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Why Is Our Planet Green? Photosynthesis!

By Olivia Wedig

Satellite image of Earth. From NPR.
Satellite image of Earth.

Satellite images of the Earth from space show a planet of blue and green: vast oceans and continents covered by dense forests, lush jungles, and grassy plains. Green, the signature hue of many of Earth’s plants, has even become the official color of movements to make human activities more environmentally friendly and sustainable. But why are plants green in the first place? Why have so many different species all over the planet evolve to be the same color? In order to answer this question, we must look inside plant cells. It turns out that the green color comes from molecules involved in photosynthesis, a process that allows plants to convert sunlight into fuel.

Prism separating white light into different colors. From Wikipedia.
Prism separating white light into different colors.

While sunlight may look white or yellow, it is really composed of light of many different colors. In fact, if you shine sunlight through a glass prism, it will separate out the individual colored beams that add together to give the pale color you end up seeing. Not all colors are present in equal amounts, though. Certain colors are much more abundant than others. So how do plants absorb this light? It turns out that certain molecules within plant cells are specifically designed for this purpose. But most of these molecules, called pigment molecules, only absorb a narrow range of colors. Since plants want to harvest light as efficiently as possible, their pigment molecules absorb the colors that are naturally the most common in sunlight. The pigment molecule found in green plants is called chlorophyll. Since chlorophyll takes in mostly red and blue light but causes green light to bounce off of it, plants look green to the human eye. Plants that grow underwater, such as red algae and cyanobacteria, have different pigment molecules and thus are different colors. This is because only certain colors of light can travel very far through the water.

Many tasks that a plant cell must carry out require energy. Just like a car needs fuel to run, the cell needs fuel to do things such as making proteins. Photosynthesis uses the energy that chlorophyll obtains from sunlight to start a long chain of reactions that ultimately create molecules called carbohydrates and sugars. These molecules store the energy until it is needed by the cell. Then they are broken down, releasing the energy again so that it can be used. This process highlights a key feature of energy: it is conserved. This means that energy can never be created or destroyed, it can only change from one form to another. For example, in photosynthesis, the energy starts off as light energy and then converts to chemical potential energy. When you eat plants, your body breaks down the chemicals, and you eventually convert that energy into mechanical energy when you move.

Process of Photosynthesis. From Organic Earth Tech.
Process of Photosynthesis.

So, all of the green covering Earth’s surface is really due to the color of sunlight and the way that plants absorb and reflect the sun’s rays. Their process is incredibly efficient, and scientists are currently working hard to learn how we can adapt their techniques in order to improve our own methods of generating energy from sunlight. That way, we can keep our planet’s beautiful green for as long as possible.



Editor: Andrew Zheng

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