Investigating photosynthesis
Introduit| English | Sens |
|---|---|
| starch/stɑːtʃ/ | A storage polysaccharide in plants, made from glucose units. |
| destarching/diːˈstɑːtʃɪŋ/ | Keeping a plant in darkness so that it uses its stored starch before a photosynthesis experiment. |
| hydrogencarbonate indicator/ˌhaɪdrədʒən ˈkɑːbənət ˈɪndɪkeɪtə/ | A solution whose colour shows whether carbon dioxide has increased or decreased relative to its starting level. |
Does a green leaf always contain newly made starch?
- A leaf may contain starch 淀粉 stored before an experiment began. Finding starch at the end does not, by itself, show that a particular treatment caused photosynthesis.
- Good evidence compares treated and control samples with similar starting conditions. Destarching 去淀粉 means keeping a plant in darkness long enough to use its stored starch, then checking a sample leaf.
Why destarch a plant before testing whether light is needed?
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Old starch could give a positive iodine result even in the covered region.
Test for starch, not directly for glucose
- In a supervised school starch test, heat a leaf in water, then use ethanol in a hot-water bath to remove chlorophyll. Rinse it in warm water to soften it, spread it on a white tile and add iodine solution.
- A blue-black result indicates starch; remaining orange-brown indicates no detectable starch. Chlorophyll removal makes the colour change visible. Ethanol is flammable: use a water bath away from flames and follow the teacher's handling instructions.
Iodine tests starch. It does not directly measure the rate of photosynthesis or detect glucose.
A decolourised leaf turns blue-black with iodine. What does this directly show?
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Iodine detects starch; it is not a direct test for glucose or photosynthetic rate.
Order the main steps in the supervised starch test.
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Removing chlorophyll allows the iodine colour to be seen. Ethanol must be kept away from flames.
Test whether light is needed
- After destarching, cover part of a leaf on both sides with opaque material and leave another part exposed. Place the plant in suitable light, then remove the cover and test the whole leaf for starch.
- Blue-black in the exposed region and orange-brown in the covered region support the need for light, with other conditions suitable. Keep the cover's position recorded so results can be matched to treatment.
A leaf is not destarched before half is covered. Both halves later test blue-black. This does not establish that the covered half photosynthesised in darkness: it may still contain old starch. Improve the starting conditions before interpreting the treatment.
A controlled test for light
Link the initial starch check, treated region and final iodine result before drawing a conclusion.
Test chlorophyll and carbon dioxide separately
- For chlorophyll, use a destarched variegated leaf with green and non-green regions. Record the regions before decolourising; expose it to light and compare the starch results. Only green regions turning blue-black support the need for chlorophyll under those conditions.
- For carbon dioxide, compare matched destarched plants in sealed transparent chambers: one with carbon dioxide absorbed and one control without an absorbent. Keep light, temperature, time and plant condition comparable; use a teacher-prepared setup for corrosive absorbents.
Why record a variegated leaf's green regions before removing chlorophyll?
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You need to match each final starch result to whether chlorophyll was initially present.
Measure a rate with controls and repeats
- An aquatic plant can release gas during illumination. Collect gas volume over a fixed time, or record the time to collect a fixed volume. Allow the plant to adjust to each treatment before measuring, and repeat readings.
- When testing light intensity, control carbon dioxide supply, temperature and plant size. Moving a lamp may also warm the water. Bubble counts are a rough proxy because bubble sizes vary; collected gas volume is usually a better measure of output.
Equal bubble counts always represent equal gas volumes.
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Bubble sizes vary, so bubble number is only a rough proxy for gas output.
An aquatic plant produces 12 cm³ of collected gas in 4 minutes. What is the mean collection rate in cm³ per minute? Enter the number only.
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12 ÷ 4 = 3 cm³ per minute. This is collected gas output, not a direct measure of every photosynthetic reaction.
Change carbon dioxide or temperature separately
- To compare carbon dioxide availability, use teacher-prepared solutions providing different carbon dioxide supplies. Keep the same light intensity, temperature, plant size and collection time; compare repeated gas-volume measurements.
- To compare temperatures, use controlled water baths and let the plant adjust before each reading. Keep light intensity and carbon dioxide supply the same. Rate may rise towards an optimum and then fall; do not assume every increase in temperature helps.
Read net carbon dioxide exchange
- Hydrogencarbonate indicator 碳酸氢盐指示剂 responds to carbon dioxide relative to its starting level: yellow indicates an increase, purple a decrease, and red/orange little change. Include an indicator-only control and equal liquid volumes.
- In sufficient light, photosynthesis may remove carbon dioxide faster than respiration adds it, giving purple. In darkness, respiration adds carbon dioxide while photosynthesis stops, giving yellow. Little colour change can mean balanced rates; it does not prove that nothing happened.
The rates are invented examples. Indicator colour reports net carbon dioxide change, not oxygen directly.
A single endpoint or colour does not identify every cause. Use the control, the initial conditions and the treatment together. A sealed apparatus must also remain suitable for the plant over the measurement period.
Match the indicator observation to net carbon dioxide change relative to the starting level.
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Interpret the colour with the initial conditions and an indicator-only control.
A plant in light causes little change in hydrogencarbonate indicator compared with a control. Does this prove that both photosynthesis and respiration stopped? Explain.
No. Photosynthesis may remove carbon dioxide at about the rate respiration releases it, giving little net change while both processes occur. Use the control and starting conditions; little colour change alone does not establish that either rate is zero.
Evaluate a lamp experiment
- A student moves a lamp closer to an aquatic plant. Bubble count rises, but water temperature also rises and no readings are repeated.
- Explain why the result cannot isolate the effect of light intensity. Propose a controlled temperature, a more reliable gas measure and repeated measurements.
A lamp is moved closer to an aquatic plant. Bubble counts rise, but the water also warms and there are no repeats. Evaluate the conclusion that light intensity alone caused the increase, and improve the method.
The conclusion does not isolate light because temperature also changed and could alter the rate. Control temperature with a monitored water bath or an effective heat filter while changing light. Collect gas volume in equal times, allow adjustment to each condition and repeat measurements to compare mean rates.
State what the evidence actually tests
- Destarch and check the starting material; iodine detects starch after the treatment.
- Test light, chlorophyll and carbon dioxide with suitable comparisons, then control other conditions.
- Gas volume per time estimates output; indicator colour shows net carbon dioxide change. Respiration continues in light as well as darkness.
Both halves of a leaf test blue-black after one half was covered, but the plant was not destarched. Which conclusion is justified?
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The starting starch condition was not established, so old starch is an alternative explanation.