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weber uiuc edu 10 September 2007 MCB150 Lecture 08 Lecture 08 10 September 2007 Announcements More broken hand notes LON CAPA don t forget to log out Stuff won t show up when new content is posted Weekly Review 2 is due Friday Second Lipid Class Phospholipids The polar head groups can vary widely in their structure The tails the hydrophobic tails can also vary o In phospholipids the two tails will be called R1 and R2 They can vary o One could be saturated the other could be unsaturated Happens frequently o One could be longer than the other though typically they are the same length Includes length and number of carbons He thinks that in biology that R1 R2 o Cells can make changes to the tails on the fly to make necessary changes to the tails they can make them saturated etc They can introduce a new tail o This is termed regulation to adjust based upon the needs of the cell o We will stick with one head group The polar head group won t change on the fly Properties of Biological Membranes Contains both hydrophilic and hydrophobic regions they are amphipathic Know the different styles of depicting the phospholipids The hydrophobic region is the non polar tail o Ex soap can remove oils because the hydrophobic end can attract the other hydrophobic regions Lipid molecules will typically be amphipathic species Phospholipid Bilayers Form Spontaneously This information comes from experimentation where the lipids are separated from all other membrane components such as proteins sterols etc Bilayers spontaneously form when purified amphipathic chemicals are present in an aqueous environment o When in water the phospholipids immediately form a bilayer Leaflets 2 leaflets per bi layer o The leaflet is a sheet of lipid o One top and one bottom with the hydrophobic regions contained in the center o The tails in the center exclude water o The result is a sandwich with a hydrophobic core and with polar groups on the outside Page 1 of 6 weber uiuc edu 10 September 2007 MCB150 Lecture 08 These do not remain flat if sufficient material is present but will spontaneously form into spherical shapes o Around the edges water can still interact with the hydrophobic tails in the planar structure Not energetically favorable o The sheet will begin to curl so that all of the outer surfaces are touching each other o The spherical shape is the most energetically favorable arrangements of the leaflets o The hollow sphere is called a liposome Liposomes are believed to be the precursors of cells Since they spontaneously form all it takes is a sufficient quantity of lipids in an aqueous environment Biological cells are not liposomes o There are other kinds of membranes and cells have proteins and other molecules embedded in those membranes o But in order to understand how lipids behave we start with liposomes The important thing to remember is that the polar groups are always exposed to the outside the aqueous regions of the environment either external or internal to the cell Planar Bilayers If you want to know what happens on the inside of a liposome need to make planar bi layers so we can control what happens on each side o Can ask what s different on each side How does this behave Use a glass plate to separate two sections of water o Leave a small hole so the lipid can move into the space o Some lipids will spontaneously form liposomes o But many will find the hole and will form a flat bilayer naturally o Now the only separation between the compartments is the stable lipid bilayer In the space filling model the polar regions are on the outside blue water gold polar head with the non polar regions in the inside red lipid There is space in the tails of the lipids that permit the inclusion of smaller molecules o Some molecules can move across via these holes o Also consider that these are not static structures The empty space oscillates wildly Third Lipid Class Steroids Recall first two are triglycerides phospholipids Steroids not all steroids are used exclusively as signals Cholesterol found in animals If you see animal in a question it cholesterol o Cholesterol helps manage and regulate the stability of a biological membrane helping to balance fluidity and stability o Plants etc will have other sterols Fungi for instance have ergosterol These are distant cousins of cholesterol o Bacteria do not use sterols in their membranes Page 2 of 6 weber uiuc edu 10 September 2007 MCB150 Lecture 08 Cholesterol is also amphipathic o A four member ring structure with a hydrocarbon tail o It has a polar head group one hydroxyl o Generalization cholesterol is almost always found near unsaturated tails It is a smaller chunkier molecule that can fit between the phospholipid tails Almost always find them near the unsaturated tails Why Because the kink in an unsaturated tail makes some space for the sterol With steroids we now have enough to build up membranes Other Membrane Lipids Glycolipids Exam Know this and what they do Know the difference between a glycolipid and a glycoprotein Glyco means sugar in anything This molecule structurally speaking looks like a phospholipids but is not o It does not have a phosphate group o The 3 carbon backbone is serine It is similar to sphingomyelin but that is actually a phospholipids o No phosphate groups are on glycolipids Can add one or more sugar groups to this molecule Usually find oligosaccharides from 4 a dozen in a chain or tree covalently linked at their root to this lipid molecule o Branching is OK o They are signaling molecules o They are only exposed to the outside world They don t face into the cytoplasm They will have oligosaccharides on the head group facing the exterior of the cell o They are NOT found on the cytosolic cytoplasmic face The cytosolic leaflet o Instead they are found on the extracellular leaflet is the outside o Oligosaccharides are found here and are used as signals to other cells among other things o Always always always expect to find the oligosaccharides on the extracellular leaflet These are NOT phospholipids Biological Membranes are Asymmetrical Look up the distribution of components of the bi layers in the diagram The general rule is that a given molecule used in the lipid is distributed differently depending upon the leaflet extracellular or cytosolic orientation o That is it is highly unlikely to have a 50 50 distribution of a phospholipid on inner and outer leaflets Composition is different depending upon the membrane Must reanalyze the distribution for


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