C4, CAM Alternate pathways of Photosynthesis: Photosynthesis V
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@Vips BiochemistryPhotosynthesis V discusses the
@Vips Biochemistry
Photosynthesis V discusses the C4, CAM Alternate pathways of Photosynthesis. The inhibitors of photosynthesis also is included in the same video. Role of photorespiration is also illustrated with animations.
C4 PATHWAY: Hatch and Slack pathway
Rubisco is a two pronged enzyme in that it has oxygenase activity (using O2) and carboxylase activity (using CO2). Under normal atmospheric conditions, rubisco adds CO2 to ribulose 1,5-bisphosphate. However, when the CO2 concentration is low, it can add O2 instead and produces phosphoglycolate and 3-phosphoglycerate. The phosphoglycolate can be salvaged and used for biosynthetic reactions but the pathway for achieving this releases CO2 and NH4+ inside mitochondria and wastes metabolic energy. Because the net result of this process is to consume O2 and release CO2, it is known as photorespiration. This is a major problem for plants in hot climates. Photorespiration reduces photosynthetic efficiency by 30-40%. The plants close the gas exchange pores in their leaves (stomata) to conserve water but this leads to a drop in the CO2 concentration within the leaf, favoring photorespiration. In addition, as temperature rises, the oxygenase activity of rubisco (using O2) increases more rapidly than the carboxylase activity (using CO2), again favoring photorespiration.
To avoid these problems, some plants adapted to live in hot climates, such as corn and sugar cane, have evolved a mechanism to maximize the carboxylase activity of rubisco. In these plants, carbon fixation using the Calvin cycle takes place only in bundle-sheath cells that are protected from the air by mesophyll cells. Since the bundle-sheath cells are not exposed to air, the O2 concentration is low. The CO2 is transported from the air passing through the mesophyll cells to the bundle-sheath cells by combining with three-carbon molecules (C3) to produce four-carbon molecules (C4). These enter the bundle-sheath cells where they are broken down to C3 compounds, releasing CO2. The C3 molecules return to the mesophyll cell to accept more CO2. This cycle ensures a high CO2 concentration for the carboxylase activity of rubisco action in the bundle-sheath cells. Since the first stable product of the pathway is a four-carbon molecule namely oxaloacetate it is called the C4 pathway and plants that use this mechanism are called C4 plants. All other plants are called C3 plants since they trap CO2 directly as the three-carbon compound 3-phosphoglycerate. Details of the C4 pathway are shown below in figure. The steps involved are as follows:
i. In the mesophyll cell, phosphoenolpyruvate (C3) accepts CO2 to form oxaloacetate (C4); a reaction catalyzed by phosphoenolpyruvate carboxylase
ii. Oxaloacetate is converted to malate (C4) by NADP+ linked malate dehydrogenase
iii. Malate enters the bundle-sheath cell and releases CO2, forming pyruvate (C3); catalyzed by NADP+ linked malate enzyme
iv. Pyruvate returns to the mesophyll cell and is used to regenerate phosphoenolpyruvate. This reaction, catalyzed by pyruvate-Pi dikinase, is unusual in that it breaks both ATP high-energy bonds to generate AMP and pyrophosphate.
The pyrophosphate from the pyruvate-Pi dikinase is rapidly degraded so that, overall, the net price the plant pays for operation of this CO2 pump is the hydrolysis of two high-energy phosphate bonds for every molecule of CO2 transported:
So when energy is calculated for the 6 molecules of CO2 that is used up from bundle sheath store through one Calvin pathway, an additional 12ATP will have to be taken in to consideration
Inhibitors of photosynthesis
Many herbicides inhibit photosynthesis by interfering with electron carriers of photosystem II. Derivatives urea (eg: diuron and fluometuron), triazine (eg: atrazine and cyanazine) and uracil (eg: terbacil and bromacil) bind to PSII and block the formation of reduced plastoquinone (QH2). These inhibitors are found to specifically bind to QB protein of PS II thus preventing it from accepting and transferring electrons to plastoquinone.
Photosynthesis V discusses the C4, CAM Alternate pathways of Photosynthesis. The inhibitors of photosynthesis also is included in the same video. Role of photorespiration is also illustrated with animations.
C4 PATHWAY: Hatch and Slack pathway
Rubisco is a two pronged enzyme in that it has oxygenase activity (using O2) and carboxylase activity (using CO2). Under normal atmospheric conditions, rubisco adds CO2 to ribulose 1,5-bisphosphate. However, when the CO2 concentration is low, it can add O2 instead and produces phosphoglycolate and 3-phosphoglycerate. The phosphoglycolate can be salvaged and used for biosynthetic reactions but the pathway for achieving this releases CO2 and NH4+ inside mitochondria and wastes metabolic energy. Because the net result of this process is to consume O2 and release CO2, it is known as photorespiration. This is a major problem for plants in hot climates. Photorespiration reduces photosynthetic efficiency by 30-40%. The plants close the gas exchange pores in their leaves (stomata) to conserve water but this leads to a drop in the CO2 concentration within the leaf, favoring photorespiration. In addition, as temperature rises, the oxygenase activity of rubisco (using O2) increases more rapidly than the carboxylase activity (using CO2), again favoring photorespiration.
To avoid these problems, some plants adapted to live in hot climates, such as corn and sugar cane, have evolved a mechanism to maximize the carboxylase activity of rubisco. In these plants, carbon fixation using the Calvin cycle takes place only in bundle-sheath cells that are protected from the air by mesophyll cells. Since the bundle-sheath cells are not exposed to air, the O2 concentration is low. The CO2 is transported from the air passing through the mesophyll cells to the bundle-sheath cells by combining with three-carbon molecules (C3) to produce four-carbon molecules (C4). These enter the bundle-sheath cells where they are broken down to C3 compounds, releasing CO2. The C3 molecules return to the mesophyll cell to accept more CO2. This cycle ensures a high CO2 concentration for the carboxylase activity of rubisco action in the bundle-sheath cells. Since the first stable product of the pathway is a four-carbon molecule namely oxaloacetate it is called the C4 pathway and plants that use this mechanism are called C4 plants. All other plants are called C3 plants since they trap CO2 directly as the three-carbon compound 3-phosphoglycerate. Details of the C4 pathway are shown below in figure. The steps involved are as follows:
i. In the mesophyll cell, phosphoenolpyruvate (C3) accepts CO2 to form oxaloacetate (C4); a reaction catalyzed by phosphoenolpyruvate carboxylase
ii. Oxaloacetate is converted to malate (C4) by NADP+ linked malate dehydrogenase
iii. Malate enters the bundle-sheath cell and releases CO2, forming pyruvate (C3); catalyzed by NADP+ linked malate enzyme
iv. Pyruvate returns to the mesophyll cell and is used to regenerate phosphoenolpyruvate. This reaction, catalyzed by pyruvate-Pi dikinase, is unusual in that it breaks both ATP high-energy bonds to generate AMP and pyrophosphate.
The pyrophosphate from the pyruvate-Pi dikinase is rapidly degraded so that, overall, the net price the plant pays for operation of this CO2 pump is the hydrolysis of two high-energy phosphate bonds for every molecule of CO2 transported:
So when energy is calculated for the 6 molecules of CO2 that is used up from bundle sheath store through one Calvin pathway, an additional 12ATP will have to be taken in to consideration
Inhibitors of photosynthesis
Many herbicides inhibit photosynthesis by interfering with electron carriers of photosystem II. Derivatives urea (eg: diuron and fluometuron), triazine (eg: atrazine and cyanazine) and uracil (eg: terbacil and bromacil) bind to PSII and block the formation of reduced plastoquinone (QH2). These inhibitors are found to specifically bind to QB protein of PS II thus preventing it from accepting and transferring electrons to plastoquinone.
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