Plants need light to grow. If they aren’t exposed to enough light they’ll get brown and withered- they’ll die. Think of light as their food source. They need enough of it to thrive and continue to grow. As a more scientific answer, plants need light to follow through with photosynthesis- the process of using sunlight to create energy.
Carbon atoms have 4 valence electrons. That allows them to make 4 covalent bonds. That means they can make really big molecules. All the organic molecules have Carbon in them.
