Cell Structure and Cell Organelles
The Cell Membrane
The cell or plasma membrane is a double layer of lipids with proteins both on its surface and embedded within. It surrounds the cell and closes the cell off from its surroundings.
All cells are divided by membranes into closed reaction spaces (compartmentalization). The cell membrane consists of a lipid bilayer that has various proteins and some cholesterol molecules embedded within it. The membrane thus primarily forms a protective barrier against water-soluble or polar substances, while lipophilic or nonpolar substances can easily pass through.
This basic behavior of the lipid bilayer is modulated by numerous membrane proteins. Receptors, ion channels, and transport or ion channel proteins are particularly important. Receptors are proteins that have binding sites for one or more different ligands. Binding of the ligand, which is often a hormone or neurotransmitter, results in a change to the protein structure that either opens an ion channel or initiates a metabolic cycle within the cell by means of the G-proteins. (second messenger).
Ion channels are transmembrane proteins that enable the influx and efflux of ions along the concentration gradient. There are voltage-, receptor-, and ligand-gated ion channels.are analogous to ion channels and enable the exchange of uncharged substances along a concentration gradient. Some porins only differentiate molecules by size while others only allow certain classes of substances to pass through. Finally, there are also proteins that actively transport molecules and ions in or out against the concentration gradient.