Year 8 Science · Unit 1 · Lesson 8
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Learning Goals
Match each leaf part to its job
Write the matching letter in the middle column. Each letter is used once.
| Term | Letter | Meaning |
|---|---|---|
| Stomata | A. Gases moving between a living thing and its environment. | |
| Guard cells | B. Tightly packed cells near the top, where most photosynthesis happens. | |
| Spongy layer | C. Tiny pores, mostly on the lower surface, for gases. | |
| Palisade layer | D. Loose cells with air spaces that let gases spread through the leaf. | |
| Gas exchange | E. A pair of cells that change shape to open or close a pore. |
Odd one out
Circle the odd one out in each row and say why it does not belong.
| Items | Odd one out, and why |
|---|---|
| 1. oxygen · glucose · carbon dioxide · water vapour | |
| 2. palisade cell · guard cell · red blood cell · spongy cell | |
| 3. less water lost · guard cells turgid · stoma open · CO₂ enters |
Gas exchange or transport?
The lesson says gas exchange and transport are different processes. Decide which one each event is, and give a reason.
| Event in a plant | Gas exchange or transport? Why? |
|---|---|
| Carbon dioxide passes into a leaf through an open stoma. | |
| Water travels up the stem inside the xylem. | |
| Sugar made in a leaf moves down to the roots. | |
| Oxygen made in the palisade cells leaves the leaf into the air. |
Cause and effect: light on a leaf
Complete the chain. Each box is caused by the box before it.
1. A leaf takes part in both transport and gas exchange. Name one substance each process moves, and one leaf structure each process depends on.
Scenario: a gum leaf over one summer day
A scientist clipped a small clear chamber over one leaf of a young eucalypt (gum) tree. A sensor measured how much carbon dioxide the leaf took in or gave out each hour. A plus sign (+) means the leaf took in carbon dioxide overall. A minus sign (−) means it gave out carbon dioxide overall.
| Time | Light | Carbon dioxide (mg per hour) |
|---|---|---|
| 2 am | dark | −1 |
| 9 am | morning sun | +6 |
| 12 pm | full sun | +12 |
| 4 pm | afternoon sun | +8 |
| 10 pm | dark | −1 |
Source: sample data for one leaf.
2. At which two times did the leaf give out carbon dioxide overall? Which process was the only one making gas exchange happen at those times?
3. How much more carbon dioxide per hour did the leaf take in at 12 pm than at 9 am? Then suggest a reason for the difference.
Answer mg per hour
4. A student looks at the 12 pm result and says: “At midday the leaf has stopped respiring, because it is only taking carbon dioxide in.” Use the table to explain why the student is wrong.
5. A leaf has no lungs, yet it is very good at gas exchange. Explain how the structure of a leaf suits it to gas exchange. Refer to its broad, flat shape, the stomata and guard cells, and the spongy layer, and say how each feature helps.
Match each leaf part to its job
Stomata → C; Guard cells → E; Spongy layer → D; Palisade layer → B; Gas exchange → A.
Odd one out
1. glucose (a sugar, not a gas passing through stomata). 2. red blood cell (animal cell; the others are leaf cells). 3. less water lost (a closed stoma; the others describe an open one).
Gas exchange or transport?
Carbon dioxide in and oxygen out: gas exchange (a gas moves between plant and air). Water up the xylem and sugar to the roots: transport (substances move within the plant).
Cause and effect: light on a leaf
Box 2: palisade cells photosynthesise, using carbon dioxide and making oxygen. Box 3: carbon dioxide in the air spaces runs low; oxygen builds up.
1. Transport: water (or sugar), using the xylem (or phloem) in the veins. Gas exchange: carbon dioxide or oxygen, using the stomata (or air spaces). [2 marks: 1 per process, substance and structure both needed]
Go Deeper
2. 2 am and 10 pm. Respiration (there is no light, so no photosynthesis). [2 marks]
3. 12 − 6 = 6 mg per hour more. The light is brighter at midday, so the leaf photosynthesises faster and uses more carbon dioxide. [2 marks: 1 calculation, 1 reason]
4. The night results (−1) show the leaf respires, giving out carbon dioxide, and respiration never stops, so it still happens at midday. In bright light photosynthesis uses carbon dioxide faster than respiration makes it, so the overall result is carbon dioxide in. [3 marks: night data shows respiration; respiration never stops; photosynthesis faster than respiration]
Show What You Know
5. Marking guide (points expected): 1 broad, flat shape gives a large surface exposed to the air; 1 stomata are pores that let gases in and out; 1 guard cells open and close them to control exchange (and limit water loss); 1 spongy air spaces let gases spread to the cells; 1 carbon dioxide in, oxygen out in the light. [5 marks]
Common error to watch for: reading a minus value as “no gas exchange”. The leaf is still exchanging gases, giving out carbon dioxide from respiration.