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Ch 7: Membranes: Their Structure, Function and Chemistry
Terms in this set (23)
What are the main functions of cell membranes?
1. Define boundaries for the cell and organelles. 2. serve as sites for enzymes and receptor proteins. 3. Provide for and regulate transport processes. 4. contain protein receptors to detect external signals. 5. provide mechanisms for cell-to-cell contact, adhesion, and communication
What is signal transduction?
Signal transduction refers to the specific mechanisms used to transmit signals such as nerve impulses from the outer surfaces of cells to the cell interior.
Describe the Fluid Mosaic Model of membrane structure.
The model says that the cell membrane is a 'fluid' lipid bilayer with a 'mosaic' of proteins embedded in or attached to it.
Describe the different types of membrane proteins.
1. Integral 2. Peripheral 3. Lipid-anchored
What are the major types of lipids in the membrane?
1. Phospholipids 2. Glycolipids 3. Sterols
What is thin-layer-chromatography?
In thin layer chromatography, a sample is spotted onto a silica plate, then carried upward by a solvent. The lipids in the sample are separated by polarity: less polar lipids do not adhere to the plate as strongly and move farther up the plate.
Phospholipids are the most abundant type of lipid. There are two types, phosphoglycerides and sphingolipids. Phospholipids are compused of a small polar head (choline, ethanolamine, serine, or inositol) attached to a lipid backbone made up of glycerol of sphingosine.
Glycolipids have an attached carbohydrate group. Most are sphingosine derivatives, or glycosphingolipids. Glycosphingolipids can be either cerebrosides (which has a single uncharged sugar head) or gangliosides (which has a oligosaccaride head with multiple negatively charged residues)
Sterols are bent chains that are necessary for stabilizing membranes. They have important function in membrane fluidity and ion permeability.
What are fatty acids and why are they important in membranes?
Fatty acids are part of all membrane lipids except sterols. They are generally between 16-18 C in length. The long hydrocarbon tail is hydrophobic. The fatty acids in lipds vary in the presence and number of double bonds. Cis fatty acids are bent and contribute to membrane fluidity.
Are membranes symmetrical? Why not?
Lipid layers of membranes are not symmetrical, lipids are distributed unequally between them. This makes sense, because each monolayer has a slightly different function (interacting with outside versus inside). Lipids can move laterally, but cannot 'flip flop' without help of an enzyme.
What are the "fluid" features of the bilayer?
The lipids are able to move laterally very quickly. This can be seen in the technique "fluorescence recovery after photobleaching" (pg 169). There is a specific transition temperature at which every lipid bilayer transitions from gel to fluid.
What affects membrane fluidity?
Lipid makeup is the primary determinant of membrane fluidity, the length of the fatty acid chain and their degree of unsaturation. Sterols also effect membrane fluididty. Cholestorol molecules are riid structures that make the membrane less fluid at higher temperature, but also increases fludiity at temperatures below the Tm.
Describe Integral membrane proteins.
Integral membrane proteins can be monotopic or transmembrane, but most have hydrophobic transmembrane segments. Transmembrane proteins can be single pass, multipass, or have multiple subunits.
Describe peripheral membrane proteins.
Peripheral membrane proteins are bound to the membrane surfaces by weak electrostatc forces and hydrogen bonding.
What are lipid-anchored proteins?
Lipid anchored proteins can be attached covalently to a fatty acid or an isoprenyl group. Many are anchored to GPI, a glycolipid in the outer monolayer.
How are membrane proteins isolated?
Membrane proteins can be extraced by changes in pH or can be solubilized using detergents such as SDS that disrupt hydrophobic interactions and dissolve the lipid bilayer.
How does SDS-PAGE work?
In SDS page, membrane fragments are first solublilized with SDS to give a negative charge. A small amount is loaded onto a polyacrilimide ge, and an electric potential with the positive side at the bottom is applied. The molecules move down the gel and are sorted by size by the gel complex. A western blot can transfer them to a membrane which can allow scientists to quantify and identify polypeptides. Sometimes SDS-Page is done 2Dimentionaly with pH gradient as well.
What are some ways to analyze 3D structure of membrane proteins?
X-Ray crystallography, hydropathic analysis
What are some functions of membrane proteins?
Membrane proteins can be transport proteins, channel proteins, receptors, involved with intercellular communication, have roles in uptake and secretion, provide cellular structure, etc.
Why are many membrane proteins glycosylated?
Glycosylation has a role in cell ell recognition. They are always positioned so that the carboyhydrate groups protrude on the outer surface of the membrane.
What is the experimental evidence for protein mobility/nonmobility?
Membrane proteins vary in their mobility. Cell fusion experiments have been used to provide evidence for protein fludidty. When human and mouse cells were fused, membranes labeled with specific fluorescent antibodyies for human or mouse began to mix with in a few minutes to finally reach random distribution. however, photobleaching experiments of certain proteins have provided evidence for limited mobility.
How is protein mobility restricted?
Proteins can aggregate to form large slow moving clusters. Membrane proteins can frm structures to bar diffusion of other membrane proteins. Proteins might anchor to structures on one side or the other of the cell membrane.
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