Here's why:

* a. used to store energy: Lipids, particularly fats and oils, are very efficient energy storage molecules. They contain more energy per gram than carbohydrates or proteins.

* b. do not dissolve in water: Lipids are hydrophobic, meaning they repel water. This is because they are primarily composed of long hydrocarbon chains, which are nonpolar.

* c. make up part of cell membranes: Phospholipids are a type of lipid that form the structural basis of cell membranes. They have a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail, which allows them to create a barrier between the cell's interior and its surroundings.

Let me know if you have any other questions!