Final answer:
The most thermodynamically unfavorable activity in a membrane is flip-flopping due to the energy required for a hydrophilic head to pass through the hydrophobic core. Membrane fluidity is maintained by varying the proportion of unsaturated and saturated fatty acids in the membrane in response to temperature changes.
Step-by-step explanation:
The most thermodynamically unfavorable membrane lipid activity in a membrane is flip-flopping. Flip-flopping refers to the movement of lipid molecules from one leaflet of the bilayer to the other, which is a rare and energetically costly event because it requires the hydrophilic head of the phospholipid to pass through the hydrophobic core of the membrane. In contrast, activities such as diffusion, rotation, and flexion are more frequent and thermodynamically favored since they involve movement within the same leaflet and do not oppose the hydrophobic nature of membranes.
To maintain membrane fluidity in fluctuating temperatures, organisms alter the composition of their membranes. In cold environments, the presence of unsaturated fatty acid tails, which have kinks due to double bonds, prevents the membrane from solidifying by creating space between the phospholipid molecules. Conversely, in high temperatures, organisms might increase the proportion of saturated fatty acid tails to maintain optimal fluidity and stability.