Biology 120, Section 10 J. Greg Doheny Chapter 7 The Cell Membrane

NOTES The Plasma Membrane of the cell divides the outside of the cell from the inside. It consists of a phospholipid bilayer, a small amount of cholesterol (for added strength), and a number of proteins, glycoproteins, and glycolipids that are either embedded in it, or are associated with proteins that are embedded in it. Many of the proteins embedded in the cell membrane are receptors, which receive signals from the outside, while others form channels that let specific compounds in or out of the cell. A phospholipid consists of a hydrophilic (literally ‘waterloving’) head, and a hydrophobic (literally ‘water-hating’) lipid tail. When placed in solution, phospholipids form a bilayer, where two layers of phospholipids line up with their hydrophobic tails facing each other (and away from the water), and their hydrophilic phosphate groups facing the water. Fluid Mosaic Model: The phospholipids inside the plasma membrane are not fixed in place, but move around, as do the proteins that are embedded in it. Movement can be either lateral (from side to side) or ‘flip-flopping,’ where a phosphate group that is facing outside can invert, and face inside. Many of the membrane channels work this way, too. While some channels are pores that can open and close to let molecules in or out, others can bind something on the outside, and then flip over to bring it inside. Evidence for the fluid mosaic model is provided by cell fusion experiments (Figure 7.7). Rigidity of the membrane: Several factors can influence how rigid a membrane is. The more saturated the hydrocarbons in the phospholipids, the more rigid it is. Likewise, the more cholesterol in the membrane, the more rigid it is. Types of proteins associated with the plasma membrane: a. Integral Proteins: are embedded in the membrane, but do not penetrate all the way through it. b. Transmembrane Proteins: pass all the way through the membrane, so that one end is outside the cell and the other end is inside. Many cell receptors are transmembrane proteins, as are many of the proteins that form channels through the plasma membrane. c. Peripheral Proteins: are not embedded in the membrane, but are associated with proteins that are. (By the way, whenever a biochemist or cell biologist says that a protein is “associated” with another protein or with DNA, they usually mean that it is bound to it with ionic or hydrogen bonds, but not covalent bonds. If you say that one protein is ‘bound’ to another, people will assume you mean two proteins are bound together with covalent bonds, which usually isn’t the case.) Signal Transduction: Signal Transduction is a very important concept in cell biology. Cells receive ‘signals’ from other cells, telling them to do things (ie-a cell receives a signal telling it to 1

divide). This happens when one cell produces a small protein called a cytokine, secretes it into the blood, and it then makes contact with a cell somewhere else in the body. The cytokine ‘docks’ with a cell receptor on the surface of the target cell. A cytokine that is meant to dock with a cell receptor is referred to as the receptor’s ligand. Thus, we talk of “receptor/ligand interactions.” Many cell receptors are transmembrane proteins. Docking of a ligand to the receptor on the outside of the cell causes a conformational change (change in shape) of the entire receptor, including a change of shape to the part of the receptor on the cytoplasmic side of the membrane. This usually causes another type of protein to bind to the cytoplasmic end of the receptor, and set off a ‘signal transduction cascade’ of activated proteins that eventually make their way into the nucleus, where they will turn specific genes on or off. Endocytosis: Endocytosis refers to taking things into the cell. What generally happens is that something makes contact with the outside of the cell membrane, the membrane then forms a pit which then pinches off from the rest of the membrane to form a vesicle inside the cytoplasm of the cell. (Remember that vesicles move around inside the cell with the help of the cytoskeleton!) There are three main types of endocytosis. 1. Phagocytosis: Food particles bind to the outside of the cell. The cell forms a pit, pinches off into a vesicle which then docks with a lysosome inside the cytoplasm, allowing digestion to begin. 2. Receptor-Mediated Endocytosis (RME): when cytokines (signal proteins) are produced and sent to a target cell, they make contact with receptors on the cell surface. The activated receptors then form a cluster, and the membrane underneath the cluster forms a pit, which then pinches off into a vesicle, taking both the receptors and cytokine ligands into the cell. 3. Pinocytosis: If phagocytosis is ‘cell eating,’ pinocytosis is ‘cell drinking.’ Sections of the plasma membrane pinch off to form vesicles containing fluid from outside the cell. Exocytosis: Toxic waste products are packaged into vesicles and sent outside the cell. These vesicles fuse with the plasma membrane. It is basically the reverse of endocytosis. Osmoregulation Osmosis, Osmoregulation and Turgor Pressure: Regulation of salt and carbohydrate concentration inside the cell is another important function of the cell membrane and the proteins associated with it. Water is able to pass freely across the plasma membrane, but most salt ions (KCl and NaCl) and carbohydrates (ie-glucose) cannot. If the salt or carbohydrate concentration is not equal on both sides of the membrane, water will move to the side with the higher salt or carbohydrate concentration to try to equalize it. Thus, if the concentration of salt is higher outside the cell, water will diffuse outside the cell to try and dilute it, causing the cell to go limp. (This is what happens when you make pickles! You put firm cucumbers into salty water, and they become soft and limp as water diffuses outside the cells.) The opposite will happen if the salt concentration is higher inside the cell. Water will flow into the cell in an attempt to dilute the salt inside the cell, so that the salt concentration is the same on both sides of the membrane. This will usually cause the cell to become more rigid as the membrane stretches to accommodate the increased volume. In some cases it might actually burst

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the cell! Stretching of the membrane will exert a force against further uptake of water, to a certain extent. This counter pressure caused by stretching is called turgor pressure. When a cell is in a solution that has the same salt concentration outside the cell as inside the cell, it is said to be in an isotonic solution. If the salt concentration outside the cell is higher than inside the cell, it is said to be in a hypertonic solution, and if the salt concentration outside the cell is lower than inside the cell it is said to be in a hypotonic solution. (“Hyper-“ means ‘above,’ and “Hypo-“ means underneath. Thus, a hypodermic needle is a needle meant to go underneath the dermis [skin].) Passive and active transport across membranes: Some proteins embedded in the cell membrane form channels that let certain types of molecules in and out freely. Other types of channels (called gated channels) will selectively let things in and out. With active transport, specific types of channels will actively pump certain ions outside the cell, requiring energy (in the form of ATP) to do so. The best example of active transport of ions across membranes is called the sodium-potassium pump, where sodium ions are actively pumped outside the cell, and potassium ions are pumped inside the cell. In both cases, ATP is consumed to pump ions in our out of the cell, against their concentration gradient. Having a high concentration of charged sodium ions outside the cell also causes there to be an electrical voltage difference between the two sides of the membrane, with the inside being up to 200 millivolts (mV) more negative than the outside. When this happens, it is called a membrane potential. (ie-there is a membrane potential of 200 mV across the membrane.) Neurons, for example, maintain large membrane potentials as a way of transmitting neural impulses. When the neuron is activated, it ‘depolarizes,’ meaning that channels open up allowing the sodium to rush inside, neutralizing the membrane potential.

Practice Questions: Short Answer Questions: 1. Will water diffuse in or out of a cell that is in an isotonic solution? 2. Will water diffuse in or out of a cell that is in a hypotonic solution? 3. Will water diffuse in or out of a cell that is in a hypertonic solution? 4. What is the difference between Phagocytosis and Pinocytosis? (2 points) 5. What is the difference between Phagocytosis and Receptor Mediated Endocytosis (RME; 2 points)? 6. What is the main macromolecule that comprises the plasma membrane? (There are many components to the plasma membrane, but this one vastly outnumbers the rest.) 7. Plasma membrane components: What is the difference between an integral membrane protein and a transmembrane protein? 8. Plasma membrane components: What is the difference between an integral membrane protein and a peripheral membrane protein? 9. List one type of protein found in the cell membrane that is often a transmembrane protein. 10. Describe what would happen if you put red blood cells, with an internal salt concentration of approximately 200 mM, into a solution of pure water. 11. What is the energy source used to power active membrane transport? 3

Essay type question: (Essay type questions are usually worth 10 points, as opposed to descriptive questions, which are usually worth two points, and short answer questions worth 1 point.) 1. Explain what is meant by the Fluid Mosaic Model of the Plasma Membrane (10 points). Compare and contrast active vs. passive transport across plasma membranes (10 points). 2. Describe what happens during the process of Receptor Mediated Endocytosis (10 points). 3. Describe what happens during the process of Phagocytosis (10 points). 4. What are the components of a typical plasma membrane of a eukaryotic cell? (10 point) 5. Describe what a ‘phospholipid bilayer’ is, and what its main function is for a cell. (10 points) 6. Describe the experiment that was used to prove the Fluid Mosaic Model of the cell plasma membrane. (10 points) (Hint: study figure 7.7 of your textbook.) 7. Describe the difference between integral, peripheral and transmembrane proteins, and how they are associated with the cell membrane. (10 points) 8. Describe what is meant by signal transduction. (10 points) 9. Describe what happens when you put a cell into a hypotonic solution vs. a hypertonic solution. (10 points) 10. Describe what is meant by a membrane potential, and what happens when a neuron ‘depolarizes.’ (10 points)

Descriptive Questions: (This time I give you the term, and you give me the definition. Short answer questions, where I give you the definition and you give me the term are generally worth 1 point, and descriptive questions, where I give you the term and you give me the description, are worth 2 points.) 1. What is an isotonic solution? 2. What is a hypotonic solution? 3. What is a hypertonic solution? 4. What is meant by active transport across a plasma membrane? 5. What is the function of lysosomes, and why are they acidic on the inside? 6. What is a membrane potential, and what causes it? 7. What is turgor pressure? 8. What is a glycoprotein? 9. What is a glycolipid? 10. What is the difference between an integral membrane protein and a transmembrane protein?

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Extended Matching Inventory: Match the term to the definition. a. b. c. d. e. f. g.

Cytokine Docking Glycolipid Glycoprotein Hydrophilic Hydrophobic Hypertonic

h. i. j. k. l. m.

Isotonic Ligand Motif Phospholipid RME Transduction

1. A pattern or characteristic feature. 2. To love water. 3. A solution having a higher salt concentration than the inside of a cell. 4. Something that binds to a receptor. 5. A protein that has a carbohydrate motif attached to it. 6. A macromolecule having a hydrophobic head and a hydrophilic tail. 7. A lipid that has a carbohydrate motif attached to it. 8. A solution having a lower salt concentration than the inside of a cell. 9. The process of a ligand binding to its specific receptor. 10. The process of taking activated receptors and their ligands into a cell. 11. To hate water. 12. A small protein produced by one cell and sent to another cell as an intercellular signal. 13. The process of relaying cytokine signals from the outside of the cell, through a series of intermediate proteins, to the nucleus. 14. A solution having the same salt concentration as the inside of a cell. © J. Greg Doheny 2014

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