However, in the form of N2, nitrogen is unusable by all organisms except for a few primitive bacteria that are capable of converting N2 gas to ammonia (NH3). This process of conversion is called nitrogen fixation, and makes the nitrogen available for use by organisms.
1 Simple carbohydrates have very simple chemical structures and are made of small molecules. 2  Complex carbohydrates have very complex structures while the molecules are large, unlike simple carbohydrates.
Simple carbohydrates are sugars made from atoms of carbon, oxygen, and hydrogen. Inside cells, sugar molecules are broken down. This process provides usable energy for the cell. Simple sugar molecules can also be linked into long chains to form more complex carbohydrates, such as starch, cellulose, and glycogen. Starch and cellulose are complex carbohydrates made by plant cells.