Photosynthetic Reaction

Posted by Waston Chen on

Photosynthesis is a chemical process where the electromagnetic energy of photons
is absorbed, transferred, and stored chemically in carbohydrate molecules through a
complex array of oxidation/reduction reactions in photosynthetic organisms
(Fig. 6.2). Photosynthetic organisms are also referred to as photoautotrophic
organisms, include bacteria, algae as well as plant species and together, are the
primary source of energy for all other life-forms on earth (Falkowski and Raven 2007). The photosynthetic process can be described by the following simplified
equation:

 

Photosynthetic-Reaction

Photosynthesis occurs within the chloroplast, a chlorophyll bearing type of
plastid organelle dedicated to energy production (Cooper 2000; Mishra 2004). These
are found within the cytoplasm of mesophyll cells, mostly the palisade and spongy
parenchyma cells located between the bounding epidermal layers of leaves (Mishra
2004). Within chloroplasts, the energy-generating photooxidation-reduction reactions
of photosynthesis occur within the third, internal thylakoid membrane system,
which forms a set of flattened thylakoid disks, often stacked in grana (Cooper 2000).
Embedded in the thylakoid membrane are five membrane protein complexes
which participate in electron transport and the concomitant synthesis of the energy
carrier molecules NADPH and ATP, which in turn serve to fuel the synthesis of
carbohydrates. Prominent among these are the two main photosynthetic light reaction
centers, membrane protein photosystem I and II complexes (PS I and PS II), named
after the order of their discovery, which is counter to that of their evolution in nature.
Also known as pigment systems I and II, these consist of arrays of associated
chlorophyll and carotenoid antenna pigments, the molecules involved in harvesting
light energy for photosynthesis, arranged in such a way as to maximize light energy
capture and transfer (Cooper 2000; Mishra 2004). Chlorophyll a (Chl a) is the main
pigment in photosynthesis, occurring at the light reaction centers in all photosynthetic
organisms (Farabee 2007). In PS II, the reaction center Chl a is known as P-680
Fig. 6.2 Schematic presentation of the photosynthetic apparatus and the chemical reactions of
photosynthesis (adapted from Falkowski and Raven 2007)
104 M.T. Naznin and M. Lefsrud
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based on its excitation wavelength, while at PS I the form of Chl a is P-700 (Mishra
2004). Accessory antennae pigments are highly conserved in higher plants and
include chlorophyll b (Chl b) and the carotenoid, b-carotene, and the carotenoids
subset xanthophylls, lutein, violaxanthin, anteroxanthin, and zeaxanthin. The carotenoid
and xanthophyll pigments are lipid soluble yellow, orange, and red secondary
plant pigments that are uniquely synthesized in plants, algae, fungi, and
bacteria (Sandmann 2001). They surround the light reaction centers where they
harvest light energy and channel it, through resonance energy transfer, to Chl a at the
reaction center. In cotton (Gossypium hirsutum L.), lutein is the predominant carotenoid
in PS II, while b-carotene is the predominant carotenoid in PS I (Thayer and
Bjorkman 1992). In the PS II complex, b-carotene is highly concentrated close to the
reaction center, while lutein is present in several light-harvesting antennae components
(Demmig-Adams et al. 1996). Photosynthesis is activated when sufficient
photon energy excites electrons in the P-680 form of the Chl a pigment in PS II,
ejecting the electrons from it, effectively oxidizing them. The electrons are then
replaced by the photolysis of water within the thylakoid lumen, which splits it into
two hydrogen ions (protons, H+) and free O2− ions. The O2− ions combine to form the
released diatomic O2, and the protons which remain in the thylakoid lumen contribute
to establishing a proton gradient across the thylakoid membrane, energizing it with a
potential energy which ultimately serves in ATP synthesis and/or photoprotection
(Cooper 2000; Falkowski and Raven 2007). The electron transport chain of PS II
(Fig. 6.2) transfers the segregated high-energy electrons to plastoquinone (PQ) in the
membrane. Plastoquinone then siphons the electrons to the second protein complex,
cytochrome bf, where they lose energy pumping additional protons into the thylakoid
lumen. Plastocyanin (PC) then transfers the depleted electrons to PS I, where photon
light energy excites the P-700 Chl a molecule, thereby raising those same electrons
back to a higher energy, excited state. When the absorption of light radiation exceeds
the capacity of photosynthesis, excess excitation energy can result in the formation of
triplet excited chlorophyll (3Chl) and reactive singlet oxygen (1O2). Carotenoid
pigments protect photosynthetic structures by quenching excited 3Chl to dissipate
excess energy (Frank and Cogdell 1996) and binding 1O2 to inhibit oxidative damage
(Demmig-Adams et al. 1996). Ferrodoxin (FD) transfers these to the fourth thylakoid
membrane protein complex, NADP reductase, where NADP+ is reduced to NADPH
in the chloroplast stroma. Finally, the fifth thylakoid membrane complex, ATP
synthase, converts ADP and inorganic phosphate to ATP, using the proton motive
force from the proton gradient established by the photolysis of water and the flow of
electrons through the cytochrome bf complex to run its proton pump in reverse. In the
chloroplast stroma, ATP and NADPH then fuel the fixation of atmospheric CO2 and
its incorporation into the three carbon sugar glyceraldehyde-3-phosphate in the
Calvin cycle reactions, mediated by the enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase), thought to be the most abundant protein on earth (Cooper 2000; Mishra 2004; Farabee 2007; Falkowski and Raven 2007).


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