Ncert Solutions for Class 12 Biology Chapter 11 Organisms and Populations

Organisms and Populations ncert solutions: Class 12th biology chapter 11 ncert solutions

Textbook	NCERT
Class	Class 12
Subject	Biology
Chapter	Chapter 11
Chapter Name	Organisms and Populations class 12 ncert solutions
Category	Ncert Solutions
Medium	English

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Question 1: List the attributes that populations possess but not individuals.

Solution 1: Populations, as groups of individuals of the same species living in a particular area, possess several attributes that do not apply to individuals. Here are some key attributes of populations:

1. Population Size (N): Refers to the total number of individuals in a population.
2. Density: The number of individuals per unit area or volume, which gives insights into how crowded or sparse a population is.
3. Distribution/Dispersion: The spatial arrangement of individuals within a habitat (e.g., clumped, uniform, or random distribution).
4. Age Structure: The composition of the population based on the age of individuals, which affects reproduction and survival rates.
5. Sex Ratio: The ratio of males to females in a population, which can influence breeding and population dynamics.
6. Genetic Diversity: The total number of genetic characteristics in the genetic makeup of a population, affecting its adaptability and resilience.
7. Birth Rate (Natality): The rate at which new individuals are born into the population over a specific time period.
8. Death Rate (Mortality): The rate at which individuals die in the population over a specific time period.
9. Immigration and Emigration Rates: The rates at which individuals move into (immigration) or out of (emigration) a population, impacting population dynamics.
10. Carrying Capacity: The maximum population size that an environment can sustain indefinitely without being degraded.
11. Population Growth Rate: The rate at which the population increases or decreases over time, influenced by birth, death, immigration, and emigration rates.
12. Allee Effect: The phenomenon where individuals have a more difficult time surviving or reproducing if the population size is too small.

Question 2: If a population growing exponentially double in size in 3 years, what is the intrinsic rate of increase (r) of the population?

Solution 2: To find the intrinsic rate of increase (r) of a population that doubles in size in a given time period, we can use the formula for exponential growth:

\(N(t) = N_0 \cdot e^{rt}\)

Where:

• N(t) is the future population size
• N₀ is the initial population size
• r is the intrinsic rate of increase
• t is the time in years
• e is the base of the natural logarithm (approximately 2.71828)

Since the population doubles in size in 3 years, we can set \( N(t) = 2N_0 \) and ( t = 3 ). Substituting these values into the formula, we get:

\(2N_0 = N_0 \cdot e^{3r}\)

Dividing both sides by N₀ (assuming \( N_0 \neq 0 )\) gives us:

\(2 = e^{3r}\)

To solve for ( r ), we take the natural logarithm of both sides:

\(\ln(2) = 3r\)

Now, we can solve for ( r ):

\(r = \frac{\ln(2)}{3}\)

Calculating the value of ( r ):

\(\ln(2) \approx 0.6931\)

\(r \approx \frac{0.6931}{3} \approx 0.2311\)

Thus, the intrinsic rate of increase (r) of the population is approximately 0.2311 or 23.11% per year.

Question 3: Name important defence mechanisms in plants against herbivory.

Solution 3: Several plants have evolved various mechanisms both morphological and chemical to protect themselves against herbivory.

Morphological defence mechanisms:

• Cactus leaves (Opuntia) are modified into sharp spines (thorns) to deter herbivores from feeding on them.
• Sharp thorns along with leaves are present in Acacia to deter herbivores.
• (In some plants, the margins of their leaves are spiny or have sharp edges that prevent herbivores from feeding on them.

Chemical defence mechanisms:

• All parts of Calotropis weeds contain toxic cardiac glycosides, which can prove to be fatal if ingested by herbivores.
• Chemical substances such as nicotine, caffeine, quinine, and opium are produced in plants as a part of self-defense.

Question 4: An orchid plant is growing on the branch of mango tree. How do you describe this interaction between the orchid and the mango tree?

Solution 4: An epiphyte, or air plant, is an entity growing on other plants. An orchid growing on the branch of a mango tree is an epiphyte. Such plants derive their nutrition and moisture from air, water and rain or from the debris around them and not from the plant on which it is growing. Hence, the relationship between a mango tree and an orchid is an example of commensalism, wherein one species receives its benefits, and the other stays unaffected. In the given interaction, the orchid is benefitted, as it gets physical support from the mango tree, but the mango tree is unaffected.

Question 5: What is the ecological principle behind the biological control method of managing with pest insects?

Solution 5: 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. 

Question 6: Define population and community.

Solution 6: Population and community are fundamental concepts in ecology that describe different levels of biological organization:

Population

Definition: A population refers to a group of individuals of the same species that live in a specific geographic area and interbreed.

Characteristics: Populations are characterized by their size, density, distribution, age structure, and reproductive rates. They interact with each other and can experience changes in size due to births, deaths, immigration, and emigration.

Community

Definition: A community is an assemblage of different populations of various species that coexist in a particular area and interact with each other.

Characteristics: Communities are characterized by species diversity, species interactions (such as competition, predation, and symbiosis), and the structure of the habitat. They can be influenced by abiotic factors (like climate and soil) and can change over time due to ecological succession.

Question 7: Define the following terms and give one example for each:
(a) Commensalism
(b) Parasitism
(c) Camouflage
(d) Mutualism
(e) Interspecific competition

Solution 7: Here are the definitions and examples for each of the specified terms:

(a) Commensalism

• Definition: Commensalism is a type of symbiotic relationship between two species where one species benefits while the other is neither helped nor harmed.
• Example: Barnacles on whales – Barnacles attach themselves to the skin of whales. The barnacles benefit by gaining access to nutrient-rich waters as the whale swims, while the whale is unaffected by their presence.

(b) Parasitism

• Definition: Parasitism is a symbiotic relationship where one species (the parasite) benefits at the expense of another species (the host), often harming it in the process.
• Example: Tapeworms in the intestines of mammals – Tapeworms attach themselves to the intestines of animals (like humans or dogs) and absorb nutrients, which can lead to malnutrition and health issues for the host.

(c) Camouflage

• Definition: Camouflage is a defensive adaptation that allows an organism to avoid detection by predators or prey by blending into its environment.
• Example: The peppered moth – The coloration of the peppered moth allows it to blend in with the bark of trees, making it less visible to predators.

(d) Mutualism

• Definition: Mutualism is a type of symbiotic relationship where both species involved benefit from the interaction.
• Example: Bees and flowering plants – Bees collect nectar and pollen from flowers for food, while helping the plants with pollination, which is essential for plant reproduction.

(e) Interspecific Competition

• Definition: Interspecific competition occurs when individuals of different species compete for the same resources (such as food, water, or space) in an ecosystem.
• Example: Lions and hyenas – Both lions and hyenas hunt similar prey in the savannah, leading to competition for food resources.

Question 8: With the help of suitable diagram describe the logistic population growth curve.

Solution 8: The logistic population growth curve is usually observed in yeast cells cultivated in laboratory conditions and includes five phases, namely:

• The lag phase – The population of a yeast cell is small initially as resources are limited in the habitat.
• Exponential phase – In this stage, the population of the yeast cell suddenly rises as a result of rapid growth leading to exponential population growth due to the availability of enough food resources and consistently favourable environmental conditions without any interspecific competition. This results in the curve rising upwards steeply.
• Positive acceleration phase – At the start of this phase, cell growth is limited. The yeast cell adjusts to the new environment and grows its population.
• Negative acceleration phase – The environmental resistance increases, and the growth rate of the population decline as a result of increased competition between the yeast cells for shelter and food.
• Stationary phase – The population is stable in this phase. The count of cells generated in a population is equivalent to the number of cells that die out. Additionally, the species’ population is said to have touched nature’s carrying capacity in its habitat. The S-shaped growth curve is also referred to as the Verhulst-pearl logistic curve.

Question 9: Select the statement which explains best 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.

Solution 9: The statement that best explains parasitism is: (d) One organism is benefited, other is affected.

Explanation: In parasitism, one organism (the parasite) derives benefit at the expense of another organism (the host), which is harmed in the process. The parasite typically relies on the host for nutrients, shelter, or other needs, leading to detrimental effects on the host’s health or well-being.

For example, tapeworms living in the intestines of animals absorb nutrients from their hosts, leading to malnutrition or other health issues for the host while the tapeworm thrives.

Question 10: List any three important characteristics of a population and explain.

Solution 10: A population can be defined 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:

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.

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.

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.