Notes for Class 12 Biology Chapter 13 Organisms and Populations

Commerce students can refer to the Organisms and Populations Notes Class 12 Biology given below which is an important chapter in class 12 Biology book. These notes and important questions and answers have been prepared based on the latest CBSE and NCERT syllabus and books issued for the current academic year. Our team of Biology teachers have prepared these notes for the benefit of students so that you can read these revision notes and understand each topic carefully.

Organisms and Populations Notes Class 12 Biology

Refer to the notes and important questions given below for Organisms and Populations which are really useful and have been recommended by Class 12 Biology teachers. Understanding the concepts in detail and then solving questions by yourself will help you to learn all topics given in your NCERT Books for Class 12 Biology. 

Very Short Answer Type Questions

Question. Give two reasons as to why a weed such as calotropis flourishes in abandoned fields.
Answer : Dry hairy seeds helps in dissemination/having xerophytic adaptations (thick hair on leaves and stems) not grazed by animals as it produces poisonous substances/cardiac glycosides.

Question. Name the interaction that exists between sucker fish and shark.
Answer : Commensalism.

Question. Provide an instance where the population size of a species can be estimated indirectly, without actually counting them or seeing them.
Answer : Tiger census in National parks and Tiger reserves was done based on counting pug marks /faecal pellets.

Question. Name the interaction that exists between Cuscuta and shoe-flower plant.
Answer : Parasitism.

Question. Very small animals are rarely found in polar regions. Give two reasons.
Answer : Small animals have a larger surface area relative to their volume, lose heat very fast, due to small size, expend much energy to generate body heat through metabolism.

Short Answer Type Questions-

Question. Shark is eurythermal while polar bear is stenothermal. What advantage does the former have and what is the constraint the latter has ?
Answer : Shark : Tolerates wide range of temperature so widespread / survives in all waters.
Polar bear : Restricted occurrence in a narrow range of temperature so constraint to live in a very cold icy environment.

Question. Many freshwater animals cannot survive in marine environment. Explain.
Answer : Freshwater animals are not able to maintain their osmotic concentration in marine conditions. If they are transferred in marine conditions, their body will shrink due to exosmosis. Hence, they cannot survive in marine environment.

Question. Why are the plants that inhabit a desert are not found in a mangrove? Give reasons.
Answer : Desert plants are not adapted to survive in saline / aquatic conditions. 
Plants are conformers / stenothermal / cannot maintain constant internal environment / temperature / osmotic concentration of the body fluids affects kinetics of enzymes through basal metabolism / activity and other physiological functions of the organisms.

Question. A moss plant is unable to complete its life cycle in a dry environment. State two reasons.
Answer : Mosses cannot complete their life cycle in a dry environment because of the following reasons :
(i) They need water for sexual reproduction as water acts as a medium for flagellated sperm to reach the egg and undergo fertilization.
(ii) Since their roots are rudimentary, they cannot absorb water. Therefore, they need to grow in a moist environment for their survival.

Question. Draw labelled diagrams of stable and declining age pyramids of human population.
Answer : Diagram : Refer Topic 2/ Revision Notes/ Important diagram/ Fig 5.3

Question. If in a population of size ‘N’ the birth rate is represented as ‘b’ and the death rate as ‘d’,the increase or decrease in ‘N’ during a unit time t’ will be : 
dN/dt = (b-d) x N
The equation given above can also be represented as : 
dN/dt = r x N where r = (b-d)
What does ‘r’ represent ? Write any one significance of calculating ‘r’ for any population.
Answer : r = intrinsic rate of natural increase; it is an important parameter for assessing the impacts of any biotic or abiotic factor on population growth .

Question. (a) Represent diagrammatically three kinds of age-pyramids for human populations.
(b) How does an age pyramid for human population at given point of time helps the policy-makers in planning for future?
Answer :  (Image 150)
(b) Planning of health / education / transport /infra-structure / finance / food / employment can depend on the age-pyramid analysis of a population / any other relevant point.

Question. Highlight the differences between the population interactions given below. Give an example of each. 
(i) Parasitism
(ii) Amensalism
(iii) Mutualism
Answer : Parasitism : Only one species benefits e.g.,Cuscuta / Tapeworm ½ + ½
Amensalism : One species is harmed whereas the other is unaffected e.g., Penicillium growing on bacterial culture / Trichoderma – biological control agent and plant pathogen 
Mutualism : Both species are benefitted e.g.,lichens exhibit mutualistic relationship with fungus that absorbs water and nutrients from soil and photosynthesizing algae / cyanobacteria

Question. Highlight the differences and a similarity between the following population interactions, competition, predation and commensalism.
Answer : (Image 153)

Question. What is predation ? Explain with the help of suitable examples why is it required in a community with rich biodiversity?
Answer : Organism of higher trophic level (predator) feeds on organism of lower trophic level (prey) is called the predation.
Importance of predation : Refer Q. No. 4. SATQ-I.

Question. (i) Explain any two defence mechanisms plants have evolved against their predators.
(ii) How does predation differ from parasitism ?
Answer : Plants develop following defence mechanisms :
(i) (a) Thorns are (morphological) means of defence. 
(b) Plants may produce / store chemicals such as nicotine, strychnine etc. for defence which inhibit digestion / disrupts reproduction /kill the predator / Calotropis produces highly
poisonous cardiac glycosides / plants may produce chemicals such as nicotine/ caffeine/ quinine/ strychnine/ opium are produced as defence.  (Image 154)

Question. In certain seasons we sweat profusely while in some other season we shiver. Explain.
Answer : To regulate body temperature. 
In summer, outside temperature is higher than body temperature, sweating causes cooling by evaporation of sweat and thus lowering the body temperature.
In winter, outside temperature is much lower than body temperature, shivering is an (involuntary) exercise which produces heat and raises the body temperature in winter.

Question. Name and explain the type of interaction that exists in mycorrhizae and between cattle egret and cattle.
Answer : Mycorrhiza is an association between fungi and the roots of higher plants. It is an example of mutualism in which both fungi and plants are dependent on each other for nutritional needs.
The fungi help the plant in the absorption of essential nutrients from the soil while the plant provides the fungi with carbohydrates.
The interaction that exists between cattle egret and cattle is known as commensalism. In this type of interaction, one species is benefitted whereas the other is neither benefitted nor harmed. The cattle egret (bird) usually forages near the grazing cattle. As cattle moves in the grass they stir up the grass and flush out the insects which then become an easy target for the egret. In this way, the cattle are neither benefitted nor harmed but the egret is benefitted.

Question. When do you describe the relationship between two organisms as mutualistic,
competitive and parasitic ? Give one example of each type.
Answer : Mutualistic : Both the interacting organisms are benefitted from each other e.g., Lichens – Algae and fungi mutually help each other. (any other appropriate example).
Competition : When two organisms belonging to closely related species / unrelated groups compete for the same resources that are limited, both are losers e.g., superior barnacle dominates and excludes the small barnacles / in some South American lakes visiting flamingoes and resident fishes compete for their common food (zooplankton) in the lake / any
other appropriate example.
Parasitic : One of the two organisms is dependent on the other (host) for nutrition and support / the host is harmed and the parasite is benefited e.g., Malarial parasite and human / Cuscuta on host plant / or any other appropriate example.

Question. “The population of a metro city experiences fluctuations in it’s population density over a period of time.”
(a) When does the population in a metro city tend to increase?
(b) When does the population in metro city tend to decline?
(c) If ‘N’ is the population density at the time ‘t’, write the population density at the time ‘t + 1’. 
Answer : (a) Population in a metro city will tend to increase when natality and immigration will be higher.
(b) Population in metro city will tend to decline when mortality and emigration will be higher.
(c) The equation Nt+1 = Nt + [(B + I) – (D + E)] represents the population density at time t + 1.
Nt+1 = Population density at time t+1.
Nt = Population density at time t.
B = Natality
I = Immigration
D = Mortality
E = Emigration

Long Answer Type Questions

Question. (i) What is an age-pyramid ?
(ii) Name three representative kinds of agepyramids for human population and list the characteristics for each one of them.
(i) What is an age-pyramid ?
(ii) Explain with the help of figures the three different types of age pyramids represented by a human population.

Answer : (i) If the age distribution (per cent individuals of a given age or age group) is plotted for the population, the resulting structure is called the age pyramid. 2
(ii) Expanding : Pre-reproductive population is greater than reproductive or post reproductive population / growing with maximum no. of individuals in pre-reproductive phase and least
no. in post reproductive phase.
Stable : Pre-reproductive and reproductive population are almost similar / ideal for population / maintains balanced continuity
/ no. of individuals in reproductive phase is almost same, less no. of individuals in post reproductive phase.
Declining : Pre-reproductive population is less than reproductive population / less no.
of individuals in pre reproductive phase than reproductive phase. 
In lieu of the above explanation the following diagram can be considered.
Diagram : Refer Topic 2/ Revision Notes/ Important diagrams/ Fig 5.3.

Question. (i) What is population density ? Why are ecologists interested in measuring it ?
(ii) Write the different ways of measuring population density. Explain any two with the help of specific examples.
Answer : (i) The population density is the number of individuals of a population found per unit area at a given time. 
Whatever ecological process we wish to investigate in a population (competition / pesticide applicable) we always evaluate in terms of any change in population size (numbers / biomass). 
(ii) Number of organism 
Biomass of organism 
Example : Three ways of measuring population density of a habitat
(a) Per cent cover for trees with larger canopy.
(b) Number of fishes caught per trap.
(c) Pug marks or faecal pellets for tiger census.

Question. (a) What is “population” according to you as a biology student?
(b) “The size of a population for any species is not a static parameter.” Justify the statement with specific reference to fluctuations in the
population density of a region in a given period of time.
Answer : (a) The total number of organisms of a species in a particular area at a particular time. 
(b) The size of a population for any species is not a static parameter because of the factors like :
l Birth rate/ Natality, number of births during a given period 
l Death rate/ Mortality, number of deaths during a given period 
l Immigration, number of individuals of the same species that have come into the habitat from elsewhere during the period under consideration l Emigration, number of individuals of the population who left the habitat and gone elsewhere during the period under consideration.

Question. (i) Name the two growth models that represent population growth and draw the respective growth curves they represent.
(ii) State the basis for the difference in the shape of these curves.
(iii) Which one of the curves represents the human population growth at present ?
Do you think such a curve is sustainable ? Give reason in support of your answer.
Answer : (i) The two types of growth models are as follows :
(a) Exponential growth : When the resources are unlimited, population tends to grow in an exponential pattern.
If the population size is N and the birth and death rates (not per capita) are b and d respectively, then increase or decrease of N at
t (time period) is given by
dN/dt = (b – d) * N
If (b – d) = r , then
dN/dt = rN
where r = Intrinsic rate of natural increase (Image 157)
Nt = Population density after time t
N0 = Population density at time 0
r = Intrinsic rate of natural increase
e = Base of natural algorithm (2.71828) 
(b) Logistic Growth : When the resources are limited leading to competition between individuals and survival of fittest, the population tends to grow logistically.
A population 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. This type of population growth is called Verhulst-Pearl Logistic Growth.  (Image 157)
N = Population density at time t
r = Intrinsic rate of natural increase
K = Carrying capacity  (image 157)

(ii) The difference in the shape of these curves is the amount of resources available for the given population. For exponential growth,the amount of resources is considered infinite,
while, logistic growth happens when there are limited resources. 
(iii) The human population represents the logistic growth curve as the numbers of human beings are increasing rapidly but available resources are not increasing at the same pace. Such a growth pattern is not sustainable because at one point the human population would reach a place where there would not be enough resources for everyone. For sustainable
growth, we must find out ways to develop and use already present resources more intelligently.

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