2. Light-Dependent Reactions
Learning Objectives for this Section
Photoautotrophs use sunlight as a source of energy and through the process of photosynthesis, reduce carbon dioxide to form carbohydrates such as glucose. The radient energy is converted to the chemical bond energy within glucose and other organic molecules.
The overall reaction for photosynthesis is as follows:
C6H12O6 + 6 O2 + 6 H2O
Note that carbon dioxide (CO2) is reduced to produce glucose (C6H12O6 ) while water (H2O) is oxidized to produce oxygen (O2).
Photosynthesis is composed of two stages: the light-dependent reactions and the light independent reactions. We will now look at the light-dependent reactions.
The exergonic (def) light-dependent reactions of photosynthesis convert light energy into chemical energy, producing ATP and NADPH. These reactions occur in the thylakoids of the chloroplasts. The products of the light-dependent reactions, ATP and NADPH, are both required for the endergonic (def) light-independent reactions.
The light-dependent reactions can be summarized as follows:
6 O2 + 12 NADPH + 18 ATP
The light-dependent reactions involve two photosystems called Photosystem I and Photosystem II. These photosystems include units called antenna complexes composed of chlorophyll molecules and accessory pigments located in the thylakoid membrane. Photosystem I contain chlorophyll a molecules called P700 because they have an absorption peak of 700 nanometers. Photosystem II contains chlorophyll a molecules referred to as P680 because they have an absorption peak of 680 nanometers.
Each antenna complex is able to trap light and transfer energy to a complex of chlorophyll molecules and proteins called the reaction center (see Fig. 1). As photons are absorbed by chlorophyll and accessory pigments, that energy is eventually transfered to the reaction center where, when absorbed by an excitable electron, moves it to a higher energy level. Here the electron may be accepted by an electron acceptor molecule of an electron transport chain (see Fig. 1) where the light energy is converted to chemical energy by chemiosmosis (def).
The most common light-dependent reaction in photosynthesis is called noncyclic photophosphorylation. Noncyclic photophosphorylation involves both Photosystem I and Photosystem II and produces ATP and NADPH. During noncyclic photophosphorylation, the generation of ATP is coupled to a one-way flow of electrons from H2O to NADP+. We will now look at Photosystems I and II and their roles in noncyclic photophosphorylation.
1. As photons are absorbed by pigment molecules in the antenna complexes of Photosystem II, excited electrons from the reaction center are picked up by the primary electron acceptor of the Photosystem II electron transport chain. During this process, Photosystem II splits molecules of H2O into 1/2 O2, 2H+, and 2 electrons. These electrons continuously replace the electrons being lost by the P680 chlorophyll a molecules in the reaction centers of the Photosystem II antenna complexes (see Fig. 2).
During this process, ATP is generated by the Photosystem II electron transport chain and chemiosmosis. According to the chemiosmosis theory, as the electrons are transported down the electron transport chain, some of the energy released is used to pump protons across the thylakoid membrane from the stroma of the chloroplast to the thylakoid interior space producing a proton gradient or proton motive force. As the accumulating protons in the thylakoid interior space pass back across the thylakoid membrane to the stroma through ATP synthetase complexes, this proton motive force is used to generate ATP from ADP and Pi (see Fig. 2 and Fig. 3).
2. Meanwhile, photons are also being absorbed by pigment molecules in the antenna complex of Photosystem I and excited electrons from the reaction center are picked up by the primary electron acceptor of the Photosystem I electron transport chain. The electrons being lost by the P700 chlorophyll a molecules in the reaction centers of Photosystem I are replaced by the electrons traveling down the Photosystem II electron transport chain. The electrons transported down the Photosystem I electron transport chain combine with 2H+ from the surrounding medium and NADP+ to produce NADPH + H+ (see Fig. 2).
Cyclic photophosphorylation occurs less commonly in plants than noncyclic photophosphorylation, most likely occurring when there is too little NADP+ available. It is also seen in certain photosynthetic bacteria. Cyclic photophosphorylation involves only Photosystem I and generates ATP but not NADPH. As the electrons from the reaction center of Photosystem I are picked up by the electron transport chain, they are transported back to the reaction center chlorophyll. As the electrons are transported down the electron transport chain, some of the energy released is used to pump protons across the thylakoid membrane from the stroma of the chloroplast to the thylakoid interior space producing a proton gradient or proton motive force. As the accumulating protons in the thylakoid interior space pass back across the thylakoid membrane to the stroma through ATP synthetase complexes, this energy is used to generate ATP from ADP and Pi (see Fig. 4).
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