Unit 3 · Topic 3.4 end-of-topic test
Suggested time: about 51 minutes. Answer everything, then press Submit the test to see the feedback and scoring guides.
Photosynthesis is the reaction carbon dioxide + water → glucose + oxygen, driven by light energy. Its balanced equation is 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, with light energy written above the arrow.
What does light contribute to the reaction?
A student examines four samples: a beech leaf cell packed with chloroplasts, a cell from deep inside a turnip root, a cyanobacterium from a lake bloom, and a cell from a bread mold.
Which samples photosynthesize?
A gardener grew a young oak for six years in a tub of dried soil and gave it only water and light. At the start, the sapling's dry mass was 1.2 kg and the dried soil's mass was 60.00 kg. At the end, the tree's dry mass was 28.7 kg and the dried soil's mass was 59.95 kg.
Where did most of the tree's 27.5 kg of new dry mass come from?
A researcher grows a single-celled green alga in three lit flasks. Flask 1: water with heavy oxygen atoms (¹⁸O) and ordinary CO₂; the O₂ released is 91% ¹⁸O. Flask 2: ordinary water and CO₂ with ¹⁸O; the O₂ released is 0.4% ¹⁸O. Flask 3: nothing labeled; the O₂ released is 0.2% ¹⁸O.
Where does the released oxygen come from?
A student picks two leaves from the same plant on a sunny day: one at dawn, one at dusk. The student removes the chlorophyll from both and adds iodine. Iodine turns blue-black where starch is present. Only the dusk leaf turns blue-black.
What does the blue-black in the dusk leaf show?
A researcher gives a lit culture of a green alga a 5-second pulse of carbon dioxide made with labeled carbon, then measures where the labeled carbon is at three times. The table shows the results.
Which of the following does the timing of the labeled carbon show?
The figure shows a chloroplast in cross-section, inside a leaf cell. Four structures are numbered.
Which number marks a single thylakoid?
A researcher breaks isolated chloroplasts open and separates their parts.
In which part does the Calvin cycle build sugar from carbon dioxide?
A student lights three flasks of a green alga with red, blue or green light of equal brightness. In ten minutes the flasks release 26, 23 and 6 bubbles of oxygen.
Why does green light give so few bubbles?
The graph shows the percentage of light chlorophyll absorbs at each wavelength.
Light of which wavelength will drive photosynthesis fastest?
A researcher splits isolated chloroplasts between two tubes containing a yellow iron compound that turns colorless when it gains electrons. The researcher lights one tube and keeps the other dark. Only the compound in the lit tube turns colorless.
Why does the compound lose its color only in the light?
A herbicide blocks the transfer of electrons out of photosystem II. Within a minute of spraying, a treated leaf stops releasing oxygen, although its chlorophyll still absorbs light.
Why does oxygen release stop?
A researcher puts isolated thylakoid membranes in a solution at pH 7.0 and shines light on them. Within a minute the solution’s pH rises to 7.8.
What produces this rise in the solution’s pH?
A researcher gives isolated chloroplasts in the light NADP⁺, ADP and Pi.
What is NADP⁺'s job in the light reactions?
In the stroma of a lit chloroplast, the Calvin cycle attaches carbon dioxide to a molecule and builds a three-carbon sugar from it.
Which of the following supplies the energy and the electrons the Calvin cycle uses to build that sugar?
A researcher gives illuminated chloroplasts isolated from pea leaves a herbicide that blocks the transfer of electrons along the electron transport chain in the thylakoid membrane. The herbicide does not affect ATP synthase or the enzymes of the Calvin cycle. Two minutes later, carbon fixation is 8% of its rate before the herbicide.
Which of the following best explains why carbon fixation fell?
Researchers compare the genes in a moss's chloroplast with genes from many organisms. Their closest matches are in free-living cyanobacteria, not in the moss's own nucleus. The chloroplast's light reactions also use photosystems, an electron transport chain and an ATP synthase built to the cyanobacterial plan.
Which claim does this evidence best support?
Geologists take a drill core through river-laid rocks. The layers younger than about 2.4 billion years are stained red by rusted iron. The older layers beneath them are dark and hold iron minerals of a kind that survive only where oxygen is absent. Fossils generally taken to be cyanobacteria occur in rocks 2.7 billion years old, and oxygen-releasing photosynthesis is the only process known to make oxygen gas in such quantity.
Which claim do these findings best support?
An archaean from deep-sea mud, a bacterium from yogurt and a maize plant all split glucose by the same ten-step glycolysis, the steps in the same order, and all three make ATP with an ATP synthase built to the same plan.
Which claim does this pattern best support?
(a) Identify photosystem X, and explain why water is split at X rather than at Y. (2 points)
A full-credit answer: X is photosystem II.
Light absorbed by the chlorophyll in X boosts an electron.
The boosted electron leaves X and passes to the electron transport chain, so X must replace it.
Water is the source: water is split, its electrons go to X, its oxygen leaves as O₂, and its hydrogen ions go into the thylakoid space.
Y also loses its boosted electrons, toward NADP⁺, but Y's replacement electrons arrive from the electron transport chain, so Y needs no water.
Check the box for each point your answer earns
The two points are separate: "X is photosystem II" alone scores 1, and the explanation with no identification scores 1. Accept "X loses electrons to the electron transport chain and refills from water; Y refills from the electron transport chain" for the explanation point. Do not award the explanation point for oxygen coming from carbon dioxide.
Common slip: Naming X and stopping earns the identification point only, and "X absorbs light" is not the explanation. The explanation point needs the reason for the water: X loses electrons to the electron transport chain and refills from water, while Y refills from the electron transport chain.
(b) Explain how the movement of electrons from X to Y leads to ATP being made at Z. (1 point)
A full-credit answer: Electrons pass down the electron transport chain from X to Y.
The energy they release pumps protons from the stroma into the thylakoid space.
So protons pile up inside the thylakoid space: a proton gradient across the membrane.
The protons can return to the stroma only through ATP synthase, Z.
Protons flowing down their proton gradient through Z drive the formation of ATP from ADP and inorganic phosphate.
Check the box for each point your answer earns
Accept "chemiosmosis" for the proton flow through Z. Do not award the point for electrons passing through ATP synthase, or for the electron transport chain making ATP directly.
Common slip: Sending the electrons through ATP synthase, or having the electron transport chain make ATP. Electrons go to Y. Protons pass through Z, and their flow makes the ATP.
(c) Identify the region of the chloroplast where carbon dioxide is fixed into sugar, and explain why carbon fixation stops within seconds when the light is switched off. (1 point)
A full-credit answer: Carbon dioxide is fixed into sugar in the stroma, by the Calvin cycle.
The Calvin cycle uses no light itself.
But the Calvin cycle uses the ATP and NADPH that the light reactions supply.
In the dark no electrons are boosted.
So ATP and NADPH stop being made.
The stroma's small supply is used up within seconds.
So carbon fixation stops.
Check the box for each point your answer earns
Both halves are required for the point: "the stroma" alone scores 0, and the explanation with a location other than the stroma scores 0. Accept "the Calvin cycle needs the products of the light reactions" for the explanation. Do not award the point for "the Calvin cycle needs light" with no mention of ATP or NADPH.
Common slip: Writing 'the Calvin cycle needs light'. The cycle needs what light makes: ATP and NADPH from the thylakoid membranes. Name them.
(d) A herbicide stops electrons leaving Y. Predict what happens to the amount of oxygen the membrane releases, and justify your prediction using the model. (1 point)
A full-credit answer: Oxygen release falls to almost nothing.
Electrons leave Y only into the stroma.
With that exit blocked, Y fills with electrons and takes no more from the electron transport chain.
So the electron transport chain fills too, and takes no more electrons from X.
So X loses no electrons.
Water is split only to replace electrons X has lost.
So no water is split, and no oxygen is released.
Check the box for each point your answer earns
Both halves are required for the point: the prediction alone scores 0, and the reasoning with no prediction scores 0. Accept any wording of: the electrons back up along the chain, so photosystem II no longer needs electrons from water, so water is not split; for example "the electron's path is one connected chain, so a block at Y stops the flow all the way back to the water". Accept "oxygen release continues briefly, then stops" as the prediction. Do not award the point for "Y releases the oxygen", or for a fall in oxygen with no reasoning through the electron transport chain.
Common slip: Predicting that oxygen release continues because water is split at X, not at Y. X splits water only to replace electrons it has passed on; with the path blocked at Y, X passes none on.
(a) Calculate the rate at which photosynthesis removes carbon dioxide from the chamber in the hour of light, and explain your working. (1 point)
A full-credit answer: The dark hour shows respiration alone: the plant releases 5 ppm of carbon dioxide an hour.
In the light the plant respires just as fast.
So the fall of 11 ppm is what photosynthesis removed minus the 5 ppm that respiration put back.
So photosynthesis removed 11 + 5 = 16 ppm of carbon dioxide in the hour.
dark hour: CO₂ rises by 5 ppm (respiration alone)
light hour: CO₂ falls by 11 ppm (photosynthesis minus respiration)
CO₂ removed by photosynthesis = net fall in the light + CO₂ released by respiration
CO₂ removed by photosynthesis = 11 + 5
CO₂ removed by photosynthesis = 16 ppm per hour
Check the box for each point your answer earns
Accept 16 ppm per hour with the reasoning stated in words. Do not award the point for 11 (the net change) or 6 (respiration subtracted), or for 16 with no explanation of why the dark reading is added.
Common slip: Reporting 11, the net change, or subtracting to get 6. Respiration continues in the light and hides 5 ppm of photosynthesis every hour. Add it back.
(b) Explain how the labeled carbon demonstrates that the carbon in the plant's new sugar comes from the carbon dioxide in the chamber air. (1 point)
A full-credit answer: The researcher put the label only in the carbon dioxide.
The plant's enzymes treat ¹³C like ordinary carbon.
So the labeled carbon follows the path every carbon atom from carbon dioxide takes.
The label appeared in the new sugar.
So the carbon in the sugar came from the carbon dioxide.
Check the box for each point your answer earns
Accept "the labeled carbon could only have come from the carbon dioxide". Do not award the point for restating that plants use carbon dioxide with no use of the label, or for the carbon coming from the minerals or the water.
Common slip: Writing that plants take in carbon dioxide, with no use of the label. The point is the evidence: the label was only in the carbon dioxide, and it turned up in the sugar.
(c) Make a claim about the effect of the drought on the plant's sugar production. (1 point)
A full-credit answer: Sugar production falls during the drought.
Check the box for each point your answer earns
Make a claim earns the point for the assertion; the reasoning is scored in part (d). Accept "carbon fixation slows". Do not award the point for a claim with no direction word, or for sugar production rising or staying the same because the light is bright.
Common slip: Claiming that sugar output holds up because the plant is still in bright light. Light is only half of what photosynthesis needs; the carbon has to come in through the stomata.
(d) The light is as bright as before, and the leaves' chlorophyll absorbs it normally. Support your claim: identify which of the Calvin cycle's needs the almost-closed stomata change, and explain how that change alters the plant's sugar production. (1 point)
A full-credit answer: The Calvin cycle needs carbon dioxide, ATP and NADPH.
Carbon dioxide enters the leaf only through the stomata.
With the stomata almost closed, little carbon dioxide reaches the stroma.
Light and water still reach the leaf, so the light reactions still make ATP and NADPH.
So carbon dioxide is the one need the stomata change.
The Calvin cycle fixes little carbon, so it builds less sugar.
Check the box for each point your answer earns
Both halves are required for the point: naming carbon dioxide alone scores 0, and an explanation that never reaches sugar production scores 0. Accept a note that ATP and NADPH pile up unused, or that the stomata close to save water. Water for the light reactions reaches the leaf from the roots, not through the stomata, so an answer naming water alone as what the stomata cut off earns no point; water may be mentioned alongside carbon dioxide. Do not award the point for blaming the light reactions or the chlorophyll, which the task says are working.
Common slip: Naming water as what the stomata cut off. Water reaches the leaf from the roots. Carbon dioxide comes in through the stomata, so that is the need the drought cuts, and the Calvin cycle is the process that slows.