Year 8 Science · Unit 1 · Lesson 8
Challenge Worksheet
Learning Goals
Find the errors
This paragraph has exactly three errors. Underline each one and correct it below.
A student’s summary
Carbon dioxide gets into a leaf through tiny pores. Each pore, or stoma, sits between two guard cells, which stay the same shape all the time. Inside, the carbon dioxide drifts through the tightly packed spongy layer to the palisade cells. There, light is used to turn carbon dioxide and water into glucose and oxygen. The oxygen then leaves the leaf by travelling down the xylem to the roots.
1. Write the three corrections.
Investigation: pond weed and light
A class counted the gas bubbles (mostly oxygen) rising from a cut sprig of pond weed (elodea) in one minute, three times at each lamp distance and three times in the dark. A tank of water between lamp and weed stopped the lamp heating it.
| Lamp distance | Count 1 | Count 2 | Count 3 | Mean |
|---|---|---|---|---|
| 10 cm | 38 | 41 | 38 | 39 |
| 20 cm | 27 | 25 | 29 | 27 |
| 30 cm | 16 | 18 | 14 | |
| 40 cm | 9 | 11 | 7 | 9 |
| lamp off (dark) | 0 | 0 | 0 | 0 |
Source: sample class results.
2. (a) Calculate the mean for 30 cm and write it in the table. (b) How many times as many bubbles per minute did the pond weed give off at 10 cm as at 40 cm? Give your answer to one decimal place.
(b) Answer times
3. Describe the pattern in Table 1 and explain it using photosynthesis. Then explain why there were no bubbles in the dark, even though the pond weed was still alive and respiring.
Evaluate the claim
Someone claims
“Plants make their own oxygen, so they never use any. If you seal a pot plant inside a big glass jar, it will keep the air in the jar full of fresh oxygen, day and night, for ever.”
4. Identify one part of the claim that is supported by science, using evidence from Table 1. Then identify three scientific errors in the claim and explain what really happens.
5. Plan a test of this question: Does a pot plant take in oxygen when it is kept in the dark? You have two identical sealed jars, a pot plant, an identical pot containing the same soil but no plant, a dark cupboard and an oxygen sensor that reads the oxygen level in a jar.
Prediction
Variables
What goes in the second jar, and why is it needed?
6. Scientists study a plant whose guard cells can never fill with water, so its stomata stay closed all the time. A student says: “This plant is better off than a normal plant, because it never loses water through its leaves.” Evaluate the student’s claim.
Find the errors
1. (i) Guard cells change shape: they swell with water to open the stoma and go floppy to close it. (ii) The spongy layer is loosely packed with air spaces. (iii) The oxygen leaves the leaf through the stomata into the air, not down the xylem (xylem carries water up from the roots). [3 marks: 1 per correction]
Your Turn
2. (a) (16 + 18 + 14) ÷ 3 = 48 ÷ 3 = 16 bubbles per minute. (b) 39 ÷ 9 = 4.33…, so about 4.3 times as many. [3 marks: 1 mean; 1 using the two means; 1 answer to one decimal place]
3. Closer lamp, more bubbles (39 at 10 cm down to 9 at 40 cm): brighter light, faster photosynthesis, more oxygen. In the dark there is no photosynthesis, so no oxygen is made; respiration uses oxygen rather than making it. [3 marks: pattern with data; brighter light, faster photosynthesis; dark explanation]
Evaluate the claim
4. Supported: plants make oxygen by photosynthesis in light (bubbles whenever the lamp was on). Errors, any two: “never use any” (cells respire all the time, using oxygen); “day and night” (in the dark there is no photosynthesis, so the plant takes in oxygen and the jar’s oxygen falls); “for ever” (photosynthesis also needs carbon dioxide and light). [4 marks: 1 supported part; 1 per explained error, up to 3]
5. Prediction: oxygen falls, because the plant respires but cannot photosynthesise in the dark. Independent: plant in the jar or not. Dependent: oxygen reading after a set time. Controlled (any two): jar size, cupboard, temperature, time, starting reading. Second jar: identical, sealed, containing the matching soil-only pot, same cupboard, so any difference between the jars can be pinned on the plant rather than soil organisms. [5 marks: prediction with reason; independent; dependent; two controlled; control jar with reason]
Show What You Know
6. Marking guide (points expected): 1 guard cells (swollen with water, stoma open; lose water, floppy, stoma closed); 1 correct part (closed stomata greatly cut water vapour loss); 1 carbon dioxide cannot get in and oxygen cannot get out freely; 1 photosynthesis slows or stops, so little glucose is made; 1 consequence (the plant cannot grow well); 1 clear judgement (not better off: stomata must open and close to balance gas exchange against water loss). [6 marks]
Common error to watch for: in Q3, saying the plant “stops exchanging gases” in the dark. It still respires; it just makes no oxygen.