There are many different parts of respiration. Carbon begins in the Calvin cycle, where it comes in as carbon dioxide. It is necessary to follow this process all the way through to discover all the intricacies of the process. Although carbon dioxide is first used in plants, it is later used in respiration. It is here that energy is actually produced. In photosynthesis, energy is not produced in the large quantities that come from respiration. Photosynthesis merely provides glucose so that the anaerobic and aerobic reactions can take place. This glucose can be provided through other means, but in plant cells, that is the major way that it is obtained. What the Calvin Cycle builds, respiration takes apart, and takes the energy from. All this seems to support the simple statement that respiration is photosynthesis backwards. If you look at it on a basic level, this is true, but when you look at the processes that are actually taking place, the perspective changes. A completely different process is used when trying to gain energy. The Krebs cycle uses enzymes as intermediates. These were barely used at all in photosynthesis. There are also many more carbons involved in respiration. Many more steps must be taken to take the glucose apart then were needed to put it together, at least those which involve carbon. Because of all of this, photosynthesis and respiration are two entirely different processes which can not be compared at all, except for the basic in and out of them.