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NCERT Solutions for Class 9th Science Chapter 2 The four questions the chapter sets out to answer — Think It Over

Book page 8 Updated on2026-09-08

Q1.
Where does a cell come from?
Answer

Every cell comes from an already existing cell that divides — never from non-living matter. This is the third statement of the Cell Theory, added by Rudolf Virchow in 1855.

pre-existing cell → grows → divides → new cells
mitosis: 1 parent cell → 2 identical daughter cells
meiosis: 1 parent cell → 4 cells with half the chromosome number
Why it happens: a cell is not just a bag of chemicals. It needs a working membrane, cytoplasm, ribosomes and above all a complete set of DNA instructions, and the only place all of these are ready-made and correctly arranged is inside another living cell. The chapter's own example proves the point: when Craig Venter's team made a synthetic Mycoplasma mycoides genome in 2010, they still had to put it inside an existing bacterial cell whose cytoplasm and membrane were intact. Only the DNA was synthetic.
Did you know? Your own body began as a single fertilised egg. Every one of the roughly 30 trillion cells you now carry traces back to that one cell through an unbroken chain of divisions.
Q2.
How have technological interventions facilitated the creation of new knowledge in understanding the world beyond the naked eye?
Answer

Each new instrument pushed the limit of what could be seen, and every push produced new biology.

ToolSmallest thing it showsNew knowledge it created
Unaided eye0.1 mm (100 µm)Whole organisms and organs only
Robert Hooke's microscope, 1665 (200–300X)a few µmThe box-like compartments in cork — he named them ‘cells’
School light microscope (10X × 10X = 100X, up to 400X)about 0.2 µmNucleus, cell wall, chloroplasts, stages of cell division in an onion root tip
Electron microscope (beam of electrons)nanometre scale (1 nm = one-billionth of a metre)Ribosomes, cristae of mitochondria, the lipid bilayer, cytoskeleton, viruses; it confirmed the Golgi apparatus decades after Camillo Golgi first saw it
Why it happens: resolution is set by the wavelength of whatever is used to ‘look’. Visible light has a wavelength of about 400–700 nm, so no light microscope can separate two points much closer than about 200 nm, however strong the lens. Electrons behave as waves of far shorter wavelength, so an electron microscope resolves structures a thousand times finer. Scientists also improved two other things — contrast (the brightness difference between parts of an object, which is why we stain onion peel with safranin and cheek cells with methylene blue) and magnification.
Did you know? The Golgi apparatus was doubted for decades because early microscopes could not resolve it clearly. Only electron microscopy settled the argument. Technology does not merely confirm knowledge — it decides which questions can be answered at all.
Q3.
How is the cell structural and functional unit of life?
Answer

It is the structural unit because every living body is built out of cells and nothing smaller, and the functional unit because every life process is carried out inside a cell.

  • Structural: similar cells form a tissue, tissues form an organ, organs form an organ system — nasal pores, nasal cavity, trachea and lungs form the respiratory system. Take the organisation apart and you always arrive at cells.
  • Functional: respiration happens in mitochondria, protein synthesis on ribosomes, photosynthesis in chloroplasts, digestion of waste in lysosomes, storage in vacuoles. All of these sit inside one cell.
  • A single cell can be a complete organism — a bacterium or a yeast lives, feeds, grows and reproduces on its own. Nothing smaller than a cell can do that.
Why it happens: the cell is the smallest unit that has all three things life needs at once — a boundary that keeps the inside different from the outside (the membrane), a set of instructions to run and copy itself (DNA), and machinery to turn those instructions into work (ribosomes, enzymes, mitochondria). Break a cell open and the parts stop working within minutes, because the boundary that held the right concentrations together is gone. Viruses have DNA and protein but no membrane-bound machinery of their own, which is exactly why they cannot live outside a host cell.
Q4.
How does a cell multiply?
Answer

By cell division — the parent cell copies its DNA and then splits. There are two types.

MitosisMeiosis
Daughter cells24
Chromosome numberSame as parentHalf of parent
Number of divisionsOneTwo, one after the other
WhereBody cells — skin, root tip, bone marrowOnly testes and ovaries in animals; anthers and ovaries in plants
PurposeGrowth, repair, maintenance, asexual reproductionMaking gametes; creating variation
Mitosis: 1 cell (46 chromosomes) → 2 cells (46 each)
Meiosis: 1 cell (46) → division I → 2 cells (23 each) → division II → 4 cells (23 each)
Fertilisation: 23 + 23 = 46 — the original number is restored
Why it happens: a cell cannot simply keep growing. As a cell gets bigger its volume rises as the cube of its size but its surface area only as the square, so the membrane soon becomes too small to supply the inside with food and oxygen. Dividing resets that ratio. Meiosis exists for a different reason: if gametes carried the full chromosome number, fertilisation would double it every generation, so the number must be halved first.
Did you know? Roughly a few hundred billion cells in your body are replaced every day — about 1 per cent of the total. That is why a cut heals and hair grows back.
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