ISC Biology Previous Year Question Paper 2010 Solved for Class 12
(Attempt All Questions)
(a) Give one significant difference between each of the following : 
(i) Aggregate fruit and Multiple fruits.
(ii) Amphicribal vascular bundle and Amphivasal vascular handle
(iii) Open vascular system and Closed vascular system.
(iv) Uricotelism and Ammonotelism.
(v) Auxetic growth and Accretionary growth.
(b) Give reasons for the following: 
(i) Most people living in hilly regions suffer from goitre.
(ii) The water potential of pure water changes when solute is added to it.
(iii) When we turn round and round, we lose our balance.
(iv) The spores of Bacillus thuringiensis are used as bioinsecticide.
(v) Owls have better night vision than day vision.
(c) Give scientific terms for each of the following: 
(i) Adjustment of the eye in order to obtain a clear vision of objects at different distances.
(ii) A device to measure growth in length of a plant.
(iii) The formation of blood cells in the bone marrow.
(iv) The response of an organism to relative length of day and night.
(v) Surgical removal of a section of vas deferens.
(vi) The act of passing out of urine.
(d) Mention the most significant role of each of the following : 
(i) Schwann cells
(ii) Organ of Corti
(iv) Sertoli cells
(vi) Gustatory cells
(e) State the best-known contribution of 
(ii) John Otto
(iv) T.R. Malthus
(f) Expand the following: 
|Aggregate fruit||Multiple fruits|
|It is a group of fruit lets which develop from the free ovaries of a single flower.||It is a group of fruit lets which is formed from a whole inflorescence including peduncle.|
|Amphicribal vascular bundle||Amphivasal vascular handle|
|It consists of a central core of xylem, surrounded completely by phloem on all sides.||It consists of a central core of phloem, surrounded completely by xylem on all sides.|
|Open vascular system||Closed vascular system|
|It contains a strip of cambium in between xylem and phloem.||It lacks cambium between xylem and phloem.|
|It is the elimination of uric acid as the main nitrogenous waste material.||It is the elimination of nitrogen waste mainly as ammonia.|
|Auxetic growth||Accretionary growth|
|In this the body grows in dimension merely by the enlargement of its cells without any increase in the number of cells.||There is production of new cells to replace the worn-out differentiated cells, by mitotic division of special undifferentiated reserve cells, present along with differentiated cells.|
(b) (i) It is caused by the deficiency of iodine in the diet, needed for synthesis of thyroid hormones. It causes enlargement of the thyroid gland.
(ii) The water potential of pure water is highest, considered as zero at normal temperature and pressure. All the water molecules are free and have maximum kinetic energy. Addition of solute decrease the kinetic energy of the water molecules (due to collision of solute and solvent molecules), thus lowering the water potential of water in the solution.
(iii) When we turn round and round, the endolymph in the semicircular ducts, does not move as fast as our body and the sensory cells of cristae continue to move, after the body stops moving. Because of this difference in the rate of movement, the sensory hair of cristae are swept through the endolymph and become bent over, to stimulate sensory cells, which set up action potential in the fibres of auditory nerve, which transmit it to brain.
The brain sends, instructions to muscles to act, hence we lose our balance.
If you turn round and round in circles, the endolymph in semicircular canal is forced to one end. When you stop moving, the endolymph rushes back, the other way. This give you the whirling sensation in the opposite direction and you feel giddy and find difficult to overcome the tendency of falling down. Thus we lose our balance.
(iv) The spores of Bacillus thuringiensis are used as bio-insecticides because they contain proteinaceous toxins- thurioside as crystals to eradicate different group of insects such as moths, flies, mosquitoes, beetles etc. ingesting the spores. These toxins are converted in to active form and kill the insects by inhibiting the ion transport chain in the midgut.
(v) Owls have better night vision than day vision because they mainly contain rod cells in
their retina. These cells possess a visual purple pigment called Rhodopsian which works in dim or diffused light. In bright light, it is broken and does not work.
(i) Power of accommodation
(d) (i) Schwann Cells form neurilemma outside myelin sheath of medullated nerve fibre. They help in saltatory conduction of impulses.
(ii) Organs of corti help in hearing by transmitting the sound impulses to the brain through the auditory nerve. In brain, the sensation of hearing is felt.
(iii) Phellogen is the cork cambium. Its cells have bipolar activity. It cuts of compactly arranged cork cells (phellem) towards outside and loose, radially arranged secondary cortex cells (phelloderm) towards inner side. Later, the cork cells become dead, lose their protoplasm. The three together are called Periderm.
(iv) Sertoli cells are present, along the wall of seminiferous tubule and play a role in nourishing the developing sperms. ,
(v) Neutrophils are the most numerous of all leucocytes, having many lobed nucleus, fine granules and phagocytic in nature.
(vi) Gustatory cells are the taste cells, large in number, present inside each taste bud. They contain taste hairs, sensitive to particular chemicals only when they are dissolved in solutions that enter taste pores.
(e) (i) Nawaschin discovered double fertilisation in plants.
(ii) John Otto discovered Haemophilia.
(iii) Went coined the term auxin for growth-promoting substance.
(iv) T.R. Malthus an economist published essay on ‘Human Population Growth’ in 1799. Both Darwin and Wallace were inspired by his writings and conceived the idea of natural selection.
(f) (i) ECG: Electro Cardiography.
(ii) AIDS : Acquired Immuno Deficiency Syndrome.
(iii) NAA: Naphthalene Acetic Acid.
(iv) ACTH : Adreno Cortico Trophic Hormone.
Section – A
(Attempt Three Questions)
(a) Draw a neat and fully labelled diagram of a T.S. of dicotyledonous stem.
(b) List three differences between micronutrients and macrountrients of plants.
(c) Describe the significance of osmosis in plants.
Fig. T.S. Primary dicot stem Sunflower (detailed structure of a part of T.S. Stem).
(b) Differences between Macronutrients and Micronutrients :
- They are present in plants in relatively large concentrations.
- Their concentration per gram of dry material is at least 1 mg.
- They are often called major elements or macronutrients.
- They build up the body structure and different protoplasmic constituents.
- Some of the macronutrients contribute to the development of osmotic potential in the cells.
- They are not toxic in slightly excessive quantities.
- They are present in plants in very small amounts.
- Their concentration is less than 1 mg per gram of dry material.
- They are known as trace elements or micronutrients.
- They do not play such a role.
- They do not play any role in the development of osmotic potential in the cell.
- They show toxic effects in slight excesses.
(c) Significance of osmosis : [Any three points]
- Entry of soil water into roots is carried out by osmosis.
- Cell to cell movement of water occurs by osmosis.
- Living cells remain distended or turgid only by the osmotic entry of water into them,
- The soft organs like leaves, fruits and young stems are able to keep themselves stretched and swollen due to turgidity of their cells which is dependent upon osmosis.
- Osmosis plays a key role in the growth of radicle and plumule during germination of seeds.
- Many plant movements like the folding and drooping of leaves in Mimosa are brought about by osmosis.
- The stomata open and close only in response to increasing of the osmotic pressure of the guard cells in relation to nearby epidermal cells.
- High osmotic pressure has been found to protect the plants against drought and frost injury. Seeds and spores are similarly able to pass through the un favourable periods due to high osmotic pressure (or low solute potential).
(a) Describe the cohesion and transpiration pull theory of ascent of sap. 
(b) Explain the sequence of events between pollination and fertilization in plants. 
(c) State and explain Blackman’s Law of Limiting Factors. 
(a) Cohesion and transpiration pull theory was proposed by Dixon and Joly in 1894. It is most widely accepted theory. It states that transpiration creates a pull over water column which is lifted upwards like a rope and is not broken due to presence of strong cohesion force amongst its molecules. It is also known as Dixon’s theory of ascent of sap.
(i) Water Column : Plants have a continuous water column in their xylem channels which begins at the base in water absorbing parts of roots and continues upto leaves where water is being lost through transpiration.
(ii) Cohesion Force : Water column remains intact despite gravitational pull because water molecules have a strong cohesion force amongst them due to presence of hydrogen bonds. Cohesion force provides a tensile strength to water column. It has a value of 45-207 atm (Dixon and Joly, 1894).
Other properties of water which account for high tensile strength (ability to remain as a column against a pulling force) and high capillarity (ability to rise in narrow tubes) are adhesion (attraction of water molecules to polar surface as of tracheary elements) and surface tension (Stretching of surface layer at the interphase) due to more attraction of water molecules in the liquid phase than water in the gaseous phase.
(iii) Transpiration Pull : Mesophyll cells transpire water and develop a strong negative water potential. As a result mesophyll cells withdraw water from xylem channels. As there are a very large number of leaves, with each leaf having thousands of transpiring mesophyll cells withdrawing water from xylem, a tension or negative pressure develops in the water column present in tracheary elements. It exerts an upwards pull over the water column and is called transpiration pull.
(iv) Ascent of Sap : Force of transpiration pull is -10 to -30 bars. It is sufficient to overcome resistance of water conducting channels, gravity, resistance in movement of water from soil to conducing channels and from conducting channels to transpiring mesophyll cells. Therefore, transpiration pull lifts the water column upwardly like a rope.
- Water contains dissolved air. As tension increase, dissolved air is changed into air bubbles. Air bubbles do not pass out of water channels but block the same.
- Overlapping cuts break the continuity of tracheary elements. They, however, do not stop ascent of sap.
- Tension created by evaporation of water can be observed with the help of instrument called atmometer.
- Rate of ascent of sap is roughly equal to the rate of transpiration,
- Tracheary elements possess a high degree of lignification which prevents their collapse even under high negative pressure,
- Air bubbles formed during day time dissolve at night because of lower tension and lower night temperature.
(b) The process of transfer of pollen grains from a mature anther to a mature stigma of the same flower or a flower of another plant is called pollination. The following is the account of events between pollination and fertilization.
Germination of Pollen Grains on the stigma : The pollen grains are already two or three celled at the time of pollination. They absorb water and nutrients from stigmatic secretions through germ pores and germinate. The intine grows out through a germ pore present in the thick exine in the germ of a form tube. The germ tube grows through the stylar canal in the form of a pollen tube. The pollen tube secretes pectinases and other hydrolytic enzymes which digests the tissues of the stigma and style to form a passage. The tube or vegetative nucleus descends to the tip of the pollen tube.
The generative cells also moves down into the pollen tube and divides into two male gametes (sperms) if it is not already divided. Each male gamate is lenticular to spherical in outline. It has a large nucleus and is surrounded by a thin sheath of cytoplasm. The tube nucleus degenerates soon after. The fertilization by means of pollen tube is known as siphonogamy. If the pollen tube enters the ovule through the microphyle, the method is known as porogamy. If the pollen tube enters through the chalaza end of the ovule it is known as chalazogamy. When the pollen tube enters the ovule through integuments it is known as mesogamy. Sometimes the empty space above the micropyle has a plug of placental origin. The plug is formed by obturator. The obturator guides the growth of the pollen tube through the empty space.
(c) Blackman (1905) formulated the principle of limiting factors. It states that when a metabolic process is conditioned as to its rapidity by a number of separate factors, the rate of the process is limited by the pace of the slowest factor. This principle is also known as Blackman’s Law of Limiting Factors.
Explanation : A metabolic process is conditioned by a number of factors. The slowest factor or the limiting factor is the one whose increase in magnitude is directly responsible for an increase in the rate of the metabolic process. Let us consider, for example, effect of CO2 and light on the rate of photosynthesis, presuming other factors to be optimum. Suppose a leaf is exposed to a light intensity that allow it (leaf) to reduce 5 mg of carbon (i.e CO2) per hours. If availability of CO2 in the environment is less than the required magnitude, photosynthesis will proceed at a slow rate. Here CO2 becomes the limiting factor in the process of photosynthesis.
If the availability of CO2 is increased there will be corresponding increase in the rate of photosynthesis till the availability of CO2 reaches 5 mg/hr (A- B). Any further increase in CO2 availability will not enhance the rate of photosynthesis because light now becomes a limiting factor (B-C). Thus, while C02 has been limiting the process in reaction, A – B of the graphic curve, light becomes the limiting in the region B-C of the curve. Photosynthetic rate will increase only if there is a corresponding increase in light intensity with the increased concentration of CO2 till CO2 again becomes a limiting factor (A – B, D – E)
(a) Write two functions each of: 
(i) Amniotic fluid ‘
(ii) Human placenta
(b) Briefly explain the procedure followed for haemodialysis. 
(c) Explain the origin and conduction of heart beat in man. 
(a) (i) Amniotic Fluid: Functions –
- Protect the embryo from injury,
- Prevents dessication of embryo.
(ii) Human placenta : Functions –
- Exchange of materials, food and oxygen pass from the .maternal blood to foetal blood,
- Wastes from foetus pass in to the maternal blood.
(b) Procedure for Haemodialysis: When the kidneys are completely damaged and do not function, the patient often receives haemodialysis (treatment with an artificial kidney). Haemodialysis is the separation of certain substances from blood by use of a selectively permeable membrane.
The pores in the membrane allow some substances to pass through, however, prevent others. The patient is connected to the machine by a tube attached to an artery often the radial artery.
Blood from the artery is pumped into a tube that runs through the dialyzer. The dialyzer is filled with dialysis fluid which contains the same quantities of electolytes and nutrients as normal plasma but contains no waste products. The cellophane tube (a tube bounded by thin membrane) is kept in the dialysis fluid. The pores in the cellophane tube tube do not allow the movement of blood cells and proteins from the blood into the dialysis fluid, but are large enough to allow smaller molecules to diffuse into the fluid. Molecules of waste substances such as urea, ammonia and waste diffuse into the dialysis fluid. Diffusiion of other substances such as glucose, amino acids and electrolytes is prevented by the presence of these substances in the dialysis fluid in the same concentration as in the normal plasma. Now the blood is returned to the patient body through a vein usually the radial vein.
(c) Heart beat is the rhythmic contraction and relaxation of the heart. Each heart beat includes one systole (contraction phase) and one diastole (relaxation phase) of the heart to distribute and receive blood to and from the body. The heart of a healthy person beats 72 times per minute. Origin of heart beat. The mammalian heart is myogenic (myo = muscle, genic = originating from). It means the heart beat originates from a muscle, (however, it is regulated by the nerves). The heart beat originates from the sinoatrial node (SANode)—pace maker, which lies in the wall of the right atrium, near the opening of the superior vena cava. The SA node is a mass of neuromuscular tissue.
Conduction of heart beat. Another mass of neuromuscular tissue, the atrio-venticular node (AV node) is situated in the wall of the right atrium. The AV node picks up the wave of contraction propagated by SAnode. Amass of specialized fibres, the bundle of His, originates from the AV node. The bundle of His divides into two branches, one going to each ventricle. Within the myocardium of the ventricles the branches of bundle of His divide into a net work of fine fibres called the Purkinje fibres. The bundle of His and the Purkinje fibres convey impulse of contraction from the AV node of the myocardium of the ventricles.
(a) What is reflex action ? Draw a neat labelled diagram of a reflex arc.
(b) Mention a cause and symptom of each : 
(c) Write four functions of the human skeletal system. 
(a) A reflex action is an automatic, spontaneous, mechanical response to a stimulation without the will of the animal through the nervous system.
Cause : Bone loses minerals and fibres from its matrix.
Symptom: Bones become weak, porous, light in weight, liable to easy fracture.
(ii) Constipation :
Cause : Insufficient fibres in the diet.
Symptom : Infrequent passage of dry, hardened faeces.
(iii) Asthama :
Cause : Long term inflammatory disease caused by an allergic reaction to irritants such as cigarette smoke, dust, or pet pander.
Symptom : Exhaling is more difficult than inhaling due to spasms of smooth muscles of bronchi and bronchioles.
(iv) Tetany :
Cause : Deficiency a parat hormone.
Symptom : Lowering of blood calcium level, increased excitability of nerves muscles, sustained, contraction (tetany) of muscles of larynx, face, hands, and feet.
(c) Functions of human skeletal system :
- It gives support to softer body parts.
- Endoskeleton also protects the body parts e.g., brain is protected by the cranium of the skull, spinal cord gets protection by vertebral column.
- Endoskeleton parts provide attachment for large muscles.
- Bones help in bringing about movements due to contraction of muscles and the bones change their positions.
- Blood cells are produced by bone marrow.
- Bones maintains calcium and phosphorus level of blood.
(a) Explain the effects of gibberllins on plants.
(b) Describe the mechanism of pulmonary gaseous exchange.
(c) Mention the site of secretion and function of the following hormones : 
(a) Physiological effects of gibberellins :
- Stem elongation. The most prominent effect of gibberellins on plant is stem intemodal elongation. Application of gibberellin on genetically dwarf plants causes them to grow well. They also help cell growth of leaves and other aerial parts except roots.
- Bolting and flowering. Some biennial plants grow as rosette of leaves on a very short condensed stem, in the first year of growth and then in the next year there develop an elongated shoot and bear flowers. This phenomenon is known as bolting. Exogenous application of gibberellin induces the bolting and flowering in the same year, e.g. Cabbage, Radish.
- Seed germination. Some of the light sensitive seeds like lettuce, barley etc. can germinate with the treatment of gibberellins even in complete dark, which otherwise require specific light conditions.
- Parthenocarpy. Gibberellins have been found to be more effective than auxins in causing parthenocarpic development of seedless Suits in pomaceous plants like apples, pears, etc.
- Breaking of dormancy. Gibberellins are effective in overcoming the natural dormancy in potato tubers and buds in winter.
- Vernalisation. Vernalisation or low-temperature treatment of some plants can be replaced by gibberllins
- Sex-expression. Gibberllins have masculinzing effect on genetically female plants of Cannabis. They can also replace female flowers with male flowers on monoecious plants of cucurbits.
(b) The process of inhaling fresh air into the lungs (Inspiration) and expelling out or exhaling stale air (expiration) from the lungs is called breathing. It is a purely mechanical process that is completed in the following two steps :
The movement of air in and out of the lungs occurs due to alternate change in the intera pulmo-nary pressure inside the lungs which is caused by the contraction and relaxation of diaphragm and the intercostal muscles.
(i) Inspiration: During inspiration, the intercostal muscles contract that lifts the ribs upwards and outwards. At the same time, diaphragm also contracts and becomes flat by moving down. As a result the volume of the thoracic cavity increases. This reduces the pressure of air in it which in turn causes the fall of pressure in the lungs below the atmospheric pressure. As a result, the atmospheric air rushes into the lungs passing through the respiratory passage and bringing about inspiration. The intercostal muscles responsible for inspiration are called inspiratory muscles.
(ii) Expiration : After inspiration, the intercostal muscles and diaphragm are relaxed and move upwards towards the thorax. The volume of the thoracic cavity is reduced which increases intrapulmonary pressure inside the lungs above the atmospheric pressure. This forces the air from the lungs to out side causing expiration. The inspiration is a active process while expiration is a passive process.
In this way, each breathing consists of one inspiration and one expiration. It is an involuntary process which is carried out automatically at a constant rate in a healthy person.
- Cholecystokinin: Pancreozymin (cck-Pz): Site of secretion : Mucosa of small intestine.
Function : Stimulate the gall bladder to release the bile and also stimulate pancreas to release its enzymes.
- Oxytocin : Site of secretion : Posterior lobe of pituitry gland.
Function : Promotes contraction of Uterine muscles, just before and during labour, contraction of myo-epithelial muscles of lactating breast, for squeezing milk for baby.
- Insulin :Site of secretion : Beta-.cells in islets of langerhans of pancreas.
Function : Decrease level of glucbse in blood, by increasing the rate of its transport out of blood and in to cells, muscles and convert it into glycogen.
Section – B
(Answer any Two Questions)
(a) Describe the procedure of hybridisation in plants. 
(b) Name the causative agent and the main preventive measure for each of the following diseases :
(c) What is manure? Explain any two types of manures. 
(a) The process of crossing two genetically different plants to obtain a progeny having a combination of desirable characteristics is called Hybridisation. It may be interspecific or intraspecific or intergeneric.
Procedure of hybridisation :
- First of all we select the plants with desirable traits.
- In bisexual plants anthers are removed from the stamens (emasculation) to avoid self-pollination. In unisexual plants and in self-sterile bisexual plants emasculation is not required. Male sterillity has been reported in many plants such as wheat, maize, sorghum, barley, sunflower etc.
- The emasculated flower is immediately enclosed within polythene bag to prevent the pollination from unwanted plants. This process is called bagging.
- The emasculated and bagged flowers must be tagged by writing every step with date and time. The bagging and pollination is incomplete without tagging.
- During artificial pollination, the mature and viable pollen grains are collected from the male parent and the pollen grains from the bag are dusted over the stigma. The bag is replaced immediately over artificially pollinated stigma.
- Hybrid. Seeds of the cross are sown to raise the F (first filial) generation progeny. The breeding of ‘hybrid corn’ is an important example of hybridization. It is produced by crossing two inbred or homozygous lines of maize. The inbred lines aree true breeding and highly uniform.
(b) (i) Amoebiasis:
Causative agent: Entamoeba histolytica (Protozoan).
Preventive measure : Proper sanitary conditions, proper coverage of eatables, proper washing of fruits and vegetables before eating.
(ii) Rabies :
Causative agent: Rabies virus (RNA virus) infection by bite of rabid, dog, monkeys, cats etc.
Preventive measure: Isolation and killing of rabid dogs. Immunization of domestic cats and dogs, injection of vaccine to person bitten by dog.
(iii) Pneumonia :
Causative agent: Diplococcus pneumoniae (bacteria).
Preventive measure : Isolation of infected persons.
(c) Manure is a partially decayed organic wastes that increase the soil fertility. They supply all essential elements required by crop plants. They also improve the physical conditions of soil by preventing erosion and leaching and increase the water holding capacity of the soil.
Manure are of two types
(i) Green Manure and
(ii) Farmyard Manure
(i) Green Manure : They are obtained by cultivating quick growing leguminous and non- leguminous crops. The crop is ploughed when the plants are about one foot height. The green manure crop supplies the organic as well as inorganic components to the soil. It also provides a protective action against erosion and leaching e.g. Dhaincha (Sesbania aculeata) cluster beans (Cymopsis tetragonoloba), sun-hemp (Crotolaria juncea), cowpea (Vigna sinensis) are some of leguminous crops commonly used as greeen manures.
(ii) Farmyard Manure: It is the most valuable decayed organic matter commonly applied to the soil. It is obtained by the partial decay of animal dung, farm refuse and crop residues. These materials are stored in heaps in a pit and kept moist. The manure becomes ready after 4-5 months. The manure is dark coloured, amorphous and rich in humus. Manure enriches the soil with organic substances and releases mineral elements in the soil. Residue of gobar gas plant is a kind of farmyard manure.
(a) State three differences between Homologous and Analogous organs and give an example of each. 
(b) Explain the resistance of mosquitoes to pesticides such as DDT. 
(c) What is the importance of preserving the germplasm of wild species? 
(a) Homologous organs:
- They have a common origin and structure.
- They perform different functions.
- Eg. Forelimbs of vertebrates.
- They have a different origin and structure.
- They perform same function.
- Eg. Wings of insect and bird.
DDT resistance of mosquitoes to pesticides: Earlier DDT (dichloro-diphenyl trichloroethane) has been supposed to be the best pesticide for controlling the population of the mosquitoes which spread malaria. Under the National Malaria Eradication Programme, DDT was extensively sprayed to kill the mosquitoes. This made the mosquitoes to adapt and become resistance to the DDT and other pesticides. This can be explained on the basis of natural selection.
Before the discovery of DDT, the mosquito population had more DDT-sensitive and less DDT- resistant mosquitoes. As DDT was not being used, the DDT resistant remained undetected, unidentified and dominated by DDT sensitive mosquitoes. But when the use of DDT as an insecticide started, the DDT resistant mosquitoes had a competitive survival mosquito with advantage over their counter parts. Then only with the resistant genotypes were able to survive and reproduce. The DDT resistant characteristic strain multiplied and spread over more and more members of the population. Gradually their number in the population increased while the DDT sensitive type were finally eliminated, making the mosquito population resistant to DDT.
(c) Genetic material present inside the germ cells is called germplasm. The germplasms are generally collected from the areas where wild relatives of our cultivated plants are still growing in wild state. Development of agriculture in the world is based on the introduction of these wild varieties of useful crop plants form their original homes to new areas. Wild species must be preserved because they have played a major role in the improvement of economically useful crop plants and rare animals.
(a) Explain the basic postulates of Darwinism.
(b) Whatis genetic erosion ? State any two factors responsible for it.
(c) What is meant by biotic potential?
(a) The main features of Darwin’s theory of Natural Selection are as follows :
1. Over production (Rapid Multiplication): All organisms possess enormous fertility. They multiply in geometric ratio. Some organisms (living beings) produce more offspring and others produce fewer off-spring. This is called differential reproduction.
2. Limited Food and Space: Despite of rapid multiplication of all types of species, food and space and other resources remain limited. They are not liable to increase.
3. Struggle for existence. The struggle for existence can be of three types.
- Intraspecific Struggle. It is the struggle between the individuals of the same species because their requirements like food, shelter, breeding places, etc. are similar.
- Interspecific Struggle: It is the struggle between the members of different species. This struggle is normally for food and shelter. For example, a fox hunts out a rabbit, while the fox is preyed upon by a tiger.
- Environmental Struggle: It is the struggle between the organisms and the environmental factors, such as drought, heavy rains, extreme heat or cold, earthquakes, diseases, etc. Thus climate and other natural factors also help in restricting the number of individuals of particular species.
4. Variations: Except the identical twins, no two individuals are similar and their requirements are also not exactly the same. It means there are differences among the individuals. These differences are called variations. Due to the variations some individuals would be better adjusted towards the surroundings than the others. Adaptive modifications are caused through the struggle for existence. According to Darwin, the variations are continuous and those which are helpful in the adaptations of an organism towards its surroundings : would be passed on to the next generation, while the others disappear.
5. Survival of the fittest or Natural Selection: The organisms which are provided with favourable variations would survive, because they are the fittest to face their surroundings, while the unfity are destroyed. Originally it was an idea of Herbert Spencer (1820-1903) who used the phrase ‘ the survival of the fittest’ first time. While Darwin named it as natural selection.
It is to be noted that only survival of the fittest is not enough. But organisms should also adapt or change themselves according to the changed conditions of the environment as environment is always changing. The small animals who could change their feeding habits ffom.herbivorous to carnivorous diet survived, because they could easely get adapted to the changed environment, while the giant reptiles like dinosaurs became extinct.
6. Inheritance of useful variations: The organisms after getting fitted to the surroundings transmit their useful variations to the next generation, while the non-useful variations are eliminated. He agreed with Lamarck’s views, because according to Darwin acquired characters which are useful to the possessor could be inherited.
7. Formation of new species: Darwin considered that useful variations are transmitted to the offspring and appear more prominently in succeeding generations. After some generations these continuous and gradual variations in the possessor would be so distinct that they form a new species.
(b) The genetic erosion is the loss of genes from the gene pool due to certain factors such as deforestation, urbanization, shifting cultivation, damage of ecosystem and adoption of certain genetically uniform crop plants.
- Deforestation : It results in depletion of plant and animal life throughout the world. Deforestation disturb the tribals and forest dwellers living in harmony with nature. It reduces natural population of plants and hence cause genetic erosion.
- Reduction in crop varieties : As new varieties develop, old varieties are discarded. With this gene pool gets depleted. With development of high yielding varieties, local low yielding varieties are discarded. Such varieties and their genes can not be regained.
(c) Biotic potential: Biotic potential is the physiological capacity to produce off spring or It is a natural capacity of a population to increase at its maximum rate under ideal conditions. The biotic potential of all organisms is very high. If unchecked the number of any species will quickly over run the world. It is also called reproductive potential. It is calculated as the multiple of number of young ones produced at each reproduction and the number of reproductions in a given period of time.
In nature, full biotic potential of an organism is never realised as conditions are never ideal. There are factors such as predators, pathogens, parasites, diseases, food scarcity, adverse weather, natural calamities like drought, floods that always prevent the population from growing and keep the biotic potential under control.
(a) Explain the origin of bread wheat (Triticum aestivum). 
(b) Write two differences between : 
(i) B cells and T cells
(ii) Antibodies and interferons
(c) Define organ transplantation. Mention a precaution which must be taken for this procedure. 
(a) The wild ancestor of wheat is diploid Einkorn wheat, Triticum boeticum (2n =14) which grew as early as 10,000 to 15,000 B.C. The cultivated diploid species of that period was T. monococcum (2n = 14). The T. monococcum was naturally fertilized by a wild grass Aegilopes speltoides (2n =14) The diploid hybrid of these species was sterile. Polyploidy occurred in those sterile individuals to give rise to tetraploid form (2n = 4x = 28). This is known as T. turgidum. It’s cultivated form is called emmer wheat. One of its natural mutant is T. durum. The tetraploid emmer wheat T. dicoccoides on natural crossing with diploid wild goat grass Aegilops squrarosa. (2n = 14) formed a triploid (3x = 21). It was found to be sterile. This triploid hybrid undergo chromosome doubling to produce fertile hexaploid modem bread wheat i.e.,T. aestivum (.In = 6x = 42). It can be illustrated as below.
(b) (i) B-Cells:
- B-cells provide hormonal or antibody-mediated immunity.
- These are formed in the bone marrow.
- They produce clone of cells which release antibodies into the lymph which enter the blood circulation.
- The antibodies attack the foreign particles and destroy them.
- They defend the body against invading bacteria or viruses. They do not react against transplants and cancerous tissues.
- They do not react against the cancerous tissue and transplants.
- Plasma cells do not move to the site of infection, only antilbodies appears in the blood and lymph, to dispose of the antigens.
- T-cells provide cell-mediated immunity.
- These are formed in the thymus gland and hence called T-cells.
- They divide to form three types of lymphoblasts-killer, helper and suppressor cells. They do not release antibodies but hold onto the pathogen or antigens.
- The whole cells directly attacks the for¬eign particles and destroys them.
- They defend the body against pathogens but also attack the transplants and cancerous cells.
- They also react against the cancerous tissues and the transplants.
- Killer cells reach the site of infection, se¬crete perforin, kills microbes by puncturing their cell walls. (Any two points)
- They are produced by plamsma cells
- Pass into blood cells and lymph to dispose of pathogen.
- Act outside the cells.
- Form body’s third line of defence.
- They are produced by microbe infected cells.
- Leave infected cells to enter healthy cells to dispose of the microbes.
- Act inside the cells.
- Form body’s second line of defence.
(c) Organ transplantation is the implanting of living or preserved tissue or organ from one site to another site in the same individual or from one individual called the donor to another individual termed the recipient with an objective of keeping the graft functional at the new place. Organ transplantation involves kidney, liver, heart, lung, skin, tendon, bones and cornea. The most commonly transplanted tissue in humans is the blood.
An important problem in organ transplantation is that the recipient body rejects the transplanted organ as non-self. It is due to cellullar immune system response of T-cells. While transplanting a precaution must be taken is that the antigens of a donor should match those of recipient as closely as possible. For preventing rejection, immunosuppressants are used after transplantation operation.