NCERT Solutions Class 9 Science Ch 12 Classification 2026-27

Biodiversity is the variety of living organisms found on the Earth, including plants, animals and microorganisms, together with the ecosystems in which they live. It exists at three levels:

1. genetic diversity,
2. species diversity, and
3. ecosystem diversity.

It is important because it keeps the ecological balance and supports food chains, nutrient cycling and the stability of the environment.

Grouping organisms (classification) divides the huge variety of living things into smaller, manageable groups based on their similarities and differences. This makes organisms easier to study and identify, and it helps us understand the evolutionary relationships among them. So classification gives a systematic way to understand the variety of life on the Earth.

They are classified on the basis of similarities and differences in their characteristics, such as cell structure, body organisation and mode of nutrition.

• Plants are grouped by features such as the presence of vascular tissues and seeds.
• Animals are grouped by their body structure and level of organisation.

Classification helps farmers to identify crop plants, pests and useful organisms. By knowing the groups to which they belong, farmers can choose suitable crops, recognise harmful pests and use proper methods to control them. This improves the crop yield and helps in managing farming practices.

Yes. Organisms with many common features are likely to have evolved from a common ancestor. The more features they share, the closer is their evolutionary relationship. This helps us to understand evolution and diversity.

A single-celled organism does all its life processes within one cell, using its specialised organelles. Each organelle does a particular job, such as respiration, digestion or excretion.

In multicellular organisms these functions are divided among many specialised cells and tissues, and a large body needs many cells to be efficient. A unicellular organism is very small and simple, so one cell is enough to carry out all its functions.

These are features of the early land plants (bryophytes and pteridophytes).

• A cuticle on the body that reduces the loss of water.
• Rhizoids (in bryophytes) or roots (in pteridophytes) that absorb water from the soil.
• Vascular tissues (xylem and phloem) in pteridophytes, which carry water and food to all parts of the plant.

Still, these plants need moist conditions, because their male gametes swim in water to reach the female gamete, so fertilisation needs water. That is why mosses and ferns grow in moist and shady places.

In a tall plant many cells are far from the source of water and nutrients, and simple diffusion is too slow to carry water, minerals and food over long distances. So specialised tissues are needed:

• Xylem carries water and minerals.
• Phloem carries food.

They distribute substances throughout the plant, which supports its growth, survival and proper functioning.

• The seed coat protects the embryo.
• Fruits help dispersal by wind, water or animals, so plants can grow in new and suitable places and compete less with the parent plant.
• Seeds can stay dormant in unfavourable conditions and germinate when conditions become suitable.

So seeds and fruits help the plant species to survive and spread.

Correct option: (a) Body with jointed legs

Insects have jointed legs and a segmented body divided into head, thorax and abdomen. An earthworm has a cylindrical body and no jointed legs.

Correct option: (c) Presence of a cell membrane

Sponge cells have a cell membrane and no cell wall, which is a defining feature of animal cells. Sponges are also heterotrophic and do not photosynthesise. These cellular features place them in Kingdom Animalia.

Example: a dog and a sparrow.

Animals with similar features are grouped together, so observing features like these helps us to identify organisms and place them in the right groups.

• Cellular organisation is a basic feature found in all living organisms. It separates organisms into major groups: prokaryotes and eukaryotes, and unicellular and multicellular forms.
• Xylem and phloem are found only in higher plants, so they can group only a small part of living things.

So cellular organisation is broader and reflects deeper evolutionary relationships, which makes it a more fundamental basis.

It most likely belongs to Kingdom Protista.

• It is single-celled (unicellular).
• It has a well-defined nucleus, so it is a eukaryote.
• Cilia are used for movement, a feature of many protists (for example, Paramecium).

Different species play different roles, such as producers, consumers and decomposers. Together they form food webs that keep the flow of energy and the cycling of nutrients going.

With greater biodiversity, an ecosystem can withstand changes in the environment better, because if one species is lost, others can take over its role. So biodiversity gives the ecosystem proper functioning and long-term stability.

Important differences would be ignored:

• Prokaryotic unicellular organisms (like bacteria) have no true nucleus, while eukaryotic ones (like Amoeba) have a well-defined nucleus.
• They also differ in the mode of nutrition, cell structure and reproduction.

Such a group would be unnatural and not very meaningful, because it would not show their true evolutionary relationships.

• Viruses are acellular: they are not made of cells and lack cellular organisation, which is a basic criterion of the five-kingdom system.
• Outside a host they behave like non-living particles and do not carry out metabolism on their own.
• Inside a living host they reproduce, which is a living feature.

Because they are on the borderline between living and non-living, they are not included in any kingdom.

This is an open question; a well-justified answer is: I would keep viruses outside the five kingdoms, rather than make a separate kingdom.

• Viruses are acellular and do not show all the characteristics of living things.
• They are inactive outside a host and become active only inside living cells, so they are different from every organism in the five kingdoms.

This shows that scientific classification is not fixed but keeps changing. As new information and discoveries come, classification systems are revised and improved.

• They are acellular and have no cytoplasm or organelles.
• They cannot carry out metabolic activities on their own.
• Outside a host they behave like non-living particles, but inside a host they reproduce like living organisms.

This shows that classification systems have limitations, because they cannot easily include organisms with mixed or unusual characteristics. It also shows that classification keeps changing with new scientific knowledge.

So the presence or absence of vascular tissues, with the level of body organisation, is the key feature that places them in separate groups.

A genus has fewer members but more features in common than a class.

Classification goes from broader groups to more specific ones. A class contains many different organisms that share only a few features. A genus contains closely related species that share a large number of features. So as we move from class to genus, the number of organisms becomes smaller but the similarity among them becomes greater.

The characters that identify a protist are a unicellular body and a well-defined nucleus (a eukaryotic cell).

Locomotion and autotrophic nutrition alone are not enough (for example, Euglena shows both). Protists are single-celled eukaryotes that often show both plant-like and animal-like features. So a unicellular, eukaryotic body confirms that the organism belongs to Protista.

The identification key is:

• Heterotrophic nutrition by absorption (saprophytic or parasitic) and no photosynthesis,
• a cell wall made of chitin.

Most fungi are multicellular, but some, like yeast, are unicellular eukaryotes. A unicellular eukaryote with a chitin cell wall and heterotrophic nutrition belongs to Kingdom Fungi, unlike protists, which may photosynthesise.

(i) Organism Q belongs to Kingdom Fungi. It grows on dead organic matter (saprophytic nutrition), has a cell wall and no chlorophyll.

(ii) Organism P belongs to Kingdom Monera. It has no true nucleus (a prokaryotic cell), which is the main feature of Monera.

(iii) R and Q: both are eukaryotes, but they are separated by the cellular organisation and the mode of nutrition.

(iv) S cannot be classified by the mode of nutrition alone, because many organisms from different kingdoms feed in the same way (heterotrophic). Features such as a multicellular body, a backbone and well-differentiated tissues are more reliable, and they place S in Kingdom Animalia.

(v) T lacks cellular organisation (it is acellular). This shows that classification systems have limitations, because they cannot fit entities like viruses that show both living and non-living features.

(vi) If only the habitat were used, organisms that live in the same place would be grouped together even though they are very different. For example, in the pond, the unicellular R, the multicellular S and bacteria like P would be put in one group. This would ignore cell structure, body organisation and evolutionary relationships, so the classification would be unscientific and misleading.

(vii) It should be placed in Kingdom Fungi. It is eukaryotic and multicellular, it has no chlorophyll, and it takes in food by absorption. These are the defining features of fungi. Animals are different, because they ingest their food.