🧬AP Biology
6. Cell membranes are composed primarily of a phospholipid bilayer. The fluidity of the membrane is critical for proper cellular function and is influenced by the structure of the fatty acid tails in the phospholipids. Cells can alter the composition of their membranes in response to environmental temperature changes to maintain homeostasis.
Scientists studied a wild-type (WT) strain of cyanobacteria and a mutant strain (desA-) that lacks a functional gene encoding a fatty acid desaturase enzyme. This enzyme introduces double bonds into fatty acid chains. Both strains were grown at two different temperatures: 20°C and 35°C.
After the cultures reached a specific density, the scientists isolated the cell membranes and analyzed them. They measured the average percentage of unsaturated fatty acids in the membrane phospholipids (Figure 1A) and the relative membrane fluidity at the growth temperature (Figure 1B).
Figure 1. (A) Mean percentage of unsaturated fatty acids in membrane phospholipids and (B) relative membrane fluidity (measured at each culture’s growth temperature) for wild-type (WT) and desA− cyanobacteria grown at 20°C or 35°C. Bars show means; vertical error bars show ±1 error unit as specified in each panel.
Based on Figure 1A, identify the specific treatment group that has the highest percentage of unsaturated fatty acids.
Based on Figure 1B, describe the difference in membrane fluidity between the wild-type (WT) cells and the mutant (desA-) cells when grown at 20°C.
Scientists hypothesize that the DesA enzyme is required for maintaining membrane homeostasis at 20°C but is not required at 35°C. Use the data in Figures 1A and 1B to support the scientists' hypothesis.
Based on the chemical structure of fatty acids, explain why the trend observed in the wild-type (WT) cells in Figure 1A allows the cells to maintain the fluidity observed in Figure 1B as the temperature decreases.
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