NCERT Solutions for class 12th Biology Chapter 13 Organisms and Populations


Question 1. How is diapause different from hibernation?

Question 2. If a marine fish is placed in a fresh water aquarium, will the fish be able to survive? Why or why not?

Sol. Seawater or marine fish have evolved osmoregulatory mechanisms that permit them to excrete excess salts in a hypertonic environment. Since the salt content in their blood is much lower than that of seawater, they constantly tend to lose water and build up salt. To replace the water loss, they continually drink seawater. Since their small kidney can only excrete relatively small amount of urine, the excretion of salt additionally takes place in the gills where chloride cells secreted excess sodium and chloride ions out into the sea.

If marine fish is placed in a fresh water aquarium, it will not be able to survive as they are not able to regulate water entering its body. The body of a marine fish has high salt concentration to surrounding fresh water so water enters the fish’s body due to osmosis. Without any active regulation of this process, fishes would swell and get bigger and bigger leading to its death. In order to survive in freshwater, marine fish must not drink much water and its kidneys must produce a large amount of urine, leading to loss of salts. In order to maintain a sufficient salt level, chloride cells in the gills must take up ions from the water. But osmoregulation of marine fish works in reverse direction, so it will not be able to survive in opposite environment.

Note: Body of marine fish are adapted to high salt concentrations of the marine environ­ ment. Therefore, the body fluid concentration of marine fish is much higher than that of fresh water.

Question 3. Most living organisms cannot survive at temperature above 45°C. How are some microbes able to live in habitats with temperatures exceeding 100°C?

Sol. In most living organisms, the metabolic reactions and hence all the physiological functions proceed optimally in a narrow temperature range such as in humans, it is 37°C. Thus, they cannot survive at temperatures above 45°C as high temperature causes denaturation of their enzymes.

Some microbes such as archaebacteria.flourish in hot springs and deep sea hydrothermal vents where temperatures far exceed 100°C. Because:

  • They have thermostable enzymes which can perform their catalytic reactions even at high temperatures, without being denatured.
  • Some archaea have membranes with more stable components called glycerol-ether lipids which help them to survive in extreme environments.
  • These microbes avoid DNA denaturation at high temperature by proteins such as type I topoisomerase that catalyses positive supercoiling of closed circular DNA. Positively supercoiled DNA appears to resist degradation more than negatively supercoiled DNA.

Note: Thermophilic proteins have several adaptations that give the protein the ability to retain structure and function in extremes of temperature. Some of the most prominent are increased number of large hydrophobic residues, disulfide bonds, and ionic interactions.

Question 4. List the attributes that populations but not individuals possess.

Sol. Population is a group of individuals of same species living in a well defined geographical area, sharing or competing for similar resources and potentially interbreeding. A population has certain attributes whereas, an individual organism does not.

The list of attributes possessed by population but not by an individual are as follows:

  • Natality or Birth rate: It is the ratio of the number of births occurring in a population in a given period to the size of the population.
  • Mortality or Death rate: It is the ratio of the number of deaths occurring in a population in a given period to the size of the population.
  • Sex ratio: It is the ratio of males to females in a population. Population age distribution: It refers to the proportionate number of individuals in successive age categories in a given population.
  • Population density: It is the number of organisms that make up a population in a defined area.

Question 5. If a population growing exponentially double in size in 3 years, what is the intrinsic rate of increase (r) of the population?

Sol. Population growth is exponential when resources are unlimited. The intrinsic rate of increase (r) can be calculated by exponential growth equation which is:

Question 7. Name important defense mechanisms in plants against herbivory.

Sol. Herbivores are the predators of plants. Herbivory is the act of eating plants or plant-like organisms. This problem is severe for plants because they cannot run away from their predators.

So as to counter the attack of herbivores, plants have evolved diverse morphological and chemical defenses.

(i) Morphological defenses: These defenses include the development of some morphological structures or changes that provide mechanical protection on the surface of the plants. Examples of morphological defenses are:

  • Plants develop spiny structures like spines, thorns, and prickles which discourage animals by causing physical damage or by inducing rashes and allergic reactions. Thorns in Acacia and cactus are the most common morphological means of defense.
  • Trichomes are hair-like outgrowths from the epidermis of stem, leaves, and even fruits, and occur in several forms such as straight, spiral, stellate, hooked, and glandular to defend the plants against many insects.
  • In some plant leaves hardened to provide defense to plants against herbivores by reducing the palatability and digestibility of the tissues.

(ii) Chemical defenses: The chemical defenses involve toxic compounds that are produced constitutively or in response to plant damage, and affect the feeding, growth, reproduction, and survival of herbivores. Examples of such defenses are:

  • Calotropis produces highly poisonous cardiac glycosides which is why any cattle or goats do not browse on this plant.
  • Volatile oils produced by mint and sage discourage predators with noxious odors.
  • Quinine prevents herbivory by its repellent tastes.
  • Other alkaloids such as caffeine and opioids affect herbivores by causing excessive stimulation and lethargy respectively.
  • Some compounds become toxic after ingestion such as glycol cyanide in the cassava root releases cyanide only upon ingestion by the herbivore.
  • Foxgloves produce several deadly chemicals, namely cardiac and steroidal glycosides which on ingestion cause nausea, vomiting, hallucinations, convulsions, or death.
  • Other chemical substances that are produced by plants as defenses against grazers and browsers are nicotine, caffeine, quinine, strychnine, opium, etc.

Question 7. An orchid plant is growing on the branch of mango tree. How do you describe this interaction between the orchid and the mango tree?

Sol. An orchid is an epiphyte that grows on the branches of other plants such as mango for physical support. Most epiphytes live in tropical or temperate rainforests, where humidity and competition for sunlight are high. Thus, by growing on the branches of high plants, orchids are elevated receive more sunlight, and get protection from herbivores. In this interaction, only orchid is benefitted whereas the mango is neither harmed nor benefitted. So, this interaction between orchid and mango is an example of commensalism.

Question 8. What is the ecological principle behind the biological control method of managing with pest insects?

Sol. Biological control is to keep pest populations below damaging levels by using other living organisms that are their natural enemies e.g., the use of ladybirds and dragonflies to get rid of aphids and mosquitoes, respectively. The ecological principle behind the biological control method adopted in agriculture to manage pest insects is based on the natural mechanism called predation.

Note: 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 prey in a habitat, thereby helping in the management of pest insects.

Question 9. Distinguish between the following:

  • Hibernation and Aestivation
  • Ectotherm and endotherm

Question 10. 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.

Sol. (a)  Adaptations of desert plants and animals

  • The adaptations that allow plants to minimize water loss in high temperatures and water-scarce conditions of desert plants are as follows:
    • Presence of thick cuticles on their leaf surfaces.
    • Arrangement of stomata in deep pits (sunken) to minimize water loss through transpiration.
    • The presence of a special photosynthetic pathway (CAM) enables their stomata to remain closed during day time.
    • Reduction of leaves into spines and photosynthesis by the flattened stems in some desert plants like Opuntia.
  • The adaptations in desert animals that help to acquire and retain water and also to regulate temperature thereby making their survival possible in the harsh conditions of the desert are as follows:
    • Avoid being out in the sun during the hottest part of the day by either staying in their burrows as in most animals or sleeping during the daytime in their burrows or dens and hunting only during the night when the temperatures drop as in nocturnals.
    • Ability to concentrate urine so as to use minimal volume of water to remove excretory products.
    • Meeting the water requirements from water produced as by-product through internal fat oxidation in the absence of an external source of water such as in the kangaroo rat.
    • Excreting concentrated urine and solid faeces.
    • Possessing long body parts that provide greater body surface to dissipate heat.

(b) Adaptations of plants to water scarcity

  • These structures remain in the state of dormancy during unfavourable conditions and germinate to form new plants under favourable moisture and temperature conditions.

(c) Behavioural adaptations in animals

  • Animals show following behavioural adaptations to cope up with variations in their environment:
  • Some animals move away temporarily from the stressful habitat to a more hospitable area and return when stressful period is over. For example, manyanimals, particularly birds, during winter undertake long-distance migrations to more hospitable areas.
  • Animals may avoid the stress by escaping in time through hibernation as in bears during winter or by aestivation as in snails and fish to avoid heat and desiccation.
  • Some organisms such as desert lizards bask in the sun and absorb heat when their body temperature drops below the comfort zone, but move into shade when the ambient temperature starts increasing.
  • Some animals make burrows into the soil to hide and escape from the above-ground heat.

(d) Importance of light to plants

  • Light affects plants through three different aspects, i.e., quantity, quality and duration.
  • It is important for following aspects of growth and development in plants:
  • Plants use light as a source of energy for photosynthesis.
  • Light is required for chlorophyll formation in many higher plants.
  • Plant leaf stomata! opening is controlled by light. Both red and blue light stimulate stomatal opening.
  • Light is responsible for transpiration which is responsible for absorption of water and pulling of water column in the upward direction (ascent of sap).
  • Sunlight is also required by the plants to meet their photoperiodic requirement for flowering.

(e) Effect of temperature or water scarcity on adaptations of animals:

  1. Animals tend to adapt to the water scarcity by
  • Excreting solid urine and faeces.
  • Having spiny skin and highly cornified skin
  • Changing the feeding habits. For this they come out during the night or early morning for feeding purposes to prevent loss of water.

Question 11. List the various abiotic environmental factors.

Sol. Various abiotic environmental factors are:

  • Temperature: It is the most important environmental factor. It affects the kinetics of enzymes and through it the metabolic activity and other physiological functions of the organism. The levels of thermal tolerance of different species determine to a large extent their geographical distribution.
  • Water: It is another most important factor influencing the life of organisms. Life is unsustainable without water. The productivity and distribution of plants is also heavily dependent on water. For aquatic organisms, the quality (chemical composition, pH) of water becomes important.
  • Light: Photosynthesis, a process through which all living organisms get their food, is only possible when sunlight is available as a source of energy. Many plants are also dependent on sunlight to meet their photoperiodic requirement for flowering. It is important for many animals also. Animals use the diurnal and seasonal variations in light intensity and duration (photoperiod) as cues for timing their foraging, reproductive and migratory activities. The spectral quality of solar radiation is also important for life.
  • Soil: The nature and properties of soil which varies in different places, is dependent on the climate, the weathering process, whether soil is transported or sedimentary and how soil development occurred. Various characteristics of the soil such as soil composition, grain size and aggregation determine the percolation and water holding capacity of the soils. These characteristics along with parameters such as pH, mineral composition and topography determine to a large extent the vegetation in any area. This in tum dictates the type of animals that can be supported.

Question 12. Give an example for:

  • An endothermic animal
  • An ectothermic animal
  • An organism of benthic zone


  • Endothermic animals generate their body heat to maintain a constant body temperature independent of environment, e.g., humans (mammals).
  • Exothermic animals cannot maintain a constant internal environment and their body temperature changes with the changes in the external environment, e.g., fish, amphibians and reptiles.
  • Benthic animals live at the bottom of water body such as a lake or ocean, e.g., sea anemone.

Question 13. Define population and community.

Sol. Population is a group of individuals of same species living in a well defined geographical area, share or compete for similar resources, potentially interbreed, e.g., cormorants (aquatic birds) in a wetland, rats in an abandoned dwelling, teakwood trees in a forest tract, bacteria in a culture plate and lotus plants in a pond.

Community is a group or association of populations of two or more different species that occupy the same geographical area and interact with each other, e.g., a population of mallard ducks, and a population of cattail (Typha Latifolia) plants in a freshwater pond.

Question 14. Define the following terms and give one example for each:

  1. Commensalism 
  2. Parasitism
  3. Camouflage
  4. Mutualism
  5. Interspecific competition

Sol. (1) Commensalism

  • It is a interspecific positive interaction in which, one species is benefitted and the other is neither harmed nor benefitted.
  • The examples of commensalism are barnacles growing on the back of a whale, the cattle egret forging close to grazing cattle and clown fish living among sea anemone. In these examples, barnacles, cattle egret and clown fish are the organisms that are benefitted whereas whale, cattle and sea anemone do not get any benefit.

(2) Parasitism

  • It is an interaction between two species in which one organism called parasite) lives on or within the other organism called the host for food or shelter.
  • Parasites harm the host and may reduce the survival, growth and reproduction of the host eventually its population density. So, in this interaction, one organism is benefited and the other is harmed.
  • Example: ticks on dogs, Plasmodium feeding upon liver cells and RBCs of man.

(3) Camouflage

  • It is an adaptation in animals that allows them to blend in with their surroundings. This strategy helps prey to avoid being detected easily by the predator and helps predator to ambush their prey.
  • Example, stone fish always sit perfectly still, on the sea floor, in their preferred habitat of coral and rocky reefs and looks like an encrusted rock or lump of coral. This helps them remain unnoticed by its prey.

(4) Mutualism

  • In this interaction, both the interacting organisms in the association are benefitted.
  • The examples of are lichens (mutualistic relationship between a fungus and photosynthesising algae or cyanobacteria), mycorrhizae (association between fungi and the roots of higher plants) and association between plants and their pollinators.

(5) Interspecific competition

  • It is a form of competition in which individuals of different species compete for the same resources in an ecosystem. It is a type of antagonistic interaction.
  • Example, competition between flamingos and fishes for their common food, the zooplankton in the lake.

Question 15. With the help of suitable diagram describe the logistic population growth curve.

Sol. When resources are unlimited, populations exhibit exponential growth, resulting in a J-shaped curve, which is not the case in the real world. In nature, a given habitat has enough resources to support a maximum possible number, beyond which no further growth is possible. This population size, which represents the maximum population size that a particular environment can support, is called the carrying capacity, or K.

When resources are limited, populations exhibit logistic growth. In such a growth, a population initially show a lag phase, followed by phases of acceleration and deceleration and finally an asymptote, when the population density reaches the carrying capacity resulting in an S-shaped curve. This type of population growth is called Verhulst-Pearl Logistic Growth and is described by the following equation:

Question 16. Select the statement which explains best parasitism.

  • One organism is benefited.
  • Both the organisms are benefited.
  • One organism is benefited, other is not affected.
  • One organism is benefited, other is affected.

Sol. (d) Parasitism is a relationship between species, where one organism called the parasite lives on or in another organism called the host for food and/ or shelter. Parasite causes some harm to its host, and is adapted structurally to this way oflife. So, in such a relationship, parasite is benefited and the 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.

Question 17. List any three important characteristics of a population and explain.

Sol. The three important characteristics of a population are :

  • Population density : It refers to the number of organisms that make up a population in a defined area.
  • Natality or Birth rate : The birth rate is determined by the number of individuals that are added to a given population during a given period of time.
  • Mortality or Death rate : The death rate or mortality represents a decrease in a given population due to deaths during a given period of time. Generally, the death of individuals in a population is expressed by specific mortality which is described as the mortality for given age group.

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