Photorespiration | Blog Prof. Djalma Santos
Photosynthesis in C 3 plants is always accompanied by photorespiration, a process that consumes edwards lifesciences O 2 and releases CO 2 in the presence of light, hence the name. This process is, however, very different from the aerobic edwards lifesciences respiration occurring in mitochondria, as it is not followed by oxidative edwards lifesciences phosphorylation, does not synthesize ATP. The photorespiration occurs due to the dual activity exerted by the enzyme r ib u lose-1, 5 - b ifosfato arboxilase c / o xigenase (Rubisco), which catalyzes two competing reactions (figure below): the addition of CO 2 to ribulose-1, 5-bisphosphate (RuBP) to form two molecules of 3-phosphoglycerate (PGA), which are used in the thermochemical step of photosynthesis and the addition of O 2 to RuBP producing a molecule of PGA and phosphoglycolate, characterized photorespiration. Thus, Rubisco present in chloroplasts, in addition edwards lifesciences to the activity of carboxylase, which initiates the Calvin cycle, is also able to exert oxygenase activity, and, in both cases, the RuBP as substrate, hence its name " ribulose-1 ,5-bisphosphate carboxylase / oxygenase. " The carboxylase activity of rubisco oxygenase or dependent concentrations of CO 2 and O 2 in stromal (chloroplast region where the thermochemical step of photosynthesis occurs). When the CO 2 concentration is high and O 2 is relatively edwards lifesciences low, rubisco carboxylase acts as connecting the CO 2 to RuBP to form PGA. When the situation is reversed, ie the O 2 concentration is relatively higher than that of CO 2, the enzyme operates as oxygenase, combining RuBP and oxygen, yielding PGA and phosphoglycolate, which characterizes, as previously mentioned photorespiration. The latter route may be a problem for plants that live in conditions of high temperature edwards lifesciences and humidity and low light intensity, which closed their stomata to prevent excessive water loss, there is a decrease of CO 2 leaves. The phosphoglycolate, from the photorespiration is recycled through a complex pathway, termed glycolysis pathway, which involves the functional cooperation of chloroplasts where it is dephosphorylated to glycolate, peroxisomes where O 2 is consumed, and mitochondria where there detachment CO 2. Due to this cooperative role, mitochondria and peroxisomes are seen, usually next to chloroplasts in the cytoplasm of photosynthetic edwards lifesciences cells of C 3 plants to. In the recycling process, two fosfoglicolatos (molecules with two carbons), generated due to the oxygenase activity of rubisco are converted into a PGA (tricarbonada molecule), recovering thus 3/4 carbons diverted from the Calvin cycle.
The photorespiration, which consumes ATP and O 2 and produces CO 2, is costly to the plant. In the face to be very active in C 3 plants, photorespiration, reducing photosynthesis, seriously limits the efficiency of these plants. To get an idea, the exacerbation of this process edwards lifesciences can inhibit the formation of biomass by 50%. It is surprising, however, that all known rubiscos catalyze photorespiration. It is likely edwards lifesciences that this enzyme, whose active center is not able to discriminate well enough CO 2 O 2, has appeared before the O 2 represents an important component of the atmosphere. In view of the MAC and C 4 plants (CAM) that study below, possess physiological strategy which allows the accumulation of CO 2, the rubisco not present in these plants, cyclooxygenase activity, avoiding therefore photorespiration.
In some plants, such as corn, sugarcane açúca r ee sorghum species adapted to arid climate, the first product of CO 2 fixation is not the PGA (tricarbonada molecule), as seen previously in the study of C 3 plant, and yes, a molecule with 4 carbons oxalacetic acid (AOA) or oxaloacetate. As a result, plants that employ this pathway, along with the Calvin cycle are called C 4 plants. The foil of these plants has two distinct types of photosynthetic cells, cells of the perivascular cuff and mesophyll cells (Figure edwards lifesciences below), including having a large number edwards lifesciences of plasmodesmata, which is important for the flow of metabolites. The mesophyll cells, and adjacent to the outer leaf epidermis contains the enzyme phosphoenolpyruvate carboxylase (PEP carboxylase), which, however, devoid of rubisco. The perivascular more internal and adjacent tissue edwards lifesciences vascular sheath, are, unlike the previous ones, endowed with Rubisco and PEP carboxylase lacking.
In C 4 plants (scheme below), CO 2 combines, in the mesophyll cells, endowed with a three carbon molecule, phosphoenolpyruvate (PEP) to form a compound having four carbon atoms, the oxalacetic acid (AOA) or oxaloacetate. This reaction edwards lifesciences is catalyzed by PEP carboxylase present in the cytosol of mesophyll cells, we did not com
Photosynthesis in C 3 plants is always accompanied by photorespiration, a process that consumes edwards lifesciences O 2 and releases CO 2 in the presence of light, hence the name. This process is, however, very different from the aerobic edwards lifesciences respiration occurring in mitochondria, as it is not followed by oxidative edwards lifesciences phosphorylation, does not synthesize ATP. The photorespiration occurs due to the dual activity exerted by the enzyme r ib u lose-1, 5 - b ifosfato arboxilase c / o xigenase (Rubisco), which catalyzes two competing reactions (figure below): the addition of CO 2 to ribulose-1, 5-bisphosphate (RuBP) to form two molecules of 3-phosphoglycerate (PGA), which are used in the thermochemical step of photosynthesis and the addition of O 2 to RuBP producing a molecule of PGA and phosphoglycolate, characterized photorespiration. Thus, Rubisco present in chloroplasts, in addition edwards lifesciences to the activity of carboxylase, which initiates the Calvin cycle, is also able to exert oxygenase activity, and, in both cases, the RuBP as substrate, hence its name " ribulose-1 ,5-bisphosphate carboxylase / oxygenase. " The carboxylase activity of rubisco oxygenase or dependent concentrations of CO 2 and O 2 in stromal (chloroplast region where the thermochemical step of photosynthesis occurs). When the CO 2 concentration is high and O 2 is relatively edwards lifesciences low, rubisco carboxylase acts as connecting the CO 2 to RuBP to form PGA. When the situation is reversed, ie the O 2 concentration is relatively higher than that of CO 2, the enzyme operates as oxygenase, combining RuBP and oxygen, yielding PGA and phosphoglycolate, which characterizes, as previously mentioned photorespiration. The latter route may be a problem for plants that live in conditions of high temperature edwards lifesciences and humidity and low light intensity, which closed their stomata to prevent excessive water loss, there is a decrease of CO 2 leaves. The phosphoglycolate, from the photorespiration is recycled through a complex pathway, termed glycolysis pathway, which involves the functional cooperation of chloroplasts where it is dephosphorylated to glycolate, peroxisomes where O 2 is consumed, and mitochondria where there detachment CO 2. Due to this cooperative role, mitochondria and peroxisomes are seen, usually next to chloroplasts in the cytoplasm of photosynthetic edwards lifesciences cells of C 3 plants to. In the recycling process, two fosfoglicolatos (molecules with two carbons), generated due to the oxygenase activity of rubisco are converted into a PGA (tricarbonada molecule), recovering thus 3/4 carbons diverted from the Calvin cycle.
The photorespiration, which consumes ATP and O 2 and produces CO 2, is costly to the plant. In the face to be very active in C 3 plants, photorespiration, reducing photosynthesis, seriously limits the efficiency of these plants. To get an idea, the exacerbation of this process edwards lifesciences can inhibit the formation of biomass by 50%. It is surprising, however, that all known rubiscos catalyze photorespiration. It is likely edwards lifesciences that this enzyme, whose active center is not able to discriminate well enough CO 2 O 2, has appeared before the O 2 represents an important component of the atmosphere. In view of the MAC and C 4 plants (CAM) that study below, possess physiological strategy which allows the accumulation of CO 2, the rubisco not present in these plants, cyclooxygenase activity, avoiding therefore photorespiration.
In some plants, such as corn, sugarcane açúca r ee sorghum species adapted to arid climate, the first product of CO 2 fixation is not the PGA (tricarbonada molecule), as seen previously in the study of C 3 plant, and yes, a molecule with 4 carbons oxalacetic acid (AOA) or oxaloacetate. As a result, plants that employ this pathway, along with the Calvin cycle are called C 4 plants. The foil of these plants has two distinct types of photosynthetic cells, cells of the perivascular cuff and mesophyll cells (Figure edwards lifesciences below), including having a large number edwards lifesciences of plasmodesmata, which is important for the flow of metabolites. The mesophyll cells, and adjacent to the outer leaf epidermis contains the enzyme phosphoenolpyruvate carboxylase (PEP carboxylase), which, however, devoid of rubisco. The perivascular more internal and adjacent tissue edwards lifesciences vascular sheath, are, unlike the previous ones, endowed with Rubisco and PEP carboxylase lacking.
In C 4 plants (scheme below), CO 2 combines, in the mesophyll cells, endowed with a three carbon molecule, phosphoenolpyruvate (PEP) to form a compound having four carbon atoms, the oxalacetic acid (AOA) or oxaloacetate. This reaction edwards lifesciences is catalyzed by PEP carboxylase present in the cytosol of mesophyll cells, we did not com
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