BIOB10 Some Relevant Midterm Readings (Highly Detailed)
BIOB10 Some Relevant Midterm Readings (Highly Detailed)

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University of Toronto Scarborough
Biological Sciences
Rene Harrison

Cell Biology, CHAPTER 4 (OCTOBER 2 READINGS) FIG 4.4, OCT2 4.2 BRIEF HISTORY OF STUDIES ON PLASMA MEMBRANE STRUCTURE p119-120 Overton, 1890s - first to investigate chem. nature of plasma memb. KNEW - non-polar solutes dissolved more readily in non-polar solvents vs. polar solvents - polar solutes dissolved more readily in polar solvents vs. non-polar solvents REASONED - substance entering cell has to first dissolve in cell's plasma memb. INVESTIGATION W/ ROOT HAIR CELLS - placed plant root hairs into many diff sol'ns containing diff. types of solute - found that the more lipid-soluble the solute, the more rapidly it enters into root- hair cells HIS CONCLUSION - dissolving ability of plasma memb akin to that of fatty oil p120 (p142) [1] Gorter & Grendel - first to propose that cell memb's may contain lipid bilayer - extracted lipid from human red corpuscles - measured amt of surface A the lipid would cover when spread over water's SA - in mature mammalian red corpuscles, there is no nuclei & cytosolic organelles => only lipid-containing structure in cell is plasma memb. - so, can assume that all of the lipids extracted from these cells were in the cell's plasma memb. - ratio of (SA of H2O covered by extracted lipid) : (SA calculated for red corpuscles from which lipid extracted) ... varied b/ween 1.8-1 and 2.2-1 - speculated that actual ratio is 2:1 - concluded that plasma memb contains [D] Lipid bilayer - Phospholipids self-assembled into a bimolecular structure based on hydrophobic and hydrophilic interactions; biologically important as the core organization of cellular membranes. (4.2) - suggested that polar groups of each molec. layer (= leaflet) facing outward towards (aq) envirnmt => each molec. layer of bilayer = LEAFLET - thermodynamically favourable arrangement b/c lipid's polar head groups can interact w/ surr. H2O molec's => polar head groups face cytoplasm on one edge & blood plasma (extracellular space for red corpuscles) on other - arrived at conclusion that memb's contain LIPID BILAYER. [2] 1920s & 1930s EVIDENCE SHOWS THAT MEMBRANE STRUCTURE IS NOT MERELY A PURE LIPID BILAYER - (ex) lipid solubility is not the only dictating factor as to whether or not substance can go thro. plasma memb. - (ex) memb's surface tension was calculated to be LOWER than that of pure lipid structures >- lower surface tension explained by presence of protein structures in memb. 1935-DAVSON, AND DANIELLI - proposed that plasma memb comprised of: = lipid bilayer that was contained layer of globular proteins on both its inner & outer surface - revised their above model in early 1950s by adding in the fact that: >- in addition to outer & inner protein layers, the lipid bilayer also has protein- lined pores in it - these can provide passageway for polar solutes & ions to enter/exit cell p121 [1] (p143) SINGER & NICOLSON - proposed the [D] FLUID-MOSAIC MODEL =Model presenting membranes as dynamic structures in which both lipids and associated proteins are mobile and capable of moving within the membrane to engage in interactions with other membrane molecules. (4.2) --> FMM - central dogma of memb. bgy since 1972 - this model focuses on phys. state of lipids w/in memb how does this model differ from prev ones? - bilayer of fluid-mosaic memb. present in FLUID state - individual lipid molec's can laterally move w/in plane of memb [2] --> FMM how does this model differ from prev ones? - by structure & arrangemt of memb proteins >- mosaic of discont. particles that penetrate the lipid sheet - cellular memb's are dynamic structures inw hich its constituents are mobile & capable of coming together to do various types of temp. & semipermanent interactions ----------- --> FIGURE 4.4 BRIEF HISTORY OF STRUCTURE OF PLASMA MEMBRANE 4.4(a) -> Davson-Danielli model (1954 revised version) - lipid bilayer >- is lined on both surfaces by 1 layer of proteins which extend thro. out memb to form protein-lined pores 4.4(b) -> Singer-Nicolson's fluid mosaic model (1972) - proteins penetrate lipid bilayer, but only partially embedded in bilayer >- this was diff. from prev. models >- but this is contradictory to what is seen today: protein capable of extending into entire lipid bilayer (ie. able to be embedded all the way thro: from one side of bilayer to other) 4.4(c) currently accepted model - has some basic elements of Singer & Nicolson's model (compare 4.4(b) to 4.4(c)) - most memb. proteins have their external surface containing short chain of sugars => glycoproteins - small % of phospholipids also contain this => glycolipids - those pts of polypep chain that extend thro. lip. bilayer typically made up of alpha helices composed of hydrophobic amino acids - 2 leaflets of bilayer contain diff. types of lipids (notice that there are diff. colours for the head groups) => all phospholipids on one leaflet are not identical - outer leaflet >- may contain microdomains (rafts) = structures comprised of clusters of specific lipid species
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