Plasma Membranes PDF

Plasma membranes -​ Plasma membranes are ca. 8nm thick (=8000x the thickness of paper) → control traffic in & out of cells -​ Plasma membranes possess selective (like all biological membranes) -​ Ability of cells to discriminate in their chemical exchange with their = a fundamental to live...

Plasma membranes -​ Plasma membranes are ca. 8nm thick (=8000x the thickness of paper) → control traffic in & out of cells -​ Plasma membranes possess selective (like all biological membranes) -​ Ability of cells to discriminate in their chemical exchange with their = a fundamental to live Structure of membranes -​ Mostly phospholipids & proteins, but: carbohydrates are also important -​ Most abundant class of molecules: phospholipids → have the inherent ability to form membranes -​ Amphipathic molecules (i.e. they have hydrophilic & hydrophobic regions) -​ Next most abundant class of molecules: proteins -​ Most membranes proteins also have hydrophobic and hydrophobic regions Fluid Mosaic Model -​ Biological membranes are fluid structure with a “mosaic” of various proteins embedded in or attached to a bilayer of phospholipids -​ Membranes are not locked rigidly in place → held together mainly by hydrophobic interactions between the tails of the phosphorlipids  → most of the phospolipids & some of the proteins can shift about laterally (sideways), but: it is rare for a molecule to flip-flop across the membrane -​ Phospholipids can be seen as the fabric of the membrane & proteins can be seen as the functional components of the membrane. Membranes can vary -​ Different types of cells & also different types of membranes within the same cell have different sets of membrane proteins -​ 2 main populations of membrane proteins: integral and peripheral proteins -​ Integral proteins - Penetrate the hydrophobic interior of the membrane. - Majority of the integral proteins are also transmembrane proteins → span the membrane -Some integral proteins extend only partway into the hydrophobic interior of the membrane -​ Peripheral proteins - Not embedded in the membrane at all → bound only to the hydrophilic outer parts of the membrane (often to exposed parts of the integral proteins) Fluid Mosaic Model  Transport of substances across membranes -​ Biological membranes are Supramolecular structures they have emergent properties -​ Includes the ability to regulate transport across cellular boundaries → selective transport -​ Biological membranes are semi-permable -​ Non-polar molecules (e.g. hydrocarbons, CO2 or O2 are hydrophobic → can dissolve in the lipid bilayer → cross easily -​ Ions & polar molecules (e.g. Na+. Cl- or H2O) are hydrophilic implended by the hydrophobic interior -​ E.g glucose & other sugars pass only slowly -​ E.g H2O does not cross rapidly either -​ E.g Ions like Na+ or Cl- are even less likely to cross -​ 2 types of transport of substances across membranes: -​ Passive transport → no energy investment from the cells required -​ Active transport → requires energy input from the cell Passive Transport -​ Based on diffusion: “ The spontaneous movement of a substance down its concentration gradient, i.e. from a region where it is more concentrated to a region where it is less concentrated.” -​ Diffusion if driven by the Brownian molecular motion i.e. the inherent kinetic energy of all molecules → molecules of all substances always remain in a state of random linear motion -​ So: each molecules moves randomly, but: diffusion of a population of molecules may be directional → formation of a dynamic equilibrium -​ Rule: in the absence of other forces, a substance will diffuse from a region where it is highly concentrated to a region where it is less highly concentrated (i.e. down its concentration gradient) -​ Note: each substance moves down its own concentration gradient, i.e. it is not impacted by other substances present.  1.​Simple Diffusion -​ Some traffic across cell membrane occurs via simple diffusion -​ E.g O2: used up inside the cell by cellular respiration → concentration gradient upheld -​ Concentration gradient = potential energy source → in this case: drives diffusion of O2 into the cell -​ Generally: -​ Small non-polar molecules and some small polar molecules may pass the membrane without hindrance via simple diffusion -​ No energy input required -​ Molecules pass between the phospholipids & move down their concentration gradient Simple Diffusion 2.​Facilitated Diffusion -​ 2a) trough tunnel proteins -​ Tunnel or channel proteins are transmembrane proteins that provide a hydrophilic tunnel across a membrane → certain molecules or ions can pass through them -​ Most Tunnel proteins are quite specific to one or maybe two types of ion -​ 2 versions of tunnel proteins: 1.​ Open channels → constantly open 2.​ Gated channels → open or close in response to a specific stimulus -​ 2b) Through carrier proteins -​ Carrier proteins undergo subtle changes in shape → translocates the solute-binding site across the membrane -​ Can be triggered by binding and release of substances -​ For both types of facilitated diffusion (i.e. facilitated diffusion through tunnels & through carrier proteins): -​ No energy input required → diffusion of molecules down their concentration gradient -​ Ca transport larger molecules & ions across the membrane which would otherwise not pass (i.e. cannot pass through the phospholipids) Facilitated Diffusion  Active Transport -​ Some transport proteins can move solutes against their concentration gradient → requires energy input -​ Active transport is performed by carrier proteins (not channel proteins) -​ Enables internal concentration of solutes that differs from that outside the cell -​ Energy for most active transport process is supplied by ATP - ATP (Adenosine Tri-Posphate) is the energy currency of the cell - Adenosine +3 phosphate groups - Transfers energy by transferring one or more of its phosphate groups directly to the transportation protein → transport protein changes shape → translocates a bound solute across the membrane Passive Transport: -​ Simple diffusion -​ Facilitated diffusion through tunnel protein -​ Facilitated diffusion through carrier proteins Active Transport: -​ Primary active transport -​ Secondary active transport -​ Bulk transport - exocytosis - endocytosis → - phagocytosis - pinocytosis -receptor-mediated endocytosis Osmosis: -​ A process by which molecules of a solvent tend to pass through a semi-permable membrane from a less concentrated to a more concentrated solution, thus equalizing the concentrations on each side of the membrane Or Diffusion of solvent molecules through a semi-permable membrane impermeable to the solutes.  Active Transport: Proton Pump ActiveTransport:Na+ -K+ -Pump  Sodium-Potassium-Pump (Na+-K+-Pump) -​ An example for an active transport system is the Na+-K+-pump in animal cells -​ Transport 3 Na+ out of the cell for every 2K+ it transports into the cell -​ Cytoplasmic Na+ binds to the carrier protein (3 Na+) → binding of Na+ stimulates phosphorylation of the carrier protein by ATP → this leads to a change in the shape of the carrier protein → translocates the bound Na+ across the cell membrane → change in shape reduces the affinity of the carrier protein for Na+ → 3 Na+ released outside of the cell → new shape has high affinity for K+ → binds 2 K+ → binding triggers release of the phosphate group → this restores the carrier protein to its original shape → lower affinity for k+ → 2 K+ released inside the cell →… cycle repeats Cotransport (or secondary active transport) -​ A single ATP-powered pump transports a specific solute → establishes a concentration gradient → this concentration gradient indirectly drives active transport of other solute: -​ The solute that is actively pumped moves back across the membrane via Facilitated diffusion (i.e. it moves down its concentration gradient) -​ A cotransporter protein couples the “downhill diffusion” to the uphill transport of a second substance against its concentration gradient -​ 2 Types of cotransport: - Symport: cotransport of both substances in the same direction - Antiport: cotransport of the two substances in two different directions - (Uniport: only one substance is transported via facilitated diffusion through a channel or via a carrier protein) Secondary active Transport: Co-Transport: Bulk transport -​ Large molecules such as proteins & polysaccharides, large particles & even whole bacteria/viruses can be transported across the cell membrane in bulk using vesicles → requires energy (ATP) → = active transport -​ Exocytosis - Secretion of substance via a fusion of vesicles with the plasma membrane -​ Endocytosis - reverse of exocytosis: uptake of molecules and particles in to the cell - 3 types of endocytosis: 1.​ Phagocytosis = “cell eating” → cell engulfs solid particles 2.​ Pinocytosis = “cell drinking” → cell engulfs droplets of extracellular fluid (Note: cell needs the solutes, not the fluid itself) 3.​ Receptor-mediated endocytosis = uptake of specific molecules Exocytosis Endocytosis