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ISC Class 12 Biology Solved Question Paper 2007

Short Answer Type

51.

Explain:

(i) Exponential population growth

(ii) Environmental Resistance

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52. Explain the origin of Bread Wheat.

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53. How does palaeontological evidence support the theory of organic evolution ? Explain with an example.

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54. What complications will arise if the blood of an Rh +ve person is transferred to a Rh -ve person and vice versa ?

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Long Answer Type

55.

What is feedback control ? Illustrate feedback mechanism of homeostasis by an example of hormones.

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56.

Explain the following :

(i) Tidal volume
(ii) Portal vein
(iii) Ultrafiltration

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57.

Give an account of the cohesion and transpiration pull theory of ascent of sap.

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58. Write the differences between C₃ and C₄ cycles.

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59. Explain the development of the different types of endosperms in angiosperms.

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60. How does the human body protect itself from infection ?

Human body is continually exposed to various microorganisms like bacteria, viruses, fungi and certain toxic substances. The overall ability of our body to fight these disease causing agents and provide protection to our body is called immunity and the system which provides protection is called the immune system.

Immunity is of two types innate immunity which is inborn immune mechanism and acquired immunity.

Innate immunity

It is the first line of defence. It is non-specificand is present since the time of birth.
Innate immunity consists of the following four barriers.

i. Physical barriers - These include the skin and the mucous membranes. These prevent the entry of the microorganisms and trap the microbes entering pur body.

ii. Physiological barriers - The acid in the stomach, saliva in the mouth and tears prvode protection against the microbes.

iii. Cellular barriers - Cells like leukocytes and polymorpho-nuclear leukocytes, monocytes and natural killer cells in the blood as well as macrophages in tissues phagocytose and destroy th organism

iv. Cytikine barriers : Virus infected cells secrete proteins called interferons which provide protection.

Acquired immunity

The acquired immunity is the ability of the immune system to adapt itself with each disease and to generate pathogen-specific immunity. Hence, it is also known as adaptive immunity.
Our acquired immune response are of two types:
Humoral immune response- The antibodies that our B-lymphocytes produce are present in the blood and they are transported all over. This is why it is called the humoral immune response; it consists of an antibody army produced by the lymphocytes.

Cell-mediated immune response - The T-lymphocytes are responsible for this immunity. Cell-mediated immunity becomes clear in the case of transplant patients. When one of our organs malfunctions, we may use another person’s organs and replace the malfunctioning one with it. But it isn’t that simple. It appears that T-lymphocytes are capable of recognising whether a tissue or an organ is from our body or someone else’s. This is the reason why we cannot take organs from just anyone and replace ours with it. Even if you find another person with the same blood group,your body might reject the transplant. The T-cells quickly recognise that the tissue is foreign and do not allow it to become a part of the body. This is why transplant receivers have to take immune-suppressant medication for the rest of their lives. This response is controlled by the T-lymphocytes.

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