In this article we will discuss about:- 1. Photosystem II (PS II) 2. Photosystem I (PS I) 3. The Light Reaction (Hill Reaction).
Photosystem II (PS II):
The light-driven reaction of photosynthesis also called light reaction (Hill reaction), referred to as electron transport chain, were first propounded by Robert Hill in 1939. The electron transport chain of photosynthesis is initiated by absorption of light by photosystem II (P68o).
When P680 absorbs light, it is excited and its electrons are transferred to an electron acceptor molecule. Therefore, P680 becomes a strong oxidising agent, and splits a molecule of water to release oxygen. This light-dependent splitting of water molecule is called photolysis.
However, manganese, calcium and chloride ions play important roles in photolysis of water. After photolysis of water, electrons are generated, which are then passed to the oxidised P680. Now, the electron deficient P680 (as it had already transferred its electrons to an acceptor molecule) is able to restore its electrons from the water molecule.
After accepting electron from the excited P680, the primary electron acceptor is reduced. The primary electron acceptor in plants is pheophytin. The reduced acceptor which is a strong reducing agent, now donates its electrons to the downstream components of the electron transport chain.
Photosystem I (PS I):
Similar to photosystem II (P680), photosystem I (P700) is excited on absorption of light and gets oxidised, and transfers its electrons to the primary electron acceptor (pheophytin), which, in turn gets reduced. While the oxidised P700 draws electrons from photosystem II, the reduced electron acceptor of photosystem I, transfers electrons to ferredoxin and ferredoxin-NADP reductase to reduce NADP to NADPH2.
NADPH2 is a powerful reducing agent, and is utilised in the reduction of CO2 to carbohydrates in the carbon reaction of photosynthesis. The reduction of CO2 to carbohydrates requires energy in the form of ATP, produced through electron transport chain. Process of ATP formation from ADP in the presence of light in chloroplasts is called photophosphorylation.
The Light Reaction (Hill Reaction):
The light reaction is thought to be responsible for the production of a ‘reducing power’ and oxygen from water as a result of light energy. This is as follows: The light energy, after absorption by chlorophyll, splits H2O.
H2O + Light → (H) + (OH)
(i) The (H) combines with an unidentified compound (probably ferredoxin) and is passed from this to NADP.
(ii) The NADPH2 can cause the reduction of phosphoglyceric acid……. Phosphoglyceraldehyde, together with some ATP production.
(iii) The (OH) forms H2O and oxygen:
4 (OH) → 2H2O + O2
The light reaction gives rise to two very important productions:
(i) A reducing agent NADPH2 and
(ii) An energy rich compound ATP.
These two products of the light reaction are utilized in the dark phase of photosynthesis.
The energy transformations in photosynthesis are as follow:
(i) The radiant energy of an absorbed quantum is transformed into the energy of an activated pigment molecule
P + Light energy → P
(pigment molecule or activated pigment)
(ii) Now the activated pigment removes an electron from the hydroxyl ion derived from the water molecule. The (OH) represents the ‘free radical’. These are uncharged, but highly reactive forms.
(iii) The free radicals react in many ways; the release of oxygen and formation of free radicals of hydrogen takes place.
2 (OH) → O2 + 2(H)
(iv) The H+ ions from water, together with the electron attached to the pigment are transferred to certain molecules, which then carry the reducing power to other reactions.
NADP + H+ + e− → NADP.H
(v)Another reaction is the recombination of the split products of water into the water molecules itself.
(H) + (OH) → H2O
This reaction is strongly energy-releasing. The chloroplast puts this reaction to work by causing it to synthesize energy-rich ATP from a precursor molecule ADP and inorganic phosphate
(6) The energy of the ATP can now be used, in the reduction of CO2 to sugar by the reducing power (NADP.H) generated in the light reaction.
This way, the radiant energy has been converted to the chemical energy of the sugar molecule by passing through a photo-activated pigment, photolyzed water fragments, and ATP. The main function of light energy in photosynthesis is to produce ATP through a complex of reactions called photophosphorylation.
The subsequent reactions leading to the formation of sugar from CO2 can proceed entirely in darkness.
With the discovery that CO2 can be assimilated in isolated chloroplasts, this came into existence that the chloroplast must contain the enzymes necessary for this assimilation and must be able to produce the ATP (adenosine tri-phosphate) essential for the formation of the main photosynthesis products.
Arnon and his co-workers (1954) demonstrated that the isolated chloroplasts can produce ATP in the presence of light. They gave the name to this process photosynthetic phosphorylation.
This was revealed for the first time that mitochondria are not the only cytoplasmic particles that produce ATP. ATP formation in chloroplasts differs from that in mitochondria in that it is free from respiratory oxidations. During this process the light energy is being converted to ATP. In other words, there is a conversion to light energy of chemical energy.
ATP is only one of the necessary requirements for the reduction of carbon dioxide to the carbohydrate level. A reductant must be formed in photosynthesis that will provide the hydrogens or electrons for this reduction. Arnon (1951) demonstrated that isolated chloroplasts are capable of reducing pyridine nucleotides in light.
The photochemical reaction and an enzyme system are capable of utilizing the reduced pyridine nucleotide as soon as this was formed, Arnon (1957) found that NADP. H2 is the reduced pyridine nucleotide in photosynthesis.
In the presence of H2O. ADP (adenosine di-phosphate) and orthophosphate (P), substrate amounts of NADP (nicotinamide adenine dinucleotide phosphate) were reduced, accompanied by the evolution of oxygen.
The equation is as follow:
2ADP+ 2P+ 2NADP+ 4H2O → 2ATP+ O2+ 2NADPH2+ 2H2O
As shown by the equation the evolution of one molecule of oxygen is accompanied by the reduction of two molecules of NADP and esterification of two molecules of orthophosphate. Together, ATP and NADPH2 provide the energy requirements for CO2 assimilation. Arnon gave name to this power assimilatory power (i.e., ATP + NADPH2).
According to Arnon (1967), in bacterial photosynthesis NADH2 is utilized of NADPH2.
In the late 1950’s the reduction of NADP+ was thought to be associated with a soluble protein factor found in chloroplasts. Arnon et al. (1957) observed that this protein reduced NADP+ accompanied by the evolution of oxygen. They termed it the ‘NADP reducing factor.’
Thereafter the NADP reducing factor was purified and called photosynthetic pyridine nucleotide reductase (PPNR), since its catalytic activity was only apparent when chloroplasts were kept in light.
Tagawa and Arnon (1962) recognized that PPNR is one of a family of nonhemenonflavin, iron-containing proteins that is universally present in chloroplasts. These proteins were given a generic name ferredoxin.
When ferredoxin was not discovered, NADP was thought to be the terminal electron acceptor of the photosynthetic light reaction. Arnon (1967) revealed that illuminated chlorophyll reacts directly with ferredoxin and not with NADP+.
The exposition of chlorophyll to light causes a flow of electrons to ferredoxin. Now the reduced ferredoxin causes the reduction of NADP+ in an enzyme catalyzed reaction that is independent of light. In other words, ferredoxin is termed as terminal electron acceptor of the photosynthetic light reaction.
The reduction of NADP takes place by ferredoxin. Under normal condition, in photosynthesis ferredoxin reduced by the acceptance of an electron is immediately reoxidized by NADP+. The reduction of NADP by ferredoxin is catalyzed by ferredoxin-NADP reductase. This shows that the mechanism of NADP+ reduction in photosynthesis completes in three steps.
These steps are:
(i) Photochemical reduction of ferredoxin;
(ii) Reoxidation of ferredoxin by ferredoxin NADP+ reductase and
(iii) Reoxidation of ferredoxin-NADP+ reductase by NADP+.
According to Arnon there are two types of photophosphorylation:
(i) Non-cyclic photophosphorylation and
(ii) Cyclic photophosphorylation.
This is a result of an interaction of photosystem I (PSI) and photosystem II (PSII). In non-cyclic photophosphorylation, the electron is not returned to the chlorophyll molecule, but is taken up by NADP ± which thereafter reduces to NADPH. Here the electron that returns to the chlorophyll molecule is derived from an outside source which is water.
In this process oxygen is released and both NADPH2− and ATP are formed. In green plants and many photosynthetic bacteria, however, illumination is known to produce also NADPH2− which provides hydrogen for the reduction of carbon dioxide in the day.
The electron lost by the excited chlorophyll is accepted by NADP along with a proton resulting in the formation of NADPH2. The light energy is now stored in the NADPH2 molecule. The proton required for the reduction of NADP is released from the dissociation of water molecule by photolysis into hydrogen H± and hydroxyl ions OH.
2H2O → 2H+ + 2(OH)
The hydroxyl ions react to produce water and molecular oxygen.
The reaction is as follows:
4 (OH) →2H2O + O2 + 4e−
Here the hydroxyl ion also releases an electron that is accepted by the cytochromes of the chloroplast. In turn, the cytochrome donates this electron to the chlorophyll molecule, which already lost an electron earlier. The energy released during this transfer of electron from the cytochrome is utilized in the formation of ATP by the photophosphorylation of ADP.
In water molecule hydrogen is strongly bound to oxygen and this can be cleaved only by the use of energy. This energy is supplied by light. This way, in non-cyclic photophosphorylation light energy takes part in two processes, i.e., the activation of chlorophyll molecule and photolysis (cleavage) of water.
In non-cyclic photophosphorylation one molecule of NADPH2 and one molecule of ATP are produced by the activation of chlorophyll molecule by a photon, while in cyclic photophosphorylation two molecules of ATP are produced for each photon absorbed by chlorophyll.
The overall reaction of photophosphorylation is as follows:
When non-cyclic photophosphorylation is stopped under certain conditions, cyclic photophosphorylation takes place. The non-cyclic photophosphorylation can be stopped by illuminating isolated chloroplasts with light of wavelength greater than 680 nm.
By this way, only photosystem I (PS I) is activated, as it has a maximum absorption at 700 nm, and photosystem II (PS II), which absorbs at 680 nm, remains inactivated.
Due to inactivation of PS II, the electron flow from water to NADP is stopped, and also CO2 fixation is retarded.
When CO2 fixation stops, electrons are not removed from reduced NADPH. Thus, NADPH will not be oxidised and NADP will not be available as an electron acceptor.
Under above-mentioned conditions, cyclic-photophosphorylation occurs.
During cyclic-photophosphorylation, electrons from photosystem I (PS I) are not passed to NADP from the electron acceptor, as NADP is not available in oxidised state to receive electrons.
Hence, the electrons are transferred back to P700.
This type of movement of electrons from an electron acceptor to P700 result in the formation of ATP from ADP, and the process is called cyclic photophosphorylation.
During cyclic photophosphorylation oxygen is not released, as there is no photolysis of water and NADPH2 is also not produced.
In cyclic photophosphorylation the excited electron lost by the chlorophyll is returned to it through vitamin K or FMN (flavin mononucleotide) and cytochromes. The chlorophyll molecule on losing an electron assumes a positive charge and subsequently the electron is transferred to a second acceptor.
This second acceptor is a group of substances collectively known as cytochrome system. All the members of cytochrome system are variants of cytochrome. Ultimately these cytochromes transfer the electron to the chlorophyll molecule from where it was lost initially.
The electromagnetic energy of the light is utilized in the formation of ATP. This means that light energy is being converted into chemical energy. Here the electron after leaving a chlorophyll travels in a cyclic way and ultimately returns to the same molecule from which it initiated, and therefore, this process has been termed by Arnon as cyclic photophosphorylation.
The final electron acceptor and the initial electron donor is the same substance—the chlorophyll. No outside material takes part in the process. During cyclic photophosphorylation, one electron and two ATP molecules are formed.
One ATP molecule is being formed when the electron travels from the cofactor (i.e., vitamin K or FMN) to the cytochromes while the other when it travels from the cytochromes back to the chlorophyll molecule.
Here the light energy is being converted into chemical energy.
In nature both processes of photophosphorylation proceed simultaneously. In green plants the non-cyclic electron transfer is essential for the production of NADPH2 and ATP.
The oxygen is evolved during the process. The cyclic electron transfer fulfils the requirement of the low yield of ATP during non-cyclic process. This way, the complete light phase of photophosphorylation produces ATP and NADPH2 and oxygen is evolved.
NADPH2 is a biological reductant that brings about the reduction of carbon dioxide to carbohydrates in the dark phase of photosynthesis. Here both NADPH2 and ATP provide energy for reduction. The assimilatory power of the cell is constituted by these two components. The energy of these components is derived from visible part of sunlight.
In the dark phase of photosynthesis the energy that is stored in NADPH2 and ATP, is being transferred to the molecules of organic substances and stored there in the form of chemical energy.
During photosynthesis the electromagnetic energy of visible light is being converted into chemical energy. Now this energy is utilized by living cells as the driving force for various vital activities. This act of the conversion of energy is brought about by the photosynthetic cells of green plants or photosynthetic bacteria.
Here the solar energy is trapped by the chlorophyll apparatus. As soon as the light energy is being transformed into chemical energy, it may be used in the formation of carbohydrates, protein synthesis and other important vital activities.
The living are so designed that they can use only chemical energy for various metabolic activities. The light energy cannot be directly used for these vital activities. The light reaction of the higher plants takes place in the grana of the chloroplasts.