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Ch 7 Membrane Structure and Function

Ch 7 Membrane Structure and Function. Cellular membrane -overall functions. - Boundary between intracellular compartments, living cells, and abiotic environment Selectively permeable Some molecules cross membranes more readily than others. Membrane Structure.

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Ch 7 Membrane Structure and Function

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  1. Ch 7 Membrane Structure and Function

  2. Cellular membrane -overall functions - Boundary between intracellular compartments, living cells, and abiotic environment • Selectively permeable • Some molecules cross membranes more readily than others

  3. Membrane Structure Predominant constituent: phospholipids • Amphipathic molecules: hydrophobic AND hydrophilic Dispersed protein components Membrane organization and properties described by: Fluid Mosaic Model Singer and Nicolson 1972

  4. Organization of membrane phospholipids LE 7-2 WATER Hydrophilic head Hydrophobic tail WATER

  5. Mosaic: something made of small pieces

  6. Mosaic: Proteins dispersed among phospholipids in membrane: LE 7-3 Hydrophilic region of protein Phospholipid bilayer Hydrophobic region of protein

  7. Freeze-fracture studies of the plasma membrane • Frozen membrane split along the middle of the phospholipid bilayer using a knife • Imaged by EM Supports mosaic part of model

  8. Extracellular layer LE 7-4 Proteins Knife Plasma membrane Cytoplasmic layer Extracellular layer Cytoplasmic layer

  9. The Fluidity of Membranes • Phospholipids move laterally within the bilayer • Some membrane proteins also drift laterally • Rarely does a phospholipid flip-flop transversely across the membrane

  10. LE 7-5a Lateral movement (~107 times per second) Flip-flop (~ once per month) Movement of phospholipids

  11. Effects of Temperature on membranes • Cool temp: membranes switch from fluid to more solid state • Solidification depends on type of lipid • What property of lipids would favor liquid versus solid state?

  12. Degree of saturation of fatty acid tails LE 7-5b Viscous Fluid Saturated hydro- carbon tails Unsaturated hydrocarbon tails with kinks Membrane fluidity

  13. Steroid cholesterol also component of membranes • Tends to moderate effects of temp. on membrane state • At warm temperatures (such as 37°C), restrains movement of phospholipids • At cool temperatures, maintains fluidity by preventing tight packing

  14. LE 7-5c Cholesterol Cholesterol within the animal cell membrane

  15. Movement of membrane proteins Can drift within the bilayer • Proteins much larger than lipids--> move more slowly • Cell fusion studies support fluidity of membrane proteins

  16. LE 7-6 Membrane proteins Mouse cell Mixed proteins after 1 hour Human cell Hybrid cell

  17. Membrane Proteins and Their Functions • Proteins determine most of the membrane’s specific functions • Peripheral membrane proteins • not embedded • attached to extracellular or cytoplasmic surface • Integral membrane proteins • penetrate the hydrophobic core of bilayer • often span the membrane

  18. Fibers of extracellular matrix (ECM) LE 7-7 Glycoprotein Carbohydrate Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Cholesterol Peripheral proteins Microfilaments of cytoskeleton Integral protein CYTOPLASMIC SIDE OF MEMBRANE

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