The Earth's atmosphere is mostly composed of nitrogen. Oxygen makes up just 21 percent of the air we breathe. Carbon dioxide, argon, ozone, water vapor and other gasses make up a tiny portion of it, as little as 1 percent. These gasses probably came from several processes as the Earth evolved and grew as a planet.
But some scientists believe that the Earth's atmosphere would never have contained the oxygen we need without plants. Plants (and some bacteria) release oxygen during photosynthesis, the process they use to change water and carbon dioxide into sugar they can use for food.
Photosynthesis is a complex reaction. In a lot of ways, it's similar to the way your body breaks down food into fuel that it can store. Essentially, using energy from the sun, a plant can transform carbon dioxide and water into glucose and oxygen. In chemical terms:
6CO2 + 12H2O + Light -> C6H12O6 + 6O2+ 6H2O
In other words, while we inhale oxygen and exhale carbon dioxide, plants inhale carbon dioxide and exhale oxygen. Some scientists believe that our atmosphere had little to no oxygen before plants evolved and started releasing it.
Without the sun to feed plants (and the plants to release oxygen), we might not have breathable air. Without plants to feed us and the animals most people use for food, we'd also have nothing to eat.
Obviously, plants are important, but not just because they give us food to eat and oxygen to breathe. Plants help control the amount of carbon dioxide, a greenhouse gas, in the atmosphere. They protect the soil from wind and from water runoff, helping to control erosion. In addition, they release water into the air during photosynthesis. This water, along with the rest of the water on the planet, takes part in a huge cycle that the sun controls. We'll look at this cycle on the next page.