Tutorial work - 10 - Cell membrane worksheet PDF

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Answers to End-of-Chapter Questions – Chapter 5 Test Yourself Questions 1. Which of the following statements best describes the chemical composition of biomembranes? a. Biomembranes are bilayers of proteins with associated lipids and carbohydrates. b. Biomembranes are composed of two layers, one layer of phospholipids and one layer of proteins. c. Biomembranes are bilayers of phospholipids with associated proteins and carbohydrates. d. Biomembranes are composed of equal numbers of phospholipids, proteins, and carbohydrates. e. Biomembranes are composed of lipids with proteins attached to the outer surface. Answer: c. Biomembranes are bilayers of phospholipids. Proteins are also associated with the bilayer, either embedded in the membrane or attached to the inner or outer surface. Carbohydrates are also associated with the bilayer, usually attached to proteins or lipids of the outer surface. 2. __________ is a lipid that helps stabilize membranes of animal cells by regulating fluidity as temperature changes. a. Cholesterol b. Prostaglandins c. Glycerol d. ATP e. Acetone Answer: a. Cholesterol stabilizes membranes by regulating fluidity of the membrane as the temperature changes. 3. The presence of double bonds in the fatty acyl tail will a. decrease fluidity because of the attraction between the unsaturated tails. b. increase fluidity due to the difficulty of the kinked acyl tail to interact. c. decrease fluidity by increasing the space between the phospholipids. d. increase fluidity by allowing more room for cholesterol to move into the membrane. e. decrease fluidity by decreasing the amount of proteins in the membrane. Answer: b. Double-bonds in the fatty acyl tail create a kink in the tail that prevents interactions with other fatty acyl tails. This results in an increase in fluidity of the membrane. 4. Carbohydrates of the plasma membrane a. are associated with a protein or lipid. b. are located on the outer surface of the membrane. c. can function as cell markers for recognition by other cells. d. All of the above. e. a and c only Answer: d. All of the statements are true of the membrane carbohydrates. 5. Which of the following can easily diffuse through a lipid bilayer? a. sodium ions. b. amino acids c. glucose d. oxygen e. DNA Answer: d. Oxygen, like other gases, can easily diffuse through membranes. The other substances listed are either too large (e.g., DNA) or not compatible with the phospholipid bilayer due to charge or polarity (e.g., sodium ions and glucose). 6. The tendency for Na+ to move into the cell is the due to a. higher numbers of Na+ outside the cell resulting in a chemical concentration gradient.

b. c. d. e.

the net negative charge inside the cell attracting the positively charged Na+. the attractive force of the K+ inside the cell pulling Na+ into the cell. All of the above. a and b only.

Answer: e. Na+ moves into the cell due to the electrochemical gradient present across cell membranes. Na+ moves into the cell down a chemical concentration gradient as well as in response to an electrical gradient. 7. The hydrostatic pressure required to stop osmosis is a. an electrochemical gradient. b. filtration. c. tonicity. d. osmotic pressure. e. partial pressure. Answer: d. Osmotic pressure is the hydrostatic pressure necessary to stop the movement of water due to osmosis. Osmotic pressure is also called turgor pressure. 8. The selectively permeable characteristic of cell membranes is mainly due to the presence of a. phospholipids. b. transport proteins. c. glycolipids on the outer surface of the membrane. d. concentration gradients across the membrane. e. cholesterol. Answer: b. Transport proteins provide passageways into and out of the cell for many molecules that cannot diffuse across a bilayer. 9. During ________________, materials are moved across the plasma membrane against their concentration gradient. a. facilitated diffusion b. osmosis c. active transport d. filtration e. simple diffusion Answer: c. Active transport requires cellular energy to move molecules across the membrane from areas of low concentration to areas of higher concentrations. This process also involves the activity of a membrane protein. 10. Large particles or large volumes of fluid can be brought into the cell by a. facilitated diffusion. b. active transport. c. endocytosis. d. exocytosis. e. all of the above. Answer: c. Endocytosis is a vesicular transport that brings large particles or volumes of material into the cell. Conceptual Questions 1. Explain the fluid-mosaic model of membrane structure. Answer: The membrane is considered a mosaic of lipid, protein and carbohydrate molecules. The membrane exhibits properties that resemble a fluid because lipids and proteins can move relative to each other within the membrane. 2. What are three types of membrane proteins?

Answer: Transmembrane proteins have one or more regions that are physically embedded in the hydrophobic region of the phospholipids bilayer. These regions contain hydrophobic amino acids that span the membrane from leaflet to leaflet. A second way for proteins to associate with the membrane is via lipid anchors, which are covalent attachments of a lipid to an amino acid side chain within a protein. Peripheral membrane proteins do not interact with the hydrophobic interior of the phospholipids bilayer but are weakly bound to the membrane components and can easily be removed. 3. What is the difference between passive diffusion and passive transport? Answer: Passive diffusion is the movement of a substance across a membrane without the aid of a transport protein. Passive transport refers to the movement across a membrane that does not require an input of energy. Passive transport can occur via passive diffusion or facilitated diffusion. Experimental Questions 1. What observations led to the experiment of Figure 5.15 to identify proteins that may increase water movement into the cell? Answer: Most cells allow movement of water across the cell membrane by passive diffusion. However, it was noted that certain cell types had a much higher rate of water movement indicating that something different was occurring in these cells. 2. How did Agre and associates choose a candidate protein that may function as a water channel in cell membranes? What was the hypothesis tested by the researchers? Briefly explain how they were able to test their hypothesis. Answer: The researchers identified water channels by characterizing proteins that are present in red blood cells and kidney cells but not other types of cells. Red blood cells and kidney cells have a faster rate of water movement across the membrane than other cell types. These cells are more likely to have water channels. By identifying proteins that are found in both these types of cells but not in other cells, the researchers were identifying possible candidate proteins that function as water channels. After identifying a protein that was abundant in red blood cells and kidney cells but not present in other cell types, Agre and associates hypothesized that the protein, CHIP28, functions as a water channel. This protein would allow for a faster rate of water movement across the membrane of these particular cells. Agre and his associates experimentally created multiple copies of the gene that produces the CHIP28 protein and then artificially transcribed the genes to produce many mRNAs. The mRNAs were injected into a frog oocyte where they could be translated to make the CHIP28 proteins. After altering the frog oocytes by introducing the CHIP28 mRNAs, they compared the rate of water transport in the altered oocytes versus normal frog oocytes. This procedure allowed them to introduce the candidate protein to a cell type that normally does not have the protein present. 3. What were the results of the experiment of Figure 5.15? Do they support the proposed hypothesis? Answer: After artificially introducing the candidate protein into the frog oocytes, the researchers found that the experimental oocytes took up water at a much faster rate in a hypotonic solution as compared to the control oocytes. The results indicated that the presence of the CHIP28 protein did increase water transport into cells. Collaborative Questions 1. Discuss the concept of solute concentrations in cells. Answer: Cells are separated from their surroundings by a membrane which regulates what goes in and out of the cell. In most cases the cell membrane prevents most solutes from simplify passing across the cell membrane. One exception is water which is somewhat permeable across the membrane. When the solute concentration inside the cell is the same as it is outside the cell then this condition is called isotonic. As a result there will be no net movement of water across the cell membrane. If the solute concentration outside the cell is greater than that which is inside the cell then the outside solution is said the hypertonic. As a result of this

condition water will flow out of the cell resulting in cell shrinking. If the concentration of solute is greater on the inside of the cell than the outside solution is said to be hypotonic and will result water will flow into the cell causing the cell to expand. 2. Discuss the two categories of transport proteins found in cell membranes. Answer: Channels – these are transmembrane proteins that form an open passageway for the direct diffusion of ions or molecules across the membrane. Most of these channels are gated, which means they can open to allow the diffusion of solutes or they can be closed to prohibit diffusion. This feature allows cells to regulate the movement of solutes. In addition, solutes move through channels from a high to a low solute concentration. Transporters – these proteins are also known as carriers. Transporters bind the solute on one side of the membrane and as a result the protein undergoes a conformational change that allows access of the solute to the opposite side of the membrane. This mechanism is the principal pathway for the uptake of organic molecules such as sugars, amino acids, and nucleotides. This mechanism can also be used to remove wastes such as lactic acid from the cell. Pumps are a type of transporter that usually use ATP to move solutes against a concentration gradient (from a low solute concentration to a high solute concentration). Because solutes are moving from a low to high concentration, the cell has to pay a price in the form of energy. This type of transport would be used if a cell wanted to accumulate nutrients inside the cell even when nutrients inside the cell are plentiful, or remove waste from inside the cell even when waste concentrations are high outside of the cell....