Mechanism of Photosynthesis

Light Reaction

Light reaction occurs in grana fraction of chloroplast and in this reaction are included those activities, which are dependent on light. Assimilatory powers (ATP and NADPH₂) are mainly produced in this light reaction.

Robin Hill (1939) first of all showed that if chloroplasts extracted from leaves of Stellaria media and Lamium album are suspended in a test tube containing suitable electron acceptors, e.g., Potassium ferroxalate (Some plants require only this chemical) and potassium ferricyanide, oxygen is released due to photochemical splitting of water. Under these conditions, no CO₂ was consumed and no carbohydrate was produced, but light-driven reduction of the electron acceptors was accompained, by O₂ evolution.
4Fe³⁺+2H₂O ↔ 4Fe³⁺+4H⁺+O₂
The splitting of water during photosynthesis is called photolysis. This reaction on the name of its discoverer is known as Hill reaction.

Hill reaction proves that

(1)    In photosynthesis oxygen is released from water.

(2)    Electrons for the reduction of CO₂ are obtained from water [i.e., a reduced substance (hydrogen donor) is produced which later reduces CO₂],

Dichlorophenol indophenol is the dye used by Hill for his famous Hill reaction.

According to Amon (1961), in this process light energy is converted to chemical energy. This energy is stored in ATP (this process of ATP formation in chloroplasts is known as photophosphorylation) and from electron acceptor NADP⁺, a substance found in all living beings NADP*H is formed as hydrogen donor. Formation of hydrogen donor NADPH from electron acceptor NADP⁺ is known as photoreduction or production of reducing power NADPH. 

Quantum Yield

(i) Rate or yield of photosynthesis is measured in terms of quantum yield or O₂ evolution, which may be defined as, "Number of O₂ molecules evolved per quantum of light absorbed in photosynthesis."

(ii) Quantum requirement in photosynthesis = 8, i.e., 8 quanta of light are required to evolve one mol. of O₂.

(iii) Hence quantum yield = 1 /8 = 0.125 (i.e., a fraction of 1) as 12%. 

Emerson effect and Red drop: R. Emerson and C.M. Lewis (1943) observed that the quantum yield of photosynthesis decreases towards the far red end of the spectrum (680nm or longer). Quantum yield is the number of oxygen molecules evolved per light quantum absorbed. Since this decrease in quantum yield is observed at the far region or beyond red region of spectrum is called red drop.

Emerson et al. (1957) further observed that photosynthetic efficiency of light of 680nm or longer is increased if light of shorter wavelengths (Less than 680nm) is supplied simultaneously. When both short and long wavelengths were given together the quantum-yield of photosynthesis was greater than the total effect when both the wavelengths were given separately. This increase in photosynthetic efficiency (or quantum yield) is known as Emerson effect or Emerson enhancement effect. 

E = (Quantum yield in combined beam - Quantum yield in red beam)/Quantum yield in far red beam

Two pigment systems: The discovery of Emerson effect has clearly shown the existence of two distinct photochemical processes, which are believed to be associated with two different specific group of pigments.  
(i) Pigment system I or Photosystem I: The important pigments of this system are chlorophyll a 670, chlorophyll a 683, chiorophyll a 695, P700' Some physiologists also include carotenes and chlorophyll b in pigment system I. P₇₀₀ acts as the reaction centre. Thus, this system absorbs both wavelengths shorter and longer than 680nm.

(ii) Pigment system IT or photosystem II: The main pigments of this system are chlorophyll a 673, P₆₈₀, chlorophyll band phycobilins. This pigment system absorbs wavelengths shorter than 680nm only. P₆₈₀ acts as the reaction centre. 

Pigment systems I and II are involved in non-cyclic electron transport, while pigment system I is involved only in cyclic electron transport. Photosystem I generates strong reductant NADPH. Photosystem II produces a strong oxidant that forms oxygen from water. 

During the light reaction, the primary function of the two pigment systems/photosystems is to interact with each other to trap light energy and convert it into the chemical energy (ATP). These reactions are cyclic and non cyclic types.

Antenna, or accessory pigments receive radiant energy and transfer it among themselves. This transfer of energy is known as resonance transfer. Then antenna gets molecules excited and transfer their energy to the chlorophyll 'a' molecules of reaction centre.

It is known as inductive resonance. Finally chI. 'a' molecules converts the light energy into electrical energy by bringing about electric charge separation

(I) Cyclic ETS and Photophosphorylation:

In cyclic ETS, only PS-I (LHC-I) works, which consists of Chl-'a' 670, Chl-a-683, Chl-'a'-695, carotenoids, some molecules of chl- 'b' & reaction centre-Chl-'a'-700/P-700 . 

Cyclic ETS OR PS-I is activated by wavelength oflight greater than 680nm.

it occurs at grana thylakoids and stroma thylakoids.

During cyclic ETS the electron ejected from reaction centre of PS-1, returns back to its reaction centre.

In cyclic ETS, no oxygen evolution occurs, because photolysis of water is absent.

Phosphorylation takes place at two places, thus two ATP generate in each cyclic ETS.

NADPH₂ (reducing power) is not formed in cyclic process.

Plastocyanin (PC) is Cu-containing blue protein in cyclic.

According to modem researches, first e^- acceptor is FRS (Ferredoxin Reducing Substance), which is a Fe-S containing protein. Earlier fd (Ferredoxin) was considered as first e^- acceptor.) 

(II)  Z-Scheme/Non-cyclic ETS and PhotophosphOrylation:

Both PS-I and PS-H is involved in non cyclic ETS.

PS-II (P-680) consists ofChl-a-660, Chl-a-673, Chl-a-680, Chl­a-690, Chl-b, or Chl-c or Chl-d, carotenoids & phycobilins. Phycobilins present only in PS II.

It occurs at grana thylakoids only.

The e- ejected from PS-II never goes back to chl-a-680 (reaction centre) & finally gained by NADP. Thus gap of e^- in PS-II is tilled by photolysis of water as a result, oxygen evolution occurs in Z-scheme.

Each turn of non cyclic ETS· produces 1 ATP and 2NADPH₂ (4 mol. of water is photolysedand 1O₂ released)

12 NADPH₂ + 18ATP are required as assimilatory power to produce one molecule of glucose in dark reaction, thus 6 turns of Z-scheme are necessary for the production of one glucose molecule by calvin cycle.

Additional 12 ATP come from 6 turn of cyclic ETS. (over all 54 ATP equivalents).

Primary e^- acceptor in non-cyclic reaction is PQ or plastoquinone. Recently pheophytin (structure like chI. a without Mg) is considered as 1^st e^- acceptor in Z-scheme.

Plastocyanin (PC) is link between PS-I and PS-II in non cyclic ETS. (Some scientists-cyto-f)

Final e^- acceptor in Zs-scheme is NADP⁺ (Hill reagent)

During non-cyclic ETS energy flow takes place from PS II to PS I.

(B) Dark Reaction:

Blackman reaction is called as dark reaction, because no direct light is required for this. Calvin presented these reactions in cyclic manner & thus called as Calvin cycle.

1^st stable compound of Calvin cycle is 3C-PGA (Phosphoglyceric acid) thus. Calvin cycle is called as C₃- cycle.

(First compound is unstable, 6C keto acid)

Study by Calvin was on Chlorel/a & Scenedesmus. During his experiment he used chromatography & radio isotopy (C¹⁴) techniques for detecting reactions of C_3-cycle.

Rubisco (Ribulose bis-phosphate carboxylase oxygenase) is in enzyme in Crcycle, whieh is present in stroma & it makes 16% protein of chloroplast. Rubisco is most abundant enzyme.

CO₂-acceptor in Calvin cycle is RuBp. This carboxylation reaction is' catalysed by Rubisco.

C₃, C₄, C₅, C₆ and C₇ monosaccharides are intermediates of Calvin Cycle.

C₃ = Phosphoglyceraldehyde and DHAP, C₄ = Erythrose,

C₅ = Xylulose, Ribose, C₇ = Sedoheptulose.

The largest monosaccharide in livings are 7C-Sedoheptulose­P(Ketose)

War burg effect: Inhibitory effect of high cone. of O₂ on photosynthesis is called as Warburg effect (It is due to Photorespiration).

6 turns of Calvin cycle are required for the formation of one glucose. 

Bariochemical Reaction of Calvin Cycle

Carboxylation:

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