A carbohydrate is a chemical compound made up of some combination of carbon, hydrogen and oxygen; different carbohydrates have different amounts of these elements. The primary role that carbohydrates serve for living organisms is that they make up sugars, which are the primary source of energy for life. Carbohydrates are made internally by plants and certain microbes during photosynthesis; other organisms must ingest their carbohydrates. One type of carbohydrate that does contain nitrogen is chitin, which is part of the outer layer of fungi cell membranes.
Proteins are a complex group of chemical compounds that serve a variety of purposes for living organisms. Within the cell, proteins form the basic structure of cells and, in multicellular organisms, they are what make up connective tissues, such as the skin on human beings and the vascular system in plants. Additionally, proteins serve as the cause (also known as "catalyst") for chemical reactions within the cell; one of the most important proteins is adenosine triphosphate (ATP), an enzyme that is a byproduct of cellular respiration.
Lipids are a chemical compound made up of carbon, hydrogen and oxygen, which, when combined in a certain manner, create water soluble compounds. As a result, lipids are the basic median through which most chemical reactions take place in the cell, as water is needed for cells to survive, but the structure of a lipid is not affected by water. Fats and oils are made up of lipids, a well as waxes and steroids.
The most important chemical compounds to contain nitrogen are nucleic acids, and the most important nucleic acids are deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Together, RNA and DNA contain the genetics of the cell, which is essentially the blueprint for how the cell should act. RNA and DNA are copied when a cell undergoes mitosis, which is the process of a cell splitting and dividing into a new cell. Proteins serve the cell by "translating" the actions encoded in RNA and DNA.