during photosynthesis carbohydrates are formed by which reaction
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Carbohydrates are formed in green plants by photosynthesis, which is the chemical combination, or "fixation", of carbon dioxide and water by utilization of energy from the absorption of visible light. The overall result is the reduction of carbon dioxide to carbohydrate and the formation of oxygen:
If the carbohydrate formed is cellulose, then the reaction in effect is the reverse of the burning of wood, and obviously requires considerable energy input.
Because of its vital character to life as we know it, photosynthesis has been investigated intensively and the general features of the process are now rather well understood. The principal deficiencies in our knowledge include just how the light absorbed by the plants is converted to chemical energy and the details of how the many complex enzyme-induced reactions involved take place.
The ingredients in green plants that carry on the work of photosynthesis are contained in highly organized, membrane-covered units called chloroplasts. The specific substances that absorb the light are the plant pigments, chlorophyll a and chlorophyll b, whose structures are shown in Figure 20-6. These highly conjugated substances are very efficient light absorbers, and the energy so gained is used in two separate processes, which are represented diagrammatically
The end result of the photochemical part of photosynthesis is the formation of O2O2, NADPHNADPH, and ATP. Much of the oxygen is released to the atmosphere, but the NADPHNADPH and ATP are utilized in a series of dark reactions that achieve the reduction of carbon dioxide to the level of a a carbohydrate (fructose). A balanced equation is
6CO2+12NADPH+12H⊕→C6H12O6+12NADP⊕+6H2O(20.9.2)(20.9.2)6CO2+12NADPH+12H⊕→C6H12O6+12NADP⊕+6H2O
The cycle of reactions that converts carbon dioxide to carbohydrates is called the Calvin cycle, after M. Calvin, who received the Nobel Prize in chemistry in 1961 for his work on determining the path of carbon in photosynthesis.
Carbon enters the cycle as carbon dioxide. The key reaction by which the CO2CO2 is "fixed" involves enzymatic carboxylation of a pentose, DD-ribulose 1,5-phosphate.88
A subsequent hydrolytic cleavage of the C2C2-C3C3 bond of the carboxylation product (this amounts to a reverse Claisen condensation.yields two molecules of DD-3-phosphoglycerate.99
In subsequent steps, ATP is utilized to phosphorylate the carboxyl group of 3-phosphoglycerate to create 1,3-diphosphoglycerate (a mixed anhydride of glyceric and phosphoric acids). This substance then is reduced by NADPHNADPH to glyceraldehyde 3-phosphate:
Two glyceraldehyde 3-phosphates are utilized to build the six-carbon chain of fructose by an aldol condensation (C3+C3→C6)(C3+C3→C6), but the donor nucleophile in this reaction is the phosphate ester of dihydroxypropanone, which is an isomer of glyceraldehyde 3-phosphate. Rearrangement of the C3C3 aldose to the C3C3 ketose therefore precedes the aldol addition.
From what we have described thus far, only one atom of carbon has been added from the atmosphere, and although we have reached fructose, five previously reduced carbons were consumed in the process. Thus the plant has to get back a five-carbon sugar from a six-carbon sugar to perpetuate the cycle. Rather than split off one carbon and use that as a building block to construct other sugars, an amazing series of transformations is carried on that can be summarized by the following equations:

These reactions have several features in common. They all involve phosphate esters of aldoses or ketoses, and they resemble aldol or reverse-aldol condensations. Their mechanisms will no be considered here, but are discussed more explicitly in Sections 20-10A, 20-10B, and 25-10. Their summation is C6+3C3→3C5C6+3C3→3C5, which means that fructose 6-phosphate as the C6C6 component reacts with a total of three C3C3 units (two glyceraldehyde 3-phosphates and one dihydroxypropanone phosphate) to give, ultimately, three ribulose 5-phosphates. Although the sequence may seem complex, it avoids building up pentose or hexose chains one carbon at a time from one-carbon intermediates.
If the carbohydrate formed is cellulose, then the reaction in effect is the reverse of the burning of wood, and obviously requires considerable energy input.
Because of its vital character to life as we know it, photosynthesis has been investigated intensively and the general features of the process are now rather well understood. The principal deficiencies in our knowledge include just how the light absorbed by the plants is converted to chemical energy and the details of how the many complex enzyme-induced reactions involved take place.
The ingredients in green plants that carry on the work of photosynthesis are contained in highly organized, membrane-covered units called chloroplasts. The specific substances that absorb the light are the plant pigments, chlorophyll a and chlorophyll b, whose structures are shown in Figure 20-6. These highly conjugated substances are very efficient light absorbers, and the energy so gained is used in two separate processes, which are represented diagrammatically
The end result of the photochemical part of photosynthesis is the formation of O2O2, NADPHNADPH, and ATP. Much of the oxygen is released to the atmosphere, but the NADPHNADPH and ATP are utilized in a series of dark reactions that achieve the reduction of carbon dioxide to the level of a a carbohydrate (fructose). A balanced equation is
6CO2+12NADPH+12H⊕→C6H12O6+12NADP⊕+6H2O(20.9.2)(20.9.2)6CO2+12NADPH+12H⊕→C6H12O6+12NADP⊕+6H2O
The cycle of reactions that converts carbon dioxide to carbohydrates is called the Calvin cycle, after M. Calvin, who received the Nobel Prize in chemistry in 1961 for his work on determining the path of carbon in photosynthesis.
Carbon enters the cycle as carbon dioxide. The key reaction by which the CO2CO2 is "fixed" involves enzymatic carboxylation of a pentose, DD-ribulose 1,5-phosphate.88
A subsequent hydrolytic cleavage of the C2C2-C3C3 bond of the carboxylation product (this amounts to a reverse Claisen condensation.yields two molecules of DD-3-phosphoglycerate.99
In subsequent steps, ATP is utilized to phosphorylate the carboxyl group of 3-phosphoglycerate to create 1,3-diphosphoglycerate (a mixed anhydride of glyceric and phosphoric acids). This substance then is reduced by NADPHNADPH to glyceraldehyde 3-phosphate:
Two glyceraldehyde 3-phosphates are utilized to build the six-carbon chain of fructose by an aldol condensation (C3+C3→C6)(C3+C3→C6), but the donor nucleophile in this reaction is the phosphate ester of dihydroxypropanone, which is an isomer of glyceraldehyde 3-phosphate. Rearrangement of the C3C3 aldose to the C3C3 ketose therefore precedes the aldol addition.
From what we have described thus far, only one atom of carbon has been added from the atmosphere, and although we have reached fructose, five previously reduced carbons were consumed in the process. Thus the plant has to get back a five-carbon sugar from a six-carbon sugar to perpetuate the cycle. Rather than split off one carbon and use that as a building block to construct other sugars, an amazing series of transformations is carried on that can be summarized by the following equations:

These reactions have several features in common. They all involve phosphate esters of aldoses or ketoses, and they resemble aldol or reverse-aldol condensations. Their mechanisms will no be considered here, but are discussed more explicitly in Sections 20-10A, 20-10B, and 25-10. Their summation is C6+3C3→3C5C6+3C3→3C5, which means that fructose 6-phosphate as the C6C6 component reacts with a total of three C3C3 units (two glyceraldehyde 3-phosphates and one dihydroxypropanone phosphate) to give, ultimately, three ribulose 5-phosphates. Although the sequence may seem complex, it avoids building up pentose or hexose chains one carbon at a time from one-carbon intermediates.
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