Cells are the unit of structure and function for all living things, according to the cell theory. Therefore, cells can carry out various functions to promote life. For example, cells can carry out the function of cellular respiration inside tiny organelles known as mitochondria. Inside the mitochondria, glucose plus oxygen are converted to adenosinetriphosphate or A.T.P. a high energy compound which the cells use as energy currency. Another function cells perform is secretion. There are various secretions cells can produce which get packaged and used inside the cell, or sent outside the cell to a target location. Secretions are packaged by the golgi apparatus, an organelle inside cells. Some cells are hormone secreting cells. Hormones are important chemical messengers and they are transported by the circulatory system to target organs in the body, causing some type of response to occur. For example, the hormone insulin produced by the pancreas cells helps convert excess glucose to glycogen, a storage product, thus maintaining homeostasis in the blood. Cells can protect the human body. Unbroken epithelial cells on the outside of the body help to keep foreign bodies such as germs out of the body. There are neurons, cells in the nervous system that help produce impulses to and from the brain and spinal cord. This helps keep us alive as we respond to the changing environment constantly. As you can see, there are many functions that various body cells perform. These are just five examples.