Practice question

Diauxic growth phenomenon first described by Jacques Monod in Escherichia coli grown on mixture of two sugars illustrates hierarchical carbon utilization strategy to maximize energetic efficiency. Cells first consume preferred carbon source glucose offering highest ATP yield per carbon and fastest uptake via phosphotransferase system, generating metabolic signals via dephosphorylated EIIA-Glc that reduce adenylate cyclase activity and cAMP levels plus cAMP-CRP catabolite activator protein thereby diminishing transcription of secondary catabolic operons needed for alternative sugars like lactose. When glucose is fully exhausted, growth rate temporarily crashes creating plateau known as diauxie lag during which derepression occurs and lac operon encoding lactose permease LacY and beta-galactosidase LacZ is induced enabling import and hydrolysis of lactose to glucose plus galactose. Growth curve thus shows two distinct exponential phases separated by brief lag. Molecular basis centers on PTS regulation, cAMP modulation and inducer exclusion where EIIA-Glc blocks lactose entry while glucose present. Oxygen depletion triggers anaerobic switch, binary fission halt corresponds to stationary entry, ATP absence equals death, none describe biphasic growth which specifically reflects sequential catabolism of two carbon sources.