How the electromagnetic energy of light is converted to
chemical energy in the form of reduced organic molecules
is complex. Nonetheless, the first principles of energy conservation
and conversions in photosynthesis may be simply
depicted. All higher photosynthetic organisms contain
two forms of the green pigment chlorophyll. More than
99% of the chlorophyll in chloroplasts, the organelles in
which photosynthesis takes place, functions in a passive,
purely physical manner. Organized in specific pigment–
protein complexes within the photosynthetic membrane,
these chlorophylls absorb visible light and transfer excitation
energy to nearby chlorophylls with efficiencies very
close to 100%. In a real sense, more than 99% of the
chlorophylls function only to gather light and as such they
are often referred to as light-harvesting chlorophylls.
Within picoseconds of the harvesting, the excitation energy
is transferred to specialized chlorophyll molecules
called reaction center chlorophylls. These reaction center
chlorophylls are identical to the majority of the lightharvesting
chlorophylls.Yet, rather than acting in a passive
manner when they are excited, the reaction center chlorophylls
perform photochemistry. The two reaction center
chlorophylls are termed P700 and P680. The “P” stands
for pigment and the numbers refer to their absorption maxima,
in nanometers, in the red region of the spectrum. The
reaction center chlorophylls were first detected by lightinduced
bleaching at 680 and 700 nm. When the reaction
center chlorophylls are excited, either directly or by
resonance energy transfer from excited light-harvesting
chlorophylls, an electron is transferred from the reaction
center chlorophyll ensemble to an electron acceptor. These
light-driven oxidation–reduction reactions occur within
picoseconds and can operate with a quantum efficiency
that is close to 100%. The reactions may be written as
|⇒ Equation 
||P700* + FeS → P700+ + FeS−
|⇒ Equation 
||P680* + Q → P680+ + Q−,
where the asterisks indicate the first excited singlet state
of the reaction center chlorophyll, and FeS and Q are the
redox active part of an iron–sulfur protein and a quinone,
respectively, the first stable electron acceptors. P700+
are chlorophyll cation radicals and Q−
is a half
reduced quinone and FeS−
is a reduced iron-sulfur protein.
The reactions shown in Eqs. (6) and (7) cannot take
place, in the direction shown, in the dark when the reaction
center chlorophylls are in the unexcited, ground
state. The ΔG0'
for both these reactions is approximately
+24 kcal/mol. The excited reaction center chlorophylls
are, however, much stronger reducing agents than the
ground state chlorophylls are. The E0'
1.3 V more reducing than that of P700 in the ground state.
These two electron transfer reactions are the only lightdriven
reactions in photosynthesis and they set the entire
process in motion. The electron transport chain of chloroplasts
is illustrated in Fig. 10.
|Figure 10 Electron transport and ATP synthesis in chloroplasts. The jagged arrows represent light striking the two
photosystems (PS I and PS II) in the thylakoid membrane. Other members of the electron transport chain shown are
a quinone (Q), the cytochrome complex (b6f ), plastocyanin (PC), and an iron–sulfur protein (FeS). The chloroplast
ATP synthase is shown making ATP at the expense of the electrochemical proton gradient generated by electron
Specific light-harvesting chlorophyll–protein complexes
are associated with the reaction center chlorophyll–
protein complexes in assemblies known as photosystems.
Photosystem I (PS I) contains P700 and the FeS acceptor,
and photosystem II (PS II), P680 and the quinone acceptor.
Electron transfer in PS I generates a relatively weak
and a strong
The primary reductant
generated in photosynthesis is nicotinamide adenine dinucleotide
), which, as the name suggests,
differs from NAD+ by a single phosphate. While the
physical properties of NADP+
are very similar,
enzymes that use these pyridine nucleotides as substrates
can discriminate between them by at least a factor of 1000.
In general NAD+
is used in catabolic metabolism as we
have seen for glycolysis and the tricarboxylic acid cycle.
The reduced form of NADP+
, NADPH, is, in contrast,
used in biosynthesis, or anabolic metabolism. The E0'
−NADPH redox pair is −340 mV. Thus, electron
transfer from the reduced iron–sulfur protein of PS I
is energetically a very favorable spontaneous
reaction. It is NADPH that provides the electrons for CO2
reduction. The ultimate electron donor is water.
Two water molecules are oxidized by PS II to yield
four protons and molecular oxygen. Water is a very weak
reducing agent. Thus, a strong oxidizing agent is needed
for water oxidation. P680+
fits the bill. The midpoint potential
of the P680+
−P680 redox pair is on the order of
+1 V. Since the water–oxygen redox couple has an E0'
+0.815 V, the oxidation of water by P680+
is an energetically
spontaneous reaction. Water oxidation is catalyzed
by a manganese-containing enzyme that is plugged into
the energy-converting thylakoid membrane.
So far, we have seen that the reduced FeS protein of
PS I is converted to its oxidized form by passing electrons
eventually to NADP+
. In PS II, P680+
is reduced to P680
with electrons extracted from water. For electron transport
to continue, the electron acceptor of PS II, Q−, and
the electron donor of PS I, P700+
, must be oxidized and
reduced, respectively. The redox potential of the Q−Q−
couple is about +0.05 V, whereas that of P700+
is near +0.450 V. Thus, electron transport from Q−
is energetically spontaneous with a free energy of
9.3 kcal/mol for each electron transferred.
Electron transport from Q−
is mediated by a
quinone, iron–sulfur, and a cytochrome protein complex
in the thylakoid membrane. This protein, the cytochrome b6f
complex, is remarkably similar to the cytochrome bc1
complex of the mitochondrial electron transport chain.