaesthetic gases, so they may not know that they have it / a relative's reaction may be the only sign that the allele is in the family

    Accept 'the phenotype depends on the environment (the anaesthetic) as well as the genotype'.

  4. 1 mark

    If a relative has reacted, the patient can be tested / given an anaesthetic that does not trigger the condition / dantrolene can be kept ready

Question 3

4 marks

Read the following passage.

Naked mole-rats are small rodents that live in large colonies in underground burrows in East Africa. The air in a crowded burrow can contain very little oxygen. In laboratory tests, naked mole-rats survived for 18 minutes in air containing no oxygen and then recovered completely. Mice die within a minute in the same conditions.

Without oxygen, the mole-rats became motionless and stopped breathing, and their heart rate fell from about 200 to 50 beats per minute. The concentration of fructose in their blood rose rapidly. Fructose is a hexose sugar with the same molecular formula as glucose.

In the first stage of glycolysis, glucose is phosphorylated twice, using two molecules of ATP, to form a hexose bisphosphate. The enzyme that adds the second phosphate group is inhibited when the pH of the cytoplasm falls. The brain and heart cells of naked mole-rats can take up fructose and convert it into triose phosphate by a different pathway that does not involve this enzyme.

Use the information in the passage and your own knowledge to answer the following question.

The enzyme that adds the second phosphate group is inhibited when the pH of the cytoplasm falls (paragraph 3).

Explain why the pH of the cytoplasm of brain cells falls when they receive no oxygen, and suggest how using fructose allows the brain cells of naked mole-rats to continue to make ATP.

Model answer

Without oxygen, the brain cells respire anaerobically: pyruvate is converted to lactate, which regenerates oxidised NAD so that glycolysis can continue. Lactate accumulates, increasing the concentration of hydrogen ions and lowering the pH of the cytoplasm, which inhibits the enzyme that adds the second phosphate group, so glucose cannot be broken down past this step. Fructose is converted into triose phosphate by a different pathway that does not use this enzyme, so triose phosphate is still produced. It is oxidised to pyruvate, giving a net gain of ATP by substrate-level phosphorylation.

Explanation

In anaerobic respiration, converting pyruvate to lactate regenerates NAD so that glycolysis can continue, but lactate accumulates and the pH falls. Each enzyme has an optimum pH: hydrogen ions alter the bonds that hold its tertiary structure. Feeding fructose into glycolysis after the pH-sensitive step lets the mole-rats' cells keep producing triose phosphate, pyruvate and ATP; answers that do not use the information about fructose cannot gain full marks.

Mark scheme
  1. 1 mark

    Without oxygen, the cells respire anaerobically / pyruvate is converted to lactate

    Max 2 if the answer does not mention fructose. Max 4. Accept 'lactic acid' for lactate. Reject 'ethanol is produced' in mammalian cells.

  2. 1 mark

    Lactate / lactic acid accumulates, increasing the concentration of hydrogen ions / lowering the pH

    Accept 'lactate lowers the pH'.

  3. 1 mark

    Converting pyruvate to lactate regenerates (oxidised) NAD, so the oxidation of triose phosphate (in glycolysis) can continue

    Accept 'NAD is reoxidised / reduced NAD is oxidised'.

  4. 1 mark

    Fructose is converted to triose phosphate without using the inhibited enzyme / bypasses the step inhibited by the low pH, so triose phosphate is still produced

  5. 1 mark

    Triose phosphate is oxidised to pyruvate, giving a net gain of ATP (by substrate-level phosphorylation)

    Accept 'the rest of glycolysis can continue and produce ATP'. Reject 'produces energy'.

Question 4

4 marks

Read the following passage.

Malignant hyperthermia is a rare inherited condition that is usually discovered during an operation. In an affected person, some of the gases used as general anaesthetics cause calcium ions to leak continuously out of the sarcoplasmic reticulum of skeletal muscle fibres into the sarcoplasm.

Within minutes, the patient's muscles become stiff and rigid. The muscle cells use ATP and oxygen very rapidly. The concentration of carbon dioxide in the air the patient breathes out rises, and so does the concentration of lactate in the blood. Body temperature can rise by 1 °C every 5 minutes. Without treatment, the patient may die.

The anaesthetist must stop giving the anaesthetic gas immediately and inject a drug called dantrolene. Dantrolene binds to the calcium ion channel proteins in the membrane of the sarcoplasmic reticulum and reduces the release of calcium ions through them.

The condition is usually caused by a dominant allele of the gene that codes for these channel proteins. Before an operation, patients are asked whether any of their relatives has had a problem with an anaesthetic.

Use the information in the passage and your own knowledge to answer the following question.

Explain why body temperature, the concentration of carbon dioxide in the exhaled air and the concentration of lactate in the blood all rise during an episode of malignant hyperthermia (paragraph 2).

Model answer

The calcium ions in the sarcoplasm keep the muscle fibres contracting, and every cycle of actinomyosin bridge formation hydrolyses ATP; ATP is also used to pump calcium ions back into the sarcoplasmic reticulum. To resynthesise this ATP, the rate of aerobic respiration increases, producing more carbon dioxide in the link reaction and the Krebs cycle; the carbon dioxide is carried in the blood and exhaled. Respiration and ATP hydrolysis are not 100% efficient, so much of the energy is released as heat and body temperature rises. Oxygen cannot be supplied fast enough, so the muscles also respire anaerobically, converting pyruvate to lactate, which passes into the blood.

Explanation

Muscle contraction is driven by ATP hydrolysis, so uncontrolled contraction increases respiration just as very intense exercise does. More respiration means more carbon dioxide and more heat, and when the oxygen supply cannot keep up, more lactate from anaerobic respiration. A common error is to write that respiration 'produces energy': energy is released, much of it as heat.

Mark scheme
  1. 1 mark

    Continuous muscle contraction / repeated formation and breaking of actinomyosin bridges uses (hydrolyses) ATP rapidly

    Max 3 if the answer does not refer to all three of body temperature, carbon dioxide and lactate. Max 4. Accept 'active transport of calcium ions back into the sarcoplasmic reticulum uses ATP'.

  2. 1 mark

    So the rate of (aerobic) respiration increases (to resynthesise ATP)

  3. 1 mark

    More carbon dioxide is produced in the link reaction / Krebs cycle (and it diffuses into the blood and is exhaled)

    Accept 'by decarboxylation'.

  4. 1 mark

    Respiration / ATP hydrolysis is not 100% efficient, so (much) energy is released as heat

    Accept 'produces heat'. Reject 'produces heat energy' or 'respiration produces energy'.

  5. 1 mark

    Oxygen cannot be supplied fast enough, so (more) anaerobic respiration occurs

  6. 1 mark

    Pyruvate is converted to lactate (using reduced NAD), and the lactate diffuses into the blood

Ready to try the rest? Every answer you write is marked against the mark scheme, so you can see exactly where the marks are.

Study Helper Hints first, answers after you try

Stuck on this one? Ask for a hint and I’ll point you in the right direction, without giving the answer away. Once you’ve checked your answer, I’ll show you how the marks are awarded.