CBSE Set Qa2 Biology Sample Test Papers For Class 12th for students online

Q21. State Lamarck’s theory on evolution. Who disapproved it and how?
Ans21. Lamarck’s theory is the ‘Theory of inheritance of acquired characters’.
According to this theory organisms undergo certain changes to adapt themselves to the environment. These characters acquired during the lifetime are passed on to progeny. Weismann refuted Lamarckism by showing that even if tails of newborn mice were cut off generation after generation, no shortening of tails was observed in any subsequent generations.

Q22. A farmer has been advised to sow soybean seeds inoculated with a bacterial culture. Name the bacterium in the culture. How is this bacterium useful to the crop?
Ans22. The bacterium is Rhizobium.
This bacterium produces nodules in the roots of leguminous plants and fixes atmospheric nitrogen.
The inoculation of legume seed with Rhizobium will ensure better nitrogen fixation and yields.

Q23. Draw a labelled diagram showing the longitudinal section of phloem of an angiosperm with its components.
Ans23.

Q24. Name any three organs homologous to human hand. Why are they considered homologous?
Ans24. The flippers of seal, the wings of bats, and the paws of cats are the organs homologous to the human hand. They are considered homologous because although they perform different functions and look superficially different, they have fundamentally similar plan and the similar structure.
Moreover the pattern of their embryonic development is essentially similar. Such organs in different animals with same basic structure and developmental origin are called homologous organs.

Q25. Give the full form of ADH. Name the gland that secretes it. Under what condition of the body is it released? What is its role in forming hypertonic urine? What disease is caused due to the failure of secretion of this hormone?
Ans25. ADH stands for Antidiuretic Hormone. It is secreted by the posterior pituitary gland. ADH is secreted when the water content of the body is less than the required, and therefore has to be conserved. ADH secretion causes the walls of the DCT, collecting tubule and collecting duct to become
permeable to water. The tissue surrounding to them is hypertonic due to the active reabsorption of Na⁺ into it and due to the retention of Na⁺ and urea by the counter-current system of vasa rectae. So, water is progressively reabsorbed from the filtrate flowing along the DCT, collecting tubule and collecting duct (rendered permeable by action of ADH) into the surrounding
hypertonic tissue and the peritubular capillaries. The filtrate in the collecting duct consequently becomes hyperosmotic and a strongly hypertonic urine flows out into the renal pelvis.

Q26. Give one reason why lysozyme is considered an enzyme and not a hormone? How does it defend the body? Name any two secretions in humans, which contain lysozyme.
Ans26. Lysozyme is not a hormone because it does not satisfy the following criteria required for a protein to become a hormone :
(i) It is not secreted in response to any change in the internal or external environment of the body,
(ii) it is not transported by blood,
(iii) it has no specific target organ of its own,
(iv) it does not stimulate or inhibit specific biological processes in the targeted organs to modify their activity.
Lysozyme destroys the cell wall of many bacteria and thus prevents many infections.

Q27. Draw a schematic diagram to show the continuous and discontinuous synthesis of DNA and label it.
Ans27.

Q28. Where does spermatogenesis take place? Describe the stages of the process.
Ans28. Spermatogenesis takes place in seminiferous tubules of testes by repeated divisions of spermatogonia. Spermatogenesis occurs in four stages, and each spermatogonium produces eight sperms :
(i) Spermatocytogenesis : Spermatogonia are diploid cells on the walls of seminiferous tubules.
Each spermatogonium divides mitotically to form two primary spermatocytes (2n) interconnected by their cytoplasm.
(ii) Meiosis I : Both of the primary spermatocytes undergo meiosis I and each gives rise to two haploid (n) secondary spermatocytes (n). All the four secondary spermatocytes remain interconnected by their cytoplasm and form a single spermatogonium.
(iii) Meiosis II : The secondary spermatocytes undergo meiosis II and each produce two haploid cells called 'Spermatids' (n).
(iv) Spermiogenesis : All the eight spermatids interconnected with each other get separated and develop into flagellated 'Spermatozoa' (n). They get released free in the lumen of the seminiferous tubule and are carried to the epididymis.


Q29. Where does cyclic photophosphorylation occur in the leaves? Explain the events in sequence. Why is the process referred to as cyclic?
Ans29. Cyclic phosphorylation occurs in the thylakoid membranes of the chloroplasts in the green leaves of plants. When the photons activate Photosystem I chlorophyll P₇₀₀ a pair of electrons are raised to a higher energy level. They are captured by the primary acceptor, which passes them on to ferredoxin (Fd), plastoquinone (PQ), cytochrome complex, plastocyanin (PC) and finally back to chlorophyll P₇₀₀.
The process is cyclic. At each step of electron transfer, the electrons lose potential energy. Their trip downhill is used by the electron transport chain to pump H⁺ across the thylakoid membrane.

The proton gradient triggers ATP synthesis :
This process is referred to as cyclic because the electron transfer starts with P₇₀₀ and the ultimate acceptor of the de-energised electron is also P₇₀₀. Thus the total transfer of the electron is a cyclic one.

Q30. Differentiate between linkage and sex-linked inheritance. Describe the mode of inheritance of any one sex-linked trait in humans.
Ans30. 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.

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