Metabolism refers to all the catabolic and anabolic processes that a cell is engaged in.
Metabolism in its most simplified "big picture" description is largely a story of chemical bonds, more specifically the electrons in these bonds. You probably recall that electrons are a subatomic particle with a negative charge. Electrons also move with considerable energy and speed as they find themselves drawn to protons. So, what if we could take some high energy electrons and store them somewhere. Later, when we wanted to harvest some of the electron energy to do some work, we could go get them. This is kind of how a battery works. Smart people have figured out how to store high energy electrons and when we want these electrons to do something (like flow through a flashlight filament) we can get them and use them. Living things also use high energy electrons to power the processes of biology.
On planet earth, the primary source of energy is the sun. This energy is called solar energy. You can feel some of this energy when you go out on a warm day. Solar energy is absorbed by plants and when a plant gets solar energy it uses this energy to excite electrons.
An electron in a bond between oxygen and hydrogen has a certain amount of energy. An electron in a bond between carbon and hydrogen has even a greater amount of energy. When sunlight strikes a plant, the solar energy is used to break an oxygen-hydrogen bond and then create a carbon-hydrogen bond. In this very incredible and complex process called photosynthesis, plants use water as the source of oxygen-hydrogen bonds and carbon dioxide (CO2) serves as the source of carbon that will accept a hydrogen with its newly energized electron. Whenever an atom receives more electrons, the atom has been reduced, and whenever an atom loses electrons, the atom has been oxidized. In the case of photosynthesis, CO2 is reduced, and H2O is oxidized.
Let's summarize to this point. Plants take CO2 and H2O, and then capture solar energy to excite some electrons in H2O and then transfer the excited electrons to a C-H bond, increase the energy of that bond. Reduced CO2 can be transformed into a variety of molecules with different numbers of C-H bonds that include the macromolecules (i.e., sugars, lipids and proteins). The structures of these macromolecules reveal lots of C-H bonds. From the broadest sense, plants create a kind of "battery." The carbon-hydrogen bonds formed by plants can exist for a long time as sugars, lipids and proteins. If the high energy electrons are allowed to go back and form an O-H bond, energy will have to be released. This is what metabolism is all about. Cells can facilitate the transfer of high energy electrons in C-H bonds back to O-H bonds and use the energy that is released as work.
This process is called the energy cycle. Organisms consume C-H bonds in the form of carbohydrates, lipids and proteins. Cells then "process" the C-H bonds in a way that allows the high energy electrons back onto Oxygen. Energy is released that energy is used to run cell processes. Also, if you note in figure 1, CO2 and H2O are created again when high energy electrons return to oxygen. Plants use the CO2, and the H2O and the cycle continues.
Energy Cycle. Image created by JS at BYU Idaho. Clipart from clker.com; License Public Domain;
The main metabolic processes that create energy in cells are Glycolysis (the breakdown of sugars), the Citric Acid Cycle (cellular respiration), the Electron Transport Chain (where ATP is created due to the transfer of electrons and protons along the membrane of the mitochondria during cellular respiration), Lipolysis and Beta Oxidation (the breakdown of fats), and Protein Metabolism. Since energy exchange is all about electrons, what follows will be a brief description of molecules categorized as electron carriers and how they contribute to each of the metabolic processes described above.