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

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Exercises 8.11–8.14 (part 2 of 2)

  1. Exercise 8.11

    What are nuclear pores? State their function.

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    Nuclear pores are minute pores that interrupt the nuclear envelope at a number of places, formed by the fusion of its two membranes.
    The nuclear envelope itself consists of two parallel membranes separated by the perinuclear space ($\displaystyle 10$ to $\displaystyle 50$ nm), and it forms a barrier between the materials inside the nucleus and those of the cytoplasm.
    Function:
    They are the passages through which the movement of RNA and protein molecules takes place in both directions between the nucleus and the cytoplasm.

  2. Exercise 8.12

    Both lysosomes and vacuoles are endomembrane structures, yet they differ in terms of their functions. Comment.

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    Lysosomes are the digestive bags of the cell, whereas vacuoles are storage and osmoregulatory spaces — that is why two organelles of the same endomembrane system behave so differently.
    Lysosomes
    Membrane bound vesicular structures formed by the process of packaging in the Golgi apparatus.
    Very rich in almost all types of hydrolytic enzymes (hydrolases — lipases, proteases, carbohydrases), which are optimally active at acidic pH.
    These enzymes are capable of digesting carbohydrates, proteins, lipids and nucleic acids.
    Vacuoles
    A membrane-bound space in the cytoplasm, bounded by a single membrane called the tonoplast.
    Contains water, sap, excretory product and other materials not useful for the cell; in plant cells it can occupy up to $\displaystyle 90$ per cent of the volume of the cell.
    The tonoplast transports ions and other materials against concentration gradients into the vacuole, so their concentration is significantly higher inside the vacuole than in the cytoplasm.
    In Amoeba the contractile vacuole is important for osmoregulation and excretion; in many cells such as protists, food vacuoles are formed by engulfing food particles.
    What they share
    Both are single-membrane bound, both belong to the endomembrane system, and their functions are coordinated with those of the ER and the Golgi complex.

  3. Exercise 8.13

    Describe the structure of the following with the help of labelled diagrams.
    (i)
    Nucleus
    (ii)
    Centrosome

    The book prints no answer for this

    NCERT publishes no answers for this textbook, so there is nothing in the book to check this working against. It has also not yet been read through by hand.

    (i) Nucleus
    A dense, membrane bound organelle, first described by Robert Brown ($\displaystyle 1831$), that contains the chromatin and controls the activities of the cell.
    It is bounded by a nuclear envelope of two parallel membranes with a space between them ($\displaystyle 10$ to $\displaystyle 50$ nm) called the perinuclear space; this forms a barrier between the contents of the nucleus and the cytoplasm.
    The outer membrane usually remains continuous with the endoplasmic reticulum and bears ribosomes on it.
    At a number of places the envelope is interrupted by nuclear pores, formed by the fusion of its two membranes; RNA and protein molecules move through them in both directions.
    The nuclear matrix (nucleoplasm) contains the nucleolus and the chromatin. The interphase nucleus has highly extended nucleoprotein fibres (chromatin) containing DNA, histones, non-histone proteins and RNA.
    The nucleoli are spherical, are not membrane bound (their content is continuous with the nucleoplasm), and are the site of active ribosomal RNA synthesis; they are larger and more numerous in cells actively making protein.
    Diagram to draw: a large circle within which is drawn a double boundary line (the two membranes of the nuclear envelope) broken at two or three places by gaps; inside, a small dense sphere and a fine network of threads. Label: Nuclear membrane, Nuclear pore, Nucleoplasm, Nucleolus (and chromatin fibres). The book prints this as Figure $\displaystyle 8.11$.
    (ii) Centrosome
    An organelle usually containing two cylindrical structures called centrioles, surrounded by amorphous pericentriolar materials.
    The two centrioles lie perpendicular to each other, and each has an organisation like a cartwheel.
    Each centriole is made up of nine evenly spaced peripheral fibrils of tubulin protein, and each peripheral fibril is a triplet; the adjacent triplets are also linked.
    The central part of the proximal region of the centriole is proteinaceous and is called the hub, which is connected to the tubules of the peripheral triplets by radial spokes made of protein.
    Function: the centrioles form the basal body of cilia or flagella, and the spindle fibres that give rise to the spindle apparatus during cell division in animal cells.
    Diagram to draw: show the two centrioles as two short cylinders set at right angles to each other, enclosed in a hazy zone. Beside them draw one centriole in transverse section as a cartwheel — a central hub, nine radial spokes running out from it, and at the end of each spoke a set of three linked tubules (a peripheral triplet fibril). Label: Centrioles, Pericentriolar material, Hub, Radial spoke, Peripheral triplet fibril. (The chapter prints a figure for the nucleus but not for the centrosome, so this one must be drawn from the description above.)
    NCERT_Solution_Class11_Biology_Ch8_Q8-13

  4. Exercise 8.14

    What is a centromere? How does the position of centromere form the basis of classification of chromosomes. Support your answer with a diagram showing the position of centromere on different types of chromosomes.

    The book prints no answer for this

    NCERT publishes no answers for this textbook, so there is nothing in the book to check this working against. It has also not yet been read through by hand.

    The centromere is the primary constriction of a chromosome, on the sides of which disc-shaped structures called kinetochores are present; it holds the two chromatids of a chromosome together.
    Based on the position of the centromere, chromosomes are classified into four types:
    Metacentric — the centromere lies in the middle, forming two equal arms.
    Sub-metacentric — the centromere is slightly away from the middle, giving one shorter arm and one longer arm.
    Acrocentric — the centromere is situated close to one end, giving one extremely short and one very long arm.
    Telocentric — the centromere is terminal, at the very end of the chromosome.
    (A few chromosomes also show non-staining secondary constrictions at a constant location, giving the appearance of a small fragment called the satellite.)
    Diagram to draw: draw four chromosomes side by side, each as a pair of chromatids pinched at one point. Move the pinch along the length from one to the next — exactly in the middle (metacentric, arms equal), a little off centre (sub-metacentric, one short arm and one long arm), near one end (acrocentric, one very short arm), and right at the end (telocentric, no upper arm). Mark the pinch centromere in each and label the four types beneath them. The book prints this as Figure $\displaystyle 8.13$; the kinetochore on either side of the centromere is shown separately in Figure 8.12.
    NCERT_Solution_Class11_Biology_Ch8_Q8-14