Chapter 13: Organisms and Populations [ Exercise,Solutions,Notes ]

Balbharati solutions for Biology 12th Standard HSC for Maharashtra State Board chapter 13 - Organisms and Populations [Latest edition]

Chapter 13: Organisms and Populations [ Exercise,Solutions,Notes ]

Multiple choice question

Exercise | Q 1.1 | Page 307
Which factor of an ecosystem includes plants, animals, and micro-organisms?
  • Biotic factor
  • Abiotic factor
  • Direct factor
  • Indirect factor
Biotic factor

Exercise | Q 1.2 | Page 307
An assemblage of individuals of different species living in the same habitat and having functional interactions is _______________.
  • Biotic community
  • Ecological niche
  • Population
  • Ecosystem
An assemblage of individuals of different species living in the same habitat and having functional interactions is a Biotic community.

Exercise | Q 1.3 | Page 307
Association between sea anemone and Hermit crab in gastropod shell is that of _______________.
  • Mutualism
  • Commensalism
  • Parasitism
  • Amensalism
Association between the sea anemone and Hermit crab in gastropod shell is that of Commensalism.

Exercise | Q 1.4 | Page 307
Select the statement which explains the best parasitism.
  • One species is benefited.
  • Both the species are benefited.
  • One species is benefited, other is not affected.
  • One species is benefited, other is harmed.
One species is benefited, other is harmed.

Exercise | Q 1.5 | Page 307
Growth of bacteria in a newly inoculated agar plate shows ____________.
  • exponential growth
  • logistic growth
  • Verhulst-Pearl logistic growth
  • zero growth
Growth of bacteria in a newly inoculated agar plate shows exponential growth.

Define the following term and give one example for each

Exercise | Q 2.1 | Page 307
Commensalism: Commensalism is an interaction between two species in which one species gets benefited 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.

Exercise | Q 2.1 | Page 307
Parasitism is a kind of relationship between two species in which one species which is termed a parasite, derives its food from the other species which is termed as host. Parasitism also involves shelter, in addition to food obtained by a parasite. Parasites may be ectoparasites or endoparasites. Ectoparasites live on the surface of their host while endoparasites live inside the body of the host.

Examples of Parasitism
(i) Cuscuta growing on a shoe flower plant is a parasitic interaction, as Cuscuta derives nutrition from Hibiscus (shoe flower), as it lacks chlorophyll.
(ii) Head lice is an ectoparasite and suck human blood
(iii) Ascaris (roundworm), Taenia (tapeworm), Malaria (Plasmodium) causing diseases in humans
(iv) Koel laying its eggs in a crow’s nest is an example of brood parasitism. Birds lay eggs in the nest of their host and the host incubates it.

Define the following term

Exercise | Q 2.1 | Page 307
Camouflage, also called cryptic coloration, is a defense mechanism or tactic that organisms use to disguise their appearance, usually to blend in with their surroundings. Organisms use camouflage to mask their location, identity, and movement.

Exercise | Q 2.2 | Page 307
Give one example for Interspecific competition
Example 1. A human requires oxygen for life and plants use carbon dioxide for photosynthesis. Here both humans and plants are mutually benefited. Humans use the oxygen given by the plants. In return, plants use carbon dioxide, which is exhaled by the Humans
Example 2. lichen is a mutualistic relationship between a fungus and algae. Algae provide food to fungus obtained from photosynthesis. The fungus provides anchoring and protection to the algae. 

Exercise | Q 2.2 | Page 307
Give one example for Mutualism
Interspecific competition:
Example 1. Imagine a cow and a horse on a piece of grassland. Both of them belong to different species but compete for the same grass (food). This type of interaction is called interspecific interaction.
Example 2.  Competition between leopards and lions for the same prey,
Example 3. Resident fish competing with migratory birds Flamingos for common food i.e. zooplankton

Exercise | Q 2.3 | Page 307
Name the type of association: Clownfish and sea anemone
Clownfish and sea anemone: The symbiotic interaction in which both partners benefit is referred to as mutualism. 
The clownfish need protection from predators, so it requires sea anemone for protection. Sea anemone, on the other hand, requires food that is given by clownfish (faeces) to live. The two species derive benefits from each other. Their partnership is thus called a mutualistic relationship.

Exercise | Q 2.3 | Page 307
Name the type of association: Crow feeding the hatchling of Koel
Crow feeding the hatchling of Koel: Brood parasitism

Exercise | Q 2.3 | Page 307
Name the type of association: Hummingbirds and host flowering plants
Hummingbirds and host flowering plants: Mutualism

Exercise | Q 2.4 | Page 307
What is the ecological process behind the biological control method of managing pest insects?
The ecological process behind the biological control method of managing pest insects is Predation. Predators regulate the population of prey in a habitat, thus helping in the management of pest insects.

Exercise | Q 3.1 | Page 307
How is the dormancy of seeds different from hibernation in animals?
i. Seed dormancy a condition in which seeds are prevented from germinating even under suitable environmental conditions, The main reason behind these conditions is that they require a period of rest before being capable of germination. whereas hibernation in animals is a state of reduced activities to escape cold winter conditions.
ii. During seed dormancy, the growth, and development of an embryo are arrested temporarily, whereas in hibernation animals enter a state of inactivity by slowing their metabolism.

Exercise | Q 3.2 | Page 307
If a marine fish is placed in a freshwater aquarium, will it be able to survive? Give reason.
i. If a marine fish is placed in a freshwater aquarium, fish would not be able to survive because marine fishes are adapted to high salt concentrations of the marine environment.
ii. Marine fishes have more osmotic concentration (more salt concentration) than marine water which prevents marine water to enter the body.
iii. When marine fish is placed in a freshwater aquarium, water enters into the body of marine fish due to osmosis, as freshwater creates a hypotonic environment outside the fish’s body.
iv. Entry of water into the body causes its body to swell leading to the death of the marine fish.

Exercise | Q 3.3 | Page 307
Name important defense mechanisms in plants against herbivores.
Defense mechanisms in plants against herbivores can be morphological like thorns (in Acacia, Cactus) or chemicals like poisonous cardiac glycosides (produced by Calotropis), secondary metabolites (for e.g.nicotine, caffeine, quinine, strychnine, opium, etc.)

Exercise | Q 3.4 | Page 307
An orchid plant is growing on the branch of the mango tree. How do you describe this interaction between the orchid and the mango tree?
An orchid is an epiphyte growing on the branch of a mango tree. 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 commensalism, where one species gets benefited while the other remains unaffected. In the above interaction, the orchid is benefited as it gets support while the mango tree remains unaffected.

Exercise | Q 3.5 | Page 307
Distinguish between the following: Hibernation and Aestivation.
i.It is also called winter sleep.It is also called summer sleep.
ii.It is a state of reduced activities in some organisms to escape cold winter conditions.It is a state of reduced activities in some organisms to escape desiccation due to heat in summer.
iii.Animals rest in warm places.Animals rest in cool, shady, and moist places.

It is shown by bears inhabiting cold regions.

It is shown by some fish and snails.

Exercise | Q 3.5 | Page 307
Distinguish between the following: Ectotherms and Endotherms.

i.These are cold-blooded animals.These are warm-blooded animals.
ii.Ectotherms do not possess the ability to generate sufficient heat to keep them warm, thus their body temperature varies with their surroundingsEndotherms do possess the ability to generate heat and keep them warm, thus they can maintain constant body temperature.
iii.They are also known as poikilothermic.They are also known as homeothermic.

They are affected by changes in environmental temperature

They remain unaffected by changes in environmental temperature.
 E.g. Most of the fishes, amphibians, reptilesE.g. Birds, mammals

Exercise | Q 3.5 | Page 307
Distinguish between the following: Parasitism and Mutualism

In parasitism, only one species (parasite) is benefited and the interaction is detrimental to other species (host).

In mutualism, both species are benefited.
ii.The parasite needs a host, but the host does not need the parasite.Both species need the presence of each other.
E.g.Cuscuta, a parasitic plant commonly found growing on hedge plants.Lichen represents the mutualistic relationship between a fungus and photosynthetic algae or cyanobacteria.

Exercise | Q 3.6 | Page 307
Write a short note on Adaptations of plants to water scarcity
Adaptations of plants to water scarcity:
  1. Plants found in deserts are well adapted to cope with water scarcity and the scorching heat of the desert.
  2. Plants have an extensive root system to tap underground water.
  3. They bear thick cuticles and sunken stomata on the surface of their leaves to reduce transpiration.
  4. In Opuntia, the leaves are modified into spines and the process of photosynthesis is carried out by green stems.
  5. Desert plants have special pathways to synthesize food, called CAM (C4 pathway). It enables their stomata to remain closed during the day to reduce water loss by transpiration.

Exercise | Q 3.6 | Page 307
Write a short note on Behavioural adaptations in animals
  1. Behavioural adaptations in animals
  2. 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, ectothermal animals and certain endotherms exhibit behavioral adaptations. Ectotherms are cold-blooded animals such as fish, amphibians, reptiles, etc. 
  3. 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. 
  4. Certain endotherms (warm-blooded animals) such as birds and mammals escape cold and hot weather conditions by hibernating during winters and aestivating during summers. 
  5. They hide in shelters such as caves, burrows, etc. to protect against temperature variations.

Short answer question

Exercise | Q 3.6 | Page 307
Write a short note on Adaptations of desert animals
Adaptations of animals for desert habitats:
1. Desert animal-like Kangaroo rat inhabiting the Arizona deserts has the potential to concentrate its urine to conserve water. This animal never drinks water in its life.
2. Snakes and desert lizards bask in the sun early in the morning and burrow themselves in the sand in the afternoons to escape the heat of the day, to prevent water loss.
3. Camels can store fat in the hump which can be metabolised for energy. A camel can survive for many days without water. Long eyelashes, ears lined with hair, and slit-like nostrils help to keep out sand.

Exercise | Q 3.7 | Page 307
Define Population.
Individuals live in groups in a well-defined geographical area, share or compete for similar resources, potentially interbreed and thus form a population.

Exercise | Q 3.7 | Page 307
Define Community.
Community: Several populations of different species in a particular area constitute a community that interacts with one another in several ways.

Exercise | Q 4.1 | Page 307
With the help of a suitable diagram describe the logistic population growth curve.
  1. Logistic growth curve of population
  2. Resources like food and space are not always unlimited. They may be plenty in the beginning; but as the population density increases, competition for those resources starts, resulting in a slowdown in the rate at which the original population was growing. This results in a logistic or sigmoid growth curve.
  3. Competition between individuals for limited resources will weed out the ‘weaker' ones. Only the ‘fittest’ individuals will survive and reproduce.
  4. A given habitat has enough resources to support a maximum possible number, beyond which no further growth is possible. This limit can be called nature’s carrying capacity (K) for that species in that habitat.
  5. A population growing in a habitat with limited resources shows initially a lag phase, followed by phases of acceleration and deceleration, and finally an asymptote when the population density reaches the carrying capacity.
  6. A plot of population density (N) in relation to time (t) results in a sigmoid curve. This type of population growth is called VerhulstPearl Logistic Growth.
  7. Since resources for the growth of most animal populations, are finite and become limiting sooner or later, the logistic growth model is considered a more realistic one.

Exercise | Q 4.2 | Page 307
Enlist and explain the important characteristics of a population.
  1. The important characteristics of a population are population size, population density, natality, mortality, sex ratio, immigration, emigration, age pyramids, expanding population, population growth forms, and biotic potential.
  2. Population density: Population density tells us the number of individuals present in per unit space, in a given time. OR The density of a population is the total number of individuals in that population present per unit area at a specific time.
  3. Natality: Natality is the birth rate of a population.
  4. Mortality: Mortality is the death rate of a population.
  5. Age distribution and Age pyramids: 1. A population consists of individuals of different ages. The entire population is divided into three age groups – pre-reproductive (0-14 years), reproductive (age 15-44 years), post-reproductive (45-85+years)The relative proportion of individuals of various age groups in the population is referred to as the age structure of the population. 2. If the age distribution (percent individuals of a given age or age group) is plotted for the population, the resulting structure is called as age pyramid.
  6. Sex Ratio: Sex ratio is the ratio of the number of individuals of one sex to that of the other sex.

organisms and populations class 12 notes

Natural world around us shows amazing diversity of forms and complexity of relations. To understand these, we have to study levels of organization in the living world viz. macromolecules, cells, tissues, organs, individual organism, population, communities, ecosystems and biomes. You have already studied in school, that ecology is a study of the interactions among organisms and between the organisms and their physical (abiotic) environment. 

Term ecology was first used by Reiter but E. Haeckel gave substance to the term (introduced) ecology. Ecological grouping of organisms is nothing but ecological hierarchy. There are four sequential levels with increasing complexity of ecological (biological) organizations viz, Organism, Populations, Communities and Biomes. Individual organism is the basic unit of ecological hierarchy. Organisms of same kind inhabiting a geographical area constitute population. 

Several populations of different species in a particular area constitute community that interact with one another in several ways. Biome constitutes a large regional terrestrial unit delimited by a specific climatic zone having major vegetation zone (plant communities) and the associated fauna. There are six major groups of terrestrial biomes. We shall explore first two levels viz, organism and populations. 

Organisms and the environment around: Ecology at the level of organism is basically the study of animal or plant physiology which helps us to understand how the organisms are adapted to their environments, not only for their survival but also for propagation (multiplication).

You have studied in earlier classes about the rotation of earth around the Sun and the tilt of its axis, cause seasons. These seasons with annual variation in precipitation in the form of rain and snow, gives rise to formation of major biomes of the earth like desert, rain forest, grassland, tundra, etc. Regional and local variations within each biome lead to the formation of a variety of habitats. Major biomes of earth are shown in Fig. 13.1. On the Earth, life exists even in extreme and harsh habitats like scorching deserts of Rajasthan, perpetually rain-soaked forests of North Eastern states and high mountain tops of Himalayas.

Here, we must remember that it is not only the physico-chemical (abiotic) components that make up the habitat of an organism, but the habitat also includes biotic components like plants, pathogens, parasites, and predators of the organism. We assume that over a period of time, the organism had through natural selection, evolved adaptations to optimize its survival and reproduction in its habitat

Habitat and Niche : Habitatis a place or the set of environmental conditions around the organism to which it must adapt to survive and prosper. The term Niche is used to denote the functional role played by an organism in its environment (J. Grinnell 1917). Niche includes various aspects of the life of an organism like diet, shelter, etc. A habitat defines the physical space of an organism with the other living or nonliving factors, while niche describes how that organism is linked with its physical and biological environment. In colloquial language habitat is a postal address while niche is the profession of organism.  

Definition of Habitat : Place or area where a particular species lives, is its habitat. Factors like the sunlight, average rainfall, annual temperatures, type of soil present and other abiotic (topographic) factors, affect the presence of organisms. These factors help in determining the presence of the particular type of species in the environment. Pond, river, ocean, etc. are the examples of habitat as many organisms are found in the same place or habitat. These habitats can be arboreal, terrestrial, aerial, aquatic, etc. The immediate surrounding of an organism, sometimes also referred to as microhabitat, is an important concept to remember when working with sedentary or weakly motile organisms

Definition of Niche : The term niche was first time used by ‘J. Grinnell’. The term ecological niche is still not well understood and is sometimes even misused. 

Niche is described as a position of a species in the environment like, what they do for their survival? how they fulfill their needs of shelter, food? etc. Niche deals with the flow of energy from one organism to another and hence, it is important to understand, what an organism eats, how it interacts with other organisms, etc. As soon as the niche is left vacant, other organisms fill that position. 

The niche is specific to each species, which means no two species can share the same niche. If the species creates its own unique niche in an ecosystem, it would be helpful in reducing competition for resources among species. By taking an example of a bird, it can be understood that how these birds differ in their eating habits, where some birds eat only insects, some only fruits and some can eat both and anything they come across. So here we can conclude that these birds living in the same habitat differ in their niches because of different eating habits.

Major Abiotic Factors : Temperature: It is the most ecologically relevant environmental factor. Average temperature on land varies from subzero levels in polar areas and high altitudes, upwards upto 50o C in tropical deserts in summer. Temperature also varies seasonally. It decreases progressively from the equator towards the poles and from plains to the mountain tops. 

There are some unique habitats such as hot springs (80 to 1000 C) and deep-sea hydrothermal vents where average temperatures usually 400o C. Ambient temperature affects the enzyme kinetics of the cell and thus, the entire metabolism, activity and other physiology of the organism. Only few organisms can tolerate and thrive in a wide range of temperatures (eurythermal), but, a vast majority of them are restricted to a narrow range of temperatures (stenothermal).

 Water: Availability of water is an important factor affecting the organisms. As we know, life on earth originated in water, its availability is so limited in deserts that only special adaptations are required to survive there. The productivity and distribution of plants are also heavily dependent on water.

Organisms living in water bodies such as oceans, lakes and rivers, have their own waterrelated problems. For aquatic organisms the chemical composition and pH of water are important. The dissolved salt concentration (measured as salinity in parts per thousand), is less than 5ppt in fresh waters of streams , lakes and rivers, and 30-35ppt in the seas and oceans. 

It may go up to 100ppt in some hypersaline lagoons. Some organisms are tolerant for a wide range of salinities (euryhaline) but others are restricted to a narrow range (stenohaline). Many fresh water animals cannot live for long in sea water and vice versa because of the osmotic problems, they would face. 

Light: Plants use light for photosynthesis, which is only source of energy for the entire ecosystem. Photosynthesis can occur only in presence of sunlight. Many species of small plants (herbs and shrubs) growing on forest floor are adapted to perform photosynthesis optimally under very low light conditions because they are constantly overshadowed by tall trees. 

For animals too, diurnal and seasonal variations in light intensity and duration (photoperiod) are clues for timing their foraging, reproductive and migratory activities. The availability of light on land is closely linked with that of temperature, since the sun is the source for both.

Soil: The nature and properties of soil are dependent on the climate, the weathering process. Various characteristics of the soil such as soil composition, grain size, determine the percolation and water holding capacity of the soil. These characteristics along with pH, mineral composition and topography, determine the vegetation of an area. Vegetation in turn dictates the type of animals. The abiotic factors that determine the type of habitat, also show considerable diurnal and seasonal variations. 

The plants and animals must adapt to these changes in order to survive and flourish in the habitat. During the course of their evolution, many species have evolved a relatively constant ‘internal’ environment that permits all biochemical reactions and physiological functions to proceed with optimum rate, and allow the species to flourish. 

The organisms try to maintain the constancy of its internal environment (homeostasis) despite variations in the external environmental conditions. To survive and flourish in any environment, organisms must adapt to the changes in the environment for which there are following possibilities :
i. Regulate: Some organisms are able to maintain homeostasis by physiological and behavioural changes which ensure constant body temperature, constant osmotic concentration, etc. All birds and mammals are capable of such regulation (thermoregulation and osmoregulation).

ii. Conform: Most of animals and plants cannot maintain a constant internal environment. Their body temperature changes with the ambient temperature. In aquatic animals, the osmotic concentration of the body fluids changes with that of the ambient water osmotic concentration. These animals and plants are simply conformers.

Some species have evolved the ability to regulate, within a limited range of environmental conditions, beyond which they simply conform. If the stressful environment is localized or only for a short period of time, the organism may migrate or suspend its activities

iii. Migrate: The organism can move away temporarily from the stressful habitat to a more hospitable area and return when stressful period is over. Many animals, particularly birds, during winter undertake long-distance migrations to more hospitable areas. 
iv. Suspend: In plants, seeds serve as means to tide over periods of stress; they germinate to form new plants under favourable moisture and temperature conditions. They do so by reducing their metabolic activity and going into a state of ‘dormancy’. In animals, the organism, if unable to migrate may go into hibernation during winter e.g. polar bear. Some snails and fish go into aestivation to avoid summer heat.

organism (morphological, physiological, and behavioural) that enables the organism to survive and reproduce in its habitat. Many desert plants have a thick cuticle on their leaf surfaces and have their stomata in deep pits to minimize loss of water through transpiration. They also have a special photosynthetic pathway (CAM - Crassulacean acid metabolism) that enables their stomata to remain closed during daytime. 

Some desert plants like Opuntia, have their leaves reduced (modified) to spines and the photosynthetic function is taken over by the flattened stems. Mammals from colder climates generally have shorter snout, ears, tail and limbs to minimize the loss of body heat (Allen’s Rule.) In the polar seas, aquatic mammals like seals have a thick layer of fat (blubber) below their skin acting as an insulator to reduce loss of body heat. 

Some organisms show behavioural responses to cope with variations in their environment. Desert lizards manage to keep their body temperature fairly constant by behavioural adaptations. They bask in the sun and absorb heat, when their body temperature drops below the comfort zone, but move into shade, when the ambient temperature starts increasing. Some species burrow into the sand to hide and escape from the heat.

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Balbharati solutions for Biology 12th Standard HSC for Maharashtra State Board
Chapter 1: Reproduction in Lower and Higher Plants
Chapter 2: Reproduction in Lower and Higher Animals
Chapter 3: Inheritance and Variation
Chapter 4: Molecular Basis of Inheritance
Chapter 5: Origin and Evolution of Life
Chapter 6: Plant Water Relation
Chapter 7: Plant Growth and Mineral Nutrition
Chapter 8: Respiration and Circulation
Chapter 9: Control and Co-ordination
Chapter 10: Human Health and Diseases
Chapter 11: Enhancement of Food Production
Chapter 12: Biotechnology
Chapter 13: Organisms and Populations
Chapter 14: Ecosystems and Energy Flow
Chapter 15: Biodiversity, Conservation and Environmental Issues

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