- The plasma membrane is the highly organized and specialized first line of defense against the world and first line of communication with it.
- It is a formidable barrier to any polar molecule larger than gaseous oxygen. Non-polar molecules may enter cells by diffusion.
- The PM is selectively permeable to uncharged, small molecules. It is permeable to oxygen and somewhat to water, but very impermeable to polar or ionized molecules/atoms like salts and sugars.
- Communication via the PM can be beneficial ( e.g. growth hormones/factors) or detrimental (e.g. invasive bacteria, viruses, or apoptotic signals)
- Protein components primarily carry out the communication functions of the PM
- PMs are primarily comprised of polar lipids (mainly phospholipids and cholesterol) and proteins
- Two PM phospholipid layers (leaflets) form a bilayer. These phospholipids rarely flip spontaneously from one leaflet to the other.
- Like lipids, protein orientation across the PM is determined at their point of biosynthesis in the endoplasmic reticulum.
- Integral membrane proteins virtually never change their orientation, because their functions often are transmembrane in nature.
- "peripheral" membrane proteins do not actually cross the membrane but are "stuck" to it by non-covalent adhesion to integral membrane proteins, specific membrane lipids, or both. They may also migrate within the cell to other locations such as the cytosol, mitochondria, or endoplasmic reticulum, as part of signaling events initiated at the exterior of the PM.
- Impermeable molecules/atoms require specific protein carrier systems to enter or exit the cell.
- ATP pumps drive ions against their concentration gradient and are responsible for intracellular concentrations of several ions; in particular potassium (K+), sodium (Na+) and calcium (Ca++) are different inside and outside cells.
- A symport type of transport couples the energy of transport down a concentration gradient (Na ion) with the need to transport the sugar in the same direction but against its concentration gradient.
- An antiport transport system uses the energy of an inward concentration gradient to transport a molecule in the opposite direction.
- There are many types of diffusion-regulated systems that permit rapid diffusion of molecules down their concentration gradients through periodically open protein channels that possess both open and closed states.
- In the plane of the PM there exist specialized ‘patches’ that are composed of different lipid combinations than in the rest of the membrane; they are called lipid rafts.
- These small areas contain high levels of glycosphingolipids and cholesterol, and they favor the aggregation of certain integral membrane proteins such as receptors.
- Lipid rafts permit specialized functions such as endocytosis, complex signaling, or the formation of protein machines that perform important functions such as the synthesis of ATP, electron transport, and recognition of extracellular signals.
- Scaffold proteins are essential parts of the cellular signaling systems and may have several functions, one of which is to bind to and immobilize other proteins. They possess patches of binding sites for other proteins, sometimes four or six patches per protein; some are called PDZ domains.
- PDZ domains bind to complementary domains on other scaffolding proteins, to proteins participating in signal transduction from PM receptors, or to other functional proteins and multiply the speed of signals within the cell or from outside to inside.
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