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NCERT Solutions for CBSE Class 12 Biology ‒ Chapter 13: Organism and Population

May 16, 2017 09:43 IST

    Download NCERT Solutions for CBSE Class 12 Biology ‒ Chapter 13: Organism and Population

    NCERT Solutions for Class 12 Biology, Chapter 13 ‒ Organism and Population are available here. You can also download these solutions with the help of download link given at the end of this article. With the help of these solutions, students can easily understand the concepts involved in the questions of this chapter. Questions from this chapter of NCERT textbooks are frequently asked in CBSE board exams and competitive exams.

    NCERT Solutions and questions for Class 12 Biology ‒ Chapter 13: Organism and Population are given below:

    Question1. How is diapause different from hibernation?

    Solution1: 

    A stage of suspended development to cope with unfavorable conditions is termed as diapause. Various species of Zooplankton and insects show diapause to tide over adverse climatic conditions during their development period.

    In hibernation or winter sleep is an animal escape winters (cold) by hiding themselves in their shelters. In this stage animal follows a state of inactivity by slowing their metabolism. The phenomenon of hibernation is exhibited by bats, squirrels, and other rodents.

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    Question2. If a marine fish is placed in a fresh water aquarium, will the fish be able to survive? Why or why not?

    Solution2: 

    Body of a marine fish is adapted to high salt concentrations of the marine environmen so, if we place the marine fish in a fresh water aquarium, then its chances of survival will diminish. In fresh water conditions, they are unable to regulate the water entering their body (through osmosis). The hypotonic environment of marine environment is helpful in entering the water in their bodies. This results in the swelling up of the body, eventually leading to the death of the marine fish.

    Question3. Define phenotypic adaptation. Give one example.

    Solution3:

    Adaptation in the physical appearance to cope the genetic mutations or certain environmental changes is called as phenotypic adaptation. For example, desert plants have thick cuticles and sunken stomata on the surface of their leaves to prevent transpiration. Similarly, elephants have long ears that act as thermoregulators.

    Question4. Most living organisms cannot survive at temperature above 45°C°. How are some microbes able to live in habitats with temperatures exceeding 100°C?

    Solution4:

    Archaebacteria (Thermophiles) are ancient forms of bacteria found in hot water springs and deep sea hydrothermal vents. They are able to survive in high temperatures (which far exceed 100°C) because their bodies have adapted to such environmental conditions. These organisms contain specialized thermo-resistant enzymes, which carry out metabolic functions that do not get destroyed at such high temperatures.

    NCERT Solutions for Class 12 Biology

    Question5. List the attributes that populations but not individuals possess.

    Solution5:

    If a group of individuals of the same species residing in a particular geographical area at a particular time and functioning as a unit then it is called as population. For example, all human beings living at a particular place at a particular time constitute the population of humans.

    Below are the main attributes or characteristics of a population residing in a given area are:-

    (a) Natality (Birth Rate): 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.

    (b) Population density: It is defined as the number of individuals of a population present  per unit area at a given time

    (c) Mortality (Death Rate): 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.

    (d) Sex ratio: It is the number of males or females per thousand individuals.

    (e) Age Distribution: It is the percentage of individuals of different ages in a given  population. At any given time, the population is composed of individuals that are present in various age groups. The age distribution pattern is commonly represented through age pyramids.

    Question6. If a population growing exponentially double in size in 3 years, what is the intrinsic rate of increase (r) of the population?

    Solution6:

    Exponential growth of a population is depends on the availability of sufficient amounts of food resources to an individual. Its exponential growth can be calculated by the following integral form of the exponential growth equation:

    Nt = No ert

    Where,

    Nt = Population density after time t

    NO= Population density at time t= zero

    r = Intrinsic rate of natural increase

    e = Base of natural logarithms (2.71828)

    From the above equation, we can calculate the intrinsic rate of increase (r) of a population.

    Now, as per the question,

    Present population density = x

    Then,

    Population density after two years = 2x

    t = 3 years

    Substituting these values in the formula, we get:

    ⇒ 2x = x e3r

    ⇒ 2 = e3r

    Applying log on both sides:

    ⇒ log 2 = 3r log e

    ⇒ (log 2)/(3 log e) = r

    ⇒ (log 2)/(3 × 0.434) = r

    ⇒0.301/1.302 = r

    ⇒ 0.2311 = r

    Hence, the intrinsic rate of increase for the above illustrated population is 0.2311.

    Question7. Name important defense mechanisms in plants against herbivory.

    Solution7:

    To protect themselves against herbivory, several plants have evolved both morphological and chemical defense mechanism.

    Morphological defense mechanisms:

    (1) Modification of Cactus leaves (Opuntia) into sharp spines (thorns) to deter herbivores from feeding on them.

    (2) In some plants, the margins of their leaves are spiny or have sharp edges that prevent herbivores from feeding on them.

    (3) Sharp thorns along with leaves are present in Acacia to deter herbivores.

    Chemical defence mechanisms:

    (1) Ttoxic cardiac glycosides present in Calotropis weeds can be prove to be fatal if ingested by herbivores.

    (2) Chemical substances such as nicotine, caffeine, quinine, and opium are produced in plants as a part of self-defense.

    Question8. An orchid plant is growing on the branch of mango tree. How do you describe this interaction between the orchid and the mango tree?

    Solution8:

    An orchid comes under the category of epiphyte which grows on the branch of a mango tree. Epiphytes are plants growing on other plants which however, do not derive nutrition from them. Therefore, the relationship between a mango tree and an orchid is an example of commensalisms, where one species gets benefited while the other remains unaffected. In the above interaction, the orchid is benefited as it gets support while the mango tree remains unaffected.

    Question9. What is the ecological principle behind the biological control method of managing with pest insects?

    Solution9:

    The ecological principle behind the biological control method of managing with pest insects is based on the concept of predation. Predation is a biological interaction between the predator and the prey, whereby the predator feeds on the prey. Hence, the predators regulate the population of preys in a habitat, thereby helping in the management of pest insects.

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    Question10. Distinguish between the following:

    (a) Hibernation and Aestivation

    (b) Ectotherms and Endotherms

    Solution10:

    (a) Hibernation and Aestivation

    Hibernation

    Aestivation

    1. State of reduction of activity in some organisms to escape cold winter conditions.

    1. State of reduction of activity in some organisms to escape desiccation due to heat in summers.

    2. Bears and squirrels inhabiting cold regions are examples of animals that hibernate during winters.

    2. Fishes and snails are examples of organisms aestivating during summers.

    (b) Ectotherms and Endotherms

    Ectotherms

    Endotherms

    1. Ectotherm are cold blooded animals. Variation of temperature with the  surroundings is ectotherm

    1. Endotherms are warm blooded animals. They maintain a constant body temperature.

    2. Fishes, amphibians, and reptiles are ectothermal animals.

    2. Birds and mammals are endothermal animals.

    Question11. 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:

    (a) Adaptations of desert plants and animals:

    (i) Adaptations of desert plants:

    Plants found in deserts are well adapted to overcome the harsh desert conditions such as water scarcity and scorching heat.to tap underground water, plants have an extensive root system. 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 photosynthesis is carried out by green stems. An special pathway that is C4 pathway is found to synthesize food. It enables the stomata to remain closed during the day to reduce the loss of water through transpiration.

    (ii) Adaptations of desert animals:

    Adaptations of desert animals are done for the same purpose as adaptation in desert plnts. It is done to conserve the water. For example Animals found in deserts such as desert kangaroo rats, lizards, snakes, etc. are well adapted to their habitat. The kangaroo rat found in the deserts of Arizona never drinks water in its life. It has the ability to concentrate its urine to conserve water.

    (b) Adaptations of plants to water scarcity:

    Plants found in deserts are well adapted to cope with water scarcity and scorching heat of the desert. Plants have an extensive root system to tap underground water. They bear thick cuticles and sunken stomata on the surface of their leaves to reduce transpiration. In Opuntia,the leaves are modified into spines and the process of photosynthesis is carried out by green stems. Desert plants have special pathways to synthesize food, called CAM (C4 pathway). It enables their stomata to remain closed during the day to reduce water loss by transpiration.

    (c) Behavioural adaptations in animals

    Various organisms are affected by various environmental conditions. To overcome the situation, organisms undergo adaptations such as migration, hibernation, aestivation, etc. These adaptations in the behaviour of an organism are called behavioural adaptations. For example, ectothermal animals and certain endotherms exhibit behavioral adaptations. Ectotherms are cold blooded animals such as fish, amphibians, reptiles, etc. Their temperature varies with their surroundings. For example, the 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. Similar burrowing strategies are exhibited by other desert animals. 

    (d) Importance of light to plants:

    The ultimate source of energy for plants is sunlight. Plants are autotrophic organisms, which need light for carrying out the process of photosynthesis. Plants need the sun because sunlight is made up of electromagnetic radiation which is given off as energy as the sun burns through its stores of hydrogen many millions of miles away from Earth. The electromagnetic radiation that plants receive from the sunlight that reaches Earth and penetrates its atmosphere is the key ingredient in a process that is called photosynthesis. Photosynthesis is important because it is the way in which plants derive their energy.

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

    The most important ecological factor is temperature. From one place to another the average temperature is varies. These variations in temperature affect the distribution of animals on the Earth. Animals which can tolerate a narrow range of temperature are called stenothermal animals. Those which can tolerate a narrow range of temperature are called stenothermal animals. Animals also undergo adaptations to suit their natural habitats. For example, animals found in colder areas have shorter ears and limbs that prevent the loss of heat from their body. Also, animals found in Polar regions have thick layers of fat below their skin and thick coats of fur to prevent the loss of heat.

    Some organisms exhibit various behavioural changes to suit their natural habitat. These adaptations present in the behaviour of an organism to escape environmental stresses are called behavioural adaptations. For example, desert lizards are ectotherms. This means that they do not have a temperature regulatory mechanism to escape temperature variations

    Water scarcity is another factor that forces animals to undergo certain adaptations to suit their natural habitat. Adaptations of desert animals are done for the same purpose as adaptation in desert plnts. It is done to conserve the water. For example Animals found in deserts such as desert kangaroo rats, lizards, snakes, etc. are well adapted to their habitat. The kangaroo rat found in the deserts of Arizona never drinks water in its life. It has the ability to concentrate its urine to conserve water.

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    Question12. List the various abiotic environmental factors.

    Solution12:

    Abiotic components are nothing but all non- living components of an ecosystem. Abiotic environmental factors are temperature, water, light, and soil.

    Question13. Give an example for:

    (a) An endothermic animal

    (b) An ectothermic animal

    (c) An organism of benthic zone

    Solution13:

    (a) Endothermic animal: Endothermic animals are those that must generate their own heat to maintain their body temperature. Birds such as crows, sparrows, pigeons, cranes, etc. and mammals such as bears, cows, rats, rabbits, etc. are endothermic animals.

    (b) Ectothermic animal: An ectothermic animal is one who cannot regulate its own body temperature. Fishes such as sharks, amphibians such as frogs, and reptiles such as tortoise, snakes, and lizards are ectothermic animals.

    (c) Organism of benthic zone: The benthic zone basically means the floor of a body of  water such as the ocean, a lake or river. Decomposing bacteria is an example of an organism found in the benthic zone of a water body.

    Question14. Define population and community.

    Solution14:

    Population:

    If a group of individuals of the same species residing in a particular geographical area at a particular time and functioning as a unit then it is called as population. For example, all human beings living at a particular place at a particular time constitute the population of humans.

    Community:

    If a group of individuals of different species, living within a certain geographical area. Such individuals can be similar or dissimilar, but cannot reproduce with the members of other species.

    Question15. Define the following terms and give one example for each:

    (a) Commensalism

    (b) Parasitism

    (c) Camouflage

    (d) Mutualism

    (e) Interspecific competition

    Solution15:

    (a) Commensalism: An interaction between two species where 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.

    (b) Parasitism:  This is a symbiotic relationship between two organisms in which one species (parasite-usually smaller) benefits for growth and reproduction to the harm of the other species (host-usually larger). An example of this is liver fluke. Liver fluke is a parasite that lives inside the liver of the host body and derives nutrition from it. Hence, the parasite is benefited as it derives nutrition from the host, while the host is negatively affected as the parasite reduces the host fitness, making its body weak.

    (c) Camouflage: The adaptation of a strategy by prey species to escape their predators. 

    Cryptically coloured so that they can easily mingle in their surroundings and escape their predators. For example many species of frogs and insects camouflage in their surroundings and escape their predators.

    (d) Mutualism: The symbiotic relationship between two species in which both species involved are benefited. For example, lichens show a mutual symbiotic relationship between fungi and blue green algae, where both are equally benefited from each other.

    (e) Interspecific competition: It is just opposite to mutualism where both species (different) get negatively affected. For example, the competition between flamingoes and resident fishes in South American lakes for common food resources i.e., zooplankton.

    Question16. With the help of suitable diagram describe the logistic population growth curve.

    Solution16:

    Observe yeast cells that is growing under laboratory conditions. Mostly logistic population growth curve is found in yeast cell growing in the laboratory condition. It consist of five phases: 

    (1) Lag phase: It is the Initial phase where the population of the yeast cell is very small. This is because of the limited resource present in the habitat.

    (2) Positive acceleration phase: 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.

    (3) Exponential phase: 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.

    (4) Negative acceleration phase: 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.

    (5) 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.

    Question17. 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.

    Solution17:

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

    Question18. List any three important characteristics of a population and explain

    Solution18:

    If a group of individuals of the same species residing in a particular geographical area at a particular time and functioning as a unit then it is called as population. For example, all human beings living at a particular place at a particular time constitute the population of humans.

    (1) Natality (Birth Rate): 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.

    (2) Mortality (Death Rate) : 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.

    (3) Age Distribution: It is the percentage of individuals of different ages in a given population. At any given time, the population is composed of individuals that are present in various age groups. The age distribution pattern is commonly represented through age pyramids.

    Download NCERT Solutions for CBSE Class 12 Biology ‒ Chapter 13: Organism and Population in PDF format

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