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Lecture 5

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Biology 1002B
Tom Haffie

Lecture 5: Membrane Biology  Membranes can be more than 50% protein  Lots of proteins embedded in the membrane and/or interacting with the membrane  Phospholipid has 2 fatty acid tails: very hydrophobic  Polar head group = very hydrophillic  Fatty acid tails can be saturated or unsaturated  Do they have a complete saturation of hydrogen? Single bonds -> saturated  Or not complete saturation? Double bond -> unsaturated  A phospholipid bilayer can be formed spontaneously -> no energy required to drive the formation  Hydrophobic tails drives the formation of the bilayer -> no biosynthetic process  If you unsaturate the fatty acid tail, it puts a kink in the tail  You can regulate the amount of saturated/unsaturated fatty acids that you have  If unsaturated = the membrane is more fluid at the same temperature Physiological importance of unsaturation  You can regulate the degree to which you have double bonds in a fatty acid  Always made in a fully-saturated form  Desaturase acts on the fatty acid which can introduce carbon-carbon double bonds  If you regulate Desaturase, you regulate the conversion of saturated -> unsaturated  In bacteria, high desaturase transcript when cold -> declines over time when it gets warmer  Maintaining proper fluidity is fundamental for electron transport -> don’t want membranes to be too fluid, or things leak; too rigid is bad too  3 different desaturase genes place double bonds in different positions Membrane Permeability  Oxygen, carbon dioxide and nitrogen move right through the membrane; does no need to be transported  As you increase size and charge, gets more difficult to go through the layer; can’t interact with hydrophobic core  How does other stuff get across?  Proteins transport ions and glucose  Interact with the phospholipid bilayer  Inner part of protein channel is very distinct from the outer part that has to interact with the membrane  Hydrophilic protein surface & channel on one side, hydrophobic on the other  You can identify the primary sequence  It’s a membrane protein because it will have regions where the amino acids tend to be predominantly made up of non-polar amino acids  Usually takes 17-20 amino acids to span the membrane  Alpha helices tend to be the dominant secondary structure of a membrane protein  Because alpha helical structure minimizes the charges of the backbone  Protein is much more easily able to interact with the hydrophobic core Membrane Transport  There’s potential energy when there’s more solutes on one side of the membrane than the other  Concentration gradient  Diffusion is dissipating that concentration gradient  Change in free energy drives diffusion  You can facilitate diffusion: a protein channel where molecules leak from high concentration to low concentra
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