NCERT Class 12 Biology Solutions: Chapter 13 - Organisms and Populations

May 13, 2019
NEET

How is diapause different from hibernation?

Solution: Diapause is defined as the stage of suspended development in order to cope with unfavourable conditions. Many species of zooplankton and insects show diapause to survive adverse climatic conditions during their development.

Hibernation or winter sleep is a resting stage shown by animals to escape winters by hiding themselves in their shelters. They escape the winter season by entering a state of inactivity by slowing their metabolism. Bats, squirrels, and other rodents exhibit the phenomenon of hibernation.

If a marine fish is placed in a fresh water aquarium, will the fish be able to survive? Why or why not?

Solution: If a marine fish is placed in a fresh water aquarium it cannot survive. This is because their bodies are adapted to high salt concentrations of the marine environment. In fresh water conditions, they fail to regulate the water entering their body through osmosis. Water enters their body due to the hypotonic environment outside resulting in the swelling up of the body; this will lead to the death of marine fish.

Define phenotypic adaptation. Give one example.

Solution: Phenotypic adaptation involves changes in the body of an organism in response to genetic mutation or certain environmental changes. These responsive adjustments takes place in an organism in order to cope with environmental conditions present in their natural habitats.

For example, desert plants have thick cuticle and sunken stomata to prevent transpiration.

Most living organisms cannot survive at temperature above 45°C. How are some microbes able to live in habitats with temperatures exceeding 100°C?

Solution: Archaebacteria (Thermophiles) are ancient forms of bacteria found in hot water springs and deep sea hydrothermal vents. They can survive in high temperatures because they have adapted to such environmental conditions. These organisms contain specialized thermo-resistant enzymes, which carry out metabolic functions at high temperatures without getting destroyed.

List the attributes that populations but not individuals possess.

Solution: The main attributes or characteristics of a population residing in a given area are:

  • Birth rate (Natality): refers to the number of births during a given period in the population that are added to the initial density.
  • Death rate (Mortality): It is the number of deaths in the population during a given period.
  • Sex ratio: It is the number of females per 1000 of males.
  • Age Distribution: It is the percentage of individuals of different ages in a given population.
  • Population density: It is defined as the number of individuals of a population present per unit area at a given time.

Name important defence mechanisms in plants against herbivory.

Solution: Several plants have evolved various mechanisms both morphological and chemical to protect themselves against herbivory.  

Morphological defence mechanisms:

  • In cactus leaves are modified into sharp spines (thorns) to deter herbivores from feeding on them.
  • In Acacia sharp thorns along with leaves are present to deter herbivores.
  • In some plants, the margins of their leaves are spiny or have sharp edges that prevent herbivores from feeding on them.

Chemical defence mechanisms:

  • All parts of Calotropis weeds contain toxic cardiac glycosides, which is fatal if ingested by herbivores.
  • As a part of self-defence chemical substances such as nicotine, caffeine, quinine, and opium are produced in plants.

An orchid plant is growing on the branch of mango tree. How do you describe this interaction between the orchid and the mango tree?

Solution: An orchid growing on the branch of a mango tree is an epiphyte. Epiphytes are plants growing on other plants which do not derive nutrition from their host directly. Therefore, the relationship between a mango tree and an orchid is an example of commensalisms, where orchids get benefit of support from mango tree while the mango tree remains unaffected.

What is the ecological principle behind the biological control method of managing with pest insects?

Solution: The biological control methods are based on the principle of prey predator relationship. Predation is a biological interaction between the predator and the prey in which the predator feeds on the prey. In this way predators regulate the population of preys in a habitat, thereby helping in the management of pest insects.

Distinguish between the following:

(a) Hibernation and Aestivation

(b) Ectotherms and Endotherms

Solution:

  • Hibernation and Aestivation

S no: Hibernation S no:Aestivation1. Hibernation is a state of reduced activity in some organisms to escape cold winter conditions.   1.Aestivation is a state of reduced activity in some organisms to escape desiccation due to heat in summers.2. For example bears and squirrels inhabiting cold regions hibernate during winters.  2.For example fishes and snails aestivate during summers.

  • Ectotherms and Endotherms

S no: Ectotherms S no: Endotherms1.Ectotherms are cold-blooded animals. Their body temperature varies with their surroundings.1.Endotherms are warm-blooded animals. They maintain a constant body temperature irrespective of the surrounding temperature.2.Fishes, amphibians, and reptiles are ectothermic animals.2.Birds and mammals are endothermal animals.

Write a short note on

(a) Adaptations of desert plants and animals

(b) Adaptations of plants to water scarcity

(c) Behavioural adaptations in animals

(d) Importance of light to plants

(e) Effect of temperature or water scarcity and the adaptations of animals

Solution:

  • Adaptations of desert plants and animals  
  • Adaptations of desert plants
  • Plants have an extensive root system to get underground water.
  • They bear thick cuticles and sunken stomata on the surface of their leaves to reduce transpiration.
  • In Opuntia, the leaves are entirely modified into spines and green stems carry out photosynthesis.
  • Desert plants have special pathway for photosynthesis, called CAM. It enables the stomata to remain closed during the day to reduce the loss of water through transpiration.
  • Adaptations of desert animals
  • The kangaroo rat never drinks water in its life. It has the ability to concentrate its urine to conserve water.
  • Desert lizards and snakes bask in the sun during early morning and burrow themselves in the sand during afternoons to escape the heat of the day. This helps to prevent the loss of water.
  • Adaptations of plants to water scarcity  
  • Plants have an extensive root system to get underground water.
  • They bear thick cuticle and sunken stomata to reduce transpiration.
  • In Opuntia, the leaves are entirely modified into spines and green stems carry out photosynthesis.
  • Desert plants have special pathway for photosynthesis, called CAM. It enables the stomata to remain closed during the day to reduce the loss of water through transpiration.
  • Behavioural adaptations in animals

The adaptations in the behavior of an organism to escape environmental stress to suit their natural habitat are called behavioral adaptations.

For example:

  • Ectotherms are cold-blooded animals such as fish, amphibians, reptiles, etc. Their temperature varies with their surroundings.
  • Desert lizard basks in the sun during early hours when the temperature is quite low. However, as the temperature begins to rise, the lizard burrows itself inside the sand to escape the scorching sun.
  • Certain endotherms (warm-blooded animals) such as birds and mammals escape cold and hot weather conditions by hibernating during winters and aestivating during summers.
  • Importance of light to plants  
  • Sunlight acts as the ultimate source of energy for plants. Plants need light for carrying out the process of photosynthesis.
  • Light also plays an important role in generating photoperiodic responses occurring in plants.
  • Photoperiodic requirements for flowering in plants are dependent on changes in intensity of light during various seasons.
  • Light also plays an important role in vertical distribution of plants in the sea.
  • Effects of temperature or water scarcity and the adaptations of animals.

Temperature is the most important ecological factor. These variations in temperature affect the distribution of animals on the Earth.

  • Animals that can tolerate a wide range of temperature are called eurythermals.
  • Those which can tolerate a narrow range of temperature are called stenothermal animals.
  • Animals found in colder areas have shorter ears and limbs that prevent the loss of heat from their body. They also have thick layers of fat below their skin and thick coats of fur to prevent the loss of heat.
  • These lizards bask in the sun during early hours when the temperature is quite low. As the temperature begins to increase, the lizard burrows itself inside the sand to escape the scorching sun.
  • The kangaroo rat never drinks water in its life. It has the ability to concentrate its urine to conserve water.
  • Desert lizards and snakes bask in the sun during early morning and burrow in the sand as the temperature rises to escape the heat of the day.

List the various abiotic environmental factors.

Solution: Abiotic component: They are the non-living component of an ecosystem such as light, temperature, water, soil, air, inorganic nutrients, etc.

Give an example for:

(a) An endothermic animal

(b) An ectothermic animal

(c) An organism of benthic zone

Solution:

  • Endothermic animal:

Birds : sparrows, pigeons, cranes

Mammals : bears, cows, rats.

  • Ectothermic animal:

Fishes : sharks,

Amphibians : frogs

Reptiles : tortoise, snakes, lizards  

  • Organism of benthic zone: Decomposing bacteria

Define population and community.

Solution:

Population:  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.

Community:  A community is defined as a group of individuals of different species, living within a certain geographical area. These individuals can be similar or dissimilar, but cannot reproduce with the members of other species.

Define the following terms and give one example for each:

(a) Commensalism

(b) Parasitism

(c) Camouflage

(d) Mutualism

(e) Interspecific competition

Solution:

  • Commensalism: (+,0) Commensalism is an interaction between two species in which one species gets benefited while the other remains unaffected. An orchid growing on the branches of a mango tree and barnacles attached to the body of whales are examples of commensalisms.
  • Parasitism: (+,-) It is an interaction between two species in which one species (usually smaller) gets positively affected, while the other species (usually larger) is negatively affected.

For example, liver fluke. Liver fluke is a parasite that lives inside the liver of the host body and derives nutrition from the host, while the host is negatively affected as the parasite reduces the host fitness, making its body weak.

  • Camouflage: It is a strategy adapted by prey species to escape their predators. Organisms can easily mingle in their surroundings and escape their predators.

Example: Many species of frogs and insects camouflage in their surroundings and escape their predators.

  • Mutualism: (+,+) It is an interaction between two species in which both species involved are benefited.

For example, lichens show a mutual symbiotic relationship between fungi and algae, where fungi gets nutrient from algae and algae gets protection from environment by fungi.

  • Interspecific competition: (-,-) It is an interaction between individuals of different species where both species get negatively affected.

For example, the competition between flamingos and resident fishes in South American lakes for common food resources i.e., zooplankton.

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

Solution: 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.

  • Lag phase: Initially, the population of the yeast cell is very small.
  • Positive acceleration phase: During this phase, the yeast cell adapts to the new environment and starts increasing its population. In this phase, the growth of the cell is very limited.
  • Exponential phase: During this phase, the population of the yeast cell increases suddenly due to rapid growth. As a result, the curve rises steeply upwards.
  • Negative acceleration phase: During this phase, the environmental resistance increases and the growth rate of the population decreases.
  • Stationary phase: 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.

Logistic curve

Related: NCERT Class 12 Biology solutions - Chapter 12 - Biotechnology and its applications

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.

Solution: (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, Liver fluke is a parasite that lives inside the liver of the host body and derives nutrition from the host, while the host is negatively affected as the parasite reduces the host fitness, making its body weak.

List any three important characteristics of a population and explain

Solution: Three important characteristics of a population are:

  • Birth rate (Natality): refers to the number of births during a given period in the population that are added to the initial density.
  • Death rate (Mortality): It is the number of deaths in the population during a given period.
  • Age Distribution: It is the percentage of individuals of different ages in a given population.

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