What Does Chlorophyll Reveal About the Origin of Life on the Planet Earth?

What Does Chlorophyll Reveal About the Origin of Life on the Planet Earth?

Majid Ali, M.D.

Chlorophyll (C55H72O5N4Mg) is the pigment that imparts green color to most plants, algae, and cyanobacteria. The term is derived from the Greek word chloros (meaning green) and phyllon (for leaf). Chlorophyll absorbs light energy most strongly in the blue and red ranges of electromagnetic spectrum, but poorly in the green region. This explains the green color of chlorophyll-containing leaves and other elements.

Chlorophyll traps solar energy and transmutes it into chemical bond energy.7-10 This process is called photosynthesis and involves absorption of light and its transfer through resonance energy transfer. To serve this role, chlorophyll molecules are arranged in specific configurations in and around pigment protein complexes called photosystems. Two main types of photosystem are called Photosystem I and Photosystem II. Both photosystems have their own distinct reaction center chlorophylls, P700 of Photosystem I and P680 of Photosystem II. The names of the pigments reflect the wavelength (in nanometers) of their red-peak absorption maximum, 700 and 680 respectively for Photosystems I and II. These two systems, as explained later, work in tandem. Photosystem II actually comes into play before Photosystem I. There are four well characterized forms of chlorophyll: a, b, c, and d. Chlorophyll a sometimes is designated as the primary chlorophyll, while chlorophyll b, c1, c2, and d are accessory pigments.

The functional (spectral properties) and structural properties of the types of chlorophyll in each photosystem are distinct and determined by the protein structure surrounding them, as well as by chlorophyll molecules themselves. The chlorophyll pigments are separated by simple paper chromatography, the identities being based on the number of polar groups between two main types, chlorophyll a and chlorophyll b.

The Reaction Center of the Photosystems

Chlorophyll molecules located in the reaction center of the photosystems use the energy absorbed by the systems and transfer it to other chlorophyll pigments in the photosystems to begin the process of charge separation—a specific redox reaction—by which the chlorophyll donates an electron into a series of electron receiving molecular intermediates called an electron transport chain. The charged reaction center of the Photosystems II (containing chlorophyll P680+) then returns to its original state —the “ground state,” so to speak—by accepting an electron (a process of reduction). In Photosystem II, the electron which reduces P680+ is drawn from the oxidation of water into O2 and H+ through several intermediates. This is the essential photosynthetic reaction by which plants and related organisms produce O2 gas. This photosynthetic process is the source for nearly all the O2 in Earth’s atmosphere.

Photosystem II typically works in tandem with Photosystem I. The P700+ of Photosystem I usually gains electrons—is reduced through many intermediates—from Photosystem II. These electron transfer events occur in complex ways in specialized membrane structures of leaves called the thylakoid membranes, the plant equivalent of human mitochondria. The electrons used to reduce P700+ are drawn from various sources.

Electron Flow Systems

The next significant chain of energetic events involves electron flow produced in the reaction centers of the photosystems by chlorophyll pigments, which drives the transport of hydrogen ions (the H+ ions shuttle) across the thylakoid membrane. Consequently, a chemosmotic potential is generated that, in turn, provides the energy for the production of high-energy phosphate bonds of adenosine triphosphate (ATP). The electrons in the electron transfer chain are ultimately used to reduce NADP+ to NADPH, a universal reduction reaction that reduces CO2 into sugars, as well as drives many other biosynthetic reduction reactions.

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