E. coli cells with a mutated Z gene of the lac operon cannot grow...

Lac operon is a cluster of genes encoding three proteins that bacteria use to obtain energy from the sugar lactose. There are three structural genes in lac operon. Promoter for lac $i$ (i) Lac Z (3063 bp) : This gene codes for the enzyme $\beta$ galactosidase which breaks lactose into glucose and galactose to be utilized in the cell. Therefore, E.coli cells with a mutated Z genes of the lac operon can not grow in medium containing only lactose as the source of energy because they cannot synthesize functional $\beta$-galactosidase. (ii) Lac Y (800 bp) : It codes for $\beta$ galactose permease enzyme which is a membrane bound protein and helps in transport of metabolites. (iii) Lac A (800 bp) : It codes for $\beta$ galactose transacetylase an enzyme that transfers an acetyl group from acetyl Co-A to $\beta$-galactosidase.