It contains a variety of biological molecules, primarily proteins and lipids, which are involved in a vast array of cellular processes. Thumbnail: The cell membrane, also called the plasma membrane or plasmalemma, is a semipermeable lipid bilayer common to all living cells. An example my mentor would use when teaching basic neuroscience to schoolchildren was a bipolar neuron that extends from the toe to the brain. This specialization allows for extremely rapid transmission of information across long distances. However, in neurons, the movement of ions has another crucial function in metazoan animals: production of action potentials used for neurotransmission. 4.4: The Action Potential in Neurons The transport of solutes in and out of cells is critical to life.Integral membrane proteins may extend completely through the membrane contacting both the extracellular environment and the cytoplasm. 4.3: Membrane Transport Proteins Membrane proteins come in two basic types: integral membrane proteins (sometimes called intrinsic), which are directly inserted within the phospholipid bilayer, and peripheral membrane proteins (sometimes called extrinsic), which are located very close or even in contact with one face of the membrane, but do not extend into the hydrophobic core of the bilayer.The reasons should be self-evident: larger molecules simply cannot fit between the lipid molecules to make their way through. However, small nonpolar molecules are able to pass through the membrane with relative ease. ethanol), but more often pass at low rates if at all (e.g. Small polar molecules can sometimes pass easily (e.g. 4.2: Membrane Permeability A pure phospholipid bilayer, whatever the lipid composition, is a semi-permeable membrane that is generally repellent to large molecules and to ions.Cellular membranes were partially defined as being composed primarily of phospholipids: molecules consisting of a phosphorylated polar head group attached to a glycerol backbone that has two long hydrocarbon tails. 4.1: Membrane Structure and Composition Since most cells live in an aqueous environment and the contents of the cell are also mostly aqueous, it stands to reason that a membrane that separates one side from the other must be hydrophobic to form an effective barrier against accidental leakage of materials or water.
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