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Lipids and Membranes, Fall 12—Worksheet - KEY Adapted from a POGIL exercise developed by J. Loertscher & V. Minderhout Crowe Work in groups of three to complete this worksheet. Your understanding will be checked using clicker questions periodically throughout the activity. Membranes are some of the most important macrostructures of biological systems. Membranes define the organism and separate it from its environment. All biological membranes contain lipids as major components. Understanding how these lipids contribute to the structure of membranes and interact with membrane proteins will help you understand many cellular processes including cell signaling, cell motility and cell respiration. 1. Based on what you know about hydrophobic interactions, draw phospholipids indicating possible ways that phospholipid molecules would be expected to interact (use circles to represent the polar head groups and wavy lines to indicate hydrophobic tails) 2. Based on what you know about the structure of a bi-lipid membrane, compare and contrast how a plastic seal surrounding the cell would be similar and how it would be different from a cell membrane. Similar: keeps things separated Different: not permeable, so doesn’t allow for certain molecules to cross 3. Based on your answer to Q2 and what you have already learned about cell organelles, list at least 2 possible purposes of membranes. Possible answers:: 1.Concentrates reactants – resulting in increased frequency of chemical reactions 2. Forms selective barrier - allows for different environments inside and outside the cell 3. Allows different environments/gradients to exist inside and outside a cell 4. Forms internal compartments within a cell B Table 1: Membrane Passage Molecule Molecule Structure Name # of molecules that pass through a lipid bilayer at 25 °C # of molecules that pass through a lipid bilayer at 42 °C Indole 30,000,000 34,500,000 # of molecules that pass through a lipid bilayer with additional unsaturated lipid tails at 25 °C 33,900,000 Adenosine Triphosphate (ATP) 2 2.3 2.5 18 45,600 27 51,300 25.5 52,000 Methane 190,000,000,000 290,000,000,000 240,000,000,000 Octane 99,800 129,200 112,300 Glycine ? 85,000 ? Ethanol 4,090,000 ? ? Potassium Fructose K+ page 2 of 5 4. Use Table 1 to answer the following questions 1) Which molecule is the least likely to pass through a membrane at a given temperature? ATP 2) Which molecule is the most likely to pass through a membrane at a given temperature? Methane page 3 of 5 Q4 Continued 3) What are two molecular characteristics that account for the rate of membrane passage? In other words, what is it about the molecules that makes them pass through differently? (Answer each in a single complete sentence.) a. Size: Smaller molecules are more likely to pass through a membrane than large bulky molecules b. Hydrophobicity: Hydrophobic molecules are more likely to pass through a membrane than hydrophilic molecules 4) Which characteristic from #3 above helps to explain the data you see for the Potassium? If neither of your characteristics helps explain this data, then you may need to go back and change your answers for #3. Potassium is a hydrophilic molecule due to its positive charge 5) Which characteristic from #3 above helps to explain the data you see for the differences between Methane and Octane? Again, if you don’t have an explaining factor then you may need to change your answer for #3. Octane is larger than methane, so does not travel through the membrane as easily, but both are hydrophobic so can pass through relatively easily compared to hydrophilic molecules. 6) Make an educated guess for the rate of Ethanol passage through a membrane at 42°C. There are many possible correct answers, but this answer should make sense based on what you see in the rest of the table. ~5,000,000 . Should be greater than the number that pass through at 25°C. Other molecules shown have an increased rate of about 1.2-1.3-fold at 42°C compared to 25°C. 7) What is happening to the molecules in the lipid bilayer with increasing temperature?. In a single sentence, develop a hypothesis that would explain your rate of Ethanol passage at 42°C. Increased fluidity of the membrane at 42°C allows ethanol to pass more easily between the nonpolar tails in the lipid bilayer resulting in increased passage at the higher temperature. [Challenge questions] 8) Fill in the table with reasonable rates of passage that agree with the rest of the data. 9) Changing the lipid tails in the membrane can change the passage of molecules through that membrane. Can you make a confident prediction about the 3rd rate for glycine? If not, why not? Cannot make a confident prediction. The effect of more unsaturated lipids on a molecule’s passage is molecule-specific, so must be tested for each molecule page 4 of 5 Information: Integral membrane proteins often contain helical segments of appropriate length to span the lipid bilayer. In a protein that has a single segment that spans the membrane, the helix usually only contains hydrophobic residues and is called a single-span membrane protein. In transmembrane proteins with multiple segments that span the membrane, hydrophilic residues are often found in sequences of the helices. 5. Why are hydrophobic residues favored in single-span membrane proteins? .&$ !"#$ !"#$ !"#$ !"#$ !"#$ /01$ 6. From Fig. 2, what is the role of Protein A? 23$ !"#$ %&$ !" #$ !"#$ !"#$ /01$ '()*+,-$.$ !"#$ !"#$ !"#$ 23$ Fig. 2. Diagram of a cell membrane either without Protein A (top) or with Protein A (bottom). OUT = outside the cell; IN = inside the cell. Protein A enables the passage of Cl- across the cell membrane, down the Cl- ion concentration gradient. 7. What effect do you predict the presence of a hydrophilic amino acid in the membrane-spanning portion of protein A to have on its function? This could be beneficial, if the hydrophilic residue is facing into the channel and can interact with chloride ions being transported through the channel. It could also disrupt the structure of Protein A if the hydrophilic residue is facing outward to the hydrophobic environment of the lipid membrane. . page 5 of 5 On your own: 1. The molecule shown to the right is a hormone that is important for sending signals from the pituitary to other organs in the body. In order to receive the signal, cells within these organs contain receptors. Where in the cell would you expect to find a receptor for this hormone? Explain your reasoning. In the cytosol or nucleus, because this steroid can cross them membrane, so the receptor does not need to be located on the cell surface. 2. The structure of cholesterol is shown in Fig. 2. What effect do you predict the presence of cholesterol in a cell membrane to have on membrane fluidity. Fig. 2. Structure of cholesterol 3. Cell membranes are described l mosaic model. Does this model fully describe the using a fluid mosaic model. Define the fluid reality of a cell membrane? Explain why or why not. Proteins floating about in a fluid membrane (definition of the fluid mosaic model) In reality, there are a lot of interactions between transmembrane proteins and intracellular proteins that impact fluidity of proteins in the membrane. Location of membrane proteins, like receptors, are also influenced by extracellular signals (like the chemical cues from bacteria!) causing them to group in on region of the membrane. So the reality is more complex. 4. Why is it important for determining the function of a membrane protein to know if it spans the bilayer or appears only on one face of the membrane? Provide examples of different proteins’ functions to support your answer. Only proteins that span the membrane would be able to function as channels or active transport molecules to ferry other molecules across the membrane. Proteins whose function is to interact with other cells would need to be located on the outer face of the cell membrane, whereas proteins whose function was to transmit signals to the cytoplasm from a cell receptor would need to be located on the inner face of the cell membrane. page 6 of 5