But ruminants, such as cows, have bacteria in their guts which have the enzymes which can break apart cellulose, releasing the glucose units. Cholesterol and saturated lipids contribute to the build up of waxy deposits on the walls of blood vessels, causing heart disease and strokes. So, lipids don't dissolve very well in water. But, lipids form the basic structure of cell membranes. We need lipids - just not on the walls of our blood vessels.
Plant cells have a rigid cell wall, outside the cell membrane.
As a result, the cell membranes can be a bit less strong. Plant cell membranes thus have a slightly different composition than animal cell membranes. They don't have cholesterol and a kind of lipid called a phospholipid is unsaturated , meaning that all the places where hydrogen can bond to the carbon backbone aren't filled. Animal cell membranes generally are saturated.