Recall the history of photosynthesis
Many scientists have contributed to understanding how plants carry out photosynthesis. These are as follows:
- Van Helmont (1648) concluded that all food of the plant is derived from water and not from the soil.
- Stephen Hales (Father of Plant Physiology) (1727) reported that plants obtain a part of their nutrition from air and light may also play a role in this process.
- Joseph Priestley (1772) demonstrated that green plants purify the foul air (i.e., Phlogiston), produced by burning of the candle, and convert it into the pure air (i.e., Dephlogiston).
- Jan Ingen-Housz (1779) concluded by his experiment that purification of air was done by green parts of the plant only and that too in the presence of sunlight. Green leaves and stalks liberate dephlogisticated air during sunlight and phlogisticated air during dark.
- Jean Senebier (1782) proved that plants absorb CO and release O in presence of light. He also showed that the rate of Oevolution depends upon the rate of COconsumption.
- Lavoisier (1783) identified the pure air (i.e., phlogiston) as oxygen (O) and noxious air (i.e., Phlogiston) produced by the burning of the candle as carbon dioxide (CO).
- Nicolas de Saussure (1804) showed the importance of water in the process of photosynthesis. He further showed that the amount of CO absorbed is equal to the amount of O released.
- Pelletier and Caventou (1818) discovered chlorophyll. It could be separated from the leaf by boiling in alcohol.
- Sachs (1864) discovered the carbohydrate as a product of the photosynthesis.
- Emerson and Arnold (1932) described the occurrence of light and dark reaction during the process of photosynthesis.
- Robert Hill (1937) demonstrated the photolysis of water in light reaction of photosynthesis.
Introduction to the history of photosynthesis
There are the scientists who explained the life-sustaining process, photosynthesis.
- Aristotle: Over two thousand years ago, he said that plant absorbs all the inorganic and organic material directly from the soil.
- Jan Baptista Von Helmont: In the year 1648, he conducted an experiment. In this experiment, He grew a weighed mass of willow tree in a pot with the weighed amount of soil. He covered the soil with the lid with small the small hole. The lid allows only rainwater to pass. After five years, he discovered that the mass of the tree and soil more than it did at the start. He concluded that major substance for plant growth is water.
- Stephan Hales: He described that plant exchanges gases with the surroundings.
- Priestley: He demonstrates that plant has the ability to reserve air.
- Igenhouz: He discovered that plants has the ability use carbon dioxide only in the presence of sunlight.
UV absorption spectrophometry
When the UV light is used to measure the concentration of compounds in the given solution is called as UV absorption spectrophotometry.
Principle- It is based on Beer's law, the absorption of light.
Application- Determine the concentration of immunoglobulin, amino acids in
proteins, purity of solution etc.
Principle- It is based on Beer's law, the absorption of light.
Application- Determine the concentration of immunoglobulin, amino acids in
proteins, purity of solution etc.
Light dependent phase or photochemical phase
The process of photosynthesis has two phases light dependent and light independent.
Light-dependent or photochemical phase
Light-dependent or photochemical phase
- The light-dependent reaction takes place in the presence of light inside the thylakoid of the chloroplast.
- It occurs in 400-700 nm wavelength of visible light.
- It is maximum in blue- red light and least in green light.
- Activation of chlorophyll: The chlorophyll becomes activated by absorbing light energy.
- Splitting of water: The water molecule splits into hydrogen, oxygen and releases electron. This process occurs due to the absorbed energy. Hydrogen ion from the photolysis of water reduced the NADP to NADPH. Electrons from photolysis convert ADP to ATP.
ATP synthesis
ATP is also called as energy currency. It is produced during the process of respiration in the presence of a respiratory enzyme (ATPase).
- It is a nucleotide that contains a large amount of chemical energy stored in its high-energy phosphate bonds.
- It releases energy when it is broken down into ADP. The energy is used for many metabolic processes. Hence, ATP is considered as the universal energy currency for metabolism.
- Its functions are for intracellular energy transport for various metabolic processes including biosynthetic reactions, motility, and cell division.
- It is also used as a substrate by kinases that phosphorylate proteins and lipids, and by adenylate cyclase to produce cyclic AMP.
Non-cyclic photophosphorylation
Non-cyclic phosphorylation
It is the normal process of photophosphorylation in which the electron expelled by the excited photocentre does not return to it.
It is the normal process of photophosphorylation in which the electron expelled by the excited photocentre does not return to it.
- It is carried out in the collaboration of both photosystem I and photosystem II.
- In this process, electrons are released by the photolysis of water. Thus, constant supply of water is required.
- Both ATP and NADPH are formed.
Z scheme of electron transport
Z-scheme is the graphical representation of the electron transport pathway according to the redox potential pathway. It occurs in non cyclic phosphorylation.
- In this process, electron flows from water to reduce NADP to NADPH.
- The energy required to move electrons is provided by absorbed sunlight.
- Fist of all PSII absorbs light and promote to the excited state.
- In excited state, electrons aquire large amount of energy and move primary acceptor Q.
- Electrons from Q pass into the PQ . The reduced form of PQ donates electron to cytochrome where ATP is formed.
- The electrons then pass to the plastocyanin and then to PS I.
- The electrons from PSI expelled to excited state from it moves to the ferredoxin protein.
- Ferredoxin protein transfers the electron to the NADP that reduce to NADPH.
Cyclic photophosphorylation
Phosphorylation is the process of formation ATP from ADP in light reaction of photosynthesis. It can be done by two ways cyclic and non-cyclic.
Cyclic phosphorylation
It is a process of photophosphorylation in which an electron expelled by the excited photocentre is returned back to it after passing through a series of electron carriers.
In this process water is not consumed.
Only ATP is formed when electrons pass ferredoxin to PQ and from PQ to cytochrom system.
Cyclic phosphorylation
It is a process of photophosphorylation in which an electron expelled by the excited photocentre is returned back to it after passing through a series of electron carriers.
In this process water is not consumed.
Only ATP is formed when electrons pass ferredoxin to PQ and from PQ to cytochrom system.
Chemiosmotic hypothesis of ATP formation
Chemiosmotic hypothesis was proposed by Peter Mitchell. It is the generation of ATP by ATP synthase in an electron transport chain.
- ETC is an oxidative phosphorylation reaction takes place in the inner membrane of the mitochondria.
- It begins with NADH and FADH that looses electrons. These electrons are transferred along the chain.
- As each complex accepts electrons and passes it on, energy is released.
- This energy is used to pump the protons move against the concentration gradient from the matrix of the mitochondria to the inner membrane space.
- Eventually, there is a high concentration of protons build up the membrane space and the protons try to move back into the matrix. However, the inner membrane is impermeable to protons.
- A proton motive force (PMF) is set up and ATP synthase undergoes conformational change and uses the PMF to make ATP from ADP.
- ATP synthase allows the proton to diffuse back down their gradient. ATP + Pi -> ATP
Dark reaction
When the reaction of photosynthesis does not require light energy is called as a light-independent or dark reaction.
- It is also called as a biosynthetic phase or Calvin cycle.
- It occurs in the stroma of the chloroplast.
- It is a multistep, enzyme-catalyzed process, where simple substrates are converted into more complex products.
- NADPH provides the electrons, required to fix the into carbohydrates.
- The is fixed in a number of steps using acceptor RuBP (ribulose bisphosphate).
Phases of Calvin cycle
Clavin cycle was given by the Melvin Calvin, James Bassham and Andrew Benson. In this cycle, phosphogyceric acid (3 carbon compound) is produced as a first product and thus called as cycle. From the given diagram we can understand the stages of clavin cycle.
Calvin cycle can be divided into three stage:
Calvin cycle can be divided into three stage:
- Carboxylation: It is the first stage where COis fixed from an antmosphere in the presenc of rubisco enzyme. One molecule of RuBP joins with one mole colecule of in the presence of enzyme and produces phosphogyceric acid.
- Reduction: In this stage, glucose is formed. ATP and NADPH are used to reduce 3-PGA into G3P; then ATP and NADPH are converted to ADP and NADP, respectively.
- Regeneration of RuBP: It is the series of enzymatic reactions. In this stage, RuBP is regenerated by the use of ATP. It again enables the system to fix CO.
Characteristics of C4 plants
Plants that are adapted to dry tropical region have pathway and called as plants e.g., grass, maize, sorghum sugarcane etc. In this pathway, first product is 4 carbon compoud hence called as . It was given by M.D. Hatch and Roger Slack and thus named as Hatch and Slack pathway.
The characteristics of plants are as follows:
The characteristics of plants are as follows:
- Leaf have special anatomy called as Kranz anatomy. Leaves have large bundle sheath cells around the vascula bundle (xylem and phloem). Cells have large number of dimorphic chloroplast (different structure and size) and thick walled. The chloroplast of bundle sheath cells have less number of grana as compared to mesophyll cells.
- Dimorphic chloroplast prevents the phorespiration and increases the tolerance of high temperature.
- The layers of bundle sheath cells are surrounded by layesr of mesophyll cells.
C4 pathway
pathway occurs by the cooperation between bundle sheath cells and mesophyll cells.
- Fist step takes place in the mesophyll cell where phosphoenolpyruvate carboxylase enzyme catalyses the formation oxaloacetate from phosphoenolpyruvate andcarbon dioxide.
- Oxaloacetate the reduced to malic acid (c4 compound)in the presence of malate dehydrogenase enzyme.
- The malic acid then transferred to bundle sheath cell through plasmodesmata.
- In bundle sheath cell, malic acid is decarboxylated in the presence of the malic enzyme and releases carbon dioxide.
- By the decarboxylation of malic acid 3 carbon compound i.e., pyruvate is formed. Carbon dioxide enters into the Calvin cycle and pyruvate return back to the mesophyll cells.
- In mesophyll cell, pyruvate in the presence of pyruvate phosphate dikinase enzyme converted into the phosphoenolpyruvate and cycle repeats.
Photorespiration
Photorespiration is the process where the enzyme RuBisCO oxygenates RuBP with the release of carbon dioxide. It is also known as a wasteful process because it prevents the plant from using ATP and NADPH to synthesize carbohydrate.
- It occurs in plants and not in plants.
- In pathway, RUBP binds with oxygen and form phosphoglycolate and phosphoglycerate. This reaction decreases the fixation as we can see in the given cycle.
- It occurs when the level of is low inside the leaf.
- It occurs on the hot dry day to close stomata and prevent excess loss of water.
- It limits the damaging product of the light reaction of photosynthesis.
- It reduces the photosynthetic efficiency in plantt by 25%.
Blackman's law of limiting factors
F.F. Blackman in 1905 proposed the principle of limiting factor. According to this principle, when a process depends on number of factors its rate is limited by the pase of the slowest factor.
- It determines the rate of photosynthesis.
- It explains that if all other factors are kept constant the factor under consideration will affect the rate of photosythesis.
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