Before you read further, take a moment to look at your dog. You are looking at a biological masterpiece whose body is a complex organism comprised of many structures that break down into interrelated systems. Organs like the pancreas, liver, kidneys, heart and lungs are composed of tissue, and tissue is the aggregation of millions of individual cells. For example, carbon, oxygen, hydrogen, nitrogen, potassium, sodium and calcium are some of the atoms, which create and maintain life at a chemical level.
Atoms combine to form molecules such as vitamins, fats and proteins.
These molecules then combine to form structures at the cellular level. Each cell is bound by a cell membrane composed of lipids and proteins. The cell membrane controls the passage of substances into and out of the cell. Cells contain jellylike cytoplasm in which are suspended a nucleus and other structures specialized to carry out specific functions geared toward construction and operation of the cell. Cells are made up of many substances, especially carbohydrates, lipids, nucleic acids and protein.