NCERT Solutions for Class 12 Biology Chapter 11 Organisms and Populations

NCERT
SOLUTIONS
CLASS - 12th

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Class : 12th
Subject : Biology
Chapter : 13
Chapter Name : Organisms and Populations

Q1 How is diapause different from hibernation?

Answer. Diapause is a stage of suspended development to cope with unfavourable conditions.
Many species of Zooplankton and insects exhibit diapause to tide over adverse climatic conditions
during their development. Hibernation or winter sleep is a resting stage where in animals escape
winters (cold) by hiding themselves in their shelters. They escape the winter season by entering a
state of inactivity by slowing their metabolism. The phenomenon of hibernation is exhibited by
bats, squirrels, and other rodents.

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Q2 If a marine sh is placed in a freshwater aquarium, will the sh be able to survive? Why or why
not?

Answer. If a marine sh is placed in a freshwater aquarium, then its chances of survival will
diminish. This is because their bodies are adapted to high salt concentrations of the marine
environment. In fresh water conditions, they are unable to regulate the water entering their body
(through osmosis). Water enters their body due to the hypotonic environment outside. This results
in the swelling up of the body, eventually leading to the death of the marine sh.

Page : 239 , Block Name : Exercise

Q3 Most living organisms cannot survive at temperature above 45⁰C. How are some microbes able
to live in habitats with temperatures exceeding 100⁰C?

Answer. 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
100⁰ 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.

Page : 239 , Block Name : Exercise

Q4 List the attributes that populations but not individuals possess.

Answer. A population can be de ned as a group of individuals of the same species residing in a

particular geographical area at a particular time and functioning as a unit. For example, a human
beings living at a particular place at a particular time constitute the population of humans. The
main attributes or characteristics of a population residing in a given area are:-
(a) 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.
(b) Death rate (Mortality): 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.
(c) Sex ratio: It is the number of males or females per thousand individuals.
(d) 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.
(e) Population density: It is de ned as the number of individuals of a population present per unit
area at a given time.

Page : 239 , Block Name : Exercise

Q5 If a population growing exponentially double in size in 3 years, what is the intrinsic rate of
increase (r) of the population?

Answer. A population grows exponentially if suf cient amounts of food resources are available to
the 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 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
⇒ = r
3 log e

log 2
⇒ = r
3×0.434

0.301
⇒ = r
3×0.434

0.301
⇒ = r
1.302

⇒ 0.2311 = r

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

Page : 239 , Block Name : Exercise

Q6 Name important defence mechanisms in plants against herbivory.

Answer. Several plants have evolved various mechanisms both morphological and chemical to
protect themselves against herbivory.
(1) Morphological defence mechanisms:
(a) Cactus leaves (Opuntia) are modi ed 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.

Page : 239 , Block Name : Exercise

Q7 An orchid plant is growing on the branch of mango tree. How do you describe this interaction
between the orchid and the mango tree?

Answer. An orchid growing on the branch of a mango tree is an epiphyte. 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 bene ted while the other remains unaffected. In the above interaction, the orchid is
bene ted as it gets support while the mango tree remains unaffected.

Q8 What is the ecological principle behind the biological control method of managing with pest
insects?

Answer. The basis of various biological control methods is 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.

Q9 Distinguish between the following:
(a) Hibernation and Aestivation
(b) Ectotherms and Endotherms

Answer. (a) Hibernation and Aestivation

(b) Ectotherms and Endotherms

Q10 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.

Answer. (a) Adaptations of desert plants and animals:
(i) Adaptations of desert plants: Plants found in deserts are well adapted to cope with harsh desert
conditions such as water scarcity and scorching heat. 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 entirely modi ed into spines and photosynthesis
is carried out by green stems. Desert plants have special pathways to synthesize food, called CAM (
C pathway). It enables the stomata to remain closed during the day to reduce the loss of water
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through transpiration.
(ii) Adaptations of desert animals: 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. Desert
lizards and snakes bask in the sun during early morning and burrow themselves in the sand during
afternoons to escape the heat of the day. These adaptations occur in desert animals to prevent the
loss of 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 modi ed into spines and the process of photosynthesis is carried out by green stems. Desert
plants have special pathways to synthesize food, called CAM (C pathway). It enables their
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stomata to remain closed during the day to reduce water loss by transpiration.

(c) Behavioural adaptations in animals Certain organisms are affected by temperature variations.
These organisms undergo adaptations such as hibernation, aestivation, migration, etc. to escape
environmental stress to suit their natural habitat. These adaptations in the behaviour of an
organism are called behavioural adaptations. For example, ectothermic animals and certain
endotherms exhibit behavioral adaptations. Ectotherms are cold blooded animals such as sh,
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. Certain endotherms
(warm- blooded animals) such as birds and mammals escape cold and hot weather conditions by
hibernating during winters and aestivating during summers. They hide themselves in shelters such
as caves, burrows, etc. to protect against temperature variations.

(d) Importance of light to plants :
Sunlight acts as the ultimate source of energy for plants. Plants are autotrophic organisms, which
need light for carrying out the process of photosynthesis. Light also plays an important role in
generating photoperiodic responses occurring in plants. Plants respond to changes in intensity of
light during various seasons to meet their photoperiodic requirements for owering. Light also
plays an important role in aquatic habitats for vertical distribution of plants in the sea.

(e) Effects of temperature or water scarcity and the adaptations of animals :
Temperature is the most important ecological factor. Average temperature on the Earth varies

from one place to another. These variations in temperature affect the distribution of animals on
the Earth. Animals that can tolerate a wide range of temperature are called eurythermals. Those
which can tolerate a narrow range of temperature are called stenothermal animals. Animals also
undergo adaptations to suit their natural habitats. or 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. These lizards bask in the
sun during early hours when the temperature is quite low. As the temperature begins to increase,
the lizard burrows itself inside the sand to escape the scorching sun. Similar burrowing strategy is
seen
in other desert animals. Water scarcity is another factor that forces animals to undergo certain
adaptations to suit their natural habitat. Animals found in deserts such as desert kangaroo rats,
lizards, snakes, etc. are well adapted to stay in 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. Desert lizards and
snakes bask in the sun during early morning and burrow in the sand as the temperature rises to
escape the heat of the day. Such adaptations can be seen to prevent the loss of water.

Q11 List the various abiotic environmental factors.

Answer. Missing

Q12 Give an example for:
(a) An endothermic animal
(b) An ectothermic animal
(c) An organism of benthic zone

Answer. (a) Endothermic animal: Birds such as crows, sparrows, pigeons, cranes, etc. and
mammals such as bears, cows, rats, rabbits, etc. are endothermic animals.
(b) Ectothermic animal: Fishes such as sharks, amphibians such as frogs, and reptiles such as
tortoise, snakes, and lizards are ectothermic animals.
(c) Organism of benthic zone: Decomposing bacteria is an example of an organism found in the
benthic zone of a water body.

Q13 De ne population and community.

Answer. Population:
A population can be de ned as a group of individuals of the same species residing in a particular
geographical area at a particular time and functioning as a unit. For example, all human beings
living at a particular place at a particular time constitute the population of humans.

Community:
A community is de ned as 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.

Q14 De ne the following terms and give one example for each:
(a) Commensalism
(b) Parasitism
(c) Camou age
(d) Mutualism
(e) Interspeci c competition

Answer. (a) Commensalism:Commensalism is an interaction between two species in which one
species gets bene ted 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: It is an interaction between two species in which one species (usually smaller) gets
positively affected, while the other species (usually larger) is negatively affected. An example of
this is liver uke. Liver uke is a parasite that lives inside the liver of the host body and derives
nutrition from it. Hence, the parasite is bene ted as it derives nutrition from the host, while the
host is negatively affected as the parasite reduces the host tness, making its body weak.
(c) Camou age:lt 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 camou age in their surroundings and escape their
predators.
(d) Mutualism: It is an interaction between two species in which both species involved are
bene ted. For example, lichens show a mutual symbiotic relationship between fungi and blue
green algae, where both are equally bene ted from each other.
(e) Interspeci c competition: It is an interaction between individuals of different species where
both species get negatively affected. For example, the competition between amingoes and
resident shes in South American lakes for common food resources i.e., zooplankton.

Q15 With the help of suitable diagram describe the logistic population growth curve.

Answer. The logistic population growth curve is commonly observed in yeast cells that are grown
under laboratory conditions. It includes ve phases: the lag phase, positive acceleration phase,

exponential phase, negative acceleration phase, and stationary phase.
(a) Lag phase: Initially, the population of the yeast cell is very small. This is because of the limited
resource present in the habitat.
(b) 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.
(c) 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 suf cient food
resources, constant environment, and the absence of any interspeci c competition. As a result,
the curve rises steeply upwards.
(d) 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.
(e) 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.

A Verhulst-pearl logistic curve is also known as an S-shaped growth curve.

Q16 Select the statement which explains best parasitism.
(a) One organism is bene ted.
(b) Both the organisms are bene ted.
(c) One organism is bene ted, other is not affected.
(d) One organism is bene ted, other is affected.

Answer. (d) One organism is bene ted, other is affected.
Parasitism is an interaction between two species in which one species (parasite) derives bene t
while the other species (host) is harmed. For example, ticks and lice (parasites) present on the
human body represent this interaction where in the parasites receive bene t (as they derive
nourishment by feeding on the blood of humans). On the other hand, these parasites reduce host

tness and cause harm to the human body.

Q17 List any three important characteristics of a population and explain

Answer. A population can be de ned as a group of individuals of the same species, residing in a
particular geographical area at a particular time and functioning as a unit. For example, all human
beings living at a particular place at a particular time constitute the population of humans. Three
important characteristics of a population are:
(a) 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.
(b) Death rate (Mortality): 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.
(c) Age Distribution: It is the percentage of individuals of different ages in a given population. At
any given time, a population is composed of individuals that are present in various age groups.
The age distribution pattern is commonly represented through age pyramids.