Practice questions · Topic 3.4

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

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 tells you what a wrong choice assumed. For the free-response questions, write your answer in full sentences and show any calculation. The first free-response question is broken into small steps, each with a sentence starter and a hint you can open if you need it; the second is at the level of the test. When you finish each one, open the scoring guide and mark your own work against it. Error bars in these questions are ±2SE.

Question 1

Green algae are sealed in a lit flask of pond water. Over an hour the dissolved carbon dioxide falls, the dissolved oxygen rises, the algae's sugar content rises, and the volume of water falls very slightly.

Which two changes are the reactants of photosynthesis being used up?

A. The fall in carbon dioxide and the rise in oxygen B. The fall in carbon dioxide and the rise in sugar C. The fall in carbon dioxide and the fall in water D. The fall in water and the rise in oxygen

Question 2

A pale yellow fungus and a green film grow side by side on a lit log. Each is scraped off and sealed in its own lit flask fitted with a carbon dioxide sensor. In the flask with the green film the carbon dioxide falls; in the flask with the fungus it rises, in the light and in the dark alike.

Which organism photosynthesizes, and what shows it?

A. The green film, because it takes carbon dioxide out of the air when it is lit B. The fungus, because it releases carbon dioxide, a product of photosynthesis C. Both, because any organism that grows in the light captures light energy D. The fungus, because it lives on the wood as a plant lives on soil, taking in its food

Question 3

A weedkiller stops the splitting of water at photosystem II; the photosystem still absorbs light normally, and the rest of the chloroplast is untouched. A leaf is sprayed and kept in bright light with plenty of carbon dioxide.

What happens in the leaf within a few minutes?

A. Oxygen release continues, because the oxygen photosynthesis gives off comes from carbon dioxide B. Photosystem II keeps sending electrons down the chain, because light itself supplies them C. The Calvin cycle runs at full speed, because it uses no light directly D. Electron flow from photosystem II stops, so oxygen release stops and ATP and NADPH production falls

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

Thylakoid membranes are taken from spinach chloroplasts and kept in the dark. They are soaked in a solution at pH 4 until the thylakoid space inside them is also at pH 4, then moved quickly, still in the dark, into a solution at pH 8 containing ADP and Pi. For a few seconds they make ATP.

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

A. Light powers ATP synthase directly, so some light must have reached the membranes as they were moved B. ATP forms when protons flow from the outside solution into the thylakoids through ATP synthase, up their gradient C. Protons flowing out of the thylakoid space, down their gradient, through ATP synthase drive the formation of ATP D. The acid is split as water is at photosystem II, and the electrons released supply the energy to make the ATP

Question 7

A bean plant in bright light is placed in a chamber of air from which all the carbon dioxide has been removed. In the first 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 has no carbon dioxide to fix into sugar C. The ATP and NADPH are spent building sugar from oxygen instead, a much slower process D. The ATP and NADPH leave the chloroplast to power the rest of the cell, so the stroma runs short of them

Question 8

A biologist compares the electron transport chain of the inner mitochondrial membrane with the electron transport chain of the thylakoid membrane. 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.

What 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 light-boosted chlorophyll) and where they end (oxygen, or NADP⁺)

Question 9

Grains of pyrite, an iron mineral that oxygen destroys, are common in river-laid sandstones older than 2.4 billion years and rare in younger ones. Fossil cyanobacteria are found in rocks 2.7 billion years old, and rocks with the chemistry of an oxygen-rich atmosphere like today's appear only in the last 0.6 billion years.

Which claim do these findings support?

A. Oxygen from cyanobacteria began to build up about 2.4 billion years ago and kept rising for over a billion years B. Oxygen reached today's level about 2.4 billion years ago, within a short time of the first cyanobacteria appearing C. Oxygen has been at today's level since the Earth formed, and the pyrite grains only mark when rivers first began to run D. Oxygen began to build up only about 0.6 billion years ago, when the first land plants began to photosynthesize

Question 10

ATP synthase from an archaean living in a salt lake, from a soil bacterium and from a mouse are compared. All three are built to the same plan and make ATP the same way, from protons flowing through them at the end of an electron transport chain.

Which claim does this pattern best support?

A. Each of the three groups invented oxidative phosphorylation separately, because it is the only way to make ATP B. The mouse acquired its ATP synthase from bacteria in its gut, and the archaean from bacteria in the lake C. The three domains inherited oxidative phosphorylation from a common ancestor that already had it D. Archaea and bacteria belong to one domain and mice to a second, so the match covers only two domains

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 a fifth 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 light reactions run and the fluid in which the 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 run in the thylakoid membranes, the flattened sacs stacked as grana, which hold the chlorophyll. The Calvin cycle runs in the stroma, the fluid inside the inner membrane in which the thylakoids sit.

Check the box for each point your answer earns

the light reactions run in the thylakoid membranes (stacked as grana) and the Calvin cycle runs 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 a photosystem 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: Absorbed light energy boosts an electron in chlorophyll to a higher energy level. The energized electron leaves the chlorophyll and is passed to a neighboring molecule: from photosystem II into the electron transport chain, and from photosystem I 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 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, 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.

Write down the values in the question:

normal strain: mean 42.0 mg, SE 1.5 mg
weakened strain: mean 18.0 mg, SE 1.2 mg

Write down the equation:

lower end = mean − 2SE
upper end = mean + 2SE

Substitute in the values, and calculate:

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: Photosystem I normally hands light-boosted electrons to NADP⁺, reducing it to NADPH, which carries those electrons into the stroma. The Calvin cycle needs NADPH, with ATP, to build carbon dioxide into sugar. In the weakened strain photosystem I passes electrons to NADP⁺ at a fifth of the normal rate, so far less NADPH reaches the Calvin cycle, less carbon dioxide is fixed, and the alga builds less sugar and less new mass, however normally its chlorophyll absorbs light.

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 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 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: Little or no change: the weakened strain's dry mass stays near 18 mg. Its Calvin cycle is starved of NADPH, because photosystem I is slow, and carbon dioxide was already plentiful. Adding more of an input that was never in short supply cannot speed the cycle up; sugar-building is held to the rate at which NADPH arrives.

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 mat of cyanobacteria from a hot spring is sealed 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 the bottle is given 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) Justify the claim that the mat is photosynthesizing, using two pieces of evidence from the data. (1 point)

A full-credit answer: Two lines of evidence show it. First, dissolved oxygen rises in the light and falls in the dark: oxygen is released only when light is captured, as photosynthesis predicts. Second, carbon from the supplied carbon dioxide (¹³C) turns up in the cells' sugar within minutes, so the cells are building sugar from carbon dioxide. The herbicide adds a third: blocking the light reactions stops both at once.

Check the box for each point your answer earns

two of: oxygen rises in the light and falls in the dark; carbon from the supplied carbon dioxide (¹³C) appears in the cells' sugar; blocking electron transfer between the photosystems stops both together, tying them to the light reactions; each linked to what photosynthesis does (it captures light energy to build sugar from carbon dioxide, releasing oxygen).

Accept the light-versus-dark oxygen contrast as one piece of evidence. Do not award the point for a single piece of evidence, or for "it is green".

Common slip: Giving one piece of evidence, or the color of the mat. Two observations from the data, each tied to what photosynthesis does, earn the point.

(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 it respires just as fast, so the rise of 1.8 mg/L is what photosynthesis made minus the 0.4 mg/L respiration used. Photosynthesis therefore produced 2.2 mg/L of oxygen in the hour.

Write down the values in the question:

dark hour: oxygen falls by 0.4 mg/L (respiration alone)
light hour: oxygen rises by 1.8 mg/L (photosynthesis minus respiration)

Write down the equation:

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

Substitute in the values, and calculate:

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 keeps running 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 chain. With transfer between them blocked, photosystem II can no longer pass electrons on, so it stops splitting water for replacements and oxygen release stops; no electrons reach photosystem I, so NADP⁺ is no longer reduced to NADPH; and with no electron flow no protons are pumped, so ATP production falls too. The Calvin cycle runs on 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 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: A complex machine, two photosystems linked by a chain to ATP synthase, built the same way in a free-living cyanobacterium and inside a plant's chloroplast, is most simply explained by inheritance rather than by two separate inventions. The chloroplast descends from a cyanobacterium that was taken into a eukaryotic cell and kept, by endosymbiosis; 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 points to a shared origin, the chloroplast being a descendant of a cyanobacterium.

Free-response score: 0 of 4

Multiple choice checked: 0 of 10 correct.

Take the end-of-topic test