As mentioned earlier,the ultimate source of all the chemical energy in cells comes from the sun. Because this source is directly available only to the cells of photosythesizers, most organisms are either directly or indirectly dependent on photosythesis. A few chemoautotrophs derive their energy and nutrients solely from inorganic substrates. The other major products of photosythesis are organic carbon compounds, which are produced from carbon dioxide through a process called carbon fixation.