Here is a brief overview of photosynthesis, a process carried out by plants. Plants use this process to obtain food, or energy, and without it they would not be alive. Photosynthesis occurs in the chloroplast of the plant, an organelle in plant cells devoted to this process. Plants take in carbon dioxide (CO₂), water (H₂O), and light from the sun and convert it to sugar for themselves and oxygen for the environment. Other organisms, other than plants, usually need oxygen to survive and should not be taking in CO₂ because it is "dirty air". Plants take the "dirty air" and convert it to "clean air", or oxygen for other organisms. Without oxygen many organisms, like humans and animals, wouldn't survive.

This is why global warming, pollution, and deforesting are big issues in the world today. The increase in pollution, which is increased CO₂ in the air, doesn't let organisms thrive. The excess CO₂ leads to warming and many other consequences.

Answer #2 Photosynthesis is the primary source of energy for life on Earth to use, and is also the primary source of oxygen in the atmosphere. Barring a few bacteria in volcanic vents using volcanic chemical energy to survive, all life on Earth depends directly or indirectly on photosynthesis.

That is quite the question. Photosynthesis is probably the #2 most important biological reaction for life on this planet. All energy that is used by living organisms on earth derived originally from the sun and sunlight (except for deep ocean trenches). Photosynthesis is the reaction by which organisms package light energy (in the form of photons) into chemical energy, namely sugars. These sugar molecules are little packets of energy that can be moved within an organism and broken down to utilize the energy for a variety of purposes. These sugars can also be passed along food chains from one organism to another. Fruit is sweet because of the sugar molecules that were generated from photosynthesis in the fruit tree. We can eat meat and gain energy from it as the molecules in the meat came from an herbivore that digested plant material and derived energy from the photosynthetic outputs of plants. Photosynthesis is the basis of virtually every food chain on the planet.

When an organism undergoes the process of photosynthesis, it generates ATP, a chemical form of energy that is usable by the organism. However, it also has by-products, such oxygen, which is used in cellular respiration by non-plant organisms. Thus, there is a sort of "symbiotic" relationship between plants and non-plant organisms: plants use carbon dioxide to make themselves ATP and have a by-product of oxygen. Animals use oxygen to make themselves ATP and have a by-product of carbon dioxide!