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Photosynthesis is a process in which a plant (includes trees) uses energy from sunlight to produce sugar and uses it to make adenosine triphosphate (ATP). Photosynthesis occurs in the chloroplasts, a feature in plant cells that is not found in animal cells.

The process of photosynthesis is done mostly in the leaves of a plant, and water from the roots travel to the leaves through passages called xylem. Water, carbon dioxide, and sunlight work together and produce sugar and oxygen.

Forestry Main Page

A diagram of a plant cell.

The Actual Workings of Photosynthesis

Electron Transport Chain / Light Reaction

The electron transport chain (ETC) takes place in the thylakoid membrane. It begins with light hitting a photosystem, which is essentially a bundle of light-recepting molecules. At the core is chlorophyll, which, when hit by a photon of light, loses an electron. This electron is later replaced by splitting a molecule of water into 2 protons, 2 electrons, and an oxygen molecule. Using the energy from the excited electron, specialized channel proteins pump protons across the thylakoid membrane from the outside fluid, called the stroma, into the inside of the thylakoid, which resembles a hollow discus. A gradient is then formed , where there are too many protons in the thylakoid, and they want to come out. They come out through a special protein channel, called ATP synthase. As they are entering the stroma, the protons release potential energy, which is harnessed by the synthase molecule to make ATP.

Meanwhile, the electron that started this whole cycle, after it loses energy, gets stuck to a molecule called NADP+, adding on another H, to form NADPH. This, and the ATP that is formed, head for the Calvin Cycle, and the oxygen is excreted as waste.

Calvin Cycle / Dark Reaction

This process takes place in the inside of the thylakoid. In the Calvin Cycle, the carbon dioxide molecule is combined via carbon fixation with a molecule of ribulose bisphosphate (RuBP) to form the molecule 3PG. 3PG is then altered to form the molecule G3P, also called PGAL. Since there are 6 G3P formed, one G3P is sent to other locations in a plant cell to be converted to glucose (which needs 2 G3P molecules). The other 5 G3P molecules are converted, with the aid of an ATP molecule, back into ribulose bisphosphate. The new RuBP molecule is then ready to fix the next carbon dioxide molecule.

Other info you might need to know.

The reason the ETC is called the light reaction is that it cannot occur without light, which is the initial energy used to drive it. Similarly the Calvin Cycle is called the Dark Reaction because no light is needed to run it, therefore, it could take place in a dark room. However, if there is no light, the ETC would soon stop, and therefore, the Calvin Cycle would stop as well, so light is still needed for photosynthesis to continually occur.

The enzyme used to fix carbon dioxide is Ribulose Bisphosphate Carboxylase, better known as Rubisco. Rubisco is the most abundant enzyme on the planet.