Ecology

Why Are Plants Green and Not Black?

3 Mins read

Let’s ask a very simple question. Why are plants green? We will start to find the answer to this question by getting to know the plants closely. Leaves are green because they contain lots of chlorophyll to absorb sunlight and chlorophyll is green. Chlorophyll is the molecule that takes light from the sun and turns it into usable energy in the plant, photosynthesis. Photosynthesis happens in the leaves of a plant. Photosynthesis is a chemical reaction that takes place inside a plant, producing food for the plant to survive. It takes place in the part of the plant cell containing chloroplasts, these are small structures that contain chlorophyll. For photosynthesis to take place, plants need to take in carbon dioxide, water, and light. If the sun’s spectrum peaks in green and even the light that makes it down through our atmosphere peaks in green, then plants are green to absorb the greenest light.

If leaves are green, that means they are reflecting green light. They are shooing away the most common color of light from the sun.

Why Are Plants Green?

If plants wanted to absorb the most energy, would not they absorb all the colors? Would not they be black? So black pigmented plants exist but they are rare, like helium. So maybe this whole getting rid of green things has its benefits. Like all plants on the Earth are pretty much green.

So why are the plants green? Seems like something scientists should know. There are several theories on this subject.

Plants Need Sunlight

Plants absorb sunlight to make food.That’s mean more sunlight equals more food. But there is a point called saturation, where shining more light on a leaf doesn’t lead to higher output. So full sunlight on, say, a spinach leaf is 3 to 5 times the saturation level. It’s like being served 3 to 5 hamburgers when you can only fit one in your stomach. So reflecting away some green light isn’t necessarily bad. If plants always hot full sunlight, but they don’t.

Overheating

One theory goes that chlorophyll being bad at absorbing green light means that plants can absorb more light overall. Here is how that works. Chlorophyll does absorb and use some green light. But because not all green light gets absorbed at the surface of the leaf, some of it can then penetrate deeper into the leaf, like burrowing worms. And even if the top of the leaf gets saturated, and get this, 90% of blue and red light is absorbed in the upper one-fifth of a spinach leaf. So that means that chlorophyll deeper down is there to absorb the green light that made it through. As a bonus, this prevents your leaf from overheating. It’s like reheating your burger in an oven versus in a fire. You don’t want a frozen patty with a burnt bun. Plus, the lower leaves of the plant get some of the green light that is reflected and scattered, which is good for them. Because you know, if you ate all 3 or 5 hamburgers, then your family would starve.

Purple Theory

Early light guzzling life on Earth is included in archaea called halobacteria that absorbs green and reflects red and blue so it looks kind of purple. It’s what gives this lake its purple color. Covered in this stuff, Earth might have looked like this, except the plates would have been in different places 3 billion years ago. 

If halobacteria were absorbing all the green light, then another, say, green algae that were layers deeper in the ocean would have to make do without a green light.

Evolution

Why might chlorophyll be evolutionarily awesome? Maybe it is a molecule that’s easier for evolution to come up with. Maybe it’s more efficient at turning sunlight to energy than other potential photosynthesizing black or purple pigments that absorb green better.

It possible that chlorophyll was the accident we got during the complex process of evolution on Earth. Maybe with slightly different conditions, it would go differently if life on Earth went through evolution again.

As a result of all these theories, we understand that plants are green for mechanisms such as producing food and protecting themselves from overheating to work well.

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