<P> The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates the interior of all cells from the outside environment (the extracellular space). It consists of a lipid bilayer with embedded proteins . The basic function of the cell membrane is to protect the cell from its surroundings . The cell membrane controls the movement of substances in and out of cells and organelles . In this way, it is selectively permeable to ions and organic molecules . In addition, cell membranes are involved in a variety of cellular processes such as cell adhesion, ion conductivity and cell signalling and serve as the attachment surface for several extracellular structures, including the cell wall, the carbohydrate layer called the glycocalyx, and the intracellular network of protein fibers called the cytoskeleton . In the field of synthetic biology, cell membranes can be artificially reassembled . </P> <P> While Robert Hooke's discovery of cells in 1665 led to the proposal of the Cell Theory, Hooke misled the cell membrane theory that all cells contained a hard cell wall since only plant cells could be observed at the time . Microscopists focused on the cell wall for well over 150 years until advances in microscopy were made . In the early 19th century, cells were recognized as being separate entities, unconnected, and bound by individual cell walls after it was found that plant cells could be separated . This theory extended to include animal cells to suggest a universal mechanism for cell protection and development . By the second half of the 19th century, microscopy was still not advanced enough to make a distinction between cell membranes and cell walls . However, some microscopists correctly identified at this time that while invisible, it could be inferred that cell membranes existed in animal cells due to intracellular movement of components internally but not externally and that membranes weren't the equivalent of a cell wall to plant cell . It was also inferred that cell membranes weren't vital components to all cells . Many refuted the existence of a cell membrane still towards the end of the 19th century . In 1890, an update to the Cell Theory stated that cell membranes existed, but were merely secondary structures . It wasn't until later studies with osmosis and permeability that cell membranes gained more recognition . </P>

The main function of the plasma membrane is to