The Living World

Biology is the science of life. But what exactly does it mean to be living? Defining life is not as simple as it seems — a virus, a seed and a hibernating animal all challenge a quick definition. Instead, biologists describe a set of characteristic features that living organisms show. The two defining properties are metabolism and the ability to respond and self-organise; the others are important but not unique to life.

• Growth — living things increase in mass and number of cells. In living organisms growth is from inside (intrinsic), unlike a non-living mountain or crystal which grows by addition from outside. Note that growth alone is not a defining feature, since dead cells can still swell with water.
• Reproduction — producing offspring with features similar to the parents. Yet some organisms (e.g. sterile worker bees, mules) do not reproduce, so reproduction too is not an all-defining feature.
• Metabolism — the sum of all chemical reactions occurring in the body. All living organisms show metabolism, and no non-living object does. This makes metabolism a defining feature of life. Metabolic reactions can even be carried out in a test tube, but such isolated reactions are not 'living' — life is a property of the organised whole.
• Cellular organisation — all living organisms are made of one or more cells, the basic unit of life.
• Consciousness / response to stimuli — the ability to sense the environment (light, heat, other organisms) and respond to it. This self-awareness of surroundings is considered the most important defining characteristic of living organisms.

So, living organisms are self-replicating, evolving, self-regulating interactive systems capable of responding to their environment. Properties like metabolism and consciousness are defining; growth and reproduction are characteristic but not exclusive to life.

There is an enormous diversity of living organisms on Earth — so far about 1.7–1.8 million species have been described, and many more remain unknown. The number and types of organisms present on Earth make up its biodiversity. To study this vast variety, biologists give each kind a name and arrange them into groups — this is classification.

If every organism were known only by local (common) names, there would be chaos: the same plant may have many names in different languages and regions, and the same common name may be used for different organisms. To bring order, scientists identify each organism and place it in a category.

The branch of biology that deals with classification — identification, naming and grouping organisms — is called taxonomy. The basis of classification has changed over time: from simple uses (food, shelter, medicine) to external features, and now to internal structure, cell organisation, development and evolutionary relationships. The study of the kinds and diversity of organisms and the evolutionary relationships among them is called systematics.

Classification is essential because it: makes the study of a huge variety of organisms easy and systematic; helps us understand how different groups are related; and forms the basis for the study of other biological sciences. Each distinct kind of organism that we classify is given the basic unit of classification — the species.

To give every organism a single, universally accepted scientific name, biologists use binomial nomenclature, a system given by Carolus Linnaeus. Each name has two parts: the genus (generic name) and the species (specific epithet) — for example, the mango is Mangifera indica and humans are Homo sapiens.

There are agreed rules for writing scientific names:

• Names are usually in Latin and written in italics.
• The first word (genus) begins with a capital letter; the second word (species) begins with a small letter.
• When handwritten, both words are underlined separately to show they are Latin.
• The name of the author who first described the species may be written after it (in abbreviated form), e.g. Mangifera indica Linn.

Organisms are arranged in a series of categories from the lowest to the highest — the taxonomic hierarchy. Each level is a taxon (plural: taxa). The categories, from lowest to highest, are: Species → Genus → Family → Order → Class → Phylum (or Division for plants) → Kingdom. As we go up, the number of common features decreases and the group becomes larger. The species is the basic unit — a group of individuals with fundamental similarities that can interbreed.

Biologists also use taxonomic aids to identify and study organisms, such as the herbarium (pressed, dried plant specimens), botanical gardens and zoological parks, museums, and identification keys.