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.
There are three main types of carbohydrates: 1  Sugar. Sugar is the simplest form of carbohydrate and occurs naturally in some foods, including fruits, vegetables, milk and milk products. 2  Starch. Starch is a complex carbohydrate, meaning it is made of many sugar units bonded together. 3  Fiber. Fiber also is a complex carbohydrate.