Biology Practice Test: Photosynthesis
- Photosynthesis
- 10 questions
- 13 minutes
This test goes deeper into photosynthesis: the two stages, the conditions that limit its rate and how plants adapt. Tip: the light-dependent reactions make ATP, NADPH and oxygen; the Calvin cycle uses ATP and NADPH to build sugar from carbon dioxide.
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Question 1
In which part of the chloroplast does the Calvin cycle take place?
Correct answer: B
The Calvin cycle runs in the stroma, the fluid around the thylakoids. The thylakoid membranes are where the light-dependent reactions happen.
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Question 2
Which enzyme attaches carbon dioxide from the air to a five-carbon sugar in the first step of the Calvin cycle?
Correct answer: C
Rubisco fixes CO2 in the Calvin cycle. ATP synthase is also found in chloroplasts, but it makes ATP in the thylakoid membrane and does not fix carbon.
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Question 3
On a hot, dry afternoon, a plant closes its stomata to save water. What is the most direct effect on photosynthesis?
Correct answer: B
Stomata are the openings through which carbon dioxide enters the leaf. When they close, CO2 inside the leaf runs low and sugar production in the Calvin cycle slows.
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Question 4
A student measures the rate of photosynthesis of a plant at different light intensities. Temperature and CO2 level are kept constant.
Light intensity (units) Rate (bubbles per minute) 10 8 20 16 30 22 40 24 50 24 What is the best explanation for why the rate stops rising above 40 units?
Correct answer: C
When raising light no longer raises the rate, something else is holding the process back. With temperature and CO2 fixed, CO2 supply or enzyme speed is now the limit, not light.
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Question 5
In autumn, the leaves of many trees turn yellow and orange before they fall. What best explains these colors?
Correct answer: B
Carotenoids are accessory pigments present in the leaf all summer, but green chlorophyll hides them. When chlorophyll breaks down in autumn, the yellow and orange carotenoids become visible.
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Question 6
Which set lists all the products of the light-dependent reactions?
Correct answer: A
The light-dependent reactions turn light energy into ATP and NADPH and release oxygen from split water. Glucose is made later, in the Calvin cycle.
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Question 7
Why do chloroplasts split water molecules during the light-dependent reactions?
Correct answer: A
Light excites electrons in photosystem II, and they are passed down the electron transport chain. Splitting water refills those electrons, with oxygen released as a by-product. Water contains no carbon, so it cannot supply carbon for glucose.
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Question 8
A scientist gives a plant carbon dioxide made with radioactive carbon (14C) and lets it photosynthesize for a short time. Where will the radioactive carbon show up?
Correct answer: D
Carbon from CO2 is fixed in the Calvin cycle and ends up in sugars. The oxygen released comes from water, so it carries no carbon at all.
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Question 9
A cactus in the desert opens its stomata only at night. It stores the carbon dioxide it takes in as an acid and uses it for the Calvin cycle during the day. What is the main advantage of this pattern?
Correct answer: A
This is CAM photosynthesis. Nights are cooler and more humid, so opening stomata then saves a great deal of water, which matters most in a desert.
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Question 10
In a lab, small disks cut from a spinach leaf are soaked in a baking soda solution (a source of CO2) until they sink. Under a lamp, the disks float to the top one by one as oxygen builds up inside them. Which change would make the disks float sooner?
Correct answer: D
More light speeds up photosynthesis, so oxygen collects in the disks faster. Removing the baking soda takes away the CO2 source, darkness stops the light reactions, and boiling kills the cells.
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