1The Living World NEET at a glance: is it a defining feature?

2What indeed is life?

“How wonderful is the living world!” Living organisms occupy extraordinary habitats: cold mountains, deciduous forests, oceans, fresh water lakes, deserts or hot springs. The beauty of a galloping horse, migrating birds, the valley of flowers or an attacking shark evokes awe and a deep sense of wonder. So do the ecological conflict and cooperation among members of a population and among populations of a community, and even the molecular traffic inside a cell. All of this makes us reflect: what indeed is life?

Two implicit questions

3Living vs inanimate: how biology began

Biology is the science of life forms and living processes. The living world comprises an amazing diversity of living organisms.

• Early man could easily perceive the difference between inanimate matter and living organisms.
• He deified some inanimate matter (wind, sea, fire etc.) and some animals and plants. The common feature was the sense of awe or fear they evoked.
• The description of living organisms, including human beings, began much later in human history.
• Societies with an anthropocentric view of biology could register only limited progress in biological knowledge.
• Systematic and monumental description of life forms brought in, out of necessity, detailed systems of identification, nomenclature and classification.
• The biggest spin-off was the recognition of the sharing of similarities among living organisms both horizontally and vertically.
• That all present-day organisms are related to each other, and to all organisms that ever lived on earth, humbled man and led to cultural movements for conservation of biodiversity.

4Diversity and the search for defining traits

Look around and you see a large variety of living organisms, and several more you cannot see with the naked eye. The larger the area you observe, the greater the range and variety, so a dense forest shows far more kinds. Each different kind represents a species. 1.7–1.8 million species are known and described; this refers to biodiversity, the number and types of organisms on earth. As we explore new areas, and even old ones, new organisms are continuously being identified.

When we try to define “living”, we conventionally look for distinctive characteristics. Growth, reproduction, and the ability to sense the environment and mount a suitable response come to mind immediately. One can add metabolism, ability to self-replicate, self-organise, interact and emergence to this list. Each one is tested below.

5Growth

How organisms grow

• A multicellular organism grows by cell division.
• In plants, growth by cell division occurs continuously throughout their life span.
• In animals, this growth is seen only up to a certain age. However, cell division occurs in certain tissues to replace lost cells.
• Unicellular organisms grow by cell division. This is easily seen in in vitro cultures by simply counting the number of cells under the microscope.

Why growth is not a defining property

If we take increase in body mass as a criterion for growth, non-living objects also grow. Mountains, boulders and sand mounds do grow. This growth, however, is by accumulation of material on the surface. In living organisms, growth is from inside. Growth, therefore, cannot be taken as a defining property of living organisms.

6Reproduction

In multicellular organisms, reproduction refers to the production of progeny possessing features more or less similar to those of the parents. Invariably and implicitly we refer to sexual reproduction. Organisms reproduce by asexual means also.

Growth has been defined as an increase in cell number or mass. So in single-celled organisms we are not very clear about the usage of the two terms, growth and reproduction.

7Metabolism

All living organisms are made of chemicals. These chemicals, small and big, belonging to various classes, sizes and functions, are constantly being made and changed into some other biomolecules. These conversions are chemical reactions, or metabolic reactions. Thousands of them occur simultaneously inside all living organisms, unicellular or multicellular. All plants, animals, fungi and microbes show metabolism.

Cell-free systems

Metabolic reactions can be demonstrated outside the body in cell-free systems.

8Cellular organisation

A metabolic reaction can run outside the body, yet the test tube does not become alive. The organisation of reactions within the body’s cells is what life requires. The conclusion follows directly:

9Consciousness: sensing and responding

• We sense our environment through our sense organs.
• Plants respond to external factors like light, water, temperature, other organisms, pollutants, etc.
• All organisms, from the prokaryotes to the most complex eukaryotes, can sense and respond to environmental cues.
• Photoperiod affects reproduction in seasonal breeders, both plants and animals.
• All organisms handle the chemicals entering their bodies.
• All organisms, therefore, are “aware” of their surroundings.
• Human being is the only organism who is aware of himself, i.e. has self-consciousness.

10The coma case

When it comes to human beings, it is all the more difficult to define the living state. We observe patients lying in coma in hospitals, virtually supported by machines which replace heart and lungs. The patient is otherwise brain-dead and has no self-consciousness. Are such patients, who never come back to normal life, living or non-living?

11Defining vs non-defining features

A feature is defining when it is present in all living organisms and absent in all non-living things.

12Emergent properties and the definition of living

All living phenomena are due to underlying interactions:

• Properties of tissues are not present in the constituent cells; they arise from interactions among the constituent cells.
• Properties of cellular organelles are not present in their molecular constituents; they arise from interactions among the molecular components of the organelle.
• These interactions result in emergent properties at a higher level of organisation. This holds true in the hierarchy of organisational complexity at all levels.

Biology is the story of life on earth: the story of the evolution of living organisms. All living organisms, present, past and future, are linked to one another by the sharing of common genetic material, but to varying degrees.

13Need for naming: nomenclature and identification

There are millions of plants and animals in the world; we know the plants and animals in our own area by their local names. These local names would vary from place to place, even within a country, and would create confusion if we did not find ways and means to refer to the organisms we are talking about.

Hence, there is a need to standardise the naming of living organisms such that a particular organism is known by the same name all over the world. This process is called nomenclature. Nomenclature is only possible when the organism is described correctly and we know to what organism the name is attached to. This is identification.

International codes

14Binomial nomenclature

Each scientific name has two components: the generic name and the specific epithet. This system of providing a name with two components is called binomial nomenclature. It was given by Carolus Linnaeus and is practised by biologists all over the world; the two-word format was found convenient.

Universal rules of nomenclature

1. Biological names are generally in Latin and written in italics. They are Latinised or derived from Latin irrespective of their origin.
2. The first word represents the genus; the second denotes the specific epithet.
3. Both words, when handwritten, are separately underlined, or printed in italics to indicate their Latin origin.
4. The genus starts with a capital letter; the specific epithet starts with a small letter (e.g. Mangifera indica).

The name of the author appears after the specific epithet, at the end of the biological name, and is written in an abbreviated form, e.g. Mangifera indica Linn. It indicates that this species was first described by Linnaeus.

15Classification, taxa and taxonomy

Since it is nearly impossible to study all the living organisms, we need a means to make this possible. This process is classification: the process by which anything is grouped into convenient categories based on some easily observable characters.

We easily recognise groups such as plants, animals, dogs, cats or insects, and the moment we use any of these terms we associate certain characters with the organism in that group. Think of ‘dogs’ and you see dogs, not cats; think of ‘Alsatians’ and you know exactly what is meant. Say ‘mammals’ and you think of animals with external ears and body hair. Say ‘wheat’ and you picture wheat plants, not rice.

All these, ‘Dogs’, ‘Cats’, ‘Mammals’, ‘Wheat’, ‘Rice’, ‘Plants’, ‘Animals’, are convenient categories. The scientific term for these categories is taxa. Taxa can indicate categories at very different levels.

Based on characteristics, all living organisms can be classified into different taxa. This process of classification is taxonomy.

Earliest classifications

Human beings have always been interested in knowing more about the various kinds of organisms, particularly with reference to their own use. In early days they needed sources for food, clothing and shelter. Hence, the earliest classifications were based on the ‘uses’ of various organisms.

16Systematics

Human beings were interested not only in different kinds of organisms and their diversities, but also in the relationships among them. This branch of study was referred to as systematics. The word is derived from the Latin word ‘systema’, which means systematic arrangement of organisms. Linnaeus used Systema Naturae as the title of his publication.

17Taxonomic categories and hierarchy

Classification is not a single step process but involves a hierarchy of steps in which each step represents a rank or category. Since the category is part of the overall taxonomic arrangement, it is called the taxonomic category, and all categories together constitute the taxonomic hierarchy. Each category, referred to as a unit of classification, represents a rank and is commonly termed a taxon (pl. taxa).

Example: insects share common features like three pairs of jointed legs. Insects are recognisable concrete objects which can be classified, and thus were given a rank or category. Groups represent category; category denotes rank; each rank or taxon represents a unit of classification. These taxonomic groups are distinct biological entities and not merely morphological aggregates.

Taxonomical studies of all known organisms have led to the common categories: kingdom, phylum or division (for plants), class, order, family, genus and species. All organisms, including those in the plant and animal kingdoms, have species as the lowest category. To place an organism in these categories, the basic requirement is knowledge of its characters, which helps identify similarities and dissimilarities.

Species

A group of individual organisms with fundamental similarities is a species. One should be able to distinguish one species from closely related species based on distinct morphological differences. In Mangifera indica, Solanum tuberosum (potato) and Panthera leo (lion), the words indica, tuberosum and leo are specific epithets; Mangifera, Solanum and Panthera are genera, a higher taxon. A genus may have one or more specific epithets: Panthera also has tigris; Solanum includes nigrum and melongena. Human beings belong to the species sapiens in the genus Homo: Homo sapiens.

Genus

A group of related species which has more characters in common in comparison to species of other genera; genera are aggregates of closely related species. Potato and brinjal are different species of Solanum. Lion (Panthera leo), leopard (P. pardus) and tiger (P. tigris) are species of Panthera, which differs from the genus Felis (cats).

Family

A group of related genera with still less number of similarities as compared to genus and species. Plant families are characterised on the basis of both vegetative and reproductive features. Solanum, Petunia and Datura are in family Solanaceae. Panthera and Felis are in family Felidae. Cat and dog show some similarities and some differences, so they are in Felidae and Canidae respectively.

Order

Order and higher categories are identified on the basis of aggregates of characters. An order is an assemblage of families which exhibit a few similar characters, fewer than among genera of a family. Plant families Convolvulaceae and Solanaceae are in the order Polymoniales, mainly on floral characters. The animal order Carnivora includes Felidae and Canidae.

Class

Includes related orders. Order Primata (monkey, gorilla, gibbon) is placed in class Mammalia along with order Carnivora (tiger, cat, dog). Class Mammalia has other orders also.

Phylum (Division in plants)

Classes such as fishes, amphibians, reptiles, birds and mammals constitute the phylum. Based on common features like the presence of notochord and dorsal hollow neural system, they are included in phylum Chordata. In plants, classes with a few similar characters are assigned to a higher category called Division.

Kingdom

All animals of various phyla are in the highest category, Kingdom Animalia. Kingdom Plantae is distinct and comprises all plants from various divisions. Taxonomists have also developed sub-categories in this hierarchy to facilitate more sound and scientific placement of taxa.

Every textbook example at a glance

18Table 1.1, memory hook and Ernst Mayr

Use the mnemonic to fix the order of the ranks, then learn Table 1.1 row by row: many questions are set directly from it.

Table 1.1 Organisms with their Taxonomic Categories

Table 1.1 indicates the taxonomic categories to which some common organisms like housefly, man, mango and wheat belong.

MCQs on Table 1.1

19Chapter summary and textbook exercises

Exercises (tap for model answers)

20High-yield NCERT lines

1. Biology is the science of life forms and living processes.
2. “What indeed is life?” has a technical question (living vs non-living) and a philosophical question (purpose of life); scientists do not attempt the second.
3. Societies with an anthropocentric view of biology registered limited progress.
4. Species known and described: 1.7–1.8 million.
5. Increase in mass and increase in number of individuals are twin characteristics of growth.
6. Plants grow throughout life; in animals growth is seen up to a certain age, and cell division in certain tissues replaces lost cells.
7. In majority of higher animals and plants, growth and reproduction are mutually exclusive events.
8. Mountains, boulders and sand mounds grow by accumulation of material on the surface; living organisms grow from inside.
9. Yeast and Hydra: budding. Planaria: true regeneration. Fungi, filamentous algae, protonema of mosses: fragmentation.
10. In bacteria, unicellular algae and Amoeba, reproduction is synonymous with growth.
11. Mules, sterile worker bees and infertile human couples do not reproduce.
12. Metabolism is the sum total of all chemical reactions in the body; no non-living object exhibits it.
13. Isolated metabolic reactions in vitro are not living things but surely living reactions.
14. Cellular organisation of the body is the defining feature of life forms.
15. Photoperiod affects reproduction in seasonal breeders, both plants and animals.
16. Only human beings have self-consciousness; consciousness is the defining property of living organisms.
17. Living organisms are self-replicating, evolving and self-regulating interactive systems capable of responding to external stimuli.
18. Local names vary from place to place, even within a country; hence standardised naming (nomenclature).
19. Nomenclature is only possible when the organism is described correctly: identification.
20. Plants: ICBN. Animals: ICZN.
21. Binomial nomenclature: generic name + specific epithet; given by Carolus Linnaeus.
22. Genus starts with a capital, specific epithet with a small letter; handwritten words underlined separately.
23. Mangifera indica Linn.: author name abbreviated, after the epithet.
24. Characterisation, identification, classification and nomenclature are basic to taxonomy.
25. Earliest classifications were based on ‘uses’ of organisms.
26. Systematics (Latin ‘systema’) takes into account evolutionary relationships; Linnaeus wrote Systema Naturae.
27. Taxonomic groups are distinct biological entities and not merely morphological aggregates.
28. Species is the lowest category in all organisms.
29. Plant families: vegetative + reproductive features; Polymoniales grouped on floral characters.
30. Chordata: notochord and dorsal hollow neural system.
31. Taxonomic studies are useful in agriculture, forestry and industry.

21MCQs: what is living

22MCQs: nomenclature, taxonomy and hierarchy

23Common mistakes

• Calling growth or reproduction a defining feature.
• Saying non-living things never grow. Mountains, boulders and sand mounds do, by surface accumulation.
• Calling an in vitro reaction a “living thing”. It is a living reaction.
• Swapping examples: yeast/Hydra = budding; Planaria = true regeneration.
• Forgetting that plants also sense and respond (light, water, temperature, other organisms, pollutants).
• Saying cell division stops completely in adult animals.
• Thinking all organisms have self-consciousness. Only human beings do.
• Writing Mangifera Indica (capital epithet) or underlining both words with one line.
• Calling taxonomy and systematics identical: systematics adds evolutionary relationships.
• Confusing ascending (species first) with descending (kingdom first) order.
• Using ‘phylum’ for plants: plants use Division.
• Placing order Primata or Carnivora in the wrong class: both are in Mammalia.
• Mixing up families: dog is Canidae, cat is Felidae, housefly is Muscidae, wheat is Poaceae.

24Common doubts