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(15) Oligosaccharides (glycoproteins) (a) Many eukaryotic membrane proteins are glycoproteins, proteins to which carbohydrate molecules of intermediate length (oligosaccharides) have been covalently attached (b) The attached oligosaccharides are always found on the extracellular side of the membrane (c) See Figure 8.5, The detailed structure of an animal cell’s plasma membrane, in cross section (d) The extracellular placement of oligosaccharides on membrane proteins makes intuitive sense since the oligosaccharides are added to these proteins within the lumen of the endomembrane system (e) See Figure 8.7, Sidedness of the plasma membrane (f) Oligosaccharides play important roles in cell-cell recognition (i.e., oligosacherides of specific monomer sequence and branching pattern are recognized by other cells) (g) [membrane oligosaccharide or carbohydrate or sugar or oligosaccharides or carbohydrates (Google Search)] [index] (16) Selective permeability (a) Lipid bilayers display selective permeability (b) In general, intact lipid bilayers are permeable to: (i) Hydrophobic molecules (including many gasses) (ii) Small, not-ionized molecules (e.g., H2O, CO2) (c) Simultaneously, lipid bilyaers are NOT permeable to: (i) Larger, polar molecules (e.g., sugars) (ii) Ions, regardless of size (d) Thus, lipid bilayers are selectively permeable barriers that allow the entry of small or hydrophobic molecules while blocking the entry of larger polar or even small charged substances (e) [selective permeability (Google Search)] [index] (17) Transport across membranes (a) Given the selective permeability of lipid bilayers, a number of mechanisms exist by which substances are moved across lipid bilayers (movement across membranes is important, for instance as a means of removing wastes from a cell or bringing food into a cell) (b) Categories of substance transport across membranes include: (i) Passive transport (ii) Facilitated diffusion (iii) Active transport (including cotransport) (c) Endocytosis, phagocytosis, and exocytosis, also considered below, technically are not mechanisms of movement of substances across lipid bilayers (though these do represent movements of substances into and out of cells; to be movement across the euakaryotic cell membrane, a substance must actually pass through an endomembrane lipid bilayer) (d) Note that in considering transport across membranes we will once again confront the concept of movement away from or towards equilibrium, i.e., endergonic and exergonic processes (e) There are three basic types of movement across membranes: simple diffusion, passive transport, and active transport
Approximate Word count = 368
Approximate Pages = 1.5
(250 words per page double spaced)
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