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United States Department of Agriculture

Agricultural Research Service

Glossary of Photosynthesis Related Terms
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ATP: Adenosine triphosphate, a small water soluble molecule that acts as an energy currency in cells.

ATP Synthase: A membrane bound protein complex that uses the energy stored across the photosynthetic membrane to add inorganic phosphate to ADP, thus creating ATP. (Also known as coupling factor.)

Calvin Cycle: The biochemical reactions, initiated by Rubisco, that result in the reduction of CO2 to a carbohydrate (also known as the photosynthetic carbon reduction cycle).

Cytochrome: Heme containing protein.

Cytochrome bc Complex: A membrane bound electron transfer protein complex found in all anoxygenic photosynthetic organisms that oxidizes reduced quinone and reduces a c-type cytochrome. The complex contains a c-type cytochrome, two b-type cytochromes and an FeS center.

Cytochrome bf Complex: A membrane bound electron transfer protein complex found in all oxygenic photosynthetic organisms that oxidizes reduced plastoquinone and reduces plastocyanin (or cytochrome c). The complex contains a c-type cytochrome, two b-type cytochromes and an FeS center.

Free Energy: The amount of energy in a reaction available to do work. Because most biochemical reactions occur at a constant temperature and pressure, the free energy is frequently the Gibbs Energy.

Light Harvesting Complex: A protein complex that harvests light energy and converts it to exciton energy that can migrate to a reaction center. The light is absorbed by pigment molecules (e.g., chlorophyll, bacteriochlorophyll, carotenoids, phycobilin) that are attached to the protein.

NADPH: reduced form of nicotinomide adenine dinucleotide phosphate, a small water soluble molecule that acts as a hydrogen carrier in biochemical reactions.

NADP+: oxidized form of nicotinomide adenine dinucleotide phosphate.

Oxidation: The removal of one or more electrons from an atom or molecule. In the case of a molecule, a protons may be involved as well, resulting in hydrogen being removed.

Phosphorylation: The covalent attachment of a phosphate group to a molecule.

Photorespiration: The removal of O2 from the atmosphere by Rubisco and the subsequent biochemical reactions that serve to recycle some of the reduced carbon.

Photosynthesis: The physical-chemical process by which certain chlorophyll (or bacteriochlorophyll) containing organisms use light energy for the biosynthesis of organic molecules.

Photosynthetic Membrane: A bilayer of lipid molecules in which are embedded proteins that transform light energy into chemical free energy. (Also known as the thylakoid membrane.)

Photosystem I: A protein complex located in the photosynthetic membrane. Photosystem I is one of two types of reaction centers found in higher plants, algae, and cyanobacteria. The photosystem I reaction center uses light energy to transfer an electron from a mobile electron transfer protein (plastocyanin or a cytochrome c) on one side of the photosynthetic membrane to a mobile electron transfer protein (ferredoxin) on the opposite side of the photosynthetic membrane.

Photosystem II: A protein complex found in the photosynthetic membrane. Photosystem II is one of two types of reaction centers found in higher plants, algae and cyanobacteria. The photosystem II reaction center uses light energy to transfer electrons from water to plastoquinone. Photosystem II is the source of the molecular oxygen in the atmosphere.

Plastoquinone: A small organic molecule involved in electron and proton transfer in photosynthesis.

Protein: A chemical structure composed of one or more polypeptides. In photosynthesis proteins serve as the scaffolding that hold the cofactors that gather light energy, transfer electrons, and catalyze biochemical reactions.

Reaction Center: A protein complex that uses light energy to create a stable charge separation by transferring a single electron energetically uphill from a donor molecule to an acceptor molecule, both of which are located in the reaction center.

Reduction: The addition of one or more electrons to an atom or molecule. In the case of a molecule, protons may be involved as well, resulting in hydrogen being added.

Rubisco: (D-ribulose 1,5-bisphosphate carboxylase/oxygenase) A water soluble protein complex responsible for the removal of CO2 from the atmosphere. The enzyme works by attaching CO2 to a five-carbon compound (1,5 ribulose bisphosphate) that is split into two identical three-carbon compounds (phosphoglycerate). In addition to catalyzing the removal of CO2 from the atmosphere, Rubisco also catalyzes the removal of O2 from the atmosphere (less efficiently). The removal of O2 is thought to be a consequence of poor design and leads to a complex set of compensatory reactions known as photorespiration.

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Last Modified: 12/24/2008
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