Gas exchange in light and dark — answer key

1. GQ1 · Practice

Options as numbered on the pupil sheet: 1 — more carbon dioxide than in air; 2 — less carbon dioxide than in air; 3 — the same carbon dioxide as in ordinary air.

Model answer. Red hydrogencarbonate indicator holds the same carbon dioxide as ordinary air. With more carbon dioxide it becomes more acidic and turns orange, then yellow. With less carbon dioxide it becomes less acidic and turns magenta, then purple.

A full answer includes

• I matched red to the same carbon dioxide as in ordinary air.
• I matched yellow to more carbon dioxide than in air.
• I matched purple to less carbon dioxide than in air.
• I did not match purple to oxygen: the indicator shows only carbon dioxide.

Also accept

• red: as in air; yellow: more carbon dioxide; purple: less carbon dioxide

Watch for

• “Purple shows oxygen.” — The indicator shows only carbon dioxide. Purple means less carbon dioxide than at the start.
• “Yellow shows less carbon dioxide.” — Yellow shows more carbon dioxide: the solution has become more acidic.

2. GQ2 · Practice

Model answer. In bright light the pondweed both respires and photosynthesises. Respiration goes on all the time. Photosynthesis is faster, so the plant takes in more carbon dioxide than respiration gives out, and overall it takes carbon dioxide in and gives oxygen out.

A full answer includes

• I chose: it respires, but photosynthesises faster, so it takes in carbon dioxide overall.
• I did not choose "stops respiring": respiration goes on all the time.

Also accept

• both; photosynthesis faster; net carbon dioxide taken in

Watch for

• “It photosynthesises and stops respiring until dark, so it takes in carbon dioxide.” — Its cells respire all the time, in light and in dark. Photosynthesis is just faster in bright light.
• “It only respires, and does not photosynthesise, so it gives out carbon dioxide.” — Light lets it photosynthesise too, and in bright light photosynthesis is the faster process.
• “It respires and photosynthesises at the same rate, so there is no overall change.” — That happens only in dim light. In bright light photosynthesis is faster than respiration.

3. GQ3 · Practice

Options as numbered on the pupil sheet: 1 — red, as at the start; 2 — orange-red, close to the start; 3 — purple; 4 — yellow.

Model answer. The living pondweed by the lamp photosynthesises faster than it respires, so it takes carbon dioxide out of the indicator, which turns purple. The living pondweed in foil cannot photosynthesise, but it still respires and gives out carbon dioxide, so its indicator turns yellow. The boiled pondweed is dead: its cells neither respire nor photosynthesise, so in the light and in the dark its indicator stays red.

A full answer includes

• I matched the living pondweed by the lamp to purple.
• I matched the living pondweed in foil to yellow.
• I matched both tubes of boiled pondweed to red, as at the start.
• I did not match the boiled pondweed in foil to yellow: dead cells do not respire.

Also accept

• living, lamp: purple; living, foil: yellow; boiled, lamp: red; boiled, foil: red

Watch for

• “The boiled pondweed in foil turns yellow, because plants respire in the dark.” — Only living cells respire. The boiled cells are dead, so they give out no carbon dioxide.
• “The boiled pondweed by the lamp turns purple, because it is in the light.” — Light alone does not change the indicator. Dead cells cannot photosynthesise.

4. GQ4 · Practice

Model answer. The bung stops gases moving between the indicator and the air in the room. Then the carbon dioxide in the indicator can change only because of what happens inside the tube, so the colour shows what the pondweed did.

A full answer includes

• I chose: it stops gases moving between the indicator and the air.
• I did not choose "keeps the light out": light passes through the glass.

Also accept

• it stops gases moving between the indicator and the air

Watch for

• “It keeps light out of the indicator inside the tube.” — Light still reaches the indicator through the glass. The foil and the gauze control the light.
• “It keeps the indicator at a steady, warm temperature.” — A bung does not hold in heat. All the tubes stand under the same lamp, so they are at the same temperature.
• “It keeps in the oxygen, so the indicator can show it.” — The indicator does not respond to oxygen; it shows carbon dioxide. The bung matters because no gas can get in or out.

5. GQ5 · Practice

Model answer. The best control is a bunged tube of the same indicator, wrapped in foil, with no plant. It is the same as the test tube in every way except the plant. If it stays red, the foil, the bung, the dark and the 3 hours do not change the indicator on their own, so the yellow colour was caused by the pondweed.

A full answer includes

• I chose: a bunged tube of the same indicator, wrapped in foil, with no plant.
• I checked that it differs from the test tube only in having no plant.
• I did not choose the tube with pondweed in the light: that differs in the light, not in the plant.

Also accept

• a bunged tube of the same indicator, wrapped in foil, with no plant

Watch for

• “A bunged tube of the same indicator with pondweed, in the light.” — That tube differs in the light. It is a second test, not a control for the plant.
• “A bunged tube of water, not indicator, with pondweed, in foil.” — Water cannot change colour, so this tube shows nothing about the indicator.
• “An open tube of the same indicator, in foil, with no plant.” — An open tube exchanges gases with the room, so it is not set up in the same way.

6. GQ6 · Practice

Model answer. Tube X is nearer the lamp, so its pondweed gets brighter light and photosynthesises faster. In both tubes photosynthesis is faster than respiration, so both take carbon dioxide out of the indicator. In X the difference is large, so in 1 hour the carbon dioxide falls a long way and the indicator turns purple. In Y, in dimmer light, photosynthesis is only a little faster than respiration, so the carbon dioxide falls only a little and the indicator turns magenta.

A full answer includes

• I wrote that tube X, nearer the lamp, gets brighter light.
• I wrote that the pondweed in X photosynthesises faster.
• I wrote that in X photosynthesis is much faster than respiration, so carbon dioxide is taken out of the indicator.
• I wrote that a lot less carbon dioxide is left in X, so it turns purple.
• I wrote that in Y photosynthesis is only a little faster than respiration.
• I wrote that so in Y the carbon dioxide falls only a little: magenta.
• I did not write that the pondweed in Y does not photosynthesise.

Also accept

• X: brighter light, faster photosynthesis, more carbon dioxide taken out: purple; Y: only a little faster than respiration: magenta

Watch for

• “Tube Y is magenta because its pondweed only respires.” — If it only respired, carbon dioxide would rise and the indicator would turn orange or yellow. Magenta means less carbon dioxide.
• “Tube X is purple because its pondweed gives out more oxygen.” — The indicator does not detect oxygen. Purple means less carbon dioxide.

7. GQ7 · Practice

Model answer. Yellow shows more carbon dioxide than in air, from the breath. In bright light the pondweed photosynthesises faster than it respires, so overall it takes carbon dioxide out of the indicator. The carbon dioxide falls, so the colour goes from yellow through orange back to red, the level in air. The pondweed does not stop there: it goes on taking carbon dioxide out, so the indicator turns magenta and then purple.

A full answer includes

• I chose: yellow, orange, red, then magenta and purple.
• I wrote that in bright light the pondweed photosynthesises faster than it respires.
• I wrote that it takes carbon dioxide out of the indicator.
• I wrote that the carbon dioxide falls back to the level in air: red.
• I wrote that it goes on falling below the level in air: purple.
• I did not write that the colour stops at red.

Also accept

• yellow, orange, red, then magenta and purple: photosynthesis faster than respiration takes the carbon dioxide out

Watch for

• “It turns red and stays red: the plant only removes the extra carbon dioxide from the breath.” — The plant cannot tell breath from other carbon dioxide. It goes on taking carbon dioxide in while photosynthesis is faster than respiration, so the indicator goes past red to purple.
• “It stays yellow all the time: it does not change.” — In bright light the pondweed photosynthesises, and the breath has given it extra carbon dioxide to use. It takes carbon dioxide out, so the colour changes.
• “It turns orange and red, then goes back to yellow as the plant respires.” — The plant respires all the time, but in bright light photosynthesis stays faster. The carbon dioxide keeps falling; it would rise again only in dim light or the dark.

8. GQ8 · Practice

Model answer. (a) The tube with 2 sheets: its colour stayed close to red, so the carbon dioxide taken in by photosynthesis roughly balanced the carbon dioxide given out by respiration. (b) Under 3 sheets the light is very dim, so photosynthesis is slower than respiration. The pondweed gives out more carbon dioxide than it takes in, so the carbon dioxide in the indicator rises a little and it turns orange. It is not yellow, because some photosynthesis still takes some carbon dioxide back in.

A full answer includes

• (a) I identified the tube with 2 sheets.
• (a) I wrote that its colour hardly changed from red.
• (a) I wrote that photosynthesis takes in about as much carbon dioxide as respiration gives out.
• (b) I wrote that under 3 sheets the light is so dim that photosynthesis is slower than respiration.
• (b) I wrote that more carbon dioxide is given out than is taken in.
• (b) I wrote that the carbon dioxide rises a little, so the indicator turns orange.
• I did not write that the pondweed under 3 sheets has stopped photosynthesising.

Also accept

• (a) 2 sheets; (b) photosynthesis slower than respiration, so carbon dioxide rises a little: orange

Watch for

• “In the tube with 3 sheets the pondweed only respires.” — In complete darkness the indicator would go yellow. Orange shows a small rise: some photosynthesis still happens.
• “Photosynthesis and respiration balance with 0 sheets.” — With no paper the light is brightest, so photosynthesis is much faster than respiration: purple.

9. GQ9 · Practice

Model answer. Carbon dioxide dissolves in water to make a weak acid. In bright light the pondweed photosynthesised faster than it respired, so it took carbon dioxide out of the indicator. With less carbon dioxide the solution became less acidic, so its pH rose from 8.3 to 9.4 and the indicator turned purple.

A full answer includes

• I chose: the pondweed took carbon dioxide out, so the solution became less acidic.
• I checked that a higher pH means less acidic.
• I did not choose the oxygen statement: the indicator does not respond to oxygen.

Also accept

• the pondweed took carbon dioxide out, so the solution became less acidic

Watch for

• “The pondweed gave out oxygen, which made the solution more alkaline.” — The indicator does not respond to oxygen. A higher pH means less acid, so less carbon dioxide.
• “The pondweed gave out carbon dioxide, which made the solution less acidic.” — Carbon dioxide makes the solution more acidic, so the pH would fall. The pH rose, so carbon dioxide was taken out.
• “The bright light warmed the indicator, and warm indicator has a higher pH.” — A control tube with no plant under the same lamp stays red. The change comes from the pondweed, not from the warmth.

10. GQ10 · Practice

Model answer. Purple means there is less carbon dioxide than in air, so something took carbon dioxide out of the control. In the light only a living plant can do that, by photosynthesis. A small piece of pondweed left in the tube from an earlier run would do it. So the control did not have "no plant", and it cannot show that the indicator does not change on its own.

A full answer includes

• I chose: a small piece of pondweed was left inside the control tube.
• I checked that purple means less carbon dioxide than in air.
• I checked that only something photosynthesising can take carbon dioxide out in the light.
• I did not choose the breath: breath turns the indicator yellow.

Also accept

• a small piece of pondweed was left in the control tube

Watch for

• “She left the bung out of the control tube.” — Open to the room, the indicator stays red, or turns orange in a crowded room. Purple needs less carbon dioxide than in air.
• “She stood the control tube closer to the lamp.” — Light alone does not change the indicator. With nothing living inside, nothing takes carbon dioxide out.
• “She breathed into the control tube before closing it.” — Breath adds carbon dioxide, so it would turn the indicator yellow, not purple.

11. GT1 · Apply

Model answer. Both tubes turn yellow. The snails respire all the time, in light and in dark, and give out carbon dioxide, so the carbon dioxide in the indicator rises. A snail has no chloroplasts and cannot photosynthesise, so the light makes no difference: nothing takes the carbon dioxide back in.

A full answer includes

• (a) I predicted yellow (or orange) for the tube in bright light.
• (b) I predicted yellow (or orange) for the tube in foil.
• (c) I wrote that a snail respires all the time and gives out carbon dioxide.
• (c) I wrote that a snail cannot photosynthesise, so the light makes no difference.
• I did not predict purple for the tube in the light.

Also accept

• both yellow; the snail respires and cannot photosynthesise

Watch for

• “The tube in the light turns purple.” — Only an organism with chloroplasts can take in carbon dioxide by photosynthesis; a snail cannot.
• “The tubes stay red, because a snail does not give out carbon dioxide.” — Animals respire too, and respiration gives out carbon dioxide.

12. GT2 · Apply

Model answer. (a) In sunlight the seaweeds photosynthesise faster than all the living things in the pool respire, so more oxygen is given out into the water than is used, and the oxygen rises. (b) In the dark there is no photosynthesis. The seaweeds, the crabs and the fish all go on respiring, using oxygen, and nothing adds more until the sea returns, so the oxygen falls the whole time and is lowest just before the pool is covered again.

A full answer includes

• (a) I wrote that in sunlight the seaweeds photosynthesise faster than the living things in the pool respire.
• (a) I wrote that more oxygen is given out than is used, so the oxygen rises.
• (b) I wrote that in the dark there is no photosynthesis.
• (b) I wrote that the seaweeds, crabs and fish all go on respiring.
• (b) I wrote that respiration uses oxygen and nothing adds more, so the oxygen falls.
• I did not write that the seaweeds respire only at night.

Also accept

• (a) photosynthesis faster than respiration: oxygen rises; (b) only respiration in the dark: oxygen falls

Watch for

• “The seaweeds respire only at night.” — They respire all the time. At night there is just no photosynthesis to make up for it.
• “The crabs and fish use all the oxygen during the day.” — In sunlight the seaweeds give out more oxygen than everything in the pool uses, so it rises.

13. GT3 · Apply

Model answer. The control should stay red if only the pondweed changes the colour. Turning orange means carbon dioxide got into it some other way: perhaps the bung was loose, someone breathed into the tube while setting it up, or the indicator was not freshly red to start with. So the same could have happened in the other tubes, and you cannot be sure that all of their change came from the pondweed. The investigation should be repeated with fresh indicator and tight bungs.

A full answer includes

• I suggested a way carbon dioxide got into the control: a loose bung, someone breathing into it, or indicator that was not freshly red.
• I wrote that the same could have happened in the other tubes.
• I wrote that you cannot be sure all the change in tubes 1 and 3 came from the pondweed.
• I wrote that the investigation should be repeated with fresh indicator and tight bungs.
• I did not write that the light turned the control orange.

Also accept

• carbon dioxide got in another way (loose bung, breath, old indicator); the results are less certain, so repeat

Watch for

• “The control does not matter, because tubes 1 and 3 changed the right way.” — The control tells you whether anything else changed the colour; here something did.
• “The light turned the control orange.” — Light does not change the indicator on its own; carbon dioxide does.