CASE 1 THE FIRST CELL: LIFE’S ORIGINS

Sunlight is valuable as a source of energy, but it can also cause damage. This is particularly true of ultraviolet wavelengths, which can damage DNA and other macromolecules. Thus, the earliest interactions with sunlight may have been the evolution of UV-absorbing compounds that could shield cells from the sun’s damaging rays. Over time, random mutations could have produced chemical variants of these UV-absorbing molecules. One or more of these variant compounds might have been capable of using sunlight to meet the energy needs of the cell—perhaps by transferring electrons to another molecule as a present-day reaction center does.

The earliest reaction centers may have used light energy to drive the movement of electrons from an electron donor outside the cell in the surrounding medium to an electron-acceptor molecule within the cell. In this way, energy from sunlight could have been used to synthesize carbohydrates. The first electron donor could have been a soluble inorganic ion like reduced iron, Fe²⁺, which is thought to have been abundant in the early ocean. Alternatively, the first forms of light-driven electron transport may have been cyclic and thus not required an electron donor. In either configuration, light-driven electron transport could have been coupled to the net movement of protons across the membrane, allowing for the synthesis of ATP.

Similarly, it is unlikely that these first photosynthetic organisms employed chlorophyll as a means of absorbing sunlight for the simple reason that the biosynthetic pathway for chlorophyll is complex, consisting of at least 17 enzymatic steps. Yet some of the intermediate compounds leading to chlorophyll are themselves capable of absorbing light. Perhaps each of these now-intermediate compounds was, at one time, a functional end product used as a pigment by an early photosynthetic organism. The biosynthetic pathway may have gained steps as chemical variants, produced by random mutations, were selected because they were more efficient or able to absorb new portions of the visible spectrum. Selection would have eventually resulted in the chlorophyll pigments that are used by photosynthetic organisms today.