CBSE Sample Papers for Class 12 Biology Term 2 Set 5 with Solutions

Students can access the CBSE Sample Papers for Class 12 Biology with Solutions and marking scheme Term 2 Set 5 will help students in understanding the difficulty level of the exam.

CBSE Sample Papers for Class 12 Biology Standard Term 2 Set 5 with Solutions

Time Allowed: 2 Hours
Maximum Marks: 40

General Instructions:

• All questions are compulsory.
• The question paper has three sections and 13 questions. All questions are compulsory.
• Section-A has 6 questions of 2 marks each; Section-B has 6 questions of 3 marks each; and Section-C has a case-based question of 5 marks.
• There is no overall choice. However, internal choices have been provided in some questions. A student has to attempt only one of the alternatives in such questions.
• Wherever necessary, neat and properly labeled diagrams should be drawn.

Section – A

Question 1.
The Green Revolution succeeded in tripling the food supply but yet it was not enough to feed the growing human population. Increased yields have partly been due to the use of improved crop varieties, but mainly due to the use of better management practices and use of agro chemicals (fertilizers and pesticides). However, for farmers in the developing world, agrochemicals are often too expensive, and further increases in yield with existing varieties are not possible using conventional breeding.
What are the advantages and disadvantages of green revolution?
Answer:
The Green revolution was a brainchild of Dr. Norman Borlaug and Dr. M.S Swaminathan who are considered as the pioneers of green revolution. The advantages and disadvantages of green revolution are:
Advantages:

1. It has lead to introduction of high yielding varieties of crops.
2. Through green revolution, it is now possible to grow number of crops on small piece of land.
3. Green revolution enabled food and nutritional security, which is an important aspect for developing countries.
4. Now more deforestation for conversion into agricultural land necessary as there is no need of shifting agriculture.

Disadvantages:

1. Over use of synthetic and chemical products degraded the land and soil fertility.
2. It increased the soil and water pollution and gave rise to soil erosion.
3. The high yielding varieties consume large amount of water which puts pressure on natural and ground water table reserves.

Question 2.
The graph in figure below shows how the number of X cells typically declines over a period of many years following the initial HIV infection. Study the graph below and answer the following questions:

(i) Name the cells X?
(ii) Why there is a decline in number of cell X with time?
OR
Biogas is produced by anaerobic digestion with methanogens or anaerobic organisms, who digest material inside a closed system, or perform fermentation of biodegradable materials. This closed system is called an anaerobic digester, bio-digester or a bioreactor. Proper use of waste material can be achieved through biogas. The complex material is digested and is acted upon by several methanogenic bacteria.
(i) Write about the microbial activity during production of a biogas.
(ii) Enlist any four advantages of it.
Answer:
(i) helper T- cells, HIV infects and destroys helper T cells.
(ii) HIV enters into helper T-lymphocytes (T_h), replicates and produce progeny viruses. The progeny viruses released in the blood attack other helper T-lymphocytes. This is repeated leading to a progressive decrease in the number of helper T lymphocytes in the body of the infected person.
OR
(i) Biogas is produced by anaerobic digestion with methanogens or anaerobic organisms, which digest material inside a closed system or perform fermentation of biodegradable materials. Biogas is primarily methane (CH₄) and carbon dioxide (CO₂) and may have small amounts of hydrogen sulphide (H₂S), moisture and siloxanes. The gases such as methane, hydrogen and carbon monoxide (CO) can be combusted or oxidised with oxygen. This energy release allows biogas to be used as a fuel; it can be used for any heating purpose, such as cooking. It is used in a gas engine to convert the energy in the gas into electricity and heat.

(ii) Advantages of using Biogas:
(a) It is an eco-friendly source of energy.
(b) Biogas generation reduces soil and water pollution.
(c) The process of producing biogas yields organic manure.
(d) It is a healthy cooking alternative as it is low cost technology and economic for developing nations.

Question 3.
The bacteria below would best be described as a/an:

Answer:
Staphylococcus: Staphylococcus is a genus of gram-positive bacteria. Under the microscope, they appear spherical(cocci), and form in grape-like clusters. These species are facultative anaerobic organisms that are capable of growing both aerobically and anaerobically.

Question 4.
Identify the group A and B in the following pie diagram which represents the proportionate number of species of major groups of invertebrata.

Answer:
A = insects, B = molluscs
Among all the species approximately 70% are animals. Insects account for 70% of animal population which is the maximum (represented as A) (2009). There are 10,25,000 species of molluscs (represented as B).

Question 5.
In our biosphere immense diversity (or heterogeneity) exists not only at the species level but at all levels of biological organisation ranging from macromolecules within cells to biomes. Biodiversity is the term popularised by the sociobiologist Edward Wilson to describe the combined diversity at all the levels of biological organisation. Give an account of causes behind the loss of biodiversity and write a note on habitat loss and fragmentation.
Answer:
In today’s time it has been observed that the rate of extinction of the species has accelerated. The four major causes that has resulted into accelerated extinction are:

1. Loss of habitat and fragmentation.
2. Over-exploitation of available natural resources.
3. Invasion of alien species and their dominance.
4. Co-extinction of host and parasite species simultaneously.

Habitat loss and fragmentation is one of the most important causes for loss of biodiversity. This drives most plant and animal towards extinction. Due to certain anthropogenic (human) activities, larger habitats get fragmented into the small units. Due to such fragmentation, the species that require larger space may enter into competition with other species. The best example is human animal conflict. As man has converted the forest area into agricultural land by way of deforestation and conversion of tropical rainforests into soya cultivation and conversion into grasslands for rearing beef cattle, the wildlife residing in the forests have entered the human territory. Hence further fragmentation may lead to either migration of the species or may result into their extinction.

Similarly, co-extinction is another reason responsible for loss of habitat. The entities of ecosystem are interconnected to each other through food chain. Decline in the population in one species may hamper the related or associated species. Hence, it may lead to the extinction of related or associated species too. This is ecologically termed as co-extinction.

Question 6.
The plant produces highly poisonous cardiac glycosides and that is why we usually do not see any cattle or goats browsing on this plant. Give two reasons as to why a weed such a Calotropis flourishes in abandoned fields.
OR
Populations rarely grow smoothly up to the carrying capacity and then remain there. Instead, fluctuations in population numbers, abundance, or density from one time step to the next are the norm. Which processes are responsible for the fluctuations of population density within the community ?
Answer:
Calotropis flourish in abandoned fields because Calotropis thrives on poor soils, particularly where overgrazing has removed competition from native grasses. Moreover, Calotropis produces highly poisonous cardiac glycosides as a means of chemical defences against herbivores such as cattle or goats.
OR
The processes responsible for such fluctuations are:

1. Natality
2. Mortality
3. Emigration
4. Immigration.

Section – B

Question 7.
Cells when start to grow in an unruly manner may develop into a mass which eventually develops into cancer .Till so far we have identified more than 100 different types of cancer. It can develop almost anywhere in the body. Commonly these different types of cancers are named on the basis of their origin. For example, Cancer that develops in lungs is lung cancer and if develops in skin then it is called as skin cancer. It may remain confined to one place or may get dislodged. Symptoms and treatment depend on the type of cancer.
(i) Visual examination of Cancer is indicated by ……………. (a) …………. and definitive diagnose is done by …………. (b) ………… :
(ii) How the cancerous cells spread to the distant site?
(iii) Medical term for the cancerous state of blood is:
OR
What makes some viruses cause cancer in humans? How does benign tumour turn malignant? How does the latter harm the human body?
Answer:
(i) (a) Uncontrolled growth of tissues forming lumps or bumps
(b) Biopsy: Cancer is uncontrolled proliferation of the cells and is caused by damage to genes that regulates the cell cycle. The cells loose the control and divide indefinitely leading to the formation of mass of cells. They can be detected by a new bump formed usual and final examination is done by using biopsy.

(ii) Cancerous cells when remain fixed at the site they form tumor and when they dislodge and circulate in the blood stream they reach to other sites and spread cancer.

(iii) Cancer of blood cells is called as leukemia.
OR
The cancer-causing viruses possess the genes known as viral oncogenes. The normal proto-oncogenes get activated into oncogenes through some viruses that result in oncogenic transformation of cells. A benign tumour can turn malignant if it remains sustained in the body for longer period of time. The cancerous cells are carried by blood or lymph towards other parts of the body where they spread secondary cancer or metastasis. Once the tumour turns malignant, it starts invading the surrounding tissue and the cancer starts to spread in the whole body.

Question 8.
Bioreactor: A bioreactor is a vessel in which a biological reaction or change takes place. The biological systems involved include enzymes, microorganisms, animal cells, plant cells, and tissues.
(i) A culture system with constant environmental conditions maintained through continual provision
of nutrient and removal of wastes is called ………………….. culture system.
(ii) For thorough mixing of medium and inoculum the part of fermenter useful is:
(iii) In agarose gel electrophoresis, DNA molecules are separated on the basis of their:
Answer:
(i) Continuous
(ii) Impeller
(iii) Size only

Question 9.
Tobacco plants are damaged severely when infested with Meloidegyne incognita. Name and explain the strategy that is adopted to stop this infestation. Name the vector used for introducing the nematode specific gene in the tobacco plant.
Answer:
The genetically resistant tobacco cultivars have been evolved through biotechnological applications.
The infestation of the nematodes called Meloidogyne incognita can be prevented by the process of RNA interference. It is present in all the eukaryotic organisms. This method envisages the silencing of a specific mRNA of the parasite due to complementary dsRNA molecule that binds to and prevents translation of the mRNA (silencing). The source of this complementary RNA could be from an infection from viruses having RNA genomes or mobile genetic elements called transposons. They replicate via RNA intermediate. The vector used for the entire process is Agrobacterium tumifaciens .

The nematode resistant genes from agrobacterium were introduced into the host plant. This produced both, the sense and anti sense RNA in the host cell. Both of them were complementary and formed . double stranded RNA that initiated the above said process called RNA interference.

Question 10.
Edward Wilson described diversity at all levels of biological organisation ranging from macromolecules inside the cells to biomes. It is of three inter-related hierarchial levels-genetic diversity, species diversity and community ecosystem diversity. Species diversity is the variety in the number and richness of the species of a region. For example, the Western ghats have a greater amphibian species diversity than the Eastern ghats.
(i) The number of species per unit area is called:
(ii) What is the significance of species diversity?
(iii) How will increasing species diversity affect ecosystem?
Answer:
(i) Species richness
Species richness can be defined as the number of species per unit area. There is a wide variation in the distribution of species on this planet. The equatorial region is usually rich in biodiversity and this richness decreases from the equator to the polar region.

(ii) The species interacts with its environment and thus performs certain functions. In a natural state,
these interactions and consequently the system is in balance. The loss of one species affects many , other species and causes imbalance.

(iii) By increasing species diversity in an ecosystem, both the efficiency and the productivity of an
ecosystem will increase. A greater species richness and diversity may cause ecosystems to function
more efficiently and productively by making more resources available for other species.

Question 11.

Observe the above graph and answer what for the letter N, r, k stand in the typical Verhulst-Pearl population growth curve.
Answer:
N-population density at the given time, r-intrinsic rate of natural increase in population and k-carrying
capacity. “,
Logistic growth model is given by dN/dt = rN (1 – N/k) where N is no. of individual at time t, K is carrying capacity r is intrinsic growth rate.

Question 12.
The nematode infects the roots of tobacco plants and causes a reduction in yield. A unique strategy based was designed by Fire and Mello in 1998. This method involves silencing of a specific mRNA. Study the flow chart below and answer the following questions:

(i) Specify a and b in the chart.
(ii) The process of RNA inactivation by siRNAs is termed as …………………
(iii) Which nematode infects the roots of tobacco plants?
Answer:
(i) Using Agrobacterium vectors, nematode-specific genes are introduced into the host
plant. The introduction of DNA produces both sense and anti-sense RNA in the host cells. These two RNAs being complementary to each other formed a dsRNA which initiated RNAi. Thus ‘a’ is sense RNA and ‘b’ is antisense RNA.

(ii) Double stranded RNA is cleaved by a nuclease called as dicer and small fragments are generated, they are about 22 nucleotides long and are known as short interfering RNAs (siRNA). The process
is called RNA interference (RNAi).

(iii) Meloidogyne incognitia is the nematode that infects the roots of tobacco plants. It is also known as ‘root- knot nematode’. It is a plant-parasitic nematode. It affects both monocots as well as dicot plants.

Section – C

Question 13.
The graph shows the level of antibody in serum following vaccination and a challenge with the same antigen 28 days later.
Study the graphs below and answer the following questions:

(i) Which of the following graphs depicts primary immune response?
(ii) What does peak A represents?
(iii) In reference to the graph above, which graph 1 or 2 represent secondary response?
OR
Microorganisms include bacteria, fungi, protozoa, some algae, viruses, viroids and also prions. Microorganisms may be single-celled like bacteria, some algae and protozoa, or multicellular, such as algae and fungi. They are very useful and can be used for different purposes in day to day activities of the humans. Choose any three microbes, from the following which are suited for organic farming which is in great demand these days for various reasons. Mention one application of each one chosen. Mycorrhiza; Monascus; Anabaena; Rhizobium; Methanobacterium; Trichoderma.
Answer:
(i) Graph 1, primary immune response is the reaction of the immune system when it contacts an ”
antigen for the first time.

(ii) Level of IgM only. In a primary immune response, naive B cells are stimulated by antigen, become
activated, and differentiate into antibody-secreting cells that produce antibodies specific for the eliciting antigen. A large amount of IgM and a small amount of IgG are produced during the primary immune response.

(iii) Graph 2. During the primary immune response to the first encounter with a specific pathogen, some lymphocytes called memory cells develop with the ability to confer long-lasting immunity to that pathogen, often for life. These memory cells recognize antigens on the pathogens they have encountered before, triggering the immune system to respond faster and more effectively than on the first exposure.
OR
A mycorrhiza is a symbiotic (generally mutualistic) association between a fungus and the roots of –
a vascular plant. In a mycorrhizal association, the fungus colonises the host plant’s roots, either ‘
intracellularly as in arbuscular mycorrhizal fungi (AMF), or extra-cellularly as in ectomycorrhizal fungi. They are an important component of soil fertility and reduce fertilizer use, watering costs and plantation management costs. The Azolla-Anabaena association is important agronomically owing to its capacity to fix atmospheric nitrogen at cheaper and faster rates and making it available to crop plants. Bacteria of the genus Rhizobium plays a very important role in agriculture by inducing nitrogen-fixing nodules on the roots of legumes such as peas, beans, clover and alfalfa. This symbiosis can relieve the requirements for added nitrogenous fertilizer during the growth of leguminous crops.