ASIM'S WEBWORLD View: Main School Main AP Biology Chapter Guides RELATED LINKS Chapter Outline		AP BIOLOGY: Chapter Six Review Answers 1. Biological membranes are composed of phospholipids. They have two fatty acids and an organic phosphorylated alcohol; they also have both polar and nonpolar ends. Typical fats have three fatty acids and are nonpolar. 2. Phospholipid molecules spontaneously form lipid bilayers when placed in water. The polar heads like water and face it, forming hydrogen bonds. The nonpolar fatty acid tails move away from water and are sheltered from it on the other side. Water can't easily move through the solid sheet of nonpolar tails. 3. Increasing the number of double bonds between carbons in phospholipid fatty acid chains would increase the fluidity of the membrane. 4. Proteins typically span cell membranes as multi-pass helices which form protein channels, or as beta-barrels, creating pores in the membrane. 5. Diffusion is the movement of molecules from an area of high concentration to an area of low concentration down a concentration gradient. Without any checks, diffusion ultimately leads to an even distribution of molecules (such as ink molecules in a beaker of water). 6. Osmosis is the diffusion of water through a selectively permeable membrane. A selectively permeable membrane is required for osmosis to occur: a membrane through which water can diffuse freely but other materials cannot. 7. A cell hyperosmotic to its environment would have a higher concentration of solutes in its cytoplasm than the surrounding environment. A cell hypoosmotic to its environment would have a lower concentration of solutes in its cytoplasm than the surrounding environment. 8. Both are forms of endocytosis, or taking extracellular materials into the cell. Phagocytosis employs the extension of pseudopods to take in comparatively large particulate material (such as a pathogen by a white blood cell); pinocytosis occurs when the cell invaginates to take in a volume of fluid containing desired microparticulate or molecular material. 9. In receptor-mediated endocytosis, a coated pit in the membrane contains a protein receptor. When triggered by the presence of a specific molecule, the pit closes over and traps the molecule within a vesicle. 10. Ion concentration on either side of and voltage across a membrane will determine direction of net movement of ions through a channel. 11. Facilitated diffusion differs from simple diffusion in that specific carrier molecules are employed for movement across a membrane, and a saturation limit can be reached when no further diffusion can occur because all carrier molecules are in use. 12. Active transport is different from facilitated diffusion in that it is an energy-requiring (ATP-using) process capable of moving molecules against their concentration gradients, but similar in that it involves the use of carrier proteins. 13. The Na+-K+ pump moves three sodium molecules out of and two potassium molecules into the cell per cycle and is powered by ATP. 14. Sodium moves sugars or amino acids into cells by cotransport when it is moving into cells. In countertransport, calcium and sodium ions move out of cells when sodium moves in. 15. Protons pumped actively through transmembrane protein channels called proton pumps are used in a process called chemiosmosis to generate ATP.