The Living World

If someone hands you a mango sapling and a plastic model of a mango sapling, you can tell them apart in a second. But if you are asked to write down the rule you used, the job suddenly gets difficult. Every property people usually suggest turns out to fit some non-living things too, or to leave out some living things. That is exactly why this chapter starts by testing each property carefully instead of simply listing them.

The properties normally put forward are growth, reproduction, metabolism, cellular organisation, ability to sense and respond to the surroundings, self-replication, self-regulation and interaction. All of these are genuinely seen in living things, so all of them are characteristics of life. The sharper question is which of them are defining.

A property is defining only if it passes two tests at once. First, it must be present in every living organism, with no exceptions. Second, it must be absent from every non-living object. A property that fails either test may still be useful for describing life, but it cannot be used as a definition. Keep this two-part test in your head and use it to justify your answers instead of reciting a list.

You will see in the next two sections that growth fails the second test, because non-living objects can also get bigger, and it partly fails the first test as well, because a fully grown adult animal or a dead organism does not grow. Reproduction fails the first test, because plenty of living organisms never reproduce in their whole lifetime. Metabolism, on the other hand, passes both tests. Cellular organisation passes both as well, and the ability to sense and respond to the environment leads us to the idea of consciousness.

One more warning before we start. Being alive is not the same as being able to move about, and it is not the same as being visible. A seed lying in a tin, a bacterium in curd, and a peepal tree in the school ground are all living. A moving toy car and a river that carves out a valley are not. Movement, size and visibility are not part of the test at all.

Growth in living organisms is judged by two things together: an increase in body mass and an increase in the number of individuals. In a multicellular organism the growth we see from outside is the result of cells dividing inside the body. In a unicellular organism, the same cell divisions increase the number of individuals, which we can watch and count in a culture in the laboratory. So in unicellular organisms such as bacteria, unicellular algae and Amoeba, growth and reproduction cannot really be separated from each other. One cell dividing into two is both growth and reproduction at the same time.

There is also a difference between plants and animals in how long growth continues. In plants, cell division at the growing regions goes on throughout life, which is why an old tree keeps adding new branches and roots. In most animals, growth of this kind happens only up to a certain age, after which cell division mainly replaces worn-out cells rather than making the animal bigger.

Now compare this with the way a non-living thing gets bigger. A heap of sand grows when more sand is blown onto it. A mound of soil grows when more soil is deposited on it. Crystals in a solution get larger when more material is laid down on their outer faces. In every one of these cases material is added on the outside. This is extrinsic growth. Living organisms grow from the inside, by dividing their own cells and building their own material. So growth is not unique to living things, and its meaning is completely different on the two sides. On top of that, a dead organism does not grow, and an adult animal that has stopped increasing in size is still very much alive. Growth therefore fails as a definition, and it is safer to say that living organisms grow from inside than to say simply that living things grow.

Reproduction is the production of new individuals of the same kind. Living organisms do it in many ways: budding in yeast and in Hydra, fragmentation followed by regeneration in Planaria, spores and broken pieces of mycelium in fungi, and sexual reproduction with the fusion of gametes in most higher plants and animals. But there are clear exceptions to the rule that living things reproduce. A mule does not reproduce. Sterile worker bees do not reproduce. Infertile human couples do not reproduce. None of them is any less alive. Because it is not present in every living organism, reproduction is characteristic of life but not an all-inclusive defining property.

Metabolism is the sum total of all the chemical reactions occurring in the body of an organism. These reactions are of two broad kinds. Anabolic reactions build larger molecules from smaller ones and generally consume energy, as when amino acids are joined into proteins. Catabolic reactions break larger molecules into smaller ones and generally release energy, as when glucose is broken down during respiration. Thousands of such reactions run at the same time inside a single cell, controlled by enzymes.

No non-living object shows metabolism. This is the property that finally separates the two worlds cleanly, so metabolism is a defining feature of living organisms without exception. But there is a careful point attached to it. If you take enzymes and substrates out of a cell and let the reaction happen in a test tube, that reaction is a real metabolic reaction, yet the contents of the test tube are not a living thing. Such isolated reactions carried out in a container are neither living nor non-living as objects, even though the reactions themselves are living reactions. This tells us that reactions on their own are not enough. They have to be housed in a cell. That is why cellular organisation of the body is described as the defining feature of life forms.

The last property is the ability to sense the surroundings and respond to them. Every organism does this in its own way. Plants respond to light, to water, to gravity, to the seasons and to the chemicals in the soil. Animals respond through sense organs to light, sound, heat, cold, touch and chemicals. Even a prokaryote handles the chemicals that enter it and reacts to what is around it, and many bacteria swim towards or away from particular chemicals. Because all organisms are aware of and respond to their environment, this awareness, called consciousness, is treated as the defining property of living organisms.

Human beings show something extra on top of this. We are aware of ourselves, which is described as self-consciousness. That raises a genuinely difficult question the chapter leaves open for you to think about: a patient lying in a coma, kept going by machines, cannot sense or respond in the ordinary way. Is such a person still living? Biology does not settle that question with a one-line rule, and the chapter is honest about the difficulty rather than pretending it does not exist.

Putting it together, at the level of the cell all living organisms are made of the same kinds of chemicals and carry out the same basic reactions. What makes them look so different is how those reactions are arranged, controlled and put together. Living organisms are self-replicating, evolving and self-regulating interactive systems that are able to respond to stimuli from outside. That description covers a bacterium and a banyan tree equally well.

Step outside for ten minutes and look properly. The plants along the road, the insects on them, the birds overhead, the fungus on a damp wall, the bacteria you cannot see at all: the number of different kinds of organisms around you is enormous. The number of species that have been described and given names so far is nearly 1.7 to 1.8 million. This variety of living organisms is called biodiversity, and it is still growing, because new organisms are constantly being discovered, especially from forests, deep soil and the sea.

You cannot study 1.8 million things one at a time. Whenever the number of items is large, human beings deal with it by putting similar things into groups. You already do this without calling it science. In a shop, medicines are kept separately from stationery. In your own bag, notes for one subject are kept together. The same idea applied to organisms is called classification: the process by which anything is grouped into convenient categories based on some easily observable characters.

The groups formed by classification are called taxa (singular: taxon). A taxon is a group of any rank at all. Mammals form a taxon, dogs form a taxon, and even a single species forms a taxon. So when a question asks you what a taxon is, do not say it means a species; say it is a group of organisms of any rank in the classification system.

Two related terms are often mixed up. Taxonomy is the study that covers characterisation, identification, classification and nomenclature of organisms. It is based on external and internal structure, along with the structure of the cell, the way the organism develops and the ecological information about the organism. Systematics is a wider term. It comes from the Latin word systema, meaning the systematic arrangement of organisms, and Linnaeus used Systema Naturae as the title of his publication. Systematics takes into account the evolutionary relationships between organisms, not just their similarities. So all taxonomy is part of systematics, but systematics asks the extra question of how the groups are related by descent.

One more distinction worth fixing early: identification means correctly recognising an organism as one that is already known and described, while classification means placing it in the right groups. You identify first, then classify, then name.

The same plant can have half a dozen different local names within India alone, and different plants can share a local name in different regions. If a student in one state writes about a plant using its local name, a researcher in another country has no reliable way of knowing which plant is meant. Nomenclature solves this by giving each organism one standardised name that scientists everywhere accept. Naming is only possible after the organism has been correctly described and identified, so identification always comes before naming.

To keep names uniform, biologists agreed on codes. Plants are named following the rules of the International Code for Botanical Nomenclature (ICBN), and animals are named following the International Code of Zoological Nomenclature (ICZN). Because these codes are separate, the same name is occasionally used once for a plant and once for an animal without either being wrong.

The system itself is binomial nomenclature, given by Carolus Linnaeus. Each name has exactly two words. The first word is the genus and the second is the specific epithet. In Mangifera indica, the mango, Mangifera is the genus and indica is the specific epithet. Be careful with the wording: the second word is not the species; the two words taken together give the species name.

The universal rules you must be able to write out are these. Biological names are generally in Latin and are written in italics, and they are Latinised or derived from Latin whatever their actual origin. The first word represents the genus while the second denotes the specific epithet. When the name is handwritten, the two words are underlined separately, and when printed, they are set in italics, both of which show the Latin origin of the name. The first word, the genus, begins with a capital letter, while the specific epithet begins with a small letter. Written correctly, the mango is Mangifera indica and the potato is Solanum tuberosum.

There is one more convention. The name of the author who first described the species is added after the specific epithet, at the end of the name, in an abbreviated form and not in italics. In Mangifera indica Linn., the abbreviation Linn. tells us that Linnaeus first described this species. The third part of such a name is never a family and never a place; it is always the author.

Two habits are worth building. First, never capitalise the specific epithet, even when the name honours a person or a country. Second, never underline the two words with a single continuous line; underline each word on its own.

Classification is not a single step. Each organism is placed in a series of groups, each contained inside a larger one, and this arrangement is the taxonomic hierarchy. The seven obligate categories, from lowest to highest, are species, genus, family, order, class, phylum (or division in plants) and kingdom. Every organism you study must be placed in all seven.

Species is the lowest category in this list. Taxonomic studies treat a group of individual organisms with fundamental similarities as a species. To decide whether two organisms are different species, a taxonomist looks for distinct morphological differences from other closely related organisms. In Panthera leo, Panthera tigris and Panthera pardus, the words leo, tigris and pardus are the specific epithets of the lion, the tiger and the leopard.

Genus is a group of closely related species that have more characters in common with one another than with species of other genera. Potato, tomato and brinjal are three different species, yet all three are placed in the genus Solanum. Lion, tiger and leopard are three species of the genus Panthera, and Panthera differs from Felis, the genus of cats.

Family is a group of related genera, with fewer similarities among them than you find within a genus. Plant families are characterised on the basis of both vegetative and reproductive features. The three genera Solanum, Petunia and Datura are placed in the family Solanaceae. Among animals, the genera Panthera and Felis are placed in the family Felidae, while dogs are placed in the family Canidae.

Order brings together families that share a few similar characters, and these characters are fewer still. The plant families Convolvulaceae and Solanaceae are included in the order Polemoniales, mainly on the basis of floral characters. Among animals, the families Felidae and Canidae are both included in the order Carnivora.

Class is a group of related orders. The class Mammalia includes the order Primata, which contains monkey, gorilla and gibbon, along with the order Carnivora. Phylum groups related classes: fishes, amphibians, reptiles, birds and mammals are all placed in the phylum Chordata, because they share a notochord and a dorsal hollow nerve cord. For plants the equivalent rank is called division rather than phylum. Finally, all animals belonging to the various phyla are assigned to the highest category, the kingdom Animalia, and plants to the kingdom Plantae.

Two worked examples are worth learning in full. Man is Homo sapiens, family Hominidae, order Primata, class Mammalia, phylum Chordata. Mango is Mangifera indica, family Anacardiaceae, order Sapindales, class Dicotyledonae, division Angiospermae. Notice the pattern behind all of this: as you climb from species towards kingdom, the number of organisms in the group increases while the number of characters they share decreases, and working out the relationships between the members becomes harder.

Two terms are easy to blur. Taxonomy is the process of naming and classifying organisms on the basis of characteristics — it covers identification, nomenclature and classification. Systematics is broader: it studies the diversity of organisms and the evolutionary relationships between them. The word comes from the Latin systema, meaning systematic arrangement, and Linnaeus used it as the title of his own work, Systema Naturae.

The difference matters. Taxonomy asks what an organism is and where it goes. Systematics asks how it is related to everything else. A classification built only on visible characteristics can group organisms that merely look alike; a systematic classification tries to reflect actual descent.

This is also why the concept of species sits at the base of everything. A species is a group of organisms with fundamental similarities, distinguishable from any other closely related group by one or more characteristics. Panthera leo and Panthera tigris share the genus but are separate species; Solanum tuberosum (potato) and Solanum nigrum (makoi) likewise. Deciding where one species ends and another begins is one of the genuinely difficult problems in biology, not a formality.

Above the level of kingdom sits a further arrangement. Carl Woese proposed the three-domain system after comparing ribosomal RNA sequences, which splits the prokaryotes into two fundamentally different groups: Archaea, Bacteria and Eukarya. The finding was that archaebacteria differ from other bacteria so deeply — in cell wall chemistry, in membrane lipids, in the machinery of transcription — that placing them in one kingdom hides a division older than the one between plants and animals.

This does not replace the five-kingdom system so much as sit above it. Domain became a rank higher than kingdom, and it is a good illustration of the point that classification is a working tool. When better evidence arrives — here, molecular sequence data rather than appearance — the arrangement changes to match it.