Membranes: Brief History of Studies on Membrane Structure
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a. first insights of outer layer of cell obtained by __
- What did he know?
- What his reasoning?
- i. knew nonpolar solutes dissolved more readily in nonpolar than polar solvents, and polar solvents had opposite solubility
- 1. reasoning: a substance entering a cell from the medium would have to dissolve in the outer boundary layer
What did Overton test?
- 1. test: placed plant root hairs into hundreds of different solutions containing numerous solutes
- 2. discovery: the more lipid-soluble the solute, the more rapidly it would enter the root hair cells
- 3. conclusion: dissolving power of the outer boundary layer of the cell matched that of a fatty oil
Who came after Overton?
Gorter and Grendel (1925)
What did Gorter and Grendel decide? Do?
- Cell membranes might contain a lipid bilayer
- i. extracted lipid from human RBCs and measured amt. of surface area the lipid covered when spread over water
- 1. all lipids in cell from membrane
- 2. ratio: 1.8 to 1 and 2.2 to 1à 2:1 ratio
Gorter and Grendel conclusion
- i. Conclusion: plasma membrane was lipid bilayer and polar groups of each layer were outward toward the aqueous environment
- 1. this arrangement is thermodynamically favored because polar head interact with water and hydrophobic fatty chains are protected
- a. polar heads face both cytoplasm and plasma
In the 1920s adn 1930s, people said that? Why?
- a. had to be more than just lipids
- i. reasons: lipid solubility not only determining factor to whether or not a substance could penetrate the plasma membrane
- 1. also, surface tensions of membranes were calculated to be much lower than those of pure lipid structures
- a. decrease in surface tension could be explained by protein presence
1935: Who came along?
Davson and Danielli
What did Davson and Danielli propose? Revise?
- i. proposal: plasma membrane was composed of a lipid bilayer that was lined on both its inner and outer surface by a layer of globular proteins
- ii. revision: in addition to outer and inner protein layers, the lipid bilayer was penetrated by protein lined pores, which could provide conduits for polar solutes and ions to enter and exit the cell
- 1. said this to account for selective permeability of membranes
After Davson and Danielli was?
a. 1972: Singer and Nicolson= fluid mosaic model (central dogma)
Singer and Nicholson model
fluid mosaic model
- i. bilayer of a fluid-mosaic membrane is present in a fluid state, and individual lipid molecules can move laterally within the plane of the membrane
- ii. membrane proteins occur as mosaic of discontinuous particles that penetrate the lipid sheet
The fluid mosaic model presents cellular membranes as __ in which components are __ and capable of __ to engage in various types of __ or __.
- dynamic structures
- coming together
- transient or semipermanent interactions
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