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NCERT Solutions for Class 12 Biology Chapter 13 - Organisms and Populations

Neet Mock Test 2022

Aakash NCERT Solutions for Chapter 13 teach students to explore ecology at organismic and population level. Important topics in the chapter are organisms and their environment, niche, major abiotic factors affecting the organisms and their environments such as temperature, water, light, soil, responses to abiotic factors such as regulation, conformation, migration, and suspension. Aakash NCERT Solutions are provided in easy and simple language for the ease of students. The chapter also consists of detailed Diagrams, illustrations and flowcharts for easy comprehension.

 

 

Q1. Bacteria cannot be seen with naked eyes, but these can be seen with the help of a microscope. If you have to carry a sample from your home to your biology laboratory to demonstrate the presence of microbes with the help of a microscope, which sample would you carry and why?
Answer: If I have to carry a sample from my home to the biology laboratory to demonstrate the presence of microbes with the aid of a microscope, I will take curd because it consists of numerous lactic acid bacteria.

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

Answer: Marine water fishes will not be able to survive in freshwater because marine water fishes are adapted to live in saline water conditions. If they are shifted to freshwater, the process of osmoregulation in them will be affected. Due to the internal high salt condition in fish, the freshwater will come inside the body of the fish causing the fish to swell up which will ultimately result in bursting.


Q3. Most living organisms cannot survive at temperatures above 45ºC. How are some microbes able to live in habitats with temperatures exceeding 100ºC?

Answer: Some microbes can survive in temperatures as high as 100ºC because these microbes are adapted to live at high temperatures. These organisms contain enzymes that can work at high temperature. That is why they are found at thermal vents, hot springs, etc. The organisms which are unable to survive high temperatures have enzymes that get deactivated at high temperature.

Q4. List the attributes that populations possess but not individual.

Answer: The term population refers to a group of individuals of any species living in a well-defined geographical area, sharing or competing for similar kinds of resources and potentially interbreeding.

Population Attributes: A population has certain attributes that an individual organism does not have. For Example, Individuals may have births and deaths, but a population has birth rates and death rates. The various population attributes are as follows:

1. 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.

2. Death rate(mortality)- It refers to the live deaths in an area to the population of an area. It is expressed as the number of individuals removed from the population.

3. Sex ratio- It is the ratio of the number of male or female individuals per 1000 individuals.

4. Age distribution- Population at a given time is composed of different individuals belonging to different age groups. If the age distribution is plotted for the population, the resulting structure is called age pyramids. The shape of pyramids reflects the growth status of the population.

5. Population density- The number of individuals of a population present per unit area at a given time.


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 enough food resources are available to the individual. 

Exponential growth equation:

Nt​= No​ ert

Here,

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 loge

log 2/3loge=r

log 2/3 x 0.434=r

0.301/1.302=r

Thus, if a population grows exponentially double in size in 3 years, the intrinsic rate of increase (r) of the population will be 0.2311.

Q6. Name important defense mechanisms in plants against herbivory?

Answer: The defense mechanisms in plants against herbivory are as follows:

1. Presence of thorn, pine, prickles and bristles

2. Stinging hairs

3. Hairy coating

4. Harboring ants e.g., Acacia

5. Sticky glandular hairs e.g., Gnaphalium

6. Chemicals like latex, alkaloids, and tannins have a bitter taste, offensive smell, 

silica, and poisonous cardiac glycosides which herbivore animals do not eat.


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

Answer: An orchid that grows on a mango tree represents an interaction called commensalism. In this type of interaction, one species is benefited while another one remains unaffected. Orchid acts as an epiphyte on the mango tree as it does not derive nutrition from the mango tree but uses it for support while the mango tree remains unaffected.


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

Answer: The biological principle involved in the biological control method of managing pest insects is checking their population through predators and parasites. The predators act as biological control by preying upon pest insects.


Q9. Distinguish between the following: (a) Hibernation and Aestivation

                                                               (b) Ectotherms and Endotherm

Answer(a): The differences between hibernation and aestivation are as follows

      Hibernation

Aestivation

A state of reduction in activity in some organisms to escape winters

A state of reduction in activity in some organisms to escape extreme summers

E.g., Bears and squirrels

E.g., Fishes and snails


Answer 9(b) Ectotherms and Endotherm

The differentiation between ectotherms and endotherms is as follows:

Ectotherms

Endotherms

These are Cold-blooded animals

These are warm-blooded animals

They Change temperature according to environment temperature variation.

They maintain a constant internal environment despite the changing environment.



Q10. Write short note on: 

(a) Behavioral adaptations in Plants

(b)Behavioral adaptations in animals

(c) Importance of light to plant

(d) Effect of temperature or water scarcity and the adaptations of animals adaptations of desert plants and animal

Answer 10 (a): Behavioral adaptations in Plants=

 Plants found in deserts are well adapted to overcome the harsh desert conditions such as water scarcity and scorching heat. Adaptations of desert plants include: 

1. Plants have an extensive root system to absorb underground water.

2. They bear thick cuticles and sunken stomata on the surface of their leaf to reduce transpiration.

3. In Opuntia, the leaves are entirely modified into spines, and photosynthesis is carried out by green stems.

4. A special pathway called CAM (C4 pathway) is found on desert plants. It enables the stomata to remain closed during the day to reduce the loss of water through transpiration.

Answer 10 (b): Behavioral adaptations in animals= 
Various organisms are affected by various environmental conditions. To overcome these environmental conditions, organisms undergo adaptation such as migration, hibernation, aestivation, etc. The adaptations in the behavior of an organism are called behavioral adaptations.  For example, ectothermic animals and certain endotherms exhibit behavioral adaptations. Ectotherms Cold-blooded animals such as fish, 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, 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.

Answer 10 (c): Importance of light to plant=

The ultimate source of energy for plants is sunlight. Plants are autotrophic organisms, which need light in order to perform the process of photosynthesis. Plants need the sun because sunlight is made up of electromagnetic radiations which are given off as energy to the plants. Electromagnetic radiation acts as the key factor in a process that is called photosynthesis. The photons of light energy are received by the chlorophyll molecules and they give off energy in the form of electrons. These electrons are then through a cyclic as well as non-cyclic flow are converted into chemical energy i.e., ATP and NADPH in the process of photosynthesis by which organic substances are formed in plants. Thus, light is a very important factor for plants and for the survival of all the components of Earth.

Answer 10 (d): Effect of temperature or water scarcity and the adaptations of animals=

Effects of temperature or water scarcity and the adaptations of animals: Temperature is considered to be the most important ecological factor. The average temperature varies from one place to another and these variations in temperature affect the distribution of plants as well as animals on the Earth. There are animals that cannot tolerate extreme cold. These animals have thick coats off to prevent the loss of heat in Polar regions. Some animals show adaptations like hibernation and aestivation to escape harsh winter and summer, Animals also undergo adaptations to their natural habitats. E.g., animals found in colder regions have shorter ears and limbs in order to prevent the loss of heat from their bodies. Similarly, animals like polar bears have thick layers of fat below their skin conditions respectively in order to respond against the temperature conditions.

Water scarcity is another factor that forces animals to undergo certain adaptations to suit their natural habitat. Adaptations of desert animals are mainly to conserve water. For example, 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.


Q11. List the various abiotic environmental factors.

Answer: Abiotic environmental factors- These are the non-living conditions present in an ecosystem. The various environmental factors are:

a. Temperature, 

b. Water, 

c. Wind, 

d. Humidity, 

e. Precipitation, 

f. Soil, 

g. Minerals


Q12. Give an example for: (a) An endothermic animal

Answer: Endothermic organisms are those which can regulate their body temperature. Examples of endothermic organisms include birds such as cows, sparrows, pigeons, etc., and mammals like bears, rats, cows, rabbits, etc.

Q12. Give the example for: (b) An ectothermic animal

Answer: Ectothermic organisms cannot regulate their body temperature. Examples of ectothermic organisms include sharks, frogs, snakes, lizards, tortoises, etc.

Q12. Give an example for: (c) Organism of the benthic zone

Answer:  The benthic zone refers to the floor of the water body. Decomposing bacteria are found in the benthic zone.


Q13. Define population and community.

Answer: Population- A group of individuals belonging to the same species and residing in a particular geographical area at a given period of time is called population. All humans living in a region constitute the population.

Community- A community refers to groups of individuals of different species living in a particular area at a given period of time. Such individuals cannot breed with the members of other species.


Q14. Define the following terms and give one example for each

a. Commensalism

b. Parasitism

c. Camouflage

d. Mutualism

e. Interspecific competition

Answer:
(a) Commensalism- It is a kind of population interaction in which one species gets benefited while the other one remains unaffected. E.g., Orchids growing on trees.

(b) Parasitism- It is the interaction wherein one species (parasite) depends on the other species (host) for food and shelter. This is a kind of negative interaction as the parasite gets benefited but the host is harmed. E.g., Taenia solium living in the intestine of human

(c) Camouflage- This is an adaptive strategy adapted by prey species to escape the predators. For example, many species of frogs and insects camouflage in their surroundings and escape their predators.

(d) Mutualism- The symbiotic relationship between two species in which both species are benefited is called mutualism. For example, lichens show a mutual symbiotic relationship between fungi and blue-green algae, where both are equally benefited from each other. For example, Rhizobium and legumes. 

(e) Interspecific competition: In this interaction, both species (different) get negatively affected. For example, the competition between flamingoes and resident fishes in South American lakes for common food resources i.e., zooplankton.


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

Answer: The growth of a population takes place according to the availability of food, presence of predators and parasites, presence of other biotic and abiotic factors affecting a population, and other conditions in a particular environment. There are two main types of models i.e., exponential growth model and logistic growth model.

The logistic growth curve represents the logistic growth model. It represents growth when there is competition between individuals of a population for food and space. In this case, the fittest organism survives, reproduces and population growth is seen. In this type of growth, firstly the lag phase is shown followed by phases of acceleration and de-acceleration.

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


The different phases of the logistic growth curve are:

A. Lag phase- In this phase, no growth is seen due to the absence of food and other resources.

B. Positive acceleration phase- Limited growth in population is seen as the resources become available

C. Exponential growth phase- The population grows suddenly and shows very rapid growth. The curve rises steeply.

D. Negative acceleration phase- In this phase, environmental resistance increases and the growth rate of the population decreases.  

E. Stationary phase- In this phase, the growth becomes stationary and becomes stable.

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

A diagrammatic representation of Population growth curves is given below. The curve represents exponential growth and curve b represents log c growth.


The equation of the logistic growth curve is as follows:


Where N = Population density at time t
R = Intrinsic rate of natural increase
K = Carrying capacity Since resources for growth for populations are finite and become limiting, the logistic growth model is considered a more realistic one.


Q16. Select the statement which explains the best parasitism

(a) One organism is benefited.

(b) Both the organisms are benefited.

(c) One organism is benefited; the other is not affected.

(d) One organism is benefited; the other is affected.

Answer:(d) One organism is benefited, other is affected. In parasitism, one organism lives in the body of another. In this interaction, the parasite gets benefited while the host is negatively affected.


Q17. List any three important characteristics of a population and explain

Answer: The term population refers to a group of individuals of any species living in a well-defined geographical area, sharing or competing for similar kinds of resources and potentially interbreeding. Some characteristics of the population are as follows:

1. 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
2. Death rate(mortality)- It refers to the live deaths in an area to the population of an area. It is expressed as the number of individuals removed from the population.
3. Sex ratio- It is the ratio of the number of male or female individuals per 1000 individuals.
4. Age distribution- A population at a given time is composed of different individuals belonging to different age groups. If the age distribution is plotted for the population, the resulting structure is called age pyramids. The shape of pyramids reflects the growth status of the population.
5. Population density- The number of individuals of a population present per unit area at a given time.

 

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Also See
Chapter 1 Reproduction in Organisms Chapter 2 Sexual Reproduction in Flowering Plants Chapter 3 Human Reproduction
Chapter 4 Reproductive Health Chapter 5 Principles of Inheritance and Variation Chapter 6 Molecular Basis of Inheritance
Chapter 7 Evolution Chapter 8 Human Health and Disease Chapter 9 Strategies for Enhancement in Food Reproduction
Chapter 10 Microbes in Human Welfare Solutions Chapter 11 Biotechnology: Principles and Processes Chapter 12 Biotechnology and its Applications
Chapter 13 Organisms and Populations Chapter 14 Ecosystems Solutions Chapter 15 Biodiversity and Conservation
Chapter 16 Environmental Issues

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