Practice question

Historical diauxie experiment performed by Jacques Monod in 1940s used Escherichia coli growing in mixture of glucose and lactose where total sugar limited growth. Growth curve displayed first rapid exponential on glucose, plateau slight lag, second slower exponential on lactose. Reasoning glucose metabolized via Embden Meyerhof pathway directly to pyruvate yielding ATP quickly, while lactose requires uptake via LacY permease and hydrolysis by beta-galactosidase LacZ to glucose plus galactose then Leloir pathway. Regulation ensures lac operon silent during glucose phase due to low cAMP and absence inducer due to EIIA-Glc blocking LacY. When glucose exhausted, rising cAMP allows CRP binding, allolactose derepresses LacI, lac mRNA transcribed, enzymes produced allowing lactose metabolism. Although other pairs peptides lipids or amino acids nitrogen also cause sequential use, classic textbook example remains glucose lactose because molecular details lac operon, catabolite repression, inducer exclusion illustrate principles of gene regulation, metabolic economy, and signal integration of PTS, cAMP, and repressor proteins foundational for molecular biology curricula and industrial media formulation.