Membrane
From EvoWiki
Biological membranes are composed primarily of lipids, molecules with a charged region and a large, hydrophobic tail. The membranes form a lipid bi-layer, with these molecules lined up tail to tail, leaving the charged regions facing outward towards the aqueous environment. The primary boundary between a cell and its environment is a membrane, termed the cell membrane. In Eukaryotes, the Nucleus, which contains the genetic material, is also surrounded by a membrane. Many Eukaryotic Organelles within the cell are also surrounded by membranes, including:
- Mitochondria
- Chloroplasts
- Endoplasmic reticulum
- Golgi complex
- Lysosomes
- Peroxisomes
- Secretory vesicles
- Vacuoles
As the majority of the lipid bi-layer is hydrophobic, a membrane serves as a barrier for any charged or polar molecules, including ions, sugars, proteins, etc. As a result, transport across membranes is mediated by proteins which are embedded in the membrane, or alterations in the membrane structure itself. Proteins that reside in the membrane generally have alpha-helical structures containing many hydrophobic amino acids, allowing them to remain embedded within the membrane.
Small molecules such as ions and sugars are generally transported through the membrane by single proteins or protein complexes termed channels. This transport can be passive, in which case molecules are free to pass in or out of the cell at random, allowing the cell to equilibrate to its environment. Alternately, active transport can couple the passage of one type of ion/molecule with the passage of a second, or the expenditure of energy. This behavior allows the cell to maintain an internal environment distinct from its external environment. Larger protein complexes, termed pores, can allow the passage of entire molecules across membranes. These do not generally act between the interior of a cell and the environment, but rather within a cell. An example is the nuclear pore, which allows proteins that structure and regulate the chromosomes to be imported into the nucleus.
Transport of collections of proteins between organelles and to the cell surface is generally accomplished by small membranous sacs termed transport vesicles. These may contain both soluble proteins and membrane bound/embedded proteins, and are trafficked within a cell along its microtubule network. Upon reaching their target, these sacs fuse with the cellular/organelle membrane, resulting in the release of their contents to the new environment in a process termed exocytosis. Membrane bound/embedded proteins become embedded in the organelle, or at the cell surface. The converse activity, termed endocytosis, results in the pinching off of a new transport vesicle, internalizing collections of membrane bound/embedded proteins and any molecules that they are bound to.
