Final answer:
It is likely that there will be a decrease in membrane fluidity when a bacterial culture is moved from 37°C to 25°C as the cell attempts to maintain homeostasis. Bacteria like E. coli change their membrane's lipid composition by increasing unsaturated fatty acids, and the U/S ratio graph would reflect this change by showing higher ratios at lower temperatures.
Step-by-step explanation:
When a bacterial culture growing at 37°C is transferred to a culture room maintained at 25°C, it is likely that there will be an initial decrease in membrane fluidity. This is because the lower temperature causes the fatty acid tails within the cell membrane to become more packed together and less dynamic, resulting in reduced fluidity. Bacterial cells, such as E. coli, adapt to environmental temperature changes by modifying the lipid composition of their plasma membranes; specifically, by altering the ratio of unsaturated (U) to saturated (S) fatty acids. An increase in unsaturated fatty acids, which have C-C double bonds, maintains fluidity at lower temperatures as these double bonds introduce kinks in the tails, preventing the lipids from packing too closely.
For instance, samples of E. coli grown at different temperatures showed variation in their plasma membrane fatty acid composition, with colder growth temperatures leading to a higher proportion of unsaturated fatty acids. This adaptation allows E. coli to ensure proper functioning of their membranes, maintaining the ability to control the transit of materials through the cell membrane even in colder environments. If we were to graph the ratio U/S versus growth temperature, we would expect to see an increase in the ratio as the growth temperature decreases, indicating a homeostatic response to maintain membrane fluidity.