IGCSE Biology Paper-3: Specimen Questions with Answers 235 - 236 of 358

Passage

Fig. shows a species of bacterium, Lactobacillus bulgaricus.

Bacteria from Other Groups

Question 235 (1 of 5 Based on Passage)

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Short Answer▾

The curve shown in Fig. A. is a sigmoid population growth curve.

Define the term growth.

Number of Living Bacterial Cells

Explanation

The increase in mass and increase in the number of individuals are twin characteristics of growth. The multi-cellular organisms grow by cell division. The animals grow up to a certain age, whereas the plants grow throughout their life. The cell division also occurs to replace lost cells. The unicellular organisms also grow by cell division but there it is a type of reproduction. In multicellular organisms, whether animals or plants, the growth and reproduction are mutually exclusive events. Though the non-livings, like mountains, sand mounds and snow also grow, if we consider the increase in body mass as criterion, but this growth is by accumulation of material on the surface, and not from inside as we see in animals. So, growth alone cannot be considered as a defining property of living organisms until and unless it is well specified.

Question 236 (2 of 5 Based on Passage)

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Short Answer▾

L. bulgaricus are added to milk to make yoghurt.

Fig. A. shows the changes in a population of L. bulgaricus during fermentation to make yoghurt.

Number of Living Bacterial Cells

Name the stages shown on Fig. A. Write your answers in the boxes on Fig. A.

Explanation

Number of Living Bacterial Cells

The bacterial growth curve represents the number of live cells in a bacterial population over a period.

  • There are four distinct phases of the growth curve: lag, exponential (log) , stationary, and death.
  • The initial phase is the lag phase where bacteria are metabolically active but not dividing.

  • The exponential or log phase is a time of exponential growth.

  • In the stationary phase, growth reaches a plateau as the number of dying cells equals the number of dividing cells.

  • The death phase is characterized by an exponential decrease in the number of living cells.