Question

Fastest phase of S-shaped growth curve is (a) lag phase (b) log phase (c) stationary phase (d) both (a) and (b).

Short answer

(b) log phase

Step-by-step solution

Understanding S-shaped (sigmoidal) growth curve

The S-shaped growth curve, also known as sigmoidal growth curve, is a pattern of growth characterized by three main phases - the lag phase, the exponential or log phase, and the stationary phase. Each phase represents different growth rates.

Identifying the fastest growth phase

The lag phase is a period of adaptation where growth is slow. The log phase (also called the exponential phase) is the period where the population size increases rapidly. The stationary phase is where growth rate decreases as resources become limited, and the population size stabilizes.

Choosing the correct option

Given the characteristics of each phase, the log phase is where the growth rate is the fastest. Therefore, we select (b) log phase as the correct answer.

Key concepts

Lag Phase

The lag phase is the initial stage in the S-shaped or sigmoidal growth curve. This period is best described as the 'adjustment phase' for organisms or cells as they adapt to a new environment or medium.

During this phase, there is minimal increase in number because the cells are busy adapting and preparing for future division. They are synthesizing the necessary enzymes, proteins, and other cellular components that are essential for growth and reproduction.

It's vital to understand that the lag phase is not a period of dormancy; rather, it's a phase of intense biochemical activity within the cell, even if population growth is not yet observable. To illustrate in a practical context, think of it as a 'warm-up' for runners before a race, preparing their bodies for the main event which corresponds, in the growth curve, to the log phase.

Log Phase

Following the lag phase, organisms enter the log phase, also known as the exponential phase. This is the period where the growth rate hits its peak and the population size increases exponentially.

Every organism has acquired the necessary resources to reproduce, and the environment is usually rich in nutrients, allowing for rapid growth. Cells divide at a constant rate, and this growth can be described by the equation \( N_t = N_0e^{rt} \) where \( N_t \) is the population size at time \( t \) \( N_0 \) is the initial population size, \( r \) is the growth rate, and \( e \) is the base of the natural logarithm.

To reinforce the concept, provide exercise improvement advice by incorporating practical examples, such as monitoring bacterial growth in a lab culture, where one can distinctly observe the exponential increase in the number of bacterial cells during the log phase.

Stationary Phase

Finally, the growth curve reaches the stationary phase, where the number of new cells being produced is roughly equivalent to the number of cells dying. At this point, essential nutrients in the environment are being depleted and waste products are accumulating, leading to a halt in population growth.

During stationary phase, survival strategies kick in, such as the formation of spores in bacteria. The culture density reaches a plateau, signifying a dynamic equilibrium between cell division and cell death. It's important to understand that while growth rate is stalled, metabolic activity is still ongoing as the population adapts to survival mode.

It's beneficial to approach this concept using real-life applications, like the importance of understanding stationary phase when optimizing fermentation processes in biotechnology or assessing the carrying capacity in ecological studies.