Practice questions · Topic 3.4

Unit 3 · Practice for the Topic 3.4 end-of-topic test

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Light energy is captured in the thylakoid membranes; sugar is built in the stroma from carbon dioxide and water; the oxygen given off comes from the water.

These are practice questions in the shape of the topic test. Work through them before you take the test; every question tells you what it wanted.

Answer every question. For each multiple-choice question, pick one option and press Check; the feedback gives the reasoning. For the free-response questions, write one short sentence per step of your reasoning, in order, and show any calculation. In the exam, write the steps as a paragraph: a bulleted list is not scored. The first free-response question walks you through the reasoning one part at a time, and you can open a hint for each part; the second is at the level of the test. When you finish a question, open the scoring guide and mark your own work against it. Error bars in these questions are ±2SE.

Question 1

A grower seals a greenhouse full of pepper plants and fits it with a carbon dioxide sensor. From dusk to dawn the greenhouse is dark.

Which of the following happens to the sensor's carbon dioxide reading between dusk and dawn, and why?

A. It rises, because in the dark the plants respire and do not photosynthesize B. It falls, because in the dark the plants keep taking in carbon dioxide for the Calvin cycle C. It stays level, because the plants stop respiring in the dark D. It stays level, because in the dark photosynthesis and respiration match each other

Question 2

A pale yellow fungus and a green film grow side by side on a lit log. A student seals a sample of each in its own lit flask with a carbon dioxide sensor. The green film's flask loses carbon dioxide in the light. The fungus's flask gains carbon dioxide in the light and in the dark alike.

Which of the two organisms photosynthesizes?

A. The green film B. The fungus C. Both of them D. Neither of them

Question 3

Under a lamp that gives out only light of 530 nm, a red tulip's petals look almost black.

Why do the petals look almost black?

A. The pigment absorbs red light, and the lamp gives out no red light for it to absorb B. Light of 530 nm carries too little energy to light up the petals C. The pigment absorbs green light and reflects only red, and the lamp gives out no red light D. Green light destroys the pigment, so the petals lose their color

Question 4

A potato tuber grows underground in the dark, yet its cells are packed with starch grains that stain blue-black with iodine.

Where did the sugar that built this starch come from?

A. The tuber's own cells made it, capturing the faint light that reaches down through the soil B. The leaves made it by photosynthesis, and the plant moved it to the tuber to store as starch C. The roots absorbed it from the soil, where starch from dead leaves had broken down into sugar D. The tuber's cells built it from minerals taken up from the soil, using energy from respiration

Question 5

Percentage of light absorbed at each wavelength by the main pigment of a diatom.

The graph shows the percentage of light absorbed at each wavelength by the main pigment of a diatom, a single-celled golden-brown alga.

Light of which wavelength will drive this diatom's photosynthesis fastest?

A. 450 nm B. 550 nm C. 650 nm D. 700 nm

Question 6

A researcher lights thylakoid membranes from spinach chloroplasts in a solution with ADP and Pi, and they make ATP. The researcher then adds a fungal toxin that plugs the channel of ATP synthase. Electrons keep passing along the electron transport chain, ATP production stops, and the thylakoid space becomes more acidic than before.

What does this result show about how the chloroplast makes ATP?

A. Light powers ATP synthase directly, and the toxin blocks the light from reaching it B. The electron transport chain makes the ATP as the electrons pass along it C. ATP synthase pumps the protons into the thylakoid space, so plugging its channel stops the pumping D. ATP is made only as protons flow out of the thylakoid space through ATP synthase, down their proton gradient

Question 7

A student places a bean plant in bright light in a chamber of air with all the carbon dioxide removed. Within minutes its sugar output falls, although its chlorophyll goes on absorbing light.

What happens to the ATP and NADPH in the chloroplasts' stroma, and why?

A. The ATP and NADPH stop being made, because the light reactions get their electrons from carbon dioxide B. The ATP and NADPH pile up unused, because the Calvin cycle that uses them has nothing to fix C. The ATP and NADPH are used up faster, because the Calvin cycle builds sugar from oxygen instead D. The concentration of ATP and NADPH in the stroma falls, because they leave the chloroplast to power the rest of the cell

Question 8

The electron transport chain of the inner mitochondrial membrane and the electron transport chain of the thylakoid membrane are alike in this: in both, electrons pass down a series of proteins, the energy released pumps protons across the membrane, and ATP synthase makes ATP as the protons flow back.

Which of the following differs between the two chains?

A. The mitochondrion uses ATP synthase, while the chloroplast makes its ATP directly from light energy B. The mitochondrion's chain takes its electrons from oxygen, while the chloroplast's chain takes them from carbon dioxide C. The mitochondrion's chain pumps protons into the matrix, while the chloroplast's chain pumps them into the stroma D. Where the electrons come from (food's NADH, or water via light-boosted chlorophyll) and where they end (oxygen, or NADP⁺)

Question 9

Fossil cyanobacteria are found in rocks 2.7 billion years old. The oldest fossils of algae are about 1 billion years old, and the oldest fossils of land plants about 0.5 billion years old. A chloroplast's photosystems, electron transport chain and ATP synthase are built to the same plan as a cyanobacterium's.

Which claim does this evidence best support?

A. Photosynthesis first evolved in land plants, which passed it to algae and then to cyanobacteria B. Photosynthesis evolved separately in cyanobacteria, algae and plants, arriving each time at the same machinery C. Photosynthesis first evolved in prokaryotes, and the photosynthesis of algae and plants was built on the prokaryotic pathways D. Algae evolved photosynthesis first, and cyanobacteria descend from algae that lost their nucleus

Question 10

A researcher collects four organisms: an archaean from the mud of a rice paddy, a cyanobacterium from a lake, a diatom (a single-celled alga) and a mushroom.

Which of the four organisms belong to the domain Eukarya?

A. The diatom only B. The diatom and the mushroom C. The diatom, the mushroom and the cyanobacterium D. The diatom, the mushroom, the cyanobacterium and the archaean

How to tackle the free-response questions. Read the verb first: describe asks what you see or know; explain asks why or how, so name the mechanism; predict asks what will happen and why; justify asks for the evidence that supports a claim. Each point is earned by one idea, stated in a sentence that names the thing and the mechanism. Extra words earn nothing; a wrong extra can lose the point. If there is a figure or table, use what it shows. When you finish, check the box for each point your answer earns and compare your sentences with the full-credit answer.

Free response 1 · Conceptual Analysis · 5 points

A single-celled green alga has a strain in which photosystem I is weakened: it holds normal chlorophyll, and its photosystem II and electron transport chain work normally, but its photosystem I passes electrons to NADP⁺ at 20% of the normal rate. Students grew the normal strain and the weakened strain side by side for five days in bright light with plenty of carbon dioxide, six flasks of each, and measured the dry mass of algae in each flask. The normal strain's mean was 42.0 mg (standard error 1.5 mg); the weakened strain's was 18.0 mg (SE 1.2 mg). The graph shows the two means with error bars of ±2SE.

Mean dry mass of algae per flask after five days (six flasks per strain). Error bars are ±2SE.

(a) Identify the membranes in which the weakened strain's light reactions take place and the fluid in which its Calvin cycle builds sugar. (1 point)

Hint: Which part of a chloroplast holds the chlorophyll, and what is the fluid around that part called?

A full-credit answer: The light reactions take place in the thylakoid membranes.
The thylakoids are flattened sacs stacked as grana, and they hold the chlorophyll.
The Calvin cycle takes place in the stroma.
The stroma is the fluid inside the inner membrane, in which the thylakoids sit.

Check the box for each point your answer earns

the light reactions take place in the thylakoid membranes (stacked as grana) and the Calvin cycle takes place in the stroma.

Accept "grana" for the membranes. Do not award the point for the two swapped, or for "the chloroplast" alone.

Common slip: Swapping the two rooms, or naming 'the chloroplast' for both. The light reactions are in the membranes. The sugar-building is in the fluid around them.

(b) Describe what happens to an electron in chlorophyll when one of the alga's photosystems absorbs light. (1 point)

Hint: What does the light's energy do to the electron's energy level, and where does the electron go next?

A full-credit answer: When light is absorbed, an electron in chlorophyll is boosted to a higher energy level, and it is then passed to a neighboring molecule.
The energized electron leaves the chlorophyll.
From photosystem II the electron enters the electron transport chain.
From photosystem I the electron goes on toward NADP⁺.

Check the box for each point your answer earns

the absorbed light energy boosts the electron to a higher energy level, and the energized electron is passed to a neighboring molecule (the first carrier of the electron transport chain from photosystem II, or toward NADP⁺ from photosystem I).

Accept "excited" for boosted. Do not award the point for "chlorophyll absorbs light" with nothing about the electron, or for the electron being created by the light.

Common slip: Stopping at 'the chlorophyll absorbs light'. The point is what happens to the electron: it is boosted, and it leaves.

(c) Calculate the ends of the ±2SE error bar on each strain's mean. (1 point)

Hint: Which formula gives the ends of a ±2SE bar from a mean and its standard error?

A full-credit answer: The normal strain's bar runs from 39.0 mg to 45.0 mg.
Two standard errors is 3.0 mg, and the bar reaches 3.0 mg either side of the mean of 42.0 mg.
The weakened strain's bar runs from 15.6 mg to 20.4 mg: 2.4 mg either side of 18.0 mg.
The two bars do not overlap.
There is a wide gap between them.

normal strain: mean 42.0 mg, SE 1.5 mg

weakened strain: mean 18.0 mg, SE 1.2 mg

lower end = mean − 2SE

upper end = mean + 2SE

normal: 42.0 − (2 × 1.5) = 39.0 mg   to   42.0 + (2 × 1.5) = 45.0 mg

weakened: 18.0 − (2 × 1.2) = 15.6 mg   to   18.0 + (2 × 1.2) = 20.4 mg

Check the box for each point your answer earns

normal strain 39.0–45.0 mg and weakened strain 15.6–20.4 mg (each mean minus and plus 2 × its own SE).

Accept 39–45 mg and 15.6–20.4 mg with the working shown. Do not award the point for ±1SE bars (40.5–43.5 mg and 16.8–19.2 mg), for one strain only, or for "42.0 ± 1.5 mg" written as the bar.

Common slip: Using ±1SE, or working out one strain's bar and stopping. Each bar is two standard errors either side of its own mean, and both strains are asked for.

(d) Explain why the weakened strain makes less sugar than the normal strain, although its chlorophyll absorbs light normally. (1 point)

Hint: Follow the electrons after photosystem I: where do they go, and which part of photosynthesis is waiting for what they make?

A full-credit answer: In the weakened strain photosystem I passes electrons to NADP⁺ at 20% of the normal rate, so far less NADPH is made.
NADPH carries electrons into the stroma.
The Calvin cycle needs NADPH, with ATP, to build carbon dioxide into sugar.
With far less NADPH, the Calvin cycle fixes less carbon dioxide.
So the alga builds less sugar and less new mass.
Its chlorophyll absorbs light normally, but the shortfall comes after the absorption.

Check the box for each point your answer earns

photosystem I reduces NADP⁺ to NADPH, which carries electrons and energy into the stroma for the Calvin cycle; with photosystem I slow, less NADPH is made, so the Calvin cycle fixes less carbon dioxide into sugar and the alga builds less new material.

Accept a note that ATP output also falls as the electron transport chain backs up. Do not award the point for "less light is absorbed" (it is absorbed normally), or for "less sugar" with no link through NADPH.

Common slip: Saying the weakened strain 'absorbs less light'. It absorbs light normally. The shortfall is downstream, in the NADPH that photosystem I fails to make.

(e) Predict what happens to the weakened strain's dry mass if the students double the carbon dioxide supplied, and justify your prediction. (1 point)

Hint: Which input is limiting the weakened strain's Calvin cycle? Compare it with the input the students are doubling.

A full-credit answer: Doubling the carbon dioxide would change the weakened strain’s dry mass little or not at all.
The Calvin cycle in the weakened strain is limited by its supply of NADPH, not by carbon dioxide.
Photosystem I is slow, so little NADPH reaches the stroma.
Carbon dioxide was already plentiful, so more of it cannot speed the Calvin cycle up.
So the dry mass stays near 18 mg.

Check the box for each point your answer earns

little or no change, because the weakened strain's Calvin cycle is limited by its supply of NADPH from photosystem I, not by carbon dioxide, which was already plentiful; more carbon dioxide cannot be fixed faster than NADPH arrives.

Accept "dry mass stays about 18 mg". Do not award the point for a large rise, or for "no change" with no reference to NADPH (or the light reactions) as the limit.

Common slip: Predicting a big rise because 'more carbon dioxide means more sugar'. That holds only when carbon dioxide is the input in short supply. Here NADPH is.

Free-response score: 0 of 5

Free response 2 · Scientific Investigation · 4 points

A researcher seals a mat of cyanobacteria from a hot spring in a clear bottle of spring water fitted with a dissolved-oxygen sensor. In one hour of darkness the dissolved oxygen falls by 0.4 mg/L. In one hour of bright light it rises by 1.8 mg/L. The mat respires at the same rate in light and in dark. When a researcher gives the bottle carbon dioxide made with the heavy carbon ¹³C, the cells' sugar becomes rich in ¹³C within minutes. A herbicide that blocks electron transfer between the two photosystems stops the rise in oxygen and stops the appearance of ¹³C in sugar together. The photosystems, electron transport chain and ATP synthase of the cyanobacteria match those of a spinach chloroplast, although the cells have no chloroplast.

(a) Support the claim that the mat is photosynthesizing with two pieces of evidence from the data. (1 point)

A full-credit answer: Dissolved oxygen rises in the light and falls in the dark.
Photosynthesis releases oxygen only while light is captured, so this pattern is what photosynthesis predicts.
Carbon from the supplied carbon dioxide, ¹³C, turns up in the cells' sugar within minutes.
So the cells are building sugar from carbon dioxide, which is what photosynthesis does.
Therefore the mat is photosynthesizing.

Check the box for each point your answer earns

two of (oxygen rises in the light and falls in the dark; ¹³C from the supplied carbon dioxide appears in the cells' sugar; blocking electron transfer between the photosystems stops both at once) AND the reasoning that links each to the claim (photosynthesis captures light energy to build sugar from carbon dioxide, releasing oxygen).

Support a claim needs the evidence and the reasoning that links it to the claim. Accept the light-versus-dark oxygen contrast as one piece. Do not award the point for one piece of evidence, for evidence with no link, or for "it is green".

Common slip: Giving one piece of evidence, or two pieces with no link to what photosynthesis does. Each observation supports the claim only when it is tied to it.

(b) Calculate the rate at which photosynthesis produces oxygen in the hour of light. (1 point)

A full-credit answer: The dark hour shows respiration alone: the mat uses 0.4 mg/L of oxygen an hour.
In the light the mat respires just as fast.
So the rise of 1.8 mg/L is what photosynthesis made minus the 0.4 mg/L that respiration used.
So photosynthesis produced 1.8 + 0.4 = 2.2 mg/L of oxygen in the hour.

dark hour: oxygen falls by 0.4 mg/L (respiration alone)

light hour: oxygen rises by 1.8 mg/L (photosynthesis minus respiration)

oxygen made by photosynthesis = net rise in the light + oxygen used by respiration

oxygen made by photosynthesis = 1.8 + 0.4

oxygen made by photosynthesis = 2.2 mg/L per hour

Check the box for each point your answer earns

1.8 + 0.4 = 2.2 mg/L per hour, because the mat respires in the light too, using 0.4 mg/L per hour, so the measured rise is photosynthesis minus respiration and the respiration must be added back.

Accept 2.2 mg/L per hour with the reasoning in words. An uncorrected 1.8 mg/L per hour (the net change) earns nothing, and neither does 1.4 mg/L per hour (respiration subtracted): the point is that the oxygen photosynthesis made equals the net rise plus the oxygen respiration used. Do not award the point for 2.2 mg/L per hour with no explanation of why the dark reading is added.

Common slip: Reporting 1.8, the net change, or subtracting to get 1.4. Respiration continues in the light and hides 0.4 mg/L of photosynthesis every hour. Add it back.

(c) Explain why the herbicide stops carbon fixation as well as oxygen release. (1 point)

A full-credit answer: The two photosystems are linked by the electron transport chain.
The herbicide blocks transfer between them.
So photosystem II can no longer pass electrons on.
So photosystem II stops splitting water for replacement electrons, and oxygen release stops.
No electrons reach photosystem I, so NADP⁺ is no longer reduced to NADPH.
No electrons flow, so no protons are pumped, and ATP production falls too.
The Calvin cycle uses that ATP and NADPH.
So carbon fixation stops with them.

Check the box for each point your answer earns

with electron transfer between the photosystems blocked, electrons no longer flow from photosystem II (so water is no longer split for replacements and oxygen release stops) and no longer reach photosystem I and NADP⁺, and proton pumping stops, so ATP and NADPH production falls and the Calvin cycle, which needs them, stops fixing carbon dioxide.

Accept "no ATP and NADPH for the Calvin cycle" with the electron block as the cause. Do not award the point for "the herbicide poisons the cells", or for the Calvin cycle needing light directly.

Common slip: Saying the herbicide 'stops photosynthesis' or 'poisons the cells'. The point follows the block through the electron transport chain to ATP and NADPH, and from there to the Calvin cycle.

(d) Explain how the match between the cyanobacteria's machinery and a chloroplast's supports the claim that chloroplasts descend from cyanobacteria. (1 point)

A full-credit answer: The cyanobacterium and the chloroplast share a complex machine: two photosystems linked by a chain to ATP synthase.
The machine is built the same way in a free-living cyanobacterium and inside a plant's chloroplast.
Inheritance explains that match more simply than two separate inventions.
So the chloroplast descends from a cyanobacterium.
A eukaryotic cell took that cyanobacterium in and kept it: endosymbiosis.
So photosynthesis first evolved in prokaryotes, and eukaryotic photosynthesis was built on those prokaryotic pathways.

Check the box for each point your answer earns

the same photosystems, chain and ATP synthase in a free-living prokaryote and in a chloroplast is most simply explained by inheritance: the chloroplast descends from a cyanobacterium taken into a eukaryotic cell (endosymbiosis), so photosynthesis first evolved in prokaryotes and eukaryotic photosynthesis was built on it.

Accept "endosymbiosis" with the shared machinery as the evidence. Do not award the point for "they look alike" with no inheritance reasoning, or for chloroplasts giving rise to cyanobacteria.

Common slip: Saying only that the two 'are similar'. The point needs the inference: shared complex machinery was inherited from a shared origin, so the chloroplast is a descendant of a cyanobacterium.

Free-response score: 0 of 4

Multiple choice checked: 0 of 10 correct.

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