In secondary active transport, lactose permease in E. coli utilizes:
Secondary active lactose permease LacY does not itself hydrolyze ATP nor perform direct phosphorylation of sugar substrate like phosphotransferase systems that import glucose as glucose-6-phosphate. Instead energy source is electrochemical proton gradient across E. coli inner membrane established by respiratory chain proton pumping. Lactose entry is strictly coupled to downhill H+ flow: one proton symported per lactose disaccharide. Experimental evidence shows that abolishing proton motive force with uncouplers carbonyl cyanide m-chlorophenyl hydrazone, nigericin, or by inhibiting respiration eliminates lactose accumulation even when ATP remains available, whereas glycolysis inhibitors that deplete ATP but preserve respiration spare transport. Sodium gradient is irrelevant for LacY though analogous bacterial MelB uses both ions. Therefore in textbook classification, lactose permease illustrates proton gradient-driven secondary active co-transport distinct from primary ABC importers requiring ATP binding proteins and sodium-dependent mammalian SGLT transporters driven by Na+ gradient. Such detailed mechanistic insight is frequently examined in competitive tests including NEET, CUET, CSIR-NET and GATE where transporter classification, energetics and disease linkage are integrated into problem-solving questions.
Ref: Kaback et al., J Gen Physiol 2008, Proton coupling LacY; Poolman et al., Mol Microbiol 2004.