26.4 Phospholipids o CHOCK H CH,OPO OCH,CH,N(CH3)3 Phosphatidylcholine (R and rare usually Phosphatidylcholine possesses a polar"head group"(the positively charged choline and negatively charged phosphate units) and two nonpolar""tails"(the acyl groups) Under certain conditions, such as at the interface of two aqueous phases, phosphatidyl- choline forms what is called a lipid bilayer, as shown in Figure 26.4. Because there are two long-chain acyl groups in each molecule, the most stable assembly has the polar groups solvated by water molecules at the top and bottom surfaces and the lipophilic acyl groups directed toward the interior of the bilayer. Phosphatidylcholine is one of the principal components of cell membranes. These membranes are composed of lipid bilayers analogous to those of Figure 26. 4. Nonpola materials can diffuse through the bilayer from one side to the other relatively easily; polar materials, particularly metal ions such as Na, K, and Ca*+, cannot. The transport of metal ions through a membrane is usually assisted by certain proteins present in the lipid bilayer, which contain a metal ion binding site surrounded by a lipophilic exterior. The metal ion is picked up at one side of the lipid bilayer and delivered at the other, sur- rounded at all times by a polar environment on its passage through the hydrocarbon-like interior of the membrane lonophore antibiotics such as monensin(Section 16. 4)disrupt the normal functioning of cells by facilitating metal ion transport across cell membranes Hydrophilic 88888 Hydrophilic head groups FIGURE 26.4 Cross section of a phospholipid bilayer. Back Forward Main MenuToc Study Guide ToC Student o MHHE Website26.4 Phospholipids 1023 Phosphatidylcholine possesses a polar “head group” (the positively charged choline and negatively charged phosphate units) and two nonpolar “tails” (the acyl groups). Under certain conditions, such as at the interface of two aqueous phases, phosphatidyl￾choline forms what is called a lipid bilayer, as shown in Figure 26.4. Because there are two long-chain acyl groups in each molecule, the most stable assembly has the polar groups solvated by water molecules at the top and bottom surfaces and the lipophilic acyl groups directed toward the interior of the bilayer. Phosphatidylcholine is one of the principal components of cell membranes. These membranes are composed of lipid bilayers analogous to those of Figure 26.4. Nonpolar materials can diffuse through the bilayer from one side to the other relatively easily; polar materials, particularly metal ions such as Na, K, and Ca2, cannot. The transport of metal ions through a membrane is usually assisted by certain proteins present in the lipid bilayer, which contain a metal ion binding site surrounded by a lipophilic exterior. The metal ion is picked up at one side of the lipid bilayer and delivered at the other, sur￾rounded at all times by a polar environment on its passage through the hydrocarbon-like interior of the membrane. Ionophore antibiotics such as monensin (Section 16.4) disrupt the normal functioning of cells by facilitating metal ion transport across cell membranes. H O RCO CH2OPO2  CH2OCR O OCH2CH2N(CH3)3 Phosphatidylcholine (R and R are usually different) Water Water Hydrophilic head groups Hydrophilic head groups Lipophilic tails Lipophilic tails FIGURE 26.4 Cross section of a phospholipid bilayer. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website