Question

A flask of nutrient medium was inoculated with a suspension of yeast cells. Every two hours a sample of the culture was taken in order to estimate the growth of the population. The sampling continued after the population had reached the carrying capacity of the environment.
Which graph best represents the rate of growth of the population?

• A. A
• B. B
• C. C
• D. D
• E. E

Answer 1

Answer: (D)

D

Growth curves, which plot time against growth all have a characteristic S-shape. This curve is divided into three parts, the lag phase, during which cells prepare for growth, the exponential phase, when actual growth occurs, and the stationary phase when the growth ceases. The lag phase represents a period of rapid growth of cytoplasm so that the cells become larger without any increase in their number. The duration of the lag phase depends on the resynthesis of the enzyme systems required for growth and the availability of the necessary raw materials. During the exponential phase, each cell gives rise to two cells, the two to four and so on, so that the number of cells after n generations is 2n. The exponential growth usually ceases (giving the stationary phase) either because of lack of an essential nutrient or because waste products produced by the cells pollute the environment.

Therefore, the correct answer is option D.