Photosynthesis (Light Reactions)

Photosynthesis is the means by which plants make use of chlorophyll and light to produce energy.

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The process of Photosynthesis produces ATP from ADP and Pi by using the energy from light to excite electrons that are passed along an electron transport chain. Coupled with the transfer of electrons is the pumping of hydrogen ions and the splitting of water molecules.

The following complexes are found in the photosynthesis electron transport chain: Photosystem II, Cytochrome b6-f, Photosystem I, Ferredoxin NADP Reductase (FNR), and the complex that makes ATP, ATP Synthase. In addition to the complexes, three mobile carriers are also involved: Plastoquinone Qb, Plastocyanin, and Ferredoxin. Other key components include: photons, chlorophyll molecules, protons, water, molecular oxygen, NADP+ and the electrons to form NADPH, and ADP and Pi, which combine to form ATP.

Photosynthesis occurs in the chloroplasts of plants and algae. The process is also found in single-cell organisms such as cyanobacteria that do not have chloroplasts. Like its mitochondrial counterpart, the chloroplast electron transport chain consists of several protein complexes and mobile electron carriers.

First, a photon of light hits a chlorophyll molecule surrounding the Photosystem II complex. This creates resonance energy that is transferred through neighboring chlorophyll molecules. When this energy reaches the reaction center embedded in photosystem II, an electron is released. The reaction center chlorophyll contains electrons that can be transferred when excited. One photon is needed to excite each of the electrons in this chlorophyll.

Once excited, two electrons are transferred to plastoquinone Qb, the first mobile carrier. In addition to the two electrons, Qb also picks up two protons from the stroma.

The two electrons lost from photosystem II are replaced by the splitting of water molecules. Water splitting also releases hydrogen ions into the lumen. This contributes to a hydrogen ion gradient similar to the one created by mitochondrial electron transport. After two water molecules have been split, one molecule of molecular oxygen is created.

Plastoquinone Qb then transfers the two electrons to the cytochrome b6-f complex. The two protons it picked up are released into the lumen. These transfers are coupled with the pumping of two more hydrogen ions into the lumen space by cytochrome b6-f.

The electrons are next transferred to plastocyanin, another mobile carrier. Next the electrons are transferred from plastocyanin to the Photosystem I complex.

It is here that photons again energize each electron and propel their transfer to ferredoxin. Ferredoxin then transfers the electrons to the ferredoxin-NADP-reductase, also known as FNR. After two electrons are transferred to FNR, NADPH is made by adding the two electrons and a hydrogen ion to NADP+.

The hydrogen ion gradient created by the electron transport chain is utilized by ATP synthase to create ATP from ADP and Pi. This is similar to the way ATP is synthesized in the mitochondria. ATP, NADPH, and molecular oxygen are the final, vital, products of photosynthesis.

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Major funding provided by the National Science Foundation.

Additional funding provided by the U.S. Department of Education's Fund for the Improvement of Postsecondary Education.

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