Set-II: Past Entrance Papers

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(b) Middle lamella of the cell wall is composed of calcium and magnesium pectate. It joins the adjoining cells together and thus acts as a cementing layer.

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(c) The ability of eukaryotic cells to adopt a variety of shapes and to carry out coordinated and directed movements depends on the cytoskeleton.
The main proteins that are present in the cytoskeleton are tubulin (in the microtubules), actin, myosin, tropomyosin, and others (in the microfilaments) and keratins, vimentin, desmin, lamin, and other (in intermediate filaments).

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(c) Plasmodesmata are fine cytoplasmic strands that connect the protoplasts of adjacent plant cells by passing through their cell walls. 

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(d) Glyoxysomes are small, spherical vesicles enclosed by a single unit membrane. Glyoxysomes are generally found in cells of yeast, Neurospora, and in germinating fatty seeds where fat is being converted into carbohydrates, i.e., glyoxylate cycle (gluconeogenesis).

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(d) In a plant cell, a vacuole may be defined as a non-living reservoir, bounded by a differentially or selectively permeable membrane, the tonoplast. It is filled with a highly concentrated solution called vascular sap or cell sap which contains many dissolved solutes such as organic acids, soluble carbohydrates, and soluble nitrogenous compounds such as nitrates, enzymes, tannins, chlorides, phosphates, amino acids, alkaloids, and anthocyanin pigments.

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(a) Ribosomes are very small organelles having a diameter of \(150 Å-250 Å\). Each ribosome is made up of two subunits, a smaller subunit, and another larger subunit. These two subunits are associated with the help of \(\mathrm{Mg}^{2+}\) ions (at \(0.001 \mathrm{M}\) concentration). If the \(\mathrm{Mg}^{2+}\) ions concentration is less in the cytoplasm, the two units of ribosome separate but when the \(\mathrm{Mg}^{2+}\) ions concentration is increased ten times, the two unites and form a dimer.

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(d) According to the fluid mosaic model there is rapid internal motion involving flexing within each lipid molecule a rapid lateral diffusion of the lipids is possible and a slow ‘flip-flop’ motion, i.e., a transfer of lipid molecules from one side of the bilayer to the other, is also possible. The lipid molecules might also rotate about their axes. The proteins of the membrane are concerned with the enzymatic activity of the membrane, with the transport of molecules, and with a receptor function whereas, the lipid bilayer provides the permeability barrier.

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(b) Chemically a biomembrane consists of lipids \((20-70 \%)\), proteins \((20-70 \%)\), carbohydrates (1-5%) and water \((20 \%)\). The important lipids of the membrane are phospholipids (some hundred types), sterols, (e.g. cholesterol), glycolipids, sphingolipid (e.g., sphingomyelin, cerebrosides). Protein can be fibrous or globular structural carrier, receptor or enzymatic.

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(a) Both chloroplasts and mitochondria contain DNA and are double membrane-bound organelles having an inner membrane and outer membrane. Mitochondria occur in cytoplasm of both plants and animal cells. A mitochondria contains two chambers. The inner membrane forms mitochondrial cristae. Chloroplast is also a double membranous organelle but found only in plants. The membrane-bound matrix of chloroplasts is stroma and inside the stroma, thylakoids are present which form grana.

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(d) Molecular assemblies are large organised sets of molecular units that make up parts of organelles. For example, one common macromolecular assembly is the microtubule which is important in forming structure in the cell related to maintaining the cell structure or related to cell movement. The cell (plasma) membrane that surrounds many organelles and the cell is a highly organised molecular assembly.

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(c) The outer membrane of mitochondrion is smooth, freely permeable to most small molecules, contains fewer enzymes, and is poor in proteins. It has porin proteins which form channels for the passage of molecules through it. It allows uptake of substrates and release of ATP. The inner membrane is semipermeable and regulates the passage of materials into and out of the mitochondrion. It is rich in enzymes and carrier proteins (permeases). It is usually produced into numerous infolds called cristae (singular crista). It bears minute regularly spaced lollipop-shaped particles known as oxysomes. The rest of the inner membrane contains the electron carrier molecules of the electron transport chain.

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(b) Cilia are fine hair-like vibratile, cytoplasmic processes borne by certain cell types. Their movement either propels the organism or moves the medium past a fixed cell. The cilia are enclosed by a unit membrane which is an extension of the plasma membrane of the cell. Within the membrane, is a fluid matrix having a supporting axial shaft, or axoneme.

The microtubules, single as well as double, are composed of the globular units of the protein tubulin. The arms of A microtubules contain a protein dynein. The latter is ATPase enzyme which catalyzes hydrolysis of ATP to ADP and transfers the free energy released directly to ciliary work.

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(b) Microscopes are used for studying cellular structures. They are used to magnify small objects. In electron microscopes, a high-energy beam of electrons is focused through electromagnetic lenses. It can magnify very small details with high resolving power. The increased resolution results from the shorter wavelength of the electron beam.

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(a) A chloroplast is a vesicle bounded by an envelope of two unit membranes and filled with a fluid; matrix called the stroma. The lamellae, after separation from the inner membrane, usually take the form of closed, flattened, ovoid sacs, the thylakoids, which lie closely packed in piles, the grana. The thylakoid membrane contains photosynthetic pigments namely, chlorophyll a, chlorophyll b, carotenoids (carotene, xanthophylls), and plastoquinone. The thylakoid membranes also contain coupling factors that bring about ATP synthesis.

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(a) Resolution of the microscope is inversely proportional to the wavelength of light used. Out of the four options given, blue light has a minimum wavelength and hence maximum resolution.

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(a) An improved model of the structure of cell membrane was proposed by Singer and Nicolson (1972) widely accepted as fluid mosaic model. According to this, the quasi-fluid nature of lipid enables lateral movement (Flip-flop) of proteins within the overall bilayer. This ability to move within the membrane is measured as its fluidity.

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(a) Centromere is the point at which the two chromatids of a chromosome are held together. During the movement of chromosomes, the spindle fibres (on which the chromatids move) are attached to the centromere. Crossing over involves physical exchange of genetic material between non-sister chromatids of homologous chromosomes.

Cytoplasmic cleavage is the division of cytoplasm. Transcription is the process in which the genetic information of DNA is transferred to mRNA as the first step in protein synthesis.

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(d) Chemiosmotic coupling hypothesis is the most widely accepted explanation for oxidative phosphorylation in mitochondria and photophosphorylation in thylakoid membranes. Mitchell proposed the idea of chemiosmotic coupling. He suggested that a concentration gradient of protons is established across the mitochondrial membrane because there is an accumulation of hydrogen ions on one side of the mitochondrial membrane. The proton accumulation is necessary for energy transfer to the endergonic ADP phosphorylation process.

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(d) The proteins formed on ribosomes pass into the ER lumen where they are modified. Then the modified proteins move on into the transitional area, where the ER buds off membranous sacs, the transport vesicles, carrying the proteins to the Golgi apparatus.

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(d) Telomeres are highly conserved elements throughout the eukaryotes both in structure and function. Telomeric DNA has been shown to consist of simple randomly repeated sequences, characterised by clusters of G residues in one strand and C residues in another strand. A short sequence of (12-16 nucleotides in length) of G rich strand as \(3^{\prime}\) overhang is another feature of the telomere.

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(c) A chloroplast is a vesicle bounded by an envelope of two unit membranes and filled with a fluid; matrix called the stroma. The lamellae, after separation from the inner membrane, usually take the form of closed, flattened, ovoid sacs, the thylakoids, which lie closely packed in piles, the grana. The thylakoid membrane contains photosynthetic pigments namely, chlorophyll a, chlorophyll b, carotenoids (carotene, xanthophylls), and plastoquinone. The thylakoid membranes also contain coupling factors that bring about ATP synthesis.

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(b) Totipotency is the ability of a living somatic plant cell to develop into a complete plant. It was first demonstrated by Steward et. al (1964) using phloem cells of carrot. This technique is now used for multiplying rare and endangered plants through micro-propagations. This technique is widely used for multiplying plants e.g., Chrysanthemum, Dioscorea floribunda, Coleus, Crotons, carnation plants, etc.

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(a) Nucleolus synthesizes and stores RNA. The ribosomal proteins are synthesized in the cytoplasm and shift to the nucleolus for the formation of ribosomal subunits by complexing with rRNA.

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(c) Chemically a biomembrane consists of lipids (20-70%), proteins (20-70%), carbohydrates (1-5%), and water (20%). The important lipids of the membrane are phospholipids (some hundred types), sterols, (e.g. cholesterol), glycolipids, sphingolipid (e.g., sphingomyelin, cerebrosides). Protein can be fibrous or globular structural carrier, receptor, or enzymatic. The lipid molecules are amphipathic, that is, they possess both polar hydrophilic (water-loving) and non-polar hydrophobic (water-repelling) ends. The hydrophilic region is in the form of the head while the hydrophobic part contains two tails of fatty acids. It results in the formation of a lipid bilayer. The most common lipid of the bilayer is phospholipids. Protein molecules also possess both polar and non-polar side chains. Usually, their poly hydrophilic linkages are towards the outer side. The non-polar or hydrophobic linkage is either kept folded inside or used to establish connections with hydrophobic parts of the lipids.

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(a) Middle lamella is the first-formed layer, present between the two adjacent cells. It is situated outside the primary cell wall. It is made up of calcium and magnesium pectate.

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(a) Microtubule is a microscopic tubular structure, with an external diameter of 24 nm and of variable length, found in a wide range of eukaryotic cells. Microtubules are composed of numerous subunits of the globular protein tubulin and occur singly or in pairs, triplets, or bundles. Microtubules help cells to maintain their shape. They also occur in cilia and eukaryotic flagella and the centrioles and form the spindle during nuclear division. A further role is in the intracellular transport of materials and the movement of organelles.

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(a) Phytochrome is a plant pigment that can detect the presence or absence of light and is involved in regulating many processes that are linked to day length (photoperiod), such as seed germination and initiation of flowering. It consists of a light-detecting portion, called a chromophore, linked to a small protein and exists in two interconvertible forms with different physical properties, particularly in the ability to bind to membranes.

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(b) A lysosome is a tiny sac bounded by a single unit membrane of lipoprotein. It contains a dense, finely granular fluid. The latter consists of glycoprotein hydrolytic (digestive) enzymes called acid hydrolases. These include proteases, lipases, nucleases, glycosidases, sulphatases, acid phosphatases, etc.

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(a) Ribosomes present in the nuclear membrane and endoplasmic reticulum take part in protein synthesis. Two or more ribosomes simultaneously engaged in protein synthesis on the same mRNA strand forming polyribosomes. The ribosome functions as a template, bringing together different components required for protein synthesis.

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(c) Telomeres are controlled by the presence of the enzyme telomerase. A telomere is a repeating DNA sequence (for example, TTAGGG) at the end of the chromosomes. Telomeres function by preventing chromosomes from losing base pair sequences at their ends. They also stop chromosomes from fusing with each other.

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(c) Glycolysation of protein means the linking of sugars to proteins which starts in the rough endoplasmic reticulum and is completed in the Golgi complex.

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(b) The spherosomes are, spherical bodies, about \(0.5-1 \mu \mathrm{m}\) wide and enclosed by a single unit membrane. They contain granular contents rich in lipids but also have some proteins. They occur in most plant cells but are abundant in the endosperm cells of oil seeds. Spherosomes, arise from the endoplasmic reticulum.

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(c) The ribosomes provide space for the synthesis of proteins in the cell. Hence, they are known as the “protein factories” of the cell. The ribosomes bound to the membranes generally synthesize proteins for export as secretions by exocytosis or for incorporation into membranes, or for inclusion into lysosomes.

The free ribosomes generally produce enzymic proteins for use in the cell itself.

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(a) The centrioles occur in nearly all animal cells and in motile plant cells, such as zoospores of algae, sperm cells of ferns, and motile algae. They are absent in amoebae, prokaryotic cells, higher gymnosperms, and all angiosperms. An interphase (undivided) cell has a pair of centrioles (diplosome) usually near the nucleus. They lie in a small mass of specialized, distinctly staining cytoplasm that lacks other cell organelles. The centrioles and the centrosphere are together referred to as centrosomes. Before cell division, the centrioles duplicate so that a dividing cell has a pair of centrioles at each pole of the spindle. Spindle fibre, cell plate, and centromere are present in all plant cells.

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(a) Chemiosmosis theory relates to the generation of ATP by the movement of hydrogen ions across the membrane of the thylakoid. The accumulation of positively-charged ions inside the lumen creates a proton gradient which is then used to drive hydrogen ions out of the ATP synthase complex to phosphorylate ADP, making ATP.

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(d) Protein synthesis in an animal cell, takes place in the nucleolus as well as in the cytoplasm. The main part of protein synthesis (transcription and translation) occurs in the nucleolus. Chain elongation occurs in the cytoplasm.

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(b) A lysosome is a tiny sac bounded by a single unit membrane of lipoprotein. It contains a dense, finely granular fluid. The latter consists of glycoprotein hydrolytic (digestive) enzymes called acid hydrolases. These include proteases, lipases, nucleases, glycosidases, sulphatases, acid phosphatases, etc. However, all the enzymes do not occur in the same lysosome. There are different sets of enzymes in different lysosomes. The lysosome enzymes can break down all major biological macromolecules present in the cells or enter the cells from outside into their building block subunits by the addition of water. The lysosome enzymes are active in an acid medium, at about pH5, hence their name.

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(c) Colchicine is an alkaloid derived from the autumn crocus, Colchicum autumnale. It inhibits spindle formation in cells during mitosis so that chromosomes cannot separate during anaphase, thus inducing multiple sets of chromosomes. Colchicine is used in genetics, cytology, and plant breeding research and also in cancer therapy to inhibit cell division.

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(a) Prokaryotic flagellum is not surrounded by any membrane. It consists of a single thread. The thread is made of numerous identical spherical protein sub-units called, flagellin. Each subunit is about 40 \(Å\) in diameter. The flagellin subunits are arranged in helical spirals and form a hollow cylinder. Each flagellum is about \(120-150 Å\) thick.

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(b) In desmosomes circular patches of cell membranes are held together by the interaction of proteins that extend through each membrane into the space between cells. The cell membrane has on the inner side a dense plate of protein for mechanical support and bears fine filaments, the tonofibrils, radiating into the cell. The desmosomes act as “spot welds” and keep the cells firmly together.

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(d) A lysosome is a tiny sac bounded by a single unit membrane of lipoprotein. It contains a dense, finely granular fluid. The latter consists of glycoprotein hydrolytic (digestive) enzymes called acid hydrolases. These include proteases, lipases, nucleases, glycosidases, sulphatases, acid phosphatases, etc. However, all the enzymes do not occur in the same lysosome. There are different sets of enzymes in different lysosomes. The lysosome enzymes can break down all major biological macromolecules present in the cells or enter the cells from outside into their building block subunits by the addition of water. The lysosome enzymes are active in an acid medium, at about pH5, hence their name.

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(d) Respiratory chain is located in the inner membrane (cristae) of mitochondria. It consists of a series of proteins containing oxidation-reduction groups. Chemical treatment of the mitochondrial membrane results in the isolation of five complexes which have been designated as complexes I, II, III, IV, and \(\mathrm{V}\).

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(c) Centromere is present in every cell. It is a specific DNA sequence in a chromosome. It is specifically useful in the process of cell division. During mitosis, the spindle fibres attach to the centromeres via kinetochore, and all the chromosomes are arranged in an equatorial plane. After this, the chromosomes move towards their respective poles and cell division takes place.

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(d) Golgi apparatus is a stack of parallel, flattened, intercommunicating sacs or cisternae and many peripheral tubules and vesicles. The cisternae vary in number from 3 to 7 in most animal cells and from 10 to 20 in plant cells. They are usually equally spaced in the pile, separated from each other by thin layers of intercisternal cytoplasm. The latter may contain a layer of parallel fibres, called intercisternal elements, that support the cisternae. The cisternae are free of ribosomes and have swollen ends. They look like the smooth endoplasmic reticulum.

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(c) The inner membrane of the mitochondria is infolded variously to form involutions called cristae. The cristae extend inward to varying degrees and may fuse with those from the opposite side, dividing the mitochondrion into compartments. The cristae are arranged in characteristic ways in different cells. They may be simple or branched, straight or zigzag, lamellar or tubular. The cristae have in them a narrow intracristal space. It is continuous with the intermembrane space. They greatly increase the inner surface area of the mitochondria to hold a variety of enzymes.

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(a) The two poles of a Golgi apparatus are called cis face and trans face, which act respectively as the receiving and shipping departments.

The vesicles lie near the ends and the concave surface of the Golgi complex. They are pinched off from the tubules of the cisternae. They are of two types: smooth or secretory vesicles, which have a smooth surface and contain secretions of the cell; and coated vesicles, which have a rough surface and elaborate membrane proteins. They carry materials to or from the cisternae. The Golgi complex gives rise to primary lysosomes by budding from the trans face of cisternae.

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(b) Active and passive transports across cell membrane differ in passive transport is due to metabolic energy. There is always expenditure of energy in active transport.

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(b) Balbiani rings (puffs) are site of RNA and protein synthesis. These chromosomes show distinct dark and light bands. Euchromatin is present in dark bands and heterochromatin is present in light bands. These bands help in the mapping of chromosomes in cytogenetic studies. These chromosomes form puffs or loops (in the region of dark bands) which are called Balbiani puffs or Balbiani rings where synthesis of mRNA occurs.

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(a) In plant cells, peroxisomes are associated with photorespiration. Peroxisomes are found only in \(\mathrm{C}_{3}\) plants where a wasteful phenomenon of photorespiration occurs. The other organelles associated with this process are chloroplast and mitochondria. Peroxisomes are also reported in animal cells, i.e., liver and kidney cells. The peroxisome contains several enzymes such as amino transferse, glycolate oxidase, glyoxylate reductase, peroxidase, catalase, etc.

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(c) Lysosomes are membrane-bounded organelles that contain 40 different types of hydrolytic enzymes that help in digestion. It acts as the “garbage disposal” of the cell. Lysosomes digest foreign macromolecules and engulfed viruses and bacteria presenting in phagosomes. Lysosomes also break down cell components that are no longer needed.

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(c) Golgi apparatus is absent in bacteria and blue-green algae. Golgi bodies are absent in prokaryotic cells and present in eukaryotic cells except in male gametes of bryophytes and pteridophytes, mammalian RBCs, sieve tubes of plants, and in cells of fungi.

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(d) Cell recognition and adhesion occur due to biochemicals of cell membranes named glycoproteins and glycolipids. These are formed due to small carbohydrate molecules present on lipids and extrinsic proteins.

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(d) In salivary gland chromosomes/polytene chromosomes, pairing is formed between homologous chromosomes. A characteristic feature of these chromosomes is that somatic pairing occurs in them and hence their number appears half of the normal somatic cells.

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(c) Viruses are an exceptions to the cell theory as they are obligate parasites (subcellular in nature). Paramecium, Rhizopus, and Vaucheria are some examples that may or may not be exceptions to the theory.

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(b) Names of Schleiden and Schwann are associated with cell theory in 1839. The concept that “All living organisms are composed of cell” is known as cell theory.

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(d) X-rays crystallography is a technique that is used to study of binding protein on regulatory DNA sequence. In this technique, X-rays pass through a crystal of a substance and form a diffraction pattern. With the help of this technique, the arrangement of atoms in the molecular structure of enzymes, proteins, DNA, etc. can be studied. Wilkins, Watson, and Crick used this technique to determine the molecular configuration of the double helix of DNA.

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(d) All the plastids have a common origin and one type of plastid can change into another. Plastids are of 2 main types leucoplasts and chromoplasts. The leucoplasts are colourless and occur in the cells not exposed to sunlight. The chromoplasts are coloured and occur in the cells exposed to sunlight.

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(b) Acetabularia used in Hammerling’s nucleocytoplasmic experiment is unicellular uninucleate green algae. Hammerling’s experiment on Acetabularia involved exchanging rhizoid and stalk. The presence of hereditary information in the nucleous was proved by the work of Hammerling on single-celled alga Acetabularia.

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(a) Apparato reticolare are Golgi apparatus. The Golgi apparatus was named after Camillo Golgi (1906) who discovered this cell structure in nerve cells (1898) and assigned it the role of a cell organelle. Inspired by its appearance, Golgi named this cell structure the inner reticular apparatus (apparato reticolare interno).

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(a) An outer covering membrane is absent over the nucleolus. The nucleolus is a dense, spherical, colloidal body that remains attached to nucleolar organizing chromosomes. It was discovered by Fontana (1781) and termed as nucleolus by Bowman (1840). The main function of nucleolus is the synthesis of ribosomal RNA and it is called the storehouse of RNA. It plays important role in cell division also.

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(b) All the plastids have a common origin and one type of plastid can change into another. Plastids are of 2 main types leucoplasts and chromoplasts. The leucoplasts are colourless and occur in the cells not exposed to sunlight. The chromoplasts are coloured occur in the cells exposed to sunlight.

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(c) Oxysomes or \(F_{0}-F_{1}\) particles occurs on the inner mitochondrial membrane. Each particle is made up of base, stalk and head and is about \(10 \mathrm{~mm}\) in length. The number of oxysomes per mitochondrion varies from \(10^{4}-10^{5}\). Chemically these are made up of phospholipid core and protein cortex. Oxysomes have ATP as enzyme molecules and therefore, are responsible for ATP synthesis. These elementary particles are also called \(\mathrm{F}_{0}-\mathrm{F}_{1}\) particles by some workers. The base of these is called \(\mathrm{F}_{0}\) subunit and the head is called \(F_{1}\) subunit.

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(c) Ribosomes are the centre for protein synthesis. Free ribosomes synthesize structural proteins, proteins normally found in microbodies, chloroplast, and mitochondria, and proteins destained to remain in cytosol such as enzymes of glycolysis or proteins of cytoskeleton. On the other hand, ribosomes attached to ER synthesize proteins for transport, integral or intrinsic proteins of the membrane, and proteins of certain organelles like Golgi complex, lysosomes, and plant vacuoles.

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(c) An angstrom (symbol Å ) is a non-SI unit of length that is internationally recognised, equal to \(0.1\) nanometer \((\mathrm{nm})\). It can be written in scientific notations as \(1 \times 10^{-10} \mathrm{~m}\). It is used in expressing the size of atoms, length of chemical bonds etc. It is named after Anders Jonas Angstrom.
Angstrom \(=0.0001 \mathrm{~mm}\).

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(b) Electron microscope has a high-resolution power. This is due to the very low wavelength of the electron beam ( \(0.005 \mathrm{mu}\) ) of the electron beam used as a light source. It is produced when a metal filament is heated in a vacuum tube at a high temperature, i.e., 50,000 volts.

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(c) Addition of new cell wall particles amongst the existing ones is intussusception. Growth of cell wall occurs by two methods – by intussusception and by apposition. By intussusception, the primary wall is stretched and materials of the secondary wall are deposited. By apposition, materials of the secondary wall are deposited in the form of thin layers.

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(a) Cell wall shows complete permeability because it helps in the transport of substances into and out of the cell. The main function of cell wall is to provide mechanical support.

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(d) Ribosomes are minute cellular, nonmembranous particles having an average diameter of \(23 \mathrm{~mm}\left(230 \mathrm{~A}^{\circ}\right)\). In-plant cells ribosomes were first of all observed by Robinson and Brown (1953) in bean roots. In animal cells, these were discovered by Palade (1955) and hence are called Palade particles. Ribosomes form a part of fraction microsome, a term given by A. Claude (made of broken ER, ribosomes, and Golgi bodies).

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(b) According to the fluid mosaic model of cell membrane structure proposed by Singer and Nicolson (1972) plasma membrane contains about \(50-60 \%\) proteins and \(50^{\wedge} 0 \%\) lipids. Lipids form a bilayer with hydrophilic heads pointing outwards. The cell membrane allows the transport of some molecules by passive transport e.g., water, and neutral solutes while some are transported actively e.g., \(\mathrm{Na}^{+} / \mathrm{K}^{+}\)pump.

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(b) Resolution power is the ability to distinguish two close objects. It is the ability of a system to distinguish two close points as two separate points. The resolving power of the microscope depends upon the numerical aperature of the objective lens system and its wavelength.

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(c) Acetabularia used in Hammerling’s nucleocytoplasmic experiment is unicellular uninucleate green algae. Hammerling’s experiment of Acetabularia involved exchanging rhizoid and stalk. Presence of hereditary information in the nucleous was proved by the work of Hammerling on single-celled alga Acetabularia.

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(c) The latest model of the plasma membrane is Fluid Mosaic Model. It was discovered by Singer and Nicolson.

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(a) Magnification of compound microscope is not connected with numerical aperature. The magnifying power is represented by the symbol ‘ \(X\) ‘. The total magnification of a microscope is obtained by multiplying the objective magnification and ocular lens magnification \(e. g\)., if the magnifying power of an ocular lens is \(10 \mathrm{X}\) and of the objective is \(40 \mathrm{X}\), then the total magnifying power of a microscope is \(10 \times 40=400 \mathrm{X}\).

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(c) A bivalent consist of four chromatids and two centromeres. Bivalent is a pair of homologous chromosomes lying together in the zygotene stage of prophase I of first meiotic division.

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(d) Nucleoproteins are compounds present in cells of living organisms that consist of nucleic acids with proteins. Nucleoproteins are synthesized in cytoplasm. These are conjugated proteins. They are of two types -Deoxyribonucleoproteins and ribonucleoproteins.

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(d) Polyribosomes are aggregates of several ribosomes held together by a string of \(m\) RNA. Polyribosomes consist of \(4-8\) ribosomes which are attached to a single strand of messenger RNA or \(m\) RNA. This mechanism helps in the synthesis of several copies of the same protein.

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(c) Plasma membrane consists of lipids (20-79%), proteins (20-70%), carbohydrates (1-5%), and water (20%). Lipid molecules possess both hydrophobic and hydrophilic ends and are thus arranged in the form of lipid bilayer. The most common lipid of the bilayer is a phospholipid. Protein molecules occur at places both inside (intrinsic proteins) and on the outer side (extrinsic proteins) of the phospholipid bilayer.

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(c) Organelles can be separated from cell homogenate through differential centrifugation. The basic principle involved here is the sedimentation of particles in a suspension by centrifugal force. In a centrifuge, the particles sediment at different rates when an accelerating force is subjected. The rate of sedimentation depends upon the size of the particles, their shape, and their density.

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(c) Acetabularia used in Hammerling’s nucleocytoplasmic experiment is unicellular uninucleate green algae. Hammerling’s experiment of Acetabularia involved exchanging rhizoid and stalk. The presence of hereditary information in the nucleous was proved by the work of Hammerling on single-celled alga Acetabularia.

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(d) Plasma membrane consists of lipids ( 20-79 %), proteins ( 20-70 %), carbohydrates (1-5%) and water (20%). Lipid molecules possess both hydrophobic and hydrophilic ends and are thus arranged in the form of lipid bilayer. The most common lipid of the bilayer is phospholipid. Protein molecules occur at places both inside (intrinsic proteins) and on the outer side (extrinsic proteins) of the phospholipid bilayer.

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\(
\text { (b) Genome }>\text { chromosomes }>\text { gene }>\text { nucleotide. }
\)

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(c) Cell wall is absent in a protoplast.
Protoplast is a biological term coined by Hanstein in 1880 to refer to the entire cell, excluding the cell wall. Protoplasts can be generated by stripping the cell wall from the plant, bacterial, or fungal cells by mechanical, chemical, or enzymatic means. All the content of the cell along with the nucleus is known as protoplasm (i.e. cytoplasm + nucleus = protoplasm). However, a plant cell without cell wall is known as protoplast (i.e. cell membrane + cytoplasm + nucleus = protoplast).

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(b) Mycoplasma lacks cell wall. 

Mycoplasma’s lack of cell wall characteristics makes them naturally resistant to antibiotics that target cell wall synthesis. They can be parasitic or saprotrophic. Mycoplasma usually colonizes the throat, bacterial biofilm, or tartar found in the oral cavity. Mycoplasma pneumonia is one of the common causes of acute respiratory infections.

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(d) In some bacteria (e.g., Bacillus subtilis) the plasma membrane forms certain invaginations or infoldings called mesosomes in the cytoplasm. The mesosomes have various functions, viz., respiratory, secretory, etc.

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(d) Capsule is a layer that lies outside the cell wall of bacteria. The capsule can protect cells from engulfment by eukaryotic cells, such as macrophages. They also exclude bacterial viruses and most hydrophobic toxic materials such as detergents.

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(d) Nostoc is a prokaryote while the rest are eukaryotes. Nuclear membrane is absent in prokaryotes.

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(a) The function of the gap junction is to facilitate communication between adjoining cells by connecting the cytoplasm for rapid transfer of ions, small molecules, and some large molecules.

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(b) Ribosomal RNA is actively synthesized in the nucleolus. The nucleolus is also known as ribosomal factory.

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(c) Under an electron microscope the nucleosomes appear as beads on a string in the chromosome, due to a short length of DNA wrapped around a core of histone proteins.
The nucleosome is the fundamental subunit of chromatin. Each nucleosome is composed of a little less than two turns of DNA wrapped around a set of eight proteins called histones, which are known as histone octamer. Each histone octamer is composed of two copies of each of the histone proteins H2A, H2B, H3, and \(\mathrm{H} 4\).

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(b) The inner membrane of mitochondria contain more than 151 different polypeptides and has a very high protein to phospholipid ratio.

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(c) Prokaryotes have simple flagella without microtubules whereas eukaryotes have complex cilia and flagella which consist of microtubules arranged in \(9+2\) fashion (an outer ring of nine pairs surrounding 1 center pair).

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(a) A chromatid is one of two identical strands of DNA making up a chromosome that are joined at their centromeres, for the process of nuclear division (mitosis or meiosis). The term is used till the centromeres remain in contact. When they separate (during anaphase of mitosis and anaphase 2 of meiosis), the strands are called daughter-chromosomes. The tips of the chromatid are called telomeres. They prevent the ends of the chromosome from attaching to other chromosomes.

It has been said that after repeated cell replication, the telomeres get shorter resulting in cell death. Thus, this means that the way telomeres work could play a role in determining the lifespan of a cell.

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(c) Mitotic spindles are composed of microtubules that are made of tubulin dimers. Actin and myosin are the contractile proteins of muscle fibres. Myoglobin is an oxygen-carrying pigment found in muscle.

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(a) Lysosomes are single membrane-bound vesicular organelles rich in acid hydrolases. They are involved in phagocytosis.

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(d) Golgi bodies are cytoplasmic organelle which take part in the elaboration and secretion of complex biochemicals. Microsomes are small single membraned cell organelles that absorb oxygen and perform direct oxidation of substrates. Glyoxisomes contain enzymes for \(\beta\)-oxidation of fatty acids.

Hint

(a) Genes located on the mitochondrial DNA are inherited through the cytoplasm. The zygote receives cytoplasm from the female gamete, hence it involves maternal inheritance.
Maternal Inheritance is caused by the genes in mitochondrial DNA. The egg passes on a lot of mitochondria, whereas the sperm passes a few or zero. Maternal effects result because of the maternal parent produces the egg and further, the genes control production of eggs.

Hint

(c) Rough endoplasmic reticulum bears ribosomes on its outer surface. These ribosomes take part in protein synthesis.

Hint

(d) Granules of condensed chromatin (heterochromatin), are found scattered in interphase nuclei. At chromocentre, the polytene chromosomes appear attached.

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(a) Organelle that has flattened membrane-bound cisternae and lies near the nucleus is Golgi apparatus.