Introduction
Photosynthesis is the single most important physico-biochemical process of the plant on which the existence of life on earth depends. It is the ability of green plants only to utilize the energy of light to produce carbon containing organic material from stable inorganic matter by photosynthetic process. It is from the carbohydrate produced by photosynthesis that directly or indirectly all the countless number of organic compounds, which compose the living world,are derived. The oxidation of organic compounds releases stored energy to be utilized by organisms to derive essential metabolic processes. Any energy released during oxidation of organic compounds is ultimately derived from light energy intercepted by green plants during photosynthesis.
Definition
In simple terms, the photosynthesis can be defined as the formation of carbon containing compounds from carbon dioxide and water by illuminated green cells, water and oxygen being the by products.
The simplest equation for photosynthesis can be:
Since Ruben and Kamen (1941) demonstrated the source of illiberated oxygen is water, the equation may be corrected as
Of these components water is obtained from soil by roots of the terrestrial plants and by general surface by hydrophytes. As a source of light sunlight is utilized. The terrestrial plants absorb CO2 from the atmosphere where it is present upto .03 percent or even more whereas hydrophytes obtain it from water where it is found in dissolved state.
Location
In green plants and algae, photosynthesis takes place in chloroplast. The light reactions occur in the thylakoid membranes and dark reactions take place in the stroma. In photosynthetic bacteria the light reactions take place in the bacterial membrane or in invaginations of its (chromatophores).
Light harvesting in green plants
Chlorophyll molecules absorb sunlight. Chlorophyll is a porphyrin in which nitrogen atoms are coordinated to a magnesium ion. Green plants contain two types of chlorophyll molecules, chlorophyll a and chlorophyll b, that differ slightly in structure and wavelength of light they can absorb. Although chlorophyll molecules trap light directly, several accessory pigments (carotenoids) also exist that absorbs light and pass the excitation energy on to chlorophyll molecules.
A process
Photosynthesis is an oxidation-reduction process in which water is oxidized and carbon dioxide is reduced to carbohydrate level, the water and oxygen being byproducts. The reduction of CO2to carbohydrate level needs assimilatory powers such as ATP and NADPH + H+. Reduction of CO2occurs in dark but the production of assimilatory powersis light dependent process. Hence the process of photosynthesis consists of two phases- Light dependent phase (Light reaction or Hill reaction) and Light independent phase (dark reaction or Blackman�s reaction).
The existence of light and dark reaction showed by Emerson and Arnold (1932) by flashing light experiments.
Mechanism of Light Reaction (Activities found in grana or thylakoids)
In 1937, Robert Hill demonstrated that isolated chloroplast evolved oxygen when they were illuminated in the presence of suitable electron acceptor, such as ferricyanide. The ferricyanide is reduced to ferrocyanide by photolysis of water. This reaction is called Hill reaction and it explains that water is used as a source of electrons for CO2fixation and oxygen is evolved as byproduct.
Light reaction is a complicated process and consists of several events:
1) Red drop, Emerson effect and two pigment system
���� Red drop and Emerson effect
���� Two pigment systems
2) Production of assimilatory powers
���� Electron transport system in photosynthesis or reduction of NADP
���� Cyclic photophosphorylation
���� Pseudocyclicphotophosphorylation
3) Energy relationship and efficiency of photsynthesis
4) Interrelationship between dark and light reaction
Green plants and algae use two types of photosystem called photosystem I (PSI) and photosystem II (PSII). The chlorophyll in the reaction center of PSI has an absorption maximum at 700nm and so is called P700 and that in the reaction center of PSII has an absorption maximum at 680nm and so is called P680. Other electron carriers link the two photosystems. When arranged according to their redox potential the various components form the so-called Z scheme because the overall shape of the redox diagram looks like a Z.
Two pigment system showing Z scheme and Non-cyclic Photophosphorylation
The cytochrome bf complex is a proton pump and during electron transport, pumps H+ ions from the stromata into the thylakoid space, creating an H+ gradient. H+ ions are also released into the thylakoid space when photosystem II oxidizes water to produce oxygen whilst the H+ ions used to reduce NADP+ to NADPH are taken up from the stroma. Both effects contribute to the H+ gradient. The proton gradient drives ATP synthesis via an ATP synthase located in the thylakoid membrane (photophosphorylation). Since the electron transport involves a linear array of electron carriers, the system is called noncyclicphotophosphorylation.
When NADP+ is available to accept electrons, an alternative electron transport pathway is used. The high-energy electron donated by photosystem I passes to ferredoxin, then the cytochrome bf complex, the plastocyanin and back to the P700 of photosystemI. The resulting proton gradient is generated by the cytochrome bf complex drives ATP synthesis (cyclic photophosphorylation) but no NADPH is made and no O2is produced.
Cyclic Photophosphorylation
Dark reaction (activities found in stroma) - The fact that non-photochemical process is involved in photosynthesis was established by Blackman (1905). Dark reaction also called the carbon fixation reactions use the ATP and NADPH produced by the light reactions to convert carbon dioxide into carbohydrate. The final products are starch and sucrose. At least more than one pathway of dark fixation exists:
Calvin cycle (C3 plants)
Hatch and Slack cycle (C4 plants)
CAM cycle (CAM plants)
A large enzyme called ribulosebiphophatecarboxylase located in stroma catalyzes the key carbon fixation. The reaction condenses a CO2molecule with ribulose-1,5-bisphosphate to produce a transient six carbon intermediate that rapidly hydrolyzes to two molecules of 3-phoophoglycerate.
The rubisco reaction forms a part of cycle of reactions, called a calvin cycle that leads to regeneration of ribulose 1,5-bisphosphate and the net production of glyceraldehyde 3-phosphate for the synthesis of sucrose and starch. Three molecles of CO2must be fixed to generate one molucule of glyceraldehyde 3-phosphate. Since three ATP and 2 NADPH are required for each CO2converted carbohydrate the overall reaction for synthesis of one moelcule of glyceraldehyde 3 phosphate is:
3CO2 + 6NADPH + 9ATP
����������������
Glyceraldehyde 3 phosphate + 6NADP+ + 9ADP +8Pi
Synthesis of sucrose
Much of the glyceraldehydes 3-phosphate produced by the Calvin cycle is exported to cytosol and used to produce the dissacahride sucrose. The chemical reactions involved are reversal of glycolysis.
UDP glucose + fructose 6 phosphate
����������������
Sucrose 6 phosphate + UDP
C3 Cycle
Synthesis of starch
Starch is produced in the stroma of chloroplast and stored as a starch grain.STrachsynthesis occurs from ADP-glucose, CDP- glucose or GDP glucose. The pathway involves conversion of glyceraldehydes 3-phosphate (from Calvin cycle) to glucose 1 phosphate that in turn is used to synthesize the nucleotide sugar derivatives.
C4 Pathway
When the CO2concentration is low, rubisco can add O2to ribulose 1,5 bisphosphate (oxygenase activity) instead of CO2(carboxylase activity) producing phosphoglycolate and 3- phosphoglycerate. Metabolism of phosphoglycolate releasesCO2and NH4+ and wastes energy. This consumption of O2and release of CO2is called photorespiration. Plants in hot climates close their stomata to reduce water loss. This causes a drop in the CO2concentration in the leaf, favoring photorespiration. To avoid this problem these plant carry out the Calvin cycle only in the bundle sheath cells that are protected from the O2in air by mesophyll cells. The CO2is transported from the air via the mesophyll cells to the bundle sheath cells by combining three carbon molecules to produce four-carbon compound. This C4 pathway ensures a high CO2concentration for carbon fixation by rubisco in bundle sheath cells.
C4 Pathway
CAM Pathway
Factors affecting Photosynthesis
Internal factors:
1. Chlorophyll
2. Protoplasmic factors
3. Photosynthetic factors
4. Photosynthetic enzyme system
5. Demand for photosynthate
6. Leaf resistance
7. Role of hormones
8. Genetic controls
External factors:
1. Carbon dioxide
2. Light (Intensity of light, wavelength of light, Duration of light, Photoxidation)
3. Temperature
4. Water
5. Oxygen
6. Mineral nutrient elements
7. Osmotic relations
