Chapter 6 (ii) –Cell Membranes, and TransportI. How Molecules Move across Lipid Bilayers?  Diffusion and Osmosis  Passive transporta. Diffusion: a spontaneous movement of molecules from high to low concentrationi. Molecules and ions have thermal energy and are in constant motionii. This produces a net movement from regions of high concentration to regions of low concentration. iii. The concentration gradient is a difference in solute concentrationsiv. At equilibrium, molecules and ions are randomly distributed with equal concentrations everywhere (they continue to move, but there is no net movement in any one direction)v. Diffusion is a spontaneous process because it increases entropy (second law of thermodynamics)b. Osmosis: a diffusion of wateri. Osmosis occurs whenever two solutions are separated by a membrane that is permeable to water but not to the solutesii. Water molecules show a net movement from high to low water concentration – that is,from low to high solute concentrationiii. Osmosis can cause shrinking or swelling of membrane-bound vesicles (liposomes or cells)1. Hypertonic solution (higher solute concentration outside than inside): water moves out, vesicle/cell shrinks2. Hypotonic solution (lower solute concentration outside than inside): water moves in, vesicle/cell expands and may burst3. Isotonic solution (same solute concentration outside and inside): no net movement of water, vesicle does not changeII. Membrane Proteinsa. What are membrane proteins like?i. Membrane proteins are amphipathic, with bothnonpolar and polar amino acids1. Polar amino acids “stick out” into the aqueoussolution on either side2. Nonpolar amino acids are stabilized in theinterior of the bilayer and keep the protein in the membraneii. Secondary and tertiary protein structures can form a channel or poreiii. (1972) The fluid-mosaic model1. Some proteins may span the membrane- protein and lipids side by side with no covalent bond formation non covalent interactions. The model was confirmed by freeze-fracture electron microscopyiv. Current understanding1. The fluid-mosaic model is largely correct2. Some proteins span the membrane entirely (transmembrane proteins), while others are on just one side (peripheral membrane proteins)3. These proteins strongly affect membrane permeabilityb. Transport I: facilitated diffusion via channel proteins  Passive transporti. Ion channels1. Ion channels are proteins that form a pore in the membrane that allows specific ions or molecules to pass passively through2. The ions move in response to the electrochemical gradient – the combined effects of the concentration gradient and electrical (charge) gradient3Example: cystic fibrosis transmembrane conductance regulator (CFTR)-CFTR is now known to be a chloride channelii. Protein structure determines channel selectivity1. A channel protein is selective, allowing only a certain ion or molecule to pass through. Example: aquaporins and wateriii. Movement through ion channels is regulated1. Ion channels are gated channels, meaning that they open or close in response to a particular signal. Example: Na+ channel2. Once a gated channel is open, movement through it is passiveiv. Ion channels allow facilitated diffusion, a form of passive transport1. Facilitated diffusion is passive transport of substances that otherwise could not cross the membrane readily on their own2. Passive transport is transport powered only by diffusion along an electrochemical (concentration and charge) gradientc. Transport II: facilitated diffusion via carrier proteins – Passive transporti. Facilitated diffusion can also occur via carrier proteins, which change shape during the process. Unlike channel proteins, carrier proteins do not form a “pore”ii. The search for a glucose transporter1. It is thought that GLUT-1 changes shape when glucose binds to it, similar to the shape change of a catalytic enzyme. The change in shape propels the glucose to the interiord. Protein transport III: Active transport by pumpsi. In active transport, ions and molecules are moved against their concentration gradient (from low to high concentration)ii. Active transport requires energy input, usually via phosphorylation from ATP1. Example: sodium-potassium pump2. For each ATP used, three Na+ are transported out and two K+ iniii. Secondary active transport (cotransport)1. Protein pumps can set up electrochemical gradients2. The electrochemical gradient can then be used to power the transport of a different molecule against its concentration gradiente. Plasma membranes and the intracellular environmenti. The lipid bilayer and the membrane proteins create an internal cellular environment thatis very different from the extracellular environment. Many factors affect membrane permeability. - 3 types of lipids found in cells?- Structure of membrane Lipids? Plasma membranes are made up of selectively permeable bilayers of phospholipids. Phospholipids are amphipathic lipid molecules – they have hydrophobic and hydrophilic regions.4- Membrane