Protein component (60%): act as ion channels; transporters, receptors, cell identity markers, enzymes and linkers.
(a) Integral proteins - extend into or through the lipid bilayer and are firmly embedded in it. Most are transmembrane proteins spanning the entire lipid bilayer and protrude into both the cytosol and the extracellular fluid –touches both sides all the way through. Most are Amphipathic (hydrophilic parts touch fluids and Hydrophobic extend among the FA tails). (b) Peripheral proteins - not firmly embedded in the membrane. They are generally located outside and inside of the membrane (phospholipid). They do not go all the way through. (c) Glycoproteins – sugar attached to protein
Functions of Membrane Proteins
1. Ion Channels (Integral) Pores or holes through which specific ions can flow to get into or out of the cell. Most are “selective” allowing only a specific ion to pass (most commonly K+ and Cl-). 2. Transporters (Carrier) (Integral) Selectively move substances or ion (polar – hydrophilic) from one side of the membrane to the other (ex: amino acids enter cells through transporters) 3. Receptors (Integral) Serve as cellular recognition sites. Each type of receptor recognizes and binds a specific type of molecule (a “Ligand” of that receptor) 4. Enzymes (Integral & Peripheral) Catalyzes a reaction(s) inside or outside the cell. (ex: lactase on the lining of the small intestine splits the lactose in your milk) 5. Cell Identity Markers (Mostly Peripheral) Distinguishes your cells from others (unless you are a twin). (ex: MHC – Major Histocompatibility Proteins or ABO blood type markers) 6. Linkers (Integral & Peripheral) Anchor proteins in the plasma membranes of neighbouring cells to one another.
Properties of the Plasma Membrane
1. Membrane Fluidity Adjacent phospholipids and proteins that make up the plasma membrane exhibit fluid- like movement with respect to each other and allows for self-repair. (Ex: Needle puncture)
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