Compared to the neutral cytoplasmic pH (of about 7.0), the endosome maintains an acidic pH due to the activity of the ATP-dependent proton pumps present in the membrane of endosomes. The endosomal pH is associated with the stage of endosomal maturation. Early endosomes maintain pH at about 6.5, while late endosomes are at about 5.5. When endosomes fully mature into lysosomes, pH is at about 4.5.