What is photoionisation?

The release of electrons from chlorophyll as it absorbs light energy.

What happens to the energy released as electrons pass along the electron transfer chain?

It is used to move protons across the thylakoid membrane, forming an electrochemical gradient.

What is chemiosmosis?

The formation of ATP as protons diffuse down their electrochemical gradient through ATP synthase.

What does ATP synthase catalyse?

The formation of ATP from ADP and inorganic phosphate.

The splitting of water by light energy into protons, electrons and oxygen.

What does NADP accept in the light-dependent reaction?

Electrons and a proton, forming reduced NADP.

What is the use of reduced NADP and ATP?

They are used in the light-independent reaction to make organic molecules.

What is DCPIP and what does it show in RP8?

A blue dye that is decolourised when reduced by electrons; the rate shows dehydrogenase activity.

Why are chloroplasts extracted in ice-cold, isotonic buffer?

To prevent damage by enzymes and by osmosis.

Why is a tube of boiled chloroplasts used as a control?

Boiling denatures enzymes, so any colour change in the other tubes is due to functioning chloroplasts.

The light-dependent reactionAQA A-Level Biology: Flashcards

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Where does the light-dependent reaction take place?

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Where does the light-dependent reaction take place?: In the thylakoid membranes of the chloroplasts.
What are the products of the light-dependent reaction?: ATP, reduced NADP and oxygen.
What is photoionisation?: The release of electrons from chlorophyll as it absorbs light energy.
What happens to the energy released as electrons pass along the electron transfer chain?: It is used to move protons across the thylakoid membrane, forming an electrochemical gradient.
What is chemiosmosis?: The formation of ATP as protons diffuse down their electrochemical gradient through ATP synthase.
What does ATP synthase catalyse?: The formation of ATP from ADP and inorganic phosphate.
What is photolysis?: The splitting of water by light energy into protons, electrons and oxygen.
Where do replacement electrons for chlorophyll come from?: The photolysis of water.
What does NADP accept in the light-dependent reaction?: Electrons and a proton, forming reduced NADP.
What is the use of reduced NADP and ATP?: They are used in the light-independent reaction to make organic molecules.
What is DCPIP and what does it show in RP8?: A blue dye that is decolourised when reduced by electrons; the rate shows dehydrogenase activity.
Why are chloroplasts extracted in ice-cold, isotonic buffer?: To prevent damage by enzymes and by osmosis.
Why is a tube of boiled chloroplasts used as a control?: Boiling denatures enzymes, so any colour change in the other tubes is due to functioning chloroplasts.

Exam questions on The light-dependent reaction

In the thylakoid membranes of a chloroplast, light energy absorbed by chlorophyll causes the chlorophyll to release electrons. The chlorophyll must then be supplied with replacement electrons so that it can go on absorbing light.
Explain why chlorophyll needs a supply of replacement electrons and state where they come from.2 marks
Isolated chloroplasts were illuminated in a solution that contained no carbon dioxide but did contain an artificial electron acceptor. Oxygen was released while the chloroplasts were in the light.
Explain why oxygen release would stop if the chloroplasts were placed in the dark.2 marks
A student extracted chloroplasts from leaves in ice-cold isotonic buffer. She added a sample to a solution of DCPIP, a blue dye that becomes colourless when it is reduced, and measured the absorbance of the mixture. In a tube in the light, the absorbance fell from 0.80 to 0.20 over 12 minutes. In a tube kept in the dark it fell from 0.80 to 0.78, and in a tube containing boiled chloroplasts in the light it fell from 0.80 to 0.79.
Calculate the rate of change of absorbance in the tube in the light. Give your answer in absorbance units per minute.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).