Gas exchange in light and dark — questions

1. identify

Hydrogencarbonate indicator changes colour with the carbon dioxide dissolved in it. Match each colour to what it shows.

Write the number of the matching option.

1. more carbon dioxide than in air
2. less carbon dioxide than in air
3. the same carbon dioxide as in ordinary air

• red: ______
• yellow: ______
• purple: ______

Nudge (only if you are stuck): The indicator starts red with ordinary air. Which way does it go with more carbon dioxide, which with less?

2. identify

Which statement about pondweed in bright light is correct?

Circle one answer.

• It only respires, and does not photosynthesise, so it gives out carbon dioxide.
• It photosynthesises and stops respiring until dark, so it takes in carbon dioxide.
• It respires and photosynthesises at the same rate, so there is no overall change.
• It respires, but photosynthesises faster, so it takes in carbon dioxide overall.

Nudge (only if you are stuck): Does a plant ever stop respiring? Which process is faster in bright light?

3. predict

Four bunged tubes hold the same volume of red hydrogencarbonate indicator. Two hold living pondweed. Two hold pondweed that was boiled first, so its cells are dead. One tube of each kind stands by a lamp for 5 hours, and the other two are wrapped in foil beside them. Predict the colour of each tube after the 5 hours.

Write the number of the matching option.

1. red, as at the start
2. orange-red, close to the start
3. purple
4. yellow

• living pondweed, by the lamp: ______
• living pondweed, in foil: ______
• boiled pondweed, by the lamp: ______
• boiled pondweed, in foil: ______

Nudge (only if you are stuck): Can dead cells carry out either process? For the living pondweed, which process needs light?

4. explain

Why is each tube closed with a rubber bung?

Circle one answer.

• It keeps the indicator at a steady, warm temperature.
• It keeps in the oxygen, so the indicator can show it.
• It stops gases moving between the indicator and the air.
• It keeps light out of the indicator inside the tube.

Nudge (only if you are stuck): Where else could carbon dioxide come from, or go to, if the tube were open?

5. identify

A pupil puts pondweed into a bunged tube of red hydrogencarbonate indicator, wraps the tube in foil and leaves it for 3 hours. The indicator turns yellow. Which tube is the best control for this tube?

Circle one answer.

• A bunged tube of the same indicator, in foil, with no plant.
• An open tube of the same indicator, in foil, with no plant.
• A bunged tube of the same indicator with pondweed, in the light.
• A bunged tube of water, not indicator, with pondweed, in foil.

Nudge (only if you are stuck): A control is the same as the test tube in every way except one. What is the one thing the pupil wants to test?

6. explain

Two bunged tubes hold the same volume of red hydrogencarbonate indicator and similar sprigs of pondweed. Tube X stands 8 cm from a lamp and tube Y stands 60 cm from it. After 1 hour, tube X is purple but tube Y is only magenta. Explain the difference.

Nudge (only if you are stuck): Which tube gets the brighter light? What does brighter light do to the rate of photosynthesis, and so to the carbon dioxide?

7. predict / explain

A pupil breathes out through a straw into a tube of red hydrogencarbonate indicator until it turns yellow. Then she puts a sprig of pondweed into it, closes it with a bung and stands it in bright light. Predict how the colour changes over the next few hours, and explain your prediction.

Circle one answer.

• Yellow, orange, red, then magenta and purple.
• Yellow, orange, red, then back to yellow.
• Yellow, orange, red, then it stays red.
• Yellow all the time: it does not change.

Explain the colour changes in terms of photosynthesis, respiration and carbon dioxide.

Nudge (only if you are stuck): What does yellow show about the carbon dioxide? In bright light, which process is faster, and where does the carbon dioxide go?

8. identify / explain

A pupil puts similar sprigs of pondweed into four bunged tubes of red hydrogencarbonate indicator under the same lamp. She covers the tubes with 0, 1, 2 or 3 sheets of white tissue paper, which cut down the light. The table shows the colours after 3 hours. (a) Identify the tube in which photosynthesis and respiration roughly balance. (b) Explain why the tube with 3 sheets turned orange.

(a) The balance tube

(b) Why orange

Nudge (only if you are stuck): In which tube did the carbon dioxide hardly change? Under 3 sheets, which of the two processes is faster?

9. explain

A pupil measures the pH of a bunged tube of red hydrogencarbonate indicator with a sprig of pondweed in it. At the start the pH is 8.3. After 2 hours in bright light it is 9.4, and the indicator is purple. Which statement explains the change in pH?

Circle one answer.

• The light warmed the indicator, and warm indicator has a higher pH.
• The pondweed took carbon dioxide out, so the solution became less acidic.
• The pondweed gave out oxygen, which made the solution more alkaline.
• The pondweed gave out carbon dioxide, which made the solution less acidic.

Nudge (only if you are stuck): Does a higher pH mean more acidic or less acidic? Which gas makes the indicator more acidic when it dissolves?

10. identify

In a four-tube investigation, the control tube should hold red hydrogencarbonate indicator and no plant. After 5 hours by the lamp, the control has turned purple. Which mistake explains this?

Circle one answer.

• She stood it a little closer to the lamp than the other three tubes.
• A small piece of pondweed from an earlier run was left stuck inside it.
• She left the bung out, so the tube was open to the air in the room.
• She breathed into it through a straw to check it before closing it.

Nudge (only if you are stuck): What does purple show about the carbon dioxide? What could take carbon dioxide out of a tube in the light?

11. predict / explain

A small water snail is put into a bunged tube of red hydrogencarbonate indicator, with no plant, and left in bright light for 6 hours. A second snail is put into an identical tube wrapped in foil. Predict the colour of each tube, and explain your predictions.

Tube in bright light

Tube in foil

Explanation

Nudge (only if you are stuck): Does a snail photosynthesise? Which process does it carry out in light and in dark?

12. explain

At low tide a rock pool full of seaweeds, crabs and small fish is cut off from the sea for several hours. (a) On a sunny afternoon, the oxygen in the pool water rises while the pool is cut off. Explain why. (b) When low tide comes in the night, the oxygen falls instead, and it is lowest just before the sea covers the pool again. Explain why.

(a) Sunny afternoon

(b) Night

Nudge (only if you are stuck): Which living things in the pool can photosynthesise? Which process stops when it is dark, and which goes on?

13. suggest

In another class, tube 1 (light) turned purple and tube 3 (dark) turned yellow, but the control tube, with no plant, turned orange. Suggest what might have happened to the control, and what this means for the other results.

Nudge (only if you are stuck): What should the control do if only the plant changes the indicator? What could add carbon dioxide to it?