Class XII – Organisms and Populations – PYQs

Ans. Population: It refers to the sum total of all organisms having similar features and potential to interbreed among themselves and produce fertile offsprings.

Community: It refers to assemblage of all the populations of different species in a specific geographical area.

Ans. Three features of plants to survive in water scarce environment are

• Roots grow very deep to explore any possibility of available underground water.
• Many desert plants have a thick cuticle on their leaf surfaces and have their stomata arranged in deep pits to minimise water loss through transpiration.
• Some desert plants like Opuntia, have no leaves. Their leaves are reduced to spines and photosynthesis occurs in flattened stems.

Three features of animals to survive in water scarce environment are

(i) They produce concentrated urine, so as to keep the water in their body.

(ii) Their sweat glands get closed, hence there is no water loss through sweat.

(iii) They undergo aestivation to escape harsh condition.

Ans. The adaptations that help halophytes to survive in salty conditions are

• Some halophytic plants possess small negatively geotropic vertical roots called pneumatophores (have lenticels for gaseous exchange), e.g. Avicennia, Aegialitis.
• Another adaptation of halophytic plants is vivipary or seed germination, while the fruit is still attached to plants, e.g. Rhizophora, Aegiceras, Ceriops.

Ans. The plants found in desert are not found in mangrove because desert plants have made modification in order to adapt their surroundings. For example, their leaves are reduced into spine and have sunken stomata to reduce water loss through transpiration their roots grow deeply to explore possibility of underground water. Their stems have cuticle and are fleshy to preserve moisture. While mangrove has salty condition so to overcome this plants have specialised root which grow negatively geotropic, called pneumatophores, leaves are thin, small and green.

Stems are usually cuticularised.

Ans. The rainforest plants have a thin, smooth bark because they do not need thick bark to prevent moisture. The smoothness of the bark makes it difficult for other plants to grow on their surface. While in wetland, plants have roots in the soil under the water but they grow above the water for respiration (pneumatophores). They have adapted to growing in very wet soil. So, rainforest plants are not found in wetland.

Ans. Mammals living in colder climates generally have shorter ears and limbs to minimise heat loss. This is called Allen’s rule. In polar areas, aquatic mammals like seals have a thick layer of fat (blubber) below their skin that acts as an insulator and reduces loss of body heat.

Ans. Small animals like humming bird or shrew, have a large surface area relative to their volume. So, they tend to lose body heat very fast during cold conditions. They need to spend more energy to generate body heat through metabolism. Due to this, smaller animals are rarely found in polar regions.

Ans. People visiting high altitude regions experience altitude sickness. This is because the body does not get enough oxygen. The person shows symptoms of nausea, fatigue and heart palpitations.

After couple of days, the body responds gradually by physiological adaptation. The body compensates low O₂ availability by

(i) increasing RBCs production.

(ii) decreasing binding capacity of haemoglobin.

(iii) increased breathing rate.

Ans. The important characteristics of a population are as follows

1. Population size or density: It is the number of individuals of a species per unit area or volume. Population Density (PD)= Number of individuals in a region (N) / Size of unit area in the region (S) ; PD = N/S

2. Birth rate or Natality: It is the number of births of new individuals per unit of population per unit time, e.g. if in a pond, there are 20 lotus plants last year and through reproduction, 8 new plants are added, taking the current population to 28. Then, birth rate = 8/20 = 0.4 offspring per lotus per year.

3. Death rate or Mortality: It is the number of loss of individuals per unit of population per unit time due to death or due to the different environmental changes, competition, predation, etc. For example, if in a laboratory population of 40 individuals, 4 fruit flies died during a specified time interval, then the death rate = 4/40 = 0.1 individuals per fruit fly per week.

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Ans. When the pre-reproductive age group individuals are comparatively fewer and both reproductive and post-reproductive stages are almost in equal stage, i.e. at same level, it is zero population growth rate. An inverted bell-shaped age pyramid is obtained for zero population growth rate.

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Ans. Predation is an interaction where one organism (predator) kills and eats the other weaker organism called prey. It is a natural way of transferring the energy fixed by plants, to higher trophic levels.

For example, snake eating a frog, tiger killing and eating a deer. Predators keep prey population under control which otherwise could achieve very high population densities and cause instability in ecosystem.

They also help in maintaining a species diversity in a community by reducing the intensity of competition among competing prey species.

Ans. The herbivores are predators of plants and nearly 25% insects are phytophagous (feeding on plants). So, plants show morphological as well as chemical defence against herbivores such as

(i) Thorns of rose, Acacia and cactus.

(ii) Certain plants produce chemicals, such as opium, quinine, caffeine, nicotine, etc., to protect them against being grazed by the animals.

(iii) Calotropis produces highly poisonous cardiac glycosides. So, the cattle and goats do not eat this plant.

Ans. Coevolution is a phenomenon where many parasites have evolved to be host specific in such a way that both the host and parasite tend to coevolve, i.e. if the host develops/evolves special mechanism for rejecting or resisting, then the parasite also have to evolve mechanism to counteract and neutralise them to succeed with the same host species.

Parasites have special adaptations according to their lifestyles such as, loss of unnecessary sense organs, presence of adhesive organ or suckers, loss of digestive system and high reproductive system.

Ans. Mycorrhiza is an association between fungi and roots of higher plants. This interaction is called mutualism. The fungi help plant in the absorption of essential nutrients from the soil, while in turn it provides the fungi with energy yielding carbohydrates. Both partners are benefitted in this relationship.

The cattle egret and grazing cattle share commensalism type of relationship. The grazing cattle flush out insects from grass while grazing. So, it becomes easier for egret to catch and feed on them. In this, one partner gets benefit, while other remains unaffected.

Ans. Mutualism is an interaction that confers benefits on both the interacting species. Two examples where organisms involved are commercially exploited in agriculture are mycorrhizae and lichen.

Ans. The relationship between fig tree and wasp shows mutualism. The wasp while searching for sites to lay its eggs, pollinates the fig’s inflorescence.

On the other hand, the fig not only provides shelter (fruit) for oviposition but also allows wasp’s larva to feed on its seeds.

Ans. Orchid is an epiphytic plant that grows on a mango tree branch and they together show commensalism. In this interaction, orchid is benefitted and the mango tree is neither harmed nor benefitted at all.

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Ans. Attributes that a population has but not an individual organism are birth rate, death rate, sex ratio, age pyramids or age distribution.

Ans. Population density is the number of individuals per unit area at a given time/period. Different ways of measuring population density are as follows

(a) Biomass or percentage cover, e.g. hundred Parthenium plants and one huge banyan tree.

(b) By measuring relative density, e.g. the number of fish caught per trap from a lake.

(c) Numbers, e.g. human population.

(d) Indirect estimation, i.e. without seeing or actually counting population sizes, e.g. the tiger census is often based on pug marks and faecal pellets.

Ans. Age pyramid displays the age distribution (% individuals of a given age or age group) for the population. Population at any given time is composed of individuals of different ages. For human population, the age pyramids generally show age distribution of males and females in a combined diagram.

The shape of the pyramids reflects the growth status of the population that whether it is growing, stable or declining.

It tells about an area’s population, birth and death rates, dependants, young people and elderly people, etc. These are important inputs that can be used for long term planning strategies, e.g. to create job opportunities (if there are more young people), to focus on control of population (in case of high birth rate), to improve and invest more on medical facilities (in case of low elderly people), etc.

Hence, through in depth analysis of the age pyramids, better planning strategies, resource allocation and other planning benefits can be obtained.

Ans. Figure A It is a ‘pyramid’-shaped age pyramid. In this figure, the base, i.e. pre-reproductive stage is very large as compared with the reproductive and post-reproductive stages of the population. This type of age structure indicates that the population would increase rapidly. Figure B It is an inverted bell-shaped pyramid. In this figure, the pre-reproductive and reproductive stages are same. This type of age structure indicates that the population is stable.

Figure C It is an urn-shaped pyramid. In the figure, the pre-reproductive and reproductive stages are less than the post-reproductive stages of this population. In this population, more older people are present. This type of age structure indicates that the population definitely is declining.

Ans. Comparisons between J-shaped and S-shaped model of population growth are as follows

Ans. Fitness of a species as mentioned by Darwin:

Darwinian fitness (high ‘r’ value) states that the population evolves to maximise its reproductive fitness in the habitat where it lives. Under a particular set of selection pressure, organisms evolve towards the most efficient reproductive strategy.

The rate of breeding varies from species to species. Some species breed only once in their lifetime (Pacific salmon fish and bamboo), while some breed many times in their lifetime (birds and mammals). Some organisms produce a large number of small-sized offsprings (oysters), whereas other produce a small number of large-sized offspring (birds and mammals).