NCERT Solutions for Class 9th Science Chapter 12 Chapter opener — Think It Over
Book page 228 Updated on2026-09-08
Q1.
What do you understand by biodiversity?
Answer
Biodiversity is the immense variety of living organisms found on the Earth — from microscopic organisms invisible to the naked eye to giant trees, from glowing jellyfish to soaring eagles, living in countless forms and habitats.
Why it matters: biodiversity is not a list of names, it is the machinery that keeps the planet running. The chapter gives four jobs that different groups of organisms do, and each one is a link the rest of life depends on.
Microscopic algae in the oceans release most of the oxygen we breathe — they photosynthesise on a scale that no forest matches.
Fungi and bacteria decompose fallen leaves and convert waste into manure, returning minerals to the soil and making it fertile.
Birds, bees and bats pollinate flowers, so seed and fruit are set.
Plants capture sunlight and prepare the food that supports nearly all life on the planet.
Remove any one link and the others weaken. That is why the chapter says these interconnections sustain ecosystems and make the Earth suitable for living organisms.
Note: biodiversity is measured at more than one level — the variety of species, the variety within a species (the different varieties of rice a farmer keeps), and the variety of habitats and ecosystems (mangrove, desert, coral reef). India is counted among the world's richest countries on all three.
Q2.
How does the grouping of organisms help us understand diversity?
Answer
Grouping turns an unmanageable list into a searchable structure — and because organisms are grouped by shared features, the structure itself carries information about how they are related.
Why it works: the chapter's own image is a library. Imagine thousands of books scattered on the floor and you want On the Origin of Species. Without subjects, authors and sections you would have to look at every book. Millions of organisms are the same problem. Sorting them by shared characters does three things at once:
It compresses information. Once you know an animal is an arthropod, you already know it has a segmented body, jointed appendages and an exoskeleton — you do not have to learn those facts separately for every insect, crab and spider.
It predicts. If a newly found organism has a cell wall of chitin and absorbs nutrients, you can place it in Fungi and expect the rest of the fungal features.
It shows relationship. Similar features suggest descent from a common ancestor, so the groups are not arbitrary boxes — they trace the actual history of life.
Try this: group ten animals from your area by habitat, then regroup the same ten by what they eat. The lists change. Which grouping told you more about how the animals are built? That comparison is exactly what Activity 12.1 asks you to do.
Q3.
On what basis, are plants and animals classified?
Answer
Both are classified on features that are shared because of common ancestry — scientists start from broad, easily visible characters and then move to finer ones. The chapter lists seven kinds of evidence.
Criterion
What is compared
External features
Shape, size, body organisation
Mode of nutrition
Autotrophic or heterotrophic
Internal structures
Skeletal patterns, presence or absence of organs, types of tissues
Cell structure
Unicellular or multicellular; eukaryote or prokaryote; cell wall present or absent
Ecological role
Producer, consumer or decomposer
Reproduction
Asexual and/or sexual methods
Genetic similarity
Similarities in inherited features, studied in detail using DNA
Within the two kingdoms the book then applies one decisive character each:
Plantae is divided into five classes on how far the plant body has separated from water — a thallus (Thallophyta), rhizoids but no vascular tissue (Bryophyta), true roots, stems, leaves with xylem and phloem (Pteridophyta), naked seeds (Gymnosperm), seeds enclosed in fruits (Angiosperm).
Animalia is divided first on the notochord — non-chordates (invertebrates) have none, chordates have one at least once in life — and then on level of organisation, symmetry, body cavity, segmentation and skeleton.
Tip: a good classifying character is one that is hard to change and shared by descent. Habitat is a poor character — a whale, a fish and a prawn all live in water but are built completely differently.
Q4.
How does classification help address problems in farming?
Answer
Because a farmer's problems — pests, disease, poor soil, drought, pollination — are all problems about particular organisms, and you cannot manage an organism you cannot correctly identify.
How it works in practice:
Telling friend from enemy. The chapter opens with exactly this question: some insects protect crops while others damage them. A ladybird beetle and a leaf-eating beetle are both Arthropoda, but correct identification down to genus decides whether you protect it or control it.
Choosing the right crop variety. Farmers have for centuries conserved diverse varieties with useful characteristics — drought tolerance, pest resistance, the ability to grow in nutrient-poor soils. Knowing which variety carries which character is a classification problem, and it is why diversity reduces the risk of crop failure and strengthens food security.
Using useful microbes.Rhizobium (Monera) fixes nitrogen in the root nodules of pulses; Lactobacillus is used in food. Fungi are decomposers that release minerals into soil. All of these are used deliberately only because they have been identified and named.
Protecting pollinators. Birds, bees and bats pollinate flowers. Fruit and seed set depend on them, so knowing which species pollinate a crop tells a farmer what not to spray and when.
Did you know? The same logic runs the other way too. Mushroom cultivation is now a growing livelihood, but only communities with reliable knowledge of which wild mushrooms are edible and which are poisonous can safely harvest them from the forest — a folk taxonomy doing exactly the job scientific classification does.