One metabolic pathway is expressed this way:
LIGHT (energy) plus #CO_2# + #H_2O# ----> (glucose)
You will find this reaction in plants and algae (and some bacteria). They take sunlight and combine carbon dioxide and water. Then they create glucose and oxygen gas. Chemists say they are fixing the atmospheric carbon (C).
Remember, plants put the energy into glucose. Glucose is in most of the food you eat and the oxygen you breathe come from these plants. Even if you have a piece of meat, that animal was originally able to get its' glucose from plants.
Cellular respiration is a 3 step process that includes glycolysis, the Krebs cycle, and a number of electrons being pushed across the membrane of the mitochondria. Together they take that energy out of the sugar related molecules.
Glucose is then combined with oxygen and releases usable energy, carbon dioxide, and water:
#C_6H_12O_6# + #0_2# ----> usable energy (ATP) plus #CO_2#
Using glucose to create energy, cells can use that extra energy to power their functions.
The energy is not just floating around but it is stored in an excitable compound called ATP (adenosine triphosphate).
ATP is the power molecule used by all cells to power the secondary reactions to keep you alive. The #C0_2# that we produced from the breakdown of glucose in our mitochondria is exhaled when we breathe.
Plants then can take that carbon dioxide and use it to make sugars, completing a cycle.