Energy Transfers

Photosynthesis

Explanation of what occurs in photoionisation: (3 points)

Chlorophyll absorbs light energy.
This causes an electron to become excited and released from the chlorophyll molecule, making the chlorophyll positively charged.
The excited electron will then enter the top of the electron transport chain.

Sources

2019 A-Level Paper 2 Q7.2 (2 marks)

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Explanation of what occurs in photolysis: (4 points)

Light energy is used to split 2 water molecules.
1 molecule of oxygen is produced as waste.
4 protons are produced which enter the thylakoid lumen for chemiosmosis.
4 electrons are produced which replace the electrons released from the chlorophyll molecules in photoionisation.

Sources

Not from a mark scheme, but useful content to know.

Explanation of what occurs in chemiosmosis and photophosphorylation (ATP synthesis): (3 points)

The electrons released from chlorophyll molecules during photoionisation travel down the electron transport chain, releasing energy which is used to pump H+across the thylakoid membrane, from the stroma to the thylakoid lumen, generating an electrochemical gradient.
The H⁺ ions then move back down this gradient via facilitated diffusion through ATP synthase enzymes, releasing energy which allows the formation of ATP from ADP + Pi.
At the end of the electron transport chain, the electrons, alongside the H⁺ ions that have just passed through the ATP synthase enzyme, are used to reduce NADP into NADPH.

Sources

Not from a mark scheme, but useful content to know.

Why does greater production of ATP and NADPH in the light-dependent reaction cause more growth in plants? (4 points):

The greater amount of ATP and NADPH would cause more light-independent reactions to occur.
Producing more sugars that can be used in respiration.
Providing more energy for growth.
This causes more synthesis of new organic materialsNote: it is important to carry this explanation all the way through.

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Specimen A-Level Paper 2 Q8.3 (4 marks)

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How are the products of the light-dependent reaction used in the light-independent reaction? (2 points)

Both ATP and NADPH are needed for the reduction of GP into triose phosphate, so TP wouldn’t form without them, meaning glucose wouldn’t form.
The energy released from the hydrolysis of ATP is needed to regenerate RuBP from triose phosphate.

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2024 A-Level Paper 2 Q3.3 (3 marks)

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Describe the Light-independent Reaction: (6 points)

Carbon dioxide combines with RuBP.
To produce two GP molecules using the Rubisco enzyme.
GP is then reduced to triose phosphate (you HAD to give the full name triose phosphate for the mark)
Using reduced NADP.
The next point is that using energy from ATP (always specify it’s the energy from ATP needed)
Triose phosphate is then converted to RuBP (with the energy released from the hydrolysis of ATP) and an organic molecule, such as glucoseNotes: give the full name ‘Triose phosphate’ and always specify it is the 'energy from ATP' that is needed.

Sources

2022 A-Level Paper 2 Q1.2 (6 marks)

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2014 A-Level Unit 4 Q8(a) (6 marks)

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June 2011 A-Level Unit 4 Q8(c) (6 marks)

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5 limiting factors of the light independent reaction:

Temperature.
NADPH or ATP, which is caused by low light intensity which limits the rate of the light dependent reactionNote: remember that light wouldn’t be a limiting factor if there was still sufficient ATP and NADPH.
The rubisco enzymeNote: it may be denatured.
RuBP.
Carbon dioxide.

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2016 A-Level Unit 4 Q2(b)(ii) (2 marks)

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Why is it beneficial for slow-growing, shade-tolerant plants found in low light intensities to have poisonous leaves that deter herbivores from eating them? (3 points)

They have a slower rate of photosynthesis.
Their leaves would take a long time to replace.
Not having their leaves eaten would provide a large SA for photosynthesis, allowing them to absorb more light.Note: I’m trying to show that if a question applies something that would be beneficial to ALL plants (poison, long roots, large leaves etc.), you still need to make sure you explain why it’s ESPECIALLY beneficial to the specific type of plant in question. Structure your answers like ‘The poison is beneficial as…. This is important for this plant as it has a slower rate of photosynthesis so their leaves would take longer to replace.’

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2022 A-Level Paper 3 Q5.5 (2 marks)

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Why is there no CO₂ uptake into a plant when it is in the dark, even if the stomata are open? (2 points)

Carbon dioxide is not being used in photosynthesis.
This means there is no diffusion gradient for CO₂ into the leaf.Note: actually, there is a diffusion gradient for CO₂ out of the leaf due to the CO₂ produced in respiration.

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2014 A-Level Unit 4 Q3(c)(ii) (1 marks)

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Suggest how a rise in the rate of photosynthesis would cause a rise in the rate of respiration in soil under trees: (3 points)

Photosynthesis produces sugars.
These sugar move to roots (important point that makes the answer question-specific)
The sugars are then used for respiration.

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2014 A-Level Unit 5 Q9(f)(i) (2 marks)

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What is the biomass of an ecosystem measured in?

Mass of carbon per area per given time.

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2022 A-Level Paper 2 Q1.1 (2 marks)

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4 reasons why most of the light falling on producers is not used in photosynthesis:

The light is reflected.
The light is the wrong wavelength;
The light misses the chlorophyll.
CO₂ concentration or temperature is a limiting factor.

Sources

2020 A-Level Paper 2 Q03.3 (2 marks)

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RP 7 – Chromatography

Description of how the chromatography practical on a chlorophyll solution should be conducted: (4 points)

Draw a line on the paper using a pencil and ruler.
Use a pipette to add the chlorophyll solution to the line.
Add the solvent below the line.
Remove the paper from the glassware before the solvent reaches the end and mark the point the solvent reaches,.

Sources

2023 A-Level Paper 2 Q01.1 (4 marks)

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Why may you observe 3 different pigments in one solvent but only 2 different pigments (from the same sample) in a different solvent?

2 of the pigments may have the same solubility in the second solvent, but different solubilities in the first solvent.

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2023 A-Level Paper 2 Q01.2 (2 marks)

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2 reasons ink should not be used to draw the origin line in chromatography:

The ink would mix with the pigments and dissolve in the solvent.
The origin line would move to a different position.

Sources

2019 A-Level Paper 2 Q07.3 (1 marks)

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Why should the solvent be added below the origin line?

Adding it below the line prevents the pigments from dissolving directly into the sample.

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2019 A-Level Paper 2 Q7.4 (2 marks)

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In the chromatography practical, why should you not move the boiling tube once you’ve put the filter paper in it?

If you move the tube the pigments would not run straight and may run off the paper.

Sources

Not from a mark scheme, but useful content to know.

In the chromatography practical, why should you measure to the centre of the pigment spot? (4 points)

It standardises readings.
And allows fair comparisons to be made.
Since the pigment is spread out.
And the centre is the mean distance moved by the pigment.

Sources

Not from a mark scheme, but useful content to know.

RP 8 – Dehydrogenase in Chloroplasts (DCPIP)

What is one cause of temperature decrease in photosynthesis practicals?

Energy being taken in to excite electrons.

Sources

Not from a mark scheme, but useful content to know.

2 reasons to up a control containing just DCPIP in light-exposed water with no chloroplasts:

To show light does not affect DCPIP.
To show chloroplasts are required (for the colour change)Note: for ‘why this control’ questions, think ‘What does it show doesn’t have an effect?’ and ‘What does it show does have an effect?’

Sources

2017 A-Level Paper 2 Q04.2 (2 marks)

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Why does DCPIP change colour (from blue to colourless)? (2 points)

It gets reduced by electrons.
From chlorophyll in the light dependent reaction.

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2017 A-Level Paper 2 Q04.3 (2 marks)

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Respiration

Description of glycolysis: (4 points)

In the cytoplasm of the cell, one glucose molecule is phosphorylated to form glucose phosphate using the energy released from the hydrolysis of 2 ATP molecules.
This glucose phosphate molecule then splits into two Triose Phosphate molecules.Note: it does this because it is unstable.
These two TP molecules are then oxidised into two Pyruvate molecules, reducing 1 NAD and synthesising 2 ATP molecules per TP molecule that gets oxidised.
This causes a net gain of 2 ATP molecules, 2 NADH molecules and 2 Pyruvate molecules for the entire process.

Sources

2021 A-Level Paper 2 Q1.1 (4 marks)

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Description of the link reaction: (3 points)

Before the link reaction can occur, the pyruvate molecules are actively transported into the matrix of the mitochondria when oxygen is available for aerobic respiration.
In the link reaction, each pyruvate molecule is oxidised and decarboxylated to reduce 1 NAD molecule into NADH and form acetate and 1 CO₂.
The acetate then combines with Coenzyme A to form acetyl CoA.Note: For the two pyruvate molecules produced from one glucose molecule, the net gain is 2 Acetyl CoA molecules, 2 NADH molecules and 2 CO₂ molecules (waste).

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2025 A-Level Paper 2 Q1.2 (2 marks)

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2015 A-Level Unit 4 Q6(a) (2 marks)

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Description of the Krebs Cycle: (4 points)

First, the Coenzyme A releases acetate and returns to be used in the link reaction.Note: The Krebs Cycle occurs in the matrix of the mitochondria.
Then, the acetate combines with a 4C molecule to form a 6C molecule.
The 6C molecule is then oxidised to a 5C intermediate molecule, releasing 1 CO₂ and reducing 1 NAD.
The 5C molecule is then oxidised to a 4C molecule, releasing 1 CO₂, reducing 2 NAD and 1 FAD, and synthesising 1 ATP.Note: The products are 2 CO₂, 3 NADH, 1 FADH, and 1 ATP per acetate molecule.

Sources

2025 A-Level Paper 2 Q1.2 (2 marks)

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Description of oxidative phosphorylation: (6 points)

Oxidative phosphorylation occurs on the inner membrane of the mitochondria, as it involves channel proteins found in the inner membrane and involves the pumping and diffusion of H⁺ ions from the matrix to intermembrane space and in reverse. The large surface area of the folded cristae provides many locations for oxidative phosphorylation to occur.
First, the NADHs and FADHs formed during earlier stages are oxidised, releasing electrons and H⁺ ions.
The electrons enter the top of the electron transport chain and release energy as they pass between electron carriers and travel down it.
The energy released is used to actively transport protons across the inner membrane from the matrix to the intermembrane space.
The protons then move down a concentration gradient from the intermembrane space back into the matrix via facilitated diffusion through proton channels associated with ATP synthase enzymes. This provides the energy for ATP synthase to form ATP from ADP + Pi.
At the end of the chain, the electrons pass to oxygen, which acts as the final electron acceptor and combines with electrons and protons to form H₂O as a waste product.Note: Describing oxygen as the ‘final electron acceptor’ is key wording.

Sources

2025 A-Level Paper 3 Q1.3 (3 marks)

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June 2010 A-Level Unit 4 Q6(a) (3 marks)

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January 2012 A-Level Unit 4 Q8(b) (6 marks)

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What are 3 advantages of having spare ATP?

It can be used for growth, synthesis and replication.
It allows a lower rate of respiration to occur in cells.
It can be used for phosphorylation.

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2022 A-Level Paper 2 Q3.3 (2 marks)

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What occurs during anaerobic respiration and why is it therefore needed? (3 points)

Pyruvate produced from glycolysis is reduced into lactate.
This oxidises NADH to regenerate NAD so it can be reduced again in glycolysis.
This allows glycolysis to occur so more ATP can be formed, meaning cells can still produce ATP and use energy when oxygen is in short supply.

Sources

2019 A-Level Paper 2 Q4.3 (2 marks)

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2015 A-Level Unit 4 Q6(c)(i) (2 marks)

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2 reasons anaerobic respiration is ‘worse’ than aerobic:

It produces less ATP per glucose molecule.
It causes lactate to build up, which causes muscle fatigue.

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January 2010 A-Level Unit 4 Q5(d) (3 marks)

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2019 A-Level Paper 2 Q4.3 (2 marks)

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What happens when oxygen returns to tissue that has been respiring anaerobically and why is this beneficial? (2 points)

The lactate produced in anaerobic respiration is oxidised back into pyruvate.
This is beneficial as lactate is toxic and causes muscle fatigue.

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2015 A-Level Unit 4 Q6(c)(ii) (1 marks)

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Explanation of anaerobic respiration in plants: (2 points)

Pyruvate is decarboxylated to form ethanal, releasing 1 CO₂.Note: I don’t believe this is actually tested.
Ethanal is then reduced into ethanol, oxidising NADH to regenerate so it can be reduced again in glycolysis.

Sources

Not from a mark scheme, but useful content to know.

RP 9 – Respiration

During the respiration practical that investigates the rate of respiration of yeast in different temperature solutions, why should you wait 10 minutes before starting to measure the rate of respiration?

To allow the temperature of the solutions to equilibrate.Note: the solutions need to reach the temperature of the water bath they were placed in.
To allow for the rate of respiration of the yeast to stabilise.Note: if it were a practical where you measure the volume of gas produced, allowing time for the pressure changes that occur when temperature increases to happen is another reason for waiting 10 minutes.

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Specimen A-Level Paper 2 Q01.1 (3 marks)

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Energy and Ecosystems

What causes a high NPP relative to GPP and what is its advantage?

It is caused by low respiration.Note: Remember NPP = GPP - R (respiratory losses)

The advantage of it is that more energy is available for growth and biomass.Note: this is because NPP is the amount of energy actually available for growth.

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2021 A-Level Paper 3 Q4.1 (2 marks)

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3 reasons not all energy is transferred from secondary consumers to tertiary consumers:

Heat loss from respiration.Note: you had to say ‘heat’
Food isn’t all digested or eaten.
Excretion.Note: Say this not ‘faeces’

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2023 A-Level Paper 2 Q7.3 (3 marks)

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Why is farming cows less energy efficient than farming crops for human consumption? (2 points)

Energy is lost between trophic levels.Note: you could’ve said feeding levels, but other than that you had to say ‘trophic’ or describe it.
As energy is lost via respiration and excretion.

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2022 A-Level Paper 2 Q2.4 (2 marks)

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6 reasons for conserving rainforests:

Conserve species for biodiversity.
Conserve habitats.
Reduces climate change as the trees take up carbon dioxide.
Source of medicine and wood.
Reduces erosion and eutrophication.
Tourism.

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2023 A-Level Paper 2 Q7.2 (2 marks)

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Description of eutrophication: (6 points)

Leeching of minerals causes algae to grow/ algal bloom (use this terminology)
Light gets blocked out.
Plants can’t photosynthesise so die out.
Saprobionts breakdown the dead plants.
These bacteria respire aerobically and use up oxygen.
This means there is less oxygen available for fish to respire so they die.

Sources

2021 A-Level Paper 2 Q10.3 (4 marks)

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June 2013 A-Level Unit 4 Q8(b) (5 marks)

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June 2011 A-Level Unit 4 Q3(b)(i) (3 marks)

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Nutrient Cycles

Explain the role of saprobionts in the nitrogen cycle: (2 points)

They use enzymes to decompose proteins, DNA, RNA and urea
This releases ammonium ions.

Sources

2019 A-Level Paper 3 Q2.1 (2 marks)

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How do microorganisms break down dead organisms to produce nitrates? (2 points)

Saprobionts break down the nitrogen-containing remains, such as proteins, into ammonium ions.Note: Always name a nitrogen containing molecule - could also be DNA or amino acids, for example.Note: this process is ammonification.
Nitrifying bacteria then convert the ammonium ions to nitrites then nitrates.Note: it is important you mention nitrites ‘then’ nitrates.Note: this process is nitrification.

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2025 A-Level Paper 2 Q10.3 (4 marks)

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Which of the 4 processes in the nitrogen cycle are anaerobic and which are aerobic?

Nitrogen fixing - Anaerobic only

Ammonification - Both

Nitrification - Aerobic only

Denitrification - Mainly anaerobic.

Sources

2021 A-Level Paper 3 Q4.2 (2 marks)

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2015 A-Level Unit 4 Q2(c) (2 marks)

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Which of the 4 processes in the nitrogen cycle is the only one to require ATP?

Nitrogen fixing.Note: the rest produce ATP.

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2022 A-Level Paper 2 Q3.3 (2 marks)

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Why do anaerobic conditions create soil with a high concentration of ammonium ions and a low concentration of nitrite and nitrate ions? (4 points)

Nitrifying bacteria require oxygen, so in anaerobic conditions there will be fewer of them.
This causes less nitrification of ammonium ions into nitrites and nitrates.
Denitrifying bacteria do not require oxygen, so in anaerobic conditions there will be more of them.
This causes more denitrification of nitrites and nitrates to nitrogen gas.

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2021 A-Level Paper 3 Q4.2 (2 marks)

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3 advantages of natural fertilisers over artificial:

Less leaching.Note: artificial are water soluble so leach a lot.
Production of artificial fertilisers is energy-consuming.Note: references to cost were ignored.
They contain a wider range of elements - more than just NPK.

Sources

June 2011 A-Level Unit 4 Q3(b)(ii) (1 marks)