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3.5.1 Photosynthesis

AQA A-level Biology 3.5.1 practice on Photosynthesis, with free MCQs, clues and worked explanations drawn from the Energy transfers in and between organisms section of specification 7402.

Specification route
3.5.1
Question bank
21 questions
Course stage
Year 13 / A-level only

Sample questions

QUESTION 1 · 3.5.1 · LEVEL 2

During the light-dependent reaction of photosynthesis, what process leads to the photoionisation of chlorophyll?

  • The absorption of light energy excites electrons in the chlorophyll molecule, causing them to leave the molecule.
  • The splitting of water molecules releases electrons that forcefully eject protons from the chlorophyll.
  • Light energy directly hydrolyses ATP, releasing phosphate ions that bind to the chlorophyll.
  • Carbon dioxide binds to the chlorophyll, altering its tertiary structure and releasing electrons.
Show clue

The prefix 'photo-' means light, and 'ionisation' refers to the loss or gain of electrons to form an ion.

Show answer and explanation

Answer: The absorption of light energy excites electrons in the chlorophyll molecule, causing them to leave the molecule.

Photoionisation occurs when chlorophyll absorbs light energy, which excites electrons to a higher energy level, causing them to leave the chlorophyll molecule.

QUESTION 2 · 3.5.1 · LEVEL 1

What are the direct products of the photolysis of water in the light-dependent reaction?

  • Protons ($H^{+}$), electrons ($e^{-}$), and oxygen ($O_{2}$)
  • Reduced NADP, ATP, and oxygen ($O_{2}$)
  • Hydrogen gas ($H_{2}$), electrons ($e^{-}$), and oxygen ($O_{2}$)
  • Protons ($H^{+}$), hydroxide ions ($OH^{-}$), and ATP
Show clue

Water ($H_{2}O$) is split. Think about the atomic components it breaks down into.

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Answer: Protons ($H^{+}$), electrons ($e^{-}$), and oxygen ($O_{2}$)

The photolysis of water uses light energy to split water molecules into protons ($H^{+}$), electrons ($e^{-}$), and oxygen ($O_{2}$). The electrons replace those lost by chlorophyll.

QUESTION 3 · 3.5.1 · LEVEL 3

In the light-dependent reaction, how is the proton gradient required for ATP synthesis generated?

  • Energy released as electrons move down the electron transfer chain is used to pump protons into the thylakoid space.
  • The breakdown of ATP releases energy that pumps protons across the chloroplast envelope.
  • Rubisco actively transports protons from the stroma into the thylakoid.
  • The photolysis of water in the stroma creates a high concentration of protons outside the thylakoid.
Show clue

The movement of electrons through carrier proteins provides the energy for this active transport.

Show answer and explanation

Answer: Energy released as electrons move down the electron transfer chain is used to pump protons into the thylakoid space.

As electrons move down the electron transfer chain, they lose energy. This energy is used to actively pump protons ($H^{+}$) from the stroma into the thylakoid space, establishing a concentration gradient for chemiosmosis.

QUESTION 4 · 3.5.1 · LEVEL 1

During the light-independent reaction (Calvin cycle), which enzyme catalyses the reaction between carbon dioxide and ribulose bisphosphate (RuBP)?

  • Rubisco
  • ATP synthase
  • NADP reductase
  • Amylase
Show clue

This is often considered the most abundant enzyme on Earth.

Show answer and explanation

Answer: Rubisco

Carbon dioxide reacts with ribulose bisphosphate (RuBP) to form two molecules of glycerate 3-phosphate (GP). This reaction is catalysed by the enzyme rubisco.

QUESTION 5 · 3.5.1 · LEVEL 3

What is the specific role of reduced NADP in the light-independent reaction?

  • It provides the reducing power (electrons and hydrogen) to reduce glycerate 3-phosphate (GP) into triose phosphate (TP).
  • It provides the energy to regenerate ribulose bisphosphate (RuBP) from triose phosphate (TP).
  • It acts as an enzyme to catalyse the fixation of carbon dioxide.
  • It absorbs light energy to drive the splitting of carbon dioxide molecules.
Show clue

GP is an acid, while TP is a sugar. Converting an acid to a sugar requires reduction.

Show answer and explanation

Answer: It provides the reducing power (electrons and hydrogen) to reduce glycerate 3-phosphate (GP) into triose phosphate (TP).

The reduction of GP to triose phosphate (TP) requires energy from ATP and reducing power (electrons/hydrogen) from reduced NADP.

QUESTION 6 · 3.5.1 · LEVEL 2

In the Calvin cycle, what happens to the majority of the triose phosphate (TP) molecules produced?

  • They are used to regenerate ribulose bisphosphate (RuBP) using ATP.
  • They are immediately converted into glucose and stored as starch.
  • They are broken down to release carbon dioxide and ATP.
  • They are transported to the mitochondria to undergo glycolysis.
Show clue

For the cycle to continue, the initial $5$-carbon acceptor molecule must be reformed.

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Answer: They are used to regenerate ribulose bisphosphate (RuBP) using ATP.

While some TP is converted into useful organic substances (like glucose), the majority ($5$ out of every $6$ molecules) is used to regenerate RuBP in the Calvin cycle, a process that requires ATP.

QUESTION 7 · 3.5.1 · LEVEL 2

A farmer wishes to maximize plant growth in a glasshouse. If carbon dioxide concentration and light intensity are high, what is the most likely limiting factor for the rate of photosynthesis?

  • Temperature
  • Oxygen concentration
  • The amount of chlorophyll in the leaves
  • The concentration of rubisco
Show clue

Photosynthesis is heavily reliant on enzyme-controlled reactions in the Calvin cycle.

Show answer and explanation

Answer: Temperature

The rate of photosynthesis can be limited by light intensity, carbon dioxide concentration, or temperature. If two of these are high and not limiting, the third (temperature) is the most likely limiting factor, affecting enzyme kinetics.

QUESTION 8 · 3.5.1 · LEVEL 3

In a laboratory experiment investigating the rate of the light-dependent reaction, what is the role of the indicator DCPIP?

  • It acts as an alternative electron acceptor to NADP, changing from blue to colourless as it is reduced.
  • It acts as an artificial source of carbon dioxide, changing colour as $CO_{2}$ is consumed.
  • It selectively breaks down the chloroplast envelope, releasing chlorophyll into the solution.
  • It acts as a competitive inhibitor of rubisco, halting the light-independent reaction.
Show clue

DCPIP picks up the electrons generated by the photolysis of water and the photoionisation of chlorophyll.

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Answer: It acts as an alternative electron acceptor to NADP, changing from blue to colourless as it is reduced.

DCPIP (or methylene blue) accepts electrons from the electron transfer chain instead of NADP. As it becomes reduced, it turns from blue to colourless, providing a measurable rate of the light-dependent reaction.

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