NCERT Solutions for Class 12 Biology Chapter 13 Organisms and Populations

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NCERT Solutions for Class 12 Biology Chapter 13 Organisms and Populations

Question 1.
How is diapause different from hibernation ?
Diapause is different from hibernation. The table below shows the differences between them :
NCERT Solutions for Class 12 Biology Chapter 13 Organisms and Populations Q1.1

Question 2.
If a marine fish is placed in a freshwater aquarium/will the fish be able to survive ? Why or why not ?
If a marine fish is placed in a freshwater aquarium, the fish will not be able to survive because of osmoregulation problems. The marine fish is adapted to live in saline water, so, if it is kept in freshwater, it will not be able to cope with outside hypotonic environment and it would face death.

Question 3.
Define phenotypic adaptation. Give one example.
Phenotypic adaptations are non-genetic changes occurring in living organisms due to various extreme environmental conditions, such as stress, extreme temperature, change of habitat. These includes acclimatization, behavioural changes, etc.

Question 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?
organisms survive at a temperature range of 0° to 40°C or less. However, there are some notable exceptions. Certain microorganisms live in hot springs and deep sea hydrothermal vents where temperature far exceeds 100°C. They survive at the high temperature due to occurrence of branched chain lipids in their cell membrane that reduce fluidity of cell membranes and occurrence of minimum amount of free water in their cells that provides resistance to high temperature

Question 5.
List the attributes that populations but not individuals possess.
population has some attributes which are not shown by its individual members. An individual born and dies, whereas the population has a birth rate (natality) and a death rate (mortality). Each population has certain characteristics, such as age distribution, genetic composition, growth models. An individual is either male or female but a population has a sex ratio.

Question 6.
If a population growing exponentially double in size in 3 years, what is the intrinsic rate of increase (r) of the population ?
The intrinsic rate of increase(r), can be calculated by the following exponential growth equation:
NCERT Solutions for Class 12 Biology Chapter 13 Organisms and Populations Q6.1

Question 7.
Name important defence mechanisms in plants against herbivory.
There are various defence measures for animals against predators. But plants, as they cannot move away, have certain defence mechanisms against herbivory. Their main defences are chemical toxins, such as strychnine, a poison produced by tropical vine, morphine by opium poppy, nicotine produced by tobacco plant. Apart from these chemicals, the common defence measure is presence of spines (modified leaves) on the leaves, stems of the plant, modifications of leaves into thorns, development of sharp silicated edges in leaves which prevent them against damage caused by herbivores.

Question 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?
An orchid growing as an epiphyte on a branch of mango tree is an example of commensalism. Commensalism is the relationship between individuals of two species of which one is benefited and the other is almost unaffected, i.e., neither benefited nor harmed. A commensal may get shelter (protection), or ride, or support instead of or in addition to food. Epiphytes are space parasites, they use trees only for attachment and manufacture their own food by photosynthesis. In Vanda, an epiphytic orchid, a special kind of aerial roots (hanging roots) hang freely in the air and absorb moisture with the help of their special absorptive tissue called velamen.

Question 9.
What is the ecological principle behind the biological control method of managing with pest insects?
The ecological principle behind the biological control method of managing with pest insects is predator-prey relationship. It is based on the ability of the predator to regulate prey population.

Question 10.
Distinguish between the following:

  1. Hibernation and Aestivation
  2. Ectotherms and Endotherms


  1. Differences between hibernation and aestivation are as follows :
    NCERT Solutions for Class 12 Biology Chapter 13 Organisms and Populations Q10.1
  2. Differences between ectotherms and endotherms are as follows:
    NCERT Solutions for Class 12 Biology Chapter 13 Organisms and Populations Q10.2

Question 11.
Write a short note on :
(a) Adaptations of desert plants and animals
(b) Adaptations of plants to water scarcity
(c) Behavioral adaptations in animals
(d) Importance of light to plants
(e) Effect of temperature or water scarcity and the adaptations of animals.
(a) Desert plants have developed various adaptations to increase absorption of water and to reduce water loss. They have well developed root system that penetrates deep into soil surface to absorb water, leaves have thick waxy covering and are mostly reduced into spines to reduce rate of transpiration. Stems are green (carry out photosynthesis) and fleshy (to store water and mucilage). Animals in arid areas have the ability to tolerate dry conditions. During daytime desert animals move to burrows and seldom come out. They minimize loss of water by producing dry feces and concentrated urine. Some of them have special physiological adaptations to cope with scarcity of water and hot arid desert conditions. E.g., kangaroo rat seldom drinks water and 90% of its water requirements are met from metabolic water produced by respiratory breakdown of fats. Likewise, camel does not sweat till body temperature rises upto 55° – 66°C. Its body cells are capable of tolerating upto 40% dehydration, etc.

(b) The xerophytes have special adaptations to withstand prolonged period of drought. These are of four types – ephemerals (drought escapers), annuals (drought evaders), succulents (drought resistants) and non-succulent perennials (drought endurers).Ephemerals are xerophytes which evade dry conditions by remaining in the form of seeds but live for a brief period and complete their life cycle during the rains. Common examples are Euphorbia prostrata, Tribulus terrestris, etc. Annuals are the xerophytes which continue to live for a few months even after rains in hot dry conditions. They have modifications to reduce transpiration. Common example is Echinops echinatus. Succulents have fleshy organs to store large amounts of water. Plants like Opuntia, Euphorbia, Asparagus have fleshy stems which are green and photosynthetic. Non-succulent perennial plants have many morphological modifications to withstand dry conditions. These have extensive root system that spreads deep in the soil and may reach water table to absorb maximum amount of water. They also possess waxy coatings or hairs on leaves, sunken stomata, reduced leaf blades etc. to reduce transpiration, Examples include Acacia nilotica, Calotropis procera, etc.

(c) Some organisms show behavioral adaptations to cope with variations in their environment. E.g., desert lizards. They lack the physiological ability that mammals have to deal with the high temperature. They keep their body temperature fairly constant by behavioral means. They enjoy in the sun and absorb heat when their body temperature drops below the comfort zone, but move into shade when the surrounding temperature starts increasing. Some species are capable of burrowing into the soil to hide and escape from too much heat.

(d) Light is the visible part of electromagnetic spectrum. It is an important ecological factor as it affects different physiological processes of plants, e.g., photosynthesis, transpiration, movements, flowering, seed germination, etc.

  • Photosynthesis: Light is essential for photosynthesis. The amount of photosynthesis depends upon the quality, intensity and duration of light.
  • Growth: It is favoured by increased availability of food, moderate light intensity and red light. UV radiations favour rosette habit in plants. Blue light favours moderate but normal growth.
  • Transpiration : Stomata generally open in light and close in darkness. Because of it light promotes transpiration. Transpiration is further enhanced by heating effect of light.
  • Germination: A number of seeds are sensitive to light. They are called photoblastic seeds. Positively photoblastic seeds germinate only in the presence of light, e.g., Vtscum, Lactuca, Rumex.
  • Movements : Small photosynthetic organisms show positive phototaxis in moderate light, e.g., Chlamydomonas, Euglena, Volvox. Plant shoots bend towards the source of light. It is positive photo- tropism. Flowers of some plants open or close in response to light and darkness.
  • Photoperiodism: Different plants require different periods of light for flowering which is known as photoperiod and the response of the plant to photoperiod in terms of flowering is called photoperiodism.

(e) Animals facing water scarcity have ability to tolerate arid conditions and reduce water loss. For example kangaroo rat seldom drinks water ; 90% of its water needs are met from v metabolic water i.c., water produced by respiratory breakdown of fat and remaining 10% is fulfilled from food. It is nocturnal in habit and seldom comes out of its burrow during the daytime. It has a thick body covering to minimise water loss due to evaporation. The kangaroo rat excretes nearly solid urine and faeces. Another example is that of camel, commonly called the ‘ship of desert’. Camel being very economical in water consumption, tolerates fluctuations in temperature, maintains blood stream moisture with body cells capable of tolerating extreme heat stress, does not sweat till body temperature rises to 55°-66°C, produces dry faeces’and concentrated urine. During periods of non-availability of water, the animal does not produce urine and stores urea.

Question 12.
List the various abiotic environmental factors.
Abiotic factors are non living factors and conditions of the environment which influence survival, function and behaviour of organisms. Various abiotic factors are :

(i) Temperature – Temperature is one of the most important environmental factors. The average temperature varies seasonally. It ranges from subzero level in polar areas and high altitudes to more than 50°C in tropical deserts in summer and exceeds 100°C in thermal springs and deep-sea hydrothermal vents.

(ii) Water – Next to temperature, water is the most important factor which influences the life of organisms. The productivity and distribution of land plants are dependent upon availability of water. Animals are adapted according to the water availability. E.g., aquatic animals are ammonotelic while xerophytic animals excrete dry feces and concentrated urine.

(iii) Light – Plants produce food through photosynthesis for which sunlight is essential source of energy. Light intensity, light duration and light quality influences the number of life processes in organisms, such as – photosynthesis, growth, trans-piration, germination, pigmentation, movement and photoperiodism.

(iv) Humidity – Humidity refers to the moisture (water vapour) content of the air. It determines the formation of clouds, dew and fog. It affects the land organisms by regulating the loss of water as vapour from their bodies through evaporation, perspiration and transpiration.

(v) Precipitation – Precipitation means rainfall, snow, sleet or dew. Total annual rainfall, seasonal distribution humidity of the air and amount of water retained in the soil are the main criteria that limit the distribution of plants and animals on land.

(vi) Soil – The soil is one of the most important ecological factor called the edaphic factor. It comprises of different layers called horizons. The upper weathered humus containing part of soil sustains terrestrial plant life.

Question 13.
Give an example for:

  1. An endothermic animal
  2. An ectothermic animal
  3. An organism of benthic zone.


  1. Hedgehog
  2. Frog
  3. Sponges

Question 14.
Define population and community.
Population can be defined as the total number of individuals of a species or any other class of an organism in a defined area or habitat or a group of individuals of the same species within a commnity.
Community can be defined as a naturally octurring assemblage of species living within a defined area or habitat.

Question 15.
Define the following terms and give one example for each.
(a) Commensalism
(b) Parasitism
(c) Camouflage
(d) Mutualism
(e) Interspecific competition
(a) Commensalism is the interaction between two living individuals of different species in which one is benefited while the other is neither harmed nor benefited except to a negligible extent. Example, the pilot fish (Naucrates) always accompanies shark without getting attached to its body. It feeds on falling pieces of food when the shark is eating the prey.

(b) Parasitism is an interaction between two living organisms of different species in which one organism called parasite obtains its food directly from another living organism called host. The parasite spends a part or whole of its life on or in the body of the host. Thus it is an interaction between two individuals wherein the parasite gets the benefit at the expense of the host.

(c) Camouflage (Cryptic appearance) is the ability to blend with the surrounding or background. It is the most common type of adaptation by animals to remain unnoticed for protection or aggression. Camouflage is protective to animals which are preyed upon by others. It is advantageous in predation for predators like praying mantis (Mantis religiosa) which are green coloured, resembling a thin stem and remain unnoticed till the prey comes within their striking range.

(d) Mutualism is an interaction between two species in which both species benefit. (The term symbiosis is often used synonymously with mutualism.) A well-known example of mutualism is the association between termites and the specialised protozoans that inhabit their guts. The protozoans, unlike the termites, are able to digest the cellulose of the wood that the termites eat and release sugars that the termites absorb. The termites benefit by being able to use wood as a foodstuff, while the protozoans are supplied with food and a suitable environment.

(e) In interspecific competition two or more populations usually belonging to the same trophic level or feeding habit compete with one another for the available natural resources. For example, in a forest area trees, shrubs, herbs and vines compete with one another for sunlight, nutrients, water, pollinators and dispersal agents.

Question 16.
With the help of suitable diagram describe the logistic population growth curve.
Logistic population growth curve or S-shaped or sigmoid growth curve is shown by the populations of most organisms. It has following phases : lag phase, log phase, exponential phase and stationary phase. In lag phase there is little or no increase in population. In log phase increase in population starts and occurs at a slow rate in the beginning. During exponential phase, increase in population becomes rapid and soon attains its full potential rate. This is due to the constant environment, availability of food and other requirements of life in plenty, absence of predation and interspecific competition and no serious intraspecific competition so that the curve rises steeply upward. The growth rate finally slows down as environmental resistance increases.

Finally, the population becomes stable during the stationary phase because now the number of new cells produced almost equals to the number of cells that die. Every population tends to reach a number at which it becomes stabilized with the resources of its environment. A stable population is said to be in equilibrium, or at saturation level. This limit in population is a constant K and is imposed by the carrying capacity of the environment. The sigmoid growth form is represented by the following equation :
NCERT Solutions for Class 12 Biology Chapter 13 Organisms and Populations Q16.1
r = intrinsic rate of natural increase
N = population density at time t; K = carrying capacity.

Question 17.
Select the statement which best explains 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.
(d) Parasitism is an association of two organisms of different sizes and species in which one is benefited and the other is harmed. The organism which is benefited is called parasite, and the organism that suffers is termed host. The parasite is weaker and smaller than host and gets nourishment and often shelter also from the host. The host can live without the parasite, but the parasite cannot survive without the host.

Question 18.
List any three important characteristics of a population and explain.
The three important characteristics of a population are :

  1. Birth and death rate
  2. Age structure
  3. Sex ratio

(i) The birth rate (natality) of a population refers to the average number of young ones produced per unit time (usually per year). In the case of humans, it is commonly expressed as the number of births per 1,000 individuals in the population per year. The death rate (mortality) of a population is the average number of individuals that die per unit time (usually per year). In humans, it is commonly expressed as the number of deaths per 1,000 persons in a population per year.

(ii) The age structure of a population is the percentage of individuals of different ages such as young, adult and old. Age structure is shown bv organisms in which individuals of more than one generation coexist. The ratio of various age groups in a population determines the current reproductive status of the population. It also indicates what may be expected in the future. Population is divided into three age groups; pre-reproductive, reproductive and post-reproductive.

(iii) The sex ratio of a population refers to the number of females per thousand male individuals. There were 933 females per 1,000 males in our country in 2001 census. The number of females in a population is very important as it is often directly related to the number of births. The number of males may be less significant because in many species a single male can mate with several females.

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