The it to carry out different functions. Some of

The plasma membrane or cell membrane defines the cell, separating the inside from the outside. The biomembrane is made up of a basic structure: a phospholipid bilayer in which proteins are embedded. For smaller cells, like the Prokaryotes, each cellular membrane has its own set of proteins that allow it to carry out different functions. Some of these proteins catalyze ATP synthesis and initiation of DNA replication. Others include the many types of membrane transport proteins that enable specific substances to cross the otherwise impermeable phospholipid bilayer to enter the cell. Receptor proteins allow the cell to recognize chemical signals present in the its environment and adjust its metabolism in response. Eukaryotes also have a plasma membrane studded with many different proteins that are in charge of a variety of functions, including membrane transport, cell signalling, and connecting cells into tissues. In addition, eukaryotic cells have a variety of internal membrane-bound organelles and each type has a unique complement of proteins that enable it to carry out particular functions, such as ATP generation in mitochondria and DNA synthesis in the nucleus. Many plasma-membrane proteins also bind components of the cytoskeleton, a dense network of protein filaments that provide mechanical support for cellular membranes.Despite playing a structural role in the cells, the plasma membrane is not a rigid structure. According to the fluid mosaic model, first proposed in the 1970s, the phospholipid bilayer works in some respects like a two-dimensional fluid, with individual lipid molecules able to move past one another as well as spin in place. This fluidity and flexibility confers on the membrane the dynamic property that enables membrane budding and fusion.A bilayer of phospholipids about 3nm thick provides the basic architecture of the plasma membrane. Besides phospholipids, biomembranes contain smaller amounts of other amphipathic lipids, such as glycolipids and cholesterol.All phospholipids are amphipathic molecules that consist of two segments with different chemical properties: a fatty acid-based hydrocarbon tail that is hydrophobic and a polar head that is strongly hydrophilic and tends to interact with water molecules. The hydrophobic fatty acyl chains in each “leaflet” (phospholipid layer) minimize their contact with water by aligning themselves tightly together in the center of the bilayer, forming a hydrophobic core that is about 3-4 nm thick. The close packing of these nonpolar tails is stabilized by van der Waals interactions between the hydrocarbon chains. Ionic and hydrogen bonds stabilize the interactions of the polar head groups with one another and with water. A phospholipid bilayer can be almost unlimited in size and can contain tens of millions of phospholipid molecules. Although biomembranes contain other molecules, it is the phospholipid bilayer that separates two aqueous solutions, acting as a permeability barrier