Louise Weston
Plant metabolism
Is photorespiration an effective mechanism for protecting against photoinhibition?
CO2 and O2 are competitive substrates for the enzyme Rubisco (Ribulose bisphosphate carboxylase/oxygenase). Photorespiration is the process where by the enzyme Rubisco fixes molecular oxygen as apposed to carbon, which ultimately leads to the evolution of CO2 from plants. The process occurs concurrently with photosynthesis in green leaves of C3 plants. A high CO2 and low O2 atmospheric concentration favours the carboxylase action, whereas a high O2 and low CO2 concentration favours the oxygenase action. Therefore due to the existence of photorespiration, photosynthesis is inhibited by oxygen. When the release of CO2 by photorespiration equals CO2 fixation by photosynthesis, a concentration of CO2 is reached called the CO2 compensation point. When the light is extinguished, continued CO2 production from photorespiration results in a post illumination burst (PIB).
The enzymic pathways for CO2 production from oxygen fixation are known as the photosynthetic carbon oxygen cycle (PCO). It involves the coordinated activities of the chloroplast, peroxisome and the mitochondria as well as transport of the compounds through the cytoplasm between these organelles and it is shown below;
The cost of fixing one molecule of O2 is 9.5ATP equivalents and in the presence of photorespiration, the consumption of ATP and reducing equivalents per CO2 fixed (5.375 mol ATP and 3.5 mol NADPH) is higher than in the absence of photorespiration (3mol ATP and 2 mol NADPH). Therefore photorespiration results in considerable loss of fixed carbon and utilization of energy, which subsequently decreases the productivity and yield. CAM and C4 plants have developed a supplementary system of fixation of CO2 into C4 acids. In both plants the decarboxylation of these C4 acids elevates the CO2 concentration in the vicinity of Rubisco to enhance carboxylation and suppress oxygenation. Therefore here photorespiration occurs, in some conditions but is greatly reduced. Although in C3 plants the rate of the carboxylase reaction is four times that of the oxygenase reaction under atmospheric conditions, why C3 plants haven’t evolved a mechanism for CO2 concentration like C4 and CAM plants is an interesting point. It seems a wasteful reaction and maybe with the increasing CO2 concentration of the atmosphere it may diminish photorespiration and make CO2 concentrating mechanism more important. Or, photorespiration may have a necessary purpose in the plant. For example the PCO cycle generates metabolites, such as glycine, serine or one-carbon units for other processes in plants. It is also involved in stress protection, including light stress and therefore elicits protection against photoinhibition. The focus of this essay will be to evaluate the effectiveness of photorespiration as a mechanism for protecting against photoinhibition.
