When life originated on this planet some 3.8 billion years ago, the first life forms were heterotrophs. They were depending upon both the organic and inorganic compounds available in ‘Mother Ocean’. With time, organic soup created earlier years, almost exhausted by the fast growing and multiplying population of then existing organisms. There was an imminent danger for the survival of heterotrophs. At this juncture, new life forms arose and they had the unique potentiality to capture solar energy and use the same for the synthesis of simpler organic compounds where the solar energy is stored in the form of chemical bond energy. The origin of photosynthetic organisms saved the heterotrophic organisms by providing food materials to them. It is these organisms to produce oil reserves found all over the world, thankless are those who are exploiting the reserves and getting richer and richer. At the same time, they also provided an environment for the diversification and sustenance of organisms.
In the light of knowledge gained in recent years, the process of photosynthesis is defined as “a series of processes in which green plants capture, convert and conserve solar electromagnetic energy in the form of chemical energy” in organic compounds.
Ever since man learnt the art of agriculture, he understood the importance of light in the development of green plants. Nevertheless, it was left to the great Aristotle, a philosopher who noted the relationship between light and greening of plants. Later, Joseph Priestley demonstrated that green plants are capable of purifying air by liberating oxygen, which is called dephlogistication. Considering the purifying effects of plants, Jan Ingenhauze, working alone for a decade or so, cam out with a conclusion that light i.e., solar energy is absolutely essential for the green plants to purify the air. It almost amounted that green plants liberate oxygen in the presence of light.
Green Plants ->O2 (J. Priestley)
Green Plants + light ->O2 (J. Ingenhauze)
Later Jean Seedier (1782), a Swiss Pastor, pointed out that the fixed air (i.e. CO2) is very essential for the purification of air, for oxygen that is liberated during photosynthesis comes from CO2.
Co2 + light -> C+O2 (Jean Senebier)
Nicholas Theodore de Saussure (1804) elucidated the process of photosynthesis in which carbon combines with H2O to produce organic matter. The gain in organic matter, during the growth of green plants in sunlight is due to this process.
C+H2O Green plants (CH2O) n (Nicholas De Saussure)
CO2 + H2O light energy (CH2O) n (Robert Mayer)
Dutrochet (1837) clearly demonstrated the relationship between photosynthesis and green colored chlorophyll. However Julius Robert Mayer (1845) came out with a view that during photosynthesis, plants capture fleeting solar energy particles and store it in the form of chemical energy in organic matter. Sachs (1864), who was a pioneer in experimental botany, demonstrated that the ultimate product of photosynthesis starch.
Co2+H2O+light energy/green plants = starch
Few decades later, Robert Hill (1937) demonstrated that the oxygen liberated during photosynthesis comes from H2O and not from CO2. For the first time radioactive isotopes were used to demonstrate the source of O2 during photosynthesis in isolated chloroplasts. The splitting of H2O into hydrogen and oxygen was then called as Photolysis. Later studies revealed that the splitting of water is not due to photolysis but it is due to photo ionization. This process is called Hill’s Reaction.
H2O+ light / Chloroplast ->(H ) + OH
4OH - > 2H2O + O2
Von Neil showed similar reactions in photosynthetic bacteria, where the hydrogen source supplied was not H2O but H2S.
H2S + light = (CH2O) +S (von Neil)
Though Lei big (1845) and Balckmann (1905) showed the importance of CO2 and other factors essential for photosynthesis, it is Melvin Calvin and his associate Benson (1940-45) elucidated the pathway of carbon fixation. This was a remarkable piece of work for which Calvin was awarded a Nobel Prize. Since then, a large number of workers have contributed their might in understanding and elucidating many facts of this complicated process.
With the exception of fungi almost all-eukaryotic plants contain well-developed chloroplasts. The color of plastids depends upon the dominance of specific pigments. Chloroplasts are bounded by two unit membranes within which liquid is found, it is called stoma or stromatic fluid. Suspended within the stroma there are a number of highly organized membranous structures called Grana.
Each granum is made up of 20-60 pigment containing membranous sacs called Thylakoids. Besides granal structures, stroma also contains all the enzymes necessary for carbon fixation, protein metabolism, nucleic acid metabolism, fatty acid synthesis etc. Even certain phytohormones like Gibberellic acid (GA) and Abscisic Acids (ABA) are synthesized within chloroplasts. Chloroplast membranes also contain unique pigments called phytochromes. The presence of 70s ribosomes and nucleic acids like DNA and RNAs (the genetic material) present in chloroplasts provide information for the development and functions of plastids. However, nuclear coded gene products are also required for the structural organization and functional efficiency of the organelle. That is why plastids do not enjoy complete autonomy, but functions as semi autonomous organelles.
Thylakoid is a circular flat membranous sac and such structures are arranged one above the other similar to a stack of carom coins. Such a stack of thylakoids is called Granum. However some of the thylakoid membranes found in a granum are continuous and they are in contact with other granal structures. Such lamellae are called intergranal lamellae or stromal lamellae.
Structurally, as shown in the figure, the surface A is the core of the thylakoid membrane in which a number of large globular shaped structures are located in such a way they span the entire cross section of the membrane and also a part of them protrude out at both the surfaces. Such larger particulates are called photosynthetic units II or photo system II, often they are called Quantosomes.
The membrane surface also contains a number of smaller particles. Some of them are photo system I structures. The others are little bigger particles. They also contain Cyt F/b6 protein complexes in thylakoid membrane exposed to stromatic fluid. The enzyme complexes like ATP synthetase (F1) and RUBP carboxylase are located on this surface. In fact, a part of this complex of enzymes is buried in the membrane. In addition, there are some more protein complexes like ferrodoxin reducing protein, NADP reductases and other electron transporting protein complexes within the thylakoid membranes. Most of these components are vectorially organized and moreover the above said particles show lateral movement within the dynamic fluid. The thylakoid membranous sac is filled with a fluid which is mostly acidic when chloroplasts are active. The intergranal lamellae contain mostly PSI system and its associated components. The near absence of large PS II particles is a distinct feature of the stromal lamellae or intergranal lamellae. The presence of granular structures was first observed by Park and his associated members and such structures were then called as Quantosomes.
Not all chloroplasts show the structure as described above. Tropical grass members contain leaves where vascular bundles in leaves are surrounded by a distinct layer of cells called bundle sheath. Both mesophyll cells and bundle sheath cells contain chloroplasts, but of different structures and functions. While chloroplasts found in the mesophyll cells contain organized grana with intergranal lamellae, bundle sheath chloroplasts are totally lacking in granal membranes but contain only stromal lamellae with many large starch granules. Interestingly, the chloroplast membranes of mesophyll cells contain both PSI and PS II, but bundle sheath chloroplasts contain just PSI system and PS II is totally absent. Another important feature that distinguishes mesophyll chloroplasts from bundle sheath chloroplasts is the presence of C4 pathway enzymes in the latter. Moreover, most of the chloroplasts found in bundle sheath cells are disposed towards their neighboring mesophyll cells and one can find a large number of protoplasmic connections between sheath cells and surrounding mesophyll cells. The plants which posses such chloroplasts which fix carbon by Hatch and Slack pathway are called C4 plants. Ex. Tropical grass members like crab grass, sugar grain, Zea mays, etc. Even some dicots like Atriplex etc. also show C4 characters.
PHOTOSYSTEM I (PS I)
Recent techniques have been successfully used in isolating PS I and PS II systems into almost pure forms. The biochemical analysis of PS I reveal the presence of various plant pigments and different kinds of proteins. Each photosynthetic unit is made up of 250-300 chlorophyll molecules of which Chl.A is found in greater amounts (25 times) than Chl. B. There are just 40-50 molecules of Carotenoids in the entire complex. Along with the pigments a
large number of light harvesting antenna proteins are present. The mol. Wt. of these proteins vary from 11 KD to 44 KD. The light harvesting proteins (LHPs) are associated with pigments in such a way; the captured light energy is conducted in a manner befitting a semi conductor. Among the LHPs, the core proteins are located in the dome shaped region where they are associated with two distinct Chl.A700 molecules; which act as active centers. The other proteins called peripheral proteins are complexes with pigments they are located around the central dome. The peripheral proteins are capable of conducting photons channeled towards Chl.A 700, the active centre of the PS I system.
In granal membranes, photo system I particulate are also associated with ferrodoxin reducing Fe-S containing proteins (FRS), non-heme iron containing ferrodoxin proteins, FD dependent FD-NADP reductase and copper contained plastocyanin containing protein complex. Some of the above mentioned protein complexes are as large as PS I systems and they are vectorially arranged in the membranes, so as to facilitate the electron flow from Chl. A 700 towards NADP. It is now certain that the ferrodoxin NADP reductase is topographically located towards the outer surface i.e. stromal side of the thylakoid membranes. However, the PS I units found in the intergranal lamellae contain most of the said pigments and protein complexes for light harvesting and photochemical reactions but they are not associated with FD NADP reductase. Instead, PS I am associated with
Cyt.b6-Cyt.f electron transport structures, which are mobile from granal to intergranal membranes. These photosynthetic units perform cyclic photophosphorylation reactions. In the granal membranes, photo system I in association with PS II is responsible for the reduction of NADP to NADPH2 liberation of oxygen and non cyclic photo phosphorylation.
Photo system II (PS II)
Photosystem II are larger photosynthetic units found in thylakoid membranes and but absent from intergranal membranes. They are also made up of pigments and protein complexes. The number of chlorophyll molecules found are 250-300, of which the number of Chl. A present is 7.5 times the number of Chl. B. That means the number of Chl. B molecules present in PS II are many times greater than Chl.B found in PS I system. The PS II also contains about 80-100 molecules of carotinoids. The protein molecules found in PS II vary in their mol. Wt from 25 KD To 55 KD, of which the core proteins have a mol. wt. of 42-55 KD. This forms the central dome shaped structures which again possesses two unique chlorophyll molecules called Chl.A 680. They act as reaction centers. The peripheral complexes act as light harvesting structures. And most of these proteins are complex with various pigments.
The above said photosynthetic units also possess another important protein called ‘Z’ protein, which is complexed with 4-6 Mn2+ molecules. The Z protein is highly hydrophobic. The other complexes which are associated with PS II are cyt.B559 and cyt.f reducing protein complex, pheophytin containing proteins and plastoquinones.
Recently, Kenneth Miller (1984) used techniques like deep etching, neutron diffraction and rotator shadowing to understand the structural organization of photosynthetic units in the chromatophores of Rhodopseudomonos viridis. In these bacterial cells, the photo synthetic units are organized within the chromatophores membrane as an array of regular crystal lattices. Each photosynthetic unit is 10 mm thick at the base and it has a central elevated dome of 7 mm thick. The central dome shaped complex acts as the photoreaction centre. This central dome in turn is surrounded by six peripheral units of 3 mm thickness each. The photoreaction centre is made up of special Chl. A molecules which are complexed with five LHP of mol. Wt. 24-44 KD. The peripheral complexes also contain pigments associated with LHP of 11-16 KD sizes. The entire photosynthetic units weight about 423 KD. Now, it is speculated that the structural organization of photosynthetic units found in higher plants also have similar structural organization.
When, chloroplasts in toto are allowed to absorb white light at different wavelengths they exhibit maximum absorption (peak) at red, blue, green and yellow part of VIBGYOR spectrum and it is called absorption spectrum.
On the other hand, if specific pigments are allowed to absorb light at different wavelengths, chlorophyll shows maximum absorption at 435 mm and 670-680 mm. Similarly Chl. B shows absorption peak at 480 and 650 mm, but carotinoids show a broad absorption spectrum ranging from 420-524 mm .
The part of light that is mainly responsible for photochemical reactions is called action spectrum. Though leaves absorb light at red, blue, green, orange and yellow regions, all are not used for photochemical reactions. It is possible to distinguish between the absorption spectrum and action spectrum by subjecting green leaves, unicellular algae or isolated chloroplasts to single beam of monochromatic light at a particular wavelength or a combination of wavelengths and observe for the release of oxygen or carbon fixation into glucose. Whichever beam of light or a combination of lights that effectively initiates and sustains photosynthesis either in the formation of glucose or release of oxygen is considered as action spectrum. Studies in this regard have revealed that blue and red lights are the action spectrum. If blue or red light are alone used, though there is an initial reaction, the process of photosynthesis does not continue. For the sustained photosynthetic reactions both blue and red lights are required. Such an affect is called Emersion effect or Emersion’s Enhancing Effect. Furthermore, Emersion and others found that if the wavelengths of red light beyond 700 mm along with blue light are used photochemical reactions are inhibited. Such an effect is called Red drop effect. The above observations suggests though plants absorb light at various wavelengths of VIGGYOR spectrum but only blue and red light at particular wavelengths are effective in photosynthesis.
From the analysis of absorption spectrum and action spectrum, as discussed above, it is clear that though plants absorb light at different wavelengths, the absorption bands at 435 and 680-700 are very important for they alone initiate photochemical reactions.
Nonetheless, the white light that is absorbed at other wavelengths is not wasted but used by photo systems. Photons absorbed at different but specific wavelengths by different pigments, transfer the light energy as quantisized units from molecule to molecule through light harvesting proteins. The unit of light energy that is transferred is often referred to as excitons and the process by which excitons are transferred is resonance mechanism.
The photons absorbed by carotinoids are transferred to Chl. B or Chl.A. Similarly, light energy absorbed by Chl. B is transferred to Chl.A. Again among the Chl.A molecules, light energy absorbed at 683 mm is transferred to Chl.A 680 or Chl.A 700. In this process, generally the pigments that absorb light energy at shorter wavelengths are handed over to the pigments that absorb or capable of absorbing light at longer wavelengths. The light harvesting antenna proteins play an important role in transferring the excitons are semiconductors. Whatever may be wavelength of the light absorbed or whatever may be the pigments that absorbs light, ultimately the light energy in the form of excitons are channeled to photo reactive centers in the photosynthetic units. In PS II the light energy is ultimately conveyed to Chl. A680. Similarly in PS I system, light energy is transferred to Chl. A700. The transfer of energy from molecule to molecule is not cent percent. For example, the transfer of energy among the accessory pigments or accessory to primary pigments is 80-90%, but among Chl. A pigments it is 100%.
SOLAR ENERGY AND ITS PRIMARY EFFECT IN PHOTOSYNTHESIS
All living organisms require chemical free energy for their biological activities. The ultimate source of this form of energy comes from the sun as solar electro magnetic radiations. Among all the living organisms, plants with the exception of fungi are alone capable of capturing, converting and conserving the solar energy in chemical bonds in organic molecules like glucose and its derivatives. The same is made available for other living organisms. That is why the food that we eat is considered as bottled sunshine. In fact, the flesh that develops in animals is derived from the green grass. And all the sources available for mankind in the form of fossil, fuel or bio fuel is nothing but photosynthetic capital.
Sun, for that matter any other living star in our universe, by its nuclear reactions liberates enormous amount of energy in the form of solar electromagnetic radiations. These radiations have a wide spectrum of which the visible i.e. perceptible to human eye is mainly responsible for initiating photochemical reactions in plants. The red and blue part of VIBGYOR spectrum is mostly utilized in the process of photosynthesis. Out of the total amount of solar energy that strikes the surfaces of photosynthetically active plants parts, only a small portion of it is fixed in the form of chemical energy and the rest of it is reflected or lost as radiation energy. The total amount of solar energy that is fixed by the plants on this planet has been estimated to be 13 x 18 K.cals per year which is actually used to fixation of 160 to 175 billion tons of carbon per year, out of which nearly 130-135 billion tons of carbon is fixed by the plants living in oceans and the rest by land plants. Furthermore only 40-50% of it is tapped by human beings and other organisms, the rest is fixed as biomass for future use.
Where h=n Planck’s constant (6.624+10-27 erg sec.) = frequency of light waves per sec. C=velocity of light (2.998 x 1010 cms/sec.) and x = wavelength in centimeters. According to Einstein’s law of Photochemistry, in any photochemical act, one atom or one molecule can absorb only one photon at any given time and this brings about only one reaction at a time.
In order to relate the photochemical effect and the molecule that is affected by it, Avogadro’s molar values have been used to explain mass action i.e. E=NHV, where E is total energy found in mole quanta of light at any wavelength. Thus one Einstein of red light at 600 mm i.e. one mole of red light photons (6.2 x 1023 photons) possess 47.667 K.cals of energy. So different bands of VIBGYOR show different quantum of energy. Shorter the wavelength higher is the amount of energy present in one Einstein of light and light at longer the wavelength contains less energy. Hence blue light has more energy than red light
400 mm = 71.5 K.cals/mole
500 mm = 57.2 K.cals/mole
600 mm = 47.667 K.cals/mole
EXCITATION AND FLUORESCENCE
In photochemical reactions one photon of a particular wavelength is absorbed by one molecule at a time.
When a photon is absorbed by one multiatomic molecule like chlorophyll the energy level of this molecule is raised to higher state, the photonic energy moves randomly within the molecule and at a particular site, an electron found in the outer orbit, accepts the energy, with its higher energy, it jumps to the next higher orbit. Now the electron said to be in a higher state of energy called excited state or singlet state. Generally, when two electrons are present as pairs in the outer orbit they exhibit opposite spins. According to Pauli’s exclusion principle a pair of electrons in an orbit mentioned above, exhibit magnetic momentum as zero because of the opposite spin. If one of the two electrons is raised to higher state of energy it retains the same spin and such a state is called singlet state.
The same electron can be further raised to the next higher orbital provided the energy is sufficient. The electron in the single state cannot remain not more than 10^9 sec. It always tends to fall back to its normal orbit, called Ground state. While falling back, electrons change their spin other way. So the spin of the energized electron and the other electron found ground state is rendered same. But according to Pauli’s exclusive principle two electrons with the same spin is forbidden to be present in the same orbit. Thus the energized electron remains suspended for a period of time till it changes its spin into opposite direction.
Such a state of electron is called triplet state and the duration of such metastable state is 10^6 sec. This time period is quite long enough for any photochemical reaction. If nothing happens in this period, the electron in triplet state changes its spin by internal conversion and falls back to the ground state. The energy thus released is lost as phosphorescent energy or radiant energy.
Now it is known that most of the photochemical reactions take place at the triplet state. This is actually the principle of photochemical act where electrons are boosted to higher energy state, and then they are made to fall back in a stepwise fashion. It is during the descent, the light energy that is released in quantum is used up in the formation of energy rich chemical bond. Furthermore, it is now clear that in photosynthetic units, electrons are actually transported one by one (unpaired) as in the case of semiconductors. Such a transport is possible only through proteins and its associated chlorophyll molecules. Such events have been detected by observing electron spin resonance signals in isolated chloroplasts even at a temperature as low as O 0C, where enzymatic reactions are totally ruled out.
Photosynthesis is a series of processes of which some photochemical events are independent of temperature and some are temperature dependent enzymatic processes. The structural organization of chloroplasts is so designed the granal structures perform light induced photochemical reactions and then the products are released into the stromatic fluid, where carbon dioxide is fixed into carbohydrates. So, photosynthetic events have been broadly divided into two important sets of reactions called light reactions and dark reactions.
This process is also known as Hill’s Reaction. It takes place mainly in granal and intergranal membranes. Light is primarily responsible for initiating a series of events, in which some are temperature dependent photochemical reactions. In this process three important products are produced i.e. liberation of oxygen, production of reducing power, and ATP synthesis by photophosphorylation.
As the solar radiations strike the leaf surface, the chlorophyll molecules with their antenna protein complexes present in Quantosomes absorb the photons belonging to specific wavelengths and transfer the same from molecule to molecule by a mechanism called resonance. Thus chlorophyll molecules with the absorbed light energy are raised to higher state of energy called excited state.
The excited chlorophyll protein complexes as they are surrounded by water molecules exert electrical effect on water. So the water molecules are cleaved into the respective ions like hydroxyl ions (OH) and hydrogen ions (H+). This process is called photo ionization.
The hydroxyl ions thus produced are immediately accepted by Z-protein Mn2+ complex found in PS II system. The OH ions are transitorily accepted by Mn2+ ions. Each Mn2+ can take up two OH ions, so totally four OH are now bound two energized Z-Mn proteins via arginine residues. Then OH ions by rearranging their orbitals react to produce a molecule of H2O and two atomic oxygens. As atomic oxygens are highly unstable they immediately combine to produce molecular oxygen. In these rearrangement reactions of four hydroxyl ions four electrons are also released.
Interestingly, in this reaction four OH ions are accepted at four equital steps by Z-Mn complex. Each step requires a photon. Hence the Z-Mn2+ Complex goes through different states called So, S1, S2, S3. Such reactions have been confirmed by detecting the electron spin resonance (ESR) signals.
Reduction of NADP to NADPH+H takes place at the outer surface of thylakoids and the NADPH+H+ thus produced is called reducing power. In thylakoids after absorbing light by PSI electrons are transferred from molecule to molecule and ultimately they reach the reaction centre i.e. Chl. A 700. There are two such chla.700 molecules for every photosynthetic unit.They are associated with their respective protein complexes. With the exitons reaching Chl. A 700, the pigment gets excited, because at C-O site (in all probabilities) an electron is raised to higher orbital where they may undergo internal conversion and state triplet state for a period of 107 to 108sec.
It is at this juncture, the high energy electrons are accepted by ferrodoxin reducing substance. From two Chl. 700 molecules two electrons are transferred to FRS and the same are transferred to non-heme iron containing protein called ferrodoxin. Then these electrons are transferred to NADP through NADP reductase. Simultaneously, the energy rich NADP accepts two protons (H+). These reactions results in the formation of reducing power called NADPH+H. Though some amount of energy is lost during the transfer of electrons from Chl. A 700 to NADP, a significant amount of solar energy is conserved in NADPH+H as chemical energy.
Most importantly, in this process, as the electrons are lost from the Chl. 700 molecules they are rendered electrically positive. In this state Chl. 700 molecules cannot remain for a long time. But the photosynthetic units i.e. PS I and PS II are organized in such a way the positively charged Chl. 700 molecules receive electrons from Chl.A 680 from PS II system through an electron transport chain. So Chl.700 molecules are rendered neutral till they go through another set of reactions leading to the formation of another Molecule.
The PS II system also absorbs light energy and the same is conveyed to Chl.A 680 active centers where the electrons are boosted to a higher state of energy then the energy rich electrons are accepted by pheophytin. Pheaophytin is similar to chlorophylls but lack Mg in the center of porphyrin ring.
From pheophytin electrons are transported down the hill through an electron transport chain towards Chl.A 700. In this process, Chl. 680 molecules are rendered positively charged. However, the Chl. 680 molecules receive electrons from Z-Mn2 (OH) protein mediated oxygen liberation reactions. Thus Chl. A 680 are rendered neutral. It should be noted that the flow the electrons from Chl. A 700 to NADP and from Chl. A 680 to Chl. A 700 plus from 2nMn2+ complex to Chl. A 680 is non cyclic process .
Energy trapping and conversion and conservation of the same in the form of chemical energy is one of the fascinating aspects of photosynthesis. Besides, producing energy rich NADPH molecules, Quantosomes, i.e. PS I and PS II are also capable of generating high energy chemical bonds in ATP, while transporting electrons from one acceptor to an other. Such a process is called photophosphorylation. In fact, plants have evolved to produce energy rich molecules like ATP by two mechanisms namely non cyclic photophosphorylation and cyclic photophosphorylation.
Non cyclic photophosphorylation takes place in thylakoids where both PS I and PS II are in close proximity with each other and function co-ordinatingly. The flow of electrons in this case is discontinuous because the electrons boosted to higher state of energy from Chl. A 700 of PS I are accepted by FRS and then they are transferred to NADP via FD. The electrons lost from dimer Chl. A 700 are replaced by electrons boosted by Chl. 680 molecules of PS II. The high energy electrons from Chl. 680 dimers are first accepted by pheophytin complex, and then they are transferred from one acceptor to another sequentially finally to Chl. A 700.
It is while the electrons flow from Chl.A 680 to Chl.A 700, because of difference in the redox potential between Cyt. B. 559 and Cyt.f, a significant amount of electronic energy is liberated and the same is used up by ATP synthetase to bring about the synthesis of an ATP molecule. For every pair electron flow from one molecule of ATP can be synthesized in this process. However, recent investigations suggest that there is one more site for non – cyclic photophosphorylation between Z-Mn2+ complex and Chl. A 680.
This process occurs in the intergranal lamellae or stromal lamellae which contain mostly PS I systems and its associated electron transporting component. When the solar energy is absorbed by antenna Chlorophyll protein complexes found in PS I system, the energy is used to boost electrons from Chl. 700 dimers. The energy rich electrons are immediately accepted by FRS, whose red ox potential is 0.6 Ev. From FRS the energy rich electrons (in pails) are transferred to FD and then t cyt. B6 is quite substantial and the energy released in this step is used to produce a molecule of ATP. From cyt. B6 electrons are transferred to cyt. F and from cyt. F to Chl. A 700 through plastocyanin. Again the energy that is released between cyt. B6 and Cyt. F. redox couple is used in the formation of another molecule of ATP. The rest of the energy that is not utilized in the ATP synthesis is lost as thermal energy. In this scheme electrons complete one cycle, that is why this process is called cyclic photophosphorylation (Fig. 11.18 & 10)
The release of energy during electron transport in itself is into enough for the formation of a terminal bond between ADP and PI. The formation of ATP requires the activity of ATP synthetase. This enzyme consists of protein components called CF 1 and HFI, the former is located on the outer surface of the thylakoid and intergranal lamellae and the later is buried is the core of the membrane. Many theories have been proposed to explain the synthesis of ATP, of which Peter Mitchell’s chemiosmotic hypothesis and Boyer’s conformational chemical coupling hypothesis are very significant.
According to chemiosmotic hypothesis, the hydrogen ions produced during photoionisation (it need not be) are accepted up by plastoquinones which also accepts electrons donated by pheophytin. The reduced plastoquinones i.e. PQH2 shuttles within the membrane and discharges only H+ into the space found in the thylakoid sac, but electrons on the other hand are channeled towards Chl.A 700 through electron transport chain. Because of charge separation and accumulation of H+ ions, the liquid present in the thylakoid space, becomes more acidic and the liquid present at the outer surface of the thylakoid more basic.
This creates proton motive force and a charge in the membrane potential. In order to neutralize these charges. The protons are pumped out by the hydrogen transporting protein located at the base of the ATP synthetase complex. It is during the transfer of protons the energy generated is used in the formation of a chemical bond between ADP and Pi. With the release of OH into the thylakoid space and H into the stromatic fluid the charges are neutralized. In fact, Jogendorf provided a very good evidence for chemiosmotic mode of ATP synthesis. When isolated chloroplasts are first suspended in an acidic pH medium and then if they are transferred to a medium with basic pH, containing ADP and Pi ,energy rich ATP molecules are generated.
In recent years, chemiosmotic hypothesis has been questioned because intergranal lamellae containing PS I system also synthesize ATP by cyclic photophosphorylation. This process takes place in spite of the intergranal lamellae do not contain inner space as found in thylakoid membranous sacs. Furthermore, the time required for generating such strong proton motive force requires at least few milliseconds, but in actuality the synthesis of ATP takes place in microseconds. In recent years new microelectrodes have been developed which could measure slightest change in pH in shortest possible time. Such studies suggested that chemiosmotic hypothesis is no more tenable but it is too early to deny the hypothesis once and for all. However to day every scientist has agreed to chemiosmotic hypothesis in generating ATP molecules not only in chloroplasts but also in mitochondria.
But Boyers conformational chemical coupling hypothesis appears to be more valid. For his proposal many evidences have come up. He was also awarded a Nobel Prize, much later than Peter Mitchell received Nobel Prize. Accordingly, during the electron transport along the electron transport chain energy released causes conformational change in the contractile proteins found in the granal membrane. The presences of such contractile proteins have been identified in chloroplast membranes and they are believed to be associated with ATP synthetase. The consisted contractile protein later relaxes by releasing the energy to ATP synthetase which by virtue of its activity, traps this energy, as a result the enzyme itself undergoes conformational change where it brings the enzyme bound ADP and inorganic Pi close to each other to bring about an anhydride bond formation between ADP and Pi to produce ATP molecules. This process takes place in a few microseconds and added to this, it does not require any kind of proton motive force for the synthesis of ATPs. Boyer’s hypothesis is now more or less accepted by most of the workers in the field of Bioenergetics. Yet chemiosmotic process of loading thylakoid space with hydrogen ions is a fact.
Dark reaction, which is also called by names like carbon pathway, Blackman’s reaction, C3 pathway etc, is a temperature dependent enzymatic process, where the products of light reactions like ATP and NADPH are used in fixing carbon dioxide into simple carbohydrates like glucose. However, the mechanism of carbon fixation or dark reaction varies in different plants. Accordingly three different pathways have been recognized so far. They are Calvin’s cycle or C3 pathway. Hatch and Slack pathway C4 pathway and the third Crassulacian acid metabolic (CAM) pathway. Most of the dicot members (with certain exceptions) and majority of monocots operate Calvin’s pathway. But tropical grass members and some dicots like Atriplex, etc. exhibits C4 pathway. The succulents and some orchids on the other hand show CAM pathway as a method of carbon fixation.
Utilizing radioactive tracers like 14C bicarbonate and auto radiographic techniques, Calvin and his associates have elucidated various biochemical steps involved in this pathway. This involves three major steps viz. Carboxylation, Reduction and Regeneration (CRR).
Biochemical analysis of photosynthetic cells like chlorella and others show that a 5 carbon sugar called Ribulose diphosphate (RUDP or RuBP) acts as an acceptor of carbon dioxide. When the cells are illuminated the concentration of RUBP decreases and Phosphoglycerate (PGA) increases. On the contrary, when light is switched off, the concentration of RUBP increases while the concentration of PGA goes down in the chloroplasts. Based on these experimental evidences RUBP has been identified as the primary acceptor of CO2.
Initially Ribulose monophosphate (RUMP) is converted to RUBP by a kinase enzyme. The five carbon RUBP with binding of enzyme undergoes a transitory stage which facilitates the binding of CO2. The resultant 6C compound, still bound to the enzyme surface, is highly unstable and breaks up into two 3C compounds called phosphoglycerate. They are the first stable products of carbon fixation.
The enzyme that is responsible for this reaction is called RUBP carboxylase, which is known to be the most abundant protein is nature because more than
50% of the leaf proteins is RUBP carboxylase. In chloroplasts this enzyme is located as a granular particle on the outer surface of thylakoid membrane. It consists of 4 smaller subunits coded for by nuclear genome and 4 smaller larger subunits coded for by plasto genome.
Phosphoglycerate is then converted to an energy rich di phosphoglycerate by the activity of an enzyme called phosphoglycerate kinase. One molecule of ATP is used in this process. Then the DIPGA is reduced to phosphoglyceraldehyde by the action of glyceraldehyde phosphate dehydrogenase. NADPH + H that is synthesized during light reaction donate hydrogen to this reaction. In fact, in the above reactions, energy stored in ATP and NADPH+H is transferred to PGALD.
The process of regeneration involves a series of inter linked reactions in which the initial substrate i.e. RUMP used in Carboxylation step is regenerated; simultaneously glucose is also synthesized as an ultimate product of CO2 fixation. In this process some of the PGALD are converted to dihydroxyacetone phosphate by an enzyme called isomerase.
The symbols used denote, PG = phosphoglycerate (3C), DPGA = Diphosphoglycerate (3C), NADP = (Nicotinamide di nucleotide), GAP + Phosphoglyceraldehyde –p, DAP = Dihydroxy acetone phosphate, FDP = Fructose1,6-diP, F6P = Fructose6–P, SDP = Sedoheptulose–diP, S7P = Sedoheptulose 7-P, E4P = Erythrose 4-P, Ru5P = Ribulose 5-P, G6P = Glucose6-P, ATP = Adenose triphosphate,
Then 3 molecules of PGALD and 3 molecules of DiHAP (DAP) are condensed head to head to synthesize 3 molecules, of 6C fructose 1, 6 diphosphate. The enzyme responsible for this reaction is aldolase.
The fructose diphosphate then undergoes dephosphorylation at 1st carbon position by the action of phosphotase enzymes.
Two of the three fructose-6P thus produced combine with two PGALD to generate 2 xylulose 5 P and 2Erythrose 4, P molecules. This reaction is performed by the enzyme called transkelotase.
The 2 Erythrose phosphates fuse with 2 molecules of DHAP to produce 2 molecules of seven carbon Sedoheptulose diphosphate. The enzyme that brings about this reaction is Aldolase. The Sedoheptulose diphosphates are then dephosphorylated at first carbon atom by the phosphotase enzyme.
The transkeletolase enzymes brings about another rearrangement reaction between 2 Sedoheptulose 7(P) and 2 PGALD to generate 2 molecules of ribose 5 (P) and 2 molecules of xylulose 5(P).
The ribose phosphoisomerase and the xylulose epimerase convert ribose 5, p and xylulose 5P into Ribulose 5P respectively.
One of the Fructose 6P remained in the reaction (3) is converted to glucose 6P by glucophosphate isomerase. The glucose may dephosphorylated by another phosphotase enzyme.
The overall reactions in this regenerative steps results in the formation of 6 molecules of RUMP. And one molecule of glucose as the net product carbon fixation.
7PGALD + 5 DHAP –> 6 RUMP + C6H12O6 + 6 Pi
Tropical grass members like crab grass, sugar cane, Zea mays, etc., while performing photosynthetic reactions operate light reactions similar to that of other dicot members belonging to C3 class of plants. But with regard to carbon fixation, they fix carbon first into a C4 organic acid, which is later used in the formation of glucose. Nevertheless these plants use both C3 cycle as well as C4 pathway but in different0 cell types found in the same leaf.
Grass leaves contain a layer of bundle sheath cells around vascular bundles, thus separates mesophyll cells from the veins. Detailed studies suggest that the chloroplasts of bundle sheath cells differ structurally and functionally from that of chloroplasts found in the mesophyll cells.
During carbon fixation by mesophyll chloroplasts; first pyruvate is primed up to produce an energy rich 3 carbon compound called phosphoenol pyruvate. The enzyme responsible for this reaction is pyruvate di kinase which uses ATP and Pi for the priming reactions; This is an unusual enzyme.
The phosphoenol pyruvate now acts as an acceptor of carbon dioxide and the product of this reaction is oxoloacetate. The enzyme called PEP carboxylase has a very low Km for CO2 and efficiently fixes carbon dioxide. In these reactions. The first product of carbon fixation is a 4 carbon organic acid that is why these reactions are called C4 pathway.
OAA is then further reduced to malate by malate dehydrogenase or it may be converted to aspartate by transmutation reactions.
Malate produced in mesophyll chloroplasts is then transported across the cell walls into bundle sheath cells through the protoplasmic stands.
The chloroplasts found in bundle sheath cells use malate and the same decarboxylated to pyruvate by malate decarboxylase enzyme. In this process a molecule of NADPH2 is formed by dehydrogenation reaction.
The CO2 released in this process is assimilated by RUBP with the help of RuBp carboxylase enzyme. The product of this reaction is, PGA which is then subjected to Calvin’s cycle process to produce a molecule of glucose as well as RUBP. And this process is repeated as long as carbon dioxide is available.
The enzyme RUBP carboxylase present is also very efficient in carbon fixation.
Because of the low km action of carbon fixing enzyme C4 plants are considered as highly efficient in terms of net product of photosynthesis. The above plants are also tolerant to high temperatures and they are capable of fixing about 40-80 mg of C2 per dm2 of the leaf area per hour. In addition, as there plants contain very few micro bodies, they do not operate photo respiratory process, which is very common in C3 plants; thus they don’t loose any of its photosynthetic products.
Plants belonging to Crassulacian family are mostly succulents. Such members are also found in other groups of plant kingdom. These plants possess large cells containing a number of chloroplasts. The cell sap has certain chemicals which prevent the loss of water from the surface of the plant body. Thus they prevent loss of water and survive even under drought conditions. Another interesting feature of CAM plants is the diurnal change in the opening and closing of the stomata parallel with the diurnal change in the organic acid content of the cell sap.
The Crassulacian plants exhibit closure of the stomata at day period and the same open during night time. During night tie, the cellular structures fix CO2 into malate to such an extent, the pH of the cell sap is rendered very acidic. But during the day period. The acidity of cell sap disappears because malate is decarboxylated and the products of these reactions are used for fixing carbon dioxide by C3 cycle.
The CAM plants have developed certain structural and functional features which are well suited for fixing CO2 both at day and night periods. Though the chloroplasts do not show any structural dimorphism or compartmentalization, the switching on the one set of enzymes at daytimes and the other set at night is very remarkable. These plants fix CO2 efficiently under extreme conditions like drought, intense sun light wish temperature and poor availability of CO2. That is why these plants grow well in xerophytic conditions.
Chloroplasts present in the cells of succulents do possess photosynthetic units as in any other plants. The light reactions and their products are same as that of C3 plants. But CAM plants, at night times, fix CO2 liberated due to respiration and at the same time, they also use atmospheric CO2 fix into
malate. The primary acceptor of CO2 in these plants is PEP, which by the action of PEP carboxylase fixes most of the available CO2 into OAA, which is then converted into malate by the activity of malate dehydrogenase. The required NADPH2 is supplied by Hexose monophosphate pathway. Thus, malate is continuously produced and stored in the cell sap during night times. Though the cell sap is highly acidic, it won’t inhibit normal cytoplasmic activities because of compartmentalization. Recent investigations suggest that at night times, RUBP also acts as the acceptor of CO2 and C3 cycle operates to some extent, as a result certain amount of glucose is also produced at night times. However, malate pathway is preponderant and predominant over C3 pathway.
Nonetheless, during day periods chloroplasts start fixing CO2 by normal C3 pathway. Meanwhile, the stored malate is decarboxylated and CO2 thus released is fixed by RUBP carboxylase into PGA. The same is subjected to reduction and regeneration steps of C3 pathway. Thus, Hexose is continuously synthesized. The remarkable feature of CAM plants is that they operate C3 and C4 pathway at different times of the same both operate sometimes, simultaneously.
Biochemical reactions are always controlled by one or more factors. The rate of reaction depends upon the availability of the required factors at optimal concentrations to achieve the maximal level. If any one of the factors is missing or difficult, then the rate of reactions suffers. Such factors are called as the limiting factors.
The optimal level of each of the factors can be determined by monitoring the rate of reaction by varying the concentration of the said factor while keeping the other factors more than needed for the maximal activity. When the rate of reaction is plotted against the concentration of the said factor a bell shaped curve is obtained. From this, it is possible to determine the optimal concentration at which the rate of reaction is at maximum. Nevertheless in many situations, even under optimal concentration of a particular factor, the rate of reactions remains constant. This is due to the deficiency of some other factor which controls the said reaction. If the concentration of deficient factor is increased, then the rate of reaction also increases, till other acts as a limiting factor. If such a factor is added the rate of reaction further increases. Thus in a multiple factor dependent reactions, all the said factors are required of the optimal level for the maximal rate of reaction. This concept of law of limiting factors was introduced by Balckmann. The factors that control the process of photosynthesis are Co2, pigments light, temperature, oxygen, water, genetic factors. Etc.
During the course of evolution, heterotrophic organisms have acquired the ability to trap the solar energy into chemical energy and such auto tropic organisms were called plants and the same colonized the entire landscape of the earth. The successful story of photosynthetic plants is endowed with their genetic potentiality to produce light harvesting pigments, proteins and other enzymes to convert light energy into chemical energy and to fix CO2 into glucose. About 350 million years ago, the concentration of CO2 in the atmosphere was believed to be 20%. Since then the concentration ofCO2 is depleted and today the concentration is just 0.3% or less than that.
The enzyme that is remarkable efficient in the fixation of CO2 is RUBP carboxylase, which is present in plants in large quantities. In fact, more than 50% of the total proteins found in leaves in RUBP carboxylase. On the other hand, the concentration of COP is so low; it acts as the limiting factor which affects the total output of photosynthate. It has been demonstrated that the increase in the concentration of CO2 up to 1% or little more, increases the rate of photosynthesis and its products.
Calculations based on the total population of photosynthetic plants on this planet including plants growing in ocean and on land, the total amount of CO2 fixed amounts to about 175 billion tons per year, out of this only, 30-40% is consumed or utilized by biological activities. The rest is stored in the form of organic matter.
Photo synthetically active plants contain various kinds of pigments of which Chl. A acts as the primary component and the others act as accessory pigments. Such pigments, in association with specific proteins, are capable of absorbing solar energy and the same is made available for other enzymes to convert into chemical energy, which is ultimately stored in carbohydrates.
Though Chl. A pigments perform the primary process of photosynthesis, but accessory pigments too, absorb light energy and transfer the energy from molecule to molecule and finally to Chl.A. Apart from this function, the accessory pigments, particularly carotinoids protect the primary pigments from the bleaching effect of sunlight called solarization.
Almost all living organisms on this planet are depending upon the solar energy directly or indirectly as the main source of energy, but plants are endowed with the potentiality of utilizing the solar energy directly. Light plays an important role, in initiating photochemical reactions, as well as activating the process of chlorophyll and other related protein synthesis. The effect of quality of light, quantity of light and intensity of light of photosynthesis and its products is very interesting.
When light rays of fall on the surface of photo synthetically active parts, plastids absorb photons of the light and initiate photochemical reactions. Spectral analysis of sunlight and its effect on photosynthesis suggests that only visible part of solar electromagnetic radiation is used in photochemical reactions. Though blue, green, yellow, orange and red lights of the VIBGYOR spectrum are absorbed by the functional chloroplasts, the action spectrum is blue and red light. The other bands absorbed by the photosynthetic pigments is transferred to the primary pigments i.e. Chl. A.
The intensity of light is measured in terms of foot candles or lux. At low intensities of light, the rate of photosynthesis is slow. But with the increase in the intensity of red and blue lights the rate of photosynthesis also increases up to a certain point, but beyond 2000-2500 ft. candles of intensity the rate of photosynthesis remains constant, which indicates the saturation level of light. Intense sunlight beyond 2500 ft. candles damages chlorophylls either by photo bleaching or by denaturing the enzymes due to resultant high temperature. However, green plants do not get bleached in spite of very high level of sunlight, because carotinoids protect the green pigments from photo oxidation. Intense light also activates photorespiration because of higher levels of oxygen available to plants. This has a greater impact on the total photosynthetic products in C3 plants, but not in C4 or CAM plants.
Total amount o sunlight required either for the release of one mole of oxygen or for fixing one mole of CO2 into carbohydrates can be calculated by analyzing the requirement of energy for each step and the total amount of energy stored in one mole of glucose. The difference gives quantum efficiency.
It is possible to calculate what is the total number of quanta of light energy is required to produce the required ATPs and NADPH + H. in non cyclic photophosphorylation process, 4 mole quanta of light is required to produce 1 mole of NADPH+H and 1 mole of ATP. So for the synthesis of 12 moles of NADPH+H and 12 moles of ATP, the required light energy is 48 mole quanta of light. For the other 6 moles of ATP, six mole quanta of light energy are required for cyclic photophosphorylation to produce the same. All in all, 54 mole quanta of light energy is required 1 to fix 6 mole of CO2 into one mole of glucose. In fact, one mole quanta of red light possesses about 47.6 K.cals. Therefore 54 mole of CO2 into one mole of glucose. In fact, one mole quanta of red light possesses about 47.6 K.cals. Therefore 54 mole quanta of red light is equal to 54 x 47.6 = 2470 K.cals of energy. This is the amount of energy absorbed and used to produce one mole of glucose, but 1 mole of glucose stores just 696 K. cal of energy. So the quantum efficiency of photosynthesis to produce one mole of glucose is
696 K.cals x 100 / 2470 K.cals = 28.1%
There calculations are exclusive of the energy required for the liberation of oxygen. For the liberation of every mole of oxygen, 4 mole quanta of light are required. If calculations are made to relate the liberation of 6 mole of oxygen 4 x 6 = 24 mole quanta of light and fixation of 6 mole of CO2 into 1 mole of glucose (54 mole quanta), the overall quantum efficiency will be 18.4%.
696 x 100 / 2470 + 1122 = 18.4%
Based on the above calculation it is suggested that all the plants put together store about 13 x 10 K.cals of sunlight energy while fixing 175 x 10 tons of CO2 per year.
Any biochemical reaction that is mediated by enzyme is temperature dependent. With the increase in temperature the rate of reactions also increases. Certain photochemical processes in Hill’s reaction such as capturing of photons, transfer of excitons and the excitation of molecules, radiations in the form of fluorescence are temperature insensitive. But the synthesis of ATP by photophosphorylation, reduction of NADP to NADPH H and other carbon pathway reactions are temperature sensitive because they are enzyme dependent reactions. So for their maximal activities, they require an optimal temperature of 25-30 degree C, but some tropical grass members require 30-40 degree C as the optimal temperature. However, the temperature higher than this is deleterious because enzymes undergo denaturation and they loose their functional abilities.
Oxygen is one of the products of photosynthetic reactions. The atmospheric concentration of oxygen is 18-20% and it is the product of photosynthetic activity of plants. If the concentration of oxygen increases intracellularly, the C3 plants resort to photorespiration, because the peroxisomal enzymes become active. At the same time, the RUBP carboxylase, which is responsible for carbon fixation, now it acts as oxygenase and starts breaking down RUBP. Thus the total yield of the photosynthate gets reduced. In certain plants it has been estimated that photorespiration oxidizes nearly 18-20% of the total photosynthate.
Another deleterious effect is that the green pigments in the presence of intense sunlight and oxygen get oxidize and damage the functionally active chlorophyll pigments. This process is called photo oxidation. But C4 plants by virtue of not having micro bodies tolerate excess oxygen and do not suffer from it.
Water is an important substrate for photosynthesis, for it provides hydrogen ions for the production of NADPH+H. In addition, it is also used in many biochemical pathways during carbon fixation. The concentration of water in the plant body is very important because deficiency of it causes closing of the stomata, and also it affects various metabolic activities, hence the rate of photosynthesis is also affected.
Whatever may be the optimal levels of required factors available for the plant, it is ultimately the genetic factors that control the efficiency of plants potentiality to perform photosynthesis. For example, C4 plants because of their dimorphic chloroplasts, they are capable of fixing CO2 more efficiently than C3 plants. Similarly, CAM plants are capable of fixing CO2 both at night and day times.
The number of chloroplasts, structural adaptation of chloroplasts, functional efficiency of chloroplasts, the number of foliages, orientation of foliage and the overall cellular enzymatic features. The high yield of plants in terms of photosynthesis is ultimately controlled by the potentiality of plant genome.
Besides the oxidation of glucose into CO2 and H2O by regular respiratory process, many dicot plants with C3 characters oxidize pentose sugars in the presence of light higher concentration oxygen by a process called photorespiration. It differs from dark respiration in being insensitive to cyanide inhibition.
Photorespiration is due to the activity of micro bodies found in mesophyll cells of C3 plants. In mesophyll cells, micro bodies are closely associated with chloroplasts on one side and mitochondria at the other. On the contrary, C4 plants do not possess any micro bodies, even if present; there will be only a few. Hence, C4 plants do not oxidize their photosynthetic products but C3 plants loose about 18% or more of their total photosynthate by photorespiration. That is why C4 plants are considered as efficient when compared to that of C3 plants.
MECHANISM OF PHOTORESPIRATION
On a bright day, when the process of photosynthesis is at its peak, the concentration of CO2 goes down from 0.03% to 0.005% or so. At the same time, the concentration of oxygen increases considerably.
In the presence of higher amounts of oxygen, RUBP carboxylase acts as RUBP oxygenase. Normally RUBP carboxylase fixes carbon dioxide, but in these conditions of high O2, RUBP carboxylase exhibits oxygenase activity. This form of enzyme oxidizes RUBP molecules into phosphoglycerate and phosphoglycollate. For this oxidation reaction the enzyme uses the available oxygen. This process takes place in functional chloroplasts. The phosphoglycollate is then dephosphorylated to glycol late by a phosphotase enzyme.
The glycollate is then transported across the membrane to peroxisomes which are very closely associated with chloroplast membranes. In peroxisomes, glycol late is acted upon by glycollate oxidase to produce glyoxylate and H2O2. A molecule of oxygen is utilized in this process. However, the H2O2 produced in this reaction is immediately catalyzed by peroxidase enzymes to liberate H2O and O2. Then the glyoxylate is converted to glycine by transaminase reactions.
Glycine produced in the peroxisomes is not further metabolized. Glycine is then transported across the membranes into mitochondria which is also associated with peroxisomes. In mitochondria, two molecules of glycine are condensed to a 3 carbon serine by a series of decarboxylation and deamination reactions.