Photosynthesis is a metabolic pathway that converts light energy into chemical energy. Its initial substrates are carbon dioxide and water; the energy source is sunlight (electromagnetic radiation); and the end-products are oxygen and (energy-containing) carbohydrates, such as sucrose, glucose or starch. This process is one of the most important biochemical pathways, since nearly all life on Earth either directly or indirectly depends on it as a source of energy. It is a complex process occurring in plants, algae, as well as bacteria such as cyanobacteria. Photosynthetic organisms are also referred to as photoautotrophs.

The word comes from the Greek (photo-), "light," and (synthesis), "placing with."

Overview

Photosynthesis uses light energy and carbon dioxide to make triose phosphates (G3P). G3P is generally considered the first end-product of photosynthesis.Fact: date=June 2008 It can be used as a source of metabolic energy, or combined and rearranged to form monosaccharide or disaccharide sugars, such as glucose or sucrose, respectively, which can be transported to other cells, stored as insoluble polysaccharides such as starch, or converted to structural carbohydrates, such as cellulose or glucans.

A commonly used slightly simplified equation for photosynthesis is:

6 CO_2(g) + 12 H₂O_(l) + photons → C₆H₁₂O_6(aq) + 6 O_2(g) + 6 H₂O_(l)

carbon dioxide + water + light energy → glucose + oxygen + water

The equation is often presented in introductory chemistry texts in an even more simplified form as:

6 CO_2(g) + 6 H₂O_(l) + photons → C₆H₁₂O_6(aq) + 6 O_2(g)

Photosynthesis occurs in two stages. In the first stage, light-dependent reactions or photosynthetic reactions (also called the Light Reactions) capture the energy of light and use it to make high-energy molecules. During the second stage, the light-independent reactions (also called the Calvin-Benson Cycle, and formerly known as the Dark Reactions) use the high-energy molecules to capture and chemically reduce carbon dioxide (CO₂) (also called carbon fixation) to make the precursors of carbohydrates.

In the light reactions, one molecule of the pigment chlorophyll absorbs one photon and loses one electron. This electron is passed to a modified form of chlorophyll called pheophytin, which passes the electron to a quinone molecule, allowing the start of a flow of electrons down an electron transport chain that leads to the ultimate reduction of NADP to NADPH. In addition, this creates a proton gradient across the chloroplast membrane; its dissipation is used by ATP Synthase for the concomitant synthesis of ATP. The chlorophyll molecule regains the lost electron from a water molecule through a process called photolysis, which releases a dioxygen (O₂) molecule.