Solved Board Paper For ICSE Class 12 Biology

(ii) Development of megaspore within the ovule: (megasporangium) is known as megasporogenesis. A hypodermal cell of the nucellus at the micropylar end differentiates as archesporial cell. This cell functions as megaspore mother cell. It undergoes meiosis to form four haploid megaspores; out of which one is functional and remaining three degenerate.

The functional megaspore is the mother cell of the female gametophyte. It grows in size and forms an embryo sac. The haploid nucleus of the megaspore divides mitotically, thus forming 8-nuclei which organise in a definite manner within the embryo sac. Three nuclei at the micropylar end organise into an egg apparatus, three at the chalazal end form antipodal cells and the remaining two, called polar nuclei, migrate to the center of the embryo sac and later fuse to form a single diploid secondary nucleus. The central large cell of the egg apparatus is female gamete which is surrounded by two lateral synergid cells. The mature gametophyte thus has two polar nuclei, three antipodals, one egg and two synergids.

Since this embryo sac has developed from a single megaspore and has eight nuclei, it is said to be monosporic 8-nucleate embryo sac or Polygonum type of embryo sac. It is the most common type of embryo sac. It is the most common type of embryo sac and is found in about 81% of flowering plants

(iii) Significance of Osmosis:
a) It helps in the absorption of water from the soil.
b) It helps to maintain the turgidity of cells.
c) It helps in opening and closing of stomata.

(ii)
The advantages of asexual reproduction are:
(a) Numerous offspring can be produced without spending great amount of time and energy.
(b) Animals that remain in one particular place and are unable to look for mates can reproduce asexually.
(c) Stable environments that experience very little alterations are of advantage for asexually reproducing organisms.
The disadvantages of asexual reproduction are:
(a) It does not produce genetic variation.
(b) All of the organisms are genetically identical and therefore, share the same defect.
(c) If the stable environment changes, the results could be deadly for all of the individuals.

(iii)
(a) Nitrogen:- It is a major component of living systems and is present in the compound state as purines, pyramidines, amides etc. in the living beings both in plants or animals.

Deficiency:- causes yellowing of leaves i.e- chlorosis.

(b) Calcium:- Strengthen cell wall structure as calcium is an essential part of plant cell wall.

Deficiency:- Leaf tips of plants take a hooked shape.
(c) Boron:- works as a hormone and aids in flowering and fruiting.

Deficiency:- Stem and root apices die with lack of boron.

a. Holocrine glands – are those glands where the entire cell disintegrates while discharging its secretion like sebaceous glands.

b. Merocrine glands – secretion of these glands simply diffuse out of the cell; while the cells remain intact like sweat glands, salivary glands and intestinal glands.

c. Apocrine glands – In these glands, apical parts of the cells are shed off to discharge secretions like mammary glands.

(ii)
a. The neuron consists of –
b. Cell body or cyton
c. Dendrite
d. Axon

Cyton – It is an enlarged structure with nucleus. Cytoplasm consists of basophilic Nissl’s granules. It has numerous mitochondria which provide energy for nerve impulse conduction. It also contains Golgi bodies, nucleolus and ER.

Dendrite – These are cytoplasmic processes which conduct impulses towards cyton.

Axon – It arises from cell body as conical elevation. It conducts impulses away from the cyton. Axon is surrounded by neurilemma, which consists of Schwann cells. Axons may be myelinated or non-myelinated depending upon the presence of myelin sheath. It is insulative and nutritive in function.

Myelin sheath is periodically interrupted at nodes of Ranvier. Axon terminal branches profusely thus forming synapses with the dendrite of cell bodies of another neuron.

(iii)

Muscle contraction is initiated by a neural signal from the central nervous system through a motor neuron.
a. Neuromuscular junction is a place where motor neuron meets sarcolemma of muscle.
b. Motor neuron and the connected muscle fibre together form a motor unit.
c. When a signal reaches this junction, the neurotransmitter acetylcholine is released.
d. Acetylcholine originates an action potential in sarcolemma, which spreads along the muscle fibre.
Sarcoplasmic reticulum releases calcium ions, which binds with the specific sites, troponin, present on the thin actin filament.
e. As a result of conformational changes in the tropnin, the active sites on the F-actin are exposed.These are the active sites specific to myosin head, which exhibits myosin depedent ATPase activity
f. The myosin heads act as hooks and attach to F-actin to form cross bridges.

► Digestion of food is initiated in buccal cavity.

► Food is partly broken down by the process of chewing by teeth.

► Starch digestion is minimal in the mouth, as food stays here for short duration. The chemical action of enzymes Salivary amylase converts starch into maltose, isomaltose and a- dextrins.

► Stomach is a sac-like organ that churns the food and bathes it in strong HCl.

► Gastric juice contains no carbohydrate splitting enzyme. Hence, there is no digestion of carbohydrates in the stomach.

► Food in the stomach mixed with HCl is called chyme.

Two types of juices poured into the lumen of small intestine help in carbohydrate digestion:

I. Pancreatic Juice: It contains Pancreatic amylase or Amylopsin.

► It hydrolyses partially digested starch into disaccharides.

► Optimum alkaline pH required for its action is 8.8, made available by HCO₃^- in pancreatic and bile juices.

II. Intestinal juice:

► Carbohydrases of the small intestine act under optimum alkaline pH of 8.8.
► Complete digestion of starch into simple sugars is accomplished in the small intestine.
► Glucose is the most abundant of all the end products of sugars

(ii) Plasma contains plenty of sodium chloride. It dissociates into sodium (Na⁺) and chloride (Cl^-) ions. When the negatively charged bicarbonate ions move out of RBCs (into the plasma) then the negatively charged chloride (Cl^-) ions move inside the RBCs (from the plasma) in order to maintain the balance of electrolyte. This is called chloride shift or Hamburger phenomenon. The hydrogen ions are buffered by hemoglobin inside the cell, whereas the HCO₃ ^– ions combine with sodium ions in the plasma and form sodium bicarbonate. It is transported in blood in this form. The HCO₃ ^– ions are converted back to CO₂ by the reverse chloride shift.
(iii)In the oviducts, the sperms undergo capacitation due to which the acrosome cap is lost and the enzyme at the sperm head is exposed. The sperm releases the enzyme Hyaluronidase which helps to remove follicle cells from the ovum so that the sperm can enter it. As the sperm enters the ovum, the second polar body is extruded form the secondary oocyte. After the entry of the sperm into the ovum, the sperm head becomes the male pronucleus while the chromatin of the ovum forms the female pronucleus. They fuse together to form zygote.

This theory was proposed by Dixon and Jolly in 1894. This is most accepted theory of ascent of sap in plants. It was further improved by Dixon. This theory is based upon two points:
a. Transpiration pull exerted on water column.
b. Cohesive property of water molecule so as to form unbroken column of water in the xylem.

Water is lost from mesophyll cells as a result of transpiration and moves out through stomata. Due to this loss of water DPD of mesophyll cells increases, hence they absorb water form adjoining cells and ultimately from xylem elements of vascular bundle of the leaf. Due to continuous water column in xylem elements, a tension or pull is transmitted down finally to roots causing upward movement of water. The water molecules are held together from mesophyll to root hairs by the cohesive force which helps in maintaining the water column in xylem elements. Thus according to this theory, water ascends in the plants due to transpiration pull and cohesive force of water molecules.

(ii) In mammals, placenta performs the following functions:
a. It helps in the nutrition of embryo as the nutrients like amino acids, mono-sugars, vitamins etc. diffuse from maternal blood into foetal blood through placenta.
b. It helps in respiration of the embryo as O₂ of the maternal blood and CO₂ of foetal blood diffuse through placenta.
c. It also helps in excretion of the embryo as nitrogenous wastes of foetal blood like urea diffuse into maternal blood through placenta.
d. It also acts as an endocrine gland as it secretes certain hormones like estrogen, relaxin, progesterone and Human Chorionic Gonadotropin (hCG).
e. It stores glycogen till the formation of liver.
f. It also acts as an effective barrier for certain toxic chemicals like histamine.
iii)
a) Cartilage is a type of skeletal tissue. It is a solid, semi-rigid and flexible connective tissue. It consists of cells distributed in the intercellular matrix. The matrix is a homogeneous ground substance with fibers embedded in it. Matrix of cartilage is firm and provides it characteristic consistency.
b) The cells of cambium present between primary xylem and primary phloem is called as intrafascicular cambium.
c) Hydroponics is the technique of growing plants with their roots in soil less medium.

a) Gene migration or gene flow
b) Genetic drift
c) Mutation
d) Genetic recombination and,
e) Natural Selection.

According to Hardy-Weinberg equilibrium, allele frequencies in a population are stable and constant from generation to generation. The gene pool remains a constant. Sum total of all the allelic frequencies is one.

(ii)
(a) The process of evolution of different species in a given geographical area starting from a point and laterally radiating to other areas of habitat is called adaptive radiation. Darwin’s finches (small black birds)is the best example of this.
(b) Directional selection favors those individuals who have extreme variations in a trait within a population.
(c) Single step large mutation is called saltation.

(iii)

• Migration - The migration of individual from one place to another offer an important mechanism by which new alleles are added or subtracted in a local population. Whenever an organism leaves one population and enters another, it subtracts its genetic information from the original population and adds it to the population it joins; leaving a population, the gene pool will change.
• Genetic drift – It is the variations in gene frequencies that occur within a population by chance. It is an important mechanism of evolution in small or isolated populations. Random genetic drift is unpredictable. In small populations it can lead to the extinction of the population or result in better adapted population.
• Gene flow – It is the movement of alleles within a population. The random introduction of new alleles into the recipient population and their removal from the donor population affects the allele frequency of both populations and leads to increased genetic variation.

(ii)

The basic postulates of Darwinism are -

• Enormous fertility in living beings – All living organisms have tremendous capacity to increase their own types. However, the space and food are in limited supply but the organisms increase in geometric ratio.
• Struggle for existence – Since the number of individuals produced is far more than the number that will ultimately survive, there is an everlasting competition between organisms at all levels of life.
This struggle for existence is of three types:

a. Intraspecific struggle – It is the struggle between the members of the same species since their needs are precisely similar.

b. Interspecific struggle – It is the struggle between the members of different species living together. Every living form depends upon some other organisms for its food and thus it is the struggle between the aggressor and the victim.

c. Extraspecific struggle – This is a struggle with the environment where excess of moisture, drought, heat, cold play a major role. A large number of organisms die due to these natural calamities.

• Variation – No two individuals are exactly similar. They have certain difference called variations. Variations are responsible for evolution. Variations give rise to new characters.
• Natural selection and survival of fittest – Due to struggle for existence and useful heritable variations, only those individuals survive which are best fitted to new conditions of life. The individuals which survive show high selective value and in the course of time they develop various adaptive modifications to suit the changed conditions of life. It is known as natural selection.
• Origin of species – During the course of time, the best suited and fitted individuals survived and became adjusted to the nature. As environment is ever changing, further changes occur and thus new adaptations appear in organisms which after several generations become distinct from their ancestors and forms new species.

(iii) The concept of biological species allows populations that are morphologically indistinguishable to be classified as separate species because they do not interchange genes.
A species is a group of interbreeding natural populations that are reproductively isolated from other such groups. It was put forward by Ernst Mayer in 1969. Species consists of individuals who share a common gene pool, different and separated from other distinct individuals belonging to another species and gene pool. Biological species are maintained by reproductive isolation.

The animal cell cultures are used for a diverse range of research and development. These areas are:

a) Production of antiviral vaccines that requires the standardisation of cell lines for the multiplication and assay of viruses.
b) Cancer research, which requires the study of uncontrolled cell division in cultures.
c) Cell fusion techniques
d) Genetic manipulation, which is easy to carry out in cells or organ cultures.
e) Production of monoclonal antibodies that requires cell lines in culture.
f) Production of pharmaceutical drugs using cell lines.
g) Chromosome analysis of cells derived from womb.
h) Study of the effects of toxins and pollutants using cell lines.
i) Use of artificial skin
j) Study the function of nerve cells

(ii) Stem cells are found in all organisms and are capable of dividing to form new cells, which can be differentiated into various types of specialised cells. These cells are of two main types: embryonic stem cells, which are found in the blastocyst and can develop into any type of body cell during embryonic development. The adult stem cells are present in developed organisms. It has two main features: the ability to divide and form new cells and differentiate into cells different from itself.

Uses of stem cells in medicine are:

1. These cells have been used for treatment of various cancers like leukemia and lymphoma.

2. It may also be used for treatment of severe autoimmune diseases like multiple sclerosis.

(iii)

Heterosis is the phenomenon in which the F1 hybrid of two unrelated individuals, is superior in its performance to both its parents, e.g., A cross between line A and line B of the following genotype:

Difference:

The causes of drug addiction are:
a) Inability to adjust oneself to the stress and strain, misery etc. in everyday life.
b) Encouragement by friends and peer group pressure.
c) Persecution or indifference shown by family.
d) The intake of drugs may provide excitement and adventure to the young.

(iii)

(iii)

Replacement of diseased or damaged organ by a healthy one is called organ transplant.
Before such a transplant, it must be made sure that the transplanted organ would not be rejected by the recipient. This is done by tissue matching. This will stop or slow down rejection.

(i)

(ii)

(iii)

(iv)

(v)

b)

i. Older leaves show deficiency symptoms first because they need those nutrients in greater amounts that they have not been getting for a long time.

ii. Cyanobacteria are added in the paddy fields because they are capable of fixing free N₂ into NH₃ which is readily assimilated by the plant. The ability to synthesise the nitrogen fixing enzyme, Nitrogenase, is restricted to prokaryotes like Cyanobacteria.

iii. Ventricles have to pump the blood to the entire body by force while the auricles have to pump the blood into the ventricles, lying just below them. Hence, the walls of the ventricles have to be thicker than the auricles.

iv. During an emergency situation, adrenaline is poured into the blood stream in more amounts to increase the rate of heart beat, breathing, blood pressure and glucose in the blood. It also helps to dilate the blood capillaries so that more energy is produced in the muscle to face the emergency. Hence, it is called an emergency hormone.

v. No vision occurs at the blind spot because it does not contain the sensory cells, rods and cones, to pick up the light sensation of image.

c)

(i) Animals in which the main excretory product is urea, are called ureotelic.

(ii) Fascicular cambium is the cambium of vascular bundle of dicot stem.

(iii) Efferent fibres transmit impulses from CNS to periphery.

(iv) Plant Growth Regulators or (PGRs) are also known as phytohormones

(v) Chromosomes of higher plants are made up of DNA and histone.
(vi) In man, the ribs are attached to
sternum

(d)
(i) Schwann cells: Insulate the axons.
(ii) Organ of Corti: Help in hearing by changing vibrations into nerve impulse which goes to brain from cochlea.
(iii) Phellogen: Forms a protective layer in place of epidermis of woody plants and protects the inner, soft tissues.
(iv) Sertoli cells: Provide mechanical support, protection and nutrition to the developing the spermatozoa.
(v) Neutrophils: Destroy bacteria by phagocytosis and protect the body from their infection.
(vi) Gustatory cells: Detect taste of food.

(e)
(i) Discovered blood circulation
(ii) Gave the structure of insulin
(iii) Robert Coach discovered Bacillus anthracis, which causes anthrax.
(iv) Gave cohesion and adhesion theory of transpiration pull

(f)(i) LH: Luteinising hormone

(ii) ACTH: Adreno cortico trophic hormone.

(iii)IPM: Integrated Pest Management.

(iv) IUD: Intra Uterine Device