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NCERT Solutions for Class 9th Science Chapter 3 Chapter opener — Think It Over

Book page 28 Updated on2026-09-08

Q1.
How is the study of cells and tissues significant for understanding the life processes and human welfare?
Answer

Because every life process finally happens inside a cell, and no single cell in a large body can do all of them at once.

A unicellular organism like Amoeba manages everything — nutrition, respiration, excretion, movement — with one cell. In a multicellular body that is impossible, so cells of one kind group into a tissue and take up one job. Studying tissues therefore tells us where each life process is carried out and why that particular structure suits it:

  • Muscle tissue → movement, because its cells are long fibres that can contract.
  • Nervous tissue → control and coordination, because a neuron has a long axon that carries a message far.
  • Xylem → water transport, because its cells are dead, hollow tubes with no cross-walls to block flow.
  • Phloem → food transport, because its sieve tubes are living and are loaded by companion cells.
Why it matters for human welfare: if you know the normal structure of a tissue, you can tell what has gone wrong when it fails, and sometimes repair it. Bone-marrow stem cells are transplanted to treat leukaemia and thalassaemia. Plant tissue culture, which began with F. C. Steward showing in 1958 that a single carrot phloem cell can regenerate a whole plant, now gives us disease-free planting material and improved crops. Sipra Guha Mukherjee and S. C. Maheshwari raised a complete plant from anther culture, a technique still used in crop improvement.
Tip: remember the hierarchy in one line — cells → tissue → organ → organ system → organism. Each level exists because it makes the division of labour finer.
Q2.
How are tissues in plants and animals different, and why?
Answer

They differ because plants and animals live in different ways — plants stay fixed and make their own food, animals move about and take food in.

PointPlant tissuesAnimal tissues
SupportCell walls give rigidity; sclerenchyma and lignin form the woody skeleton. The plant is held upright by dead tissue.No cell wall, so cells change shape easily. Support comes from a living skeleton of bone and cartilage.
GrowthGrowth is restricted to meristems and continues all life. A tree adds girth every year.Growth is spread over the whole body and mostly stops at maturity.
NutritionTissues for photosynthesis (chlorenchyma-type parenchyma) — food is made inside.Tissues for digestion and absorption (epithelium of the gut) — food is taken in from outside.
TransportXylem and phloem — largely non-motile tubes; xylem is dead.Blood — a fluid connective tissue pumped by cardiac muscle.
Energy costMany supporting cells are dead, so they cost nothing to maintain.Nearly all cells are living and must be supplied constantly.
Why it happens: an animal must be able to bend, run and change shape, so it cannot afford a rigid wall around every cell; it pays instead with a living skeleton and a constant food supply. A plant does not move, so it can spend cheap dead lignified cells on support and keep growing from its tips all its life.
Q3.
How is the division of labour at various levels of organisation in multicellular organisms correlated with their structure and function?
Answer

At every level, the structure is shaped by the one job that level has to do — and each higher level exists to combine jobs that a lower level cannot combine.

LevelStructureFunction it makes possible
CellA neuron is drawn out into a long axonCarries a signal over a long distance
TissueMany muscle fibres bundled and lying parallelTheir small individual pulls add up to a large force
OrganThe heart — cardiac muscle + epithelium + connective tissue + nervesPumps blood rhythmically for a lifetime
Organ systemMusculoskeletal system — bones, muscles, tendons, ligaments, cartilage, jointsPosture, locomotion, protection of delicate organs
OrganismAll systems working under nervous controlA complete, coordinated life

The same logic runs through the plant: epidermal cells are flat and waxy (protection), parenchyma cells are thin-walled with spaces (storage and gas diffusion), xylem vessels are hollow tubes (conduction) — and together they form the dermal, ground and vascular tissue systems of the whole plant body.

Why division of labour raises efficiency: a cell that does only one job can be built entirely for that job. A parenchyma cell that only stores food does not need thick walls; a sclerenchyma cell that only gives strength does not even need to stay alive. Specialisation removes the compromises that a jack-of-all-trades cell like Amoeba is forced to make.
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