Biological membranes, formed of lipids, are important in biological systems, allowing the separation of metabolic pathways, localisation of proteins and lipids, and the formation of electrochemical gradients. Metabolic pathways need to be separated within cells - this is done with mitochondria and chloroplasts. Due to common enzymes and substrates in these pathways, it would be possible for uncontrolled futile cycles to occur if they were completed in the same compartment. By separating these, the cell is able to control the metabolic flux carefully, allowing futile cycles to be completed as needed. A futile cycle is where the starting material is regenerated by reaction pathways, without the use of the metabolic intermediates (such as pyruvate) - this is common in glycolysis, where gluconeogenesis is then used to convert pyruvate back to glucose. While this may seem excessive, it can be beneficial when tightly regulated, allowing an organism to maintain a constant body temperature. Protein and lipid localisation requires membranes. These can be tagged with their destination, and through a series of nudges by the microtubule cytoskeleton, are able to reach their intended location. The formation of electrochemical gradients is another important purpose of biological membranes. The proton motive force, an important electrochemical gradient in the synthesis of ATP, is formed by the mitochondria and chloroplast. This allows the ATP synthase enzyme to catalyse the synthesis of ATP from ADP and $$\text{P}_\text{i}$$.
The plasma membrane is a good barrier from the exterior of the cell, being selectively permeable, and able to change its permeability. By allowing certain molecules to diffuse in, a cell can quickly uptake small uncharged molecules including gases ($$\text{O}_2$$, $$\text{CO}_2$$, and others), as well as water (amongst others). This selective permeability also provides protection against toxins, preventing them from simply diffusing in. Transport proteins can be used to selectively increase the rate of transport of larger species, as well as charged ions, allowing control of uptake and release. The maintenance of electrochemical gradients is another feature of the plasma membrane, especially in nerve cells, preventing the transport of ions. This allows the maintenance of a constant resting potential, while allowing for depolarisation in a controlled manner in response to a specific stimulus. Signalling is another important aspect membranes are involved in. This is typically through signal transduction pathways, whereby an integral membrane protein is embedded through the bilayer. This allows an extracellular ligand (such as a hormone, cytokine, or other signalling molecule) to exert an influence on cellular function. This may be through GPCRs, receptor dimerisation (in membrane rafts) or other receptor proteins.