Bio notes (1.3, 1.4, 2.1, 2.2).docx

Bio notes (1.3, 1.4, 2.1, 2.2).docx - 1.3 Membrane...

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1.3 Membrane Structure Essential Idea: The structure of biological membranes makes them fluid and dynamic Function of the plasma membrane o Holds the cell together o Controls what goes in and out (diffusion, osmosis, active transport) o Protect the cell o Cell signaling and immunity o Binds to other cells o A site for biochemical reactions (enzymes) Phospholipids form bilayers in water due to amphipathic properties of phospholipid molecules Oil molecules are hydrophobic and are non-polar Water molecules are polar Phospholipid molecules are polar (charged) o The head is hydrophilic o Tails are hydrophobic An emergent property is that phospholipids will self-organize to keep their heads ’wet’ and their tails ‘dry’ The phospholipid bilayer is selectively permeable (control the entry/exit of molecules) Some molecules pass through easily (diffusion), or go through a tunnel (facilitated diffusion) Others need energy to get them through ( active transport ) Large molecules use their membrane to get them through ( endo/exo-cytosis ) Polar heads : attracted to other polar ( charged) molecules Non-polar tails : will repel any charged molecule , therefore preventing passage of ions through the membrane Analysis of the falsification of the Davson-Danielli model that led to the Singer-Nicholson model Our current model of the cell membrane is called the Single-Nicholson fluid mosaic model o Key features Phospholipid molecules form a bilayer- they’re fluid and move laterally Peripheral proteins are bound to either the inner or outer surface of the membrane Integral proteins permeate the surface of the membrane The membrane is a fluid mosaic of phospholipids and proteins Proteins can move laterally long membrane The Davson-Danielli model o A protein-lipid sandwich o Lipid bilayer composed if phospholipids o Proteins coat the outer surface o Proteins do not permeate the lipid bilayer
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Experimental evidence showed that this model had major flaws Membrane proteins are diverse in terms of structure, position in the membrane, and function Proteins o Integral proteins are permanently embedded, many go all the way through and are polytopic o Peripheral proteins usually have a temporary association with the membrane, they can be monotopic or attach to the surface Glycoproteins: o Are proteins with few sugar chains attached (oligosaccharide) o They’re important for cell recognition by the immune system as hormone receptors Protein Functions o Cell adhesion molecules to make tight connections to another one o Channel proteins: allow to help ions and large molecules to pass through the membrane by facilitated diffusion o Protein pumps move ions across the membrane to create and maintain concentration gradients They require energy (ATP) to carry out this active transport o Hormone binding sites (hormone receptors) bind to specific hormones and start signaling processes to change the behavior of the cell o
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