|MadSci Network: Cell Biology|
You've asked a good question about oxygen and its necessity for cells. There are a couple of different ways to approach it, though, so I will give you a couple different ways to approach the question.
Basically, most of an organism is made up of 6 basic elements, building blocks put together in many different ways. Those elements are C H O N S and P. Just there you see how important oxygen must be, it's involved in proteins, in sugars, in lipids, and in DNA! Long term, that means the cell would run out of things to build or renew is parts.
The biggest use of oxygen in the cell, though, is as a way to transfer energy. You see, oxygen has a lot of energy in its chemical bonds, so it can function in some ways like the insides of a battery. In this case, the energy is stored in molecules like sugars. In the cell, there are organelles called the mitochondria which are known as the "powerhouses of the cell." The process by which mitochondria use oxygen to release the chemical energy stored in food is called cellular respiration. In the early 1900's, it was discovered that the biochemical reactions of this type of respiration fall into two main groups: the carbon pathway, in which sugar is broken down into carbon dioxide and hydrogen; and the hydrogen pathway, which transfers hydrogen to oxygen in stages, forming water and releasing energy. Learn more about mitochondria here. In the presence of oxygen (aerobic respiration), this process is highly efficient, but in its absence (anaerobic respiration), it releases only about 5% as much energy. That's the main reason that cells need oxygen.
You can find out a bit more about how important oxygen is to cells in some of the following other MadSci answers:
You can also do some more searches using terms like "cellular respiration" and "oxygen." You might find some useful things in a cell biology book, such as Molecular Biology of the Cell by Alberts et al. (my old text that I used for some of the above).
I hope that helps. Keep asking good questions.
Try the links in the MadSci Library for more information on Cell Biology.