FIGURE 12-38 Overview of the four stages of photosynthesis. In stage 1, light is absorbed by light-harvesting complexes (LHCs) and the reaction center of photosystem II (PSII). The LHCs transfer the absorbed energy to the reaction centers, which use it, or the energy absorbed directly from a photon, to oxidize water to molecular oxygen and generate high-energy electrons (electron paths shown by blue arrows). In stage 2, these electrons move down an electron-transport chain, which uses either lipid-soluble (Q/QH₂) or water-soluble (plastocyanin, PC) electron carriers to shuttle electrons between multiple protein complexes. As electrons move down the chain, they release energy that the complexes use to generate a proton-motive force and, after additional energy is introduced by absorption of light in photosystem I (PSI), to synthesize the high-energy electron carrier NADPH. In stage 3, flow of protons down their concentration and voltage gradient through the F₀F₁ ATP synthase drives ATP synthesis. Stages 1–3 in plants take place in the thylakoid membrane of the chloroplast. In stage 4, in the chloroplast stroma, the energy stored in NADPH and ATP is used to incorporate CO₂ into the three-carbon molecule glyceraldehyde 3-phosphate, the first step in a process known as carbon fixation. These molecules are then transported to the cytosol of the cell for conversion to hexose sugars in the form of sucrose. Glyceraldehyde 3-phosphate is also used to make starch within the chloroplast. Inset: Three-dimensional reconstruction from cryoelectron tomography of a chloroplast in the unicellular green alga Chlamydomonas reinhardtii, showing thylakoid membranes (dark green), thylakoid lumen (light green), inner and outer membranes (blue), and one small starch granule (tan).

[Inset from Engel, B. D., et al., “Native architecture of the Chlamydomonas chloroplast revealed by in situ cryo-electron tomography,” eLIFE, 2015; 4: e04889.]