Model Paper 2
BIOLOGY
CBSE- XII

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Maximum Marks: 70
Time allowed: 3 Hours

SECTION -A

Q.Nos. - 1-8 are very short answer type carrying 1 mark each. Answer these in 1-20 words or in one sentence.

Q 1 What is cytoplasmic inheritance? (1 marks)

Ans 1 Cytoplasmic inheritance is the inheritance of the characters from the mothers to their progeny because of the factors residing in their cytoplasm and not because of the nuclear factors.

Q 2 Why dogs are used in crime detection? (1 marks)

Ans 2 Dogs have their olfactory lobes very evolved and are therefore very efficient is distinguishing various smells even when they are very low. This property makes the dogs a good candidate for tracking the criminals from the scene of crime.

Q 3 Name the phenomenon by which DNA isolated from one type of cell, when introduced into another is able to express some phenotypic characters of the donor in the host cell. (1 marks)

Ans. 3 Transformation

Q 4 Define green manure. (1 marks)

Ans 4 The Green manure is when a quick growing crop is grown and ploughed under, to incorporate it in soil. A green manure crop supplies organic matter, additional nitrogen and also exercises a protective action against erosion and leaching.

Q 5 How raw fruits are ripened artificially? (1 marks)

Ans. 5 Ethylene gas is used to ripen fruits artificially.

Q 6 What is the full form of ACTH? (1 marks)

Ans 6 Adrenocarticotrophic hormone

Q 7 Name the chemical released by WBC's which causes rise in temperature of the human body that is infected. (1 marks)

Ans 7 Histamines

Q 8 What is the function of vessopressin? (1 marks)

Ans 8 Conservation of body water and contraction blood vessels

SECTION-B

Q. Nos. - 9-18 are of short answer type carrying 2 marks each. Answer this in 20-30 words each.

Q 9 What is hypanthodium? Give one example. (2 marks)

Ans. 9 Hypanthodium is a special type of inflorescence in which main axis forms the fleshy, hollow and pear shaped receptacle with a narrow apical opening guarded by small overlapping bracts. There are large number of flowers borne on the inner side of the hollow receptacle. There are three types of flowers, male female and gall flowers (sterile flowers). Male flowers are present towards the opening and female flowers are at the base of the cavity. Gall flowers are present in between the male and female flowers
Example: Found in the Ficus species

Q 10 Differentiate between introns and exons. (2 marks)

Ans. 10 1. Exons are the regions of a gene, which have the information to form the mRNA, while Introns are the regions of a gene, which do not have the information to form the mRNA.
2. Introns are spliced but Exons are not spliced during the maturation of mRNA.

Q 11 How do mycorrhizae help in increasing the crop yield? (2 marks)

Ans. 11 Mycorrhizae help in increasing the crop yield by enhancing the water and nutrient uptake such as nitrogen, phosphorous, potassium and calcium and give them to plant. Vesicular Arbuscular Mycorrizae (VAM) are important in the phosphate nutrition of plants

Q 12 What is corpus luteum? Give its function. (2 marks)

Ans. 12 The ovum is shed from the ovary by the rupturing of the Graafian follicle. After the extrusion of ovum, the remains of the Graafian follicle is called corpus luteum.

Function: When the ovum is fertilised corpus luteum persists and secretes progesterone hormone, which bring about most of the pregnancy changes.

Q 13 Differentiate between a tendon and a ligament. (2 marks)

Ans. 13 Tendons forms the strong inextensible attachment of a skeletal muscle to a bone while the ligament connects bones at the joints and holds them in position.

Q 14 Why do men suffer from haemophilia? Can women also suffer from this disorder? Explain. (2 marks)

Ans. 14 Haemophilia is a sex- linked recessive disorder. The gene of this disease is located on the X chromosome. Haemophilia is commonly found in men because Y chromosome is silent chromosome and it is not able to mask the effect of the recessive hemophilic gene on the accompanying X chromosome.

Yes, women also suffer from this disorder when the recessive genes are in homozygous condition i.e. both XX chromosomes have recessive gene

Q 15 Define vestigial organ. Give two examples of vestigial organs in human body. (2 marks)

Ans. 15 Vestigial organs are those organs, which are nonfunctional in possessors but were functional in their ancestors and related animals.
A vermiform appendix at the end of caecum, a coccyx (tail vertebrate), muscles are two examples of vestigial organs.

Q 16 Why annual rings are comparatively well marked in plants growing in hilly area than those in coastal areas? (2 marks)

Ans. 16 There are seasonal variations (spring and autumn) in hills. In spring, the cambium is more active than autumn. The vessels formed in spring are wider than those formed in autumn. Thus an annual ring of two regions of vessels of different width are formed each year. In coastal areas there is no seasonal variation and therefore, the cambium activity remains same throughout the year and no distinction is seen in the width of the vessles.
Thus the annual rings are comparatively well marked in plants growing in hilly area than those in coastal areas.

Q 17 Bile juice does not contain any enzyme yet it is essential for digestion. Explain. (2 marks).

Ans. 17 Bile contains bile pigments and organic salts called bile salts. Bile juice is secreted from the gall bladder to small intestine.
It is essential for digestion because
- Bile provides alkaline pH for the activity of enzymes.
- Bile salts are of prime importance of digestion of fat.
- In Intestinal lumen they reduce the surface tension of fat droplets, causing their breakdown into many small ones which increase the lipase action on these fat droplets.

Q 18 Life originated from the earth's inorganic atmosphere in the past but this no longer happens today. Give two reasons. (2 marks)

Ans 18 Life originated from the earth's inorganic atmosphere in the past but this no longer happens today .
The two reasons are:
a. The essential blocks (nucleotides, amino acid) were formed in inorganic atmosphere (in the absence of free oxygen) because free oxygen could have destroyed these formed building blocks. That is why today, aerobic condition can not favour the life from inorganic atmosphere.
b. If some molecules are formed they would be absorbed and digested by the pre-exiting organisms.

SECTION-C

Q. Nos. - 19-27 are of short answer type carrying 3 marks each. Answer them in 30-60 words.

Q 19 Explain Lamarckian theory of evolution. (3 marks)
Ans. 19 Lamarck's theory of evolution is mainly based on the following four basic postulates:
1. Growth: living organisms and their component parts tend to increase in size continuously.
2. The formation of the new organ or part in the body is the result of new need.
3. Use and disuse: constant use of organs strengthens the organ while disuse in its degeneration.
4. Inheritance of acquired characters: All changes, while organism acquires during their life, are transmitted their offsprings by the process of inheritance.
Examples of Lamarckism:
a. Giraffe's neck became longer as it stretched its neck to reach the leaves of tall trees. This trait has become permanent generation to generation.
b. The ancestors snakes possessed short limbs but snakes did not use them for locomotion and ultimately lost or degenerated their limbs.

Q 20 Why are plants that consume more than usual 18 ATP to produce one molecule of glucose favored in tropical region? (3 marks)

Ans. 20 The plants, which require more than 18 ATP to produce one molecule of glucose in tropic areas, are called C₄ plants.
Photorespiration does not occur in the C₄ plants. It happens because the carbon dioxide is fixed in the mesophyll cells and malic acid or aspartic acid is formed by sequence of reactions which is then transported to and is built up in the bundle sheath cells. CO₂ is liberated from malate or aspartate and the concentration of CO₂ increases in the bundle sheath cells. Because of the high carbon dioxide concentration there RuBISCO participates in Calvin Cycle and not in the photorespiration unlike that in C₃ plants.
The yield from C₄ plants is thus more than that from C₃ plants because they can conserve as much as half the photosynthetically fixed carbon dioxide. C₄ pathway is thus of an adaptive advantage for such plants in the tropics.

Q 21 Give three functions of lymph. (3 marks)
Ans. 21
Functions of lymph:
1. The lymph serves to return interstitial fluid to blood.
2. Plasma protein macromolecules (formed in liver) enter to blood through lymph.
3. It also carries fat droplets (as chylomicron) to blood.

Q 22 What is cancer? List its danger signals? (3 marks)

Ans 22: Cancer: is the uncontrolled proliferation of cells (tumors). Cancer cells are carried by blood and making the other cells cancerous through metastasis.
Cancer cells are classified into three major categories, Carcinomas, Sarcomas and Leukaemias.
Danger Signals of cancer:
- A lump or hard area in the breast
- A change in wart or mole
- A persistent change in digestive and bowel habits
- A persistent cough or hoarseness
- Excessive loss of blood during monthly periods in women
- Unexplained loss of weight.

Q 23 How is heartwood different from sapwood? Which one is more durable and why? (3 marks)

Ans. 23 Differences
a. In the most of the tree the outer light part is distinct from the inner dark part. The outer region and inner region are called sap wood and heart wood respectively.
b. Sap wood has living cells and heart wood consists of dead cells which become impregnated with resin, gummy, tannin like substances.
c. Hard wood is also known as porous wood because it has vessles (pores) while sap wood does not have vessles and thus known as non-porous wood.
Heart wood is more durable because it is made of dead cells and more resistant to microorganisms and insects.

Q 24 What is aneuploidy and polyploidy? Give one examples of each. (3 marks)

Ans. 24 Diploid organism has two complete sets of chromosome number. A haploid organism has only one set.
Aneuploidy: An organism that does not contain all the chromosomes in equal numbers so that the total number is not exact multiple of the haploid
sets is called Aneuploid and this condition is known as Aneuploidy.

Example: Down's Syndrome (Trisomy 21)
Polyploidy: When the complete sets of chromosome are more than two, the organism is called polyploid and this condition is known as Polyploidy. Polyploidy is of two types:
Autopolyploidy: means increase in chromosome sets within a species
Allopolyploidy: means increase in chromosome sets due to interspecific hybridization.
Example: Triticum aestivum (Hexaploid wheat)

Q 25 When a person is not particularly thirsty and is made to drink lot of water, his urine flow increases greatly and urine is very dilute. Explain. (3 marks)

Ans. 25 When a person is not particularly thirsty and is made to drink lot of water, his urine flow increases greatly and urine is very dilute. This happens because ADH or Vasopressin is not secreted in this condition. The wall of the distal convoluted tubules (DCT) and collecting tubules remain impermeable to water. Thus the reabsorption of the water does not take place. The reabsorption of the Na+ ions from the filtrate in these tubules continues. Due to this reabsorption, the filtrate becomes more and more dilute and urine becomes hypotonic.

Q 26 What are casparian strips? Where they are located? What are their functions. (3 marks)

Ans. 26 Casparian Strips: The root endodermis has peculiar bands like thickening made of lignin and suberin on the radial walls. These bands are known as casparian strips.
Location: Root Endodermis
Functions:
- It prevents plasmolysis in the cells of the endodermis.
- It allows the soil water to pass radially.

Q 27 Population growth is determined by interaction of biotic potential and environmental resistance. Comment on this statement. (3 marks)

Ans. 27 Population growth: is the number of organisms added to the population minus the number of organisms lost.
Biotic potential (BP): is the ability of an organism to produce off spring.
Environment resistance (ER): is the checking of population size by some factors.
If BP = ER, the population growth would be Zero.
If BP > ER, the population growth would increase
If BP < ER, the population growth would decrease
The environmental resistance does not allow the population growth to increase towards infinity and thus the resulting population curve is sigmoid.

SECTION-D

Q. Nos. - 28-30 are long answer type carrying 5 marks each. Answer there in 80-120 words each.

Q 28 Describe various phases of menstrual cycle emphasizing the role of hormones. What is the period where there is maximum chance of conception? (3 marks)

Ans. 28 Menstrual Cycle: The rhythemic series of changes in the sex organs that occur about every 28 days throughtout the reproductive life of female primate and including woman is called menstrual cycle.
Changes in Menstrual cycle. the changes during this cycle can be divided into three phases; proliferative, secretory and bleeding phases.
(i) Proliferative phase:
- the endometrium( mucous membrane of the uterus) is thinnest as all the superfisial layers cast off during the menstrual bleeding.
- The ovarion cycle progresses side by side.
- During proliferative phase, an immature follicle ripens into a Graffian follicle, this phase is called follicular phase.
- This phase extends for about 10 to 12 days.
(ii) Secretory phase:
- In this phase, the ruptured follicle changes into a corpus luteum in the ovary and begins to secrets the progesterone hormone.
- This phase is known as progesteronal phase.
- If the ovum is not fertilised, the corpus luteum undergoes degeneration and the level of the progesterone hormone declines.
- This phase extends for about 12 to 14 days.
(iii) Bleeding phase:
- If pregnancy fails to occur, the bleeding phase starts.
- In this phase, the spiral arteries of the endometrium undergo spasm.
- The 2/3 portions of the endometrium are sloughed off and this is accompanied by menstrual bleeding.
- This phase extends for about 3 to 5 days and is followed by the next menstrual cycle.

Q 29 Differentiate between linkage and sex-linked inheritance. Describe the mode of inheritance of any one sex-linked trait in humans. (3 marks)

Ans. 29 A sex-linked Inheritance is the one where the trait is linked to a sex-chromosome, and is shown by that specific sex, carrying that sex chromosome. A linked inheritance (linkage) is the one where more than one genes located on the same chromosome are inherited together. The linked genes do not follow the Mendelian law of Independent Assortment. Haemophilia in humans is a sex-linked inheritance. Fathers pass the X-linked allele trait to their daughters but not to their sons. Mothers pass the allele of the trait to both the sons and daughters. The mutant allele which causes the haemophilia is recessive and therefore expresses in the daughters only if they are homozygous for the trait, i.e., have recieved it from both their parents. If the daughters are heterozygous, they do not express the disease and serve as just a carrier for the trait. The sons develop the disease, if their mother passes a mutant trait to them, because they have only one X-chromosome. All the sons of a diseased mother get the disease and half the sons of a carrier mother get the disease.

Q 30 What is organic evolution? Explain how homologous, analogous and vestigial organs provide evidence by giving one example of each. (3 marks)

Ans. 30 Organic evolution refers to the origin and evolution of more complex organisms from simpler organisms.
Homologous Organs: The organs which have the same basic structure and developmental origin. Example is: the seal's flipper, the bat's wings, the horse's foot, the cat's paw and the human hand
Homologous organs show common ancestry of the organisms.
- They perform different functions and look superficially different, but all of them have:
1. fundamentally similar plan,
2. approximately the same number of bones,
3. similar pattern of embryonic development
Analogous Organs: The organs, which perform the same function but are not similar in structural details and origin. Example is: the wings of a bird, a butterfly and a bat
Analogous organs show different ancestry and common habitat.
Vestigial Organs: The organs which are not of much use to an animal today but is still present in it. They are believed to be remnants of organs, which were complete and functional in their ancestors. Example is: a large caecum in man, which is reminiscent of herbivorous mammals. Vestigial organs hint at the anatomy and habitats of the ancestors.

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