Light-reactions

The photosynthetic reactions that require light occur within the thylakoid membrane. These light-dependent photosynthetic reactions employ the thylakoid membrane-embedded antenna system to harness energy delivered by a photon. The light-dependent, photosystem reactions ultimately transduce the energy of light to generate molecules of ATP and NADPH, which act as energy-transfer molecules in the light-independent, “dark” reactions of the Calvin cycle.

The thylakoid-embedded antenna complex comprises closely packed pigment molecules and the reaction center. Any of the pigment molecules can be excited by a photon of light, and pass their energy along to nearby pigment molecules until the excitation eventually reaches a specialized molecule of chlorophyll called the reaction center, which delivers an excited electron to the electron acceptor molecule in an electron transfer chain. The antenna, including the reaction center, and the electron transport molecules together make up a photosystem.

There are two kinds of photosystems in eukaryotes – PSI and PSII. The reaction center chlorophyll (im) molecule within the antenna of photosystem I responds most strongly to 700 nm light, and is therefore termed P700. The reaction center within the antenna of photosystem II responds most to 680 nm light, and is accordingly called P680. Photosystem I evolved very early, and it is found in nonoxygenic phototrophs; photosystem II evolved later. Because the PSII photosystem is most sensitive to shorter wavelength 680 nm light, it absorbs slightly more energy than the P700-PSI system.

The electron transport system of each photosystem is embedded within the thylakoid membrane and functions in the production of ATP. The system comprises membrane-bound electron carriers that pass electrons from one molecule to the next.

Water is split to generate an electron, hydrogen ions, and oxygen:
2H2O → 4e^- + 4H⁺ + O2

Mechanism of Photophosphorylation
By receiving the energized electron (reduction), the first carrier of the P680 (PSII) electron transport system gains energy. It utilizes some of the energy to pump H⁺ into the thylakoid lumen, then passes the less energetic electron to the second of four carrier molecules. Each successive carrier in the electron transport chain utilizes some of the energy of the received electron to pump H⁺ from the stroma into the thylakoid lumen, and then passes the further depleted electron along to the next carrier. Thus, the electron transport system functions to generate a concentration gradient of H⁺ inside the thylakoid. The chemical potential energy of the H⁺ concentration gradient is employed to synthesize ATP.

In the process called photophosphorylation, ATP synthase produces ATP from ADP and Pi when hydrogen ions pass out of the thylakoid. The electron, with its energy almost spent, is passed to the P700 antenna of the PSI photosystem and a second electron transport chain. The P680-PSII system has thus generated ATP, a H⁺ concentration gradient, and energy (Z-scheme (image) of noncyclic photophosphorylation).

The antenna of the evolutionarily older PSI system absorbs a photon of light, and, like the PSII antenna, passes the energy along to the PSI reaction center. Like the PSII system, the P700 reaction center passes the energized electron to an electron acceptor. However, unlike noncyclic photophosphorylation, the electron is retained in the PSI system (cyclic photophosphorylation). So, unlike the PSII system, the electron acceptor of the PSI system utilizes the delivered energy to reduce only molecules of NADP⁺ to NADPH.

The “light” reactions of noncyclic photophosphorylation produce both ATP and NADPH, which act as energy-transfer molecules in the light-independent (“dark”) reactions of the Calvin cycle. The “dark”, or light-independent Calvin anabolic reactions occur in the stroma of the chloroplast in either light or dark conditions. The light-independent reactions function to reduce CO2 to glucose:

6CO2 + 6H2O → Energy + C6H12O6 + 6O2.

The manganese-calcium oxide cluster, also referred to as the "Oxygen Evolving Complex", OEC, or photosynthetic water oxidase. The OEC is located on the oxidizing side of Photosystem II (PSII).

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