What is more likely reason for presence of lipids in acid insoluble pool during analysis of chemical composition of tissues?

Lipid is a biomicromolecule. But it is present in the acid-insoluble pool along with other bio-macromolecules. When we grind a tissue, we are disrupting the cell structure. Cell membrane and other membranes are broken into pieces, and form vesicles which are not water soluble. Therefore, these membrane fragments in the form of vesicles get along with the acid insoluble pool and hence in the macromolecular fraction. Lipids are not strictly macromolecules.