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6.1 Biological membranes are fluid layers of lipid.
The Phospholipid Bilayer
• Biological membranes are composed of phospholipid bilayers with hydrophilic, polar heads and hydrophobic, nonpolar, fatty acid tails. (p. 106)
• Phospholipid molecules orient their polar heads toward water and their nonpolar tails away from water. (p. 106)
• The nonpolar interior of a lipid bilayer impedes the passage of any water-soluble substances through the bilayer. (p. 106)
The Lipid Bilayer Is Fluid
• A lipid bilayer is stable because water's affinity for hydrogen bonding never stops, and the hydrogen bonding holds the membrane together in a liquid form. (p. 107)

6.2 Proteins embedded within cell membranes determine their character.
The Fluid Mosaic Model
• In the fluid mosaic model, a mosaic of proteins floats in the fluid lipid bilayer. (p. 108)
• Cell membranes are assembled from four components: phospholipid bilayer, transmembrane proteins, interior protein network, and cell surface markers. (pp. 108—109)
Examining Cell Membranes
• Electron microscopes must be used to visualize the structure of a cell membrane. (p. 110)
• Different procedures can be employed for preparing a specimen for viewing, including embedding tissue in a hard matrix and freeze-fracturing a specimen. (p. 110)
Kinds of Membrane Proteins
• Six key classes of membrane proteins include transporters, enzymes, cell surface receptors, cell surface identity markers, cell adhesion proteins, and attachments to the cytoskeleton. (p. 111)
Structure of Membrane Proteins
• Transmembrane proteins are anchored into the bilayer by nonpolar segments. (p. 112)
• Cell membranes contain a variety of different transmembrane proteins, including single-pass anchors, multiple-pass channels and carriers, and pores. (p. 112)

6.3 Passive transport across membranes moves down the concentration gradient.
• Random motion of molecules causes a net movement of substances from regions of high concentration to regions of lower concentration, and this movement continues until all regions exhibit the same concentration. (p. 114)
• Each transport protein in the plasma membrane is selectively permeable, and thus only allows certain molecules to diffuse through. (p. 114)
Facilitated Diffusion
• Facilitated diffusion occurs as molecules move from an area of higher concentration to an area of lower concentration via specific carriers. (p. 115)
• The essential characteristics of facilitated diffusion are specificity, passivity, and saturation. (p. 115)
• During osmosis, water moves across a membrane toward a solution with a higher solute concentration. (p. 116)
• The direction of net diffusion of water across the membrane is determined by the osmotic concentrations of the solutions on either side. (p. 116)
• Organisms have developed many solutions to being hyperosmotic to their environment, including extrusion, isomotic solutions, and turgor. (p. 117)

6.4 Bulk transport utilizes endocytosis.
Bulk Passage Into and Out of the Cell
• Endocytosis occurs when the plasma membrane envelops food particles and brings them into the cell interior. Three major forms of endocytosis are phagocytosis, pinocytosis, and receptor-mediated endocytosis. (pp. 118—119)
• Exocytosis refers to the discharge of materials from vesicles at the cell surface. (p. 119)

6.5 Active transport across membranes requires energy.
Active Transport
• Active transport is the movement of a solute across a membrane against its concentration gradient, requiring the use of protein carriers with the expenditure of ATP. (p. 120)
• Active transport involves highly selective membrane protein carriers. (p. 120)

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