Light Reactions and Pigments

Light reaction (Photochemical reactions) : 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.

\mathop {4F{e^{3 + }}}\limits_{\begin{subarray}{l} #xA0{\text{Electron}} \\ #xA0{\text{acceptor}} \end{subarray}}#xA0#xA0+ \mathop {2{H_2}O}\limits_{\begin{subarray}{l} #xA0{\text{Electron}} \\ #xA0\,\,{\text{donor}} \end{subarray}}#xA0\overset {\,\,\,\,\,\,\,} \longleftrightarrow \mathop {4F{e^{2 + }}}\limits_{\begin{subarray}{l} #xA0{\text{Reduced}} \\ #xA0\,{\text{Product}} \end{subarray}}#xA0#xA0+ 4{H^ + } + {O_2} \uparrow

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 Arnon (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.

Light phase can be explained under the following headings :

(1) Transfer of energy : When photon of light energy falls on chlorophyll molecule, one of the electrons pair from ground or singlet state passes into higher energy level called excited singlet state. It comes back to hole of chlorophyll molecule within 10^𠄹 seconds.

This light energy absorbed by chlorophyll molecule before coming back to ground state appears as radiation energy, while that coming back from excited singlet state is called fluorescence and is temperature independent. Sometimes the electron at excited singlet state gets its spin reversed because two electrons at the same energy level cannot stay for some time it fails to return to its partner electron. As a result it gets trapped at a high energy level. Due to little loss of energy, it stays at comparatively lower energy level (Triplet state) from excited singlet state. Now at this moment, it can change its spin and from this triplet state, it comes back to ground state again losing excess of energy in the form of radiation. This type of loss of energy is called as phosphorescence.

When electron is raised to higher energy level, it is called at second singlet state. It can lose its energy in the form of heat also. Migration of electron from excited singlet state to ground state along with the release of excess energy into radiation energy is of no importance to this process. Somehow when this excess energy is converted to chemical energy, it plays a definite constructive role in the process.

(2) 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%.

(3) 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 = \frac{{{\text{Quantum#xA0yield in combined beam}} - {\text{Quantum#xA0yield in red beam}}}}{{{\text{Quantum#xA0yield in far red beam}}}}

(4) 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, chlorophyll 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 II or photosystem II : The main pigments of this system are chlorophyll a 673, P₆₈₀, chlorophyll b and 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.

(5) Photophosphorylation : Light phase includes the interaction of two pigment systems. PS I and PS II constitute various type of pigments. Arnon showed that during light reaction not only reduced NADP is formed and oxygen is evolved but ATP is also formed. This formation of high energy phosphates (ATP) is dependent on light hence called photophosphorylation.

Photophosphorylation is of two types :

(i) Cyclic photophosphorylation : The system is found dominantly in bacteria. It involves only PS I. Flow of electron is cyclic. If NADP is not available then this process will occur. When the photons activate PS I, a pair of electrons are raised to a higher energy level. They are captured by primary acceptor which passes them on to ferredoxin, plastoquinone, cytochrome complex, plastocyanin and finally back to reaction centre of PS I i.e., P₇₀₀. At each step of electron transfer, the electrons lose potential energy. Their trip down hill is caused by the transport chain to pump H⁺ across the thylakoid membrane. The proton gradient, thus established is responsible for forming (2 molecules) ATP. No reduction of NADP to NADPH+ H⁺. ATP is synthesized at two steps.

(ii) Non cyclic photophosphorylation : The system is dominant in green plants. It involves both PS-I and PS-II. Flow of electrons is unidirectional. Here electrons are not cycled back and are used in the reduction of NADP to NADPH₂. Here H₂O is utilized and O₂ evolution occurs. In this chain high energy electrons released from ‘P-680’ do not return to ‘P-680’ but pass through pheophytin, plastoquinone, cytochrome b₆-f complex, plastocyanin and then enter P-700. In this transfer of electrons from plastoquinone (PQ) to cytochrome b₆-f complex, ATP is synthesized. Because in this process high energy electrons released from ‘P-680’ do not return to ‘P-680’ and ATP (1 molecules) is formed, this is called Noncyclic photophosphorylation. ATP is synthesized at only one step.

This non-cyclic photophosphorylation is also known as Z-scheme (because of shape of path of electron-flow) and this was given by Hill and Bendall (1960). Non-cyclic photophosphorylation or Z-scheme is inhibited by CMU and DCMU.

(DCMU is a herbicide which kills the weed by inhibiting CO₂ fixation as it is a strong inhibitor of PS-II).

Pseudocyclic photophosphorylation : Arnon and his coworker (1954) demonstrated yet another kind of photophosphorylation. They observed that even in absence of CO₂ and NADP, if chlorophyll molecules are illuminated, it can produce ATP from ADP and iP (Inorganic phosphate) in presence of FMN or vit. K and oxygen. The process is thus very simple and requires no net chemical change for the formation of ATP and water. Arnon called this oxygen dependent FMN catalysed photophosphorylation or pseudocyclic photophosphorylation which involves the reduction of FMN with the production of oxygen. FMN is an auto-oxidisable hydrogen acceptor with the effect that the reduced FMN is reoxidised by oxygen. Thus the process can continue repeatedly to produce ATP.

Since this process can be continuously self repeated, it appears that a single molecule of water should be sufficient to operate pseudocyclic photophosphorylation to meet the requirement of ATP.