We Peeked Inside Photosynthesis! — Hunting for Starch in Waterweed Leaves Under the Microscope
I’m Ken Kuwako, the Science Trainer. Every day is an experiment.
Textbooks tell us that plants use light to make starch. But have you ever stopped and wondered, “Does that really happen?” This time, we carried out an experiment to witness the process with our own eyes. The moment tiny granules inside the leaf suddenly turned dark under the microscope, it was impossible not to say, “Whoa!”
What You’ll Need
A microscope, Elodea (Canadian waterweed), slide glass, cover glass, tweezers, iodine solution, electric pot, and a light source.
Preparing the Experiment — “Light” Leaves vs. “Dark” Leaves
For this experiment, we prepared two different types of leaves under different conditions. The Elodea was collected about a day and a half beforehand. One group of leaves was kept in complete darkness for 1.5 days so the plants would consume as much stored starch as possible.
We placed the plants inside a shelf to block out all light.
The other group was exposed to artificial light for the same 1.5-day period so photosynthesis could continue actively. This difference — “light” versus “no light” — is the key to the entire experiment. Here is the light we used. (Plant Growth Light (Amazon))
This light is specially designed for plant photosynthesis. Some teaching guides recommend using infrared or longer-wavelength light. But why would longer wavelengths work better?
Photosynthesis actually relies on two reaction systems called Photosystem I and Photosystem II, and each absorbs slightly different wavelengths of light. Far-red light around 700 nm is said to drive Photosystem I especially efficiently.
I wonder whether this effect is unique to Elodea or true for plants in general. If anyone knows more about this, I’d love to hear your thoughts.
Boiling the Leaves — Why Boil Them?
Next, we boiled the prepared leaves in an electric pot for about five minutes. The electric pot turned out to be incredibly useful.
If you’re wondering, “Why boil the leaves?” — great question.
Boiling breaks down the leaf cells, allowing the iodine solution to penetrate more easily. It also softens the leaves, making them easier to observe under the microscope. We used two aluminum containers: one for the light-exposed leaves and one for the dark-treated leaves, so we wouldn’t mix them up during processing.
Staining with Iodine — Hunting for Starch
After boiling, we picked up a leaf with tweezers, placed it on a slide glass, and gently removed excess water with kitchen paper. Then we added iodine solution and waited about one minute. After covering the sample with a cover glass, we observed it under the microscope.
Iodine reacts with starch and turns blue-purple to black. This famous reaction is called the iodine-starch reaction. In other words, wherever the sample turns dark, starch is present.
First, we observed the leaf without adding iodine solution.
Zooming in closer…
The structures believed to be chloroplasts were not stained.
Next, we observed a leaf treated with iodine solution. This sample had been kept in the dark.
Maybe there’s a slight amount of staining?
Now for the leaves that stayed under the light.
Magnified view:
The black-stained structures are chloroplasts inside the leaf cells.
That’s right — the true “worksite” of photosynthesis is inside these chloroplasts. Let’s compare the results side by side.
You can clearly see a difference in how strongly the chloroplasts are stained. Chloroplasts are tiny green structures found only in plant cells. Inside them, light energy is used to turn water and carbon dioxide into starch (sugars) and oxygen.
In this experiment, only the chloroplasts from the light-exposed leaves turned black with iodine solution, while the leaves kept in darkness showed little to no staining. This gave us direct visual evidence that photosynthesis truly takes place inside chloroplasts.
Those tiny black dots seen through the microscope may look small, but they are producing the energy source used by nearly every living thing on Earth — powered by sunlight. Even the starch in the rice we eat originally came from photosynthesis inside plant chloroplasts.
Chloroplasts are like microscopic factories supporting life on our planet.
If you ever get the chance, take a look at the “scene of photosynthesis” through a microscope yourself.
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キーンと冷えるドライアイス！気温が上がってくるこの時期・ドライアイスを使った昇華・凝結・等速度直線運動の実験はいかが？
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