Mechanism of Photosynthesis: The Light Reaction

CHAPTER: 10 (C)

Mechanism of Photosynthesis: The Light Reaction

Mechanism Of Photosynthesis:

Ø  Photosynthesis is a complex process which involved successive oxidation and reduction process.

Ø  This involved oxidation of water and reduction of Carbondioxide.

Ø  The reaction of photosynthesis occur in two distinct phases:

            1. The light rxn: Uses light energy to synthesize NADPH and ATP.

            2. The dark rxn: Uses the NADPH and ATP to synthesize carbohydrates from

                                        CO₂    & H₂O.

1. The light rxn:

(Light Dependent phase, Photochemical rxn., or Hill’s reaction)

•      It is a photochemical reaction which takes place in grana of thylakoids of  chloroplast in the presence of sunlight.

•      It explains that water is used as a source of electrons for CO₂  fixation & O₂   is evolved as a by product.

•      The final products of this reaction are NADPH₂  , ATP and Oxygen.

Ø  The light reaction can be discussed in the following headings:

            1.         Red Drop & Emersion Enhancement Effect

            2.         Photosystem or Two pigment system

            3.         Photo Excitation of Chlorophyll- a

            4.         Photolysis of Water

            5.         Photophosphorylation

1. Red Drop & Emersion Enhancement Effect:

•      Quantum yield  is defined as the number of oxygen molecules released per light quanta absorbed.

•      Robert Emerson  concluded that 8 quanta of light energy would be required for the reduction of one molecule of carbondioxide to carbohydrate ( or for produucing one molecule of oxygen). The quantum yield (yield per quantum) is thus 1/8 or 12% only.

•      The transfer of four electrons are required in the reduction of one CO₂ molecule. It was suggested that it takes two light quanta to move each electron.

•       Emerson and Lewis (1943) determined the quantum yield of photosynthesis under different wavelength and found that there was a pronounced decrease in quantum yield at wavelength greater than 680 nm in the red zone. This discovery of Emerson was termed as “Red Drop”.

Emerson and his Co-worker  later found that the decrease could be prevented & photosynthetic rate can be enhanced by using shorter wavelength (red light) with longer wavelength infrared light. The quantum yield is increased when light of two different wavelength are given simultaneously. This photosynthetic enhancement is referred to as the Emerson Effect.

Emerson et al. (1957) found that both the red and the far-red light gave low rates of photosynthesis when given individually but gave a rate greater than the sum of the individual rates when the two were given simultaneously

2. Photosystem or Two pigment system:

•      From the discovery of Red Drop and Emerson Enhancement Effect, it was concluded that photosynthesis is driven by two photochemical processes.

•      These processes are associated with two groups of photosynthetic pigments called as pigment system I and pigment system II.

•      Chlorophyll- a exists in its two different forms, one of them absorbs light with maximum 683 nm , called pigment system I (PS I) and the other with a maximum of 672 nm , called pigment system  II (PS II).

•      For PS I the light is absorbed by Chlorophyll-a , P-700 nm, Chlorophyll-b and carotenoids.

•      For PS II light is absorbed by chlorophyll- b, phycobilins and carotenoids. The P-680 nm constitte the rxn. Center for PS II.

•      Each photosystem contains about 300 chlorophyll molecules that trap light energy.

•      In each photosystem, a molecule of chlorophyll –a is the primary pigment.

•      The other chlorophyll molecules are known as accessory pigments, as they absorbs light energy & transfer to chlorophyll-a

•      The accessory pigments include carotenoids, phycobillins & chlorophyll b, c & d.

3. Photo Excitation of Chlorophyll- a

•      Light energy is absorbed by different pigments like chlorophyll-a, b, c, d , e Carotenoids, phycobillins but this energy is ultimately transferred to chlorophyll –a, which is the main pigment regulating light reaction.

•      Chlorophyll-a absorbs photon of light & becomes activated.

•      At this state chlorophyll-a expels one electron & develops a positive charge.

•      The expelled electron contains extra amount of energy which is used during the formation of ATP.

5. Photophosphorylation:

•      The process of formation of ATP from ADP and inorganic phosphate (Pi) during photosynthesis process is known as photophosphorylation.

Ø  It takes place in two different ways:

                        1. Non-Cyclic Photophosphorylation

                        2. Cyclic Photophosphorylation

1. Non-Cyclic Photophosphorylation:

•      In this process, the electron expelled from chlorophyll molecule is not cycled back to the same chlorophyll P- 680.

•      Electron transport in two photosystem (PSI & PS II ) takes place in three steps:

            1.         From H₂O to P-680

            2.         From P-680 to P-700

            3.         From P-700 to NADPH

•      Electrons are transported from water to P-680 (PS II) through Mn containing enzymes.

•      The ultimate source of electrons within the chloroplast is water.

•      One molecule of oxygen is evolved for every four electrons donated to P-680 and one NADPH molecule requires two electrons for its production.

•      Two quanta (Photons) are needed for excitement of one electron.

Ø  The successive steps involved in transfer  of an electron from : H₂O to PS II to PS I to NADP are given below:

1. Electron passes from water to PS II or P-680 to fill up the vacancy created by excitement of chlorophyll molecule and emission of electron.
2. P-680 emits its electron & the electrons are received by a substance Q.
3. This electron is then passed to electropositive substance called photoquinone (PQ).
4. The next acceptor of electron is cytochrome which exists in two forms cytochrome b₆  and Cytochrome-f (Cyt b₆  & Cyt f)

In this step one molecule of ATP is formed by ADP & inorganic Phosphate (Pi).

5. Cyt f then passes its electron to next compound plastocyanin (PC) .
6. Plastocyanin handover the electrons to P-700 (PS I).
7. After absorption of second photon in PS I, this electron is then reached to NADP from PS I.
8. The electrons from P-700 are received by Ferredoxin reducing substance (FRS).
9. This FRS finally emits electrons and reduces Ferredoxin (FD).
10. Here NADP is reduce to NADPH in presence of an enzyme Ferredoxin-NADP – reductase. This reduced NADPH is utilized for the reduction of CO₂ to Carbohydrates.

•      H⁺  ions accumulate inside the thylakoids membrane as a result of proton gradient & diffuse across the thylakoid membrane & is used to produce ATP. This process is also known as Chemiosmosis.

2. Cyclic Photophosphorylation:

•      In this process, the electron expelled from chlorophyll molecule is cycled back to the same chlorophyll P- 700.

•      The electrons released by P₇₀₀ of PS-I in the presence of light are taken up by the primary acceptor and are then passed on to ferredoxin (Fd), plastoquinone (PQ), cytochrome complex, plastocyanin (PC) and finally back to P₇₀₀ i.e., electrons come back to the same molecule after cyclic movement. Due to this cyclic movement of electrons , this process is known as cyclic photophosphorylation.

•      The cyclic photophosphorylation also results in the formation of ATP molecules just like in non - cyclic photo phosphorylation.

•      Two molecules of ATP are produced in this cycle.

•      During cyclic photophosphorylation, electrons from photosystem - I are not passed to NADP from the electron acceptor. Instead the electrons are transferred back to P₇₀₀.

•      This downhill movement of electrons from an electron acceptor to P₇₀₀ results in the formation of ATP and this is termed as cyclic photophosphorylation.

•      It is very important to note that oxygen and NADPH₂ are not formed during cycle photophosphorylation.