Photosynthesis is defined as the process of biosynthesis of simple carbohydrates like glucose by utilizing carbon dioxide and water in the presence of sunlight inside the chlorophyll-containing cells of green plants.
Photosynthesis Equation:
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A photosynthetic unit is the smallest group of collaborating pigment molecules necessary to affect a photochemical act or conversion of light energy into chemical energy. Photosynthetic units are located in small particle units called quantasome.
A typical photosynthetic unit consists of a reaction center/photo center/trapping center which is fed by about 200-300 light-harvesting pigment molecules/ The reaction center is a special type of chlorophyll A molecule designated P700 in the case of Pigment System I or Photo System I (PSI) and P680 in the case of Pigment System II or Photo System II (PSII).
The light-harvesting pigment molecules are of two types viz: core molecules and antenna molecules. The light-harvesting pigment molecules consist of chlorophyll-b, carotene, xanthophyll, etc. The antenna molecules absorb light energy of wavelength lower than that absorbed by core molecules and transfer it to the core molecules by electron spin resonance.
The core molecules absorb light of wavelength lower than those absorbed by the reaction center. The light energy absorbed by the antenna molecules and core molecules is transferred to the reaction center.
Until 1930 AD, scientists believed photosynthesis was just the reverse of respiration, so the photosynthesis equation was given as follows.
In 1931 AD, Van Niel discovered that in the photosynthetic bacteria (green sulfur bacteria), photosynthesis occurred in the presence of Carbon dioxide but without the evolution of oxygen. If carbon dioxide was the source of oxygen, oxygen should’ve evolved. Those bacteria utilize H2S instead of water as a hydrogen donor so, elemental sulfur is produced instead of oxygen as follows:
So, Van Niel predicted that if elemental sulfur comes from H2S photosynthetic bacteria, oxygen should come from water in green plants. Finally, the experimental proof that oxygen evolved during photosynthesis comes from water was given by radio isotopic studies using heavy isotopes of oxygen (O 18 ) by Samuel Ruben, Randal Kamen, and Hyde. When they grew the experimental plant in labeled CO2 18 , the oxygen evolved was of normal type (O 16 ). However, when the oxygen of water was labeled (H2O 18 ), the evolved oxygen was off (O 18 ).
Hill Reaction
The phenomenon of the evolution of oxygen from water in the absence of carbon dioxide by the isolated chloroplast when illuminated in the presence of suitable hydrogen acceptors like ferric oxalic, quinone, etc.
A : suitable hydrogen acceptor
AH2 : reduced form of A
The process of photosynthesis is a complicated oxidation-reduction process, ultimately resulting in the oxidation of water and the reduction of carbon dioxide. The mechanism of photosynthesis consists of two phases:
It is the first phase in photosynthesis during which the light energy of the sun is converted into chemical energy which is stored in the form of ATP and NADPH + H + . It occurs in the grana region of the chloroplast.
It consists of the following steps
Light energy travels in the form of discrete energy packets called photons. Each photon contains 1 quantum of light energy. When a chlorophyll-a molecule (reaction center) absorbs a photon of light, it becomes excited. Sometimes, during excitation, an electron gets ejected from the chlorophyll-a molecule. The ejected electron contains an extra amount of energy received from the sun.
A part of this energy is used in the photolysis of water, and another part of the energy is used for photophosphorylation. Before the absorption of a photon of light, the chlorophyll molecule was in the ground state. But, after the ejection of an electron, an electron hole is formed in the chlorophyll-a molecule and the molecule is in the excited state.
It is the process of splitting water molecule into H + and OH – ions during the light reaction of photosynthesis by utlising the light energy of sun. It required the presence of Mn 2+ and Cl – .
The H + released is used for the reduction of NADP + into NADPH + H + . The OH – ions after releasing their electrons recombine to form water and oxygen gas is released. The released electron is used for filling the electron-hole present in chlorophyll-a of PSII.
The process of formation of ATP from ADP and inorganic phosphate (Pi) during the light reaction of photosynthesis by utilizing light energy is called photophosphorylation. It is of two types:
It is the most common path of light reaction involving PSI and PSII. The PSI becomes activated after absorption of light of wavelength 700nm. When chlorophyll-a of PSI designated P700 absorbs a photon of light of wavelength 700nm it becomes excited. The excited chlorophyll-a ejects one of its electrons which is trapped by a Feredoxin-reducing substance (FRS). From FRS, the electron is transferred to Ferodoxin (FD). The FD transfers the electron NADP + which gets reduced to form NADPH + H + . The proton used in this reduction is obtained from the photolysis of water. As the electron released by PSI is used for reducing NADP + to NADPH + H + an electron-hole is formed in the chlorophyll-a molecule.
Meanwhile, the PSII is activated after absorption of light of wavelength 680nm. When chlorophyll-a designated P680 absorbs a photon of light of wavelength in 680nm, it becomes excited. The excited chlorophyll-a ejects one of its electrons which is trapped by Quinone (Q). From Q, the electron is shuttled down to plastoquinone (PQ). From PQ, the electron is shuttled down to cytochrome (CYTF). During this electron shuttle, ATP is produced ADP and Pi. From CYTF, electrons make it to plastocyanin (PC) and then to chlorophyll-a of PSI.
Thus, the electron hole present in PSI is filled by the electron flowing from PSII. These electrons are released during the photolysis of water. Thus, we see that the movement of the electron donor and final electron acceptor are different. Hence, this type of photophosphorylation is called non-cyclic photophosphorylation.
This type of photophosphorylation occurs when the plants are illuminated with light of wavelength greater than 680nm. Hence, cyclic phosphorylation involved only PSI. The PSI is activated after absorption of light of wavelength 700nm. When the chlorophyll-a of PSI designated P700 absorbs a photon of light of wavelength 700nm, it becomes excited.
The excited chlorophyll-a accepts one of its electrons which is trapped by FRS. From FRS, the electron is transferred to FD. On the availability of NADP + , FD transfers its electrons to cytochrome b6. During this electron shuttle, ATP is produced from ADP and Pi. From cytochrome b6, the electrons move downhill to PQ. From PQ, the electron is shuttled down to CYTF. During this electron shuttle, ATP is produced from ADP and Pi. From CYTF, electrons move downhill to PC and then back to chlorophyll-a of PSI.
Thus, we see that the movement of electrons is cyclic i.e. initial electron donor and final electron acceptor are the same. Hence, this type of photophosphorylation is called cyclic phosphorylation.
It is the second step in photosynthesis which is independent of light and during which the reducing power (ATP and NADPH + H + ) formed during light reaction is utilized for the reduction of CO2 to glucose. It occurs in the stroma region of the chloroplast.
Scientists knew that the first stable compound formed after CO2 fixation is a three-carbon compound called Phosphoglyceric Acid (PGA). So, they were searching for a two-carbon compound that could act as an initial CO2 acceptor. But these two-carbon compounds weren’t found. The solution to these was given by Calvin and his co-workers by using the green algae Chlorella and the technique of autoradiography C 14 . They found that the initial CO2 acceptor wasn’t a two-carbon compound. Instead, it was a five-carbon compound called Ribulose-1,5-biphosphate (RuBP) which combined with the atmospheric CO2 to form an intermediate 6-carbon compound that immediately breaks down into two molecules of 3-PGA.
There are different pathways of CO2 fixation in different species of plants. These pathways are-
Calvin’s cycle consists of the following three stages: Carboxylation, Glycolytic reversal, and Regeneration of RuBP.
The initial CO2 reversal acceptor is a five-carbon compound called ribulose-1,5-phosphate (RuBP) which combines with atmospheric CO2 in the presence of the enzymes ribulose-1,5-biphosphate carboxylase or oxygenase (RuBisCO) to form an intermediate 6 carbon compound which immediately breaks down to form 2 molecules of a 3-C compound called 3-phosphoglyceric acid (3PGA).
RuBP + CO2 → 3PGA
It consists of the following steps:
Some of the fructose-6-phosphate molecules are utilized to produce hexose sugars.
It consists of the following steps:
The RuBP so formed restarts the Calvin Cycle again.
For a considerable period, the Calvin cycle as described earlier was 3rd to the only photosynthetic reaction sequence occurring in higher plants and algae. In 1965, Hartt and Burr reported that 4 carbon-containing di-carboxylic acids malate and aspartate were the major labeled product. When sugarcane leaves oil is allowed to photosynthesize for short periods in 14CO2. This finding was confirmed and greatly elaborated by Hatch and Slack who observed every labeling of oxaloacetate, malate, and aspartate only when sugarcane leaves oil exposed CO2 14 for 1 second. This and further study by these workers led to the establishment of another CO2 reduction pathway because C4 dicarboxylic acid pathway.
Besides sugarcane leaves, this pathway has been found to operate in many plant species of the family Graminea. Examples: maize, sorghum, Amaranthus, etc. These are all known as C4 plants distinguished by the absence of photorespiration and Krantz’s Anatomy of the Leaf. In the leaves of these plants, the vascular bundles are surrounded by a bundle sheath of large parenchymatous cells that in turn are surrounded by mesophyll cells. The chloroplast in cell of the bundle sheath is larger and usually lacks grana, the chloroplast in mesophyll cells is smaller and always contains grana.
C4 cycle involves two carboxylation reactions; one taking place in the chloroplast of mesophyll cells and the other taking place in the chloroplast of bundle sheath cells.
During the night, large amounts of starch are consumed during acidification which indicates that carbohydrates are the source of malic acid synthesis. The distinctive diurnal fluctuation in acidity in plants showing the CAM cycle is predominantly due to the changes in the amount of vacuolar malic acid that is synthesized, utilizing CO2, which then accumulates in the vacuole and may account for 85% of the total acid content.
Malic acid is synthesized during the night in reaction. Pyruvate or PEP is carboxylated to produce malic acid either directly or first forming oxaloacetic acid which is then reduced to malate. At day time, during the following day, when acidified organs are exposed to light, rapid consumption of malic acid occurs resulting in deacidification due to decarboxylation of malic acid.
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Bristy Shrestha graduated as a topper of her class with a Bachelor’s in Microbiology from Tribhuvan University. Her final year of research focused on the ‘Bacteriological Analysis and Antibiotic Susceptibility Test of Isolates of Jujudhau from Bhaktapur.’ She is a feminist activist and the co-founder of the campaign ‘SheTheKranti.’ Bristy is deeply passionate about her Newa: heritage and is proficient in reading and writing Nepal Lipi, the script of Nepal Bhasa. She is an avid traveler who likes to learn new skills during her leisure time. She describes herself as a mindful individual.
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