<P> Carbon dioxide is converted into sugars in a process called carbon fixation . Carbon fixation is an endothermic redox reaction, so photosynthesis needs to supply both a source of energy to drive this process, and the electrons needed to convert CO into a carbohydrate . This addition of the electrons is a reduction reaction . In general outline and in effect, photosynthesis is the opposite of cellular respiration, in which glucose and other compounds are oxidized to produce CO and water, and to release exothermic chemical energy to drive the organism's metabolism . However, the two processes take place through a different sequence of chemical reactions and in different cellular compartments . </P> <P> Most organisms that utilize photosynthesis to produce oxygen use visible light to do so, although at least three use shortwave infrared or, more specifically, far - red radiation . </P> <P> A 1993 review of scientific greenhouse studies found that a doubling of CO concentration would stimulate the growth of 156 different plant species by an average of 37% . The amount of gain varied significantly by species, with some showing much greater gains, and a small number showing a loss . For example, a 1979 greenhouse study compared the dry weights of cotton and maize plants grown in different greenhouses, one with double the CO concentration of the other . In the enriched CO air, the dry weight of 40 - day - old cotton plants doubled, but the dry weight of 30 - day - old maize plants increased by only 20% . </P> <P> Because of concerns that greenhouse experiments might not adequately simulate the actual environment (such as competing plants), scientists developed Free - air carbon dioxide enrichment (FACE) experiments . In this procedure, plants are grown outdoors, and the CO concentration of the surrounding air is artificially elevated . FACE experiments, using generally lower CO levels than the greenhouse studies, showed lower gains in growth than greenhouse studies, with the gains depending heavily on the species under study . A 2005 review of 12 experiments at 475 - 600 ppm showed an average gain of 17% in crop yield, with legumes typically showing a greater response than other species, and C4 plants generally showing less . The review also stated that the reviewed experiments have their own limitations . The studied CO levels were lower, and most of the experiments were carried out in temperate regions . </P>

The amount of carbon dioxide dissolved in the oceans depends in part on the ____