<Li> Defensive proteins, which can include everything from antibodies of the immune system, to toxins (e.g., dendrotoxins of snakes), to proteins that include unusual amino acids like canavanine </Li> <P> A bilayer of phospholipids makes up the membrane of cells that constitutes a barrier, containing everything within the cell and preventing compounds from freely passing into, and out of, the cell . Due to the selective permeability of the phospholipid membrane only specific compounds can pass through it . In some multicellular organisms they serve as a storage of energy and mediate communication between cells . Carbohydrates are more easily broken down than lipids and yield more energy to compare to lipids and proteins . In fact, carbohydrates are the number one source of energy for all living organisms . </P> <P> All organisms consist of structural units called cells; some contain a single cell (unicellular) and others contain many units (multicellular). Multicellular organisms are able to specialize cells to perform specific functions . A group of such cells is a tissue, and in animals these occur as four basic types, namely epithelium, nervous tissue, muscle tissue, and connective tissue . Several types of tissue work together in the form of an organ to produce a particular function (such as the pumping of the blood by the heart, or as a barrier to the environment as the skin). This pattern continues to a higher level with several organs functioning as an organ system such as the reproductive system, and digestive system . Many multicellular organisms consist of several organ systems, which coordinate to allow for life . </P> <P> The cell theory, first developed in 1839 by Schleiden and Schwann, states that all organisms are composed of one or more cells; all cells come from preexisting cells; and cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells . </P>

What is not necessary for an organism to survive