What are lipids primarily characterized by?

Lipids are primarily characterized by their nonpolar nature, which makes them insoluble in water. This characteristic stems from their molecular structure, which typically includes long hydrocarbon chains or rings that do not interact favorably with polar solvents like water. The nonpolar nature of lipids allows them to serve various biological functions, such as forming cell membranes (phospholipids) and storing energy (triglycerides), where their ability to aggregate and exclude water helps in creating a barrier or energy reservoir.

Other characteristics, such as solubility in water, being polar molecules, or high reactivity with carbohydrates, do not accurately describe lipids. Their primary function and properties revolve around their nonpolar nature, which distinguishes them from other macromolecules in biology.