The Living World

Written and reviewed by Anjali Sharma, Biology faculty at UniDrill (M.Sc. Botany, Banaras Hindu University · 8 years teaching NCERT Biology). Based on NCERT Biology Class XI, Chapter 1.

A flame grows, eats and spreads. Why is it not alive?

A candle flame takes in fuel, releases energy, gets bigger when you add more wax and can even spread to a second candle. Yet nobody calls it a living thing. This chapter starts by asking what separates the living from the non-living, and then deals with a very practical problem: Earth has about 1.7 to 1.8 million known species, so how does a scientist in Kerala and one in Japan make sure they are talking about the same organism?

The answer is a system of naming and ranking that you will use in every chapter of Biology, so it is worth getting the rules exactly right.

What makes something living?

Biology treats "what is living?" as a technical question, not a philosophical one. It does not ask why we are here. It asks what features a living thing has that a stone, a river or a flame does not. Five features are discussed, and the interesting part is that only some of them are reliable.

The one-line summary is that living organisms are self-replicating, evolving, self-regulating and interactive systems that respond to external stimuli. So the flame fails: it does not grow from inside, it has no cells and it has no metabolism of its own.

Pause and predict: a mule cannot reproduce. By the reproduction test alone, would you call it non-living?

Check your answer: No. Reproduction is not a defining feature, because many perfectly healthy living organisms never reproduce. Metabolism, cellular organisation and consciousness are the dependable markers.

Why we need scientific names

Every language has its own name for the same animal, and even within India one plant can have several local names. Common names also mislead: a silverfish is an insect and a jellyfish is not a fish. Scientists therefore needed one name per species that is the same in every country, and that no other species is allowed to use.

This is nomenclature, the standardisation of names. It works only after correct identification, which means describing an organism well enough that the right name can be attached to it. To keep names unique, two international codes exist: the International Code of Botanical Nomenclature (ICBN) for plants and the International Code of Zoological Nomenclature (ICZN) for animals.

Beyond the textbook. For the exam: plants are named under ICBN and animals under ICZN. In reality: the botanical code has been renamed the International Code of Nomenclature for algae, fungi and plants (ICN), because it also covers algae and fungi. Why it matters: write ICBN in the exam, but do not be surprised when a recent article uses ICN.

Binomial nomenclature: two words, strict rules

The system was introduced by Carolus Linnaeus and is used by biologists everywhere. Every organism gets a two part name made of the genus (generic name) and the specific epithet. Together they name the species. Mango is Mangifera indica, where Mangifera is the genus and indica is the epithet.

The universal rules are short, and CUET loves to test them as "which of these is written correctly".

There is one more habit that is not numbered among the four rules. The author's name is written at the end of the full name, in abbreviated form and not in italics. In Mangifera indica Linn., "Linn." tells you that Linnaeus first described the species.

Pause and predict: a newspaper prints "Panthera Leo" in italics. What is wrong with it?

Check your answer: the epithet must begin with a small letter, so it should be Panthera leo. The capital letter is for the genus only, even if the epithet is a place or a person's name.

Classification, taxonomy and systematics

Once you can name and identify an organism, you can group it. Classification is grouping organisms into convenient categories based on easily observable characters. Each such group, at any level, is a taxon (plural taxa). "Plants", "wheat", "mammals" and "dogs" are all taxa, at very different levels.

Taxonomy is the process of classifying all living organisms into taxa, using external and internal structure, cell structure, development and ecological information. Four processes are basic to it: characterisation, identification, classification and nomenclature. The earliest classifications were not scientific in this sense. They were based on the uses of organisms, such as food, clothing and shelter. Relationships came later. Taxonomy is useful in agriculture, forestry and industry, and for knowing our bio-resources and their diversity.

Systematics is the wider study. The word comes from the Latin systema, and Linnaeus used it in the title of his book Systema Naturae. Its scope was later enlarged to include identification, nomenclature and classification, and it also takes into account the evolutionary relationships between organisms. So whenever a question says "evolutionary relationships", the answer is systematics.

The seven ranks

Taxa are stacked in a ladder of ranks called the taxonomic hierarchy. Species is the lowest rank for every organism, and kingdom is the highest. Going up, each rank is a group of related groups from the rank below.

The ladder in one line: Species → Genus → Family → Order → Class → Phylum or Division → Kingdom. A mnemonic: Small Gardens Feed Our Calm, Peaceful Kingdom.

Why does the ladder matter? As you climb from species to kingdom, the number of common characters falls. Members of one species are nearly identical. Members of one kingdom share only the broadest features. At the same time, it becomes harder to work out how two higher taxa at the same level are related, which is why classification gets more complex towards the top.

Why it works this way. Think of addresses. A house number tells you very little about a neighbour, but "same street" tells you a lot and "same country" tells you almost nothing. A rank is the same idea: the lower the rank, the more specific and the more shared detail.

Beyond the textbook. For the exam: a species is a group of organisms with fundamental similarities, separated from close relatives by distinct morphological differences. In reality: many biologists define a species by whether its members can interbreed and produce fertile offspring, an idea strongly associated with Ernst Mayr. This is why a horse and a donkey are different species even though they can produce a mule, which is sterile. The interbreeding definition does not work for organisms that reproduce only asexually, such as bacteria, so no single definition fits all life. Why it matters: the morphological definition is the one to write in the exam, but the interbreeding idea explains why species are real units and not just a human convenience.

Beyond the textbook. For the exam: the order containing Convolvulaceae and Solanaceae is Polymoniales, and the order of humans is Primata. In reality: modern plant classification places both families in the order Solanales, and the primate order is spelled Primates. Why it matters: spell and answer exactly as given here in the exam, because the options are built from these spellings.

Worked placements to memorise

Four organisms are worked through the whole ladder below. A match-the-column question is almost certain to come from here.

The genus is already inside the species name: Homo, Musca, Mangifera and Triticum. Notice that the plants use Division (Angiospermae) where animals use Phylum, and that mango and wheat share the same division but differ in class.

Pause and predict: a lion (Panthera leo) and a domestic cat (Felis) are in different genera. Are they in different families too?

Check your answer: No. Both genera sit in family Felidae. They differ at genus level and agree at family level, because rank goes up from genus to family, not down.

Exam traps

60-second revision

Where this chapter connects

• Next: Biological Classification applies this hierarchy and explains how the five kingdoms were separated.
• Later in Unit 1: Plant Kingdom and Animal Kingdom fill in the divisions and phyla that sit just below the kingdom rank here.
• Class XII link: Evolution builds on the idea in systematics that classification should reflect evolutionary relationships.