Bihar Board - Class 12 Biology - Chapter 13: Organisms and Populations Long Answer Question

1. Describe the specific adaptation of xerophytes with respect to the root system, stem   & leaves.

Ans: 

(i) Adaptations in Roots - Well developed and extensively branched long root system is seen in Xerophytes. Though some perennial xerophytes of succulent nature possess extensive but shallow root systems. They seem to absorb water from dewdrops & small rain droplets.

(ii) Adaptations in Stem - Stems of woody xerophytes are comparatively stunted hard & rigid. In the case of acacia, the main stem & branches may occur as a thick, fleshy, flattened & green modified structure called phylloclade.

(iii) Adaptations in Leaves - Leaves are basically short sized which decreases the chances of getting overheated when exposed to solar radiation & thus by reducing the rate of transpiration. Leaves of xerophytes are known to store water, they are usually thick, fleshy green & leathery.

2. List the important characteristics of a population Explain? 

Ans: A population has the following four major characteristics:

(i) Population Density - The size of a population defined with respect to a definite unit of space is known as population density. The maximum limit of density depends upon energy flow in an ecosystem, nutritional status of trophic level & metabolic equilibrium. Population density can

be mathematically calculated as:

(ii) Birth Rate / Natality - The birth rate or natality denotes the produced new number of individuals produced by a natural method in per unit time. 

(iii) Death Rate / Mortality - It denotes the death rate of individuals in the population. It is expressed as the number of individuals dying in a given period. 

(iv) Carrying Capacity - A finite number of organisms can accommodate in a certain space of a particular habitat or ecosystem depending on its size & productivity. This is called carrying capacity.

3. Describe the specific adaptations of hydrophytes with respect to roots, stem & leaves?

Ans: 

(i) Adaptations in Roots - Root systems are haphazardly developed & unbranched, some floating plants or submerged plants lack roots. There is an absence of root hairs except in rooted floating hydrophytes.

(ii) Adaptations in Stem - Stems are long, slender & flexible in submerged hydrophytes whereas in the case of free-floating hydrophytes stems are modified as thick, stout, and stoloniferous occur horizontally on the water surface.

(iii) Adaptations in Leaves - Leaves are thin, long, ribbon shapes and submerged forms. In free-floating plants, the petioles of leaves show indefinite power of growth.

4. Mention the different defense mechanisms to reduce the impact of predation? 

Ans: Plant species evolved various defense mechanisms to reduce impact of predation:-

i) Certain insect species & frogs have camouflage or cryptic coloration to avoid detection by their predators.

ii) Some animals like monarch butterflies are highly distasteful to their predators because they accumulate a certain chemical by feeding on poisonous weeds during its caterpillar stage.

iii) Some prey are poisonous & hence are avoided by predators.

iv.) Plants have evolved certain morphological, or chemical defense mechanisms against herbivores e.g. thorns in bougainvillea.

v) plants also produce certain chemicals which function as:-

a. They make animals feel sick. 

b. They may inhibit them from feeding. 

c. They may interfere with digestion. 

d. They may directly kill them.

5. Mutualism often involves the co-evolution of mutualists. Describe taking the example of an animal-plant (wasp-fig) relationship.

Ans: Plants need the help of animals for pollination of their flowers & dispersing their seeds.Animals must certainly be compensated for the services they provide to plants. Plants offer rewards or fees in the form of seed dispersers “plant-animal interactions often involve co-evolution of the mutualists, that is, the evolution of the flower & its pollinator species can be pollinated only by its partner wasp species & no other species. The female wasp not only uses the fruit as an oviposition site, but it also uses a growing seed within the fruit to feed her larvae. While looking for suitable egg-laying locations, the wasp pollinates the fig inflorescence. The fig feeds the wasp some of its growing seeds as nourishment for the developing wasp larvae in exchange for pollination services.

6. What is the Age pyramid? What are the different types of age pyramids?

Ans: In a population of an organism, the geometrical diagrammatic representation of different age groups is referred to as the Age of pyramids. These are of three types:-

i) Expanding pyramid:- It is a broad-based, triangular pyramid that represents a population containing a large number of young people. It is a rapidly expanding population with a high birth rate.

ii) Stable pyramid:-  A moderate proportion of young to old is represented by this pyramid. As the rate of growth becomes slow & stable i.e.- pre-reproductive & reproductive age groups become more or less equal in size.

iii) Declining Pyramid:- The type of pyramid of population decreasing in size is characterized by a narrow base because there are fewer pre-reproductive individuals than in the other two age categories.

7. Differentiate between regulators & conformers? Why do small animals not show regulations?

Ans: Those organisms who maintain homeostasis by physiological or behavioral means & ensure a constant body temperature & constant osmotic concentration etc. are called regulators e.g. all birds, mammals some lower vertebrates & invertebrates, for example in summer, when the outside temp is more than our body temperature we sweat profusely evaporative cooling brings the body temp – down. Whereas those organisms cannot maintain a constant internal environment. The body temperature of these organisms changes with ambient temperature. e.g., In the case of the majority of animals & nearly all plants.

Thus,  thermoregulation is an energy-expensive process, small organisms do not show regulation. Small animals tend to lose body heat very fast when it is cold outside because of a large surface area relative to volume. They have to expend a lot of energy to generate body heat through metabolism.