Final answer:
A cell membrane with a greater proportion of unsaturated phospholipid tails will be more fluid due to increased spacing between the molecules caused by kinks from double bonds, as opposed to the densely packed saturated fatty acids.
Step-by-step explanation:
A cell membrane with a greater proportion of unsaturated phospholipid tails will have greater membrane fluidity at a given temperature. Unsaturated fatty acids in phospholipid tails contain double bonds between carbon atoms, which introduce kinks that prevent the fatty acids from packing tightly together. This increased spacing allows for more mobility among the molecules, resulting in a more fluid and less rigid membrane, particularly at lower temperatures.
Contrastingly, saturated fatty acid tails lack these double bonds and are straighter, allowing them to pack closely together and form a denser and more rigid membrane structure, especially in cooler environments. Organisms can respond to temperature changes by adjusting the balance between unsaturated and saturated fatty acids within their membranes to maintain optimal fluidity and function.