Biology Class 12 NCERT Solutions: Chapter 13 Organisms and Populations Part 8 (For CBSE, ICSE, IAS, NET, NRA 2022)

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Q. 16 With the help of suitable diagram describe the logistic population growth curve.

Answer:

  • The logistic population growth curve is commonly observed in yeast cells that are grown under laboratory conditions. It includes five phases: the lag phase, positive acceleration phase, exponential phase, negative acceleration phase, and stationary phase.
  1. Lag phase:
  2. Initially, the population of the yeast cell is very small. This is because of the limited resource present in the habitat.
  3. Positive acceleration phase:
  4. During this phase, the yeast cell adapts to the new environment and starts increasing its population. However, at the beginning of this phase, the growth of the cell is very limited.
  5. Exponential phase:
  6. During this phase, the population of the yeast cell increases suddenly due to rapid growth. The population grows exponentially due to the availability of sufficient food resources, constant environment, and the absence of any interspecific competition. As a result, the curve rises steeply upwards.
  7. Negative acceleration phase:
  8. During this phase, the environmental resistance increases and the growth rate of the population decreases. This occurs due to an increased competition among the yeast cells for food and shelter.
  9. Stationary phase:
  10. During this phase, the population becomes stable. The number of cells produced in a population equals the number of cells that die. Also, the population of the species is said to have reached nature՚s carrying-capacity in its habitat.
Growth Curve

A Verhulst − pearl logistic curve is also known as an S-shaped growth curve.

Q. 17 Select the statement which explains best parasitism.

(a) One organism is benefited.

(b) Both the organisms are benefited.

(c) One organism is benefited, other is not affected.

(d) One organism is benefited, other is affected.

Answer:

(d) One organism is benefited, other is affected.

  • Parasitism is an interaction between two species in which one species (parasite) derives benefit while the other species (host) is harmed.
  • For example, ticks and lice (parasites) present on the human body represent this interaction where in the parasites receive benefit (as they derive nourishment by feeding on the blood of humans) . On the other hand, these parasites reduce host fitness and cause harm to the human body.

Q. 18 List any three important characteristics of a population and explain

Answer:

  • A population can be defined as a group of individuals of the same species, residing in a particular geographical area at a particular time and functioning as a unit. For example, all human beings living at a particular place at a particular time constitute the population of humans.
  • Three important characteristics of a population are:
  1. Birth rate (Natality) :
  2. It is the ratio of live births in an area to the population of an area. It is expressed as the number of individuals added to the population with respect to the members of the population.
  3. Death rate (Mortality) :
  4. It is the ratio of deaths in an area to the population of an area. It is expressed as the loss of individuals with respect to the members of the population.
  5. Age Distribution:
  6. It is the percentage of individuals of different ages in a given population. At any given time, a population is composed of individuals that are present in various age groups. The age distribution pattern is commonly represented through age pyramids

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