What characterizes the exponential (log) phase?

• A. Cells divide at a constant, maximum rate, resulting in a straight line on a log scale growth plot
• B. Cells undergo adaptation with little or no net increase in cell number
• C. Nutrient depletion rapidly halts growth
• D. Cells exit the growth curve due to sporulation

Answer: (A)

During exponential (log) phase, the population grows at a constant, maximal rate because nutrients are plentiful and cells are in a balanced, healthy state. Each cell divides with essentially the same generation time, so the total cell number rises rapidly and predictably. On a plot of the logarithm of cell numbers versus time, this constant rate appears as a straight line, reflecting exponential growth (N = N0 e^(μt)). This straight-line behavior on a log scale is the telltale sign of exponential growth and a constant specific growth rate.

The other scenarios describe different phases or responses: an initial adaptation period with little net increase corresponds to lag phase, nutrient depletion leading to halted growth points to stationary or death phases, and sporulation occurs under stress and removes cells from active growth—not part of exponential growth.