Photosynthesis in plants converts solar energy into chemical energy using electrons and protons from water.

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The process of photosynthesis in plants involves a series of steps and reactions that use solar energy, water, and carbon dioxide to produce organic compounds and oxygen. There are two main sets of reactions: energy-transduction reactions (commonly called light reactions) and carbon-fixation reactions (commonly called dark reactions).

In the energy-transduction reactions, solar energy is converted into chemical energy in the form of two energy-transporting molecules, ATP and NADPH. When solar energy reaches plant cells and excites special chlorophyll molecules, they release a high-energy electron (check out the related strategy here). The release of this electron sets off a chain of electron-trading and energy-transferring events between several intermediary molecules, and the last molecule to form and hold the electrons in this chain is NADPH.

The excited chlorophyll molecule’s electrons need to be replaced, and these electrons come from water. With the help of enzymes and solar energy, water is split (photolysis) into electrons, protons (H+), and oxygen. The electrons go to the chlorophyll, while the protons contribute to a proton gradient that is used to power the synthesis of a second energy-carrying molecule, ATP. The oxygen is a byproduct of the whole process.

The chemical energy in NADPH and ATP is then used to power steps in the subsequent carbon-fixation reactions.

Learn more about other parts of the photosynthetic process in these related strategies:

Pigment molecules absorb and transfer solar energy: Cooke’s koki’o

Catalyst facilitates water-splitting: plants

Photosynthesis makes useful organic compounds out of CO2: plants

Energy transduction (light-dependent) reactions in a photosynthesizing plant. Illustration by Anna Guerrero, [email protected] CC-BY-NC-SA.

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“Light is captured by a set of light-harvesting complexes (LHCs) that funnel light energy into photochemical reaction centres, photosystem (PS) I and PSII (Fig. 1) (see review by Ort and Yocum, 1996). Special subsets of chlorophyll molecules in these photosystems are excited by light energy, allowing electrons on them to be transferred through a series of redox carriers called the electron transfer chain (ETC), beginning from the oxygen evolving complex (OEC) of PSII (which oxidizes H2O and releases O2 and protons) (Diner and Babcock, 1996), through the plastoquinone (PQ) pool, the cytochrome (cyt) b6f complex (Sacksteder et al., 2000) and plastocyanin (PC), and finally through PSI (Malkin, 1996). Electrons from PSI are transferred to ferredoxin (Fd), which, in turn, reduces NADP+ to NADPH via ferredoxin:NADP+ oxidoreductase (FNR) (Knaff, 1996). This linear electron flux (LEF) to NADP+ is coupled to proton release at the OEC, and ‘shuttling’ of protons across the thylakoid membrane by the PQ pool and the Q-cycle at the cyt b6f complex, which establishes an electrochemical potential of protons, or proton motive force (pmf) that drives the synthesis of ATP by chemiosmotic coupling through the chloroplast ATP synthase (McCarty, 1996; Mitchell, 1966).” (Cruz et al. 2005:395)

Journal article
Plasticity in light reactions of photosynthesis for energy production and photoprotectionJournal of Experimental BotanyJanuary 1, 2005
Cruz JA; Avenson TJ; Kanazawa A; Takizawa K; Edwards GE; Kramer DM

Journal article
Artificial photosynthesis: water cleavage into hydrogen and oxygen by visible lightAccounts of Chemical ResearchJanuary 1, 1981
Grätzel M

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