CHAPTER 5. MEMBRANES THE ESSENTIALS Students need to know: why cell membranes need to be selectively permeable. the components of the cell membrane (phospholipids, proteins, carbohydrates, and cholesterol) and the function of each. in what direction a solute will flow when a gradient exists across a selectively permeable membrane. in what direction water will flow when a cell is placed in a hypotonic, isotonic, or hypertonic solution. the differences between simple diffusion, facilitated diffusion, and active transport as well as cellular examples of each. bulk transport (the differences between endocytosis and exocytosis as well as cellular examples of each). Key Terms diffusion osmosis solvent solutes osmotic hyperotonic (hyperosmotic) hypotonic (hypoosmotic) isotonic (isoosmostic) aquaporins plasma membrane phospholipids phospholipid bilayer selectively permeable ion channels fluid mosaic model transmembrane proteins carriers facilitated diffusion osmotic pressure turgor pressure endocytosis phagocytosis pinocytosis exocytosis active transport sodium-potassium pump Strategy Class Time: The AP Acorn Book recommends devoting 10% of the course to the Cells unit which includes Chapters 4, 5, 9, and 10. Below is a suggested schedule based on a year-long class meeting three 45-minute periods every two days: Lecture 1 (40 45 minutes): Cell Membranes and Diffusion Lab 1 (120 minutes): AP Lab 1. Osmosis and Diffusion (required) Lab 2 (40 45 minutes): Cell Membrane Model (optional) Approach: One of the first steps in creating life is creating a barrier that separates inside from the outside environment; hence the cell membrane becomes one of the boundaries of life. The material in this chapter can be presented from this perspective. The cell must control its internal environment, so the cell 34
membrane functions as a barrier, but the cell must exchange material with the environment raw materials must come in and waste products and synthesized biomolecules must be released so the cell membrane must function as a semi-permeable barrier. This chapter is closely coupled with both the biochemistry chapter (in regards to protein and phospholipid structure) and the cell structure chapter. Students were introduced to phospholipids as components of membranes in Chapter 3, however they now need to expand their concept of membranes to include the many different components that make up the fluid mosaic model of a living, functional cell membrane. One again, it is better to anchor this new information to what students already know. Students already understand that cells must exchange material with the environment raw materials in and waste products out so now help them extend that knowledge through the logical exercise of puzzling out how these materials can move through the cell membrane if it has a broad layer of hydrophobic lipids. Bring students to the realization that to move polar and hydrophilic molecules like water, sugars, amino acids, ammonia through the membrane there needs to be specialized chemical channels that allow passage. This is an opportunity to weave in the theme of structure-function relationships the structure of proteins enabling them to function as membrane channels. It is useful to review protein structure from Chapter 3 to help students understand why proteins are ideally suited in this role: the varied R groups of amino acids make some hydrophobic and some hydrophilic proteins fold into 3-dimensional globular molecules the secondary structure of proteins create α-helices and β-pleated sheets which establish the structure of transmembrane protein channels transmembrane proteins are anchored in the membrane by hydrophobic, non-polar areas of the protein and hydrophilic, polar regions of the protein extend outward into the aqueous solution of both the cell and the environment The AP Osmosis and Diffusion lab demonstrates the principle of movement across a membrane very well. This lab enables students to empirically experience diffusion across a membrane, the effect on diffusion of size of molecules, osmosis across a living cell membrane, and the effect on osmosis of solute. It is well-worth devoting several class periods to learning these principles through investigation rather than solely through lecture. In fact, this lab and its follow-up discussion can replace lecture time on the topic. 35
Concept Map Cell Membranes structure function fluid mosaic model selectively permeable barrier bulk transport of large molecules phospholipid bilayer proteins cholesterol passive transport active transport endocytosis exocytosis receptor proteins transmembrane proteins peripheral protein glycoprotein down gradient against gradient phagocytosis pinocytosis transport proteins simple diffusion facilitated diffusion protein pumps requires energy nonpolar molecules polar molecules example sodium potassium pump example aquaporin channel for water Osmosis diffusion of water through a semi-permeable membrane hyperosmotic / hypertonic solution hypoosmotic / hypotonic solution isoosmostic / isotonic solution higher solute lower water lower solute higher water equilibrium animal cell shrivels plant cell plasmolyzes animal cell swells & may burst (lyse) turgid plant cell "normal" animal cell flaccid plant cell 36
Student Misconceptions and Common Pitfalls Students have difficulty reconciling the concepts of diffusion and osmosis. They often see the two as distinct processes. Emphasize that they are one and the same, osmosis is just singled out as a special case because life occurs in a watery environment, Students have difficulty distinguishing the concepts of hypertonic (hyperosmotic) and hypotonic (hypoosmotic). Be sure to only use them as comparative terms describing the relationship between two solutions rather than descriptive of a single solution. It is helpful to have students step through the logical thought process of how to determine if a solution is hypertonic or hypotonic as compared to another and then to determine which way water will flow between the solutions 1. Does Solution A have more or less dissolved material than Solution B? 2. What does this mean about the of water in Solution A as compared to Solution B? 3. What does this mean about the movement of water across a membrane between the two solutions? Students may lose sight of the fact that molecules continue their movement across a membrane once equilibrium is reached. While discussing osmosis, students may get sloppy with their language and talk about salt pushing (or pushing) water across the membrane. Insist that they refer to the process correctly to dispel misconceptions movement of water down its gradient. Activity Effects of Temperature and Solvents on the Cell Membrane If you have ever cooked fresh beets, you know how much beet dye leaks out of the cells into the cooking water. This pigment is betacyanin, a pigment stored in the large central vacuole of beet cells. When membranes are damaged, this pigment can cross the vacuole membrane and the cell membrane. The purpose of this lab is to illustrate the effects that temperature and solvents have on the cell membrane. In this exercise, students use pieces of beet root to test what types of environmental stress disrupts cellular membranes. The amount of membrane damage is directly related to the amount of pigment that leaks through the membrane and therefore the intensity of the color in the fluid, and the intensity of the color can be quantitatively assessed using a spectrophotometer. This lab can be presented as a teacher-directed activity or a student-directed inquiry activity. Students prepare uniform beet core using a cork borer (a 8 mm inside diameter is a good size) and trim each core to 15 mm in length. The cores are rinsed off by placing in a beaker of room temperature water for 2 minutes to remove betacyanin that has leaked from damaged (cut) cells. Then each beet core is placed in a separate test tube and exposed to a treatment (temperature bath or solvent) for 15 minutes. 37
Temperature Each beet cylinder is exposed to a different temperature condition. A good range of temperatures is 0 C, 20 C, 40 C, 60 C, 80 C, and 100 C. The frozen (0 C) beet cylinder must be prepared by the teacher ahead of time, so there s enough time for the whole cylinder to be frozen. After treatment, each beet core is placed in 10 ml of room temperature distilled water for 20 minutes, allowing any betacyanin to leak out. The solution from each test tube is then poured into a cuvette and tested in the spectrophotometer, measuring absorbance at 460 nm (the wavelength at which betacyanin absorbance is the highest). Make sure to calibrate or blank the spec first with a cuvette of distilled water. The students should find higher levels of betacyanin in the solution from the freezing temperature and boiling temperature treatments. The freezing temperature causes water to crystallize as ice, and expand because of hydrogen bond alignment, rupturing membranes. High temperatures cause violent molecular collisions that can physically destroy a membrane. Organic Solvents Follow the same procedure as temperature, but each beet cylinder is exposed to a different organic solvent. Solvents to consider testing are: 25% methanol, 50% methanol, 25% ethanol, 50% ethanol, 25% isopropyl alcohol, 50% isopropyl alcohol, 1% liquid soap (1 ml soap dissolved in 99 ml water), 5% liquid soap (5 ml soap dissolved in 95 ml water), and distilled water as a control. Add 20 ml of the solvents to each test tube, keep covered at room temperature for 20 minutes, and shake each tube occasionally. The solution from each test tube is then poured into a cuvette and tested in the spectrophotometer, measuring absorbance at 460 nm (the wavelength at which betacyanin absorbance is the highest). Make sure to calibrate or blank the spec first with a cuvette of 50% of the respective solvent being measured. For example, for tubes with solutions in methanol, make sure to use 50% methanol to blank the spec. Organic solvents will dissolve a membrane s lipids, reducing the membrane to tatters. Students will find high levels of betacyanin in the solution from any of these organic solvents. Web Resources Nobel Prize in Chemistry 2003: Peter Agre for the discovery of aquaporins http://www.nobelprize.org/chemistry/laureates/2003/ http://nobelprize.org/chemistry/laureates/2003/public.html Peter Agre was awarded the Nobel Prize for the discovery of aquaporins water channels. The Information for the Public article is written at a level students can understand and is a good description of a wonderful piece of research. 38
Cell Biology Animations http://www.johnkyrk.com/cellmembrane.html This site has an incredible collection of Flash animations showing many different cellular functions. This one illustrates membrane structure step-by-step, starting from phospholipid bilayer, and then adding in cholesterol, proteins, etc. Cell Membrane Tutorial http://www.bio.davidson.edu/people/macampbell/111/memb-swf/membranes.swf This site offers an extensive interactive tutorial to help students review membrane structure. Quizzes are included to assess whether students understand the key concepts of each lesson. Cell Membrane http://www.pediatric-orthopedics.com/topics/muscle/basic_science/membranes/membranes.html This site offers a brief explanation about membrane structure and some good molecular models showing phospholipids in the membrane. On the Lighter Side Cell Membrane Model To push students to puzzle out and learn the details of the structure of the cell membrane, they can be assigned an in-class activity to build a model of the cell membrane using materials at hand such as macaroni and pipe cleaners. The many shapes of macaroni and cereal lend themselves to illustrating phospholipids, protein channels, glycoproteins, and active transport pumps. The students can build a cross-section of the cell membrane by gluing their model on cardboard or manila file folders. Although this activity may seem elementary for an AP level class, cell structure is abstract since it cannot be easily viewed, so it is beneficial to offer tangible reinforcement. Multiple Choice Questions 1. Cell membranes are composed primarily of a. carbohydrates and proteins. b. phospholipids and proteins. c. carbohydrates and phospholipids. d. nucleic acids and phospholipids. e. ribosomes and lipids. 39
2. The phospholipids of the cell membrane form a. a bilayer that is impermeable to water and water soluble molecules. b. a long polymer that folds into a non-polar sphere. c. a bilayer that hides the non-polar alcohol groups inside and exposes the polar fatty acids to the surrounding water. d. an alpha helix. e. a spherical fat globule which minimizes the contact of non-polar lipids with the surrounding water. 3. All cell signal receptor proteins must a. be embedded in the plasma membrane. b. have a DNA binding domain. c. have a signal molecule binding site. d. have seven transmembrane domains. e. act as protein kinase enzymes. 4. Which of these types of molecules can diffuse across the cell membrane without the assistance of carrier proteins? a. large polar molecules such as starch b. carbon dioxide and oxygen c. ions such as calcium (Ca +2 ) d. proteins e. DNA 5. Receptor proteins embedded in the cell membrane a. are polar and diffuse easily out of the cell membrane into the cytoplasm. b. are covalently bound to the phospholipids in the cell membrane and unable to move. c. contain one or more non-polar regions that passes through the cell membrane and polar regions that interact with soluble molecules in the cytoplasm and outside the cell. d. are non-polar and remain completely embedded in the lipid region of the membrane to avoid contact with water. e. are synthesized in the nucleus and transported to the cell membrane by mrna. 6. Which of the following is NOT a function of proteins embedded in the cell membrane? a. forming pores to transport of ions and polar molecules across the membrane b. active transport c. protein synthesis d. anchors for the cytoskeleton e. hormone receptors 40
7. Which of the following requires an input of energy from the cell? a. the increase in cell volume by osmosis when it is in a hypoosmotic medium b. the decrease in cell volume by osmosis when it is in a hyperosmotic medium c. the diffusion of uncharged molecules from high to lower through a protein channel d. facilitated diffusion across the plasma membrane by carrier proteins from a region of high solute to a region of lower e. active transport across the plasma membrane from a region of lower s to a region of higher 8. Paracrine signaling involves a. direct contact between membrane bound protein of one cell and receptors of another cell. b. signal molecules that diffuse only a short distance in intracellular fluid before they are destroyed by extracellular enzymes or removed by other cells. c. signal molecules that move through the circulatory system and may have effects on cells in distant organs. d. signal molecules that travel only a short distance between the specialized signaling and target structures of a synapse. e. receptor mediated endocytosis. 9. A gated ion channel a. allows a specific type of ion or molecule to diffuse through the cell membrane very rapidly. b. moves ions through the cell membrane by active transport. c. allows messenger RNA to move out of the nucleus. d. is usually open, but closes when is it activated by a specific signaling molecule. e. acts as a membrane bound enzyme. 10. The primary function of tight junctions between cells is to a. provide physical strength for tissues that experience stress such as muscle and skin. b. allow for direct movement of ions and other water soluble molecules from one cell to another. c. seal a sheet of cells into an impermeable layer, so that water and soluble molecules must move through the cells of the sheet. d. identify cells as self to the immune system. e. pride a direct connection from proteins in the extracellular matrix to cytoskeletal filaments. Answers: 1. b, 2. a, 3. c, 4. b, 5. c, 6. c, 7. e, 8. b, 9. a, 10. c 41
Essay Questions 1. Describe the process of receptor mediated endocytosis, and explain why it is necessary for cells to have a system to transport large molecules across the plasma membrane. Specific regions of the cell membrane contain groups of receptors that are specific for one or more large molecules that need to be imported into the cell. This region of the membrane forms a clathrin lined pit. When the target molecules bind the receptor, the pit folds into the cell to form a vesicle. The molecules bound to the receptor are now on the inside of the vesicle. The vesicle transports the target molecules to the lysosome where they can be digested. This system moves large molecules from the outside of the cell to the inside of an organelle, without the need to cross two membranes. 2. How can a cell maintain internal s that are much higher than the surrounding environment of some small molecules such as Na + ions while allowing rapid diffusion of other molecules across the cell membrane? The plasma membrane is selectively permeable. Some molecules are allowed to diffuse through freely, others cannot pass, and may be pumped against the gradient by active transport 3. Describe three different adaptations that allow cells and/or organisms to live in a hypotonic (hypoosmotic) environment. contractile vacuole expels excess water from the cell sea creatures may become isoosmotic to sea water plant cells prevent swelling and cell rupture by pressing against the cell wall (turgor pressure) multicellular animals (and some plants) have an outer layer (skin) that is impermeable to water, while internal cells are kept in isoosmotic fluid (blood) 4. Explain how cell receptors are able to receive information from many different kinds of signaling molecules (peptide and steroid hormones, neurotransmitters, amino acids), and translate these into different actions inside the cell. Cell surface receptor proteins have variable shapes for the region that extends outside the cell membrane. These different shapes can form specific bonds with a wide variety of different signaling molecules. After a receptor protein is bound by its specific ligand, the portion of the molecule that extends inside the cell membrane is activated and transmits its signal to the cell. The effect of this signal may to be to activate a G-protein, open a gated membrane channel, or phosphorylate another protein. 42