BIS2A T.M. Murphy Page 1 CELL PROBLEMS A. Structure 1. Decide whether microscopy or cell fractionation would be the best way to answer each of the following questions. a. What is the nucleus made of? b. What happens when a plant cell is placed in 0.6 M NaCl? c. Do all plants cells have mitochondria? d. Are mitochondria and chloroplasts structurally related? e. Do mitochondria divide like bacteria? f. How is sugar used for food by an animal cell? g. Do muscles and cilia move by the same basic mechanism? 2. Assume a cell is a cube 10 µm on a side. What is the volume of the cell in cubic µm? What is the volume in cubic meters? How many of these cells would fill a teaspoon (approximately 5 cubic centimeters)? 3. Many textbooks say that the reason cells are small relates to the surfaceto-volume ratio: this ratio is greater for small objects than for large ones, and a large ratio is needed for the transport of substances in and out of the cell. (See Purvis et al., 5 th edn. p. 66.) What is the surface-to-volume ratio of a sphere 20 µm in diameter? Of a sphere 200 µm in diameter? Despite this reasoning, many large cells are known: for instance, the internode cell of an alga, Chara, can be 1 millimeter (1000 µm) wide and 2 centimeters long. What implicit assumption makes the reasoning suspect? (Hint: Chara internodes are cylinders that grow in length.) 4. Again considering surface-to-volume ratio, what organelle, found in plant cells but not in animal cells, allows the cells of plants to be larger than the cells of animals? 5. Assume that you are responsible for designing a new space probe to continue the search for life on Mars. What would you test for, and how would you design the test,... a) if you have to design the one best experiment? b) if you had an unlimited budget? 6. You have five electron micrographs depicting cells from five different kingdoms. The labels have dropped off, and you have to figure out which cells belong to which kingdoms. The only clues you have are the organelles you can recognize from the pictures. Two of the micrographs (which for convenience you re-label A and B) show cells with large central vacuoles; three cells (D, E, and F) do not have such vacuoles. Among the cells with large central vacuoles, one (A) has chloroplasts and the other does not. Among the cells without large central
BIS2A T.M. Murphy Page 2 vacuoles, one (C) has cilia; the others do not. Among the cells without cilia, one (D) has a cell wall; the other does not. What are the most likely identities of the five cells? 7. In the diagram below question 8, the best word for A is: a. organelles b. nucleus c. cell wall d. motility e. microtubules 8. In the diagram below question 8, the best word for B is: a. plastids b. vacuoles c. motility d. nucleus e. Organelles 9. In the diagram below, the best word for C is: a. cell walls b. DNA c. photosynthesis d. nucleus e. ribosomes 10. Cytochalasin is a poison that inhibits the polymerization and thus the function of microfilaments. A scientist observed that cells derived from liver and placed in a tissue culture medium secreted proteins into the medium; when treated with cytochalasin, that secretion quickly stopped. Provide a hypothesis to explain why. 11. A plant cell is said to have over 20 compartments that is, places to which newly synthesized proteins may be directed. How many of these can you name? (Hint: the plasma membrane is one.) 12. The pathway of newly synthesized proteins through the cell can be followed by a pulse-chase experiment. Proteins are synthesized for a short time with a radioactive isotope (the pulse ), then they are given the same
BIS2A T.M. Murphy Page 3 compound non-radioactive (the chase ), the presence of the radioactivity in different compartments of the cell is followed by isolating cell organelles and quantifying their radioactivity. What results would you expect for a) a lysosomal enzyme? b) a protein that is released from the cell? c) a protein that is accumulated in the mitochondrial matrix? d) a protein of the plasma membrane? e) a histone (nuclear protein)? B. Osmosis 13. Red blood cells are normally kept in a solution containing 0.15 M NaCl. When red blood cells are placed in water, they normally burst. Which of the following treatments would keep them from bursting? a. Adding 0.3 M glucose to the water. b. Soaking them in 0.15 M NaCl before putting them in water. c. Adding a substance (Hg 2+ ) that blocks aquaporins. d. Placing the cells and water in a pressure chamber and exerting a hydrostatic pressure of 7 atmospheres. 14. When red blood cells are placed in a solution of 0.15 M glucose, they swell and some will burst. Why are they stable in 0.15 M NaCl and not in 0.15 M glucose? 15. When red blood cells are placed in a solution of 0.3 M urea, they burst quickly; in 0.3 M glycerol, they burst (fairly suddenly) in approximately 15 min; in 0.3 M glucose they do not burst for hours, if at all.
BIS2A T.M. Murphy Page 4 Explain why they burst in urea and glycerol and not in glucose and why bursting in glycerol is delayed. 16. Is a plant cell in water always turgid? 17. Assume that two plant cells have identical volumes in water. When the cells are placed in 0.3 M sucrose (table sugar, consisting of linked glucose and fructose molecules), cell A shrinks very little and is still turgid; cell B shrinks more and is flaccid (no turgor pressure). A. When they were in water, did the two cells have the same turgor pressure? If not, which had the higher pressure? Did the cells have the same internal (osmotically effective) concentration of solutes? If not, which had the higher concentration? B. When the cells are placed in 0.6 M sucrose, both are plasmolyzed. At this point, do the cells have the same turgor pressure? If not, which has the higher pressure? Do the cells have the same internal (osmotically effective) concentration of solutes? 18. The osmotic force pulling water across a membrane is proportional to the difference in concentration of solutes across the membrane. A difference of 0.1 M produces a force of 2.4 atmospheres. A certain plant cell in water has a turgor pressure of 7 atmospheres. A. What concentration of sucrose would be needed for incipient plasmolysis? (Assume that the cell wall does not shrink noticeably as it loses turgor.) B. What concentration would be needed to plasmolyze the cell to the point that the volume of the protoplast (contents inside the plasma membrane) is one-half the volume of the space inside the cell wall. 19. Penicillin kills bacteria by interfering with their cell wall synthesis. When they grow in penicillin, breaks develop in their cell walls. Suggest reasons why under certain conditions bacteria might survive in media containing penicillin. C. Enzymes 20. Certain proteins called channels allow hydrophilic substances (e.g., glucose) to cross a cell membrane. Are these proteins enzymes? Explain why they might be considered to be enzymes (and why not). 21. You purified an enzyme protein (lysozyme) from your tears, but when you try to assay its activity, it doesn t work. Suggest reasons why not.
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BIS2A T.M. Murphy Page 6 ANSWERS TO CELL PROBLEMS 1. a. fractionation b. light microscopy c. electron microscopy d. electron microscopy, fractionation e. light, electron microscopy f. fractionation g. electron microscopy, fractionation 2. 1000 cubic µm; 10-15 cubic meters; 5 x 10 9. 3. 3/10 µm -1 ; 3/100 µm -1 ; think about the surface-to-volume ratio of different shapes. 4. The vacuole: it confines cytoplasm to the sides of the cell and thus keeps cytoplasm in close contact with the cell surface. 5. This is an open-ended question. Possible answers depend on the assumptions you make about the parameters for life. General possibilities: the presence of complex molecules that are unstable under the ambient redox conditions; reproduction in the presence of an energy source (light, food); a heat-destructible catalytic system (but note the problem with the last test in the Viking missions). More Earth-life-specific tests could look for RNA, DNA, proteins. 6. A, plant; B, fungus; C, protist; D, bacterium; E, animal 7. cell wall 8. plastids 9. nucleus 10. The secretion of proteins occurs through the function of the endomembrane trafficking system--er to Golgi to plasma membrane--with vesicles moving proteins between components. The vesicle movement is powered and directed by motor proteins moving along microfilaments. 11. cytosol, plasma membrane, ER membrane, ER lumen, golgi membrane, golgi lumen, lysosome, other vesicle (peroxisome, lipid body), nuclear envelope, nucleoplasm, extracellular space (cell wall), tonoplast, vaculolar solution, mitochondrial outer membrane, mitochondrial inner membrane, mitochondrial intermembrane space, mitochondrial matrix, chloroplast outer envelope, chloroplast inner envelope, thylakoid membrane, stroma, thylakoid lumen 12. a) (rough) ER, ER vesicle, Golgi, lysosome b) (rough) ER, ER vesicle, Golgi, Golgi vesicle, extracellular compartment c) polysome, mitochondrial outer membrane,... matrix
BIS2A T.M. Murphy Page 7 d) (rough) ER, ER vesicle, Golgi, Golgi vesicle, plasma membrane e) polysome, nuclear envelope (pore complex), nucleoplasm 13. a and c 14. NaCl dissociates, so the effective osmotic concentration is 0.3 M. 15. Urea and glycerol are smaller molecules than glucose, and they enter the cell faster. As they enter the cell, water also will enter the cell by osmosis, and the cell will expand. If the concentration of solute (urea or glycerol) becomes the same inside and out, then the difference in total concentration of solutes is as great as if the cells were placed in water. 16. Yes, assuming that its plasma membrane is intact. 17. A. No, cell A had the higher pressure and the higher osmotic concentration. B. Yes, both cells have the same pressure and the same osmotic concentration. 18. A. 0.29 M B. 0.58 M 19. The medium has a concentration of solutes higher than the concentration inside the bacterial cells. The cells are not growing, perhaps because the medium lacks an essential nutrient. 20. Glucose does not cross a membrane rapidly, because it takes an input of energy to associate the hydrophilic glucose molecule (dissolve it) in the hydrophobic lipid phase. That energy is analogous to activation energy of a chemical reaction. The channel allows the glucose to cross without acquiring the energy (or as much energy). There is, however, no chemical reaction during the crossing. 21. Many enzymes need a particular ph and sometimes a specific cofactor (metal ion, small organic molecule). You may have purified it away from its cofactor. You may have irreversibly denatured the enzyme during purification. You may have associated it (possibly covalently) with an inhibitor.