Part have metabolic activity is the ability to move materials into and out of cells. This is important in cell communication and normal cell function for example in order for the cells have your nervous system to function properly ions water proteins and other molecules need to be able to pass into and out of cells the cells have the nervous system form networks .they like all cells are able to function because they can control what substances are inside the cell and what substances state outside the cell .these materials movie in and out of cells by passing through the plasma membrane have the cell .the plasma membrane surrounds the cell and separates the interior the cell from its environment .this semi-permeable membrane is composed of a lipid by layer phospholipids make up a large part of the membrane and former by layer the structure of the by layer is due to the tail to tail packing I've the nonpolar hydrophobic tails composed of two fatty acid chains and the polar hydrophilic heads composed of glycerol and phosphor related alcohol the lipid by layer is five to 10 nanometers thick and is embedded with proteins some cell membranes also contain cholesterol a plasma membrane contains different types of proteins which are specific to the particular function of the cell these proteins also enable the cell to interact with its environment the entire structure of the plasma membrane can be described as a fluid mosaic model the phospholipid by layer has properties resembling fluids and the different proteins and their attachments on either side of the membrane resemble a mosaic the plasma membrane is the gatekeeper have the cell allowing certain substances in an out of the cell at certain times in certain amounts diffusion is a process in which substances move across a membrane from an area of high concentration to an area of lower concentration or between areas are opposite electrical charges .this is called the electrochemical gradient small non charged particles primarily gases such as oxygen and carbon dioxide can diffuse through the plasma membrane by moving in between the phospholipids in the by layer however the cell needs to control what enters and leaves and soul transport proteins aid in the selective movement other molecules across the membrane without the input %uh energy through facilitated diffusion larger molecules polar molecules and charged ions use channel proteins embedded in the by layer.