FIGURE 12-39 Structure of chlorophyll a, the principal pigment that traps light energy. Electrons are delocalized among three of chlorophyll a’s four central rings (yellow) and the atoms that interconnect them. In chlorophyll, a Mg²⁺ ion, rather than the Fe²⁺ ion found in heme, sits at the center of the porphyrin ring, and an additional five-member ring (blue) is present; otherwise, the structure of chlorophyll is similar to that of heme, found in molecules such as hemoglobin and cytochromes (see Figure 12-20a). The hydrocarbon phytol “tail” facilitates the binding of chlorophyll to hydrophobic regions of chlorophyll-binding proteins. The CH₃ group (green) is replaced by a formaldehyde (CHO) group in chlorophyll b.