12. Patterns in Life: Diversity and Classification

Answer: (ii) Body with jointed legs

Insects belong to arthropods, which have jointed legs. Earthworms have no legs.

Answer: (iii) Presence of a cell membrane

Animal cells are bounded only by a cell membrane and have no cell wall. Sponges have mitochondria, cannot photosynthesise and have no cell wall, so the other options do not fit.

Example: a cockroach and a sparrow.

The cockroach has no backbone and has jointed legs, so it is placed among invertebrates (Arthropoda, insects). The sparrow has a backbone and feathers, so it is placed among vertebrates (birds).

Cellular organisation is found in all living organisms. Whether a cell is prokaryotic or eukaryotic, and whether the body is unicellular or multicellular, can be checked for every organism.

Xylem and phloem are found only in some plants. They cannot be used to group bacteria, fungi or animals. So cellular organisation is a more basic feature and is used at the higher (kingdom) level.

It belongs to Kingdom Protista (like Paramecium).

• It is single-celled.
• It has a well-defined nucleus, so it is a eukaryote (not Monera).
• It moves with cilia, which is seen in many protists.

• Different organisms play different roles — producers, consumers and decomposers.
• Many food chains are linked into food webs. If one species decreases, others can take its place as food.
• Decomposers recycle nutrients and keep the soil fertile.
• Pollinators help plants reproduce, and predators control the number of pests.

So greater diversity makes an ecosystem more stable and better able to recover from changes.

• Prokaryotes (bacteria) and eukaryotes (Amoeba, Euglena) would be placed together, though their cell structure is very different.
• Autotrophs and heterotrophs would be mixed.
• Yeast, a unicellular fungus, would be separated from other fungi.

The group would be too varied. It would not show real similarities or relationships.

• Viruses are acellular. They are not made of cells, but the five kingdoms are based on cell structure.
• They show life only inside a living host cell. Outside, they are inactive like non-living particles.
• They have no cell organelles and cannot carry out life processes on their own.

I would keep viruses outside the five kingdoms in a separate group of acellular entities. The kingdoms are based on cellular organisation. Viruses have no cells, so they do not fit into this system. But they must still be studied and grouped because they cause many diseases.

This shows that classification is not fixed. It changes as new organisms and new information are discovered. For example, two kingdoms grew into four and then five.

Features: no cell structure, no cell membrane or organelles, no nutrition or respiration of their own, and they reproduce only inside a host cell.

Limitation: a classification system works only for organisms that share its basic features. Nature does not always fit into neat groups. So every system has limits and needs to be updated.

The presence of vascular tissue and true body parts places pteridophytes in a separate, more advanced group.

Genus. As we go down the hierarchy (kingdom → phylum → class → order → family → genus → species), the number of members decreases and the common features increase. Genus is lower than class, so its members are fewer and more similar.

• It should be unicellular.
• It should be eukaryotic (with a well-defined nucleus).
• It should have no cell wall, or a cell wall made of cellulose.

For example, Euglena moves with a flagellum and also makes food by photosynthesis.

Key: the organism has a cell wall made of chitin, has no chlorophyll and absorbs its food (heterotrophic). If these are true, it belongs to Fungi even though it is unicellular. Yeast is an example.

Q — it has a filamentous body with a cell wall, no chlorophyll, and grows on dead organic matter.

P — it has no true nucleus (prokaryotic).

Both can be classified based on level of organisation. R is unicellular and Q is multicellular. R moves with flagella and can make food in light. Q cannot move, has no chlorophyll and absorbs food from dead matter. So R is in Protista and Q is in Fungi.

S is heterotrophic. But fungi, many protists and many bacteria are also heterotrophic. Nutrition alone cannot separate them. S must be classified using other features: multicellular body, well-differentiated tissues and a backbone. These place S in Animalia (a vertebrate, like a frog).

T lacks cellular organisation (it is acellular, like a virus). The five kingdoms are based on cell structure, so there is no place for acellular entities. This shows that classification systems have limits.

Organisms from the same place would be grouped together even if they are very different. For example, R (protist) and S (vertebrate) from the pond, or P (bacterium) and Q (fungus) found in soil. Such grouping would hide real similarities in body structure. It would give wrong ideas about relationships and make study confusing.

It should be placed under Fungi. Fungi absorb food from outside their body (from dead matter or a host). Animals take in (ingest) food and digest it inside the body. A cell wall made of chitin would confirm that it is a fungus.