Where does the oxygen released in photosynthesis come from?

From the splitting (photolysis) of water, not from carbon dioxide.

In the overall reaction, where is the hydrogen from water stored?

In glucose, the fuel made when it combines with carbon dioxide.

What did the ¹⁸O isotope experiments show?

Oxygen released comes from water, because ¹⁸O appeared in the oxygen only when the water was labelled.

What is the chloroplast envelope?

The double membrane surrounding the chloroplast, controlling what enters and leaves.

A flattened, membrane-bound sac in the chloroplast, with chlorophyll and electron carriers in its membrane.

The fluid around the grana containing the enzymes of the light-independent reactions, starch grains, DNA and ribosomes.

Why do the grana give a good surface for photosynthesis?

Their large surface area holds many photosystems, electron carriers and ATP synthase molecules.

Why do chloroplasts contain DNA and ribosomes?

They can make some of their own proteins.

What is the Hill reaction?

The light-dependent reaction in isolated chloroplasts, with an artificial electron acceptor.

What does DCPIP do in the Hill reaction practical?

It accepts electrons in place of NADP and changes from blue to colourless when reduced.

Why are chloroplasts isolated in an ice-cold, buffered, isotonic solution?

Isotonic stops osmosis damaging them, cold slows damaging enzymes and the buffer keeps the pH constant.

Why is a tube with boiled chloroplasts used in the Hill reaction practical?

As a control: boiling denatures the proteins, so there is no reduction of DCPIP.

Chloroplasts and the overall reaction of photosynthesisEdexcel A-Level Biology A: Flashcards

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What is the overall equation for photosynthesis?

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What is the overall equation for photosynthesis?: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in light, with chlorophyll).
Where does the oxygen released in photosynthesis come from?: From the splitting (photolysis) of water, not from carbon dioxide.
In the overall reaction, where is the hydrogen from water stored?: In glucose, the fuel made when it combines with carbon dioxide.
What did the ¹⁸O isotope experiments show?: Oxygen released comes from water, because ¹⁸O appeared in the oxygen only when the water was labelled.
What is the chloroplast envelope?: The double membrane surrounding the chloroplast, controlling what enters and leaves.
What is a thylakoid?: A flattened, membrane-bound sac in the chloroplast, with chlorophyll and electron carriers in its membrane.
What is a granum?: A stack of thylakoids.
What is the stroma?: The fluid around the grana containing the enzymes of the light-independent reactions, starch grains, DNA and ribosomes.
Why do the grana give a good surface for photosynthesis?: Their large surface area holds many photosystems, electron carriers and ATP synthase molecules.
Why do chloroplasts contain DNA and ribosomes?: They can make some of their own proteins.
What is the Hill reaction?: The light-dependent reaction in isolated chloroplasts, with an artificial electron acceptor.
What does DCPIP do in the Hill reaction practical?: It accepts electrons in place of NADP and changes from blue to colourless when reduced.
Why are chloroplasts isolated in an ice-cold, buffered, isotonic solution?: Isotonic stops osmosis damaging them, cold slows damaging enzymes and the buffer keeps the pH constant.
Why is a tube with boiled chloroplasts used in the Hill reaction practical?: As a control: boiling denatures the proteins, so there is no reduction of DCPIP.

Exam questions on Chloroplasts and the overall reaction of photosynthesis

A student examines an electron micrograph of a palisade mesophyll cell from a spinach leaf. Each chloroplast is about 5 µm long and is surrounded by two membranes. Inside, there are flattened membrane-bound sacs arranged in stacks, linked by thin membranes, in a surrounding fluid that contains starch grains and a small circular piece of DNA.
Explain how the stacking of the thylakoids into grana is related to the role of the chloroplast in the light-dependent reactions.2 marks
Two experiments were carried out with the alga Chlorella. In experiment 1 the water supplied contained the heavy isotope oxygen-18 (¹⁸O), and the carbon dioxide contained only oxygen-16 (¹⁶O). In experiment 2 the water contained only ¹⁶O and the carbon dioxide contained ¹⁸O. In each experiment the algae were illuminated and the oxygen gas that was released was analysed to find which isotope it contained.
Use the overall equation for photosynthesis to explain what these experiments show about the origin of the oxygen that is released and what happens to the hydrogen from water.2 marks
A biologist is preparing a talk on chloroplasts in the leaf cells of green plants. Chloroplasts are the site of photosynthesis, which uses light energy to make glucose from carbon dioxide and water.
Describe the structure of a chloroplast.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).