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Holly Smith

An overview of the structure of membranes - Plasma membrane acts as a selectively permeable barrier that allows for the uptake of key nutrients and elimination of waste products while maintaining the protected environment for cellular processes to occur. A membrane consists of proteins in a fluid of lipid molecules - Fluid mosaic model is a model that shows that membranes are not ridged with molecules locked into place but rather consist of proteins within a mixture of lipid molecules - Lipid molecules exist in the bilayer [less than 10nm thick] - Lipids in the membrane are very dynamic because they exchange places, vibrate, flex back and forth, move sideways and spin around their long axis. - The mosaic part of the fluid mosaic model refers to the fact that most membranes contain an assortment of different types of proteins [proteins involved in transport and attachment, signal transduction and processes such as electron transport]- proteins move slower because they are larger than lipid molecules - The proportions of lipid and protein within a membrane depends on the type of membrane - An important characteristic of membranes is that the proteins and other components of one half of the lipid bilayer are different from those that make up the other half of the bilayer. [Membrane asymmetry- reflects differences in the functions performed by each side of the membrane] Experimental evidence in support of the fluid mosaic model - fluid mosaic model of membrane structure is supported by two major pieces of experimental evidence: 1. Membranes are fluid: Based on the measured rates at which molecules mix in biological membranes, the membrane bilayer appears to be about as fluid as olive oil or light machine oil [experiment conducted by David Frye and Michael A. Edidin- grew human cells and mouse cells separately and were able to tag the human or mouse membrane proteins with dye molecules, then they fused the two types of cells together and observed that they began to fuse] 2. Membrane asymmetry: An important technique used is the freeze-fracture technique [in this technique a block of cells is rapidly frozen by dipping it in liquid nitrogen, then the block is fractured- usually the fracture splits the bilayer into inner and outer halves] Based on these images it is clear that the particles on either side of the membrane differ in size, number, and shape. The lipid fabric of a membrane - The term lipid refers to a diverse group of water-insoluble molecules that includes fats; phospholipids, which are the dominant lipids in membranes; and steroids. [it is important to keep the membrane in a fluid state] Phospholipids are the dominant lipids in membranes - The bilayer is formed of phospholipids [Each phospholipid is formed of a head group attached to two long chains of carbon and hydrogen (hydrocarbon) called a fatty acid, the head group consists of glycerol linked to one of several types of alcohols or amino acids by a phosphate group] - Phospholipids are amphipathic- the molecule contains a region that is hydrophobic and a region that is hydrophilic [Polar molecules tend to be hydrophilic and nonpolar molecules tend to be hydrophobic] - When added into an aqueous solution, phospholipids associate with each other and form into a bilayer spontaneously because it represents the lowest energy state- they form bilayers because of the hydrophobic effect. [Most stable] - Phospholipids can differ in the degree of unsaturation of their fatty acids. Fatty acid composition and temperature affect membrane fluidity - the fluidity of the lipid bilayer is dependent on how densely the individual lipid molecules can pack together. This is influenced by 2 factors 1. Composition of the lipid molecules- more unsaturated fats on the tails results in a more fluid membrane 2. Temperature- the lower the temperature the more densely the molecules are packed (less movement) Organisms can adjust fatty acid composition - exposure to low temperatures may result in membrane viscosity to decrease to the point where normal membrane permeability is inhibited - if the membrane solidifies, electron transport ceases to operate - membranes may become too fluid and liquid due to the increase in molecular motion, which can result in membrane leakage, irreversible disruption of cellular ion balance can lead to death - most organisms can adjust the fatty acid composition of their membranes such that proper fluidity is maintained over a relatively broad range of temperatures - desaturases are enzymes that produce unsaturated fatty acids through fatty acid synthesis. Act on saturated fatty acids by catalyzing a reaction that removes two hydrogen atoms from neighboring carbon atoms and introduces a double bond - the abundance of desaturases increases are the temperature is lowered to maintain membrane fluidity - sterols act as membrane buffers: at high temperatures, they help restrain the movement of lipid molecules, thus reducing the fluidity of the membrane Membrane proteins The key functions of membrane proteins 1. Transport: of substances into and out of the cell through channels, or transport proteins 2. Enzymatic activity: enzyme membrane proteins such as those associated with the electron transport chain 3. Signal transduction: receptors that bind to signal molecules 4. Attachment/recognition: attachment points for a range of cytoskeleton elements, as well as components involved in cell to cell recognition Integral membrane proteins - Integral membrane proteins are proteins that are embedded in the phospholipid bilayer, most are trans membrane proteins, spanning the entire membrane bilayer - Composed of both hydrophobic (nonpolar) and hydrophilic (polar) domains to interact with the bilayer and the external and internal aqueous environments - Generally 17-20 amino acids long to span the entire bilayer, predominantly nonpolar, and often coiled into alpha helices Peripheral membrane proteins - Peripheral membrane proteins are on the surface of the membrane and thus do not inte
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