Year 8 Science · Unit 1 · Lesson 21
Challenge Worksheet
Learning Goals
Find the errors
This paragraph contains exactly three errors. Underline each one and write the correction below.
A student’s summary
In an ecosystem, both energy and matter go around in cycles. Plants take in carbon dioxide during respiration, and animals give it out during photosynthesis. When a wombat dies, fungi and bacteria break its body down and nutrients go back into the soil. The carbon cycle is only about carbon dioxide gas in the air.
1. Write the three corrections.
Investigation: bags on a branch
Students tied a clear plastic bag over a leafy branch of a young eucalypt in full sun, and another over a branch of the same size on the same tree in deep shade. Each branch had 40 leaves. After 4 hours they poured the water from each bag into a measuring cylinder.
| Branch | Number of leaves | Water collected (mL) |
|---|---|---|
| In full sun | 40 | 12.0 |
| In deep shade | 40 | 4.8 |
Source: sample class results.
2. Each branch had 40 leaves. Work out the volume collected per leaf on the sunny branch, then how many times more water came from the sunny branch than the shady one.
Each leaf mL Sunny ÷ shady
3. Name the process that put the water in the bags. If the bags were removed, describe the next two stages of the water cycle this water would go through.
4. A classmate argues that the water did not come from the leaves at all: it was just moisture from the air trapped inside the bag. Describe one extra bag the students could set up to test this, and state what result would show the water came from the plant.
Compare two ways back to the soil
Two fallen logs
Two identical fallen logs lie in the same forest. Log X is slowly broken down by fungi and bacteria. Log Y is burnt in a bushfire.
5. Both logs end up returning nutrients to the soil. Compare the two routes: how fast the nutrients return, and one risk to the nutrients from Log Y that Log X does not face.
Someone claims
“If we plant more trees, we can solve climate change on our own.”
Fact file
Trees take in carbon dioxide during photosynthesis and store the carbon in wood and leaves. When a tree dies and rots, or burns, most of that carbon returns to the air as carbon dioxide. Burning coal, oil and gas releases carbon that was stored underground in rocks for millions of years.
6. Use Claim, Evidence and Reasoning to evaluate the claim. Use the carbon cycle and the fact file.
Find the errors
1. (i) Energy does not cycle: it flows one way through an ecosystem, while matter is recycled. (ii) The processes are swapped: plants take in carbon dioxide during photosynthesis, and living things give it out during respiration. (iii) The carbon cycle includes carbon in plants, animals, soil, oceans and rocks; carbon dioxide is only one form. (The wombat sentence is correct.) [3 marks: 1 per correction]
Your Turn
2. 12.0 ÷ 40 = 0.30 mL per leaf (in 4 hours). 12.0 ÷ 4.8 = 2.5 times more. [3 marks: 1 per-leaf value with unit, 1 division set up, 1 answer]
3. Transpiration. Next, the water vapour rises and cools and condenses into cloud droplets (condensation); then, when the droplets are heavy enough, it falls as rain (precipitation). [3 marks: 1 each]
4. Tie an identical bag over something with no leaves (a leafless stick, or an empty sealed bag) in the same sunny spot for 4 hours. If that bag collects little or no water while the leafy bag collects 12.0 mL, the water came from the plant. [3 marks: 1 control bag, 1 kept the same (place, time, bag), 1 result that supports the plant]
Compare two ways back to the soil
5. Log X: decomposers release the nutrients slowly, over years. Log Y: fire releases them almost at once as ash on the topsoil. Risk for Log Y: rain can wash the ash nutrients away before plants can use them, especially if fires come too often or burn too hot. [3 marks: 1 slow, 1 fast, 1 risk]
Show What You Know
6. Marking guide: 1 clear claim (partly agree or disagree is best supported); 1 trees remove carbon dioxide in photosynthesis and store carbon; 1 that carbon returns to the air when trees die, rot or burn, so the store is not permanent; 1 burning coal, oil and gas adds carbon from rocks that trees did not take out; 2 reasoning that links the evidence to the claim (planting trees helps but cannot balance the extra carbon dioxide on its own, so we must also reduce burning of fuels). Model answer: I partly agree. Trees take in carbon dioxide and lock carbon in wood, but when they die, rot or burn most of it returns to the air. Burning coal, oil and gas adds carbon stored in rocks for millions of years, which trees never took out. So planting trees helps but cannot solve climate change alone; we must also release less carbon dioxide. [6 marks]
Common error to watch for: treating a tree as a permanent carbon store. The carbon in wood is part of the cycle and goes back to the air when the tree rots or burns.