Concepts: What is living and why classification matters

1.1 What the living world contains

Biology studies life in every form it takes: bacteria that survive in hot springs, moss on a damp wall, a banyan tree, a shark, a human being. The habitats are wildly different, so the question that opens the subject is a plain one. What does a living thing have that a stone does not?

1.1.1 Features that mark out living forms

Growth is the first feature most students name. In a living body, growth comes from cell division, so new cells are added and the body gets larger. A crystal or a snowball also increases in size, but by adding material on the outside. Biologists therefore treat growth as a sign of life only when it happens from inside, through multiplication of cells. Plants keep growing through their whole life, while most animals stop growing after a certain age. A dead organism does not grow, and a non-living object that gets bigger is not growing in the biological sense.

Reproduction is the second feature. Living organisms produce young ones of their own kind, either sexually or asexually. Reproduction is not needed for the survival of one individual. A mule cannot reproduce, and many people never have children, yet both are alive. The feature belongs to the species and to the line of descent, not to a single body.

Metabolism is the group of chemical reactions that run inside a living cell. Building a large molecule from smaller ones is anabolism, and breaking a large molecule into smaller ones is catabolism. Two reactions can be running in opposite directions at the same time. A seed that looks dormant still carries out slow chemical reactions inside its cells, and those reactions go on as long as the cell stays organised. That is why metabolism is treated as a defining feature of life, and why the living and the non-living are separated at the level of organised cellular chemistry.

Consciousness is the ability to sense the surroundings and respond to them. A plant bends towards light, a snail withdraws into its shell when touched, and a person flinches at a loud sound. Consciousness is the feature that separates the living from the non-living most sharply, because even a single cell responds to its environment.

1.1.2 A question that looks simple

The amoeba gives a good test case. It multiplies by mitotic division, so one individual becomes two. Is that growth or reproduction? The increase in cell number is growth of the individual, and the division of the parent into two independent cells is reproduction. The same event can be described both ways, and the chapter accepts this double reading. The point is that a definition has to fit the cell, the organism and the species at the same time.

Cellular organisation is the base on which all the other features rest. Every living organism is made of cells, and each cell has a membrane that keeps its chemistry separate from the world outside. A cell that loses that organisation stops being alive.

1.2 Diversity in the living world

Look around a garden, then a forest, then a sea shore. The number of different kinds of organisms keeps rising with the area you search. Each distinct kind of plant or animal is a species. About 1.7 to 1.8 million species have been described so far, and that figure is only the known part of the total. New species are described every year, and many small organisms are still unrecorded.

The number and variety of organisms in a place is its biodiversity. The forests of the Western Ghats, the coral reefs of the Andaman Sea and the grasslands of the Deccan each hold a different mix of species, and that mix changes with the climate and the soil.

1.2.1 Why names have to be fixed

People in different regions give the same plant different local names, and one local name may stand for two different plants. A field worker cannot build a record on such names. Two things are needed before any study can be shared. The organism must be identified correctly, which means finding out which known group it belongs to. Then it must get a standard name that every biologist in the world uses for the same organism.

1.2.2 The four processes of taxonomy

Characterisation describes the features of an organism, its form, its cells and its development. Identification decides which known taxon the specimen belongs to. Classification places the organism in a group with others that share its features. Nomenclature gives it a scientific name under a written code. These four processes together make up taxonomy.

Systematics is wider. It carries out the same work and also studies the evolutionary relationships between organisms, so it asks what descended from what. The word comes from the Latin systema, a systematic arrangement, and Linnaeus used the title Systema Naturae for his own catalogue of organisms.

1.2.3 Practice from the NCERT exercises, questions 1 to 4

Exercise 1. The exercises begin by asking why living organisms are classified. Classification makes a huge diversity manageable. It groups organisms by shared features, lets a worker place a new specimen quickly, and gives every group a name that can be used in experiments, in agriculture and in conservation.

Exercise 2. Classification systems keep changing because the evidence behind them changes. New species are found, old ones are regrouped, and molecular data now shows relationships that shape and anatomy had missed. A system is a working model of relationships, so better evidence produces a better model.

Exercise 3. A person can be classified by several criteria: age, blood group, language, home region, height or the features of the face. If the aim is medical, blood group and genetic traits matter most. If the aim is social or cultural, language and region matter. The criterion has to be chosen for the question being asked.

Exercise 4. Identification of individuals and populations tells a worker about variation inside a species. It shows which features stay the same and which change from one population to another, and that is the raw material for recognising subspecies, breeds and local varieties.