The mechanism of proton-coupled lactose transport in E. coli is an example of:
In Escherichia coli, lactose accumulation during lactose operon induction exemplifies secondary active transport powered by proton motive force, not direct ATP hydrolysis or simple diffusion. LacY permease co-transports one H+ and one lactose molecule; downhill H+ movement through transporter allows uphill lactose concentration. Gradient is sustained by primary H+ extrusion by respiratory chain complexes pumping protons outward, generating proton electrochemical gradient consisting of pH and membrane potential. When respiration is active, periplasmic proton binds outward-open LacY, increases affinity for lactose, transition to inward-open releases both in cytoplasm where subsequent beta-galactosidase action maintains low free lactose. Depletion of proton motive force by uncouplers such as CCCP abolishes accumulation without affecting respiratory enzymes. This chemiosmotic logic, conceptualized by Mitchell, conserved in human SGLT and PepT systems replacing H+ with Na+, demonstrates how bacterial nutrient uptake can be classified as secondary active proton symport rather than ABC transporter or passive diffusion mechanism. 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, Biochim Biophys Acta 2015, LacY proton coupling; Alberts, Chapter 11 secondary transport.