The membrane is made up of a phospholipid bilayer of lipids with their hydrophilic heads facing outwards and their hydrophobic tails facing inwards. This bilayer is also associated with cholesterol, proteins and carbohydrates. The main function of the plasma membrane is to separate the extracellular fluid on the outside of the cell from the intracellular fluid on the inside of the cell. In other words, it protects the cell from its surroundings so it cannot be infected or harmed by anything outside the cell. In order to regulate the movement of what goes into and out of the cell the membranes embedded proteins move specific solutes into and out of the cell when instructed to do so by the cell. Receptors are also positioned on the membrane to receive and relay signals from neighbouring cells and are called transmembrane receptors. Their job is to conduct signal transduction, converting signals received into intracellular signals that can be converted by the cell. The 3 main types of membrane receptors include; Ligand-gated ion channel receptors, enzyme coupled receptors and G-protein-coupled receptors. Ligand-gated ion channel receptors work when a ligand binds and the channel opens allowing the ion through. Enzyme coupled receptors are also activated when the ligand binds causing the enzyme to get to work. G protein coupled receptors are the largest and once the ligand binds, GTP is activated which in turn activates an effector protein. Each of these proteins are of a specific shape for their specific ligand to activate them. A slight change in this structure and the ligand can no longer bind making the receptor inactive and the signal can no longer be received. The membrane plays a vital role in ATP synthesis and is produced by a process called chesmiotic coupling. This is made possible by protein complexes that are embedded into the phospholipid bilayer, with the help of activated carrier molecules such as NAD and FAD. These molecules are called carriers as they carry high energy electrons through electron carriers. This occurs by travelling through covalent bonds and jumping across a 2mm gap via electron tunnelling. It is this structure of the enzyme complexes allowing for the crucial flow of electrons, resulting in our bodies production of energy in the form of ATP.