NCERT Exemplar Class 11 Biology Solutions Cell The Unit of Life
Multiple Choice Questions
1.A common characteristic feature of plant sieve tube cells and most of mammalian erythrocytes is
(a) absence of mitochondria
(b) presence of cell wall
(c) presence of haemoglobin
(d) absence of nucleus.
Soln.(d) : Both the mammalian erythrocytes and sieve tube cells of plants lack the nucleus. Although nucleus is present during early stages of their development but later on they become enucleated.
2.Select one which is not true for ribosomes.
(a) Made of two sub units
(b) Form polysome
(c) May attach to mRNA
(d) Have no role in protein synthesis
Soln.(d) Ribosomes are cell organelles that are formed of two subunits. Cytosolic eukaryotic ribosomes (80S) are made up of 60S and 40S subunits, while prokaryotic ribosomes (70S) are made up of 50S and 30S subunits. Many ribosomes bound to a strand of mRNA and forms polyribosome. Ribosomes are also called protein factories, as they are the sites of protein synthesis.
3.Which one of these is not a eukaryote?
(a) Euglena (b) Anabaena
(c) Spirogyra (d) Agaricus
Soln.(b) : Anabaena is a cyanobacterium which belongs to Kingdom Monera. Monerans are prokaryotic organisms.
4.Which of the following stains is not used for staining chromosomes?
(a) Basic Fuchsin (b) Safranin
(c) Methylene green (d) Carmine
Soln.(b)
5.Different cells have different sizes. Arrange the following cells in an ascending order of their size and select the correct option.
(i) Mycoplasma(ii) Ostrich eggs
(iii) Human RBCs(iv) Bacteria
(a)(i) -> (iv) -> (iii) -> (ii)
(b)(i) -> (ii i-> (iv) -> (ii)
(c)(ii) -> (i) -> (iii) -> (iv)
(d)( iii)->(ii) ->(i) -> (iv)
Soln.(a): The smallest cell is Mycoplasma (0.1 – 0.5 pm). Bacterial cell measures 3-5 pm in length. Human RBCs are 6-8 pm in diameter. The largest cell is the egg of an ostrich.
6.Which of the following features is common to prokaryotes and many eukaryotes?
(a) Chromatin material present
(b) Cell wall present
(c) Nuclear membrane present
(d) Membrane-bound subcellular organelles present
Soln.(b): Cell wall is found in bacteria (prokaryotes), fungi, algae and plants (eukaryotes). Bacterial cell wall is made up of peptidoglycan (murein). Algal cell wall is made up of glycoproteins and polysaccharides. The cell wall of fungi is composed of chitin. Plant cell wall consists of
a variety of polysaccharides, lipids, proteins, etc. Prokaryotes lack nuclear membrane, and membrane bound subcellular organelles and their genetic material is naked, called nucleoid.
7.Who proposed the fluid mosaic model of plasma membrane?
(a) Camillo Golgi
(b) Schleiden and Schwann
(c) Singer and Nicolson
(d) Robert Brown
Soln.(c): Fluid mosaic model of plasma membrane was proposed by Singer and Nicolson in 1972. According to this model, the membrane does not have a uniform position of lipids and proteins but is instead a mosaic of the two. Further, the membrane is not solid but is quasifluid. This nature of biomembrane is responsible for properties like repair, dynamic nature, ability to fuse, endocytosis etc. Fluid-mosaic model postulates that the lipid molecules are present in a viscous bilayer as in lamellar model. Protein molecules occur at places both inside and on the outer side of lipid bilayer i.e., protein icebergs in a sea of lipids. The internal proteins are called intrinsic or integral proteins while the external ones are known as extrinsic or peripheral proteins. Some of the integral proteins run throughout the lipid bilayer. They are called tunnel proteins or transmembrane proteins. Glycolipids as well as glycoproteins are found on the external face of lipid bilayer membrane.
8.Which of the following options is true for a secretory cell?
(a) Golgi apparatus is absent.
(b) RER is easily observed in the cell.
(c) Only SER is present.
(d) Secretory granules are formed in nucleus.
Soln.(b): Rough endoplasmic reticulum (RER) has ribosomes attached on its cytoplasmic surface which makes the surface look rough or granular. Ribosomes present on surface of ER are engaged in synthesis of proteins and enzymes. The RER often occurs deep in the cytoplasm. It is particularly highly developed in the cells that synthesise and secrete proteins, these include liver cells, pancreatic cells, goblet cells, plasma cells, fibroblasts,etc.
9.What is a tonoplast?
(a) Outer membrane of mitochondria .
(b) Inner membrane of chloroplast
(c) Membrane boundary of the vacuole of plant cells
(d) Cell membrane of a plant cell
Soln.(c): Tonoplast is a single membrane that bounds the vacuoles and separates it from cytoplasm. In plants, tonoplast facilitates uphill transport of many ions and other materials, making their concentration higher in vacuole than cytoplasm.
10.Which of the following is not true for a
eukaryotic cell?
(a) Cell wall is made up of peptidoglycans.
(b) It has 80S type of ribosome present in the cytoplasm.
(c) Mitochondria contain circular DNA.
(d) Membrane bound organelles are present.
Soln.(a): A eukaryotic cell has 80S ribosomes present in the cytoplasm. Membrane-bound organelles like, mitochondria, ER, Golgi apparatus, chloroplasts, etc. are present inside the cell. Bacterial cell wall is made up of peptidoglycans. Cell wall in eukaryotes is composed of polysaccharides, chi tin, cellulose, glycoprotein etc. Mitochondria are semi- autonomous organelles, which have their own ribosomes (70 S) and DNA (circular).
11.Which of the following statements is not true
for the cell membrane?
(a) It is present in both plant and animal cells.
(b) Lipids are present in it as bilayer.
(c) Proteins may be peripheral or integral in it.
(d) Carbohydrates are never found in it.
Soln.(d): Chemically a biomembrane consists of lipids (20-79%), protein (20-70%), carbohydrates (1-5%) and water (20%). The ratio of protein and lipid varies in different membranes. Carbohydrates present in the membrane are branched or unbranched oligosaccharides, e.g., hexose, fucose, hexoamine, sialic acid, etc. According to most accepted fluid mosaic model of cell membrane lipids are present in bilayer and protein molecules occur both inside and on outer side of lipid bilayer as protein icebergs in a sea of lipids.
12.Plastids differ from mitochondria on the basis of which of the following features?
(a) Presence of two layers of membrane
(b) Presence of ribosome
(c) Presence of thyla koids
(d) Presence of DNA
Soln.(c): Thylakoids are structural elements of chloroplast. They are membrane lined flattened sacs present in stroma or matrix of chloroplast (plastid). Thylakoid membranes possess photosynthetic pigments and thus they take part in photosynthesis.
13.Which of the following is not a function of cytoskeleton in a cell?
(a) Intracellular transport
(b) Maintenance of cell shape and structure
(c) Support of the organelles
(d) Cell motility
Soln. (None of these): Cytoskeletal structures; microtubules, intermediate filaments and1 microfilaments occur only in eukaryotic cells. They are minute, fibrous and tubular structures which maintain cell shape,support the organelles, help in intracellular transport and movement of cells.
14.The stain used to visualise mitochondria is (a) fast green (b) safranin
(c) acetocarmine (d) janus green.
Soln.(d) : Janus green is a basic dye and vital stain which is used to visualise mitochondria.
Very Short Answer Type Questions
1.What is the significance of vacuole in a plant cell?
Soln. Vacuoles store substances like water, sap, excretory product and also contain pigments and toxic molecules. Tonoplast of vacuole facilitates the transport of materials and some ions against the concentration gradient inside the vacuole.
2.What does ‘S’ refer to in a 70S and an 80S ribosome?
Soln.‘S(Svedberg’s unit) in 70S and 80S ribosomes stands for sedimentation coefficient (measure of size and density).
3.Mention a single membrane bound organelle which is rich in hydrolytic enzymes.
Soln.Lysosomes are rich in several hydrolytic digestive enzymes (hydrolases-lipases, proteases, carbohydrases, etc.).
4.What are gas vacuoles? State their functions.
Soln.Gas vacuole is an air vacuole. They are found only in prokaryotes. It consists of a number of smaller submicroscopic vesicles. Each vesicle is surrounded by a protein membrane and encloses metabolic gases. Air vacuoles not only store gases but provide buoyancy, mechanical strength and protection from harmful radiations.
5.What is the function of a polysome?
Soln.Different ribosomes held together by strand of mRNA form polysome. Polysome produce number of copies of the same polypeptide. It is formed during period of active protein synthesis.
6.What is the feature of a metacentric chromosome?
Soln. In metacentric chromosome, centromere lies in the centre and both the arms are of equal length.
7.What is refered to as satellite chromosome?
Soln. Few chromosomes have a non-staining secondary constrictions being present at a constant location which gives the appearance of a small fragment known as satellite. A chromosome having satellite is called (sat) chromosome Or marker chromosome.
Short Answer Type Questions
1.Discuss briefly the role of nucleolus in the cells actively involved in protein synthesis.
Soln. Nucleolus is a naked, round and slightly irregular structure, which is attached to the chromatin at a specific region. The content of nucleolus is continuous with the rest of the nucleoplasm as it is not a membrane bound structure. It is a site for active ribosomal RNA synthesis. Larger and more numerous nucleoli are present in cells actively carrying out protein synthesis.
2.Explain the association of carbohydrate to the plasma membrane and its significance.
Soln. Carbohydrates constitute about 1-5% of chemical composition of plasma membrane. These,are associated with the phospholipids or with the peripheral proteins to form glycolipids and glycoproteins respectively. Carbohydrates present in membrane are branched or unbranched oligosaccharides. These function as recognition centre, site of attachment, antigens etc. They also provide negative charge to the outer surface.
3.Comment on the cartwheel structure of centriole.
Soln.Centrosome is an organelle that generally have two cylindrical structures known as centrioles. Both the centrioles in a centrosome lie perpendicular to each other in which each has an organisation like the. cartwheel. They are usually made up of nine evenly spaced peripheral fibrils (triplet in nature) of tubulin protein, with which adjacent triplets are also being linked. The centre part of the proximal region of the centriole possess rod-shaped proteinaceous mass known as hub, which is connected with tubule of the peripheral triplet fibrils known as radial spokes made of protein.
4.Briefly describe the cell theory.
Soln.Fundamental features of cell theory given by Schleiden and Schwann and modified by Rudolf Virchow are:
(i) All living organisms are composed of cells and their products.
(ii)Each cell is made of a small mass of protoplasm containing a nucleus and a plasma membrane with or without a cell wall on its outer side.
(iii)All cells are basically alike in their chemistry and physiology.
(iv)Activities of an organism are the sum total of activities and interactions of its constituent cells.
(v)All cells arise from pre-existing cells.
5.Differentiate between Rough Endoplasmic Reticulum (RER) and Smooth Endoplasmic Reticulum (SER).
Soln.Differences between SER and RER are:
6.Give the biochemical composition of plasma membrane. How are lipid molecules arranged in the membrane?
Soln. Every living cell is covered by thin, elastic, transparent, semi-permeable membrane called plasma membrane or cell membrane. Chemically a biomembrane consists of lipids (20-79%), proteins (20%-70%), carbohydrates (1-5%) and water (20%). The lipid molecules are amphipathic, that is, they possess both polar hydrophilic (water loving) and nonpolar hydrophobic (water repelling) ends. The hydrophilic region is in the form of a head while the hydrophobic part contains two tails of fatty acids. Hydrophobic tails usually occur towards the centre of the membrane. It results in the formation of a lipid bilayer. Protein molecules occur at places both inside and on the outer side of lipid bilayer and appear as protein icebergs in a sea of lipids.
7.What are plasmids? Describe their role in bacteria.
Soln. Plasmids are self-replicating, extra chromosomal segments of double stranded, circular, naked DNA.
These plasmid confer certain unique phenotypic characters to the bacteria like antibiotic resistance, sex factor, fertility factor etc. They are also used as vectors in genetic engineering.
8.What are histones? What are their functions?
Soln. Hi stones are lysine and arginine rich basic proteins involved in DNA packing in eukaryotes. There are five types of histone proteins – H,, H2A, H2B, H3 and H4. Four of them (H2A, H2B, H3 and H4) occur in pairs to produce histone octamer, called nu body or core of nucleosome. Being positively charged (due to basic amino acids) they attract negatively charged strands of DNA. ,
Long Answer Type Questions
1.What structural and functional attributes must a cell have to be called a living cell?
Soln.Both prokaryotic and eukaryotic organisms are made up of the cell. Cell is the structural and fundamental unit of all living organisms. The main structural attributes of a cell are:-
(i) Cell membrane – Every living cell is . covered by a thin, elastic, transparent, semi-permeable and regenerative membrane called cell membrane or plasma membrane. It is made up of lipids, proteins and carbohydrates. It separates the interna] environment of the cell from external environment and helps in regulating the entry and exit of molecules into and out of the cell.
(ii)Genome – In prokaryotes the genetic material is not bounded by a definite nuclear membrane. It is called nucleoid or genophore. It consists of single membrane circular strand of DNA duplex embedded freely in the cytoplasm, with evolution, eukaryotes developed.
In eukaryotes, nucleus is bounded with well defined nuclear envelope. Nucleus contains nuclear envelope, nucleoplasm, nucleolus and chromatin. It contains all the genetic information for controlling all essential processes related to metabolism and transmission.
In prokaryotes, there are no well defined organelles but with evolution, in eukaryotes well defined membrane bound organelles developed e.g., mitochondria, plastids, ER.
Functional attributes of the cell are growth, cell division, response to stimuli. These functional attributes are found in every cell.
2.Briefly give the contributions of the following scientists in formulating the cell theory.
(a) Rudolf Virchow
(b) Schleiden and Schwann
Soln. (a) Rudolf Virchow – Rudolf Virchow proposed theory of cell lineage, Omnis cellula-e cellula, cells develop by division of
pre-existing cells.
(b) Schleiden and Schwann:- In 1838, Matthias Schleiden, a German botanist observed that all plants are composed of different kinds of cells, which form the tissues of the plant. At the same time, Theodore Schwann (1839), a British zoologist studied different types of animal tissues including development of embryos. He found that animal cells lack a cell wall, but they are covered by a membrane. Otherwise cells of both plants and animals are similar. Schwann proposed the hypothesis that bodies of animals and plants are composed of cells and products of cells. Both scientist Schleiden and Schwann together gave cell theory. This theory states that bodies of all living organisms are made up of cells and their products. All cells are basically alike in their chemical composition and physiology. Activities of an organism are the sum total of activities and interactions of its constituent cells.
3.Is extra genomic DNA present in prokaryotes and eukaryotes? If yes, indicate their location in both the types of organisms.
Soln. Yes, .extra genomic DNA is present in both prokaryotes and eukaryotes. In prokaryotes, extra genomic DNA is present in form of plasmids and as mitochondria and plastids in eukaryotes.
In Prokaryotes plasmids are self replicating, extra chromosomal segments of double stranded, circular, naked DNA. Plasmids provide unique phenotypic characters to bacteria. They are independent of main nucleoid. Some of them contain important genes like fertility factor, nif genes and resistance factors. Plasmids are used as vectors in genetic engineering.
In Eukaryotes : mtDNA (mitochondrial DNA) and cpDNA (chloroplast DNA) are present as a extra genomic DNA in mitochondria and chloroplast respectively. Mitochondria have their own naked, circular or linear DNA which can replicate independently. Mitochondrial DNA produces its own mRNA, fRNAand rRNA. Plastids are another semi-autonomous organelles having DNA and double membrane envelope which store or synthesise various types of organic compounds. Chloroplast DNA is naked, circular or occasionally linear.
4.Structure and function are correctable in living organisms. Can you justify this by taking plasma membrane as an example?
Soln. In living organisms, the structure and function of the cell organelle are correlated. E.g., every living cell is covered by a thin, elastic, transparent, semi-permeable and regenerative membrane called plasma membrane. It is made up of lipids, proteins and carbohydrates. The cell membrane is composed of lipid which forms a bilayer with protein molecules embedded in it at places and carbohydrates. The lipid molecules are amphipathic in nature and have polar hydrophilic head and nonpolar hydrophobic tail. Depending upon the ease of extraction, the ratio of protein and lipid varies considerably in different cell types.
According to fluid mosaic model, the lipid bilayer and integral proteins appear like a mosaic arrangement and the quasi-fluid ‘nature of lipid enables the lateral movement of the proteins within the overall bilayer.
This ability of proteins to move within the membrane indicate the fluidity of the lipid part. The fluid nature of the membrane is important for interactions of molecules within the membrane as well as other functions like formation of intercellular junctions, cell growth, secretion, endocytosis, cell division, etc. It is a selectively permeable or semi- permeable membrane, allows only selected substances to pass inwardly. Membranes have carrier proteins for active transport. Cell membrane contain enzymes which perform certain reaction on their surface, e.g., ATPase, phosphatase, etc.
5.Eukaryotic cells have organelles which may
(a) not be bound by a membrane
(b) bound by a single membrane
(c) bound by a double membrane
Group the various sub-cellular organelles into these three categories.
Soln. (a) Organelles not bound by membrane – Ribosomes.
(b) Organelles bound by single membrane – Lysosomes, Sphaerosomes, Glyoxysomes, Vacuole.
(c) Organelle bound by double membrane – Mitochondria, Plastids, Nucleus.
6.The genomic content of the nucleus is constant for a given species where as the extra chromosomal DNA is found to be variable among the members of a population. Explain.
Soln.As we know that no. of chromosomes are fixed in a species, so the genomic content of the nucleus is constant for a given species. But the extrachromosomal DNA is variable among the members of a population. For example in bacteria (prokaryotes) in addition to genomic DNA, extrachromosomal circular naked DNA is present in some species.
In Eukaryotes plastids and mitochondria are extra chromosomal DNA and are found to be variable among the members of a population. The number of chloroplasts per cell of algae is usually fixed for a species.
Mitochondria number in different cells of an individual or different individuals vary. For example, a resting cell will have lower number of mitochondria than that in activity swimming cells. In higher plants, a photosynthetic leaf chlorenchyma cell has 20-40 chloroplasts whereas a root cell has no chloroplasts.
This difference in number of chloroplasts and mitochondria in different cells and different species is the reason that the amount of extra¬chromosomal DNA is not species specific.
In animals, there are no plastids but there are there are more number of mitochondria which again depend, upon cell activities. That is why there is varying number of extra chromosomal DNA in a population.
7.Justify the statement, “Mitochondria are power houses of the cell”.
Soln. Mitochondria are cell organelles
of aerobic eukaryotes which take part in oxidative phosphorylation and Krebs cycle of aerobic respiration. They are called power houses of cell because they are the major centres of release of energy in the aerobic respiration.
Mitochondria are miniature biochemical factories where food stuffs or respiratory substances are completely oxidised to carbon dioxide and water. The energy liberated in the process is initially stored in the form of reduced coenzymes and reduced prosthetic groups. The latter soon undergo oxidation and form energy rich ATR ATP comes out of mitochondria and helps to perform various energy requiring processes of the cell like musclecontraction, nerve impulse conduction, biosynthesis, membrane transport, cell division, movement etc. Because of the formation of ATP, the mitochondria are called power houses of the cell.
8.Is there a species specific or region specific type of plastids? Flow does one distinguish one from the other?
Soln.Plastids are small bodies found free in the cytoplasm of most plant cells.
Plastids are of three types, chloroplast, chromoplast and leucolast. Each type of plastid shows specific position or occurrence in a plant body, i.e., plastids are region specific and occur in various plant parts according to their function.
Leucoplasts are colourless plastids which generally occur near the nucleus in nongreen cells and possess internal lamellae. Grana and photosynthetic pigments are absent. Amyloplasts are starch containing leucoplasts, elaioplasts (Lipidoplasts) are the colourless plastids which store fat and aleuroplasts are protein containing plastids.
Chromoplasts are yellow or reddish in colour because of the presence of carotenoid pigments. They provide colour to many flowers for attracting pollinating insects, provide bright red or orange colour to fruits for attracting animals for dispersal and are the site of synthesis of membrane lipids. Chloroplasts art- greenish plastids which possessphotosyntheticpigments, chlorophylls and carotenoids, and take part in the synthesis of food from inorganic raw materials in the presence of radiation energy. They occur in major number in the photosynthetic mesophyll cells of leaves and green stem. Apart from being region specific plastids may show species specificity e.g., chloroplast present in euglenoids have three envelopes whereas those1 present in higher plants are covered by two envelopes.
Moreover, distribution pattern of plants in a region also affect type and positioning of plastids e.g., plants occuring in deep layers of a water body have abundance of red pegment phycoerythrin whereas plants occuring in illuminated shallow water contain more chlorophylls.
9.Write the functions of the following
(a) Centromere (b) Cell wall
(c) Smooth ER (d) Golgi apparatus
(e) Centrioles
Soln.(a) Centromere – It is required for proper chromosome segregation. The centromere consists of two sister chromatids. It is also necessary at the point chromosome attaches to the spindle apparatus during mitosis and meiosis.
(b) Cell wall – Cell wall helps in providing a definite shape to the cell and also protects protoplasm against any mechanical injury, /.c., damage and infection. It also helps in cell-to-cell interaction and provides barrier to undesirable macromolecules and attack of pathogens.
(c) Smooth ER – It acts as a major site for the synthesis of lipid and also helps in synthesis of steroidal hormone in animal cells.
(d) Golgi apparatus –
(i) It is involved in the formation of lysosomes, vesicles that contain proteins and remains within the cell.
(ii)It performs the function of packaging material.
(iii)It acts as an important site for the formation of glycoproteins and glycolipids.
(iv)It helps in the production of complex
carbohydrates other than glycogen and starch. .
(v) It helps in the formation of cell wall.
(e) Centrioles- Centrioles help in cell division by forming microtubule – organising centrer (MTOCS). It also, form the basal body of cilia and flagella of the cells.
10.Are the different types of plastids
interchangable? If yes, give examples where they are getting converted from one type to another.
Soln.Yes, different types of plastids are interchangable in form. Plastids are of three main types – leucoplasts, chromoplasts and chloroplasts. Depending upon the condition and maturity stage one type of plastid change into another plastid.
Chromoplasts are formed either from leucoplast or chloroplasts. e.g., Change of colour from green to reddish during the ripening of tomato and chilli is due to transformation of chloroplasts to chromoplasts.
In capsicum, the cells of ovary consist of leucoplasts. When ovary changes into fruit, leucoplasts are transformed into chloroplasts.
When the fruit ripens, chloroplast are converted into chromoplast. The leucoplast in stem tubers of potato, on exposure to sunlight transform to chloroplasts.
The post NCERT Exemplar Class 11 Biology Solutions Cell The Unit of Life appeared first on Learn CBSE.