Atp Formation Is Coupled to Electron Transport During the Light Phase of Photosynthesis, Discuss

INTRODUCTION Overall, light energy drives a flow of electrons along a system of carriers from H2O to NADP+. The carriers are bound to the membrane between reaction centres. Though their precise arrangement is not known, the carriers seem to be organized so that the electron flow cause H+ to move from the stroma to the space within the thylakoids. The resulting difference in H+ concentration across the membrane represents a store of energy that is though to drive he formation of ATP. According to current ideas, the enzymes that form ATP are bound to the thalakoid membrane and are arranged so that the formation of ATP release H+ to the stroma and OH- to the space with in the thylakoid. The OH- combines with H+ to form H2O. These events would decrease the H+ difference across the membrane, “discharging the battery” to form ATP. Electrons move spontaneously along the electron transport chain because each carrier in the chain has a greater tendency to capture and hold electron than the carrier before it.

PHOTOSYNTHESIS Photosynthesis which requires the energy of sunlight by the green chlorophyll pigment for the manufacture of carbonhydrate, carbondioxide and H2O. Van Neil propose a general equation for photosynthesis as follows: CO2 + 2H2A light (CH2O) + H2O + 2A Carbon Hydrogen carbonhydrate Water Dioxide donator The hydrogen donator H2A can be H2O, H2S, H2 or any other substance capable of donating hydrogen to CO2 in the process of photosynthesis. Classic experiments performed by Blackman in 1920 laid the basis that the reaction of photosynthesis were of two types, those requiring light (the light reaction or the photochemical) and those which would proceed in its absence (the dark reaction or the chemical). This separation of photosynthesis was later amplified by the work of Emerson and Arnold (1932) who showed that the light and dark reactions could to separated in time. Cells of the unicellular green alga chlorella were exposed to brief flashes of light (3msec) followed by variable periods of dark.

CHLOROPLAST Chloroplasts, the organelles that perform photosynthesis, are abundant in the cells of leaves and other green parts of plants; the green color is due to light absorbing compounds or pigments within the chloroplast. Tissue that is homogenized in aqueous or sucrose produces two types of chloroplast, which may be separated by density gradient or differential centrifugation. One form of the organelle is devoid of envelope and stroma and consists only of the lamellar systems, while the other form appears to be relatively intact. It has been stated that each chloroplast has three principal parts; 1. A surrounding envelope consisting of a pair of membranes. 2. A liquid called stroma that is enclosed by the envelope. 3. A set of flattened membrane-lined sacs called thylakoids (grana & frets) that are embedded in the stroma.

ENERGY TRANSFER Light energy absorbed by one pigment molecule is thought to be transferred through many other pigment molecule before reaching its site of action. This transfer of light energy may be from one chlorophyll a molecule to another, from chlorophyll b to chlorophyll a, from carotenoids to chlorophyll a, or from phycobilins to chlorophyll a. To have move understanding on how energy can be transferred from molecule to molecule, we must first have a working knowledge of the excited states of molecules, including the ground or singlet state, the excited singlet state, and the triplet state.

The energy absorbed by the chlorophyll may be lost again in a number of different ways; 1. The molecule may lose the energy by the emission of light as the electron reverts to the ground state; this process is known as fluorescence. 2. The molecule can lose the energy as heat by collision or vibration. In some cases the electron may be lose some energy as heat and change to a more stable state known as the triplet state in which the electron spin has been reversed. 3. Excitation energy can also be lost by transfer of the energy to another molecule, either by electric dipole interactions or electron conductance in overlapping orbitals.

ELECTRON TRANSPORT With the discovery that CO2 can be assimilated in isolated chloroplast came the realization that the chloroplast must contain the enzymes necessary for this assimilation and must be able to produce the ATP essential for the formation of the main photosynthetic products. Arnon demonstrated that the isolated chloroplast can, in the presence of light, produce ATP. They termed the process photosynthetic phosphorylation. ATP is only one of the necessary requirements for the reduction of CO2 to the carbohydrate level. A reductant must be formed in photosynthesis that will provide the ‘hydrogens’ or ‘elections’ for this reduction. As far back as 1951, Arnon demonstrated that isolated chlotoplasts are capable of reducing pyridine nucleotide when exposed to light. The photochemical reaction had to be coupled with an enzyme system capable of utilizing the reduced pyridine nucleotide as quickly as it was formed. It was found that NADPH2 is the reduce pyridine nucleotide active is photosynthesis. In the presence of H2O, ADP, and orthophosphate (P), substrate amounts of NADP were reduced, accompanied by the evolution of oxygen in accordance with the equation.
2ADP + 2P + 2NADP + 4H2O 2ATP + O2 + 2NADPH2 + 2H2O There is one location on the photosynthesis electron transport chain where it is theoretically possible for ATP synthesis to occur i.e between cytochromeb6 and cytochromef. Electrons from H2O are transported in a unidirectional manner to ferodoxin and are eventually utilized to reduce NADP+.

CONCLUSION The passage of electrons from H2O to ferrodoxin through electron carriers requires the participation of both photosystems and the results in the synthesis of ATP i.e the excess electron energy that results from the absorption of light by photosystem I causes an electron to be lost from P700 (chlorophyll a in the ground state) and transformed to NADP+. The electron which got lost from P700 leaves it in the oxidized state with a ‘hole’ requiring the return of an election, which is believed to be provided by the weak reductant of photosystem II. The transfer of electrons is postulated not to be direct but to take place down an electron transport chain enabling the formation of ATP coupled to election transport.

REFERENCES
Govindjee, and W. Coleman, “How does photosynthesis make oxygen?” Scientific American 262 (1990).

Nicholls DG, Fergucon SJ. Bioenergetics 3. Academic Press 2002.

Robert M. Devlon, Plant physiology, Devan Nostranal Company. New York.

Stumm W. Morgan JJ. Aquatic Chemistry third edition Wiley; 1996

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Light quatum

Ground State

Excited Singlet State

fluorescence

Diagram depicting the absorption of light quatum by an election.

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References: Govindjee, and W. Coleman, “How does photosynthesis make oxygen?” Scientific American 262 (1990).