Raven holds a leaf from the garden flat against the window. Rocket studies it through the hand lens.
"Top side is darker green," he says. "Bottom side is paler. And there are tiny dots on the bottom."
"What do you notice about the green?" Raven asks. "It is not paint. It is inside the cells."
Nova hovers beside the leaf, her light shining through it. "The green is a pigment," she says. "What does a pigment do?"
"Colors things," Rocket says. Then, slower, "Catches light. That is why the leaf is dark on top. More light lands there."
Raven writes three words under the sketch: chloroplast, chlorophyll, stomata.
"The dots on the bottom are doors," she says. "Air goes in and out."
In plants, photosynthesis takes place primarily in leaves. A leaf consists of many layers of cells with different top and bottom sides.
The process does not happen in the surface layers. It happens in a middle layer called the mesophyll.
Carbon dioxide gets in and oxygen gets out through small, regulated openings called stomata. Those are the dots Rocket saw.
On a hot, dry day, plants close their stomata to conserve water. With the doors closed, less carbon dioxide can get in.
Photosynthesis takes place inside an organelle called a chloroplast. In plants, cells with chloroplasts fill the mesophyll.
Inside the chloroplast are molecules of chlorophyll, a pigment. A pigment is a molecule that absorbs light.
Chlorophyll is responsible for the green color of plants. Other pigments help, but chlorophyll is by far the most important.
You met chloroplasts in week 4 as a part plant cells have and animal cells do not. Now you know their job.
Plants use sunlight and carbon from carbon dioxide to build carbohydrates. So the carbon in a plant's sugar came out of the air.
Carbon dioxide is called CO2 because it has two oxygen atoms attached to one carbon atom. Plants take the carbon and release oxygen.
All living things on Earth contain carbon, including you. When you breathe out, you exhale carbon dioxide back into the air.
So matter cycles. Carbon moves from the air into plants, into the animals that eat them, and back to the air.
Energy does not cycle. It flows in from sunlight, is stored in sugar, and is released when an organism breaks down food.
| Statement | True or false? |
|---|---|
| Photosynthesis happens mostly on the surface layers of a leaf. | ? |
| Plants close their stomata on a hot, dry day to save water. | ? |
| Energy cycles around and around like carbon does. | ? |
| You exhale carbon dioxide. | ? |
Clear thinking. Tomorrow is Windowsill Lab: you will cover one leaf and start seed jars in light and dark.