Biological membranes are the basis for many important properties of the cell, not the least of which is to physically define the cell boundary, and in eukaryotes, the boundaries of each intracellular organelle. However, they are not completely impermeable boundaries, and through embedded proteins, the membrane serves as the gatekeeper for the passage of specific molecules into (e.g. nutrients) and out of (e.g. waste) the cell. Other embedded proteins can identify the cell to other cells, and participate in numerous interactions with the environment or other cells. Finally, the membrane, or more precisely, the chemical gradients across the membrane, is an important energy source for the cell.
- 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. 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.
- 4.2: 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. Small polar molecules can sometimes pass easily (e.g. ethanol), but more often pass at low rates if at all (e.g. water). However, small nonpolar molecules are able to pass through the membrane with relative ease. The reasons should be self-evident: larger molecules simply cannot fit between the lipid molecules to make their way through.
- 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. Integral membrane proteins may extend completely through the membrane contacting both the extracellular environment and the cytoplasm.
- 4.4: The Action Potential in Neurons
- The transport of solutes in and out of cells is critical to life. However, in neurons, the movement of ions has another crucial function in metazoan animals: production of action potentials used for neurotransmission. This specialization allows for extremely rapid transmission of information across long distances. An example my mentor would use when teaching basic neuroscience to schoolchildren was a bipolar neuron that extends from the toe to the brain.