The light reactions of photosynthesis in green plants are mediated by four large protein complexes embedded in the thylakoid membrane of the chloroplast: Photosystem I (PSI), Photosystem II (PSII), the cytochrome b6/f complex, and the ATPase. These complexes are organized into supercomplexes with variable amounts of membrane-bound peripheral antenna complexes. PSI consists of a monomeric core complex with four LHCI proteins and binding sites for additional LHCI and/or LHCII complexes. PSII supercomplexes are dimeric and contain two to four copies of trimeric LHCII complexes. These supercomplexes can further associate into megacomplexes or crystalline domains. The structural features of these complexes, along with the lipid composition of the thylakoid membrane, determine the segregation of PSII and LHCII from PSI and ATPase into stacked grana membranes. The margins of the grana, which are strongly folded regions connecting the grana, are protein-free, and protein-protein interactions in the lumen determine the shape of the grana. The supramolecular organization of pigment-protein complexes in the thylakoid membrane and their flexibility play roles in various regulatory mechanisms of green plant photosynthesis. PSI is composed of a core complex with two large subunits (PsaA and PsaB), three extrinsic subunits (PsaC–E), and several small intrinsic subunits. Green plants have three additional intrinsic membrane proteins (PsaG, PsaH, and PsaO) and one extrinsic protein (PsaN) instead of PsaM and PsaX found in cyanobacteria. The PSI core complex binds 96 Chl a and 22 β-carotene molecules. In green plants, the complex occurs in a monomeric aggregation state, while in cyanobacteria, it occurs in trimers. The PsaH subunit binds to PsaL, which may explain the absence of trimers in green plants. The PsaO subunit may also contribute to this effect. The presence of Chl b can prevent trimerization in green plants, but not in prochlorophytes. PSI binds an additional membrane-bound peripheral antenna called LHCII, consisting of four different polypeptides from the Lhc super-gene family. In green plants, these are Lhca1–4, with protein masses of around 25 kDa. In Arabidopsis, two additional genes (Lhca5 and Lhca6) have been identified, but their expression is low. Lhca1 and Lhca4 form a heterodimer, while Lhca2 and Lhca3 may also form a heterodimer. Each Lhca protein binds 10 Chl a or Chl b molecules and a few xanthophylls. TheThe light reactions of photosynthesis in green plants are mediated by four large protein complexes embedded in the thylakoid membrane of the chloroplast: Photosystem I (PSI), Photosystem II (PSII), the cytochrome b6/f complex, and the ATPase. These complexes are organized into supercomplexes with variable amounts of membrane-bound peripheral antenna complexes. PSI consists of a monomeric core complex with four LHCI proteins and binding sites for additional LHCI and/or LHCII complexes. PSII supercomplexes are dimeric and contain two to four copies of trimeric LHCII complexes. These supercomplexes can further associate into megacomplexes or crystalline domains. The structural features of these complexes, along with the lipid composition of the thylakoid membrane, determine the segregation of PSII and LHCII from PSI and ATPase into stacked grana membranes. The margins of the grana, which are strongly folded regions connecting the grana, are protein-free, and protein-protein interactions in the lumen determine the shape of the grana. The supramolecular organization of pigment-protein complexes in the thylakoid membrane and their flexibility play roles in various regulatory mechanisms of green plant photosynthesis. PSI is composed of a core complex with two large subunits (PsaA and PsaB), three extrinsic subunits (PsaC–E), and several small intrinsic subunits. Green plants have three additional intrinsic membrane proteins (PsaG, PsaH, and PsaO) and one extrinsic protein (PsaN) instead of PsaM and PsaX found in cyanobacteria. The PSI core complex binds 96 Chl a and 22 β-carotene molecules. In green plants, the complex occurs in a monomeric aggregation state, while in cyanobacteria, it occurs in trimers. The PsaH subunit binds to PsaL, which may explain the absence of trimers in green plants. The PsaO subunit may also contribute to this effect. The presence of Chl b can prevent trimerization in green plants, but not in prochlorophytes. PSI binds an additional membrane-bound peripheral antenna called LHCII, consisting of four different polypeptides from the Lhc super-gene family. In green plants, these are Lhca1–4, with protein masses of around 25 kDa. In Arabidopsis, two additional genes (Lhca5 and Lhca6) have been identified, but their expression is low. Lhca1 and Lhca4 form a heterodimer, while Lhca2 and Lhca3 may also form a heterodimer. Each Lhca protein binds 10 Chl a or Chl b molecules and a few xanthophylls. The