Definition
Chlorophyll is the green pigment inside the leaf and stem cells of plants, green algae, and similar organisms. It catches the energy of sunlight and uses it to turn water and carbon dioxide into food like glucose, which makes it the key player in photosynthesis.
The Green Antenna That Catches Sunlight
Plants spread their wide leaves in every direction to catch as much sunlight as they can. When sunlight hits the surface of a leaf, chlorophyll goes to work. It sits inside tiny cell parts called chloroplasts, and it acts like an antenna tuned to light.
The white sunlight we see actually mixes every rainbow color: red, orange, yellow, green, blue, indigo, and violet. Out of all these wavelengths, chlorophyll mainly absorbs blue light and red light. The light energy it absorbs becomes the power plants use to cook their own meals.
With the light energy chlorophyll catches, a plant splits the water molecules it drew up from its roots. The hydrogen (electrons) from that water goes through several steps and combines with carbon dioxide to become glucose. The leftover oxygen is released into the air, helping many living things breathe.
The Real Reason Plants Look Green
So why do plants full of chlorophyll look green to us? Chlorophyll absorbs green light less than other colors, so a lot of green light leaves the leaf.
The color we see in an object is the color of the light it reflects instead of absorbing. Chlorophyll soaks up blue and red light very well, but it absorbs green light less. That doesn't mean plants ignore green light, though. Green light that travels deep into a leaf is largely absorbed too and used for photosynthesis. The green light that bounces back out of the leaf is what reaches the retina of your eye, and that's why leaves look so fresh and green.
If chlorophyll absorbed every color of the rainbow, leaves would look pitch black. If it reflected all the light and used none, they would look bright white. Because chlorophyll absorbs blue and red light especially well and green light less, forests around the world are full of green.
Where Does Chlorophyll Go in the Fall?
When fall arrives, green mountains and fields turn beautiful shades of red and yellow. This happens because, as the weather cools and sunlight fades, the chlorophyll in the leaves breaks down and disappears.
Leaves have always held other pigments besides chlorophyll, such as yellow carotenoids. In spring and summer, there was so much chlorophyll that the yellow pigments were completely hidden under the green. We just couldn't see them.
To be a bit more precise, plants get ready to drop their leaves to survive the cold winter. To recover the valuable nutrients still in the leaves, they break down the proteins in the leaves and move nutrients such as nitrogen into the stem. Chlorophyll breaks down along with them. Once the green curtain of chlorophyll is gone, the yellow and orange pigments hiding behind it finally show up.
Common misconceptions
All the pigments in fall leaves are newly made as the leaves change color.
The pigments that make leaves yellow (carotenoids) were in the leaf all through spring and summer. They were hidden by all that green chlorophyll, and they only show up once the chlorophyll breaks down. The red pigments (anthocyanins), on the other hand, are newly made in fall from the sugar in the leaf.
Where you meet it
If you put a houseplant by a sunny window, its chlorophyll absorbs the light and the leaves grow a deep, healthy green.
Ginkgo leaves turn yellow in fall because, as the tree pulls nutrients back into its stem, the green chlorophyll breaks down and the hidden yellow pigment shows up.
In one sentence
Chlorophyll is the pigment plants use to absorb blue and red light and make food. Because it absorbs green light less and reflects it, plants look green to our eyes.