Photosynthesis is the process by which plants absorb the Sun’s energy to drive a series of chemical reactions to produce carbohydrates in the form of glucose and release oxygen in the process. These carbohydrates are then used by the plant and other organisms as a source of energy. Plants, algae and cyanobacteria (photoautotrophs) are some organisms that produce their own internal energy by converting sunlight into chemical energy. Photosynthesis occurs in a two-step process known as light dependent and light independent reactions. Photosynthesis maintains normal levels of oxygen in the Earth’s atmosphere and it is also responsible for the energy needed by other living organisms on the planet.


Nearly all organic compounds needed by living organisms are produced by photosynthetic organisms, including plants, algae and cyanobacteria. These organisms use the electrons from water, carbon dioxide (CO2) and the energy from the Sun to produce organic compounds, including fats, proteins, nucleic acids and carbohydrates, needed to maintain their internal processes. They’re called photoautotrophs because they make their own food from light. The ability of photoautotrophs to use sunlight and convert it into organic compounds that other organisms can use, places them at the bottom of the food chain, making them important sources of organic compounds that other animals can use as energy.

Photosynthesis is a two stage process

Photosynthesis begins when the sun’s energy is absorbed by a green pigment known as chlorophyll. Chlorophyll resides inside an organelle called chloroplast. Chlorophyll absorbs the red and blue wavelengths of light while reflecting the green wavelength. That is the reason plants acquire a greenish hue. The overall process of photosynthesis occurs in two distinct stages known as light dependent reactions and light independent reactions. In the light dependent reactions, light from the Sun is captured and used to make ATP and NADPH. In the light independent reactions, ATP and NADPH are utilized to capture and reduce CO2 to produce glucose.

Light-dependent reactions

In this set of reactions, one photon of light is absorbed by chlorophyll, exciting an electron. The electron moves along an electron transport chain in the thylakoid membrane. In the process, water is split into hydrogen ions and oxygen. When two oxygen atoms join, they form molecular oxygen O2, which if given off as a waste product of photosynthesis. During the electron-transport process, hydrogen ions are pumped across the thylakoid membrane, leading to the synthesis of ATP in the stroma. Another reaction that occurs in the thylakoid membrane is the conversion of NADP into NADPH, another energy carrier molecule.

Light-independent reactions

The ATP and the NADPH generated during the light-dependent reactions provide the energy for the reduction of CO2 into glucose (carbon fixation). This food making reaction occurs in the stroma, which is a fluid that surrounds the thylakoid disks. During this reaction, CO2 is captured into longer carbon chains utilizing the energy supplied by ATP and NADPH, and it starts when three molecules of CO2 are captured by the enzyme ribulose bisphosphate carboxylase to generate 2 molecules of glyceraldehyde 3-phosphate (G3P), which plants can assembled into any kind of carbohydrate, such as glucose, sucrose, maltose or starch.

It is believed that the first photosynthetic organisms may have evolved approximately 3.5 million years ago, when most forms of life on Earth were microorganisms which used hydrogen and hydrogen sulfide as a source of electrons for photosynthesis instead of water. Cyanobacteria, which appeared in the evolution of life 3,000 million years ago, are thought to be the first organisms to use electrons of water for the synthesis of CO2. The oxygen produced by cyanobacteria is thought to have changed life on the face of the Earth to the way in which we know it today. According to, plants function as carbon sinks, removing CO2 from the atmosphere and converting it into organic compounds, which other organisms can use as a source of energy.