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Plasma Membrane (PM) bilayer

Plasma Membrane (PM) bilayer. Function & characteristics Composition Spontaneous closure Asymmetry Fluidity. What features make lipid bilayer an advantageous container?. Permeability -> nutrients, waste, membrane potential Deformability ->movement, division

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Plasma Membrane (PM) bilayer

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  1. Plasma Membrane (PM) bilayer • Function & characteristics • Composition • Spontaneous closure • Asymmetry • Fluidity

  2. What features make lipid bilayer an advantageous container? • Permeability -> nutrients, waste, membrane potential • Deformability ->movement, division • Fluidity ->reactions, subdomain assembly • Asymmetry ->specialization of each face 5nm What accounts for these features?

  3. Properties arise from composition • Fundamental structure and function of all cell membranes depends on lipids (phospholipids, steroid derivatives) • Specific function of each membrane depends on the membrane proteins that are present in that specific membrane • Membrane lipids and proteins may be glycosylated Lipid/protein ratios weight 50:50 # 50:1

  4. Phosphoglycerides (phospholipid): most abundant type think: glycerol, 2 acyl chains, phosphate, polar head group Sphingolipids: may be a phospholipid e.g. sphingomyelin may be a glycolipid key is sphingosine (no glycerol) Cholesterol: polar OH steroid back bone acyl chain 3 main types

  5. 5.3 Phospholipid structure Figure 5-27a Figure 5-28

  6. Sphingolipids (including glycolipids)

  7. One type is phospholipidand sphingolipid

  8. Cholesterol, polar steroid with acyl chain

  9. 5.3 Due to the amphipathic nature of phospholipids, these molecules spontaneously assemble to form closed bilayers Figure 5-30

  10. Experimental formation Of pure phospholipid bilayers

  11. 5.3 Each closed compartment has two faces The two faces of a membrane are asymmetric in terms of lipid and protein composition Proteins do not flip flop! Lipids may with aid of flipase Figure 5-31

  12. Phospholipases demonstrate asymmetry Outside cellPLC releases choline, (SM) Inside cell PLC releases inositol, ethanolamine, serine => exo: PC, SM cyto:PS, PE, PI

  13. 5.3 Lipids and integral proteins demonstrate (rotational &) lateral mobility in biomembranes “The Fluid Mosaic Model” Figure 5-35 • Mobility (diffusion) of a given membrane • components depends on: • the size of the molecule • its interactions with other molecules • temperature • lipid composition (tails, cholesterol) • Mobility can be measured by “FRAP”

  14. 5.3 Fluorescence recovery after photobleaching (FRAP) Figure 5-36

  15. Fluidity depends on temperature and composition • Temp: phase transition • Composition: acyl chain length & saturation, cholesterol • Short or kinks=>fewer van der Waals interactions • Cholesterol has opposing effects & is tightly regulated: • Polar head group restricts phospholipid head group movement-> decreases fluidity • Planar steroid separates phospholipid acyl tails->increases fluidity

  16. 5.3 Functions of the plasma membrane • Regulate transport of nutrients into the cell • Regulate transport of waste out of the cell • Maintain “proper” chemical conditions in the cell • Provide a site for chemical reactions not likely to occur in an aqueous environment • Detect signals in the extracellular environment • Interact with other cells or the extracellular matrix (in multicellular organisms)

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