How many times more oxygen is produced during photosynthesis than is used up in respiration?
It depends on whether one is considering individual organisms or communities. At a community level in a climax ecosystem (defined as the rate of fixation of energy equal to its rate of consumption) there will be a balance between respiration and photosynthesis so that the net gas exchange is zero. A tropical rain forest is oxygen neutral – in that the output of oxygen in photosynthesis equals the rate of carbon dioxide output due to respiration. This is in contrast to the notion that forests ‘soak up’ carbon dioxide – they do, but they also produce it, and unless the forest is actively growing – increasing in biomass – (i.e. it is not a climax forest) it will produce as much carbon dioxide as it absorbs. In a plant at night (in the absence of photosynthesis) the pattern of gas exchange will be an output of carbon dioxide and an uptake of oxygen – this will be reversed during the day since photosynthesis happens at a much faster rate than respiration. At the compensation point (commonly
