What is the source of the oxygen released in photosynthesis?

Water, which is split by light energy (photolysis).

The splitting of water by light energy, producing hydrogen and releasing oxygen.

What happens to the hydrogen from water in photosynthesis?

It is used to reduce carbon dioxide, so it is stored in glucose.

What evidence shows that oxygen comes from water?

When algae are given H₂¹⁸O the oxygen released contains ¹⁸O, but it does not when they are given C¹⁸O₂.

Adenine, the sugar ribose and three phosphate groups.

Write the equation for the hydrolysis of ATP.

ATP + H₂O → ADP + Pi + energy, catalysed by ATP hydrolase (ATPase).

What is the reverse reaction of ATP hydrolysis called?

A condensation (phosphorylation of ADP), catalysed by ATP synthase.

What is photophosphorylation?

The formation of ATP from ADP and inorganic phosphate using light energy.

Why is ATP a suitable immediate energy source?

Its hydrolysis releases a small, manageable amount of energy in a single-step reaction, and it can be quickly resynthesised.

Give two processes that use energy from ATP.

Active transport, muscle contraction, protein synthesis or cell division.

Why can a cell not store large amounts of ATP?

ATP is unstable and is recycled continually, so it is made as it is needed from glucose or by photophosphorylation.

Why is glucose not used directly as the immediate energy source?

Its energy is released in many steps in respiration, so it is first used to make ATP.

Overview of photosynthesis and ATPEdexcel International A Level Biology: Flashcards

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State the overall equation for photosynthesis.

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State the overall equation for photosynthesis.: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (using light energy).
What is the source of the oxygen released in photosynthesis?: Water, which is split by light energy (photolysis).
What is photolysis?: The splitting of water by light energy, producing hydrogen and releasing oxygen.
What happens to the hydrogen from water in photosynthesis?: It is used to reduce carbon dioxide, so it is stored in glucose.
What evidence shows that oxygen comes from water?: When algae are given H₂¹⁸O the oxygen released contains ¹⁸O, but it does not when they are given C¹⁸O₂.
What is ATP made of?: Adenine, the sugar ribose and three phosphate groups.
Write the equation for the hydrolysis of ATP.: ATP + H₂O → ADP + Pi + energy, catalysed by ATP hydrolase (ATPase).
What is the reverse reaction of ATP hydrolysis called?: A condensation (phosphorylation of ADP), catalysed by ATP synthase.
What is photophosphorylation?: The formation of ATP from ADP and inorganic phosphate using light energy.
Why is ATP a suitable immediate energy source?: Its hydrolysis releases a small, manageable amount of energy in a single-step reaction, and it can be quickly resynthesised.
Give two processes that use energy from ATP.: Active transport, muscle contraction, protein synthesis or cell division.
Why can a cell not store large amounts of ATP?: ATP is unstable and is recycled continually, so it is made as it is needed from glucose or by photophosphorylation.
How many moles of oxygen are released for each mole of glucose made?: Six.
Why is glucose not used directly as the immediate energy source?: Its energy is released in many steps in respiration, so it is first used to make ATP.

Exam questions on Overview of photosynthesis and ATP

Algae were supplied with water that contained the heavy isotope ¹⁸O (H₂¹⁸O) and with carbon dioxide that contained only the normal isotope ¹⁶O. The algae were illuminated and the oxygen gas that they released was collected and analysed.
The experiment was repeated with water containing only ¹⁶O and carbon dioxide containing ¹⁸O. Predict where the ¹⁸O would be found. Explain your answer.2 marks
A hummingbird hovers in front of a flower while it feeds. Its flight muscles use ATP at a very high rate, but each muscle cell contains only enough ATP for a few seconds of activity.
Explain how the hummingbird's muscles can keep contracting rapidly, although the amount of ATP in each cell is small.2 marks
Chloroplasts were isolated from leaf cells and placed in a suspension that contained ADP, inorganic phosphate and a compound that accepts hydrogen. When the suspension was illuminated, oxygen was released and ATP was formed. In darkness, neither oxygen nor ATP was produced.
Explain these observations.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).