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.
Carbohydrates: Complex Carbs vs Simple Carbs. Carbohydrates are the main source of calories in a healthy diet and are the primary fuel for the brain and muscles. Typically, about three-fourths of daily calories should come from carbohydrates. It's also important to choose the best carbohydrate sources. That means two things: 1  Choose complex carbohydrates, rather than simple carbohydrates. 2  Choose carbohydrates that still have their fiber, like brown rice or brown bread, rather than white rice or white bread, from which the fiber has been stripped away.