NCERT Solutions for Class 9th Science Chapter 12 End-of-chapter questions — Revise, Reflect, Refine

(ii) Body with jointed legs.

(iii) Presence of a cell membrane. The intended point is that a sponge cell is bounded by a cell membrane and has no cell wall — and in Fig. 12.5, absence of a cell wall is exactly the fork that leads to Animalia.

Take two animals almost anyone can find — a house crow and a garden lizard — and compare them on the characters the chapter uses for classification.

Because cellular organisation applies to every living thing, while xylem and phloem apply to a small corner of one kingdom. A criterion is fundamental when it can sort all of life at the first cut; xylem and phloem cannot even begin to.

Kingdom Protista — and most likely a ciliate such as Paramecium.

Because a diverse ecosystem has many organisms doing overlapping jobs, so when one fails another carries the function — and because the jobs themselves form closed loops that keep matter cycling.

1. Every role is filled, and the cycles close.

• Plants and microscopic algae produce food and release oxygen — microscopic algae in the oceans release most of the oxygen we breathe.
• Animals consume, and in doing so keep any one population from exploding.
• Fungi and bacteria decompose fallen leaves and convert waste into manure, returning minerals to the soil. Without them, as the chapter says, the decay of dead plants and animals would be greatly reduced, adversely affecting soil fertility and ecological balance.
• Birds, bees and bats pollinate; hornbills and other animals disperse seeds.

2. Diversity gives the system spare capacity. If twenty insect species pollinate the plants of a meadow and a disease removes three of them, the plants are still pollinated. If only one species did the job, seed set fails completely. The same argument holds for decomposers, predators and nitrogen-fixing bacteria.

3. Complex food webs absorb shocks. A predator with several prey species can switch when one becomes scarce, so neither predator nor prey crashes. A single-link chain has no such give.

4. Diversity itself buffers against disaster. Villages with more mangroves suffered less destruction in the 1999 Orissa super cyclone; forest diversity in the Western Ghats acts as a biological barrier against Kyasanur Forest Disease because many animals are hosts in which the virus cannot replicate.

You would be forced to put organisms with completely different cell architecture, nutrition and ancestry into one box — which is exactly the mistake that history had to correct twice.

1. Prokaryotes and eukaryotes would sit together. A bacterium has no membrane-bound nucleus; an amoeba has a true one. This is the single deepest division in the living world. Grouping them hides it. The chapter records that this is precisely why bacteria were pulled out and put in Monera, creating the four kingdom system in 1938.
2. Producers, consumers and decomposers would be mixed. Photosynthetic cyanobacteria, a heterotrophic amoeba, an absorptive yeast and a Euglena that switches between the two would all be in one kingdom with nothing in common but size.
3. The grouping would say nothing about ancestry. Being one cell is not an inherited character shared from a common ancestor — it is a lack of multicellularity. Grouping by an absence tells you nothing about relationship, and the whole point of classification is that similar features suggest common ancestors.
4. Multicellular exceptions would be stranded. Yeast is unicellular but is placed in Fungi because of its chitin cell wall. A "unicellular kingdom" would have to tear yeast away from the moulds and mushrooms it is plainly related to.
5. It would have no predictive value. Knowing an organism was "unicellular" would tell you almost nothing else about it — no cell structure, no nutrition, no ecological role. A good group is one where membership predicts many other facts.

Because the five kingdom system classifies organisms by their cells — and a virus has no cell. Every criterion in Fig. 12.5 is unusable on it.

Both positions can be defended. The stronger one is to give viruses a separate category of their own — placed outside the five kingdoms, not inside them — and to say clearly why.

The case for a separate category

• Viruses are real, extremely numerous, and biologically important — they cause disease, they transfer genes between organisms and they shape whole populations. Simply ignoring them leaves a large part of biology unclassified.
• They can be classified among themselves perfectly well, on their own criteria: type of genetic material (DNA or RNA), shape of the coat, presence of an envelope, and which host they infect. A working system already exists.
• Putting them in a category of their own is honest about what they are — it records that they are different in kind, rather than forcing them into a kingdom where they fit none of the criteria.

The case for keeping them outside

• The five kingdoms are defined by cellular organisation. Admitting an acellular entity would break the logic of the whole system, and once broken, the criteria no longer mean anything.
• A virus cannot do anything on its own. If it is not independently alive, a classification of living organisms is arguably the wrong document to list it in.

The features that block them are exactly the features the system is built on.

1. No cellular organisation. A virus is acellular — genetic material in a protein coat, nothing more. The very first question of the five kingdom system, "prokaryote or eukaryote?", cannot be answered.
2. No cell membrane, cell wall, cytoplasm or organelles. So the "cell structure" criterion has nothing to measure.
3. No metabolism and no mode of nutrition. A virus neither makes food nor takes it in, and it does not respire. It is neither autotrophic nor heterotrophic.
4. No independent reproduction. It can multiply only inside a host cell, using the host's machinery. Outside a host it remains completely inactive.
5. No ecological role in the producer / consumer / decomposer sense.

They are separated by what they have, not by what they both lack — and the decisive character is the presence of vascular tissue. Lacking flowers and seeds is a shared absence, and an absence is a weak character; it groups plants that may be quite unlike each other.

Genus. A genus has far fewer members than a class, and its members share far more features.

Worked example, using the tiger from Fig. 12.17:

Locomotion and autotrophic nutrition together do not settle it. The characters that would place it in Protista are: (1) it is a single cell, and (2) that single cell has a true, membrane-bound nucleus.

The key is the cell wall: it must be made of chitin. That single character overrides the usual "multicellular" requirement of Fungi — which is exactly the reasoning the chapter uses for yeast: "yeast is a unicellular organism, since its cell wall is made up of chitin, it has been put under fungi."

Suggested identification key

First, place all five organisms, then answer each part.

(i) Which organism clearly belongs to Kingdom Fungi, and one supporting observation

Organism Q. The supporting observation: it has a cell wall but no chlorophyll and grows on dead organic matter — that is saprophytic, absorptive nutrition. A multicellular organism with a cell wall must be Plantae or Fungi; the absence of chlorophyll rules out Plantae. The filamentous body matches the fungal mycelium described in section 12.6.3.

(ii) Which organism belongs to Kingdom Monera, and the characteristic that justifies it

Organism P. The justifying characteristic: it has no true nucleus — it is a prokaryote, and prokaryotes are Monera by the very first fork of Fig. 12.5. Its survival at high salinity and temperature also fits, since bacteria and archaea are found in hot springs and other extreme environments where most organisms cannot survive.

(iii) Criteria separating R and Q, though both are eukaryotic

Being eukaryotic only tells you which branch of Fig. 12.5 to take. The level of organisation is what parts them immediately, and mode of nutrition and cell wall material confirm it.

(iv) Why S cannot be classified using mode of nutrition alone

Because "heterotrophic" is shared by three of the five kingdoms — Animalia, Fungi and many Protista and Monera. Knowing that S depends on others for food narrows the possibilities barely at all. What actually places S is its structure: multicellular with well-differentiated tissues (so not Protista or Monera), no cell wall implied and true tissues and organs (so not Fungi, which absorb through a mycelium), and above all a backbone — which places it not merely in Animalia but in the vertebrates. Aquatic respiration in the early life stage then narrows it further, to an amphibian. Nutrition is a weak character used alone; structure is what carries the information here.

(v) The fundamental characteristic T lacks, and what this reveals

T lacks cellular organisation. It is acellular — genetic material without a cell membrane, cytoplasm, organelles or any metabolism, and it is inert outside a host. Every criterion of the five kingdom system (cell type, level of organisation, cell structure, mode of nutrition, ecological role) is a question about cells, so none of them can even be asked of T.

(vi) If classification were based only on habitat

Organisms would be grouped by where they live rather than by what they are. From this very study:

• R (a protist) and S (a vertebrate) would go together because both were collected from the freshwater pond — a single eukaryotic cell filed with a backboned animal.
• P and T could both be grouped as "found in the digestive tract of animals", putting a living prokaryote with a non-living virus.
• Q (damp soil near decaying logs) would be grouped with earthworms, insects and mosses found in the same soil, though these belong to four different kingdoms.

(vii) The new organism — Fungi or Animalia?

Place it under Fungi. The organism is multicellular, eukaryotic, lacks chlorophyll and absorbs nutrients from a host externally.