Science Practice Challenge Questions

5.2 Passive Transport


Membrane fluidity is influenced by the number of C-C double bonds (unsaturation) in the hydrocarbon tails of the lipids composing cell membranes. Fluidity is also dependent on temperature. The transit of materials through the cell membrane is controlled by fluidity. To maintain homeostasis, all organisms, including the simple bacterium E. coli, must sense the temperature of the environment and adapt to changes.

Samples of E. coli were grown at four different temperatures, and then researchers determined the fatty acid composition of their plasma membranes. The data are shown in the following table.

Fatty Acid 10 °C 20 °C 30 °C 40 °C
Myristic 17% 14% 14% 16%
Palmitic 18% 25% 29% 48%
Palmitoleic 26% 24% 23% 9%
Oleic 38% 34% 30% 12%
Ratio (U/S)        

Fatty acid compositions of the plasma membrane of E. coli were incubated at the temperatures shown. Myristic and palmitic acid are saturated, while palmitoleic and oleic acids each have one C-C double bond.

  1. Analyze the data to calculate the ratio of the fraction of unsaturated (U) to the fraction of saturated (S) fatty acids in the plasma membrane, and complete the table.
  2. Graph the ratio U/S versus growth temperature.
  3. Explain the response of E. coli to the temperature of the environment.
  4. We know that the temperature of the environment is sensed by E. coli through the temperature-dependent confirmation of enzymes that convert a single bond in the lipid tail to a double bond, and vice versa. Explain how the discovery of a mutant strain of E. coli could lead to this insight.

Aquaporins that allow for the movement of water across a cell membrane are gated. Both low and high pH within a plant cell can cause alterations of the membrane-spanning protein. Describe the advantage of this feedback mechanism. Predict how conditions of flooding or drought could activate this mechanism.

5.3 Active Transport


Rice plants grown in high-salt environments can actively transport sodium ions into the vacuole by the antiporter movement of protons out of the vacuole. In a study aimed at the development of salt-tolerant rye, researchers produced several varieties of transgenic rye. Measurements of height and stem diameter for the transgenic varieties (TG1 – TG4) are compared with the wild type varieties WT1 and WT2. Shown in the table below are the mean and standard deviation from measurements of a very large sample size.

Variety Height (cm) Stem thickness (cm)
WT1 9.667±0.333 1.975±0.095
WT2 11.867±0.376 2.238±0.204
TG1 15.420±1.146 2.723±0.261
TG2 15.600±0.909 2.903±0.323
TG3 14.925±0.767 2.633±0.073
TG4 16.100±0.682 3.160±0.169
Table 5.4
  1. Analyze the data. Are the heights and stem thicknesses in the transgenic plants significantly different than in the wild type plant? Justify your claim with evidence.
  2. Are the heights and stem thicknesses among the transgenic plants significantly different? Justify your claim with evidence.
  3. Plants from which these data were taken were grown in 10 mM NaCl solutions. Pose one question that researchers can investigate by growing the same varieties in a series of lower-salinity conditions.
  4. The Na+/H+ antiporter is an active transport system. Briefly explain negative feedback regulation of the movement of sodium into the vacuole of rye cells.