NCERT Solutions Class 12 Biology Chapter 13

Q.1 How is diapause different from hibernation?

Ans- Diapause and hibernation are both ways to avoid the stress of the unfavourable environment. When animals want to avoid cold winter season, they undergo a state of suppressed metabolic activities or become dormant and this is called hibernation. They hide in their shelters, thereby avoiding the harsh winter climate. This phenomenon is commonly seen in bears, rodents, etc. Diapause is a stage of suspended development and reduced physiological activity under unfavourable environmental conditions. This is mostly seen in zooplankton species in lakes and ponds.

Q.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 salt concentration in the sea is in the range of 30-35 (measured as salinity in parts per thousand) as compared to the freshwater aquarium in which salt concentration remains less than 5. The body of a marine fish is adapted to high salt concentration. If they are placed in freshwater, they cannot survive due to osmotic pressure problems. The water will enter their body through the process of osmosis (from low to high salt concentration) and the fish will not be able to bear the sudden change in the osmotic pressure. The body will swell and eventually burst.

Q.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?
Most living organisms cannot survive at temperatures above 45°C. However, some microbes, known as thermophiles are able to live in habitats with temperatures exceeding 100°C due to the presence of enzymes which are stable to heat. These thermostable enzymes do not denature under high temperatures allowing the metabolic activities of bacteria to go on.

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

Ans- The attributes that populations but not individuals possess are mentioned below:

a. Birth Rate: It is the rate at which new individuals are added to the population due to birth and is also known as natality. The birth rate is the total number of births per 1,000 individuals of a population each year.

b. Death Rate: It is also known as mortality and is the number of deaths in a given population per unit of time. The mortality rate is typically expressed in units of deaths per 1000 individuals per year.

c. Sex Ratio: The ratio of males and females in a population. The sex ratio is an important attribute of a population and tells about the percent of females and males in the population.

d. Age Distribution: Age distribution is the proportionate number of persons in successive age categories in a given population at any given time. If the age distribution (percent individuals of a given age or age group) is plotted for a population, the resulting structure is called age pyramid. The shape of the pyramid reflects the growth status of the population and tells whether it is expanding, stable or declining.

e. Population Density: Population density is the total number of individuals of a species per unit area or volume at a given time e.g. number of animals per square kilometre or the number of trees in a given area etc. The population density in an area may increase or decrease due to many factors like birth rate (natality), the death rate (mortality), migration (immigration and emigration) etc. Population density reflects the success of a species in any given area.

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

Ans-

Q.6 Name important defence mechanisms in plants against herbivory.

Ans- In nature, herbivores are the predators of plants. Plants cannot run away from predators unlike animals, so over the years, they have evolved several defence mechanisms to protect themselves from herbivores. These involve both morphological and chemical defence mechanisms, a few of which are listed below:

a. Morphological Defence Mechanisms

1. Thorns e.g. Cactus, Acacia
2. Mechanical protection on the surface of the plants e.g., hairs, trichomes, spines, and thicker leaves

b. Chemical Defence Mechanisms

1. Many plants produce and store chemicals which are harmful to the herbivores e.g. weed Calotropis growing in the fields produce highly poisonous cardiac glycosides and thus, cattle do not eat it.
2. Chemicals like nicotine, caffeine, quinine, opium, strychnine, that are extracted from plants for commercial reasons are actually produced by these plants as part of their defence mechanism.
3. Indirect defences against insect herbivory are mediated by the release of a mixture of volatile compounds that specifically attract natural enemies of the herbivores and/or by providing food (e.g., extrafloral nectar) and housing to enhance the effectiveness of the natural enemies, thereby killing the herbivore.

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

Ans- This type of interaction, where one species gets benefitted and other is neither benefitted nor harmed, is called commensalism. Mango tree provides elevation to the orchid plant so that it can receive a sufficient amount of sunlight and air. The orchid plant receives nutrients mainly from the moist air around them and sometimes from decaying organic waste of host plant or other insects. They do not parasitize on mango tree, rather take only physical support. Thus, the orchid plant gets benefitted but it neither harms nor benefits the mango tree.

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

Ans- The ecological principle behind the biological control method of managing with pest insects is based on a very simple fact that the predators keep the prey population under check by consuming it e.g. all insect species are suppressed by naturally occurring organisms and environmental factors.

Q.9 Distinguish between the following:

1. Hibernation and Aestivation
2. Ectotherms and Endotherms

Ans- (a) Hibernation and Aestivation

(b) Ectotherms and Endotherms

Q.10 Write a short note on
(a) Adaptations of desert plant and animals
(b) Adaptation 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 plant and animals: Adaptations of desert plants are the morphological and behavioural responses to maximise their chances of survival in desert (arid) regions. Scarcity of water is the main limiting factor that requires these plants to adapt to such an environment. Plants evincing adaptation to arid conditions are termed xerophytes.

• Succulent plants store water in fleshy leaves, stems or roots from which it is not lost easily. All succulent plants have extensive, shallow root systems that allow them to absorb water efficiently. Example: Cacti, agaves, aloes, elephant trees and many euphorbias.
• The stems and leaves of most species have waxy cuticles that make them nearly waterproof when the stomates are closed.
• Water is further saved by reduced surface areas; most succulents have few leaves, no leaves (most cacti), or leaves that are deciduous in dry seasons.
• Also, many succulents possess a water-efficient variant of photosynthesis called CAM (Crassulacean Acid Metabolism). CAM plants open their stomata for gas exchange at night. During the day, the stomata are closed. This helps the plant prevent water loss.

Desert animals face similar challenges for survival and have developed a multitude of adaptations to ensure their survival in the harsh conditions. Some of the adaptive responses to desert-dwelling are as follows:

Avoiding Heat:

• They are active in the early morning and late evening when the temperatures are not very high.
• Many animals are completely nocturnal such as bats, snakes, rodents foxes and skunks.
• Various animals use shade and burrows or dens during the heat of the day.
• Aestivation, dormancy during periods of heat and dryness is also witnessed.

Dissipating heat:

• They keep their mouth open to exhaust body heat.
• They have long appendages and enormous ears that act like the radiator of a car.
• They are light coloured which reflects heat and acts as camouflage in desert surroundings.
• They excrete faeces on the legs, where evaporation cools the rest of the body (birds).

Retaining water:

• Many burrows into moist soil where water is absorbed through the skin.
• Some obtaining their moisture needs from the food they eat.
• Excreting metabolic wastes in the form of uric acid to conserve water.

Acquiring water:

• Deriving water directly from plants, particularly succulents, such as cactus
• Specialised kidneys that extract water from their urine.
• Specialised organs that recapture exhaled moisture in the nasal cavities.
• Manufacturing water metabolically from the digestion of dry food items.

b. Adaptation of plants to water scarcity: Environmental stresses of low rain, low relative humidity with desiccating winds, and high summer temperatures characterise climates of deserts and coupled with low nutrient availability, produce severe limitations to the plant growth. Despite such stresses, desert contains surprisingly large amounts of plant biomass, and possess remarkable diversity of plant growth forms. The following strategies are adopted by various plants to survive the water scarcity of deserts:

• Drought-escaping plants: Plants that germinate and grow only when there is sufficient moisture available to complete their life cycle. Only their seeds persist during times of drought.
• Drought-evading plants: Non-succulent perennials which restrict their growth activity to periods when moisture is available. Typically, they are drought-deciduous shrubs which go dormant or die back during dry periods.
• Drought-enduring plants: Evergreen shrubs. The extensive root system is coupled with various morphological and physiological adaptations of their aerial parts enable these hardy xerophytes to maintain growth even in times of extreme water stress.
• Drought-resisting plants: succulent perennials. The water stored in their swollen leaves and stems is usually used very sparingly.

c. Behavioural adaptations in animals: Adaptations help organisms survive in their ecological niche or habitat and it can be anatomical, behavioural or physiological. Behavioural adaptations can be inherited or learnt, including tool use, language and swarming behaviour and allow animals to respond to life needs. Behavioural adaptations are the things organisms do to survive usually in response to some type of external stimulus. They can be instinctive or learnt. Instinctive behaviour is natural and need not be taught to animals. Some examples are what the animals eat, how it moves around and how it protects itself. Migration, hibernation, dormancy, ways of finding food, raising young ones and camouflage are ways of instinctive behavioural adaptations shown by various animals across a range of ecosystem. Learned behaviour is learnt by interaction with the environment and cannot be passed on to the next generation except by teaching the young ones. In animals, behavioural adaptation depends on the level of intelligence shown by the animal, the wise animal will quickly learn to make behavioural changes, in order to survive and this change can happen quickly over time. Moving in large groups is an example of behavioural adaptation; it helps protect the members of the group from predators.

d. Importance of light to plants: Light plays an important role in the development of a plant. Plants produce food by the process of photosynthesis and light is essential for this process as it is the source of energy. Without light, plants cannot carry out photosynthesis and thus, no food will be produced. The phototropism (hormones that control plant growth begin to accumulate in the plant’s stem, forcing the plant towards the light) shown by plants is also dependent upon the availability of light. Thus photosynthesis and phototropism depend on the availability of light sources for plants. The flowering in many plants is dependent upon the availability of sunlight and season. The vertical distribution of aquatic plants in any water/marine ecosystem is also dependent upon the availability of sunlight.

e. Effect of temperature or water scarcity and the adaptations of animals: The abiotic factors like temperature and water play a very important role in the distribution and adaptation of animals. As we know, the average temperature on land varies seasonally, across the globe. It decreases from the equator towards the poles and from plains to the mountain tops. It ranges from subzero levels in polar regions and high altitudes to >50°C in tropical deserts in summer. Other unique ecosystems are thermal springs and deep-sea hydrothermal vents where the average temperature exceeds 100°C. Many animals can tolerate a wide range of temperature however most animals are restricted to a narrow temperature range. Snow leopards and polar bears are adapted to polar regions – they have a thick layer of fat below their skin and have thick fur. Plants and animals found in desert areas have adapted themselves well so that they can face high temperatures and shortage of water.

Water is another very important abiotic factor that affects the living beings and depending upon its availability and quality, life has adapted itself. Where there is water scarcity, living beings have adapted themselves by making several modifications in their morphology and physiology. The succulent leaves, thorns, long roots in plants are few modifications that have occurred over the years in desert plants. Organisms living in lakes, oceans and rivers also face water problems – it is quality of water like chemical composition, pH, which makes the impact. The salt concentration in water also affects the life forms – some can tolerate a wide range of salinities (euryhaline) but others are restricted to a narrow range (stenohaline).

Q.11 List the various abiotic environmental factors.

Ans- The major abiotic environmental (non-living components) factors are temperature, water, light and soil.

Q.12 Give an example for:
(a) An endothermic animal
(b) An ectothermic animal
(c) An organism of the benthic zone

Ans- (a) An endothermic animal: Cow
(b) An ectothermic animal: Crocodile
(c) An organism of the benthic zone: Sea star

Q.13 Define population and community.

Ans- A population is defined as a group of all the organisms of the same species, who live in the same defined geographical area, have the capability of interbreeding and share common resources.

A community is defined as a group of various populations that share a common environment.

Q.14 Define the following terms and give one example for each:
(a) Commensalism
(b) Parasitism
(c) Camouflage
(d) Mutualism
(e) Interspecific competition

Ans- (a) Commensalism: The interaction between two species in a given area where one species is benefitted and the other is neither benefitted nor harmed is called commensalism. For example, the cattle and egret relationship is a very classical example of commensalism. The egrets always forage close to where the cattle are grazing because the cattle, as they move, stir up and flush out the vegetation insects that otherwise might be difficult for the egrets to find and catch.

(b) Parasitism: The relationship or interaction between two species in a given area which is beneficial for one species (parasite) but detrimental to the other species (host) is called parasitism. For example, a tick feeding on a dog, a mosquito feeding on a human or a leech feeding on a human.

(c) Camouflage: The act, means, or result of obscuring because of colouration or patterns that help an animal to appear to blend with its surroundings is called camouflage. Camouflage is common among invertebrates. It is also common in a variety of other animals. It is used by prey to help hide from predators and it is used by predators to help them conceal themselves when they are out to prey. For example, leaf mimic Katydids, Sea dragon, Australian Leaf Insect

(d) Mutualism: The relationship between two species that confer benefits on both the interacting species is called mutualism. For example, the association between the sea anemone and hermit crab is another example of mutualism. The sea anemones give protection to the crab using its stinging cells, and it remoulds its shell to fit the crab while the hermit crab allows the sea anemones to consume the remains of its food, thus it also provides the sea anemones with food supply.

(e) Interspecific competition: Interspecific competition is a form of competition in the ecosystem where individuals of different species living in the same area compete for the food or living space. For example, competition among carnivores for prey is an example of interspecific competition. It does not have an equal impact on each competitor: the larger competitor usually wins. Lions are much larger than hyenas, so in this case, hyenas are subordinate.

Another example is the competition between plants which is observed in forests where large trees dominate the canopy and thus allow little light to reach smaller competitors living below.

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

Ans- A population growing in a habitat with limited resources initially shows a lag phase, followed by a phase of exponential growth. The exponential growth of all populations is eventually curtailed by food availability, competition for other resources, predation, disease, or some other ecological factor. Once growth is limited by resources such as food, the exponential growth of the population begins to slow as competition for these resources increases. The fittest individual survives and reproduces. The growth of the population eventually slows to zero as the population reaches the carrying capacity (K) for the environment which is the stage where the resources available in that habitat are just sufficient to sustain that population level in that geographical area. The result is an S-shaped curve of population growth known as the logistic curve. A plot of N (population density at time t) in relation to time (t) results in a sigmoid curve graph. This type of population growth is called Verhulst-Pearl Logistic Growth.

Q.16 Select the statement which best explains parasitism.
(a) One organism is benefitted.
(b) Both organisms are benefitted.
(c) One organism is benefitted, other is not affected.
(d) One organism is benefitted, other is affected.

Ans- (d) Parasitism is best described as an interaction where one organism is benefitted while the other is affected. It is the interaction between two species in a given area which is beneficial for one species (parasite) but is detrimental to the other species (host).

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

Ans- Three important characteristics of a population are population size, density, and growth rate.

1. Population size: In nature, single individuals of any species are not found in isolation. They live in well-defined geographical areas and share and compete for similar resources. At a particular time, they function as a unit that can be termed as population. The size of the population is governed by birth rates and death rates. The rates are expressed as change in numbers with respect to members of the population. The population also is characterized by sex ratio which means the number of males versus the number of females in a given population. The age distribution (percent individuals of a given age or age group) is another characteristic that is shown by a population. This governs the growth status of the population.

2. Population Density: Population density is the total number of individuals of a species per unit area or volume at any given time. For example the number of animals per square kilometre or the number of trees per hectare. The population density in an area may increase or decrease due to many factors like birth rate (natality), the death rate (mortality), migration (immigration and emigration) etc. Population density reflects the success of a species in any given area. Population density increases if the number of births and the number of immigrants is more than the number of deaths plus the number of emigrants, otherwise, it decreases. Under normal condition, birth and death are two factors which influence population density the most.

3. Growth Rate: The rate at which a population increases in a given time is called the growth rate. The availability of food and space is the most important criteria that affect the growth rate. When resources in a habitat are unlimited, each species can grow to its maximum capacity. This kind of population growth shows exponential growth. Nature cannot sustain exponential growth due to limitation in resources. This leads to competition among the members of the populations. The fittest survives and reproduces. A population growing in a habitat with limited resources initially show a lag phase, followed by a phase of exponential growth. The exponential growth of all populations is eventually curtailed by food availability, competition for other resources, predation, disease, or some other ecological factor. Once growth is limited by resources such as food, the exponential growth of the population begins to slow as competition for those resources increases. The growth of the population eventually slows nearly to zero as the population reaches the carrying capacity (K) for the environment which is the stage where the resources available in that habitat are just sufficient to sustain that population level in that geographical area. The result is an S-shaped curve of population growth known as the logistic curve. A plot of N (population density at time t) in relation to time (t) results in a sigmoid curve graph.