A bacterial cell moved from 25C to a new environment increases its unsaturated fatty acid composition from 40% to 50%. What does this indicate about the new environment?

Question

BlackTom AI · Accepted Answer

Membrane fluidity is controlled by the ratio of saturated to unsaturated fatty acids: unsaturated fatty acids (with cis double bonds) introduce kinks that prevent tight packing and lower the lipid melting point, increasing fluidity; saturated fatty acids pack tightly and raise membrane rigidity. Bacteria adjust this ratio (homeoviscous adaptation) to keep membrane viscosity appropriate for function.

1) Relevant concepts
- Unsaturated fatty acids increase membrane fluidity; saturated fatty acids increase rigidity.
- At lower temperatures membranes tend to become too rigid; cells compensate by increasing unsaturation.
- At higher temperatures cells often decrease unsaturation to prevent excess fluidity.

2) Step-by-step reasoning
- Starting condition: cell at 25°C with 40% unsaturated fatty acids.
- After transfer to a new environment the unsaturated fraction rises to 50%.
- Increasing unsaturation is a compensatory response to counteract decreased fluidity caused by a colder environment.

3) Calculation (intermediate steps)
- Absolute change = 50% − 40% = 10 percentage points.
- Relative change = (50 − 40) / 40 = 10 / 40 = 0.25 = 25% relative increase in unsaturated content.

4) Why the correct answer is correct
- The observed increase in unsaturated fatty acids is exactly the expected homeoviscous response when temperature drops: more unsaturated lipids keep the membrane fluid enough to permit proper protein function and transport and to prevent membrane "freezing" or excessive rigidity at lower temperature.

5) Why the other options are incorrect
- b: Warmer conditions normally lead cells to decrease, not increase, unsaturation to avoid overly fluid membranes.
- c: States warmer and making membrane more rigid; increasing unsaturation actually makes membranes less rigid, so this contradicts both the temperature direction and the effect.
- d: Major shifts in membrane unsaturation are classic temperature adaptations; pH changes do not selectively explain a rise in unsaturated fatty acids from 40% to 50%.

6) Conclusion
The increase in unsaturated fatty acids indicates the new environment is colder and the cell is increasing membrane fluidity to prevent freezing. This matches answer a.

A bacterial cell moved from 25C to a new environment increases its unsaturated fatty acid composition from 40% to 50%. What does this indicate about the new environment?单项选择题

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