Once the energy that was in sunlight is transformed into chemical energy, often by photosynthesis, the organism has to now convert the chemical energy into a usable form. It may seem a bit odd for there still to be more steps. After all, when you eat a candy bar isn't the sugar in the candy bar "burnt" by the body to provide energy? Well the answer is yes and no. First of all when we burn something normally in the air we combine that substance with oxygen releasing energy from the substance. Indeed, an analogous process does happen in our bodies. 

What goes on in living things is not really like burning because the molecules from which we harvest energy give up their energy in a controlled fashion rather than all at once as what happens in a fire. Think of your car. All the energy in the gas tank when you get in your car is not released all at once but rather in small bursts which allow you to control the car's movement. In the same way cells take the energy from the "food" and package that energy into manageable bursts that provide just the right amount of energy for the organism's activities be those activities driving a car or flashing a light to attract a mate. 

The Point of Cellular Respiration

The point of cellular respiration is to harvest electrons from organic compounds such as glucose and use that energy to make a molecule called ATP. ATP in turn is used to provide energy for most of the immediate work that the cell does. ATP can be thought of as being like a small package of energy that has just the right amount of energy that can be used in a controlled manner.

ATP: Adenosine tri-phosphate,ATP is a nucleotide with three phosphate groups instead of one phosphate group. The point of cellular respiration is to harvest chemical energy from food and store it in the chemical bonds of ATP. In this diagram Nitrogens=blue, Phosphorus=yellow, Carbon = grey, Oxygen = red. Hydrogen atoms are not shown.

More about the role of of ATP in metabolism is here.

Types of cellular respiration: There are two basic types of cellular respiration aerobic cellular respiration and anaerobic cellular respiration. Aerobic respiration requires the use of oxygen and anaerobic respiration which does not use oxygen. There are several types of anaerobic respiration, most familiar is a process called fermentation.

Aerobic Respiration.

Aerobic respiration is the process by which ATP is produced by cells by the complete oxidation of organic compounds using oxygen . In aerobic respiration oxygen serves as the final electron acceptor, accepting electrons that ultimately come from the energy rich organic compounds we consume. We will use glucose as an illustration of an organic molecule used in cellular respiration since glucose is a common energy source for cells. In this figure the energy rich molecules (and heat) are in red, energy poor molecules(relatively speaking) are in black.

Stages in Aerobic Respiration:

Aerobic Respiration takes place in three stages which are summarized here starting with the original glucose molecule.

Glycolysis. Glycolysis is the first step in cellular respiration and all cells regardless of the type of cellular respiration they do are able to carry out glycolysis. Because of this we believe that glycolysis probably arose very early in the evolution of life on the planet. In glycolysis glucose is partially oxidized and broken down into two 3 carbon molecules called pyruvate or pyruvic acid. In the process, glycolysis produced 4 ATP for a net gain of two ATP and two molecules of NADH. Each NADH is carrying two energy rich electrons away from the glucose and these electrons can be used by the cell to do work.

After glycolysis the pyruvate is processed to harvest 2 more NADH molecules and remove one carbon per pyruvate. The carbon and two oxygens is removed since it no longer has any useful energy. So it is waste. This little step is the source of some of the carbon dioxide we produce.

Note that glycolysis itself is anaerobic, in that oxygen is not required.

Kreb's cycle.

The remaining two carbons from the pyruvate feed into a complicated set of reactions called the Kreb's cycle. The Kreb's cycle produces 8 more NADH molecules and two molecules of FADH2. Again both of these are carrying energy rich electrons.

Electron transport phosphorylation. Most of the NADH and FADH2 travel to special membranes in the cell which have a series of molecules called the electron transport system that harvest the energy rich electrons from the NADH and FADH2 and use that energy to male lots of ATP by a process called electron transport phosphosphorylation. If we are dealing with aerobic respiration this is where the oxygen becomes important.

Role of oxygen in Aerobic Respiration. As the energy rich electrons from food are used to make ATP by electron transport phosphorylation they loose energy and once they are no longer useful they have to be removed. Oxygen is a great electron acceptor and so the electrons are combined with hydrogen ions and oxygen to make water. This prevents electrons from building up in the electron transport system.

Some forms of anaerobic respiration also use the electron transport phosphoylation but differ in that they use other inorganic molecules as the final electron acceptor instead of oxygen.

In eukaryotic organisms aerobic respiration is compartmentalized. Glycolysis takes place in the cytoplasm and the Kreb's cycle and electron transport taking place in the mitochondrion.