3.4 Photosynthesis

1. ASoil minerals and H₂O are combined, using light energy, into carbohydrate and O₂.
   A student who thinks a plant's food comes from the soil picks this. The carbon in carbohydrate comes from CO₂ taken in from the air; soil minerals are not a reactant in photosynthesis.
2. BLight energy is used to make carbohydrate and O₂ from CO₂ and H₂O. Correct
   Photosynthesis is the series of reactions that use CO₂, H₂O and light energy to make carbohydrates and O₂. The atoms of the products come from CO₂ and H₂O; light provides the energy.
3. CLight energy splits CO₂ into O₂, and its carbon joins with H₂O to form carbohydrate.
   A student who thinks the O₂ released comes from CO₂ picks this. The O₂ comes from water split in the light reactions; the carbon and oxygen of CO₂ go into carbohydrate.
4. DLight energy is turned into carbohydrate, and CO₂ and H₂O are used to make the O₂.
   A student who thinks light energy becomes the matter of sugar picks this. Light supplies energy, not atoms; the atoms of carbohydrate come from CO₂ and H₂O.

CED 3.4.A.1 · Read this in Fix

1. ARegion W Correct
   W is the stroma, the fluid inside the inner membrane and outside the thylakoids. The Calvin cycle reactions that convert CO₂ into carbohydrate take place there.
2. BRegion X
   A student who thinks every stage of photosynthesis happens where the chlorophyll is picks this. X is a granum, a stack of thylakoids, where the light reactions occur; CO₂ is fixed in the stroma.
3. CRegion Y
   A student who thinks the stroma is the fluid inside the thylakoids picks this. Y is the thylakoid space, a separate compartment; the stroma is outside the thylakoids.
4. DRegion Z
   A student who thinks the stroma lies between the chloroplast's two outer membranes picks this. Z is the space between the outer and inner membranes; the stroma is inside the inner membrane.

CED 3.4.A.2.i · Read this in Fix

1. AATP and NADPH from the light reactions to the Calvin cycle; no arrow going back to it
   A student who thinks ATP and NADPH are used up for good picks this. The Calvin cycle converts them to ADP, Pᵢ and NADP⁺, which return to the light reactions; without this return the light reactions would stop.
2. BATP and NADPH from the light reactions to the Calvin cycle; ADP, Pᵢ and NADP⁺ back Correct
   The light reactions yield ATP and NADPH, which power the production of carbohydrate in the Calvin cycle. Using them returns ADP, inorganic phosphate (Pᵢ) and NADP⁺ to the light reactions, where they are recharged.
3. CATP and NADPH from the Calvin cycle to the light reactions; ADP, Pᵢ and NADP⁺ back again
   A student who thinks the Calvin cycle makes ATP and NADPH picks this. The light reactions make ATP and NADPH, and the Calvin cycle uses them.
4. DAbsorbed light energy from the light reactions to the Calvin cycle; no arrow going back
   A student who thinks light energy passes straight into making sugar, with no energy carriers between the stages, picks this. Energy is carried from the light reactions to the Calvin cycle by ATP and NADPH, and ADP, Pᵢ and NADP⁺ return to the light reactions to be recharged.

CED 3.4.A.3 · Read this in Fix

1. AH₂O, which becomes O₂
   A student who thinks reduction means losing electrons picks this. Water loses electrons when it is split, which is oxidation; NADP⁺ gains them and is reduced.
2. BCO₂, which becomes sugar
   A student who thinks electrons from the light reactions go straight into sugar picks this. In the model the electron path ends at NADP⁺; CO₂ is reduced later, in the Calvin cycle, using NADPH.
3. CADP, which becomes ATP
   A student who thinks ATP is made by adding electrons to ADP picks this. No electron arrow reaches ATP synthase; ATP is made by adding phosphate to ADP, which is not a reduction.
4. DNADP⁺, which becomes NADPH Correct
   Electrons start in water, which loses them when it is split at photosystem II, so water is oxidized. They end on NADP⁺, which gains them after photosystem I and is reduced to NADPH.

CED 3.4.B.1 · Read this in Fix

1. AIt releases the energy that boosts electrons in photosystem II.
   A student who thinks breaking bonds releases energy picks this. The energy that boosts the electrons comes from light absorbed by chlorophyll; splitting water requires energy rather than releasing it.
2. BIt supplies the oxygen atoms that are built into carbohydrate.
   A student who thinks the O₂ released comes from CO₂, so that water's oxygen must go elsewhere, picks this. The oxygen atoms from split water are released as O₂.
3. CIt supplies electrons to replace those lost from photosystem II. Correct
   Light boosts electrons in photosystem II to a higher energy level, and they leave along the ETC. Splitting water supplies electrons to replace them, releasing O₂ and H⁺.
4. DIt gives electrons that reduce CO₂ directly to carbohydrate.
   A student who thinks light-driven electrons go straight into sugar picks this. Electrons from water pass through the photosystems and ETC to NADP⁺; NADPH then supplies electrons to the Calvin cycle.

CED 3.4.B.2 · Read this in Fix

1. AO₂ release is unaffected, but NADPH production decreases.
   A student who thinks a block affects only later steps picks this. Photosystem II can take more electrons from water only if it can pass its own on; with X blocking the path, water splitting and O₂ release slow as well.
2. BO₂ release decreases, while NADPH production is unaffected.
   A student who thinks photosystem I gets its own electrons from water picks this. The model shows photosystem I receiving electrons only through the ETC from photosystem II, so blocking X also cuts the supply for NADPH.
3. CO₂ release and NADPH production both remain unaffected.
   A student who thinks light is all the light reactions need picks this. Light is still absorbed, but with the electron path blocked at X, electrons cannot flow from water to NADP⁺.
4. DO₂ release decreases, and NADPH production decreases. Correct
   X lies on the electron path from photosystem II to the ETC. Electrons can no longer leave photosystem II, so it cannot take electrons from water and O₂ release falls; photosystem I receives no electrons from the ETC, so NADPH production falls too.

CED 3.4.B.3 · Read this in Fix

1. AH⁺ diffuses into the thylakoid space because the thylakoid needs H⁺ there to make ATP.
   A student who explains movement by need picks this. H⁺ is moved into the thylakoid space against its gradient using energy from electron transport; particles do not move toward where they are needed.
2. BATP synthase uses the energy of ATP to pump H⁺ into the thylakoid space.
   A student who thinks ATP synthase is a pump that builds the gradient picks this. ATP synthase lets H⁺ flow out of the thylakoid space and uses that flow to make ATP; the ETC builds the gradient.
3. CEnergy released by splitting water is used to move H⁺ from the stroma into the thylakoid.
   A student who thinks breaking bonds releases energy, so that splitting water powers the light reactions, picks this. Splitting water takes in energy supplied by light rather than releasing it; it adds H⁺ to the thylakoid space and supplies electrons, and the energy that moves H⁺ in from the stroma is released as electrons pass along the ETC.
4. DEnergy released as electrons pass along the ETC is used to move H⁺ into the thylakoid space. Correct
   As electrons are transferred through the ETC in a series of oxidation–reduction reactions, the energy released is used to move H⁺ from the stroma into the thylakoid space, establishing an electrochemical gradient that is high inside the thylakoid.

CED 3.4.B.4 · Read this in Fix

1. AIlluminate thylakoids with the chemical and without it, then compare the ATP formed. Correct
   The two groups differ only in whether the H⁺ channel of ATP synthase is blocked, so a difference in ATP formation can be attributed to H⁺ flow through ATP synthase. Electron transport, and so the H⁺ gradient, is the same in both.
2. BIlluminate thylakoids with the chemical, and compare them with a tube of no thylakoids.
   A student who thinks the control is always the tube with nothing in it picks this. Neither tube has thylakoids with an open ATP synthase, so the comparison cannot show whether blocking the channel matters.
3. CIlluminate thylakoids with the chemical, and compare with untreated ones in the dark.
   A student who thinks several variables can change at once picks this. The groups differ in both the chemical and the light, so a difference in ATP could be caused by either.
4. DIlluminate thylakoids with the chemical, and measure the ATP; no comparison is needed.
   A student who thinks a treated sample can be judged alone picks this. Without untreated, illuminated thylakoids for comparison, the amount of ATP formed cannot show whether the chemical reduced it.

CED 3.4.B.5 · Read this in Fix

1. A38 ppm/min
   A student who divides the final value by the time picks this: 304 ÷ 8 = 38. The rate uses the change in concentration, 400 − 304 = 96 ppm, divided by 8 min.
2. B24 ppm/min
   A student who counts each interval between readings as 1 minute picks this: 96 ÷ 4 = 24. The readings are 2 minutes apart, so the period is 8 minutes and the rate is 96 ÷ 8 = 12 ppm/min.
3. C12 ppm/min Correct
   The CO₂ concentration fell from 400 to 304 ppm, a change of 96 ppm, over 8 minutes: 96 ÷ 8 = 12 ppm/min. The leaf takes up CO₂ because the Calvin cycle, powered by ATP and NADPH from the light reactions, fixes it into carbohydrate.
4. D96 ppm/min
   A student who takes the total change as the rate picks this. The concentration fell by 96 ppm over 8 minutes, so the rate is 96 ÷ 8 = 12 ppm/min.

Working Change in CO₂ concentration = 400 − 304 = 96 ppm over 8 − 0 = 8 min. Mean rate of net uptake = 96 ppm ÷ 8 min = 12 ppm/min. Distractors: dividing the final concentration by the time, 304 ÷ 8 = 38 ppm/min; dividing by the 4 intervals between readings as if each were 1 min, 96 ÷ 4 = 24 ppm/min; the total change, 96, given as the rate.

CED 3.4.B.6 · Read this in Fix

1. AMost of the dry mass the plants gained came from minerals taken up from the soil.
   A student who thinks a plant's mass comes from the soil picks this. The soil lost only 0.1 g in each group, far less than the 3.2 g gained, and plants with soil but no CO₂ gained no mass.
2. BMost of the dry mass the plants gained came from water taken up by the roots.
   A student who thinks plant mass comes mainly from water picks this. Both groups had the same water supply, but only the group given CO₂ gained dry mass; water supplies hydrogen atoms, but most of the dry mass is carbon and oxygen from CO₂.
3. CMost of the dry mass the plants gained came from light turned into matter.
   A student who thinks light energy becomes the matter of a plant picks this. Both groups had the same light, but only the group given CO₂ gained mass; light supplies energy, and the atoms of the new mass come from CO₂ and water.
4. DMost of the dry mass the plants gained came from the CO₂ taken in from the air. Correct
   Only the plants given CO₂ gained dry mass (+3.2 g); plants with the same light, soil and water but no CO₂ lost mass, and the soil lost only 0.1 g in each group. The gain depended on CO₂, whose carbon and oxygen atoms are built into carbohydrate in photosynthesis.

CED 3.4.A.1 · Read this in Fix

1. ASugars made by photosynthesis in the leaves are moved to the roots, then used or stored. Correct
   Photosynthetic organisms produce sugars that can be used in biological processes or stored. Sugars made in the leaves are transported to the roots, where they supply energy and carbon for growth, and the excess is stored.
2. BSugars and other foods are taken up from the soil by the roots, independently of the leaves.
   A student who thinks a plant's food comes from the soil picks this. Roots take up water and minerals, but the sugar they use and store is made by photosynthesis in the leaves.
3. CRoot cells carry out photosynthesis themselves, using the small amount of light that reaches them.
   A student who thinks every plant cell photosynthesizes picks this. Light does not reach far into soil, and the cells of underground roots such as these do not photosynthesize; they depend on sugar made in the leaves.
4. DLight energy absorbed by the leaves travels to the roots and is made into root tissue.
   A student who thinks light energy becomes matter picks this. Light energy is stored as chemical energy in sugars made in the leaves; it is the sugars, not the light, that reach the roots.

CED 3.4.A.1.i · Read this in Fix

1. AStarch content rises in the dark period and falls during the light period.
   A student who thinks the Calvin cycle runs in the dark picks this. The graph shows the opposite: starch accumulates while the leaves are lit, when ATP and NADPH from the light reactions power carbon fixation, and is used up at night.
2. BStarch is being made fastest at 12 hours, when the starch content is highest.
   A student who reads the height of a line as a rate picks this. At 12 hours the amount stops rising and starts falling; the line is steepest, and starch is made fastest, while it is rising during the light period.
3. CStarch content rises in the light period and falls in the dark period. Correct
   The starch content rises steadily from about 1 to 8 mg/g during the 12 hours of light and falls steadily back to about 1 mg/g during the 12 hours of darkness: sugar made in the light is partly stored as starch, which the plant then uses at night.
4. DStarch content stays at about the same level throughout the dark period.
   A student who thinks a plant only stores the sugar it makes picks this. The starch content falls by about 7 mg/g during the dark period as the plant breaks down its stored starch and uses the sugar.

CED 3.4.A.1.i · Read this in Fix

1. AIt first evolved in prokaryotes, before any eukaryotic cells existed. Correct
   Prokaryotes were the first organisms in which photosynthesis evolved. Photosynthetic prokaryotes were present long before the first eukaryotic cells, and eukaryotes gained photosynthesis later through chloroplasts descended from a cyanobacterium.
2. BIt first evolved in land plants, and bacteria later gained it.
   A student who thinks photosynthesis began in plants picks this. Photosynthetic prokaryotes existed billions of years before land plants, and chloroplasts descend from cyanobacteria, not the other way round.
3. CIt first evolved in eukaryotes, as photosynthesis requires chloroplasts.
   A student who thinks photosynthesis needs chloroplasts picks this. Photosynthetic prokaryotes have no chloroplasts and carry out photosynthesis on their own membranes.
4. DIt evolved separately in each group of organisms that needed food.
   A student who thinks traits arise because organisms need them picks this. Photosynthesis first evolved in prokaryotes, and eukaryotic photosynthesis is built on the prokaryotic pathways inherited through chloroplasts.

CED 3.4.A.1.ii · Read this in Fix

1. AO₂ began to build up in the atmosphere only after the first land plants had appeared.
   A student who thinks land plants produced the atmosphere's O₂ picks this. The model shows O₂ rising about 2 billion years before the earliest land plants.
2. BO₂ began to build up when the only organisms in the fossil record were prokaryotes. Correct
   O₂ rises from trace levels at about 2.4 billion years ago. The earliest fossil eukaryotes appear at about 1.8 and land plants at about 0.47 billion years ago, so when O₂ began to accumulate the only organisms present were prokaryotes.
3. CO₂ has been present at about today's level since the first organisms appeared.
   A student who thinks the atmosphere has always held as much O₂ as now picks this. The model shows only trace O₂ for more than a billion years after the earliest fossil prokaryotes.
4. DO₂ began to build up in the atmosphere at about the time the first eukaryotes appeared.
   A student who thinks O₂-releasing photosynthesis needs chloroplasts, and so eukaryotes, picks this. O₂ began to rise at about 2.4 billion years ago, well before the earliest fossil eukaryotes at about 1.8 billion years ago.

CED 3.4.A.1.iii · Read this in Fix

1. ACyanobacteria release more O₂ each year than all the land plants on Earth do today.
   A student who thinks today's main O₂ producers must also have made the early atmosphere picks this. Present-day O₂ output says nothing about when O₂ first accumulated or which organisms were present then; the statement is also false, since land plants release more O₂ each year than cyanobacteria do.
2. BCyanobacteria are simpler than eukaryotes, so they must have photosynthesized first.
   A student who thinks simpler organisms must be older picks this. Simplicity is not evidence of age, and the claim concerns where the O₂ came from, which needs evidence about when O₂ appeared.
3. CSigns of free O₂ first appear in rocks formed when cyanobacteria existed but no eukaryotes did. Correct
   Evidence from rocks shows free O₂ first accumulating at a time when O₂-releasing cyanobacteria were present but eukaryotes, including all algae and plants, had not yet evolved. The only available source of the O₂ was prokaryotic photosynthesis.
4. DEarly organisms needed O₂, so cyanobacteria evolved photosynthesis in order to make it.
   A student who thinks traits evolve because organisms need them picks this. Traits are not produced by need, and this statement is an assumption about purpose, not evidence about when O₂ appeared.

CED 3.4.A.1.iii · Read this in Fix

1. AThe land plant's chloroplast gene is most closely related to the gene in the plant's own nucleus.
   A student who thinks DNA from the same organism must be most closely related picks this. On the tree, the plant's nuclear gene branches off first, and the chloroplast gene's closest relatives are the alga's chloroplast gene and the cyanobacteria.
2. BCyanobacteria descend from chloroplasts that escaped from the cells of algae and land plants.
   A student who reads one living group on a tree as the ancestor of another picks this. The chloroplast genes and cyanobacterium species 2 share a common ancestor at a branch point; the cyanobacterial branches split off earlier, and neither descends from the other.
3. CThe land plant's chloroplast is the most advanced, since it is on the last branch of the tree.
   A student who reads later branches as more advanced picks this. A tree shows common ancestry, not rank; the order of tips does not make one group more advanced than another.
4. DChloroplasts descend from a cyanobacterium, so eukaryotic photosynthesis has a prokaryotic origin. Correct
   The two chloroplast genes are nested among the cyanobacterial genes, sharing a more recent common ancestor with cyanobacterium species 2 than with the plant's own nuclear gene. This fits chloroplasts descending from a cyanobacterium, so the pathways of eukaryotic photosynthesis were inherited from prokaryotes.

CED 3.4.A.1.iv · Read this in Fix

1. ACorrect: evolution tends toward complexity, so the plant pathway has replaced the bacterial one.
   A student who thinks evolution moves toward greater complexity picks this. Plants did not replace the bacterial pathway; they inherited it through chloroplasts, and cyanobacteria still use it today.
2. BMistaken: chloroplasts came from cyanobacteria, so both share a two-photosystem pathway. Correct
   Eukaryotic photosynthesis was built on the photosynthetic pathways of prokaryotes. Chloroplasts descend from a cyanobacterium, and cyanobacteria, like chloroplasts, have thylakoid membranes with two photosystems linked by an ETC, split water and release O₂.
3. CMistaken: cyanobacteria took their reactions from plant chloroplasts, so both pathways match.
   A student who thinks photosynthesis began in plants picks this. The direction is reversed: photosynthesis evolved first in prokaryotes, and chloroplasts descend from cyanobacteria.
4. DCorrect: cyanobacteria lack chloroplasts, so they must use a simpler and different process.
   A student who thinks photosynthesis requires chloroplasts picks this. Cyanobacteria carry out photosynthesis on their own internal membranes, using the same kind of two-photosystem pathway as chloroplasts.

CED 3.4.A.1.iv · Read this in Fix

1. AOnly the thylakoids release O₂, and only the thylakoids build the ¹⁴C into carbohydrate.
   A student who thinks every stage of photosynthesis happens in the thylakoids picks this. CO₂ is fixed by the Calvin cycle, which takes place in the stroma, so thylakoids alone cannot make carbohydrate.
2. BOnly the thylakoids release O₂, and only the stroma builds any of the ¹⁴C into carbohydrate.
   A student who thinks the Calvin cycle runs independently of the light reactions picks this. The Calvin cycle in the stroma needs ATP and NADPH from the light reactions; with none added, the stroma fraction cannot fix CO₂.
3. COnly the thylakoids release O₂, and neither fraction builds the ¹⁴C into carbohydrate. Correct
   The thylakoid membranes hold the photosystems and ETC, so in light they split water and release O₂, passing electrons to the dye. The Calvin cycle is in the stroma, so the thylakoids cannot fix CO₂, and the stroma, given no ATP or NADPH, has no energy source to fix it either.
4. DOnly the stroma releases O₂, and only the thylakoids build ¹⁴C into carbohydrate.
   A student who places the light reactions in the stroma and the Calvin cycle in the thylakoids picks this. The light reactions, including water splitting, occur in the thylakoid membranes, and the Calvin cycle occurs in the stroma.

CED 3.4.A.2 · Read this in Fix

1. ALight energy absorbed by chlorophyll goes straight into making sugar, not into NADPH.
   A student who thinks light energy goes directly into sugar picks this. Absorbed light boosts electrons that, in intact thylakoids, pass along the ETC to reduce NADP⁺ to NADPH; NADPH then powers the Calvin cycle.
2. BNo water was split, and splitting water is what releases the energy that is needed to make NADPH.
   A student who thinks splitting water releases energy picks this. Splitting water supplies electrons; the energy comes from light, which this solution is still absorbing.
3. CNADPH is made in the Calvin cycle, which cannot run without CO₂ and its enzymes present.
   A student who thinks the Calvin cycle makes NADPH picks this. NADPH is made in the light reactions, when electrons from photosystem I reduce NADP⁺; the Calvin cycle uses NADPH.
4. DIts excited electrons have no ETC to enter, since it is no longer organized into photosystems. Correct
   In the thylakoid membrane, chlorophylls are organized with proteins into photosystems that pass energized electrons to an ETC and on to NADP⁺. Free chlorophyll still absorbs light, but its excited electrons have no acceptor, so they fall back and the energy is released as light and heat.

CED 3.4.A.2.ii · Read this in Fix

1. AThey are stacks of thylakoids, the site of the light reactions. Correct
   Thylakoids are organized in stacks called grana, and the light reactions of photosynthesis occur in the grana.
2. BThey are stacks of thylakoids, where CO₂ is made into sugar.
   A student who thinks every stage of photosynthesis happens in the thylakoids picks this. Grana are stacks of thylakoids, but CO₂ is converted into carbohydrate in the stroma.
3. CThey are the fluid around the thylakoids, where the Calvin cycle runs.
   A student who swaps the terms grana and stroma picks this. The fluid around the thylakoids, where the Calvin cycle runs, is the stroma; grana are the stacks of thylakoids.
4. DThey are folds of the inner membrane, where the light reactions happen.
   A student who models the chloroplast on the mitochondrion picks this. Grana are stacks of thylakoids, a membrane system separate from the inner membrane.

CED 3.4.A.2.iii · Read this in Fix

1. ACO₂ fixation is a dark reaction, so it continued because darkness switched it on.
   A student who thinks the Calvin cycle is switched on by darkness picks this. CO₂ fixation did not rise in the dark; it declined to zero within a minute, because it depends on ATP and NADPH from the light reactions.
2. BO₂ production stopped because it relies on CO₂ fixation, which splits CO₂ to release O₂.
   A student who thinks the O₂ comes from CO₂ picks this. O₂ production stopped within seconds, while CO₂ fixation was still near full rate, so O₂ release does not depend on CO₂ fixation; O₂ comes from water split in the light reactions.
3. CCO₂ fixation went on briefly, as it used ATP and NADPH made before the light went off. Correct
   O₂ production, part of the light reactions, stops within about 5 s of darkness. CO₂ fixation stays near its full rate for about 10 s and then declines to zero by about 90 s: the Calvin cycle runs on the ATP and NADPH already made and slows as they are used up, since no more are being produced.
4. DCO₂ fixation stopped the moment the light went off, as it uses light energy directly.
   A student who thinks the Calvin cycle absorbs light directly picks this. The graph shows CO₂ fixation still at about 92% of its light rate 10 s after the light went off; it slows only as ATP and NADPH run out.

CED 3.4.A.3 · Read this in Fix

1. AIt stops at once, as the O₂ released is made from the CO₂ the chloroplasts take in.
   A student who thinks O₂ comes from CO₂ picks this. O₂ comes from water split in the light reactions; it falls as NADP⁺ runs low, not because CO₂ supplied its oxygen.
2. BIt is unchanged, as the light reactions run independently of the Calvin cycle.
   A student who thinks the two stages are independent picks this. The light reactions need NADP⁺, ADP and inorganic phosphate returned by the Calvin cycle, so stopping the Calvin cycle slows them.
3. CIt increases, as light energy that the Calvin cycle no longer uses goes to the light reactions.
   A student who thinks the Calvin cycle absorbs light directly picks this. Only the photosystems absorb light; stopping the Calvin cycle frees no light for the light reactions and instead deprives them of NADP⁺.
4. DIt decreases, as the Calvin cycle stops returning NADP⁺ to the light reactions. Correct
   Without CO₂ the Calvin cycle stops using NADPH and ATP, so NADP⁺ and ADP are no longer regenerated. With no NADP⁺ to accept electrons, electron transport from water slows, and so does O₂ release, even though light is still absorbed.

CED 3.4.A.3 · Read this in Fix

1. AEach group evolved the mechanism separately because each group needed a way to make ATP.
   A student who thinks traits arise because organisms need them picks this. A need does not produce a trait, and the organelles that carry ETCs descend from prokaryotes that already had them.
2. BThe mechanism arose in eukaryotic organelles and was later passed on to prokaryotic cells.
   A student who thinks electron transport needs membrane-bound organelles picks this. Prokaryotes carry out electron transport across their plasma membranes, and the organelles descend from prokaryotes, not the other way round.
3. CThe mechanism arose in prokaryotes and was kept in organelles that descend from prokaryotes. Correct
   ETC reactions occur across prokaryotic plasma membranes, in mitochondria and in chloroplasts. Mitochondria and chloroplasts descend from prokaryotes taken into ancestral eukaryotic cells, so their ETCs are best explained as inherited from those prokaryotic ancestors.
4. DOrganelle chains are more advanced versions that replaced prokaryotic chains in eukaryotes.
   A student who thinks evolution moves toward more advanced forms picks this. The organelle ETCs are inherited from prokaryotes, which still use theirs; nothing in the distribution shows one form replacing or improving on another.

CED 3.4.B.1 · Read this in Fix

1. AIn the carbohydrate, since the O₂ released comes from CO₂
   A student who thinks the O₂ released comes from CO₂ picks this. The O₂ comes from water, so the label from the water appears in the O₂, not first in the carbohydrate.
2. BIn the O₂ released, since water is split to supply electrons Correct
   In the light reactions, water is split to replace the electrons lost from photosystem II, and its oxygen atoms are released as O₂. The ¹⁸O from the water should therefore appear in the O₂ released.
3. CIn both products equally, since each gets atoms from each reactant
   A student who thinks every product contains atoms from every reactant picks this. The oxygen atoms released as O₂ come from water; atoms are not shared out evenly among the products.
4. DIn neither product, since the water's atoms are converted into energy
   A student who thinks atoms can be turned into energy picks this. Atoms are conserved: splitting water supplies electrons, and the oxygen atoms of the water are released as O₂, so the ¹⁸O appears in the O₂.

CED 3.4.B.2 · Read this in Fix

1. AO₂ production is highest in blue and red light and lowest in green light. Correct
   The rate peaks at about 450 nm (blue, 100%) and about 675 nm (red, 90%) and falls to about 18% near 550 nm (green). This matches the strong absorption of blue and red light by chlorophylls, which boosts electrons in the photosystems.
2. BO₂ production is highest in green light, which is the color that the alga appears.
   A student who thinks a green organism absorbs green light picks this. The rate is lowest near 550 nm, in the green region; the alga looks green because it absorbs little green light.
3. CO₂ production rises steadily as the wavelength increases from 400 nm to 700 nm.
   A student who thinks longer wavelengths carry more energy picks this. The graph rises and falls twice, with peaks in the blue and the red and a low point in the green, and drops sharply at 700 nm.
4. DO₂ production is about the same at every wavelength of visible light.
   A student who thinks all colors of light are used equally picks this. The rate ranges from about 18% to 100% of the maximum, depending on how strongly each wavelength is absorbed.

CED 3.4.B.2 · Read this in Fix

1. A20% Correct
   The bright-light mean is (58 + 62 + 60) ÷ 3 = 60 μL/min and the dim-light mean is (47 + 49 + 48) ÷ 3 = 48 μL/min. The decrease is (60 − 48) ÷ 60 × 100 = 20%. In dimmer light, chlorophyll absorbs less light energy, so fewer electrons are boosted and fewer water molecules are split each minute.
2. B25%
   A student who divides the change by the new value picks this: 12 ÷ 48 × 100 = 25%. A percent decrease is calculated relative to the starting (bright-light) mean of 60 μL/min.
3. C80%
   A student who confuses the percentage remaining with the percent change picks this: 48 is 80% of 60, so the rate fell by 100% − 80% = 20%.
4. D12%
   A student who treats the difference in μL/min as a percentage picks this. The means differ by 12 μL/min, which is 12 ÷ 60 × 100 = 20% of the bright-light mean.

Working Mean rate in bright light = (58 + 62 + 60) ÷ 3 = 60 μL/min. Mean rate in dim light = (47 + 49 + 48) ÷ 3 = 48 μL/min. Percent decrease = (60 − 48) ÷ 60 × 100 = 12 ÷ 60 × 100 = 20%. Distractors: dividing the change by the dim-light mean, 12 ÷ 48 × 100 = 25%; the dim-light mean as a percentage of the bright-light mean, 48 ÷ 60 × 100 = 80%; the absolute difference of 12 μL/min read as 12%.

CED 3.4.B.2 · Read this in Fix

1. ATubes 3 and 4, as tube 4 has no chloroplasts and is thus the control
   A student who thinks the control is always the tube with the main ingredient left out picks this. Tubes 3 and 4 differ in whether chloroplasts are present at all, not in whether their proteins are intact, so the comparison cannot answer her question.
2. BTubes 1 and 3, which differ only in whether the chloroplasts were boiled Correct
   Boiling denatures the photosystem and ETC proteins. Tubes 1 and 3 are both lit and both contain chloroplasts and DCPIP, so a difference in how much DCPIP is reduced can be attributed to the boiling, that is, to whether the proteins are intact.
3. CTubes 2 and 3, which between them cover darkness and boiling
   A student who thinks several variables can change at once picks this. Tubes 2 and 3 differ in both light and boiling, so any difference between them could be caused by either factor.
4. DTube 3 alone, as its result by itself shows the effect of boiling
   A student who thinks a treated sample can be judged alone picks this. Without the unboiled, lit chloroplasts of tube 1 for comparison, the result for tube 3 cannot show what boiling changed.

CED 3.4.B.2 · Read this in Fix

1. AMutant A releases O₂ at the same rate as the wild type does.
   A student who thinks overlapping error bars show equal means picks this. The overlap means only that the data do not show a difference; mutant A's true rate could still be somewhat lower.
2. BOnly mutant B shows a rate likely to differ from that of the wild type. Correct
   The ±2 SE bars for the wild type (100–120) and mutant B (44–60) do not overlap, so mutant B's lower rate is likely a real difference. The bars for the wild type and mutant A (86–110) overlap, so these data do not show a difference for mutant A.
3. CBoth mutants likely release O₂ more slowly than the wild-type strain.
   A student who treats any difference between sample means as real picks this. Mutant A's mean is lower, but its ±2 SE bar (86–110) overlaps the wild type's (100–120), so the data do not show that mutant A's rate is likely to differ.
4. DMutant B's rate is exactly 58 μmol/mg/h lower than the wild type's rate.
   A student who treats sample means as exact values picks this. The difference between the sample means is 58, but each mean has uncertainty, shown by its error bar, so the true difference is known only approximately.

Working Approximate 95% intervals (mean ± 2 SE): wild type 100–120, mutant A 86–110, mutant B 44–60 μmol/mg/h. Wild type and mutant A overlap (100–110), so the data do not show a significant difference between them. Wild type and mutant B do not overlap (mutant B's upper limit, 60, is below wild type's lower limit, 100), so mutant B's rate is likely lower than wild type's.

CED 3.4.B.3 · Read this in Fix

1. ARegion W
   A student who thinks H⁺ collects where ATP is made picks this. W is the stroma, where ATP is made, but its H⁺ concentration is low; H⁺ flows into it from the thylakoid space through ATP synthase.
2. BRegion Z
   A student who models the chloroplast on the mitochondrion's inner membrane picks this. Z is the space between the outer and inner membranes; the chloroplast's ETC is in the thylakoid membrane, so H⁺ accumulates inside the thylakoids.
3. CRegion Y Correct
   Y is the thylakoid space. As electrons pass along the ETC in the thylakoid membrane, H⁺ is moved from the stroma into the thylakoid space, and water splitting releases more H⁺ there, so the membrane separates low H⁺ outside the thylakoid from high H⁺ inside it.
4. DRegion V
   A student who pictures H⁺ being pumped out across the chloroplast's outer boundary, as across a prokaryote's plasma membrane, picks this. V is outside the chloroplast; the ETC moves H⁺ into the thylakoid space.

CED 3.4.B.4 · Read this in Fix

1. AIn the light, the H⁺ concentration rises in the thylakoid space and falls in the stroma. Correct
   In the light, the pH of the thylakoid space falls from 7.2 to 5.5 and the pH of the stroma rises from 7.2 to 8.0. A lower pH means more H⁺, so H⁺ is moved from the stroma into the thylakoid space, building a gradient that is high inside the thylakoid.
2. BIn the light, the H⁺ concentration decreases in the thylakoid space and rises in the stroma.
   A student who thinks a higher pH means more H⁺ picks this. The pH of the thylakoid space falls in the light, which means its H⁺ concentration rises.
3. CIn the light, the H⁺ concentration rises by the same amount in both compartments.
   A student who thinks H⁺ crosses the lipid bilayer freely picks this. The two lines move in opposite directions, so the membrane keeps very different H⁺ concentrations on its two sides.
4. DAfter the light is switched off, the difference in H⁺ concentration remains.
   A student who thinks a gradient stays in place once built picks this. By 10 minutes, 2 minutes after the light goes off, both compartments are back near pH 7.2, so the gradient has disappeared.

CED 3.4.B.4 · Read this in Fix

1. A3 times
   A student who treats the pH scale as linear picks this. A difference of 3 pH units is a 10³-fold difference in H⁺ concentration, not a 3-fold one.
2. B1000 times Correct
   Each pH unit is a tenfold difference in H⁺ concentration. The thylakoid space is 3.0 pH units lower than the stroma, so its H⁺ concentration is 10 × 10 × 10 = 1000 times greater, which reflects the steep gradient built by electron transport.
3. C30 times
   A student who multiplies the number of pH units by 10 picks this. The tenfold steps multiply: 10 × 10 × 10 = 1000.
4. D0.001 times
   A student who thinks a higher pH means more H⁺ picks this and concludes the stroma has more H⁺. The thylakoid space has the lower pH, so it has the higher H⁺ concentration.

Working pH = −log[H⁺], so each pH unit is a tenfold difference in H⁺ concentration. The difference is 8.0 − 5.0 = 3.0 units, so [H⁺] in the thylakoid space ÷ [H⁺] in the stroma = 10^3.0 = 1000. Distractors: treating pH as linear gives 3; multiplying the number of units by 10 gives 30; taking a higher pH to mean more H⁺ gives 10^{5.0 − 8.0} = 0.001.

CED 3.4.B.4 · Read this in Fix

1. AA low pH inside the thylakoids is, by itself, enough to make the ATP synthase produce ATP.
   A student who thinks acidity itself drives ATP synthesis picks this. Thylakoids at pH 4 moved into a pH 4 solution formed no ATP; ATP formed only when there was a difference in pH across the membrane.
2. BATP synthase made the ATP as it pumped the H⁺ into the thylakoid space, against its gradient.
   A student who thinks ATP synthase is a pump picks this. In this experiment H⁺ moved out of the thylakoids, from pH 4 inside to pH 8 outside, down its gradient, and that flow drove ATP synthesis.
3. CA difference in H⁺ concentration across the membrane drove ATP synthesis without light. Correct
   ATP formed only when the inside (pH 4) had a much higher H⁺ concentration than the outside (pH 8), and it formed in the dark. As H⁺ flowed out through ATP synthase, the energy of the gradient drove ATP synthesis, showing that the gradient, not light itself, powers photophosphorylation.
4. DAny movement of H⁺ across the thylakoid membrane, by any route, produces ATP.
   A student who thinks any H⁺ flow makes ATP picks this. The results show that a gradient is needed but say nothing about routes other than ATP synthase; H⁺ that leaks across the membrane by another route does not make ATP.

CED 3.4.B.5 · Read this in Fix

1. AMore ATP is made, and NADPH is made too.
   A student who thinks any H⁺ flow across the membrane makes ATP picks this. H⁺ leaking across the membrane bypasses ATP synthase, so its energy is not captured as ATP.
2. BATP and NADPH are both made as before.
   A student who thinks ATP is made by adding ETC electrons to ADP picks this, since electron transport is unaffected. ATP synthesis depends on H⁺ flowing through ATP synthase, and the leak removes the gradient that drives it.
3. CNeither ATP nor NADPH is made any longer.
   A student who thinks the H⁺ gradient powers electron transport picks this. Energy flows from electron transport to the gradient, not the reverse, so electrons still reach NADP⁺ when the gradient is lost.
4. DLess ATP is made, but NADPH is still made. Correct
   Electron transport from water to NADP⁺ continues, so NADPH is still made. But H⁺ now leaks back into the stroma across the membrane instead of flowing through ATP synthase, so the gradient cannot drive photophosphorylation and less ATP is made.

CED 3.4.B.5 · Read this in Fix

1. AH⁺ flows down its gradient through ATP synthase, and this flow powers ATP synthesis. Correct
   The model shows many H⁺ in the thylakoid space and few in the stroma, with H⁺ passing up through the ATP synthase channel into the stroma as ATP is made. Energy stored in the gradient is released as H⁺ flows down it and is used to make ATP from ADP and Pᵢ: chemiosmosis.
2. BATP synthase uses ATP to pump H⁺ against its gradient, from the stroma into the thylakoid.
   A student who thinks ATP synthase is a pump picks this. In the model H⁺ moves through ATP synthase from the thylakoid space, where it is concentrated, to the stroma, down its gradient, and ATP is made, not used.
3. CLight absorbed by ATP synthase supplies the energy that joins ADP and Pᵢ to make ATP.
   A student who thinks light makes ATP directly picks this. ATP synthase does not absorb light; the energy comes from the H⁺ gradient that light-driven electron transport built.
4. DElectrons that pass through ATP synthase are added to ADP to form ATP.
   A student who thinks ATP is made by adding ETC electrons to ADP picks this. In the model only H⁺ passes through ATP synthase; electrons move through the ETC, and ATP is made by adding phosphate to ADP.

CED 3.4.B.5 · Read this in Fix

1. ACO₂ fixation takes place in darkness but stops when light is present.
   A student who thinks the Calvin cycle runs only in the dark picks this. Sample 3, in light with ATP and NADPH, fixed as much ¹⁴C (4,900 cpm) as sample 2 in the dark.
2. BCO₂ fixation needs light directly, in addition to ATP and NADPH.
   A student who thinks the Calvin cycle absorbs light directly picks this. Sample 2 fixed 4,800 cpm in the dark when given ATP and NADPH, so light is not needed directly.
3. CCO₂ fixation needs thylakoid membranes to be present in the extract.
   A student who thinks every stage of photosynthesis happens in the thylakoids picks this. The extract had no thylakoids, yet samples 2 and 3 fixed large amounts of ¹⁴C; the Calvin cycle occurs in the stroma.
4. DCO₂ fixation needs ATP and NADPH but not light directly. Correct
   Stroma given only NADPH (35 cpm) or only ATP (40 cpm) fixed almost no ¹⁴C, but stroma given both ATP and NADPH fixed about 4,800–4,900 cpm whether in light (sample 3) or dark (sample 2). The Calvin cycle in the stroma is powered by ATP and NADPH; light matters only because the light reactions make them.

CED 3.4.B.6 · Read this in Fix