What are the major steps for synthesis of carbohydrates in plants
Answers
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 in Figure 20-7.
The structure of chlorophyll a and chlorophyll b, showing cis-trans relationships of the substituents.
Simplified representation of the photoreactions in photosynthesis. The oxidation of water is linked to the reduction of NADP⊕ by an electron-transport chain (dashed line) that is coupled to ATP formation (photophosphorylation).
The other important photoreaction is oxidation of water to oxygen by the reaction:
The oxygen formed clearly comes from H2O and not from CO2 , because photosynthesis in the presence of water labeled with O18 produces oxygen labeled with O18 , whereas carbon dioxide labeled with O18 does not give oxygen labeled with O18 . Notice that the oxidation of the water produces two electrons, and that the formation of NADPH from NADP⊕ requires two electrons. These reactions occur at different locations within the chloroplasts and in the process of transferring electrons from the water oxidation site to the NADP⊕ reduction site, adenosine diphosphate (ADP) is converted to adenosine triphosphate (ATP; see Section 15-5F for discussion between the importance of such phosphorylations). Thus electron transport between the two photoprocesses is coupled to phosphorylation. This process is called photophosphorylation (Figure 20-7).
The end result of the photochemical part of photosynthesis is the formation of O2 , NADPH , and ATP. Much of the oxygen is released to the atmosphere, but the NADPH 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
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 CO2 is "fixed" involves enzymatic carboxylation of a pentose, D -ribulose 1,5-phosphate. 8
A subsequent hydrolytic cleavage of the C2 - C3 bond of the carboxylation product (this amounts to a reverse Claisen condensation; Section 18-8B) yields two molecules of D -3-phosphoglycerate. 9
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 NADPH 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) , but the donor nucleophile in this reaction is the phosphate ester of dihydroxypropanone, which is an isomer of glyceraldehyde 3-phosphate. Rearrangement of the C3 aldose to the C3 ketose (of the type described in Section 20-2D) therefore precedes the aldol addition. (For a discussion of the mechanism of the enzymatic aldol reaction, see Section 17-3F.) The fructose 1,6-diphosphate formed is then hydrolyzed to fructose 6-phosphate:
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.