Does a plant need chlorophyll, light and CO₂?
How to test a leaf for starch, and the experiments that show chlorophyll, light and carbon dioxide are needed for photosynthesis, with controls. With Vietnamese subtitles. A “Check yourself” section with typed questions is below the video on the site.
Practice
- Check yourself: Does a plant need chlorophyll, light and CO₂? – Type the missing words, put the steps in order and write a short answer. No multiple choice: you have to remember it. Vietnamese help for every question.
Transcript
To show that photosynthesis happened, we test a leaf for starch. First, we boil the leaf in water. This kills the cells. Then we heat it in ethanol, in a hot water bath, because ethanol burns easily. The ethanol removes the chlorophyll.
Next, we rinse the leaf in water, to soften it. Then we add iodine solution. If starch is present, the leaf turns blue black.
Is chlorophyll needed? We use a variegated leaf. The green parts have chlorophyll. The white parts do not. After the starch test, only the green parts turn blue black. So chlorophyll is needed.
Is light needed? We cover part of a leaf with black paper, and leave the plant in the light. After the test, only the uncovered parts turn blue black. So light is needed.
Is carbon dioxide needed? We put one plant in a sealed jar with soda lime. Soda lime absorbs carbon dioxide. A second plant is in a jar without soda lime. This is the control.
After some hours in the light, we test a leaf from each plant. The plant without carbon dioxide has no starch. The control plant has starch. So carbon dioxide is needed.
Every experiment needs a control. For chlorophyll, the control is the green part. For light, it is the uncovered part. For carbon dioxide, it is the jar without soda lime. We change only one thing.
Remember. To test for starch, boil the leaf, use ethanol, rinse, and add iodine. Blue black means starch. Chlorophyll, light, and carbon dioxide are all needed for photosynthesis. And always use a control.
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