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Week 1.docx

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Jane Mitchell

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PART 2: prof MITCHELL Lecture 1: - Slide 6: o Saccharomyces>> budding yeast a single cell euk and it’s an important model sys for many experiments b/c it allows you to investigate a simple type of eukaryotic cell and it’s also important for the amount of genetics that can be done>> genes can be deleted and added quite easily and the effect of those different genes can be studied o Most euk cells look similar with similar organelles o Budding yeast has a cell wall that surrounds it which gives it protection from the external environments since it’s a single celled org it needs a lot of protection from harsh external environments - Slide 7: o Human body has many disctinct cell types>> over 200 types o Ex: skin, lung alveloer cells, apathelial cell >> although they are made up from the same genetic material they are functionally very different and this is b/c of diff genes that are expressed in these cell types o Some cells have more of one type of organelle than the other but they all have similar organelles - Slide 8 o Most cells have the same compliment of diff organelles o Orgnalles specific to animal cells:  Extracellular matrix: secreted by cells and provides the support for cells within tissue environments  Lysosome: digests components that are taken up by the environment and digests damaged organelles within the cell - Slide 9 o Image of a plant leaf cell o Same organelles as animal cells but there are some compartments that are unique to them  Chloroplasts  Cytophotosynthesis  Vacuoles: provides plant cell with rigidity, forces plant cell againt the cell wall  Enzymatic activity or have a storage func where nutrients are used for later use  Cell walls: type of extracellular matrix but a very specific type  Vacuoles and cell walls are found in some single celled orgs - Slide 10 o Membrane structure o Cancer cell that’s getting ready to matasticise o Change in morphology.. surrounding cells are flattened and connected to each other and the cancer cells round up and its membranes lose connections with other cells in the surrounding tissue so what’s going on in the mem that allows this cancer cells to that is very important and there are many important processes that occur in cellular membranes - Slide11: o Cytoplasm: includes liq component of outside the nucleous as well as the other membrance bound organelles o Cytosol: aqueous part of cytoplasm so it doesn’t include membrane bound organelles but does include other protein stuctures like ribosomes and the cytoskeleton o Lumen: inside of orgenells>> major function of membranes is compartmatilization>> how the lumen of an organelle is different froom the cytoplasm so talking about the inside of the organelle - Slide 12: o Many ceulluar functions occur at membranes>> main functions is compartments to form diff compartments in cells>> it allows processes to be specialized and a specific environment to be created in diff compartments that really supports that process o In nucleus where transcription needs to occur and where the genome needs to be replicated specific proteins can be concentrated in the nucleus and that can support those functions o It can also be a scaffold for biochemical activities  Ex: what happens in the mito to produce ATP so the electron transport chain occurs on the membrane scaffold inside the mito o Membranes present a selectively permeable barrier so some molecuels can move across membranes based on the property of the lipids within the membrane and the permeablility can be regulated depend on the lipid composition in the mem o There are specific proteins in the membrane that can transport solutes across the membranes o Plasma membranes the outer membranes of the cell also allows cells to respond to external signals so specialized proteins called receptors in the plasma membrane can bind to external signalling moelcules and b/c they span the membrane they can interact with proteins on the inside of the membrane and cause changes inside cells o Interactions b/w diff cells occur at membranes so other classes of mem proteins can allow molecules to be transferred b/w different cells which allow cells to function in a coordinated manner in tissues - Clicker question: which of the following statements best describe the structure of cell membranes? o D  all lipids in the bilayer have 2 hydrophobic hydrocarbon tails  Cholesterol only has 1 hydrophobic hydrocarbon tail so D is wrong o B  membrane is a fluid bilayer and is composed of 100s of different lipids and proteins  500-1000 different types of lipids that can be found in cell membranes however there are 3 major components types  phospholipids, spingle lipids and glycols. And it’s hard to study the different types of lipids since they’re so complex o A is false b/c there are more than 3 types of lipids o C  bylar of amphilic lipid which is true (polar head groups and the nonpolar tails) but glycosylated on the cytoplasmic leaflet means that they would be glycosylation sticking into the cell which is FALSE b/c glycosylation is on the outside of the cell and provide protection from external environments (glycosylation of lipds are ONLY found on the outside of the plasma membrane facing into the ext environment)  But glycosylations can be found on the inside of some orgenelles - Slide 14: o Basic unit is the lipid that makes of the bilayer and the property of the lipid gives the lipid its function o Lipid bilayer is a fluid structure so it’s not rigid it’s fluid which allows the cell to change shape and there are 2 different leaflets of the byler>> inner and an outer leaflet and together the 2 leaflets make of the lipid bilayer o Also suspended within the bilayer are membrane proteins and they ARE MOBILE in the bilayer so they can move. Lipids can move and the sgtrucure is fluid and proteins can also move within the bilayer  Ex of proteins found within the bilayer:  Receptors  Channels that allows molecules to move across the bilayer o Fluid mosaic model>> if you look at this picture one second later the lipid and the proteins would have moved (MOBILE).. they don’t stay rigid so looking from one part of the membrane would look different from the other b/c they moved (why it’s called mosaic) - Slide 14 o Amphilic: both polar and nonpolar portion o Lipids:  polar head groups (hydrophilic –water loving)  nonpolar tails (Hydrophobic tails- water fearing )  it’s this property that allows them to spontaneously assemble into lipid bilayers>> if you mix lipids with water they will form a bilayer on their own b/c it’s energetically favourable >> why is it favrouable? b/c in that bilaer struc the tails are protected from the polar aq environments and the head groups are allowed to interact with the water  plasma membrane of a cell is round  leaflet::: lipid composition on the inside and outside is different (inner/outer leaflet of the membrane) - Slide 15: o One of the major components of the lipid bilyer is phospholipids:  Means they have a phosphate group in the head portion of the polar head group>> have 2 hydrocarbon tails  In an aq environment they’ll spontaneously self associate into a bilayer>> some lipids have a cone shape will spontaneously assemble into a micel>> it’s a spherical structure WITOHUT A INNER LEAFLET.. so tails all point inward towards eachother and that depends on the type of lipid lipid whether it forms a conical structure or a cylindrical structure which can form a bilayer like phospholipids do when there are 2 hydrocarbon tails o Slide 16:  Example of how a bilayer forms into a spherical type of structure  Bilayer forms into like a carpet:: hydrophobic tails are exposed around the edges  But when it’s folded into a sphere:: non of the hydrophobic inner parts of the membrane are actually exposed  This is why the structure is energetically favoured b/c only the polar part faces the external fluid  Fact that liupids form these artificial bilayer in a test tube is very useful b/c it allows it to study the property of different lipids so you can take one type of lipid and you can form these bilayer and can study the properties of these bilayers>> these are called liposomes (artificial bilayers)  Liposomes are useful in different experiments:  To study lipid properties::: how permeable they are to different molecules  Useful to study the properties of membrane proteins::: can mix mem proteins and lipids and form artificial bilayers in which membrane proteins are mobilized in the lipid bilayer  Allows molecules to be delivered into cells>> drugs might not pass the lipid bilayer very well so if we want to get in a hydrophilic drug into the cell it’s not going to pass through the hydrophobic pore very well but can use a liposome (artificial sphere of lipids) with that drug in the lumen of the liposome to deliver the drug into cells>> liposome would fuse with the plasma membrane and deliver the drug in the lumen into the cell  Another ex of something that can be delivered using a liposome is DNA >> DNA doesn’t’ cross the plasma membrane very effectively good b/c otherwise the genome would leak out of the cell>> so liposomes can be used to deliver DNA into cells and to study the effect of adding genes into the cell and also study what happens when you give that DNA into the cell (to learn what the effect is of a particular gene). - Slide 18 o 2 of the most abundant phospholipids in the bilayer:  Phophatidlcholine: phosphate group and a choline group in the head group  Sphingomyelin  These have phosphate phosphate group in the head group o These phospholipids have 2 hydrovcarbon tails that can VARY IN LENGTH (14-24 carbons long can be saturated or unsaturated) o In reality tails are not straight b/c they are mobile (can rotate)  saturated o Unsaturated tail (fatty acid): kink in the tail  cys double bond in the tail>> introduces an area in the tail that’s NOT flexible (double bond cannot rotate) therefore can’t move - Slide 19: o Another major component of lipid component of cell membranes : Steroids o Animal cells – cholesterol >> essential component of the membrane (need a balance) o Plants cells also have it but it’s plant specific o What cholesterol does when it interacts with the phospholipid membrane is that it decreases the mobility of the phospholipid tails and stiffens them and makes the phospholipid less permeable to small molecules ( changes the property of the bilayer according to how much cholesterol is present) - Slide 20: o Glycolipids are lipids that are glycosylated o Glycosylation is a sugar group attached to the lipid molecule (attached to the head group –polar head region)>> these glycosylation is added in the lumen of the golgi apparatus and they are transferred to the plasma membrane o Glycolipids are maily found in the plasmsa membrane of the outer leaflet of the plasma membrane (not found int the inner leaflet or the cytosolic region>> but are found in SOME organisms)  helps protect the cell from the environment  Ex: cells lining the intestines are heavily glycosylated on the outer leaflets that helps protect the cells from the harsh environment of the intestine that allows for the digestion of food  Some organelles digests cellular components>> also have harsh environments on the inside that’s why there are glycosylations on the inside - Slide 21 : video: o Technique called live cell imaging>> cell that’s being imaged in real time and lazer tweezers are used to manipulate the membrane (puls the membrane away from the cell>> b/c of the fluidity of the cell the membrane will move as the tweezers pull) this fluid property of the membrane is important b/c:  They need to move tight spaces like in capillaries so cell mems can’t be rigid they need to be flexible  Need to grow - Slide 21: o What allows cell membranes to be fluid?  The properites of the lipid that makes up the cell mem that allows it to be fluid o Phospholipids can rapidly diffuse in the membrane and rotate and the tails can flex and the flexible part of the cell can move and can diffuse LATERALLY  Do it WITHIN EACH LEAFLET>> motion is very fast (within a second a phospholipid can move from one end of a bacterial cell to another )  But they don’t move B/W THE LEAFLETS this is b/c the lipid would have to move through the bilayer and the head group would have to go through all the hydrophobic regions of the membrane  energetically NOT FAVOURED so lipids don’t do that on their own but there are special proteins called phospholipid translocators that can catalyze the flip flop of lipids from one leaflet to another but all the other movements like lateral diffusion happens b/c of the normal properties of the cell. - Slide 22: membrane fluidity o Regulating membrane fluidity is important for cells and it’s especially important for single celled organisms that don’t regulate their environments o Forces that affect membrane fluidit:  Temperature: at lower temps lipid bilayers can become a rigid gel , they have a phase transition from a liq to a solid  In higher temps the lipids can move around and the tails are flexible but as temp decreases they become increasingly less flexible and less mobile unil they undergo this phase transition where they form this solid gel like state>> in that state there’s not much movement and there’s also not movement of the tails  The transitions temp at which point lipids do this depends on the types of lipids that are presents  different lipids have different phase transition temperatures  Phospholipid saturation: presence of the cis double bond allows the membrane to be fluid at lower temps>> why? b/c it introduces kink that prevents the lipids from packing up into the tight arrangement  So the unsaturated tails don’t pack up as nicely as the saturated one’s do  How low long the phospholipid tails are:  The shorter tails increase fluidity at lower temps b/c the lipid tails don’t interact as much - Clicker question: the composition of phospholipids in the plasma membrane of 2 different single celled organisms that were isolated from the ocean are examined. Org A was isolated from a water surrounding a thermal vent where the temperature was 80deg. And org B was iscolated from an ocean floor where the temp was 4 deg. When you compare the phospholipid bilayers of the 2 membranes what would you expect to find? o B: org A will have more saturated phospholipids and longer hydrocarbon tails so org B has to maintain fluidity at lower temperatures to survive in the lower temperature environments so it will have more cis double bonds and more unsaturated hydrocarbon tails and it will also have shorter tails - Slide 23: o On their own phospholipids don’t move from one leaflet to another and there are enzyme
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