Why, because plants do photosynthesis. Photo what? Photosynthesis, that’s what.
So what is this photosynthesis thing? Well, it is nature’s solution to the energy and carbon drain from our biosphere.
During photosynthesis, energy from the sun is transformed into chemical energy in the form of ATP and NADPH. Then this chemical energy is used to fix carbon atoms from carbon dioxide into large organic molecules like carbohydrates. These carbohydrates are then available to other organisms like us for our own energy needs.
We can see that light energy from the sun is needed in photosynthesis. The first stage (conversion of light energy to chemical energy) in photosynthesis is a light-dependent process. The second stage (fixing of carbon into carbohydrates) is light-independent so it can occur even in the absence of light. However, it still needs the products from the light-dependent process.
In order to make use of light energy, a family of green pigment molecules called chlorophyll serves as the trap for light energy. Thus, the only organisms that can make use of light energy are those that have chlorophyll, like the plants, some bacteria and some protists. Plants store chlorophyll in their chloroplasts.
Even if we stand under the sun all day, we will never be able to do photosynthesis and make our own energy molecules. All that we will be able to do is get sunburned. Why, because we do not have chlorophyll like the plants.
This is the reason why we sometimes see some posters that asks the question, “Have you thank a green plant today?”
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