UNC-Chapel Hill BIOL 101 - L04: Membrane Structure and Function

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Ciera Ventura 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 Biologists use the fluid mosaic model to describe a membrane s structure diverse protein molecules suspended in a fluid phospholipid bilayer The plasma membrane exhibits selective permeability that is it allows some substances to cross more easily than others 2 Why was the spontaneous formation of membranes such an important step in cell evolution The formation of membrane enclosed collections of molecules would have been a critical step in the evolution of the first cells A membrane can enclose a solution that is different in composition from its surroundings If that solution included self replicating molecules such as RNA and a beneficial assortment of other molecules these combinations could be passed on to daughter cells as new membrane enclosed vesicles budded off A membrane that not only encloses a successful assembly of molecules but also regulates chemical exchanges with the environment is a basic Ciera Ventura requirement for life Indeed all cells are enclosed by a membrane similar in structure and function illustrating the evolutionary unity of life Module 5 3 3 Use the words diffusion and concentration gradient in a sentence Diffusion the tendency for particles of any substance to spread out into the available space moves along 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 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 Both O2 and CO2 are small nonpolar molecules that diffuse easily across the phospholipid bilayer of a membrane What do ions and polar molecules need to diffuse across the hydrophobic interior of a membrane They can if they are moving down their concentration gradients and if they have transport proteins to help them cross 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 Water is less concentrated in the 10 solution because there is a higher solute concentration If the 5 and 10 solution were separated by a semi permeable membrane that sucrose could not move between would the water would move the 10 solution Choose one into out of Check your answer by using Figure 5 4 Module 5 5 7 Define Isotonic referring to a solution that when surrounding a cell causes no net movement of water into or out of the cell Hypertonic referring to a solution that when surrounding a cell will cause the cell to lose Hypotonic referring to a solution that when surrounding a cell will cause the cell to take water up water Ciera Ventura 8 Is seawater hypertonic isotonic or hypotonic to drinking water Seawater is hypertonic to drinking water because seawater has a higher concentration of solute 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 Shrink because the cell could die from the water loss since it is a solution with a higher concentration Module 5 6 10 Define facilitated diffusion How is it similar and different from diffusion The passage of a substance through a specific transport protein across a biological membrane down its concentration gradient Facilitated diffusion does not require energy 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 Facilitated diffusion with the help of proteins in order to cross the membrane 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 Frog 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 The experimental egg cells exploded in 3 minutes The control eggs showed minimal swelling even for time periods exceeding an hour The researchers concluded that the newly discovered protein enabled the rapid movement of water into the cells What did it tell them about the function of aquaporins Aquaporins allow only water molecules to pass through them Ciera Ventura Module 5 8 12 How do calcium ions move against their concentration gradient How is this similar or different from facilitated diffusion Active transport moves calcium against its concentration gradient Module 5 9 11 Compare and contrast exocytosis and endocytosis A cell uses exocytosis to export bulky materials such as proteins or polysaccharides Endocytosis is a transport process through which a cell takes in large molecules or droplets of fluid Endocytosis and exocytosis are both forms of Active Transport both using energy to transport particles in and out of the cell They both transport materials across the cell membrane by forming vesicle pores Endocytosis involves cells taking in substances from outside the cell by engulfing them in a vesicle derived from the cell membrane Exocytosis is where cells shift materials from inside the cell to the extracellular space What are the distinctions between phagocytosis and receptor mediated endocytosis Phagocytosis or cellular eating is when a cell engulfs a particle by wrapping extensions called pseudopodia around it and packaging it within a membrane enclosed sac called a vacuole The vacuole then fuses with a lysosome whose hydrolytic enzymes digest the

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