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Membranes and Transport – First Topic Internal Membranes and Plasma Membrane -both plant and animal have plasma membrane Plasma (cell) membrane -5-6nm thick, very thin; extremely important Membrane functions -separates intracellular and extracellular -allows different environments to be maintained -important for maintain ion gradients – different concentrations on either outside or inside the cell -involved in signaling – several components required, receptors are embedded in the plasma membrane Membrane composition -made up of two phospholipid bilayers; each layer is called a monolayer -several proteins, glycolipids attached to plasma membrane Plasma membrane -amphipathic phospholipid bilayer – has polar and non-polar portions Phospholipids -have a glycerol backbone, and attached are two fatty acid tails, a phosphate group, and a polar unit; three hydroxyl groups -what type of bond joins the fatty acid tails to the phospholipid structure? Ester bonds Asymmetry of membranes -asymmetric properties are all conducive to different cell functions Membrane formation = spontaneous -due to the polar and non-polar, the phospholipid will decide how it forms itself -the structure arranges itself so that the hydrophobic is away from the water -does not require energy, energetically favorable Membrane repair = spontaneous -if a vesicle leaves the membrane, it does not have a hole, because it is spontaneous Membrane fluidity -always moving -lateral diffusion – move side to side – flexion – fatty acid tails move -rotation – tails can rotate -fluidity is important for exocytosis and endocytosis Fluid mosaic model: membrane is both fluid and mosaic Why fluid? – ​endo and exocytosis, help the cell accomplish what it needs quickly Why mosaic? ​-if you look at the membrane, there are a bunch of things embedded in it Fatty acid tails (non-polar, hydrophobic) ​-two major types: saturated (straight chain), and unsaturated (bend) – have a big role in determining whether more fluid or less fluid

Membrane fluidity -when you have more saturated fatty acids – it is more like a gel (viscous) – can pack closely; therefore, less fluid -when there are more unsaturated fatty acids – it is more like a fluid – less packed Factors affecting membrane fluidity: Increasing Temperature -temperature starts to increase – saturated tails start to separate, but not going to be as much as the unsaturated tails – therefore, unsaturated tails will be more fluid -temperature goes down – which will solidify first? – saturated tails since it is already densely packed Regulation of fluidity -increase fluidity – increase temperature -decrease fluidity – decrease temperature -there has to be an optimum fluidity that has to be maintained Desaturases -helps put the double bond, which allows the bend in unsaturated -when the temperature is lower, the fluidity will go down, when the temperature goes down, it will make more of the desaturases, so that there are more unsaturated tails -if there is too much fluidity, the cell will start to leak Factors influencing membrane fluidity – cholesterol -inserted between phospholipids – it has a hydrophilic end and a hydrophobic tail – cholesterol is not increased/decreased from temperature – acts like buffer – prevents over separation, and from it coming too close together -a yeast cell experiences an increase in temperature. What do you expect to see in its plasma membrane? Increased levels of saturated fat Membrane fluidity Factors that affect fluidity: -saturated vs unsaturated fatty acids -temperature -cholesterol -chain length – long – going to the tangle up, if you have a shorter chain – fluidity increases -which of the following organism will have the highest percentage of unsaturated fatty acid chains in their membranes? Antarctic fish – they live in a cold environment, in order to counter that, they need to increase their unsaturated fatty acid chains Remember, membrane composition -varies -organism, cell, organelle, regions of the same membrane -is complex and dynamic -composition is essential to function Transport across the membrane