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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.
Related
Tutorials
* Origin of Life,
Origin of Disease
*
Oxygen-Driven Energetics: An Intelligent Design
*
Beginning of Life
* What Does
Chlorophyll Reveal About the Origin of Life on the
Planet Earth?
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