Question
Beetroot cells contain a family of dark red pigments called betalains. The selectively permeable nature of the beetroot cells keeps the internal environment of the cell separate from the external environment of the cell. Researchers are interested in determining whether the selective permeability of beetroot cells is due to the cell membrane or if it is due to the cell wall.
Exposure to cellulase is known to damage the structure of the cell wall. An experiment is set up in which beetroot cells are placed in an aqueous solution with cellulase and in one without cellulase.
Which of the following results best refutes the alternative hypothesis that selective permeability is a consequence of the cell wall?
A. When beetroot cells are placed in a solution with cellulase, the solution turns dark red.
B. When beetroot cells are placed in a solution with cellulase, the solution remains clear.
C. When beetroot cells are placed in a solution, it turns dark red with or without cellulase present.
D. Since plant cells contain cell membranes, not cell walls, the alternate hypothesis cannot be tested.
▶️Answer/Explanation
Ans: B
Cellulase digests cellulose and damages the structure of the cell wall, not the cell membrane. The lack of color change in the solution indicates that the betalain is not leaking out of the beetroot cells even though the cell wall has been damaged. This refutes the alternative hypothesis.
Question
Two competing hypotheses exist regarding the cell membrane structure. One hypothesis states that membrane structure is static and membrane components throughout the bilayer are rigidly bound. Alternatively, the other hypothesis states that cell membranes are a fluid mosaic in which membrane components may drift within the bilayer around the surface of the cell. An experiment is set up in which membrane proteins of two different cells are fluorescently labeled with two different colors and then fused as shown in Figure 1.
A.
B.
C.
D.
▶️Answer/Explanation
Ans: D
The membrane proteins from each cell have mixed and drifted within the bilayer, which supports the fluid mosaic model of cell membranes. Cell membranes consist of a structural framework of phospholipid molecules that is embedded with proteins and steroids that may flow around the surface of the cell within the bilayer.
Question
The cell membrane is selectively permeable due to its structure. Thus, the internal environment of the cell is distinct from the external environment of the cell. One biologist hypothesizes that small nonpolar molecules readily pass through the membrane. Another biologist alternatively hypothesizes that these types of molecules require channel and transport proteins that are embedded in the membrane in order to move across the membrane.
Which of the following data would best refute this alternative hypothesis?
A. Ethanol is found in the cytosol of cells when they are briefly exposed to a ten percent ethanol solution.
B. Cells become oxygen deficient when membrane protein activity is blocked.
C. \(CO_2\) and \(N_2\) movement in and out of cells is unaffected when membrane protein activity is blocked.
D. Sodium ions cannot move across the cell membrane when membrane protein activity is blocked.
▶️Answer/Explanation
Ans: C
Both CO2 and N2 are small nonpolar molecules. Their ability to enter and leave cells normally when membrane protein activity is blocked would refute the alternative hypothesis.
Question
Aquaporins are channel proteins that facilitate the transport of water across the cell membrane. One group of researchers hypothesizes that without functional aquaporins, no water will be able to enter the cell. A different group proposes an alternative hypothesis, stating that even with nonfunctional aquaporins, a small amount of water will still cross the cell membrane. An experiment is set up in which plant cells with mutated (nonfunctional) aquaporins and plant cells with normally functioning aquaporins are both placed in distilled water.
Which of the following data would support the alternative hypothesis?
A. Cells with functional aquaporins exhibit low turgor pressure and are hypertonic.
B. Cells with functional aquaporins exhibit high turgor pressure and are hypotonic.
C. Cells with mutated aquaporins exhibit an absence of turgor pressure and are completely plasmolyzed.
D. Cells with mutated aquaporins exhibit moderate turgor pressure and are hypertonic.
▶️Answer/Explanation
Ans: D
Even with mutated aquaporins, a small amount of water is able to enter and leave the cell. Thus, turgor pressure is not lost and the alternative hypothesis is supported.
Question
Plant cell walls are composed of cellulose, while fungal cell walls are composed of chitin. A group of scientists hypothesize that this difference means the cell wall has largely different functions in plant cells and fungal cells. Alternatively, another group of scientists hypothesize that despite their biochemical differences, plant and fungal cell walls serve similar functions.
Which of the following observations would best support the alternative hypothesis described above?
A. Plant cell walls are found just outside the plasma membrane, while fungal cell walls are found just beneath the plasma membrane.
B. In both plant cells and fungal cells, the cell wall surrounds the outside of the cell membrane.
C. Some plant cells have secondary cell walls that confer additional rigidity, while fungal cells do not.
D. Photosynthesis occurs in plant cells, but it does not occur in fungal cells.
▶️Answer/Explanation
Ans: B
In both plants and fungi, a major function of the cell wall is to confer structural rigidity to the cell. Thus, in both cell types, the cell wall’s location is similar. This is evidence supporting the alternative hypothesis proposed above.