Biology 5090/31 — October/November 2011
Cambridge O-Level · Practical Test · worked solutions for every part, with the mark scheme
Topics Experimental Contexts · Observations and Measurements · Use of Techniques, Apparatus and Materials · Analysis, Conclusions and Evaluation · Planning Experiments and Investigations · Microscopy and Biological Drawing
You are required to investigate factors affecting photosynthesis in leaf discs.
You are provided with three samples.
A fresh, green leaf, labelled D1.
Two decolourised leaf discs, labelled D2 and D3.
- Using forceps, pick up D1 and hold just below the surface of the hot water in the beaker for about 10 seconds.
Record and explain your observations.
observations = ______
explanation = ______
Answer
Observation: bubbles of gas escape from the surface of the leaf.
Explanation: the hot water warms the air/gases trapped in the air spaces inside the leaf. The gases expand and escape through the stomata. The leaf also expands/swells slightly.
Bubbles of gas escape from the leaf surface through the stomata; the air in the leaf spaces expands when heated and the leaf expands.
Walkthrough
This is a preliminary observation before the starch test. You hold the fresh green leaf D1 just below the surface of hot water for about 10 seconds. The hot water warms the leaf, and the air trapped in the spaces between the cells inside the leaf (the intercellular air spaces) expands as it warms. Because the gas expands, it is pushed out of the leaf through the stomata — the tiny pores on the leaf surface — and you see bubbles escaping from the surface of the leaf. The leaf may also appear to swell or expand slightly because the trapped gas takes up more space.
The four marks are: bubbles escape from the surface (1), reference to the stomata (1), the air/gases in the leaf spaces (1), and the leaf expands (1). The mark scheme underlines "stomata" and "expands" — these exact words must appear.
Key Takeaways
- Leaves contain intercellular air spaces filled with air, which is used for gas exchange.
- Stomata are the pores through which gases enter and leave the leaf.
- Gases expand when heated.
- Heating a leaf makes the trapped air expand and escape as bubbles through the stomata.
Common Mistakes
- Saying the bubbles are oxygen from photosynthesis — wrong. At this stage the leaf is merely being heated; the bubbles are expanding trapped air, not a product of photosynthesis.
- Forgetting to mention the stomata — the mark scheme requires "ref. stomata".
- Saying the leaf "boils" or "steams" — the credited observation is bubbles of gas escaping.
Things to Be Careful About
- The mark scheme wants "bubbles on surface/escape" — say bubbles escape from the surface.
- Use the exact words "stomata" and "expands".
- The gases are in the "air spaces / intercellular spaces" of the leaf — use that term.
- This is a candidate-dependent observation on Paper 3, but the expected observation and explanation are as given.
- Using forceps, pick up and hold D2 in the hot water for about 10 seconds, then place it on the white tile.
- Cover D2 with iodine solution.
- Observe any changes in the disc over the next two minutes.
(i) Record your observations in Table 1.1 and state your conclusion.
- Repeat this procedure with D3.
(ii) Record your observations in Table 1.1 and state your conclusion.
Table 1.1
| D2 | D3 | |
|---|---|---|
| observation | ||
| conclusion |
[4]
Suggest why D2 and D3 were dipped in hot water.
______
Answer
To kill the cells / stop enzyme activity and make the cell membranes more permeable, so that the iodine solution can penetrate the leaf tissue.
To kill the cells and make the cell membranes more permeable so iodine can penetrate.
Walkthrough
Before the iodine test, the leaf discs are dipped in hot water. The hot water kills the cells (denaturing the enzymes and other proteins) and makes the cell membranes more permeable. This means the iodine solution can penetrate the leaf tissue and reach the starch stored inside the cells. Without this step, the iodine would not get in easily and the test would not work properly.
Key Takeaways
- Hot water kills cells and denatures enzymes.
- Heating increases the permeability of cell membranes.
- Iodine must be able to penetrate the leaf to reach the starch.
Common Mistakes
- Saying "to remove chlorophyll" — that is the job of ethanol, not hot water.
- Giving only "to soften the leaf" without the permeability or killing-cells reason.
Things to Be Careful About
- One mark: give the key reason — killing the cells / increasing permeability so iodine can penetrate.
The two leaves from which D2 and D3 were taken had been treated differently.
No chemicals were used.
Suggest how their treatment differed.
______
Answer
One plant was kept in the light and the other was kept in the dark (covered / in a container).
One plant kept in the light, the other kept in the dark.
Walkthrough
The two leaves were treated differently without any chemicals. The classic way to test whether light is needed for photosynthesis is to keep one plant in the light and one in the dark (or cover part of a leaf). The leaf kept in the light could photosynthesise and make starch; the leaf kept in the dark could not. So the treatment differed in the amount of light: one plant was kept in the light, the other in the dark (covered or in a container).
Key Takeaways
- Light is needed for photosynthesis.
- The starch test shows whether photosynthesis has occurred.
- Keeping a plant in the dark is the standard way to remove (destarch) it.
Common Mistakes
- Saying "one was watered and the other wasn't" — the classic difference is light, not water.
- Saying "one was in the sun and one in the shade" — the mark scheme wants "plant in dark / covered / in container".
Things to Be Careful About
- The mark scheme: "Ref. to light difference; Plant in dark/covered/in container". Mention both the light difference and the dark treatment.
Explain the observations and conclusions you made in Table 1.1.
Answer
D2's leaf came from a plant kept in the light: photosynthesis occurred, producing glucose which was converted to starch and stored in the leaf, so the iodine turned blue-black (starch present).
D3's leaf came from a plant kept in the dark: without light no photosynthesis occurred, so no new starch was made. Any starch already present was used up in respiration or translocated to other parts of the plant, so no starch remained and the iodine stayed brown/yellow.
D2 (blue-black) from a light-treated plant: photosynthesis made starch. D3 (brown/yellow) from a dark-treated plant: no photosynthesis, and existing starch used up in respiration or translocated away.
Walkthrough
The table shows D2 turned blue-black with iodine, so starch was present; D3 stayed the iodine's own brown/yellow colour, so no starch was present.
The leaf that had starch (D2) came from a plant kept in the light. In the light, photosynthesis occurred:
The glucose produced was converted to starch and stored in the leaf. When iodine is added, starch turns blue-black, so the disc went black.
The leaf with no starch (D3) came from a plant kept in the dark. Without light, photosynthesis could not occur, so no new starch was made. Any starch that had been present was used up: the plant respired, breaking down the starch/glucose to release energy, and/or the starch (as glucose) was translocated to other parts of the plant. So no starch remained and the iodine stayed its normal brown/yellow colour.
The four marks come from: the light/dark difference between the two plants (1), no starch remaining in the dark leaf (1), the explanation via photosynthesis (1), and the fate of the starch — respiration (1) and translocation (1); any four score.
Key Takeaways
- Iodine turns blue-black in the presence of starch.
- Light is needed for photosynthesis to make starch.
- In the dark, plants respire and use up stored starch/glucose.
- Starch/glucose can be translocated to other parts of the plant.
Common Mistakes
- Confusing which disc is which — D2 has starch (black), D3 does not (brown/yellow).
- Saying the brown colour means "no reaction" without explaining why no starch is present.
- Forgetting to mention respiration or translocation as the fate of the starch in the dark leaf.
Things to Be Careful About
- Use the exact terms "photosynthesis", "respiration" and "translocation" — they are the credited points.
- Explain the light/dark difference clearly.
- The mark scheme lists five points for four marks, so any four score — cover the main ones.
Outline the stages in the process by which the decolourised discs were prepared. Give practical details and explain the reason for each stage.
Answer
- Cut discs from the leaf using a cork borer.
- Dip the discs briefly in boiling water — to kill the cells / stop enzyme activity and increase the permeability of the cell membranes.
- Heat the discs in ethanol — to remove the chlorophyll (green colour).
- Heat the ethanol in a water bath, not directly over a flame, because ethanol is flammable (risk of fire).
Cut discs with a cork borer; dip briefly in boiling water to kill cells and increase permeability; heat in ethanol to remove chlorophyll; use a water bath for safety as ethanol is flammable.
Walkthrough
To prepare decolourised leaf discs for the starch test:
- Cut discs from the leaf using a cork borer, so all the discs are the same size and shape.
- Dip the discs briefly in boiling water. This kills the cells, stops enzyme activity and increases the permeability of the cell membranes, so that reagents such as iodine can penetrate the tissue.
- Heat the discs in ethanol. Ethanol dissolves and removes the chlorophyll (the green pigment), so the leaf becomes colourless. This is essential because the green colour would mask the colour change of the iodine test.
- Heat the ethanol in a water bath rather than directly over a flame, because ethanol is highly flammable — heating it directly is a serious fire risk.
The five marks come from: cutting the discs (1), dipping in boiling water (1), killing the cells / increasing permeability (1), heating in ethanol (1), removing the chlorophyll (1), using a water bath (1), and the safety reason — risk of fire (1); any five score.
Key Takeaways
- A leaf must be decolourised before the iodine test so the colour change is visible.
- Boiling water kills cells and increases membrane permeability.
- Ethanol removes chlorophyll.
- Ethanol is flammable, so a water bath is used for safety.
Common Mistakes
- Saying ethanol is used to "kill the cells" — that is the job of the boiling water.
- Missing the safety reason for the water bath — it is a credited point.
- Saying "boil the leaf in ethanol" without mentioning the water bath — that is a fire hazard.
Things to Be Careful About
- The mark scheme wants both "ref. use of water-bath" and "for safety/risk of fire".
- "To remove chlorophyll/colour" is the reason for the ethanol — state it.
- "Killing cells / stop enzyme activity / increase permeability" is the reason for the boiling water — state it.
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