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The assimilation of carbon into organic compounds is the result of a complex series of enzyme-regulated chemical reactions—the dark reactions. This term can be misused because these reactions can take place in light or darkness. Furthermore, some enzymes involved in the so-called dark reaction become inactive in prolonged darkness; however, they are activated when the leaves containing them are exposed to light.

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P) is valuable in identifying intermediate compounds formed during carbon assimilation. Photosynthetic plants do not clearly distinguish between the most abundant naturally occurring carbon isotopes (

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, the compounds formed will be labeled with radioactive isotopes. During very short exposure times, only the first intermediates in carbon fixation were labeled. Initial investigations showed that some radioactive products were formed even when the lights were off and

Was added immediately afterward in the dark, confirming the nature of carbon fixation as a “dark” reaction.

American biochemist Melvin Calvin, who received the Nobel Prize for his work on the carbon reduction cycle, allowed green plants to photosynthesize in the presence of radioactive carbon dioxide for a few seconds under different experimental conditions. The radiocarbon-labeled products in Calvin’s experiments included a three-carbon compound called 3-phosphoglycerate (abbreviated PGA), sugar phosphate, amino acids, sucrose, and carboxylic acids. When photosynthesis stops after two seconds, the primary radioactive product is PGA, which has thus been identified as the first stable compound formed during carbon dioxide fixation in green plants. PGA is a three-carbon compound and the photosynthetic mode is therefore called C

To oxaloacetate, a four-carbon acid, and reduces it to malate. PGA is formed from 2-carboxy-3-keto-D-arabinitol 1, 5-bisphosphate, which is a highly unstable six-carbon compound formed from the carboxylation of ribulose-1, 5-bisphosphate, a five-carbon substance.

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P led to the mapping of the carbon fixation and reduction process known as the reductive pentose phosphate (RPP) cycle, or Calvin-Benson cycle. An additional pathway for carbon transport in some plant species was later discovered in other laboratories (

Carbon fixation process in C4 plants). All steps in this pathway can be performed in the laboratory using enzymes isolated in the dark. Some steps require ATP or NADPH produced by light reactions. Additionally, some enzymes are only fully active when conditions mimic those in green cells exposed to light. In living plants, these enzymes are active during photosynthesis but not in the dark.

The Calvin-Benson cycle, in which carbon is fixed, reduced, and used, involves the formation of sugar phosphate intermediates in a cyclic sequence. A complete cycle combines three carbon dioxide molecules and produces one molecule of the three-carbon compound glyceraldehyde-3-phosphate (Gal3P). This three-carbon sugar phosphate is usually released from the chloroplast or converted to starch inside the chloroplast.

The ATP and NADPH formed in the light reactions are used for key steps in this pathway and provide the energy and reducing agent (i.e., electrons) to drive the sequence in the direction shown. town. For each molecule of carbon dioxide fixed, two molecules of NADPH and three molecules of ATP are required from the light reactions. The overall reaction can be represented as follows:

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The initial incorporation of carbon dioxide, catalyzed by the enzyme ribulose 1,5-bisphosphate carboxylase (Rubisco), proceeds by adding carbon dioxide to the 5-carbon compound ribulose 1,5-bisphosphate (RuBP) and cleaving The six-carbon compound is obtained into two molecules of PGA. This reaction occurs three times during each complete round of the cycle; thus, six molecules of PGA are produced.

The first six molecules of PGA are phosphorylated with ATP by the enzyme PGA-kinase, producing six molecules of 1,3-diphosphoglycerate (DPGA). These molecules are then reduced with NADPH and the enzyme glyceraldehyde-3-phosphate dehydrogenase to produce six Gal3P molecules. These reactions are the reversal of the two steps of glycolysis in cellular respiration (

For each complete Calvin-Benson cycle, one of the Gal3P molecules, with three carbon atoms, is the net product and can either be transferred out of the chloroplast or converted to starch inside the chloroplast. For the cycle to regenerate, the remaining five Gal3P molecules (for a total of 15 carbon atoms) must be converted back into three 5-carbon RuBP molecules. The conversion of Gal3P to RuBP begins with a series of complex enzyme-regulated reactions leading to the synthesis of the 5-carbon compound ribulose-5-phosphate (Ru5P).

Three Ru5P molecules are converted to the carboxylation substrate, RuBP, by the enzyme phosphoribulokinase, using ATP. This reaction, shown below, completes the cycle.

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Photosynthesis cannot occur at night, but the respiration process of glycolysis—which uses some of the same reactions as the Calvin-Benson cycle, except in reverse—will take place. Therefore, some steps in this cycle would be wasteful if allowed to perform in the dark, because they would interfere with the glycolytic reaction. For this reason, some enzymes of the Calvin-Benson cycle are “switched off” (i.e. inactive) in the dark.

Even in the presence of light, changes in physiological conditions frequently require adjustments in the relative rates of reactions in the Calvin-Benson cycle, so that the enzymes for some reactions will change. their catalytic activity. Changes in enzyme activity are often caused by changes in the levels of chloroplast components such as reductions in ferredoxins, acids and soluble components (e.g. P

The most important use of Gal3P is to transport it from the chloroplast to the cytoplasm of green cells, where it is used to biosynthesize products needed by plants. In land plants, the main product is sucrose, which is transported from the green cells of leaves to other parts of the plant. Other important products include the carbon skeletons of several key amino acids such as alanine, glutamate and aspartate. To complete the synthesis of these compounds, amino groups are added to the appropriate carbon skeletons generated from Gal3P. Sulfur amino acids such as cysteine ​​are formed by adding sulfhydryl groups and amino groups. Other biosynthetic pathways lead from Gal3P to lipids, pigments, and most green cell components.

Synthesis and accumulation of starch in chloroplasts occurs especially when photosynthetic carbon fixation exceeds the needs of the plant. In such cases, sugar phosphate accumulates in the cytosol, bound to P in the cell.

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In chloroplasts. These changes trigger changes in the activity of regulatory enzymes, which in turn lead to increased starch synthesis. This starch can be decomposed at night and used as a source of reduced carbon and energy for the plant’s physiological needs. However, too much starch in chloroplasts will reduce the rate of photosynthesis. In addition, high sugar levels in the cytoplasm lead to inhibition of the normal activity of genes involved in photosynthesis. Therefore, under the ideal photosynthetic conditions of a warm, sunny day, many plants actually have slower photosynthetic rates than expected.

Under conditions of high light intensity, hot weather, and limited water availability, the productivity of the Calvin-Benson cycle is limited in many plant species due to the occurrence of photorespiration. This process converts sugar phosphate back into carbon dioxide; it is initiated by the oxidation of RuBP (i.e. the incorporation of oxygen gas [O

] with RuBP). This oxidation reaction produces only one molecule of PGA and one molecule of the two-carbon acid, phosphoglycolate, which is then partially converted to carbon dioxide. The reaction of oxygen with RuBP directly competes with the carboxylation reaction (CO

+RuBP) initiates the Calvin-Benson cycle and is in fact catalyzed by the same protein, ribulose 1,5-bisphosphate carboxylase. The relative concentrations of oxygen and carbon dioxide in the chloroplast as well as the temperature of the leaf determine whether oxidation or carboxylation is favored. The oxygen concentration inside the chloroplast can be higher than atmospheric (20%) due to oxygen evolution during photosynthesis, while the carbon dioxide concentration inside can be lower than atmospheric (0.039%) due to uptake photosynthesis. Any increase in internal carbon dioxide pressure tends to help the carboxylation reaction compete more effectively with oxidation.PHOTOSYNTHESIS. What role does CO2 play in photosynthesis? Plants USE carbon dioxide to perform photosynthesis. We can determine the presence of CO.

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Presentation on topic: “SYNTHESIS. What is the role of CO 2 in photosynthesis? Plants USE carbon dioxide to perform photosynthesis. We can determine the presence of CO.”— Presentation transcript:

2 What is the role of CO2 in photosynthesis? Plants USE carbon dioxide to perform photosynthesis. We can determine the presence of CO 2 in water using an indicator called BTB (bromthymol blue). BTB is neutral (7). CO2 dissolved in water will form carbonic acid. BTB turns YELLOW in acid and BLUE in base. Please prove….

3 Procedure: 1. Label 4 test tubes A, B, C, & D 2. Fill each test tube 1/2 full with water 3. Add 20 drops of Bromthymol Blue indicator (BTB) to each test tube Water What color is the test tube in it? What is pH?

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