How is energy transformed in photosynthesis? A chemical reaction fueled by the energy of the sun, photosynthesis converts carbon dioxide, water and sunlight into glucose and oxygen. Autotrophs, organisms like plants that make their own food, use photosynthesis to fuel growth, reproduction and repair of their cells. Photosynthesis takes place in two stages: the light-dependent reactions and the light independent-reactions.

Because plants can't use the energy from the sun directly to produce glucose, sunlight is converted into chemical energy through the production of adenosine triphosphate (ATP) and nicotinamide adenine dinucleotide phosphate hydrogen (NADPH). During the light-dependent reactions of photosynthesis, ATP and NADPH are created; these are energy-carrying molecules. The chemical energy stored in ATP and NADPH is then used during the light-independent stage of photosynthesis, which is also known as the Calvin cycle, to produce carbohydrates.

Essentially, energy from the sun is stored in ATP and NADPH during the light-dependent reactions of photosynthesis. Notably, these two energy-carrying molecules are used not only in plants for photosynthesis – they are essential molecules for storing chemical energy in animals, too.

In the light-dependent phase of photosynthesis, plants absorb sunlight using a special pigment called chlorophyll. Chlorophyll, the green pigment that gives plants their color, is located within chloroplasts. Chloroplasts are organelles in plant cells that are essential to the process of photosynthesis.

Chlorophyll captures energy from sunlight by energizing electrons, and the energy from these energized electrons is used to produce NADPH. ATP is also created during the light-dependent phase of photosynthesis, using energy created by a flow of hydrogen ions.

The second part of photosynthesis, the light-independent stage, is also known as the Calvin cycle. The Calvin cycle also takes place within the chloroplasts of plant cells. There are three stages within the Calvin cycle, consisting of a series of chemical reactions.

In general terms, the chemical energy from the ATP and NADPH created in the light-dependent reactions of photosynthesis is used to create carbohydrates during the Calvin cycle. Through a series of complex reactions, carbohydrates and oxygen are made from carbon dioxide with the energy from ATP and NADPH.

ATP and NADPH are molecules that can be thought of as "full" of energy. As this energy is used during the Calvin cycle, these energy-carrying molecules are converted into their "empty" forms: ADP and NADP. These "full" and "empty" energy-carrying molecules are cycled back and forth between the light-dependent and light-independent phases of photosynthesis, providing the fuel needed to convert carbon dioxide and water into carbohydrates and oxygen.

Autotrophs like trees and algae form the foundation of all food webs on the planet. Even carnivores, animals that eat other animals, ultimately depend on the organic materials produced by autotrophs – for example, wolves eat rabbits, and rabbits are herbivores that live on plants.

Therefore, the energy-carrying molecules used in the process of photosynthesis, ATP and NADPH, are essential to all life on Earth. ATP and NADPH do not last long – some estimate their lifespans to be in the millionths of seconds – but the carbon and carbohydrates produced by their energy last for hundreds of millions of years, in the form of plants and animals.

Not only do plants use the energy from ATP and NADPH to fuel their own growth, they also produce oxygen in the process. Most of the oxygen in our atmosphere is a product of photosynthesis – so, animals depend on plants for both food and oxygen.

The energy transformations that take place during photosynthesis are critical to making the planet inhabitable, and the energy-carrying molecules ATP and NADPH make it all possible.