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Lecture 7

BIOL 112 Lecture 7: Lecture 7

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McGill University
Biology (Sci)
BIOL 112
Frieder Schoeck

Lecture 7 The lipid bilayer is extremely flexible. This flexibility is very important because the membrane is not only made up of phospholipids. There are also proteins and they have to move about in the membrane. How can we embed membrane proteins into the membrane? Proteins are amphipathic/amphiphilic (they are both hydrophobic and hydrophilic). The non-polar amino acids are hydrophobic so they are inside of the bilayers. The hydrogen is inside the alpha helix of the amino acids so the lipids do not see them. However, the polar and charged amino acids are hydrophilic so they are on the lipid bilayer membrane on the outside with the rest of the hydrophilic molecules. Therefore, the protein cannot move up or down in the membrane, it can only move sideways. The outside the proteins have different amino acids than on the inside. They cannot change their polarity. A lot of proteins have to communicate with each other. Integral/trans membrane protein: they cross the entire membrane. Peripheral membrane protein is only attached on one side or the other to other membrane proteins or some other lipids. How was it shown that there are so many proteins in the lipid bilayer? Freeze fracture electro microscopy. We freeze a cell and then fracture it with a knife. The two leaflets of the lipid bilayer separate. After the freezing the exteriorof the membrane is very strong because the hydrogen bonds are strong, but the inside of the bilayer is not because they are hydrophobic (they just have a van der waals interaction which is very weak). That is why the two leaflets separate. The membranes: transport function and barrier function. They experimented with a bowl, where glass only left asmall holewhere they put an artificial lipid bilayer WHITOUTany proteins tosee what can pass and what cant. Whatwe learned?Membranes areselectively permeable. Very smallmolecules(gases) andnonpolarcan cross very easly. Water and glycerol are polar, but they pass easly because they are small. Glucose and sucrose cannot pass because they are too large and crossing the hydrophobic interior would be too unfavorable for them. Ions are very large and charger so they can not cross at all. To get movement we need a concentration gradient: a lot of molecules on one side and very little on the other side. Molecules are always moving back and forth if they can. With a bouquet of flowers, the closer you are to the roses, the stronger they will smell. If we throw them way, the concentration gradient will slowlybecome 0becausethereis anettransportof thescentmolecules awayfromthesource.When There Is a concentration gradient, you have a negative Delta G. When the diffusion is over and there is no more concentration gradient, Delta G is 0 Molecules randomly move around (Brownian motion). Brownian motion occurs only when concentration gradient is at 0. When it is not at 0, there is diffusion happening. The deeper the concentration is, the more negative Delta G is, and the more effective the diffusion will be. The smallest the distance, the easiest diffusion will be. Higher temperature means better diffusion. The smaller the molecule, the faster the diffusion At equilibrium, the molecules still move around from one side to another, but there is no net movement just Brownian motion. For net movement, you need a concentration gradient. Osmosis is just the name we give to the diffusion of water. Water moves from regions of HIGH water concentration to regions of LOW water concentration. No difference in how water diffuses and other molecules diffuse. Hypertonic solution: a lot of solute dissolved in the water. Ex: salt water. So water flows out of our cells of our body, so the cells shrink. We do not die when we drink 200g of glucose solution because glucose has a much lower molarity (concentration) than sodium chloride. Glucose molecule are big and bulky so there are fewer glucose molecules in the resulting solution than in the sodium chloride solution. The concentration gradient is much greater when drinking sodium chloride. If we add a lot of glucose to the solution,
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