In this lesson we will look at the main principles of the cell theory. But before we begin, we need to understand the true meaning of the term "theory". In science, a theory is a group of related principles that have been scientifically tested repeatedly and have consistently held up. They have withstood the test or test if you will of time. All evidence supports them and thus they are accepted as being true because they have never been disproven. If you hear someone say that something is "just a theory", he or she is misusing the term. Keep in mind that to be designated as a theory, something has been scientifically supported each time it was tested, so a theory is considered to be fact. The cell theory, then, is a group of related principles about cells that are well accepted as fact. There are five main principles of the cell theory. Sometimes more are included but we will explore the major five, one at a time. Number one: All organisms are composed of one or more cells. All life, as we know it, is composed of cells. Organisms may be unicellular, meaning they consist of just a single cell, or multicellular, meaning that they are made of at least two cells. The human body, for example, contains trillions of cells. Number two: Cells are the basic structural and functional units of life. Cells are the first organizational level at which we see the characteristics of life. Even the simplest cells carry out functions to maintain their own existence. Remember, some organisms are only a single cell. Cells are living entities or at least at some point in their existence they were alive. For this reason, we often say that life begins at the cell. Number three: All vital functions of an organism occur within cells. Cells conduct all of the functions needed to maintain their own lives but they also carry out all of the work needed to sustain the life of a multicellular organism. As the number of cells goes up in complex, multicellular organisms, the cells can begin to specialize in different functions, sort of developing a division of labor. For example, in us, red blood cells are loaded with a molecule called hemoglobin that allows the cells to pick up and carry oxygen to the areas in the body that need it. Nerve cells are specialized to transmit electrical signals, or nerve impulses, that control the actions of other cells, such as muscle cells, which contain protein molecules that slide across each other changing the length of the muscle cells which allows muscles to contract and to move our body parts. Number four: All cells come from preexisting cells. For humans, life begins as a zygote, which is the union of one egg and one sperm. The zygote is a single cell from which all of our other cells are derived. Cells reproduce through cell division to produce daughter cells. And the daughter cells eventually divide to produce even more cells. All of the cells currently in your body trace their heritage back to the original zygote. And, of course, the zygote traces its heritage from the cells of both of your parents and the cells of your parents are derived from their parents' cells as well. Number five: Cells contain hereditary information that regulates cell functions and is passed from generation to generation. All cells contain genetic material in the form of DNA or RNA. For us, it's DNA. Our DNA contains our genes and genes determine what proteins our cells can make. That may not sound like much, but proteins have so many functions in our bodies. The enzymes that allow chemical reactions to occur are proteins. Proteins form channels that allow various materials to enter and leave our cells. Hemoglobin that combines oxygen in our red blood cells is a protein. Hair, nails, and the pigments that give our skin and our eyes their color are proteins. To an amazing extent, your DNA determines who you are and what your body can do. Each cell with a nucleus contains all of your genetic information so it is passed from one cell to the next through cell division. Whether your cells reproduce or you reproduce, your genetic information will be passed on to the next generation, allowing continuation of yourselves and also continuation of the species.