The VIDYANKER team has thoughtfully prepared the NCERT Solutions for Class 12 Biology Chapter 13, "Organisms And Populations" These solutions are designed to help you tackle the NCERT textbook questions with ease. We recommend going through the chapter's theory before diving into the solutions for a deeper understanding. Feel free to share these NCERT Solutions for Class 12 Biology with others—learning is always better when shared!
QUESTIONS FROM TEXTBOOK SOLVED
1. How is diapause different from hibernation?
Ans: Diapause refers to the period of suspended growth or development seen in many insects as well as other invertebrates whereby metabolic activities are considerably reduced. Hibernation, on the other hand, is a sleep like state in which a few animals, such as fishes and amphibians, pass the winter season as a way of surviving food scarcity and cold weather.
2. If a marine fish is placed in a fresh water aquarium, will the fish be able to survive? Why or why not?
Ans: The chances of survival will come down in a marine fish if they are shifted to the fresh water aquarium. This is because their bodies are accustomed to the high salt concentration of the marine environment. In the condition of fresh water, they are not able to regulate the entering water into the body. This water entry is due to the hypotonic environment outside. It makes the body swell up and therefore end up killing the marine fish.
3. Define phenotypic adaptation. Give one example.
Ans: The variation in the body of an organism due to mutation in genes or some other kinds of environmental changes is phenotypic adaptation. These adaptive changes occur in an organism to overcome the environment of their natural habitats. For example, the cuticles and sunken stomata on the surfaces of the leaves of desert plants prevent transpiration. Similarly, the long ears of elephants may be considered as thermoregulators.
4. Most living organisms cannot survive at temperature above 45°C. How are some microbes able to live in habitats with temperatures exceeding 100°C?
Ans: Archaebacteria, or the thermophilic bacteria, are ancient forms of bacteria found in the hot water springs and deep sea hydrothermal vents.
They are resistant to high temperatures because their bodies have survived in such conditions. These organisms contain specialized thermo-resistant enzymes, which perform metabolic functions that are not destroyed at such high temperatures.
5. Last the attributes that populations but not individuals possess.
Ans: The attributes that populations possess, but not individuals, include:
- Population size or density: This refers to the total number of individuals in a population or the number of individuals per unit area or volume, which cannot be applied to a single individual.
- Birth rate (natality): This is the rate at which new individuals are added to the population through reproduction over a specific time period.
- Death rate (mortality): This is the rate at which individuals die in the population over a specific time period.
- Population growth: The overall change in the population size over time due to births, deaths, immigration, and emigration.
- Age distribution: The proportion of individuals in different age groups within a population, such as young, adult, and old.
- Sex ratio: The ratio of males to females in a population, which cannot be attributed to a single individual.
- Population dynamics: The changes in population size and composition over time, influenced by factors like migration, natural selection, and environmental conditions.
6. If a population growing exponentially double in size in 3 years, what is the intrinsic rate of increase (r) of the population?
Ans: If the population growing exponentially double in size in 3 years, the intrinsic rate of increase of this population will be towards maximum.
7. Name important defence mechanisms in plants against herbivory.
Ans: Several plants have evolved various mechanisms both morphological and chemical to protect themselves against herbivory.
(1) Morphological defence mechanisms:
(a) Cactus leaves (Opuntia) are modified into sharp spines (thorns) to deter herbivores from feeding on them.
(b) Sharp thorns along with leaves are present in Acacia to deter herbivores.
(c) In some plants, the margins of their leaves are spiny or have sharp edges that prevent herbivores from feeding on them.
(2) Chemical defence mechanisms:
(a) All parts of Calotropis weeds contain toxic cardiac glycosides, which can prove to be fatal if ingested by herbivores.
(b) Chemical substances such as nicotine, caffeine, quinine, and opium are produced in plants as a part of self-defense.
8. An orchid plant is growing on the branch of mango tree. How do you describe this interaction between the orchid and the mango tree?
Ans: A plant growth upon the branch of a mango tree is called epiphyte, that is plants growing on other plants or trees. This sort of interaction is commensalism wherein in orchid/ derives benefit of interaction while mango tree is not affected. The orchid growing on a branch of mango tree gets more light to grow and also, the mango is not harmed in any way.
Commensalism is referred to as a mutualistic association of two species that are of animals or plants living together normally where one species benefits from the association in such a way that its continuation does not affect the other one significantly.
9. What is the ecological principle behind the biological control method of managing with pest insects?
Ans: Most of the different biological control methods are predicated on the concept of predation. Predation is the biological interaction between a predator and prey where the predator feeds on the prey. Therefore, the predators control the population of preys in a habitat, hence aiding in the management of pest insects.
10. Distinguish between the following:
(a) Hibernation and Aestivation
(b) Ectotherms and Endotherms
Ans: (a) Hibernation and Aestivation
Hibernation:
- A state of prolonged dormancy that some animals enter during the cold winter months to conserve energy when food is scarce.
- During hibernation, an animal's metabolic rate decreases significantly, body temperature drops, and physiological functions slow down. They may go without food for extended periods.
- Examples: Bears, ground squirrels, and certain bats.
Aestivation:
- A state of dormancy or inactivity that some animals enter during hot or dry periods to avoid dehydration and conserve energy.
- Similar to hibernation, metabolic rates decrease, but it occurs in response to high temperatures or drought conditions. Animals may burrow into the ground or seek shelter to escape heat.
- Examples: Certain frogs, snails, and reptiles.
(b) Ectotherms and Endotherms
Ectotherms:
- Animals that rely on external environmental sources to regulate their body temperature.
- They do not internally regulate their body temperature; instead, they adjust their behavior (like basking in the sun or seeking shade) to maintain their temperature.
- Metabolic Rate: Typically have lower metabolic rates compared to endotherms and are more affected by ambient temperature.
- Examples: Reptiles, amphibians, and most fish.
Endotherms:
- Animals that can regulate their body temperature internally through metabolic processes, maintaining a relatively constant temperature regardless of external conditions.
- They have physiological mechanisms (like shivering and sweating) to control body heat production and loss.
- Metabolic Rate: Generally have higher metabolic rates to generate heat, allowing them to remain active in cooler environments.
- Examples: Birds and mammals.
11. 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.
Ans: (a) Adaptations of desert plants: The plants occurring in deserts have adapted to desert harsh conditions like scarcity of water and extreme heat. They have long root systems to absorb water from within the ground. Thick cuticles and sunken stomata are present on the surface of the leaves to decrease transpiration. In Opuntia, the complete leaf formation is modified into spines and green stems perform the function of photosynthesis. The desert plants have special pathways to synthesize food, named CAM (C4 pathway). It allows the stomata to stay closed during the day to minimize the wastage of water through transpiration.
Adaptations of desert animals: Animals, such as desert kangaroo rats, lizards, snakes, etc, dwelling in deserts are well adapted to their habitat. The kangaroo rat of Arizona deserts never drinks water in all its life. It has the ability to concentrate its urine to conserve water. Desert lizards and snakes spend the early mornings basking in the sun and dig themselves into the sand on afternoons to shun heat during the day time. This adaptation occurs in desert animals in order to prevent the loss of water.
(b) Adaptations of plants to water scarcity: The evergreen trees like Rhododendron, show water scarcity by an inward curling of the leaves. A more significant response is the closure of stomata, which reduces transpiration, but raises the internal temperature of die leaf affecting the rate of synthesis of proteins and photosynthesis. Deciduous trees of the temperature region drop their leaves in autumn, avoiding winter drought. Some water stressed plant's accumulate excessive amounts of inorganic ions.
(c) Behavioural adaptations in animals: Some organisms face temperature fluctuations. These organisms display adaptations such as hibernation, aestivation, migration, etc. to avoid environmental stress to match their native habitat. These adjustments in the behavior of an organism are known as behavioral adaptation. For example, some ectothermal animals and some endotherms display behavioral adaptations. Ectotherms are cold-blooded animals such as fish, amphibians, reptiles, etc. Their body temperature is dependent upon the environment. For instance, the desert lizard sunbathes in the morning when the temperature is quite low. As the temperature increases, the lizard burrows itself deep into the sand to avoid getting scorched by the sun. Desert animals implement similar burrowing tactics. Some endotherms like birds and mammals make themselves rest to avoid both the cold and the hot weather conditions. This is by hibernating during winters and aestivating in summers. They hide themselves in shelters like caves and burrows, etc. to keep away from variations in temperature.
(d) Importance of light to plants : The ultimate source of energy for plants is sunlight. These are autotrophic organisms that require light to carry out the process of photosynthesis. Light also plays an important role in generating photoperiodic responses occurring in plants. These plants react to variations in the intensity of light during different seasons to meet their photoperiodic requirements for flowering. Light also plays an important role in aquatic habitats for the vertical distribution of plants in the sea.
(e) Effects of temperature or water scarcity and the adaptations of animals: Animals are of two kinds because they are adapted to regulate their body temperature. Poikilothermous are the animals whose temperature varies with that of the environment e.g., invertebrates and vertebrates except birds and animals. On the other hand, homeotherms are the animals which can maintain their body temperature at a constant level e.g., birds and mammals.
12. List the various abiotic environmental factors.
Ans: All non- living components of an ecosystem form abiotic components. It includes factors such as temperature, water, light, and soil.
13. Give an example for:
(a) An endothermic animal
(b) An ectothermic animal
(c) An organism of the benthic zone
Ans: (a) Man, (Homo sapiens)
(b) Black bear
(c) Corals
14. Define population and community.
Ans: Population: A population refers to a group of individuals of the same species living in a particular geographical area at a given point in time and functioning as a unit. For example, all human beings living at a particular place at a particular time form a population of humans.
Community: A community is a group of different species individuals which occur within a defined geographic area. These individuals may be similar or dissimilar, but they cannot reproduce with individuals belonging to any other species.
15. Define the following terms and give one example for each:
(a) Commensalism
(b) Parasitism
(c) Camouflage
(d)Mutualism
(e) Interspecific competition
Ans: (a) Commensalism - Commensalism is an interaction where one species benefits while the other is left unaltered. Examples of commensalisms include an orchid growing on the branches of a mango tree and barnacles attached to the body of whales.
(b) Parasitism - Parasitism is an association in which one organism lives on or in the body of another, from which it obtains its food. For example, the parasites of humans include fleas and lice, various bacteria, protozoans and fungi.
(c) Camouflage - It is a strategy adapted by prey species to escape their predators. Organisms are cryptically coloured so that they can easily mingle in their surroundings and escape their predators. Many species of frogs and insects camouflage in their surroundings and escape their predators.
(d)Mutualism - Mutualism is that symbiotic association between two different species in which both the partners are benefited. Cross fertilisation or pollination of flowers of plants by insects (sometimes, by birds) is a mutual relation of very wide occurrence and much importance because many plants are sterile-self-.
(e) Interspecific competition - Interspecific competition: It is an interaction between individuals of different species where both species get negatively affected. For example, the competition between flamingoes and resident fishes in South American lakes for common food resources i.e., zooplankton.
16. With the help of suitable diagram describe the . ; logistic population growth curve.
Ans: The S-shaped growth curve is also known as the logistic growth curve. This describes a situation in which, under a new environmental condition, the population density of an organism rises slowly to establish itself and then accelerates rapidly, approaching the exponential growth rate. Many populations of microorganisms roughly follow this simple sigmoidal pattern. For example, in the inoculation of bacteria in a fresh culture medium, the sigmoidal or S-shaped growth curve is observed. The curve produced when a population approaches the environmental carrying capacity is an S-shaped curve. Carrying capacity is the maximum number of individuals which can be supported from a given population in a given time.
The S-shaped growth form is represented by the following equation- ” dNldt = rN[K-NIK]
Where, r = intrinsic rate of natural increase N = population density at time t K = carrying capacity
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.
Ans: (d) One organism is benefited, other is affected.
18.List any three important characteristics of a population and explain.
Ans: A population can be defined in simple terms as a collection of individuals belonging to the same species, living in a given geographical area at a given time, functioning as a unit. For example, all the human beings living at a particular place at a particular time form the population of humans.
There are three most important characteristics of a population.
- Birth rate (Natality): 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.
- Death rate (Mortality): It is the ratio of deaths in an area to the population of an area. It can be termed as the loss of individuals with respect to the members of the population.
- Age Distribution: It is defined as the number of people of different ages that exist within a given population. At any point in time, a population is composed of individuals that fall into different age groups. The age distribution pattern can be graphically represented through age pyramids.
