Part of 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 of 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 of 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 stay outside the cell. These materials move in and out of cells by passing through the plasma membrane of the cell. The plasma membrane surrounds the cell and separates the interior of the cell from its environment. This semi-permeable membrane is composed of a lipid bilayer. Fossful lipids make up a large part of the membrane and form a bilayer. The structure of the bilayer is due to the tail-to-tail packing of the non-polar hydrophobic tails composed of two fatty acid chains. And the polar hydrophilic heads composed of glycerol and phosphorylated alcohol. The lipid bilayer is 5 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 bilayer has properties resembling fluids and the differing proteins and their attachments on either side of the membrane resemble a mosaic. The plasma membrane is the gatekeeper of the cell, allowing certain substances in and 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 of 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 bilayer. However, the cell needs to control what enters and leaves, and so transport proteins aid in the selective movement of other molecules across the membrane without the input of energy. Through facilitated diffusion, larger molecules, polar molecules, and charged ions, use channel proteins embedded in the bilayer. The transport of other substances requires a special carrier that will bind the substance on one side of the membrane, which triggers a confirmation change in the protein carrier, causing it to release the substance on the other side. When the substance being moved across the membrane is water, the process is called osmosis. The cytoplasm of the cell, as well as the interstitial fluid, is composed of solutions. The solvent, usually water, moves across a semi-permeable membrane toward a higher solute concentration, consisting of various molecules or ions, until equilibrium of the solutions is reached. The plasma membrane contains proteins called aquaporins, which are specialized channels for the movement of water during osmosis. A cell in a hypotonic environment will have water move from the inside of the cell toward the higher concentration of solutes in the solution outside the cell. In a hypotonic solution, the concentration of solutes is higher inside the cell than the outside environment, so water will diffuse into the cell. When the solutions on either side of the membrane reach equilibrium, they are referred to as isotonic. When substances need to be driven against their concentration gradient from low to high concentrations, active transport is required. This process uses energy to move a substance across the membrane using selective protein carriers. The sodium potassium pump is a specialized membrane protein carrier, common in most animal cells, that moves sodium and potassium ions across the plasma membrane. ATP fuels the pump in the movement of these ions from low to high concentrations, moving sodium ions outside of the cell where they become concentrated and bringing potassium ions into the cell where their concentration is higher relative to the outside. In other instances of active transport, the concentration gradient created from the action of one pump can be used as the input of energy to drive a different pump, a process known as coupled transport. The large concentration gradient of sodium ions is used to bring glucose into the cell against its concentration gradient. The plasma membrane with its lipid bilayer and various embedded proteins provides a barrier between the internal cellular environment and the environment surrounding the cell. The structure and function of the plasma membrane coupled with the processes of moving materials into and out of cells allow for the metabolic action cells need to function, as well as for the continuation of life.