SolveItClass 9 · NCERT

NCERT Solutions · Class 9 Science Cell: The Building Block of Life

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Pause and Ponder 2.4–2.5 (part 7 of 14)

  1. Exercise 2.4

    material enclosed by a membrane) Primitive nucleus or nucleoid (genetic

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    No — a white petal contains no pigment at all.
    NCERT_Solution_Class9_Science_Ch2_PP_Q2-4
    Its plastids are leucoplasts, the plastids that lack pigments and are therefore colourless; their job is storing food such as starch, oils or proteins.
    Colour in petals and fruits comes from chromoplasts, whose pigments are yellow, orange or red; green comes from chlorophyll inside chloroplasts.
    With no pigment to absorb any part of the light falling on it, the petal throws nearly all of that light back, and what reaches our eyes is white.
    The chapter's point about bright colours also fits: pigments exist to attract pollinators and fruit-eating animals, so a flower that attracts by scent or shape has no need to make one.
  2. Exercise 2.5

    material without membrane around it)

    Not cross-checked

    NCERT prints no numerical answer for this exercise, so this working has not been cross-checked against the book.

    Draw the plant cell — it is the one that shows every clue, including the rod-shaped chloroplasts.
    NCERT_Solution_Class9_Science_Ch2_PP_Q2-5
    Outline, two lines and not one: a thick outer rectangle with rounded corners for the cell wall, and a thinner line drawn just inside it, following it the whole way round, for the cell membrane.
    Vacuole: one large central space filling most of the cell, bounded by a single line and labelled vacuole (cell sap). Everything else goes in the narrow band of cytoplasm between it and the membrane.
    Nucleus (clue i): a dark round body pushed to one side. Draw its boundary as two closely spaced lines — the double-layered nuclear membrane — leave two or three gaps in it for the nuclear pores, and put a small solid circle inside for the nucleolus.
    Endoplasmic reticulum (clue ii): begin it at the nuclear membrane, so it clearly runs out of the nuclear envelope, and let it spread through the cytoplasm as a branching network of long flattened tubes. Dot one stretch with tiny beads and label it rough ER (ribosomes attached); leave another stretch undotted and label it smooth ER.
    Mitochondria (clue iii): two or three rod-shaped bodies, each with a smooth outer line and an inner line thrown into finger-like folds — label those folds cristae.
    Chloroplasts (clue iii): three or four rod-shaped bodies with a double boundary, a few small discs drawn inside them, and the space around the discs labelled stroma.
    Golgi apparatus: a stack of four or five flattened, slightly curved sacs, with tiny circles budding off the ends as vesicles.
    Ribosomes: scatter a few free dots in the cytoplasm as well as the ones sitting on the rough ER.
    Label the remaining space cytoplasm, run a ruled label line from every single part to its name written outside the cell, and title the figure "A typical plant cell".
    If you pick the animal cell instead: no cell wall and no chloroplasts, an irregular rounded outline, only small scattered vacuoles, and add lysosomes as small single-membrane sacs.