Unit 3 · Topic 3.4 end-of-topic test
The balanced equation for photosynthesis is 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂.
What does light contribute to the reaction?
Four samples are examined: a spinach leaf cell packed with chloroplasts, an onion bulb cell with no chloroplasts, a cyanobacterium from pond scum, and a cell from a mushroom.
Which samples photosynthesize?
A willow was planted in a tub of soil. Over five years its dry mass rose from 2.3 kg to 76.7 kg, while the soil's dry mass fell from 90.0 kg to 89.9 kg. The tree received only water and light.
Where did most of the tree's 74.4 kg of new mass come from?
Algae are grown in three lit flasks. Flask 1: water with heavy oxygen atoms (¹⁸O) and ordinary CO₂; the O₂ released is 88% ¹⁸O. Flask 2: ordinary water and CO₂ with ¹⁸O; the O₂ released is 0.3% ¹⁸O. Flask 3: nothing labeled; the O₂ released is 0.2% ¹⁸O.
Where does the released oxygen come from?
Half of a leaf on a living plant was covered with black paper for a day in bright light. The leaf was then picked, its chlorophyll removed, and iodine added. Iodine turns blue-black where starch is present. Only the uncovered half turned blue-black.
What does the blue-black half show?
A sealed jar holds a pondweed sprig and an oxygen probe. In one hour of darkness the oxygen in the jar falls by 3 units. In one hour of bright light it rises by 9 units. The plant respires at the same rate in light and in dark.
How much oxygen does photosynthesis produce in the hour of light?
The figure shows a chloroplast in cross-section with four structures numbered.
Which label marks a granum?
Isolated chloroplasts are broken open and their thylakoid membranes are separated from the stroma fluid.
Which fraction turns carbon dioxide into sugar, and which absorbs light?
Equal sprigs of pondweed are lit with red, blue or green light of equal brightness. In ten minutes they release 18, 17 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?
Isolated chloroplasts are split between two tubes containing a blue dye that turns colorless when it gains electrons. One tube is lit, the other kept dark. Only the dye in the lit tube turns colorless.
Why does the dye 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?
Probes in an illuminated chloroplast read pH 5 inside the thylakoid space and pH 8 in the stroma.
What produced these readings?
Isolated chloroplasts in the light are given NADP⁺ and build up NADPH. With no NADP⁺ available, the flow of electrons through the photosystems slows.
What is NADP⁺'s job in the light reactions?
Stroma fluid is separated from a chloroplast's membranes and kept in the dark. Given CO₂, ATP and NADPH, it fixes the CO₂ into three-carbon sugar; given CO₂ alone, it fixes nothing.
What does this show about the Calvin cycle?
Illuminated chloroplasts are given a herbicide that blocks electron transfer along the thylakoid chain but touches neither ATP synthase nor the enzymes of the Calvin cycle. Three minutes later the stroma holds 32% of its normal ATP and 29% of its normal NADPH, and carbon fixation is running at 11% of normal.
Why did carbon fixation fall?
A chloroplast's photosystems, electron transport chain and ATP synthase match a cyanobacterium's, and its DNA is closer to a cyanobacterium's than to the DNA in its own plant's nucleus.
Which claim does this evidence best support?
Rusted iron in ancient soils and river deposits first appears in rocks about 2.4 billion years old, and in no older rocks. Fossil cyanobacteria are found in older rocks than that. Oxygen-releasing photosynthesis is the only process known to make oxygen gas in such quantity.
Which claim do these findings best support?
Glycolysis splits glucose by the same sequence of ten reactions in gut bacteria, in yeast, in an oak tree and in a human. ATP synthase in all four works the same way.
Which claim does this pattern best support?
(a) Describe what happens at X when light strikes it. (1 point)
A full-credit answer: Light absorbed by the chlorophyll in photosystem II boosts an electron to a higher energy level, and that electron is passed to the first protein of the chain. Water is split to replace the electrons the photosystem has lost; its oxygen leaves as O₂ and its hydrogen ions are released into the thylakoid space.
Check the box for each point your answer earns
Accept "the energized electron leaves the photosystem" for the first half. Do not award the point for "X absorbs light" alone, or for oxygen coming from carbon dioxide.
Common slip: Saying only that X 'absorbs light' or 'makes oxygen'. The point needs the boosted electron leaving and water supplying its replacement.
(b) Explain how the movement of electrons from X to Y leads to ATP being made at Z. (1 point)
A full-credit answer: As the electrons pass down the chain from X to Y, the energy they release pumps protons from the stroma into the thylakoid space, so protons pile up inside. The only way back to the stroma is through ATP synthase (Z), and the protons flowing down their gradient through it drive the formation of ATP from ADP and inorganic phosphate: photophosphorylation.
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 chain making ATP directly.
Common slip: Sending the electrons through ATP synthase, or having the chain make ATP. Electrons go to Y; what passes through Z is protons, and their flow is what 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 cycle uses no light itself, but it spends the ATP and NADPH the light reactions supply. In the dark no electrons are boosted, so ATP and NADPH stop being made, the stroma's supply is used up within seconds, and carbon fixation stops.
Check the box for each point your answer earns
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, or for a location other than the stroma.
Common slip: Writing 'the Calvin cycle needs light'. It needs what light makes: ATP and NADPH from the thylakoid membranes. Name them.
(d) Explain how this model relates to the way a mitochondrion makes ATP. (1 point)
A full-credit answer: It is the same machine. In both, electrons pass down an electron transport chain, the energy they release pumps protons across a membrane into an enclosed space (the thylakoid space here, the intermembrane space in a mitochondrion), and the protons flowing back through ATP synthase make ATP. What differs is the source and the end of the electrons: light-boosted chlorophyll and NADP⁺ here, food and oxygen in the mitochondrion.
Check the box for each point your answer earns
Accept, in addition, the differences: the electrons come from light-boosted chlorophyll here and from food (NADH) in the mitochondrion, and end on NADP⁺ here and on oxygen there. Do not award the point for "both make ATP" or "both have membranes" with no chain, gradient or ATP synthase named.
Common slip: Saying only that both 'make ATP' or 'use membranes'. The point needs the shared mechanism: chain, proton gradient, ATP synthase.
(a) Identify where the atoms of the plant's new sugar come from, and what light contributes. (1 point)
A full-credit answer: The carbon in the sugar comes from the carbon dioxide the leaves take from the air, and the hydrogen from water taken up by the roots. Light supplies only energy: it drives the building of sugar from those two reactants and contributes no atoms.
Check the box for each point your answer earns
Accept "carbon dioxide and water" for the source of the atoms. Do not award the point for atoms coming from the soil or minerals, or for light being turned into matter.
Common slip: Naming the soil, the minerals or the light as the source of the plant's matter. Minerals are a small extra; the mass is carbon dioxide and water rearranged, with light paying for the work.
(b) 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 units of carbon dioxide an hour. In the light it respires just as fast, so the fall of 11 units is what photosynthesis removed minus the 5 respiration put back. Photosynthesis therefore removed 16 units of carbon dioxide in the hour.
Write down the values in the question:
dark hour: CO₂ rises by 5 units (respiration alone) light hour: CO₂ falls by 11 units (photosynthesis minus respiration)
Write down the equation:
CO₂ removed by photosynthesis = net fall in the light + CO₂ released by respiration
Substitute in the values, and calculate:
CO₂ removed by photosynthesis = 11 + 5 CO₂ removed by photosynthesis = 16 units per hour
Check the box for each point your answer earns
Accept 16 units 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 keeps running in the light and hides 5 units of photosynthesis every hour; add it back.
(c) Predict the effect of the drought on the plant's sugar production. (1 point)
A full-credit answer: Sugar production falls. With the stomata almost closed, far less carbon dioxide enters the leaves, so the Calvin cycle in the stroma has little carbon to fix and builds far less sugar, even in bright light.
Check the box for each point your answer earns
Accept "carbon fixation slows" for the prediction. Do not award the point for a prediction with no direction word, or for sugar production rising or staying the same because the light is bright.
Common slip: Predicting that sugar output holds up because the light is bright and the chlorophyll is working. Light is only half of what photosynthesis needs; the carbon has to come in through the stomata.
(d) Justify your prediction by explaining what the light reactions and the Calvin cycle each need, and which of those needs the almost-closed stomata cut off. (1 point)
A full-credit answer: The light reactions in the thylakoid membranes need light and water, and both are still there, so ATP and NADPH are still being made. The Calvin cycle in the stroma needs those two molecules and carbon dioxide, and carbon dioxide is what the stomata let in: with them almost closed, little carbon dioxide reaches the stroma. With little carbon to fix, the cycle slows, so less sugar is made and the plant gains less mass.
Check the box for each point your answer earns
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 stem says are working, or for an answer that never reaches sugar output.
Common slip: Naming water as what the stomata cut off. Water reaches the leaf from the roots; what comes in through the stomata is carbon dioxide, so that is the need the drought cuts, and it is the Calvin cycle that runs short.