What happens when chlorophyll absorbs light?

Electrons in the chlorophyll are excited, raised to a higher energy level, and leave the molecule.

How is ATP made in the light-dependent reactions?

Excited electrons pass along an electron transport chain, which releases energy to pump H⁺ into the thylakoid space, and H⁺ flows back through ATP synthase, making ATP from ADP and Pi.

What is photophosphorylation?

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

What is chemiosmosis?

The synthesis of ATP using the energy of protons flowing down a gradient through ATP synthase.

Which photosystems are used in non-cyclic photophosphorylation?

Both photosystem II and photosystem I.

What is the final electron acceptor in non-cyclic photophosphorylation?

NADP, which is reduced to form reduced NADP.

Write the equation for photolysis.

2H₂O → 4H⁺ + 4e⁻ + O₂.

Why is photolysis needed?

It replaces the electrons lost from PSII, and it releases the oxygen and provides the protons.

What are the products of cyclic photophosphorylation?

ATP only: no oxygen is released and no NADP is reduced.

Why do electrons return to PSI in the cyclic pathway?

The electrons pass along the electron transport chain and back to PSI, so they are used again.

Why is cyclic photophosphorylation useful?

It makes extra ATP, because the light-independent reactions need more ATP than reduced NADP, and it does not need water.

What do the ATP and reduced NADP do next?

They are used in the light-independent reactions to reduce carbon dioxide to glucose.

Light-dependent reactionsEdexcel International A Level Biology: Flashcards

Card 1 of 150 of 15 known

Question

Where do the light-dependent reactions take place?

Tap or press Space to reveal

Tap card or press Space to flip

See all 15 cards

Where do the light-dependent reactions take place?: On the thylakoid membranes of the chloroplast (in the grana).
What happens when chlorophyll absorbs light?: Electrons in the chlorophyll are excited, raised to a higher energy level, and leave the molecule.
How is ATP made in the light-dependent reactions?: Excited electrons pass along an electron transport chain, which releases energy to pump H⁺ into the thylakoid space, and H⁺ flows back through ATP synthase, making ATP from ADP and Pi.
What is photophosphorylation?: The formation of ATP from ADP and inorganic phosphate using light energy.
What is chemiosmosis?: The synthesis of ATP using the energy of protons flowing down a gradient through ATP synthase.
Which photosystems are used in non-cyclic photophosphorylation?: Both photosystem II and photosystem I.
What is the final electron acceptor in non-cyclic photophosphorylation?: NADP, which is reduced to form reduced NADP.
Write the equation for photolysis.: 2H₂O → 4H⁺ + 4e⁻ + O₂.
Why is photolysis needed?: It replaces the electrons lost from PSII, and it releases the oxygen and provides the protons.
Which photosystem is used in cyclic photophosphorylation?: Photosystem I only.
What are the products of cyclic photophosphorylation?: ATP only: no oxygen is released and no NADP is reduced.
Why do electrons return to PSI in the cyclic pathway?: The electrons pass along the electron transport chain and back to PSI, so they are used again.
Why is cyclic photophosphorylation useful?: It makes extra ATP, because the light-independent reactions need more ATP than reduced NADP, and it does not need water.
What are the products of non-cyclic photophosphorylation?: ATP, reduced NADP and oxygen.
What do the ATP and reduced NADP do next?: They are used in the light-independent reactions to reduce carbon dioxide to glucose.

Exam questions on Light-dependent reactions

The light-dependent reactions of photosynthesis take place in the thylakoid membranes of chloroplasts, where molecules of chlorophyll are arranged in groups called photosystems.
Describe how the energy of excited electrons is used to make ATP.2 marks
In non-cyclic photophosphorylation, electrons that leave chlorophyll in photosystem II are replaced by electrons that come from water. Isolated chloroplasts that are illuminated in a suspension release oxygen.
Explain why photosystem II needs electrons from water for non-cyclic photophosphorylation to continue.2 marks
DCMU is a chemical that blocks the movement of electrons from photosystem II to the electron transport chain. Far-red light, with a wavelength greater than 700 nm, is absorbed only by photosystem I. A researcher illuminated suspensions of isolated chloroplasts, with and without DCMU.
Explain why oxygen is no longer released when DCMU is added to chloroplasts in white light.3 marks

See the full worksheet

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).