Figure 6.19: Photosystem Organization (A) The molecular structure of a single light-harvesting complex shows the polypeptide in brown with three helices that span the thylakoid membrane. Pigment molecules (carotenoids and chlorophylls a and b) are bound to the polypeptide. (B) Light-harvesting complexes are organized into large photosystems that span the thylakoid membrane and have a centrally placed reaction center. The light-harvesting chlorophylls absorb light and pass the energy on to a chlorophyll in the reaction center.