fundamentals
Cells
Structure of cells
Often thought of as the smallest unit of a living organism, a cell is made up of many even smaller parts, each with its own function. Human cells have a surface membrane (called the cell membrane) that holds the contents together. However, this membrane is not just a sac, it is an active participant in the life of a cell. The membrane controls what chemicals and other substances can enter and leave the cell. The membrane also has receptors that identify the cell to other cells. The receptors also react to substances produced in the body and to drugs taken into the body, selectively allowing these substances or drugs to enter and leave the cell (see Receptors on Cells ). Reactions that take place at the receptors often alter or control a cell's functions. An example of this is when Within the cell membrane are two major compartments: The cytoplasm The nucleus Thecytoplasmcontains structures that consume and transform energy and perform the cell's functions. contains the cell's genetic material ( Genes and Chromosomes ), which contain all the instructions for how the cell functions and control cell division and reproduction. Mitochondriaare tiny structures inside the cytoplasm of every cell that provide the cell with energy. Although there are different types of cells, most cells have the same components. A cell consists of a nucleus and cytoplasm and is contained within the cell membrane, which regulates what passes in and out. The nucleus contains chromosomes, which are the cell's genetic material, and a nucleolus, which produces ribosomes. Ribosomes produce proteins, which are packaged by the Golgi apparatus so that they can leave the cell. The cytoplasm consists of a fluid material and organelles, which could be considered the cell's organs. The endoplasmic reticulum transports materials within the cell. Mitochondria generate energy for the cell's activities. Lysosomes contain enzymes that can break down particles entering the cell. Centrioles participate in cell division. 