L04: Membrane Structure and Function (Guided Reading Questions to do before Mastering Biology) (Reading 5.1-5.9) Reading Objectives • Describe cell membrane structure • Differentiate the kinds of molecules that can freely cross cell membranes vs. those that can’t. • Compare and contrast the different ways in which various substances cross membranes. • Make predictions about water movement during osmosis. Module 5.1-5.2 1. What is meant by the terms “fluid mosaic” and “selective permeability” when describing membranes? Fluid mosaic refers to the membranes structure and how the phospholipids that make up the membrane aren’t pack too tightly making it “fluid” and the many proteins with different functions that also makeup the membrane make it “mosaic.” Selective permeability refers to the membrane function and how it allows some substances to enter the cell more easily than others. 2. Why was the spontaneous formation of membranes such an important step in cell evolution? It was an important step in cell evolution because these membranes enclosed an assembly of molecules and regulated chemical exchanges with the environment, which is a basic requirement for life, including cells. Module 5.3 3. Use the words diffusion and concentration gradient in a sentence. In passive transport diffusion occurs particles diffuse down a concentration gradient 4. How does the structure of the membrane relate to its selective permeability? (Think about the chemical properties of the inner core of the bi-layer.) The structure relates to its selective permeability because the interior of the bilayer is hydrophobic, so in order for molecules to permeate through the membrane it needs to be able to withstand the hydrophobic interior. • Can O2 and CO2 diffuse easily across a membrane? Why or why not? (Review and define the words polar and nonpolar if you need to) O2 and CO2 can diffuse easily across a membrane because they are small non polar molecules and are soluble in the lipid bilayer • What do ions and polar molecules need to diffuse across the hydrophobic interior of a membrane? Ions and polar molecules need transport proteins to diffuse across the hydrophobic interior of a membrane Module 5.4 5. When water moves through a selectively permeable membrane down its concentration gradient this is termed Osmosis 6. If I give you a 5% sucrose solution and a 10% sucrose solution, in which is the free water less concentrated? In the 10% sucrose solution If the 5% and 10% solution were separated by a semi-permeable membrane that sucrose could not move between, the water would move into the 10% solution. (choose one: into/out of) Check your answer by using Figure 5.4 Module 5.5 7. Define: • Isotonic: A solution that has an equal concentration of solutes compared to the cell • Hypertonic: A solution that has a higher concentration of solutes compared to the cell • Hypotonic: A solution that has a lower concentration of solutes compared to the cell8. Is seawater hypertonic, isotonic, or hypotonic to drinking water? Seawater is hypertonic to drinking water as it has more salts or solutes than drinking water 9. Practice: If a cell is put into a solution that is hypertonic to its cytosol, what would happen? (shrink or swell?) Why? Where is the water more concentrated? The cell would shrink since water would be going out of the cell into the cytosol with the high concentration of solutes. The water would be more concentrated in the cytosol. Module 5.6 10. Define facilitated diffusion. How is it similar and different from diffusion? Facilitated diffusion is the diffusion of a specific molecule across the membrane with the help of transport proteins. It is similar to diffusion because both are passive transports, but it is different because the molecules that are diffused during facilitated diffusion are usually hydrophilic molecules/ions that can’t get through the membranes hydrophobic center by themselves In our discussion of how cells use glucose as an energy resource, we will see that cells are continually bringing glucose in from the blood where the concentration is higher. What kind of transport does glucose use? Glucose uses passive transport since the transport is driven by the concentration gradient and does not require energy. Water is a polar molecule, but very small and so it can diffuse slowly into cells. What type of membrane protein would you expect to find many of in a cell that must be highly permeable to water and move water quickly? Aquaporin Module 5.7 11. Scientific Thinking in Module 5.7: Sometimes discovery is accidental. What type of cell did Dr. Agre’s team accidentally discover aquaporins in? They discovered the aquaporins in red blood cells In their work, shown in Figure 5.7 control frog eggs were compared to frog eggs that carried genetic information to make many aquaporins. What result did they get when they put these two groups of eggs in a hypotonic situation (describe Figure 5.7). When they put the two groups of eggs in a hypotonic situation, the RNA-injected eggs constantly grew in volume and eventually ruptured while the control eggs volume stayed the same What did it tell them about the function of aquaporins? It told them that aquaporins function as a channel that allows the rapid movement of water into a cell Module 5.8 12. How do calcium ions move against their concentration gradient? How is this similar or different from facilitated diffusion? Calcium ions move against their concentration gradient by attaching to a transport protein, using ATP as energy to change the proteins shape and being released onto the other side of the membrane. This is similar to facilitated diffusion because it requires the use of a transport protein, but it is different becausefacilitated diffusion is a type of passive transport, so no energy is required while the process of calcium ions moving against their concentration gradient is a type of active transport and uses ATP. Module 5.9 11. Compare and contrast exocytosis and endocytosis. Exocytosis and endocytosis are both processes that transport large molecules across membranes, but exocytosis involves the expansion of the plasma membrane while endocytosis involves the shrinking of the plasma membrane. What are the