Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that can later be released to fuel the organisms' activities (energy transformation). This chemical energy is stored in carbohydrate molecules, such as sugars, which are synthesized from carbon dioxide and water – In most cases, oxygen is also released as a waste product. Most plants, most algae, and cyanobacteria perform photosynthesis; such organisms are called photoautotrophs. 

All green plants have the ability to produce food through photosynthesis. This is the most important process of synthesis on Earth, because it is the only carbohydrate source in which solar energy is converted into chemical energy. In the process of photosynthesis, the kinetic energy (light energy) of sunlight is converted into chemical energy and stored in carbohydrate molecules. This chemical energy is used to grow and maintain our biosphere. All animals, including humans, depend on plants directly or indirectly for their food.

Photosynthesis is largely responsible for producing and maintaining the oxygen content of the Earth's atmosphere, and supplies all of the organic compounds and most of the energy necessary for life on Earth. Although photosynthesis is performed differently by different species, the process always begins when energy from light is absorbed by proteins called reaction centres that contain green chlorophyll pigments. In plants, these proteins are held inside organelles called chloroplasts, which are most abundant in leaf cells, while in bacteria they are embedded in the plasma membrane. 

In these light-dependent reactions, some energy is used to strip electrons from suitable substances, such as water, producing oxygen gas. The hydrogen freed by the splitting of water is used in the creation of two further compounds that serve as short-term stores of energy, enabling its transfer to drive other reactions: these compounds are reduced nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP), the "energy currency" of cells.

 In plants, algae and cyanobacteria, long-term energy storage in the form of sugars is produced by a subsequent sequence of light-independent reactions called the Calvin cycle; some bacteria use different mechanisms, such as the reverse Krebs cycle, to achieve the same end. In the Calvin cycle, atmospheric carbon dioxide is incorporated into already existing organic carbon compounds, such as ribulose bisphosphate (RuBP). Using the ATP and NADPH produced by the light-dependent reactions, the resulting compounds are then reduced and removed to form further carbohydrates, such as glucose.

Carbon dioxide is converted into sugars in a process called carbon fixation; photosynthesis captures energy from sunlight to convert carbon dioxide into carbohydrate. Carbon fixation is an endothermic redox reaction. In general outline, photosynthesis is the opposite of cellular respiration; in the latter, glucose and other compounds are oxidized to produce carbon dioxide and water, and to release chemical energy (an exothermic reaction) to drive the organism's metabolism. 

The two processes, reduction of carbon dioxide to carbohydrate and then later oxidation of the carbohydrate, are distinct: photosynthesis and cellular respiration take place through a different sequence of chemical reactions and in different cellular compartments.

The general equation for photosynthesis as first proposed by Cornelius van Niel is therefore:

CO2 + 2H2A + photons → [CH2O] + 2A + H2O

Carbon dioxide + electron donor + light energy → carbohydrate + oxidized electron donor + water

Important of Photosynthesis 

• The energy produced by photosynthesis forms the basis of virtually all terrestrial and aquatic food chains.
• As a result, photosynthesis is the ultimate source of carbon in the organic molecules found in most organisms
• The high oxygen concentration in the atmosphere is derived directly from the light reactions of photosynthesis.
• Prior to the evolution of photosynthesis on earth, the atmosphere was anoxic.

What we know about photosynthesis: Photosynthesis is a natural physicochemical process, on which the existence of life on the planet earth depends. This photosynthesis is the most important biological process necessary for human existence. Chlorophyll, light and carbon dioxide are major requirements for photosynthesis