Why Do Plants Need Sunlight?

Plants are autotrophs, meaning that they make their own food, sustaining life without eating other organisms. The way they survive is through a process called photosynthesis.

Significance
Photosynthesis is the process by which plants convert light energy into chemical energy. It takes place within the chlorophyll inside of chloroplasts. The process involves two steps, first the light reactions and then the calvin cycle. Two photosystems are involved in the light reactions:
1. Water-Splitting Photosystem--Extracts electrons from water and releases oxygen into the atmosphere.
2. NADPH Photosystem--Moves electrons from the chlorophyll to NADP, producing NADPH.
The two photosystems release energy to the chloroplast, which then uses it to drive cell processes essential for the plant's survival.

Function

Chloroplasts, which are highly concentrated inside the green tissue of plant leaves, are the sites of photosynthesis. Inside of its double-layered membrane, is a thick fluid called stroma, and inside of it is an apparatus of stacked green sacs(grana). It is within the thylakoid membrane of the sacs that the chloroplast molecules capture light energy. Energy from light is used to produce ATP and NADPH. ATP(adenosine triphosphate) is the molecule that supplies cells with energy to do work. NADPH is an electron carrier needed in the calvin cycle; it turns carbon dioxide (CO2) into a high-energy sugar, glyceraldehyde 3 phosphate or G3P. G3P is used by cells to make glucose and other organic molecules it needs.

Types
There are three types of chlorophyll pigments: chlorophyll a, chlorophyll b and carotenoids.
Chlorophyll a--Primarily absorbs red and blue-violet light. It is directly involved in the light reactions of photosynthesis. It is green, because it reflects green light.
Chlorophyll b--Although not directly involved in the light reactions, it expands the kind of light a plant can use. It takes the energy from that light and turns it over to chlorophyll a, which then can use it directly in the light reactions.
Carotenoids--The group of yellow-orange pigments also held inside of chloroplasts, absorbs blue-green light rays. It may pass some of the energy to chlorophyll a. Some carotenoids have a protective mechanism. They absorb harmful, excessive light so that the chlorophyll is not damaged.

Benefits
Sunlight is essential for plant survival. Not only do plants supply the earth with the oxygen we need to live, but they also serve as food for the entire world, and that includes the animals, too.

Considerations
Besides providing food and oxygen, plants are also used in manufacturing many products, including: cosmetics, medicine, clothing, paper, paint, fuel, lighting, charcoal and adhesives

Source: http://www.ehow.com/about_4598110_why-do-plants-need-sunlight.html