Thermophiles are the individuals which can survive in very high temperature like hot springs. These include some bacteria. As we all are familiar that saturated fatty acids used to have high melting point while unsaturated fatty acids have low melting point.
In case of thermophiles, they have to survive at very high temperature. If they would have unsaturated fatty acids in their plasma membranes, they would have died. So, thermophiles adapt themselves in hot springs by making more of the saturated fatty acids in their plasma membrane that help them survive at such a high temperature.
If we talk about plants that adapt in low temperature, they tend to have high unsaturated fatty acids in their plasma membrane which lower the freezing temperature. Also, the plants accumulate more solutes in their cells which give them additional help for adaptation in low temperature.
The pKa of the value of average fatty acid is mentioned as 5. When the pH of the medium is increased as 2, 5 and 7 then the solubility of the fatty acid increase. The solubility of the fatty acid at pKa 5 is maximum at pH 7.
Phospholipids are a type of lipids with polar head and non-polar tail. This property makes them best suitable for inclusion in the plasma membrane. They align a double layer making their polar heads face outside and non-polar tails face inside.
This makes the plasma membrane polar in nature allowing water soluble solutes to pass through it easily thus making membrane fluid. Thus, phospholipids are suitable for membrane function and not as storage molecule because storage molecules prefer being fat loving (lipophilic).
On the other hand, if we talk about triacylglycerols, then they are more suitable as energy storage molecule. This is because they have such a type of structure that makes them hydrophobic in nature. This is suitable for them to get stored in fat tissues for their hydrophobic or lipophilic nature and not suitable for membrane function.
Triacylglycerols and phospholipids are similar in structure except at 3 rd position of glycerol. Here phospholipids have phosphate group that make them best suitable for membrane function.