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CHAPTER 8 MEMBRANE STRUCTURE AND FUNCTION

CHAPTER 8 MEMBRANE STRUCTURE AND FUNCTION - CHAPTER 8...

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CHAPTER 8 MEMBRANE STRUCTURE AND FUNCTION I. Membrane Structure The plasma membrane is the boundary that separates the living cell from its nonliving surroundings. It makes life possible by its ability to discriminate in its chemical exchanges with the environment. This membrane: -Is about 8 nm thick -Surrounds the cell and controls chemical traffic into and out of the cell -Is selectively permeable; it allows some substances to cross more easily than others -Has a unique structure which determines its function and solubility characteristics A. Present membrane model In 1972, S.J. Singer and G.L. Nicolson proposed the fluid mosaic model which accounted for the amphipathic character of proteins. Amphipathic = Condition where a molecule had both a hydrophilic region and a hydrophobic region. They proposed: - Proteins are individually embedded in the phospholipid bilayer. - Hydrophilic portions of both proteins and phospholipids are maximally exposed to water resulting in a stable membrane structure. - Hydrophobic portions of proteins and phospholipids are in the nonaqueous environment inside the bilayer. - Membrane is a mosaic of proteins bobbing in a fluid bilayer of phospholipids.
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- Evidence from freeze fracture techniques have confirmed that proteins are embedded in the membrane. Using these techniques, biologists can delaminate membranes along the middle of the bilayer. When viewed with an electron microscope, proteins appear to penetrate into the hydrophobic interior of the membrane. B. A membrane is a fluid mosaic of lipids, proteins and carbohydrates 1. The fluid quality of membranes Membranes are held together by hydrophobic interactions, which are weak attractions. - Most membrane lipids and some proteins caan drift laterally within the membrane. Membranes must be fluid to work properly. Solidification may result in permeability changes and enzyme deactivation. -Membranes solidify if the temperature decrreases to a critical point. -Cholesterol, found in plasma membranes of eukaryotes, modulates membrane fluidity by making the membrane: -Less fluid at warmer temperatures (e.g., 337 o C body temperature) by restraining phospholipid movement. -More fluid at lower temperatures by prevennting close packing of phospholipids. -Cells may alter membrane lipid concentratiion in response to changes in temperature. Many cold tolerant plants (e.g., winter wheat) increase the unsaturated phospholipid concentration in autumn, which prevents the plasma membranes from solidifying in winter. 2. Membranes as mosaics of structure and function A membrane is a mosaic of different proteins embedded and dispersed in the phospholipid bilayer. These proteins vary in both structure and function, and they occur in two spatial arrangements: a. Integral proteins are generally transmembrane protein with hydrophobic regions
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that completely span the hydrophobic interior of the membrane.
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