Photosynthesis, the process by which green plants and certain other organisms transform light energy into chemical energy. During photosynthesis in green plants, light energy is captured and used to convert water, carbon dioxide, and minerals into oxygen and energy-rich organic compounds.
We can write the overall reaction of this process as:
Some photosynthetic bacteria use hydrogen donor other than water. Therefore, photosynthesis is also defined as the anabolic process of manufacture of organic compounds inside the chlorophyll containing cells from carbon dioxide and hydrogen donor with the help of radiant energy.
Significance of Photosynthesis:
1. Photosynthesis is the most important natural process which sustains life on earth.
2. The process of photosynthesis is unique to green and other autotrophic plants. It synthesizes organic food from inorganic raw materials.
3. All animals and heterotrophic plants depend upon the green plants for their organic food, and therefore, the green plants are called producers, while all other organisms are known as consumers.
4. Photosynthesis converts radiant or solar energy into chemical energy. The same gets stored in the organic food as bonds between different atoms. Photosynthetic products provide energy to all organisms to carry out their life activities (all life is bottled sunshine).
5. Coal, petroleum and natural gas are fossil fuels which have been produced by the application of heat and compression on the past plant and animal parts (all formed by photosynthesis) in the deeper layers of the earth. These are extremely important source of energy.
6. All useful plant products are derived from the process of photosynthesis, e.g., timber, rubber, resins, drugs, oils, fibers, etc.
7. It is the only known method by which oxygen is added to the atmosphere to compensate for oxygen being used in the respiration of organisms and burning of organic fuels. Oxygen is important in (a) efficient utilization and complete breakdown of respiratory substrate and (b) formation of ozone in stratosphere that filters out and stops harmful UV radiations in reaching earth.
8. Photosynthesis decreases the concentration of carbon dioxide which is being added to the atmosphere by the respiration of organisms and burning of organic fuels. Higher concentration of carbon dioxide is poisonous to living beings.
9. Productivity of agricultural crops depends upon the rate of photosynthesis. Therefore, scientists are busy in genetically manipulating the crops.
Magnitude of Photosynthesis:
Only 0.2% of light energy falling on earth is utilized by photosynthetic organisms. The total carbon dioxide available to plants for photosynthesis is about 11.2 x 1014 tonnes. Out of this only 2.2 x 1013tonnes are present in the atmosphere @ 0.03%. Oceans contain 11 x 1014 (110,000 billion) tonnes of carbon dioxide.
About 70 to 80 billion tonnes of carbon dioxide are fixed annually by terrestrial and aquatic autotrophs and it produces near about 1700 million tonnes of dry organic matter. Out of these 10% (170 million tonnes) of dry matter is produced by land plants and rest by ocean (about 90%). This is an estimate by Robinowitch (1951),According to more recent figures given by Ryther and Woodwell (1970) only 1/3 of total global photosynthesis can be attributed to marine plants.
Light and Dark Reactions in Photosynthesis :
During photosynthesis water is oxidized and carbon dioxide is reduced, but where in the overall process light energy intervenes to drive the reaction. However, it is possible to show that photosynthesis consists of a combination of light-requiring reactions (the “light reactions”) and non-light requiring reactions (the “dark reactions”).
It is now clear that tall the reactions for the incorporation of CO2 into organic materials (i.e., carbohydrate) can occur in the dark (the “dark reactions”). The reactions dependent on light (the “light reactions”) are those in which radiant energy is converted into chemical energy.
According to Arnon, the functional relationship between the “light” and “dark” reactions can be established by examining the requirements of the dark reactions. The “dark reactions” comprise a complex cycle of enzyme-mediated reactions (the Calvin Cycle) which catalyzes the reduction of carbon dioxide to sugar. This cycle requires reducing power in the form of reduced nicotinamide adenine dinucleotide phosphate (NADPH) and chemical energy in the form of adenosine triphosphate (ATP).
The reduced NADP (NADPH) and ATP are produced by the “light reactions”. It is thus possible to divide a description of photosynthesis into those reactions associated with the Calvin cycle and the fixation of carbon dioxide, and those reactions (i.e., capture of light by pigments, electron transport, photophosphorylation) which are directly driven by light.
Site of Photosynthesis:
Chloroplast in green plants constitute the photosynthetic apparatus and act as site of photosynthesis. Chloroplasts of higher plants are discoid or ellipsoidal in shape measuring 4 —6 μ in length and 1—2 μ in thickness. It is a double membranous cytoplasmic organelle of eukaryotic green plant cells. The thickness of the two membranes including periplastidial space is approximately 300Å.
Ground substance of chloroplast is filled with a hydrophilic matrix known as stroma. It contains cp-DNA (0.5%), RNA (2—3%), Plastoribosome (70S), enzymes for carbon dioxide assimilation, proteins (50—60%), starch grains and osmophilic droplets, vitamin E and K, Mg, Fe, Mn, P, etc. in traces. In stroma are embedded a number of flattened membranous sacs known as thylakoids. Photosynthetic pigments occur in thylakoid membranes.
Aggregation of thylakoids to form stacks of coin like structures known as granna. A grannum consists near about 20 — 30 thylakoids. Each thylakoid encloses a space known asloculus. The end of disc shape thylakoid is called as margin and the area where the thylakoids membranes are appressed together is called partition.
Some of the granna lamella are connected with thylakoids of other granna by stroma lamella or fret membranes. Thylakoid membrane and stroma lamella both are composed of lipid and proteins. In photosynthetic prokaryotes (blue-green algae and Bacteria) chloroplast is absent. Chromatophore is present in photosynthetic bacteria and photosynthetic lamellae in blue-green algae.
Mechanism of Photosynthesis:
Photosynthesis is an oxidation reduction process in which water is oxidized and carbon dioxide is reduced to carbohydrate.
Blackmann (1905) pointed out that the process of photosynthesis consists of two phases:
(1) Light reaction or Light phase or Light-dependent phase or Photochemical phase
(2) Dark reaction or Dark phase or Light independent phase or Biochemical phase.
During light reaction, oxygen is evolved and assimilatory power (ATP and NADPH2) are formed. During dark reaction assimilatory power is utilized to synthesize glucose.
(i) Oxygenic photosynthesis (with evolution of O2) takes place in green eukaryotes and cyanobacteria (blue-green algae).
(ii) An oxygenic photosynthesis (without the evolution of O2) takes place in photosynthetic bacteria.
Photosynthetic Pigments :
Photosynthetic pigments are substances that absorb sunlight and initiate the process of photosynthesis.
Photosynthetic pigments are grouped into 3 categories(Photosynthesis):
These are green coloured most abundant photosynthetic pigments that play a major role during photosynthesis. Major types of chlorophylls are known to exist in plants and photosynthetic bacteria viz., Chlorophyll a, b, c, d and e, Bacteriochlorophyll a, b and g, and Chlorobium chlorophyll (Bacterio viridin).
The structure of chlorophyll was first studied by Wilstatter, Stoll and Fischer in 1912. Chemically a chlorophyll molecule consists of a porphyrin head (15 x 15Å) and phytol tail (20Å). Porphyrin consists of tetrapyrrole rings and central core of Mg. Phytol tail is side chain of hydrocarbon. It is attach to one of the pyrrole ring. This chain helps the chlorophyll molecules to attach with thylakoid membrane.
Out of various types of chlorophyll, chlorophyll a and chlorophyll b are the most important for photosynthetic process. Chlorophyll a is found in all photosynthetic plants except photosynthetic bacteria. For this reason it is designated as Universal Photosynthetic Pigment or Primary Photosynthetic Pigment.
These are yellow, red or orange colour pigments embedded in thylakoid membrane in association with chlorophylls but their amount is less. These are insoluble in water and precursor of Vitamin A. These are of two of types viz., Carotene and Xanthophyll (Carotenol/Xanthol).
Carotenes are pure hydrocarbons, red or orange in colour and their chemical formula is – C40H56 Some of the common carotenes are -α, β, γ and δ carotenes, Phytotene, Neurosporene, Lycopene (Red pigment found in ripe tomato). β—carotene on hydrolysis gives Vitamin A.
Xanthophylls are yellow coloured oxygen containing carotenoids and are most abundant in nature. The ratio of xanthophyll to carotene in nature is 2:1 in young leaves. The most common xanthophyll in green plant is Lutein (C40H56O2) and it is responsible for yellow colour in autumn foliage. Both carotene and xanthophylls are soluble in organic solvents like chloroform, ethyl ether, carbondisulphide etc.
(iii) Phycobilins (Biliproteins):
These are water soluble pigments and are abundantly present in algae, and also found in higher plants. There are two important types of phycobilins-Phycoerythrin (Red) and Phycocyanin (Blue). Like chlorophyll, these pigments are open tetrapyrrole but do not contain Mg and Phytol chain.
Nature of Light :
The source of light for photosynthesis is sunlight. Sun Light is a form of energy (solar energy) that travels as a stream of tiny particles. Discrete particles present in light are called photons. They carry energy and the energy contained in a photon is termed as quantum. The energy content of a quantum is related to its wave length.
Shorter the wave length, the greater is the energy present in its quantum. Depending upon the wave length electro magnetic spectrum comprises cosmic rays, gamma rays, X-rays,-UV rays, visible spectrum, infra red rays, electric rays and radio waves.
The visible spectrum ranges from 390 nm to 760 nm (3900 – 7600A), however, the plant life is affected by wave length ranging from 300 – 780 nm. Visible spectrum can be resolved into light of different colours i.e., violet (390-430 nm), blue or indigo (430-470 nm), blue green (470-500 nm), green (500 – 580 nm), yellow (580 – 600 nm), orange (600 – 650 nm), orange red (650 – 660 nm) and red (660 – 760 nm). Red light above 700 nm is called far red. Radiation shorter than violet are UV rays (100 – 390 nm). Radiation longer than those of red are called infra red (760 – 10,000 nm).
A ray of light falling upon a leaf behaves in 3 different ways. Part of it is reflected, a part transmitted and a part absorbed. The leaves absorb near about 83% of light, transmit 5% and reflect 12%. From the total absorption, 4% light is absorbed by the chlorophyll. Engelmann (1882) performed an experiment with the freshwater, multicellular filamentous green alga spirogyra.
In a drop of water having numerous aerobic bacteria, the alga was exposed to a narrow beam of light passing through a prism. The bacteria after few minutes aggregated more in that regions which were exposed to blue and red wave length. It confirms that maximum oxygen evolution takes place in these regions due to high photosynthetic activities.
Absorption Spectrum :
All photosynthetic organisms contain one or more organic pigments capable of absorbing visible radiation which will initiate the photochemical reactions of photosynthesis. When the amount of light absorbed by a pigment is plotted as a function of wave length, we obtain absorption spectrum (Fig. 6.4).
It varies from pigment to pigment. By passing light of specific wave length through a solution of a substance and measuring the fraction absorbed, we obtain the absorption spectrum of that substance. Each type of molecules have a characteristic absorption spectrum, and measuring the absorption spectrum can be useful in identifying some unknown substance isolated from a plant or animal cell.
Action Spectrum :
It represents the extent of response to different wave lengths of light in photosynthesis. It can also be defined as a measure of the process of photosynthesis when a light of different wave lengths is supplied but the intensity is the same. For photochemical reactions involving single pigment, the action spectrum has same general shape as the absorption spectrum of that pigment, otherwise both are quite distinct (Fig. 6.5).
Quantum Requirement and Quantum Yield :
The solar light comes to earth in the form of small packets of energy known as photons. The energy associated with each photon is called Quantum. Thus, requirement of solar light by a plant is measured in terms of number of photons or quanta.
The number of photons or quanta required by a plant or leaf to release one molecule of oxygen during photosynthesis is called quantum requirement. It has been observed that in most of the cases the quantum requirement is 8.
It means that 8 photons or quantum’s are required to release one molecule of oxygen. The number of oxygen molecules released per photon of light during photosynthesis is called Quantum yield. If the quantum requirement is 8 then quantum yield will be 0.125 (1/8).
Photosynthetic Unit or Quantasome :
It is defined as the smallest group of collaborating pigment molecules necessary to affect a photochemical act i.e., absorption and migration of a light quantum to trapping centre where it promotes the release of an electron.
Emmerson and Arnold (1932) on the basis of certain experiments assumed that about 250 chlorophyll molecules are required to fix one molecule of carbon dioxide in photosynthesis. This number of chlorophyll molecules was called the chlorophyll unit but the name was subsequently changed to photosynthetic unit and later it was designated as Quantasome by Park and Biggins (1964).
The size of a quantasome is about 18 x 16 x l0nm and found in the membrane of thylakoids. Each quantasome consists of 200 – 240 chlorophyll (160 Chlorophyll a and 70 – 80 Chlorophyll b), 48 carotenoids, 46 quinone, 116 phospholipids, 144 diagalactosyl diglyceride, 346 monogalactosyl diglyceride, 48 sulpholipids, some sterols and special chlorophyll molecules (P680 and P700).
‘P’ is pigment, 680 and 700 denotes the wave length of light these molecule absorb. Peso and P700 constitute the reaction centre or photo centre. Other accessory pigments and chlorophyll molecules are light gatherers or antenna molecules. It capture solar energy and transfer it to the reaction centre by resonance transfer or inductive resonance.
It is the phenomenon of re-radiation of absorbed energy. It is of two types:
(1) Fluorescence and
The normal state of the molecule is called as ground state or singlet state. When an electron of a molecule absorbs a quantum of light it is raised to a higher level of energy a state called Excited Second Singlet State. From first singlet state excited electron may return to the ground state either losing its extra energy in the form of heat or by losing energy in the form of radiant energy. The later process is called fluorescence. The substance which can emit back the absorbed radiations is called fluorescent substance. All photosynthetic pigments have the property of fluorescence.
The excited molecule also losses its electronic excitation energy by internal conversion and comes to another excited state called triplet state. From this triplet state excited molecule may return to ground state in three ways-by losing its extra energy in the form of heat, by losing extra energy in the form of radiant energy is called phosphorescence. The electron carrying extra energy may be expelled from the molecule and is consumed in some other chemical reactions and a fresh normal electron returns to the molecule. This mechanism happens in chlorophyll a (Universal Photosynthetic Pigment).
Light Trapping Centres (PSI & PSII):
The discovery of red drop effect and the Emerson’s enhancement effect concluded in a new concept about the role played bychlorophyll-a and accessary pigments in photosynthesis that photosynthesis involves two distinct photochemical processes. These processes are associated with two groups of photosynthetic pigments called as Pigment system I (Photoact I or Photosystem I) and Pigment system II (Photoact II or Photosystem II).
Each pigment system consists of a central core complex and light harvesting complex (LHC). LHC comprises antenna pigments associated with proteins (viz.., antenna complex). Their main function is to harvest light energy and transfer it to their respective reaction centre. The core complex consists of reaction centre associated with proteins and also electon donors and acceptors.
Wave length of light shorter than 680 nm affect both the pigment systems while wave length longer than 680 nm affect only pigment system I. PSI is found in thylakoid membrane and stroma lamella. It contains pigments chlorophyll a 660, chlorophyll a 670, chlorophyll a 680, chlorophyll a 690, chlorophyll a 700. Chlorophyll a 700 or P700is the reaction centre of PS I. PS II is found in thylakoid membrane and it contains pigments as chlorophyll b 650, chlorophyll a 660, chlorophyll a 670, chlorophyll a 678, chlorophyll a 680 – 690 and phycobillins.
P680-690 is the reaction centre of PS II. Chlorophyll a content is more in PS I than PS II. Carotenoids are present both in PS II and PS I. PS I is associated with both cyclic and non-cyclic photophosphorylation, but PS II is associated with only non-cyclic photophosphorylation.
Both the pigment systems are believed to be inter-connected by a third integral protein complex called cytochrome b – f complex. The other intermediate components of electron transport chain viz., PQ (plasto quinone) and PC (plastocyanin) act as mobile electron carriers between two pigment systems. PS I is active in both red and far red light and PS II is inactive in far red light (Fig. 6.7).
Evidence in Support of Two Phases of Photosynthesis:
1. Physical Separation of Chloroplast into Granna and Stroma Fraction(Photosynthesis):
It is now possible to separate granna and stroma fraction of chloroplast. If light is given to granna fraction in the presence of suitable hydrogen acceptor and in complete absence of carbon dioxide then assimilatory power, ATP and NADPH2, are produced. If these assimilatory powers are given to stroma fraction in the presence of carbon dioxide and absence of light then carbohydrate is synthesized.
2. Temperature Coefficient (Q10):(Photosynthesis)
Q10 is the ratio of the rate of reaction at a given temperature and a temperature 10°C lower. Q10 value of photosynthesis is found to be two or three (for dark reaction) when photosynthesis is fast, but Q10 is one (for light reaction) when photosynthesis is slow.
3. Evidence from Intermittent Light:(Photosynthesis)
Warburg observed that when intermittent light (flashes of light) of about 1/16 seconds were given to green algae (Chlorella vulgaris and Scenedesmus obliquus), the photosynthetic yield per second was higher as compared to the continuous supply of same intensity of light. This confirms that one phase of photosynthesis is independent of light.
4. Evidence from Carbon dioxide in Dark:(Photosynthesis)
It comes from tracer technique by the use of heavy carbon in carbon dioxide (C14O2). The leaves which were first exposed to light have been found to reduce carbon dioxide in the dark It indicates that carbon dioxide is reduced to carbohydrate in dark and it is purely a biochemical phase.
I. Light Reaction (Photochemical Phase):
Light Reaction :
Light reaction or photochemical reaction takes place in thylakoid membrane or granum and it is completely dependent upon the light. The raw materials for this reactions are pigments, water and sunlight.
It can be discussed in the following three steps:
1. Excitation of chlorophyll
2. Photolysis of water
1. Excitation of Chlorophyll:
It is the first step of light reaction. When P680 or P700 (special type of chlorophyll a) of two pigment systems receives quantum of light then it becomes excited and releases electrons.
2. Photolysis of Water and Oxygen Evolution (Hill Reaction):
Before 1930 it was thought that the oxygen released during photosynthesis comes from carbon dioxide. But for the first time Van Neil discovered that the source of oxygen evolution is not carbon dioxide but H2O. In his experiment Neil used green sulphur bacteria which do not release oxygen during photosynthesis. They release sulphur. These bacteria require H2S in place of H2O.
The idea of Van Neil was supported by R. Hill. Hill observed that the chloroplasts extracted from leaves of Stellaria media and Lamium album when suspended in a test tube containing suitable electron acceptors (Potassium feroxalate or Potassium fericyanide), Oxygen evolution took place due to photochemical splitting of water.
The splitting of water during photosynthesis is called Photolysis of water. Mn, Ca, and CI ions play prominent role in the photolysis of water. This reaction is also known as Hill reaction. To release one molecule of oxygen, two molecules of water are required.
The evolution of oxygen from water was also confirmed by Ruben, Randall, Hassid and Kamen (1941) using heavy isotope (O18) in green alga Chlorella. When the photosynthesis is allowed to proceed with H2O18 and normal CO2, the evolved oxygen contains heavy isotope. If photosynthesis is allowed to proceed in presence of CO218 and normal water then heavy oxygen is not evolved.
Thus the fate of different molecules can be summarized as follows:
3. Photophosphorylation :
Synthesis of ATP from ADP and inorganic phosphate (pi) in presence of light in chloroplast is known as photophosphorylation. It was discovered by Arnon et al (1954).
Photophosphorylation is of two types.
(a) Cyclic photophosphorylation
(b) Non-cyclic photophosphorylation.
(a) Cyclic Photophosphorylation :
It is a process of photophosphorylation in which an electron expelled by the excited photo Centre (PSI) is returned to it after passing through a series of electron carriers. It occurs under conditions of low light intensity, wavelength longer than 680 nm and when CO2 fixation is inhibited. Absence of CO2 fixation results in non requirement of electrons as NADPH2 is not being oxidized to NADP+. Cyclic photophosphorylation is performed by photosystem I only. Its photo Centre P700 extrudes an electron with a gain of 23 kcal/mole of energy after absorbing a photon of light (hv).
After losing the electron the photo Centre becomes oxidized. The expelled electron passes through a series of carriers including X (a special chlorophyll molecule), FeS, ferredoxin, plastoquinone, cytochrome b- f complex and plastocyanin before returning to photo Centre. While passing between ferredoxin and plastoquinone and/or over the cytochrome complex, the electron loses sufficient energy to form ATP from ADP and inorganic phosphate.
Halobacteria or halophile bacteria also perform photophosphorylation but ATP thus produced is not used in synthesis of food. These bacteria possess purple pigment bacteriorhodopsin attached to plasma membrane. As light falls on the pigment, it creates a proton pump which is used in ATP synthesis.
(b) Noncyclic Photophosphorylation (Z-Scheme) :
It is the normal process of photophosphorylation in which the electron expelled by the excited photo Centre (reaction centre) does not return to it. Non-cyclic photophosphorylation is carried out in collaboration of both photo system I and II. (Fig. 6.9). Electron released during photolysis of water is picked up by reaction centre of PS-II, called P680. The same is extruded out when the reaction centre absorbs light energy (hv). The extruded electron has an energy equivalent to 23 kcal/mole.
It passes through a series of electron carriers— Phaeophytin, PQ, cytochrome b- f complex and plastocyanin. While passing over cytochrome complex, the electron loses sufficient energy for the synthesis of ATP. The electron is handed over to reaction centre P700of PS-I by plastocyanin. P700 extrudes the electron after absorbing light energy.
The extruded electron passes through FRS ferredoxin, and NADP -reductase which combines it with NADP+ for becoming reduced through H+ releasing during photolysis to form NADPH2. ATP synthesis is not direct. The energy released by electron is actually used for pumping H+ ions across the thylakoid membrane. It creates a proton gradient. This gradient triggers the coupling factor to synthesize ATP from ADP and inorganic phosphate (Pi).