Adenine, ribose and three phosphate groups.

Write the equation for the hydrolysis of ATP.

ATP + H₂O → ADP + Pi (catalysed by ATP hydrolase), releasing about 30.6 kJ mol⁻¹.

Why is ATP an immediate source of energy?

Hydrolysis is a single, quick reaction releasing energy in small amounts, but ATP is not stored so is resynthesised constantly.

What type of reaction makes ATP from ADP?

A condensation reaction (phosphorylation) needing energy, catalysed by ATP synthase.

Where does the energy for phosphorylation of ADP come from in photosynthesis?

Light, through excited electrons and the proton gradient (photophosphorylation).

Where do the light-dependent reactions take place?

In the thylakoid membranes of the chloroplast.

What happens when chlorophyll absorbs light?

Electrons are excited to a higher energy level and leave the chlorophyll (photoionisation).

What happens to the excited electrons?

They pass along an electron transport chain, releasing energy used to pump H⁺ into the thylakoid space.

How is ATP made in the light-dependent reactions?

H⁺ flow down the gradient through ATP synthase, and the energy is used to join ADP and Pi.

What is chemiosmosis?

Protons flowing down a concentration gradient through ATP synthase, releasing energy to make ATP.

What is the final electron acceptor of the light-dependent reactions?

NADP, which is reduced (with H⁺) to reduced NADP.

What is photolysis of water?

Splitting of water by light energy: 2H₂O → 4H⁺ + 4e⁻ + O₂.

What does photolysis provide?

Electrons to replace those lost from chlorophyll, protons, and oxygen as a by-product.

ATP and the light-dependent reactionsEdexcel A-Level Biology A: Flashcards

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What is ATP made of?: Adenine, ribose and three phosphate groups.
Write the equation for the hydrolysis of ATP.: ATP + H₂O → ADP + Pi (catalysed by ATP hydrolase), releasing about 30.6 kJ mol⁻¹.
Why is ATP an immediate source of energy?: Hydrolysis is a single, quick reaction releasing energy in small amounts, but ATP is not stored so is resynthesised constantly.
What type of reaction makes ATP from ADP?: A condensation reaction (phosphorylation) needing energy, catalysed by ATP synthase.
Where does the energy for phosphorylation of ADP come from in photosynthesis?: Light, through excited electrons and the proton gradient (photophosphorylation).
Where do the light-dependent reactions take place?: In the thylakoid membranes of the chloroplast.
What happens when chlorophyll absorbs light?: Electrons are excited to a higher energy level and leave the chlorophyll (photoionisation).
What happens to the excited electrons?: They pass along an electron transport chain, releasing energy used to pump H⁺ into the thylakoid space.
How is ATP made in the light-dependent reactions?: H⁺ flow down the gradient through ATP synthase, and the energy is used to join ADP and Pi.
What is chemiosmosis?: Protons flowing down a concentration gradient through ATP synthase, releasing energy to make ATP.
What is the final electron acceptor of the light-dependent reactions?: NADP, which is reduced (with H⁺) to reduced NADP.
What is photolysis of water?: Splitting of water by light energy: 2H₂O → 4H⁺ + 4e⁻ + O₂.
What does photolysis provide?: Electrons to replace those lost from chlorophyll, protons, and oxygen as a by-product.
What are the products of the light-dependent reactions?: ATP, reduced NADP and oxygen.

Exam questions on ATP and the light-dependent reactions

A resting adult holds only about 50 g of ATP in the whole body at any moment, yet turns over roughly their own body mass of ATP in a day. A student reading about this wants to understand how a cell can use so much ATP while storing so little.
Using the information given, explain why a cell must continually resynthesise ATP.2 marks
A student investigates the light-dependent reactions using isolated chloroplasts suspended in a cold, buffered solution. The suspension is mixed with DCPIP, a blue dye that is colourless when reduced and that can accept electrons in place of NADP. No carbon dioxide is supplied. One tube is placed in bright light and an identical tube is wrapped in foil.
Explain why the DCPIP in the illuminated tube changes from blue to colourless.2 marks
In 1966 the biochemist André Jagendorf soaked isolated chloroplasts in an acidic buffer at pH 4 until the space inside the thylakoids was also at about pH 4. He then moved them, in complete darkness, into a buffer at pH 8 that contained ADP and inorganic phosphate (Pi). The chloroplasts made ATP for a short time and then stopped.
Suggest how the chloroplasts made ATP in the dark in this experiment.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).