SolveItClass 11 · NCERT

NCERT Solutions · Class 11 Biology Cell: The Unit of Life

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Exercises 8.1–8.10 (part 1 of 2)

  1. Exercise 8.1

    Which of the following is not correct?
    (a)
    Robert Brown discovered the cell.
    (b)
    Schleiden and Schwann formulated the cell theory.
    (c)
    Virchow explained that cells are formed from pre-existing cells.
    (d)
    A unicellular organism carries out its life activities within a single cell.

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    (a) Robert Brown discovered the cell — this is the statement that is not correct.
    Robert Brown discovered the nucleus ($\displaystyle 1831$), not the cell; it was Antonie von Leeuwenhoek who first saw and described a live cell.
    (b) is correct — Matthias Schleiden ($\displaystyle 1838$) and Theodore Schwann ($\displaystyle 1839$) together formulated the cell theory.
    (c) is correct — Rudolf Virchow ($\displaystyle 1855$) first explained that cells divide and new cells are formed from pre-existing cells (Omnis cellula-e cellula).
    (d) is correct — a unicellular organism is capable of independent existence and performs all the essential functions of life within its single cell.

  2. Exercise 8.2

    New cells generate from
    (a)
    bacterial fermentation
    (b)
    regeneration of old cells
    (c)
    pre-existing cells
    (d)
    abiotic materials

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    (c) pre-existing cells.
    Rudolf Virchow ($\displaystyle 1855$) first explained that cells divide and that new cells arise only from pre-existing cells — Omnis cellula-e cellula.
    This is the second statement of the cell theory: all cells arise from pre-existing cells.
    Cells are therefore never formed de novo from abiotic materials, nor by fermentation or by regeneration of old cells.

  3. Exercise 8.3

    Match the following Column I
    (a)
    Cristae
    (b)
    Cisternae
    (c)
    Thylakoids Column II
    (i)
    Flat membranous sacs in stroma
    (ii)
    Infoldings in mitochondria
    (iii)
    Disc-shaped sacs in Golgi apparatus

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    (a) Cristae – (ii) Infoldings in mitochondria
    (b) Cisternae – (iii) Disc-shaped sacs in Golgi apparatus
    (c) Thylakoids – (i) Flat membranous sacs in stroma
    Reasons:
    Cristae are the infoldings formed by the inner mitochondrial membrane towards the matrix; they increase the surface area.
    Cisternae are the flat, disc-shaped sacs ($\displaystyle 0.5$ μm to $\displaystyle 1.0$ μm diameter) stacked parallel to each other in the Golgi apparatus.
    Thylakoids are the organised flattened membranous sacs present in the stroma of the chloroplast, stacked like piles of coins to form grana.

  4. Exercise 8.4

    Which of the following is correct:
    (a)
    Cells of all living organisms have a nucleus.
    (b)
    Both animal and plant cells have a well defined cell wall.
    (c)
    In prokaryotes, there are no membrane bound organelles.
    (d)
    Cells are formed de novo from abiotic materials.

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    (c) In prokaryotes, there are no membrane bound organelles — this is the correct statement.
    Prokaryotic cells possess no organelles like those of eukaryotes except ribosomes, which are non-membrane bound.
    (a) is wrong — prokaryotic cells lack a membrane-bound nucleus, and some mature eukaryotic cells also lack a nucleus, e.g. erythrocytes of many mammals and sieve tube cells of vascular plants.
    (b) is wrong — the cell wall is a unique character of plant cells; animal cells have only a plasma membrane as the delimiting structure.
    (d) is wrong — cells are never formed de novo; all cells arise from pre-existing cells.

  5. Exercise 8.5

    What is a mesosome in a prokaryotic cell? Mention the functions that it performs.

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    A mesosome is a specialised, differentiated form of the cell membrane that is characteristic of prokaryotes, formed by extensions of the plasma membrane into the cell.
    These extensions take the form of vesicles, tubules and lamellae.
    Functions:
    Help in cell wall formation.
    Help in DNA replication and its distribution to the daughter cells.
    Help in respiration.
    Help in secretion processes.
    Increase the surface area of the plasma membrane and its enzymatic content.

  6. Exercise 8.6

    How do neutral solutes move across the plasma membrane? Can the polar molecules also move across it in the same way? If not, then how are these transported across the membrane?

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    Neutral solutes move across the plasma membrane by simple diffusion, along the concentration gradient, i.e. from higher to lower concentration.
    This is passive transport — it needs no energy. (Water moving the same way, from higher to lower concentration, is osmosis.)
    No, polar molecules cannot move in the same way, because they cannot pass through the nonpolar lipid bilayer of the membrane.
    Polar molecules require a carrier protein of the membrane to facilitate their transport across it.
    A few ions or molecules are moved against their concentration gradient, from lower to higher concentration. This is active transport, an energy-dependent process in which ATP is utilised, e.g. the \(\displaystyle Na^{+}/K^{+}\) pump.

  7. Exercise 8.7

    Name two cell-organelles that are double membrane bound. What are the characteristics of these two organelles? State their functions and draw labelled diagrams of both.

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    The two double membrane bound cell-organelles are the mitochondrion and the chloroplast (a plastid).
    (i) Mitochondrion
    Characteristics: sausage-shaped or cylindrical, diameter $\displaystyle 0.2$–$\displaystyle 1.0$ μm (average $\displaystyle 0.5$ μm) and length $\displaystyle 1.0$–$\displaystyle 4.1$ μm; the number per cell varies with the physiological activity of the cell.
    Bounded by two membranes: the outer membrane forms the continuous limiting boundary, while the inner membrane forms a number of infoldings called cristae towards the matrix, which increase the surface area.
    The two membranes divide the lumen into two aqueous compartments — the outer compartment (inter-membrane space) and the inner compartment, which is filled with a dense homogeneous substance, the matrix.
    Each membrane has its own specific enzymes associated with mitochondrial function.
    The matrix possesses a single circular DNA molecule, a few RNA molecules, 70S ribosomes and the components required for protein synthesis; mitochondria divide by fission.
    Function: they are the sites of aerobic respiration and produce cellular energy in the form of ATP, hence they are called the ‘power houses’ of the cell.
    (ii) Chloroplast
    Characteristics: lens-shaped, oval, spherical, discoid or even ribbon-like, of length $\displaystyle 5$–$\displaystyle 10$ μm and width $\displaystyle 2$–$\displaystyle 4$ μm; found mainly in the mesophyll cells of leaves — $\displaystyle 1$ per cell in Chlamydomonas to $\displaystyle 20$–$\displaystyle 40$ per cell in the mesophyll.
    Double membrane bound; of the two, the inner membrane is relatively less permeable. The space limited by the inner membrane is the stroma.
    The stroma contains flattened membranous sacs called thylakoids, stacked like piles of coins to form grana; flat membranous tubules called stroma lamellae connect the thylakoids of different grana. The thylakoid membrane encloses a space called the lumen.
    Chlorophyll and carotenoid pigments are present in the thylakoids; the stroma contains enzymes required for the synthesis of carbohydrates and proteins, along with small double-stranded circular DNA molecules and 70S ribosomes (smaller than the 80S cytoplasmic ribosomes).
    Function: the pigments trap light energy essential for photosynthesis — the grana are the site of the light reactions and the stroma of the dark reactions.
    Diagrams to draw
    Mitochondrion (longitudinal section) — draw an elongated sausage shape with two boundary lines: a smooth outer membrane and, just inside it, an inner membrane thrown into finger-like shelves projecting into the interior. Label: Outer membrane, Inner membrane, Crista (one infolding), Inter-membrane space (the gap between the two membranes) and Matrix (the dense interior). The book prints this as Figure $\displaystyle 8.7$.
    Chloroplast (sectional view) — draw a lens-shaped/oval body with an outer and an inner membrane. Inside, show several stacks of coin-like discs (each stack a granum, each disc a thylakoid), joined between stacks by flat tubules (stroma lamellae), all lying in a ground substance. Label: Outer membrane, Inner membrane, Stroma, Thylakoid, Granum, Stroma lamella, Lumen. The book prints this as Figure $\displaystyle 8.8$.
    NCERT_Solution_Class11_Biology_Ch8_Q8-7

  8. Exercise 8.8

    What are the characteristics of prokaryotic cells?

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    Characteristics of prokaryotic cells:
    They are represented by bacteria, blue-green algae, mycoplasma and PPLO (Pleuro Pneumonia Like Organisms).
    They are generally smaller and multiply more rapidly than eukaryotic cells, and vary greatly in shape and size — the four basic bacterial shapes being bacillus (rod like), coccus (spherical), vibrio (comma shaped) and spirillum (spiral).
    There is no well-defined nucleus: the genetic material is basically naked, not enveloped by a nuclear membrane, and is a single chromosome of circular DNA.
    Many bacteria carry plasmids — small circular DNA outside the genomic DNA — which confer unique phenotypic characters such as resistance to antibiotics.
    No membrane bound organelles of the eukaryotic kind are present, except ribosomes, which are 70S (50S + 30S subunits) and are associated with the plasma membrane.
    A cell wall surrounds the cell membrane in all prokaryotes except mycoplasma; the semi-fluid matrix filling the cell is the cytoplasm.
    The cell envelope is a tightly bound three layered structure — outermost glycocalyx (slime layer or capsule), then the cell wall, then the plasma membrane; the staining response of this envelope separates Gram positive from Gram negative bacteria.
    A mesosome, a specialised differentiated form of cell membrane formed by infoldings of the plasma membrane, is characteristic of prokaryotes; some prokaryotes such as cyanobacteria also have pigment-bearing chromatophores.
    Motile forms bear flagella made of filament, hook and basal body; pili and fimbriae are surface structures that do not take part in motility.
    Reserve material is stored as inclusion bodies (phosphate granules, cyanophycean granules, glycogen granules), which are not bound by any membrane and lie free in the cytoplasm.

  9. Exercise 8.9

    Multicellular organisms have division of labour. Explain.

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    Division of labour means that different kinds of cells in a multicellular body are specialised for different jobs, so no single cell has to perform every activity of the organism.
    A unicellular organism has no such option — it is capable of independent existence and must carry out all the essential functions of life within its one cell.
    In a multicellular organism the body is composed of many cells: Schleiden observed that all plants are composed of different kinds of cells which form the tissues of the plant, and Schwann, studying animal cells, proposed that the bodies of animals and plants are composed of cells and products of cells.
    The chapter's evidence for this is that cells differ greatly in size, shape and activities, and that the shape of a cell may vary with the function it performs (Figure $\displaystyle 8.1$): red blood cells are round and biconcave, white blood cells amoeboid, nerve cells branched and long (some of the longest cells), columnar epithelial cells long and narrow, mesophyll cells round and oval, and a tracheid elongated.
    Division of labour also operates inside each eukaryotic cell: the cytoplasm is compartmentalised by membrane bound organelles, and all the cell organelles perform different but specific functions — the nucleus controls the activities of the organelles, mitochondria supply ATP, chloroplasts trap light energy, ribosomes make proteins, lysosomes digest.

  10. Exercise 8.10

    Cell is the basic unit of life. Discuss in brief.

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    The cell is the basic unit of life because nothing less than a complete cell can live independently or carry out the essential functions of life — "anything less than a complete structure of a cell does not ensure independent living".
    Structural unit: all living organisms are composed of cells and products of cells. Some organisms are made of a single cell (unicellular), others of many cells (multicellular); the bodies of both animals and plants are built entirely of cells.
    Functional unit: a unicellular organism performs all the essential functions of life within its single cell. In every cell the cytoplasm is the main arena of cellular activities, where the various chemical reactions occur that keep the cell in the ‘living state’.
    Within the cell, the organelles perform different but specific functions, and the nucleus not only controls the activities of the organelles but also plays a major role in heredity.
    Unit of continuity: all cells arise from pre-existing cells (Omnis cellula-e cellula), so the cell is also the unit through which life is passed on.
    Hence the cell is the fundamental structural and functional unit of all living organisms.