The Living World
What Is ‘Living’?
Defining 'living' can be challenging. While we easily recognise living organisms, finding a universal definition that unequivocally distinguishes living from non-living is complex. Living organisms exhibit a set of characteristics that are not found in non-living things.
Characteristics of Living Organisms
Some of the common characteristics exhibited by living organisms are:
- Growth: Increase in mass and increase in number of individuals.
- In multicellular organisms, growth occurs by cell division.
- In plants, growth by cell division occurs continuously throughout their lifespan.
- In animals, growth occurs up to a certain age. However, cell division occurs in certain tissues to replace lost cells.
- Growth is also an increase in mass. Increase in body mass is considered as growth.
- Growth can also occur in non-living objects, such as mountains, boulders, and sand dunes. This growth is by accumulation of material on the surface.
- In living organisms, growth is from inside.
Conclusion regarding Growth: Growth by increase in mass and growth by increase in number are characteristics of living organisms. However, growth is not a defining property of living organisms, as non-living things can also grow by external accumulation. A dead organism does not grow.
- Reproduction: The process by which living organisms produce new individuals of their own kind.
- Reproduction is characteristic of living organisms.
- Organisms reproduce by asexual or sexual means.
- Fungi multiply by asexual spores. Yeast and Hydra bud. Planaria (flatworms) regenerate (fragmented organism regenerates the lost part and becomes a new organism - true regeneration).
- Many organisms do not reproduce. Examples: Mules, sterile worker bees, infertile human couples. These are living but cannot reproduce.
Conclusion regarding Reproduction: Reproduction is a characteristic of living organisms, but it is not a defining property of all living organisms, as there are living organisms that cannot reproduce.
- Metabolism: The sum total of all chemical reactions occurring within a living organism.
- All living organisms are made of chemicals. These chemicals, small or big, belonging to various classes, molecular weights, etc., are constantly being made and changed into some other biomolecules. These conversions are chemical reactions.
- Thousands of metabolic reactions occur simultaneously inside all living organisms (unicellular and multicellular).
- No non-living object exhibits metabolism.
- Metabolic reactions can be demonstrated outside the body in cell-free systems (e.g., a test tube). An isolated metabolic reaction outside the body is neither living nor non-living, but is certainly a living reaction.
Conclusion regarding Metabolism: Metabolism is a defining property of all living organisms without exception. Isolated metabolic reactions in vitro are living reactions, but not living things.
- Cellular Organisation: Living organisms are composed of cells.
- Cells are the basic structural and functional units of life.
- All living organisms, from the simplest bacteria to the most complex animals and plants, are made up of cells.
- Non-living things do not have cellular organisation.
Conclusion regarding Cellular Organisation: Cellular organisation of the body is the defining feature of all living organisms.
- Consciousness: The ability of living organisms to sense their surroundings or environment and respond to these environmental stimuli.
- Stimuli can be physical, chemical, or biological.
- Organisms sense their environment through sensory organs.
- Plants also respond to external factors like light, water, temperature, other organisms, pollutants, etc.
- All organisms, from prokaryotes to eukaryotes, can sense and respond to environmental cues.
- Humans are the only organisms with self-consciousness (awareness of oneself).
Conclusion regarding Consciousness: Consciousness is the defining property of living organisms. Self-consciousness is a defining property only of humans.
- Other characteristics: Growth, development, ageing, death, adaptation, movement, homeostasis are also characteristics of living organisms, but are consequences of underlying metabolic and cellular processes and are not considered fundamental defining properties in the same way as metabolism, cellular organisation, and consciousness.
Summary of Defining Properties
Based on the above analysis, the defining properties of living organisms are:
- Metabolism
- Cellular Organisation
- Consciousness
Growth and Reproduction are characteristics, but not defining properties, because they have exceptions or can be exhibited by non-living entities (in a different context).
The living world is incredibly diverse, with millions of different types of organisms. Each organism is unique, yet they share these fundamental defining characteristics of life.
Diversity In The Living World
The Earth is home to an enormous variety of living organisms. This variety is called biodiversity. Millions of species of plants, animals, microbes, and fungi exist around us, in diverse habitats like forests, oceans, deserts, mountains, and even inside other organisms.
The estimated number of known and described species ranges between 1.7 to 1.8 million. This refers to the number of species identified and named to date. Many more species are yet to be discovered and described.
Need for Classification
With such a vast number of organisms, it is impossible to study every single one individually. To make the study of life manageable, scientists have developed systems for grouping and classifying organisms based on their characteristics. This process is called classification.
Classification helps in:
- Organising the vast diversity of life into manageable categories.
- Identifying new organisms.
- Studying the relationships among different groups of organisms.
Nomenclature and Identification
For classification to be effective, organisms need to be identified and named. This leads to two important processes:
- Identification: Determining if a newly discovered organism is the same as any known organism, or if it is a new species. This involves comparing its characteristics with those of known organisms.
- Nomenclature: Providing a standardised name to an organism so that it is known by the same name all over the world. There are specific scientific rules and criteria for naming organisms. For plants, naming is based on the principles of the International Code of Botanical Nomenclature (ICBN). For animals, it's based on the International Code of Zoological Nomenclature (ICZN).
The process of classification, identification, and nomenclature forms the basis of taxonomy. Taxonomy is the scientific study of classifying organisms.
Systematics is a branch of biology that deals with the classification of organisms based on their evolutionary relationships. It uses taxonomic principles along with evolutionary history to understand the diversity of life.
Taxonomic Categories
Classification is not a single-step process but involves a hierarchy of steps, in which each step represents a rank or a category. Since a category is a part of overall taxonomic arrangement, it is called a taxonomic category, and all categories together constitute the taxonomic hierarchy.
Each category, in fact, represents a unit of classification. These categories are distinct biological entities and not merely morphological aggregates. There are seven obligate (essential) taxonomic categories, arranged in descending order of hierarchy:
Kingdom $\rightarrow$ Phylum/Division $\rightarrow$ Class $\rightarrow$ Order $\rightarrow$ Family $\rightarrow$ Genus $\rightarrow$ Species
(Division is used for plants, Phylum for animals)
*(Image shows a hierarchical pyramid or steps illustrating the levels from Kingdom (broadest) down to Species (most specific))*
As we go higher from species to kingdom, the number of common characteristics decreases. Lower the taxa, more are the characteristics that the members within the taxon share. Higher the category, greater is the difficulty of determining the relationship to other taxa at the same level.
Species
Species is the lowest obligate taxonomic category. It is a group of individual organisms with fundamental similarities.
According to the biological concept of species, a species is a group of individuals that can interbreed naturally to produce fertile offspring.
Example: Mangifera indica (Mango), Solanum tuberosum (Potato), Panthera leo (Lion).
In Mangifera indica, indica is the species. In Solanum tuberosum, tuberosum is the species. In Panthera leo, leo is the species.
Genus
Genus comprises a group of related species which have more characteristics in common compared to species of other genera.
Example: Potato ($Solanum \: tuberosum$) and Brinjal ($Solanum \: melongena$) are two different species, but both belong to the same genus Solanum.
Lion ($Panthera \: leo$), Leopard ($Panthera \: pardus$), and Tiger ($Panthera \: tigris$) are all species belonging to the genus Panthera. This genus is different from the genus Felis, which includes cats.
Family
Family is a group of related genera with still less number of similarities as compared to genus and species.
Family is characterised on the basis of both vegetative and reproductive features of plant species.
Example: Genus Solanum, Petunia, and Datura are placed in the family Solanaceae.
Genus Panthera (Lion, Tiger, Leopard) and genus Felis (Cats) are included in the family Felidae.
In plants, the family Felidae (cats) is included in the order Carnivora.
In animals, the family Felidae is included in the order Carnivora.
Order
Order is a group of related families exhibiting a few similar characters. Characters are less general as compared to Family.
Example: Plant families like Convolvulaceae and Solanaceae are included in the order Polymoniales based on floral characters.
Animal families like Felidae (cats) and Canidae (dogs) are included in the order Carnivora.
Class
Class is a group of related orders.
Example: Order Primata (monkeys, gorillas, gibbons) and Order Carnivora (tiger, cat, dog) are included in the Class Mammalia.
Class Mammalia includes animals that possess hair on the skin and mammary glands.
Phylum (for animals) / Division (for plants)
Phylum is a group of related classes. It is the category just below the Kingdom.
Example: Class Mammalia, along with other classes like Pisces (fishes), Amphibia (amphibians), Reptilia (reptiles), and Aves (birds), are included in the Phylum Chordata. All these classes share common features like the presence of a notochord and a dorsal hollow neural system.
In plants, related classes are placed in a Division (e.g., Angiosperms, Gymnosperms).
Kingdom
Kingdom is the highest taxonomic category. It is a group of related phyla (or divisions).
All animals are included in the Kingdom Animalia. All plants are included in the Kingdom Plantae.
Example: Kingdom Animalia includes all phyla from Porifera to Chordata.
Taxonomic Hierarchy with Examples:
| Taxonomic Categories | Mango ($Mangifera \: indica$) | Potato ($Solanum \: tuberosum$) | Human ($Homo \: sapiens$) | Housefly ($Musca \: domestica$) |
|---|---|---|---|---|
| Kingdom | Plantae | Plantae | Animalia | Animalia |
| Division/Phylum | Angiospermae (Division) | Angiospermae (Division) | Chordata (Phylum) | Arthropoda (Phylum) |
| Class | Dicotyledonae | Dicotyledonae | Mammalia | Insecta |
| Order | Sapindales | Polymoniales | Primata | Diptera |
| Family | Anacardiaceae | Solanaceae | Hominidae | Muscidae |
| Genus | Mangifera | Solanum | Homo | Musca |
| Species | indica | tuberosum | sapiens | domestica |
Taxonomists have also developed sub-categories (like sub-species, sub-genus, sub-family, etc.) to make the taxonomic hierarchy more precise.
Taxonomical Aids
Taxonomical aids are techniques, procedures, and stored information useful in identification and classification of organisms. These studies require correct identification of organisms. Collection of actual specimens of plants and animal species is essential for taxonomic studies.
Some important taxonomical aids are:
- Herbarium
- Botanical Gardens
- Museums
- Zoological Parks
- Key
Other aids include flora, manuals, monographs, and catalogues.
Herbarium
- A herbarium is a store house of collected plant specimens that are dried, pressed, and preserved on sheets.
- These sheets are then arranged according to a universally accepted system of classification (e.g., Bentham and Hooker's system).
- Herbarium sheets carry a label providing information about the date and place of collection, English, local, and botanical names, family, collector's name, etc.
- Herbaria serve as quick referral systems for taxonomic studies. By examining a herbarium sheet, a taxonomist can quickly learn about a specific plant species and compare new specimens.
*(Image shows several dried and pressed plant specimens mounted on paper, possibly with labels)*
Botanical Gardens
- These are specialised gardens where living plants are collected and grown for identification purposes and study.
- Plants in these gardens are grown for identification purposes, and each plant is labelled with its botanical/scientific name and its family.
- Examples of famous botanical gardens include:
- Royal Botanical Garden, Kew, England.
- Indian Botanical Garden, Howrah, India.
- National Botanical Research Institute (NBRI), Lucknow, India.
*(Image shows a well-maintained garden area with various plants and labels)*
Museum
- Biological museums are generally set up in educational institutes like schools and colleges.
- Museums contain collections of preserved plant and animal specimens for study and reference.
- Specimens are preserved in jars containing preservative solutions (like formalin).
- Plant and animal specimens may also be preserved as dry specimens.
- Insects are preserved in insect boxes after collecting, killing, and pinning.
- Larger animals like birds and mammals are usually stuffed and preserved.
- Museums often have collections of animal skeletons as well.
*(Image shows jars containing preserved biological specimens on shelves in a museum setting)*
Zoological Parks
- These are places where wild animals are kept in protected environments under human care.
- This allows for study of their behaviour, habitat, food habits, and overall morphology in a semi-natural setting.
- These parks provide opportunities to learn about different animal species.
- Examples: Various zoological parks in India, like the National Zoological Park in Delhi.
*(Image shows an animal in an enclosure within a zoo)*
Key
- Key is a taxonomical aid used for identification of plants and animals based on similarities and dissimilarities.
- Keys are based on the contrasting characters generally in a pair called a couplet.
- A couplet consists of two contrasting statements. One of these is accepted, and the other is rejected.
- Each statement in the key is called a lead.
- Separate taxonomic keys are required for each taxonomic category such as family, genus, and species, for identification purposes.
- Keys are generally analytical in nature.
Other Taxonomical Aids
- Flora: Contains the actual account of habitat and distribution of plants of a given area. They provide an index to the plant species found in a particular area.
- Manuals: Provide information for identification of names of species found in an area.
- Monographs: Contain information on any one taxon (e.g., a monograph on the genus Solanum would describe all known species within that genus).
- Catalogues: Lists of species found in an area with brief descriptions.
These taxonomical aids are crucial for conducting research, identification, and conservation of biodiversity.
Nomenclature
Nomenclature is the process of giving scientific names to organisms. It is essential for studying and communicating about the vast diversity of living beings. A universally accepted system of naming ensures that a particular organism is known by the same name throughout the world, avoiding confusion caused by local names.
Need for Standardized Naming
Common names vary from place to place and even within the same region. A single organism may have many local names, and one local name might be used for different organisms in different areas. For example, 'Gulmohur' is a common name for several different flowering trees in India. 'Sparrow' might refer to different bird species in different countries.
To overcome this ambiguity, a standardised system of naming is followed by biologists all over the world.
Binomial Nomenclature
The most widely accepted system of naming is Binomial Nomenclature, first given by Carolus Linnaeus in his book "$Species \: Plantarum$" (for plants) and "$Systema \: Naturae$" (for animals).
According to Binomial Nomenclature, every scientific name has two components:
- Generic name: Represents the genus.
- Specific epithet: Represents the species.
Example: The scientific name of mango is Mangifera indica. Here, Mangifera is the generic name, and indica is the specific epithet.
Universal Rules of Nomenclature
Biologists follow universally accepted rules for naming organisms, which are governed by International Codes:
- International Code of Botanical Nomenclature (ICBN) for plants.
- International Code of Zoological Nomenclature (ICZN) for animals.
The main rules of Binomial Nomenclature are:
- Biological names are generally in Latin and written in italics. They are Latinised or derived from Latin irrespective of their origin.
- The first word in a biological name represents the Genus, while the second component denotes the specific epithet.
- Both the words in a biological name, when handwritten, are separately underlined. When printed, they are written in italics to indicate their Latin origin.
- The first word (Genus) starts with a capital letter, while the specific epithet starts with a small letter.
Example: Mangifera indica (correct handwritten format: Mangifera indica)
- The name of the author (who first described the species) is written in an abbreviated form after the specific epithet and is not underlined.
Example: Mangifera indica Linn. (Linn. stands for Linnaeus).
Advantages of Scientific Names
- They are universally accepted and understood.
- Each organism has only one scientific name.
- They provide information about the generic relationship (organisms in the same genus are related).
- They follow a stable system (once a name is given according to the rules, it is generally not changed).
Scientific naming provides a common language for biologists worldwide and is fundamental to the study of biodiversity and classification.