Chloroplasts bear chlorophyll; they give the green world its color, and they carry out the business of photosynthesis. Around the inside perimeter of each gigantic cell trailed a continuous loop of these bright green dots. They spun. . . They pulsed, pressed, and thronged. . . They shone, they swarmed in ever-shifting files around and around the edge of the cell; they wandered, they charged, they milled, raced. . . They flowed and trooped greenly. . . All the green in the planted world consists of these whole, rounded chloroplasts. . . If you analyze a molecule of chlorophyll itself, what you get is one hundred thirty-six atoms of hydrogen, carbon, oxygen, and nitrogen arranged in an exact and complex relationship around a central ring. At the ring’s center is a single atom of magnesium. Now: If you remove the atom of magnesium and in its place put an atom of iron, you get a molecule of hemoglobin. The iron atom combines with all the other atoms to make red blood, the streaming red dots in the goldfish’s tail.

Pilgrim at Tinker Creek