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Material Type: Notes; Professor: Walker; Class: Eukaryotic Cell &Devlp Bio/Hon; Subject: Biology; University: University of New Hampshire-Main Campus; Term: Spring 2010;
Typology: Study notes
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Membranes: Organizing the Second Solvent System Without membranes, there would be no cells. All cells are surrounded by membranes. Membranes can be internal as well as external – in plants there are 17-20 different membrane systems that are all distinctly different from each other. The cell produces membranes; they are all distinct from one another and go to the correct places to perform their various functions. Membranes are formed from a bilayer of lipids and the two layers are distinct from one another (can cause cell death if layers get reversed or mixed up with each other). Plasma Membrane An important boundary between the inside and outside of the cell (NOT the outermost boundary – ECM) Acts as a ‘gatekeeper’ between the internal and external environments; keeps molecules in and out, lets certain molecules pass through Lipid soluble molecules pass easily through membranes, water does as well even though membranes are mostly hydrophobic Water soluble molecules pass through membranes with more difficulty Internal Membranes Compartmentalize cells so specialized functions are organized at certain locations Allow cells to organize biochemical reactions in a sequential fashion; increases the efficiency of a cell Organize the second solvent system o 1 st^ solvent system = water (most numerous molecule in cells); associated with water soluble molecules (hydrophilic molecules) o 2 nd^ solvent system = lipids; associated with lipid soluble molecules (hydrophobic molecules); membrane lipids are the largest part of this solvent system, though lipids exist elsewhere in cells o Molecules associate with one or the other of the solvent systems General Characteristics All true membranes are lipoic (made from lipids) Most contain proteins They are held together by some of the weakest bonds in biological systems – hydrophobic interactions Water must be present in order to have a functioning membrane In the 1950s the existence of membranes was proven with the invention of the electron microscope (membranes are too thin to be seen with a light microscope) In 1972 the structure of membranes was discovered Structure Made from amphipathic molecules (hydrophobic tail and hydrophilic head) Hydrophilic portion has electrical charge, hydrophobic portion is neutral Phospholipids are hydrophobic fatty acid tails connected to a phosphorylated glycerol molecule that is often connected to another amino acid or charged molecule (serine, choline, ethanolamine) Glycerol and fatty acid tails can also be attached to sugars Sphingolipids are fatty acid tails connected to sphingosine molecules that are attached to sugars; are hooked together by the amino acid serine
Components of membranes can function as part of the cell signaling system with the cleavage of their fatty acid tails Fatty acid tails can contain double bonds – the more double bonds present, the more kinky the tails are and this affects the fluidity of the membrane Membranes also contain cholesterol – a mostly hydrophobic molecule that alters the fluidity of the membrane (up to a certain point makes it more fluid, then begins to make it more rigid) and is the source of steroid hormones Conformation Gel phase – lower temperatures; hydrocarbons are tightly packed Fluid phase – higher temperatures; bilayer ‘melts’ and more movement is allowed Never present as flat sheets because the ends being exposed to water is unfavorable; must form closed bags (in theory, perfect spheres, but this is not the case in many biological circumstances) Free phospholipids form ring-like structures called micelles In 30-40 minutes a cell can completely alter the character of the membrane; it is constantly recycling o Lipids rotate, flex, bob up and down, move laterally, and can flip from one layer to the other (can cause problems since the layers are distinct) o Phospholipid translocase (Flippase) – moves lipids from one layer to the other; corrects mistakes Assembly Built in the ER – fatty acids are inserted into the outer layer of the ER and attached to glycerol, phosphorylated, and maybe groups are also attached As the outer layer grows, flippase moves some phospholipids to the inner layer Proteins Integral – dissolved in the membrane. Large regions of hydrophobic (alpha helices) molecules next to large regions of hydrophilic (beta pleated sheets) molecules; weave in and out of the membrane (usually). Some beta pleated sheets violate this law and go completely through the membrane; important in molecular transport into and out of cell Peripheral – loosely associated with membranes Many are glycosylated with sugar trees o Dolichol , a lipid molecule, carries sugar trees and will transfer its tree to a protein when it comes across a certain amino acid sequence (Asparagine – X – Serine or Threonine) Number of proteins in membranes varies – this affects the function of the membrane Also move throughout membrane but more slowly because they are bigger Lipid Rafts Mostly made of sphingolipids and cholesterol; stick together and move together throughout membranes; like icebergs; signaling and cell receptors are associated with lipid rafts.