Photosynthesis has a stage that needs light and a stage that uses the products of that light. The second stage does not need darkness. It needs what the first stage made.
The idea
In the light stage, chlorophyll absorbs light energy. That energy splits water. Oxygen from the water is released as a gas. Hydrogen is held by a carrier, and ATP is made. These events happen in the chloroplast, in the membranes of the grana. If light stops, this stage stops, and oxygen is no longer given off.
The dark stage uses the hydrogen and the ATP to reduce carbon dioxide to carbohydrate. It happens in the stroma of the chloroplast. It can run in the light. The old name "dark stage" means it does not use light directly, not that it waits for night. A leaf is built for both stages: a broad thin blade, palisade cells packed with chloroplasts, stomata for gas exchange, air spaces, and veins that deliver water and take sugar away. The rate is limited by whichever of light, carbon dioxide and temperature is in short supply.
What earns the marks
Do not write that the dark stage happens only at night. That is the error the question is often set to catch. Oxygen comes from water, not from carbon dioxide. Say so if the question mentions the source of the gas. Name the part of the chloroplast if you are asked where. Grana or thylakoid membranes for the light stage, stroma for the dark stage. A leaf adaptation needs the feature and the benefit. "It has stomata" alone is half a point.
Practice questions
These are practice questions in the style of the paper, with a model answer and a mark scheme. They are not a promise of what your paper will ask.
1Question 1
4 marks
(a) State two products of the light stage that the dark stage uses. (2 marks)
(b) Explain why the dark stage can continue for a short time after the light is switched off. (2 marks)
Model answer
(a) ATP and hydrogen on a carrier.
(b) A small supply of those products is already in the chloroplast, so carbon dioxide can still be reduced until the supply runs out.
Mark scheme
ATP, or energy from the light stage1 mark
Hydrogen, or reduced hydrogen carrier. NADPH is accepted1 mark
The products are already present in the chloroplast1 mark
They are used up, so the dark stage then stops1 mark
2Question 2
2 marks
State the source of the oxygen released in photosynthesis, and the stage that releases it. (2 marks)
Model answer
The oxygen comes from water, and it is released in the light stage when water is split.
Mark scheme
From water, not from carbon dioxide1 mark
Light stage, or photolysis of water1 mark
3Question 3
4 marks
Explain two ways the structure of a leaf suits it to photosynthesis. (4 marks)
Model answer
The palisade layer has many chloroplasts, so a large amount of light is absorbed near the upper surface. Stomata open to let carbon dioxide diffuse in, and air spaces inside the leaf give that gas a path to the cells.
Mark scheme
One feature, such as palisade cells, many chloroplasts, stomata, air spaces or veins1 mark
The matching benefit for light, gas exchange or transport1 mark
A second feature, different from the first1 mark
The matching benefit of the second feature1 mark
Practise Biology with Anna
More questions on this topic are inside GLearn, marked as you answer, with the reason written out. The first ones need no account.
Does the dark stage of photosynthesis need darkness?+
No. It does not use light directly, which is why it was called the dark stage. It needs ATP and hydrogen from the light stage, and it runs in the daytime. When the light goes off, it continues only until those products are used up.
Where does the oxygen released by a leaf come from?+
From water. In the light stage, water is split. The oxygen is given off. The hydrogen is used later to reduce carbon dioxide. The oxygen does not come from the carbon dioxide. All WASSCE Biology past questions