Exponential (log) phase dynamics: During the exponential phase of batch culture, how do cell number and cell mass change with time?

Introduction / Context:
The exponential (log) phase is characterized by balanced growth in which cellular components and cell numbers increase proportionally. Understanding this phase is essential for physiological experiments, enzyme production, and estimating generation times.

Given Data / Assumptions:

• Nutrients are not yet limiting and waste products have not accumulated to inhibitory levels.
• Environmental conditions (temperature, pH, aeration) remain constant.
• Population is genetically uniform and adapted to the medium (post-lag phase).

Concept / Approach:
In exponential phase, each cell divides after a roughly constant generation time, so both total cell number and biomass increase geometrically. On a semi-log plot, growth appears as a straight line; on a linear plot, it appears as an accelerating curve. Biochemical composition per cell remains relatively constant (balanced growth).

Step-by-Step Solution:
Recognize that exponential phase implies constant generation time. Conclude that population doubles by a fixed factor over equal intervals. Therefore, both cell number and mass increase at a constant rate of doubling. Select the option describing doubling at a constant rate.

Verification / Alternative check:
Calculate generation time g from slope: log10(N) = log10(N0) + (t/g)*log10(2). Consistent slopes confirm balanced growth.

Why Other Options Are Wrong:

• Increase-then-decrease, decrease, or constant mass describe lag, stationary, or death phases, not exponential.
• Unpredictable oscillations do not occur in simple batch culture under stable conditions.

Common Pitfalls:
Confusing “constant rate” with “linear increase”; exponential growth doubles over equal time intervals but is not linear in absolute numbers.

Final Answer:
In exponential phase, cells double at a constant rate.