Accumulation of lipophilic substances, many of which may be environmental chemicals, affects the membrane lipid order and consequently affects the functions of these proteins. Since, the function of important cellular proteins, such as the H+-ATPase strongly depends upon the integrity of the lipid bilayer, the activity of the H+-ATPase may be used as a sensitive indicator of the effect that a chemical may have on the viability of the cell. - [Read Yeast Plasma Membrane H+ -ATPASE Toxicity Test]