Chapter 2 Cell Membranes - Elsevier

[Pages:32]Chapter 2 Cell Membranes

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Figure 2?1 The hydrophobic effect drives rearrangement of lipids, including the formation of bilayers. The driving force of the hydrophobic effect is the tendency of water molecules to maximize their hydrogen bonding between the oxygen and hydrogen atoms. Phospholipids placed in water would potentially disrupt the hydrogen bonding of water clusters. This causes the phospholipids to bury their nonpolar tails by forming micelles, bilayers, or monolayers. Which of the lipid structures is preferred depends on the lipids and the environment. The shape of the molecules (size of the head group and characteristics of the side chains) can determine lipid structure. (A) Molecules that have an overall inverted conical shape, such as detergent molecules, form structures with a positive curvature, such as micelles. (B) Cylindrical-shaped lipid molecules such as some phospholipids preferentially form bilayer structures. (C) Biological membranes combine a large variety of lipid molecular species. The combination of these structures determines the overall shape of the bilayer, and a change in composition or distribution will lead to a change in shape of the bilayer. Similarly a change in shape needs to be accommodated by a change in composition and organization of the lipid core.

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Figure 2?2 The principle of the fluid mosaic model of biological membranes as proposed by Singer and Nicolson. In this model, globular integral membrane proteins are freely mobile within a sea of phospholipids and cholesterol.

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Figure 2?3 Structure of phospholipids. All phospholipids have a polar hydrophilic head group and nonpolar hydrophobic hydrocarbon tails. Glycerophospholipids are characterized by their glycerol backbone. Long carbon chains connected to the first and second carbon of glycerol provide the hydrophobic part of the molecule. The phosphate and additional head group structure provide the hydrophilic portion of the molecule. In sphingomyelin the backbone is sphingosine. A long-chain fatty acid provides the second hydrophobic tail. Note that both phosphatidylcholine and sphingomyelin have a choline-containing polar head group.

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Figure 2?4 Structure of the glycerophospholipids. DPPC, dipalmitoylphosphatidylcholine; POPE, palmitoyl-oleoyl phosphatidylethanolamine; and cholesterol.

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Figure 2?5 Endocytosis and exocytosis. Particles and other entities can be taken up by the cell by an active process called endocytosis. The plasma membrane rearranges its lipids and encloses the particle to be taken up. As a last step the membrane fuses and closes. Lipids in the membrane have to be remodeled to restore the lipid bilayer to its original composition. Examples are resorption processes in the gut, or phagocytosis. Exocytosis is a similar process in the reverse direction. Examples are secretion of enzymes and hormones and release of neurotransmitters.

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Figure 2?6 Repair of an oxidatively damaged phospholipid. Reactive oxygen species (ROS) oxidize unsaturated fatty acid in phospholipids (PL). This changes the polarity of the fatty acyl chain and the phospholipid tilts toward the water phase. Phospholipase A2 recognizes this breach in the structure and hydrolyzes the phospholipid to lysophospholipid (LPL). FAs are activated to acyl coenzyme A (FA-CoA) by acyl-CoA synthetase (ACSL) using ATP. FA-CoA and LPL are used by LPL acyl-CoA acyltransferase (LAT) to form phospholipids, releasing CoA for the next cycle. Lipid-binding entities like acyl-CoA binding domain proteins (ACBD) modulate this process.

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Figure 2?7 Phospholipases hydrolyze phospholipids. The ester bond hydrolyzed by phospholipases determines the nomenclature of these enzymes. Phospholipase A2 (PLA2), phospholipase D (PLD), and phospholipase C (PLC) are shown.

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