The enzyme catalyzing fixation btn RUBP and CO2 is not selective, can either fix O2 or CO2 . what does this implies for C3 plants during bright and hot day?
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The O2 and CO2 compensation points (O2 and CO2) of plants in a closed system depend on the ratio of CO2 and O2 concentrations in air and in the chloroplast and the specificities of ribulose bisphosphate carboxylase/oxygenase (Rubisco). The photosynthetic O2 is defined as the atmospheric O2 level, with a given CO2 level and temperature, at which net O2 exchange is zero. In experiments with C3 plants, the O2 with 220 ppm CO2 is 23% O2; O2 increases to 27% with 350 ppm CO2 and to 35% O2 with 700 ppm CO2. At O2 levels below the O2, CO2 uptake and reduction are accompanied by net O2 evolution. At O2 levels above the O2, net O2 uptake occurs with a reduced rate of CO2 fixation, more carbohydrates are oxidized by photorespiration to products of the C2 oxidative photosynthetic carbon cycle, and plants senesce prematurely. The CO2 increases from 50 ppm CO2 with 21% O2 to 220 ppm with 100% O2. At a low CO2/high O2 ratio that inhibits the carboxylase activity of Rubisco, much malate accumulates, which suggests that the oxygen-insensitive phosphoenolpyruvate carboxylase becomes a significant component of the lower CO2 fixation rate. Because of low global levels of CO2 and a Rubisco specificity that favors the carboxylase activity, relatively rapid changes in the atmospheric CO2 level should control the permissive O2 that could lead to slow changes in the immense O2 pool.
Photorespiration is a wasteful pathway that occurs when the Calvin cycle enzyme rubisco acts on oxygen rather than carbon dioxide.
The majority of plants are \text C_3C
3
start text, C, end text, start subscript, 3, end subscript plants, which have no special features to combat photorespiration.
\text C_4C
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start text, C, end text, start subscript, 4, end subscript plants minimize photorespiration by separating initial \text {CO}_2CO
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start text, C, O, end text, start subscript, 2, end subscript fixation and the Calvin cycle in space, performing these steps in different cell types.
Crassulacean acid metabolism (CAM) plants minimize photorespiration and save water by separating these steps in time, between night and day.
Introduction
High crop yields are pretty important—for keeping people fed, and also for keeping economies running. If you heard there was a single factor that reduced the yield of wheat by 20\%20%20, percent and the yield of soybeans by 36\%36%36, percent in the United States, for instance, you might be curious to know what it was^1
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