What Is ‘Living’?

The vast range and incredible diversity of organisms in various habitats worldwide inspire wonder and curiosity about the fundamental nature of life.

Scientists primarily focus on the technical definition of life, exploring characteristics that distinguish living organisms from non-living entities.

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Key characteristics commonly associated with living organisms include:

• Growth
• Reproduction
• Ability to sense the environment and respond
• Metabolism
• Ability to self-replicate
• Self-organisation
• Interaction
• Emergence (properties arising from interactions at higher levels)

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Growth

Growth is typically characterised by an increase in mass and an increase in the number of individuals.

In multicellular organisms, growth occurs through cell division. This process is continuous throughout the lifespan in plants but usually restricted to a certain age in animals, although cell division for repair and replacement persists.

Unicellular organisms also grow via cell division, which leads to an increase in their population size. For single-celled life forms, growth is essentially synonymous with reproduction.

It's important to note that in most higher plants and animals, growth and reproduction are distinct events.

Increase in body mass is a criterion for growth. However, non-living objects like mountains, boulders, and sand mounds can also increase in mass due to external accumulation of material.

A key difference is that growth in living organisms happens from within.

Since non-living objects can exhibit growth by external accretion and dead organisms do not grow, growth alone cannot be considered a defining property applicable to all living organisms under all conditions.

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Reproduction

Reproduction is the process by which living organisms produce offspring that are similar to themselves.

Multicellular organisms primarily reproduce sexually, involving the production of progeny resembling parents.

Many organisms also employ asexual means of reproduction, such as:

• Spore formation (e.g., fungi)
• Budding (e.g., yeast, hydra)
• True regeneration (regrowth of a whole organism from a fragmented part, e.g., Planaria)
• Fragmentation (breaking into pieces, each growing into a new organism, e.g., filamentous algae, moss protonema)

In unicellular organisms like bacteria or Amoeba, reproduction (increase in cell number) is equivalent to growth.

However, several living organisms exist that do not reproduce, for example, mules, sterile worker bees, and infertile human couples.

Because not all living organisms reproduce, reproduction cannot serve as an all-inclusive defining characteristic of life.

Conversely, no non-living object possesses the inherent capability to reproduce or self-replicate.

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Metabolism

All living organisms are composed of various chemicals (biomolecules) that are constantly being synthesized and converted into other forms through chemical reactions.

The sum total of all chemical reactions occurring within an organism's body is called metabolism.

Thousands of metabolic reactions take place simultaneously in all living beings, from the simplest microbes to complex plants and animals.

Metabolism is unique to living organisms; no non-living object exhibits metabolic processes.

While isolated metabolic reactions can be demonstrated outside a living body in cell-free systems (like a test tube), these reactions themselves are living reactions, but the isolated system performing them is neither living nor non-living.

Therefore, metabolism is a defining feature of all living organisms without exception.

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Building upon metabolism, the fundamental characteristic of life forms is cellular organisation.

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Ability to Sense Environment and Consciousness

A crucial and technically complex trait of living organisms is their capacity to perceive their surroundings and respond to environmental stimuli, which can be physical, chemical, or biological.

Animals sense their environment through specialized sense organs.

Plants also react to external factors such as light, water, temperature, the presence of other organisms, and pollutants.

All organisms, spanning from simple prokaryotes to intricate eukaryotes, possess the ability to detect and respond to environmental cues.

Environmental factors like photoperiod influence reproduction in seasonal breeders.

Living organisms are also capable of handling chemicals that enter their bodies.

Essentially, all organisms are 'aware' of their immediate environment.

A distinguishing feature of humans is self-consciousness, the awareness of one's own self.

Due to the presence of self-consciousness in humans, consciousness is considered a defining property of living organisms.

The living state can be particularly challenging to define in cases like patients in a coma, who are brain-dead and lack self-consciousness, despite being kept alive by mechanical support.

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All biological phenomena arise from interactions between constituent components at different levels of organisation.

For instance, the properties of a tissue emerge from the interactions among its cells, not just from the properties of individual cells.

Similarly, properties of cellular organelles result from interactions between their molecular components.

These interactions lead to emergent properties at higher levels of structural organisation.

In summary, living organisms are intricate systems that are capable of self-replication, evolution, self-regulation, interaction, and response to external stimuli.

Diversity In The Living World

Our planet hosts an immense variety of living organisms, ranging from visible plants and animals to microscopic microbes.

The number of species that have been identified and described is estimated to be between 1.7 to 1.8 million. This vast array of life forms is referred to as biodiversity.

Exploration of new regions, and even existing ones, continuously reveals new organisms.

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Dealing with such extensive diversity requires a standardized system for naming organisms, as local names vary widely.

The process of providing a standard name for an organism by which it is known globally is called nomenclature.

Nomenclature is only possible after an organism has been accurately described and its identity established. This process is known as identification.

Scientists have developed universal standards and procedures for assigning scientific names that are accepted by biologists worldwide.

For plants, the rules and criteria for scientific naming are provided by the International Code of Botanical Nomenclature (ICBN).

For animals, the rules are governed by the International Code of Zoological Nomenclature (ICZN).

These codes ensure that each organism has a unique scientific name and that the description allows others to arrive at the same name, while also preventing the reuse of a name for a different organism.

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Binomial Nomenclature

Biologists follow universally accepted principles to provide scientific names, primarily using a system called Binomial Nomenclature, introduced by Carolus Linnaeus.

This system assigns a scientific name with two components:

1. The Generic name (Genus)
2. The specific epithet (species)

Example: The scientific name for mango is Mangifera indica. Here, Mangifera is the genus, and indica is the specific epithet.

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Universal Rules of Nomenclature

The universally accepted rules for binomial nomenclature are:

1. Biological names are derived from Latin and are written in italics to indicate their Latin origin, regardless of their original language.
2. The first part of the name represents the Genus, starting with a capital letter. The second part is the specific epithet, starting with a small letter.
3. When handwritten, both parts of the biological name must be separately underlined. When printed, they are in italics.
4. Example: Mangifera indica (handwritten) or Mangifera indica (printed).

The name of the author who first described the species is sometimes added in an abbreviated form after the specific epithet, e.g., Mangifera indica Linn.

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Since studying every single organism is impractical, organisms are grouped into convenient categories based on easily observable characteristics. This process is called classification.

Groups of organisms sharing similar characters form categories (e.g., plants, animals, dogs, cats, wheat, rice).

The scientific term for these categories is taxa (singular: taxon).

Taxa can represent categories at different levels of classification (e.g., 'animals', 'mammals', and 'dogs' are all taxa but represent different levels).

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Taxonomy is the branch of biology that deals with the principles and procedures of characterisation, identification, classification, and nomenclature of organisms. These four processes are fundamental to taxonomy.

Early classifications were often based on the practical 'uses' of organisms to humans (food, clothing, shelter).

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Systematics is the study focused on understanding the diversity of organisms and the evolutionary relationships among them. The word 'systematics' comes from the Latin 'systema', meaning systematic arrangement.

Carolus Linnaeus used *Systema Naturae* as the title of his important publication.

Modern systematics is broader, encompassing identification, nomenclature, and classification, and crucially, it considers the evolutionary relationships between organisms.

Taxonomic Categories

Classification is not a single step but a series of steps forming a hierarchy, where each step represents a rank or category.

All the categories together form the taxonomic hierarchy.

Each category, which serves as a unit of classification, is called a taxon (plural: taxa).

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Taxonomic groups or categories are not just artificial groupings but represent distinct biological entities.

Common taxonomic categories, from highest to lowest rank, are: Kingdom, Phylum (for animals) or Division (for plants), Class, Order, Family, Genus, and Species.

Species is the lowest taxonomic category.

To classify an organism into these categories, one needs detailed knowledge of its characteristics, including similarities and differences compared to other organisms.

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As we move up the hierarchy from species to kingdom, the number of common characteristics shared by members within a taxon decreases.

Conversely, lower taxa (like species or genus) share a greater number of common features.

Higher categories present a greater challenge in determining the relationships between different taxa at the same level, making classification more complex at higher ranks.

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Below is a table illustrating the taxonomic classification of some common organisms:

Taxonomic Rank	Man	Housefly	Mango	Wheat
Biological Name	Homo sapiens	Musca domestica	Mangifera indica	Triticum aestivum
Genus	Homo	Musca	Mangifera	Triticum
Family	Hominidae	Muscidae	Anacardiaceae	Poaceae
Order	Primata	Diptera	Sapindales	Poales
Class	Mammalia	Insecta	Dicotyledonae	Monocotyledonae
Phylum/Division	Chordata	Arthropoda	Angiospermae	Angiospermae

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Species

Species is considered a group of individual organisms that share fundamental similarities.

Species are distinguished from closely related ones primarily based on distinct morphological differences.

In binomial nomenclature, the specific epithet denotes the species (e.g., indica in Mangifera indica, tuberosum in Solanum tuberosum, leo in Panthera leo).

A genus can include one or more species (e.g., Genus Panthera includes species like leo, pardus, and tigris).

Humans belong to the species sapiens, grouped under the genus Homo, hence the scientific name is Homo sapiens.

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Genus

Genus is a category comprising a group of related species.

Species within the same genus share more common characteristics with each other compared to species belonging to different genera.

Genera can be viewed as aggregates of closely related species.

Examples:

• Potato (Solanum tuberosum) and brinjal (Solanum melongena) are distinct species but belong to the same genus, Solanum.
• Lion (Panthera leo), leopard (Panthera pardus), and tiger (Panthera tigris) are species grouped in the genus Panthera. The genus Panthera is distinct from the genus Felis (which includes cats).

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Family

Family is a higher category that groups related genera.

Genera within a family have fewer similarities compared to the similarities found between species within a genus.

For plants, families are characterized based on both their vegetative and reproductive features.

Examples:

• Plants: The genera Solanum, Petunia, and Datura are all placed in the family Solanaceae.
• Animals: The genus Panthera (lions, tigers, leopards) is grouped with the genus Felis (cats) in the family Felidae.
• Despite some similarities, cats (Family Felidae) and dogs (Family Canidae) are placed in separate families based on their distinct characteristics.

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Order

Order is a higher category that groups related families.

Orders are generally identified based on an aggregation of a few similar characters shared among the families included, with similarities being less numerous than those defining lower categories.

Examples:

• Plants: Plant families like Convolvulaceae and Solanaceae are included in the order Polymoniales, primarily based on floral characteristics.
• Animals: The order Carnivora includes families such as Felidae (cats) and Canidae (dogs).

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Class

This category includes related orders.

Example: The order Primata (including monkeys, gorillas, and gibbons) and the order Carnivora (including animals like tigers, cats, and dogs) are both placed in the Class Mammalia. The Class Mammalia encompasses other orders as well.

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Phylum

Phylum is a higher category that includes related classes (for animals).

Organisms in different classes might be grouped into a phylum based on a few common, fundamental features.

Example: Classes of fishes, amphibians, reptiles, birds, and mammals are all included in the Phylum Chordata because they share common features like the presence of a notochord and a dorsal hollow neural system.

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Kingdom

Kingdom is the highest taxonomic category in the classification hierarchy.

Kingdom Animalia includes all animals belonging to various phyla.

Kingdom Plantae comprises all plants from various divisions.

These are the broadest classifications used to group diverse organisms.

Taxonomical Aids

Taxonomic studies are vital for various fields, including agriculture, forestry, industry, and for understanding our biological resources and their diversity.

Accurate classification and identification of organisms are essential for these studies.

Identification often requires detailed investigation in laboratories and in the field, involving the collection of actual specimens.

Collection and preservation of plant and animal specimens are the primary basis for taxonomic studies and training in systematics.

Information gathered about an organism is stored along with the preserved specimen for future reference.

Biologists have developed various techniques and resources, known as taxonomical aids, to help in the processes of identification, naming, and classification.

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Herbarium

A herbarium is essentially a collection of dried, pressed, and preserved plant specimens mounted on sheets.

These sheets are systematically arranged according to a widely accepted classification system, serving as a repository and quick reference system for taxonomic studies.

Each herbarium sheet includes a label providing crucial information such as the date and place where the specimen was collected, its English, local, and botanical names, the family it belongs to, and the collector's name.

Herbaria are invaluable aids for taxonomic research and identification.

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Botanical Gardens

Botanical gardens are specialized gardens dedicated to collecting and cultivating living plant species for reference and study.

Plants in these gardens are grown specifically for identification purposes.

Each plant is clearly labelled with its botanical/scientific name and its family.

Notable examples include the Royal Botanic Gardens, Kew (England), the Indian Botanical Garden in Howrah (India), and the National Botanical Research Institute in Lucknow (India).

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Museum

Biological museums are typically found in educational institutions like schools and colleges.

They house collections of preserved plant and animal specimens for study and reference.

Specimens are commonly preserved in containers or jars filled with preservative solutions.

Some plant and animal specimens may also be preserved in a dry form.

Insects are usually collected, killed, pinned, and stored in insect boxes.

Larger animals, such as birds and mammals, are often stuffed and preserved.

Museums frequently also contain collections of animal skeletons.

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Zoological Parks

Zoological parks, also known as zoos, are places where wild animals are kept in protected environments under human care.

These parks allow visitors and researchers to learn about the animals' food preferences and behaviour.

Efforts are made to provide conditions within the zoos that are as similar as possible to the animals' natural habitats.

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Key

A key is another taxonomical aid used for the identification of plants and animals.

It is based on comparing the similarities and dissimilarities between organisms.

Taxonomic keys utilize contrasting characters, which are generally presented in pairs called a couplet.

The couplet presents two opposing options. By choosing one option and rejecting the other, the identity can be determined.

Each separate statement within the key is referred to as a lead.

Different taxonomic keys are needed for identifying organisms at various taxonomic categories, such as family, genus, or species.

Keys are typically analytical in nature, helping to systematically work through characteristics to reach an identification.

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Other aids for recording descriptions and aiding identification include:

• Flora: Contains the actual account of the habitat and distribution of plants within a specific geographical area. It serves as an index to the plant species found there.
• Manuals: Provide information useful for identifying the names of species found in a particular area.
• Monographs: Contain detailed information about any one specific taxon.
• Catalogues: Provide a list of species with brief descriptions.

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