The chemical formula of glucose is C6H12O6, which means it is made up of six carbon atoms, 12 hydrogen atoms and six oxygen atoms. Due to it composition, glucose is classified as a carbohydrate; specifically, it is a type of sugar. Most organisms (the exception being some organisms in the Archaea kingdom of life which do not require carbon) requires some amount of glucose to survive. Organisms that can produce their glucose are called autotrophs, and they make their glucose through the process of photosynthesis. Heterotrophs are organisms that must consume other organisms in order to obtain glucose.
Glucose is broken down during cellular respiration. The overall process of cellular respiration ends up creating 36 ATP molecules for every molecule of glucose involved in the process (with six oxygen molecules being required to start the reaction, and six carbon dioxide molecules and six water molecules as additional products). Glucose is broken down in the first stage of cellular respiration, which involves breaking the six carbons of the glucose molecule in to two molecules of three carbons (C3), also known as pyruvate. In addition to initially breaking down glucose, glycolysis also creates the first two ATP molecules.
Adenosine triphophaste (ATP) is the molecule that transfers energy within the cell and, at different stages, it is broken down to produce more energy. ATP has different levels of energy in the cell; once the energy is completely used up, it becomes ADP (adenosine diphosphate), though ADP can later be converted back in to ATP if the ADP gains additional energy. ATP molecules are used to power all sorts of functions in the cell, including repairing damaged organelles and transferring nutrients.
In eukaryotic organisms, cellular respiration occurs in the mitochondria. A eukaryote is an organism that has its DNA contained in an organelle called the nucleus, as opposed to prokaryotes (which is what bacteria are) which have their DNA free floating in the cell. Interestingly, the mitochondria has its own DNA, and it is the only organelle besides the nucleus to have independent DNA. This leads biologists to believe that the mitochondria in eukaryotic cells may have evolved from a prokaryote.