Prior to 1969 organisms were classified into two kingdoms: the Plant Kingdom and the Animal Kingdom and on the basis of a cell, organisms were classified into two categories Prokaryotae or Monera (which comprised bacteria) and Eukaryotae (which comprised animals, plants, fungi, and protists). The concept of three domains of life was proposed by Carl Woese and others in 1969. The evolutionary model proposed by them is based on the difference in the sequence of nucleotides in ribosomal RNAs (rRNA) in cells and lipid structure of cell membrane and its sensitivity to antibiotics. According to them, all organisms can be classified into three different domains – Archaebacteria, Eubacteria, and Eukarya. All living things share certain genes, yet no two types of organisms have the same full sets of genes.

Scientists think that all living things have descended with modification from a single common ancestor. Thus, all of life connected. Yet, there are many different lineages representing different species. This diversity stems from the fact that genetic changes accumulate over the years. Also, organisms change as they become suited to their own special environments.

Archaea and Bacteria share a few common characteristic traits but do not have common ancestors. At the same time, they show some peculiar traits of their own. Carl Woese divided Prokaryotae into two groups – Archaea and Bacteria, and thus the concept of three domains of life came into existence.

Reasons for Selecting  rRNA for Categorization:

• It is present in all organisms and is the most conserved structure throughout nature
• It is functionally similar between organisms and is involved in protein synthesis
• Its sequence changes slowly and hence can be observed across long periods of time
• The rRNA sequences can be aligned, or matched up, between 2 organisms.
• The nucleotide sequence of rRNA gives a good indication of the relationship in different living groups.

Domain Archaea or Archaebacteria (Greek – archae – ancient):

• These are the most primitive form of life.
• These are the most ancient bacteria. Some fossils found with these bacteria are 3.5 billion years old. As they were from the time of harshest conditions on the earth, they adapted themselves to live in any harshest condition. These bacteria are special since they live in some of the harshest habitats such as extreme salty areas (halophiles), hot springs (thermoacidophiles) and marshy areas (methanogens).
• They have unique cell membrane chemistry. Archaebacteria have cell membranes made of ether-linked phospholipids, while in case of bacteria and eukaryotes both make their cell membranes out of ester-linked phospholipids. The presence of this ether containing linkages in Archaea adds to their ability to withstand extreme temperature and highly acidic conditions.
• Their cell membrane has no peptidoglycans. Archaebacteria use sugar that is similar to, but not the same as, the peptidoglycan sugar used in bacterial cell membranes.
• They are not influenced by antibiotics that destroy bacteria.
• Their rRNA is unique and is much different from the rRNA of bacteria. Their t-RNA and rRNA possess unique nucleotide sequences found nowhere else.
• Most of the archaebacteria are autotrophs. They use pigment bacteriorhodopsin for photosynthesis.

Examples: Extreme halophiles – i.e. organisms which thrive in the highly salty environment, and hyperthermophiles – i.e. the organisms which thrive in the extremely hot environment, are best examples of Archaea.

Classification of Archaebacteria on the Basis of Habitat and metabolic activities:

Methanogens or Methanogenic Archaebacteria:

As they are anaerobic autotrophs, they produce methane as a result of their metabolic activities. They produce methane gas from carbon dioxide and acetic acid from sewage in the marshy condition.

CO₂ + 4H₂ →  CH₄ + 2H₂O

CH₃COOH →  CH₄ + CO₂

Methanogens are present in the gut of several ruminant animals such as cows and buffaloes and they are responsible for the production of methane (biogas) from the dung of these animals. Methane is greenhouse gas that leads to global warming. Methanogens die in the presence of oxygen. Hence they can be found in swamp and marshes in which all oxygen is consumed. The typical smell in these areas is due to the production of methane. Methanogens help in the fermentation of cellulose. They do not decompose the organic matter but utilize the end products of decomposition.

Examples: Methanobacillus, Thiobacillus etc.

Thermoacidophiles or Thermoacidophilic Archaebacteria:

They are aerobic or facultative anaerobic chemoautotrophs. They are adapted to live in extremely hot (about 80 °C) and extremely low temperature (below freezing point) and acidic conditions (pH up to 2). They are found in hot springs (Sulfolobus), in refuse piles of coal mines (Thermoplasma) or geothermal area of Iceland (Thermoproteus).

Most of the thermoacidophiles use hydrogen sulphide as their energy source. They are chemotrophs

2S   +   2H₂O +  3O₂   →   2H₂SO₄ + Energy  (aerobic condition)

Under anaerobic condition, sulphur is reduced to hydrogen sulphide. They precipitate bicarbonate into carbonate due to their activities.

Examples: Thermoplasma, Picrophilus, Thermococci, Pyrococcus, Sulfolobus, etc.

Halophiles or Halophilic Archaebacteria:

They are aerobic or facultative anaerobic heterotrophs. They live in salty environments such as a Great Salt Lake or the Dead Sea, marshes, brine, salt-rich soil where the salt concentration is in range of 2.5 M to 5 M. They have high intracellular concentrations. Their enzymes and ribosomes function efficiently at higher salt concentration.

They contain special photoreceptor pigment called bacteriorhodopsin. Due to which they acquire a purple colour. Bacteriorhodopsin protects halophiles from strong solar radiations. It helps in the synthesis of ATP. It shows the chemotrophic nature of nutrition.

Examples: Halobacteria, halococcus, etc.

Domain Bacteria or Eubacteria:

• These are prokaryotes.
• The cell walls of bacteria; unlike the domains of Archaea and Eukarya, contain peptidoglycan.
• Their membranes are made of unbranched fatty acid chains attached to glycerol by ester linkages.
• They are sensitive to antibiotics.
• They are autotrophs; synthesize their own food, or heterotrophs. Most of the bacterial species are heterotrophs. They get their food from organic matter.
• Naked DNA molecule lies in the cell cytoplasm.
• Only one set of genes, usually in a single-stranded loop is present.
• There is a great deal of diversity in this domain, such that it is next to impossible to determine how many species of bacteria exist on the planet.
• Cyanobacteria and mycoplasmas are the best examples of bacteria.

Domain Eukarya:

• Cells have a eukaryotic organization.
• The cell membrane is composed of a tri-laminar protein-lipid-protein layer similar to that in bacteria.
• Peptidoglycans are not found.
• They are resistant to traditional antibiotics.
• Cells are organized into tissues in case of kingdom Plantae as well as kingdom Animalia.
• The cell was is present only in the kingdom Plantae.
• Eukaryotes are further grouped into Kingdom Protista (euglenoids, algae, protozoans), Kingdom Fungi (yeast, mold, etc.), Kingdom Mycota (Phycomycetes, zygomycetes, ascomycetes, basidiomycetes, Deuteromycetes) Kingdom Plantae (bryophytes, pteridophytes, gymnosperms, and angiosperms) and Kingdom Animalia (all animals).

Another system of grouping organisms divides all life into six major categories called kingdoms. The six kingdoms consist of four kingdoms within the domain Eukarya (the Kingdoms Animalia, Plantae, Fungi, and Protista), one kingdom in the domain Archaea (Kingdom Archaea) and one kingdom in the domain Bacteria (KingdomBacteria). Many biologists recognize these six kingdoms and three domains, but some biologists use other systems of grouping.

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Scientists have identified and named 1.7 to 1.8 million species of living organisms. Of these, about 1.2 million are animal species while 0.5 million are plant species. The group of insects is the largest group with 1.025 million species. According to biologists some 5 to 30 million species of organisms exist on the earth. The variety that we see in the living things that exist on the earth is called biological diversity or biodiversity. There is variety in their shapes, sizes, bodies apart and lifespan. We should remember here that as we explore new areas, and even old ones, new organisms are continuously being identified. In this article, we shall study the need for classification of living organisms.

Time Line of Classification:

Aristotle, Greek philosopher (384 – 322 B.C.):

Aristotle developed the first classification system, which divided all known organisms into two groups: plants and animals. Aristotle’s system of classification was not full proof because many animals were there they didn’t fit in the classification. Aristotle’s limited classification system was used for nearly 2000 years.

Parasara (Indian sage) (Before Christ):

On the basis of comparative morphology, he classified plants, whose detail is given in his compilation called Vrikshayurveda. He group families of plants under name ganas. These ganas, can be clearly distinguished and recognized even today.

Charaka Indian Doctor and Father of Ayurveda (first century A.D.):

In his book ‘Charak Sanhita’ he classified 200 kinds of animals and 340 kinds of plants.

John Ray British Botanist (1628-1705):

He introduced the term ‘species’. He collected plant species from all over Europe and give an improved form of classification of plants.

Carlous Linnaeus Swedish Naturalist (1707 – 1778) :

He introduced the binomial nomenclature system. He listed about 5900 species of plants in his book ‘Species Plantarum’ (1753). He listed about 4200 species of animals in his book ‘Systema Naturae’ (1758). He is called the father of taxonomy.

George Cavier American biologist (1769 – 1832):

He introduced the natural classification system. He took into account not only the structure but also the functions of various structures and the ancestral history of the organism. He studied related fossils.

Sir Julain Huxley (1940):

He introduced the term ‘New Systematics’ for the classification of living organisms based on the theory of evolution and phylogeny.

Classification and its Need:

The term classification was coined by A. P. de Condole. Classification is the process by which anything is grouped into convenient categories based on some easily observable characters. There are a large number of organisms found on Earth. They show variations in their shape, size, structure, habit, habitat, nutrition, etc. It is difficult to remember the characteristics of all the organisms without their proper arrangement.

The classification helps us to explain unity in the diversity of the organisms. The classification places an organism amongst those which have common characteristics.

Systematics and Taxonomy:

• Systematics:  Systematics is a scientific study of similarities and differences among different kinds of organisms and also includes identification, nomenclature, and classification.
• Taxonomy: It is the branch of biology which deals with the collection, identification, nomenclature, description, and classification of plants and animals.
• Generally, the terms taxonomy, systematics and classification are used interchangeably. But Simpson said that these are three separate fields of study and should not be confused with each other.
• 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. Hence, there is a need to standardize 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.

Classical or Old Systematics:

Classical systematics is based on the study of mainly mor­phological traits of one or a few specimens with supporting evidence from other fields. In classical systematics, species were considered to be an independent and immutable (changeless) entity and work of the creator.

New Systematics or Modern Systematics:

The term new systematics was coined by Julian Huxley (1940). New systematics is the systematic study which takes into consideration all types of characters including those from classification morphology, anatomy, cytology, physiology, biochemistry, ecology, genetics, development (embryology), behaviour, etc. of the whole population instead of a few typo­logical specimens.

Characteristics of New Systematics:

• The importance is given to subspecies and populations instead of species.
• The biological definition is replaced by a morphological definition. It considers other branches of biology like cytology, physiology, biochemistry, genetics, etc.
• New systematics is based on the study of all types of variations in the species.
• Along with morphological characters, other investigations are also carried out to know the variety of traits.
• Delimitation of species is carried out on the basis of all types of biological traits. It is also called biological delimitation.
• Statistical data and techniques are used to know the traits in the degree of primitiveness, advancement and to find Inter-relationships.
• According to new systematics, species are not fixed or static but highly dynamic.

Basics of Systematics:

• Characterization: The organism to be studied is described for all its morphological and other characteristics.
• Identification:
• Based on the studied characteristics, the identification of the organ­ism is carried out to know whether it is similar to any of the known groups or taxa.
• Classification: The organism is now classified on the basis of its resemblance to different taxa. It is the arrangement of organisms into groups based on their relationship. If the organism cannot be classified under known groups, then a new group or taxon is created to accommodate it.
• Nomenclature: After placing the organism in various taxa, its correct name is determined.

Objectives of Systematics and Taxonomy:

• To know various kinds of plants on the earth with their names, affinities, geographical distribution, habit, characteristics, and their economic importance.
• To have a reference system for all organisms with which scientists can work.
• To demonstrate manifold diversities of organisms and their phylogenetic (evolutionary) relationship.
• To ascertain nomenclature.

Advantages of Systematics:

• It is used in a study of other disciplines of biology. The knowledge gained through systematics is assembled for use in the field of morphology, physiology, anatomy, pathology, genetics, evolution, medicine, agriculture, forestry, and industries.
• It gives an idea about the organic diversity, its origin, and evolution. Using a few representatives from each group we can acquire knowledge of other organisms.
• It helps in the identification of crop pests and in solving the problem of many epidemic diseases.
• It helps in finding out new food resources such as fishes, arthropods, algae, etc.
• Many organisms are indicators of pollution, fossil fuels and types of minerals present in the soil. This can be achieved using systematics.

Phylogeny:

The evolutionary history of a particular species is called phylogeny. Classification based on their phylogenic relationship or on the basis of evolution is called evolutionary or phylogenetic classification.

Many groups of organisms are now extinct, and without their fossils, we would not have a picture of how modern life is interrelated. We express the relationships among groups of organisms through diagrams called cladograms, which are like genealogies of species.

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Scientists have identified and named 1.7 to 1.8 million species of living organisms. Of these, about 1.2 million are animal species while 0.5 million are plant species. The group of insects is the largest group with 1.025 million species. According to biologists some 5 to 30 million species of organisms exist on the earth. The variety that we see in the living things that exist on the earth is called biological diversity or biodiversity. There is variety in their shapes, sizes, bodies apart and lifespan. The term biological diversity (or Biodiversity) was coined by Walter G. Rosen in 1986. The term biodiversity includes diversity within species, between species and of ecosystems. In this article, we shall study the basis of biodiversity.

Variety in Size:

There are shortest lawn grasses, at the same time redwood trees of California of approximate size 100 m. We have microscopic bacteria of a few micrometres in size. At the same time, we have a blue whale of approximate sizes of 30 m.

Variety in Shape:

There are plants like Banyan, guava with branches, while there are plants like coconut and palm which has no branches. There are tiny animals like bacteria, amoeba which can only be seen through a microscope. At the same time, we have gigantic animals like an elephant.

Variety in Body Parts:

Some plants are flowering plants while some plants are non-flowering plants. In some animals limbs are present for locomotion while in some plants flagella or cilia are present. Amoeba moves by forming pseudopodia.

Variety in Life Span:

Some pine trees live for thousands of years while insects like mosquitoes die within a few days.

Variety in Complexity:

Some animals like an amoeba, paramoecium are unicellular while animals like monkey, elephant, human are multicellular.

Variety due to Habitat:

There are some plants like hydria which are freshwater dwelling. Some algae are marine. While trees like Banyan are terrestrial. Fishes are aquatic (freshwater or marine). Tigers, humans are terrestrial (land-dwelling). Birds and monkeys are arboreal (tree-dwelling). Frog and tortoise are amphibians i.e. they can live on the land and in the water. Animals and plants of a desert, snow region, and coastal areas and of same class show differences in their body structure.

Variety due to Mode of Nutrition:

Plants are autotrophic because they are the producer. They produce their own food material. Animals are heterotrophs. They depend on plants and other animals for their food. They are consumers. Bacteria are saprophytic. They depend on dead decaying matter for their nutrition. They are decomposers. Planta like Cuscuta is parasitic. It depends on another plant for nutrition without giving any return to the host plant. Some animals are vegetarian (e,g. elephant), some are nonvegetarian i.e. carnivorous (e.g. tiger). Humans are both vegetarian and non-vegetarian(omnivorous).

Variety due to Colours:

We can find colourless or even transparent worms, At the same time, we can find brightly coloured birds and flowers.

Variety in the Same Class:

There is more variety in the body structure, life patterns and habitats of species that belong to the same class. Let us consider class Pisces of the animal kingdom which includes fishes of all kind. Some fishes are of freshwater while some leaves in seawater (marine). Some have a tiny shape while some are gigantic. Some use tail fin for changing direction while some use it as a weapon of self-defence. Some have a shorter life while some have a very long life. Thus variety in habitat, size, body structure and lifespan can be observed in the same class.

The basis of Classification:

This variety of life around us has evolved on the earth over millions of years. We look for similarities among the organisms, which will allow us to put them into different classes and then study different classes or groups as a whole. For this, we need to decide which characters decide fundamental differences among organisms. This would form the basis of classification.

Importance of Biodiversity:

• Each organism in an ecosystem has a special role to play, hence biodiversity Increases ecosystem productivity.
• It promotes soil formation and prohibits soil erosion.
• It provides more fruit resources.
• It provides employment to local people by offering an environment for recreation and tourism.
• It Provides medicinal resources and pharmaceutical drugs.
• It provides security against natural disaster and provides speedy recovery from them.
• They contribute to environmental and climatic stability.
• It reduces pollution.
• It protects freshwater resources.
• It is required for breeding programmes in agriculture, horticulture, sericulture, and apiculture.
• Biodiversity maintains the balance of the ecosystem.
• As the human being is part of the ecosystem any damage to biodiversity will cause damage to the support system and it may lead to a threat to human existence. Hence biodiversity should be conserved.

The Threat to Biodiversity:

Increase in Human Population:

Due to the increase in the population more and more land is required for agriculture, housing, for making roads, constructing a dam, bridges, electrical power stations, and industries. In the last 70 years, there is a rapid decline in biodiversity due to above reasons.

Deforestation and Overgrazing:

Indiscriminate cutting of trees for wood causes deforestation. Overgrazing by cattle and sheep causes a decline in grassland. This creates a loss of habitat for wild animals.

Pollution:

Insecticides used in agricultural practices, toxic elements released by industries, petroleum products pollute water and air. The species which are unable to tolerate this pollutant level in air or water get eliminated.

Introduction of Exotic Species:

An introduction of a new species from some other area in a new area is called the introduction of exotic species. These species compete with native species in that area. It may lead to the extinction of local species.

Climatic Changes:

Global warming, Increase in temperature, changing rain pattern and melting glaciers are causing great danger to biodiversity.

Human Greed:

International trade in wildlife and wildlife products for the decorative, medical purpose has threatened many species.

Role of Biodiversity:

Biodiversity maintains equilibrium in nature because of which all kinds of organisms are able to survive. The bacteria and fungi recycle organic matter from dead decaying organisms or living organisms to feed other diverse organisms.
Green plants and algae trap solar energy during photosynthesis and produce food which is utilized by all living organisms. Insects and bats pollinate flowers. Animals are medium for dispersion of seeds. Ecosystems such as the forests, deserts, aquatic bodies, wetlands are self-sufficient and sustain their own typicality. Some ecosystems are part of their unique food chains and food webs.

Measures to Conserve Biodiversity:

It is the duty of every human being to protect biodiversity. Conservation keeps ecosystems stable. Many plants have become extinct. Some are close to extinction. Endangered species need to be protected. Fish and mollusc stocks have to be conserved and prevented from overexploitation by humans for food.

Government and non-government organizations are working for the conservation of biodiversity through legislation. Banning animal killing, banning illegal tree cutting, making zoos, national parks, botanical gardens, and biosphere reserves etc. “Operation Tiger” and “Operation elephants” are projects that have helped in preventing the decline in their numbers due to habitat destruction.

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Scientists have identified and named 1.7 to 1.8 million species of living organisms. Of these, about 1.2 million are animal species while 0.5 million are plant species. The group of insects is the largest group with 1.025 million species. According to biologists some 5 to 30 million species of organisms exist on the earth. The variety that we see in the living things that exist on the earth is called biological diversity or biodiversity. There is variety in their shapes, sizes, bodies apart and lifespan. The term biological diversity (or Biodiversity) was coined by Walter G. Rosen in 1986. The term biodiversity includes diversity within species, between species and of ecosystems.

Types of Species Diversity:

Genetic Diversity:

Genes give specific characteristics to an individual. Each member of any animal or plant species differs from other individuals in its genetic material because of a large number of possible combinations in the genes. Due to this genetic variability, a healthy breeding population of a species is assured.

The diversity in wild species forms the ‘gene pool’ from which our crops and domestic animals have been developed over thousands of years. Using this gene pool new varieties of more productive and diseases resistant crops are obtained. Similarly, the breed of better domestic animals is obtained. In modern biotechnology and genetic engineering techniques, genes are manipulated for developing better types of medicines and a variety of industrial products.

Species Diversity:

The numbers of species of plants and animals that are present in a region constitute its species diversity. This diversity can be observed both in natural ecosystems and in agricultural ecosystems. Natural tropical forests have much greater species richness than plantations. A natural forest ecosystem provides fruit, fuelwood, fodder, fiber, gum, resin and medicines to local people. Areas that are rich in species diversity are called ‘hot spots’ of diversity.

Ecosystem or Community Diversity:

There are a large variety of different ecosystems on earth, Ecosystem diversity can be described for a specific geographical region. Distinctive land ecosystems include landscapes such as forests, grasslands, deserts, mountains, etc. Aquatic ecosystems include rivers, lakes, and the sea. Due to overuse or misuse productivity of the ecosystem is decreases and the ecosystem becomes degraded.

Types of Ecosystem Community Diversity:

R.H. Whittaker proposed a four-level of diversity.

• Point Diversity: This is the diversity on the smallest scale. It is diversity in microhabitat.
• Alpha Diversity:  It is diversity over the comparatively larger area. It is also called local diversity. It includes a variety of living organisms occurring in a particular habitat. It is usually expressed by the number of species in that ecosystem. This is measured by counting the number of taxa (distinct groups of organisms) within the ecosystem.
• Gamma Diversity: It is diversity over larger areas or regions such as islands or landscapes. It is a measure of the overall diversity of the different ecosystems (alpha diversity) within a region. It is the inclusive diversity of all the habitat types within an area (region).
• Epsilon Diversity: The epsilon or regional diversity is defined as the total diversity of a group of areas of gamma diversity.
• Explanation:
• A single plant can be considered as an example of a unit of alpha diversity, then a leaf of a plant can be considered as point diversity.  The group of plants together in a region can be considered as gamma diversity. The forest in which this region is located can be considered as epsilon diversity.

Mathematical Approach Towards Biodiversity:

Beta Diversity:

R.H. Whittaker defined it as “the extent of change in community composition, or degree of community differentiation, in relation to a complex-gradient of the environment, or a pattern of environments”. Beta diversity is defined as the ratio between gamma (regional) and alpha (local) diversities. Beta diversity does not only account for the relationship between local and regional diversity but also informs about the degree of differentiation among biological communities. It is a bridge from the alpha (local) diversity to the gamma (regional) diversity.

Delta Diversity:

Delta diversity is defined as the change in species composition and abundance between areas of gamma diversity, which occur within an area of epsilon diversity. It shows differentiation diversity over wide geographic areas.

Region of Mega Diversity:

The entire world is divided into six biogeographic regions. They are Palearctic (Europe and Asia), Nearctic (North America), Neotropical (Mexico, Central, and South America), Ethiopian (Africa), Indian (Southeast Asia, Indonesia) and Australian (Australia and New Guinea). The organisms found in these regions are adapted to the climate of these regions. Certain kinds of organisms are common to all regions while some are restricted to certain regions only. e.g. elephants are found only in Asia and Africa and nowhere else in the world. The grass is found all over the world.

The warm and humid tropical regions of the earth between the tropic of Cancer and Tropic of Capricorn, are rich in diversity of life i.e. plants, animals, and microorganisms. This region is called the region of mega diversity.

More than half of the biodiversities of the world are concentrated in 12 countries. They are Brazil, Colombia, Ecuador, Peru, Mexico, Zaire, Madagascar, Australia, China, India, Indonesia, and Malaysia.

‘Hotspots’ are regions of the world where many different kinds of organisms live. Many of these organisms are not found elsewhere e.g. many species of frogs live only in the Western Ghats of India. India has two biodiversities ‘hotspots’. The Western Ghats and North Eastern regions (including Eastern Himalayas).

The Uniqueness of Indian Biodiversity:

India is one of the 12 mega diversity countries in the world. India is divided into 10 biogeographical regions. India has a variety of physical features and climatic conditions. India has forests, grasslands, deserts, rivers, wetlands, coastal and marine regions which act as ecosystem and habitat for a variety of animals. Hence India has a great biodiversity.

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Biology is a branch of science which studies living beings that all plants and animals including humans. Biology examines the structure, function, growth, origin, evolution, and distribution of living things. It classifies and describes organisms, their functions, how species come into existence, and the interactions they have with each other and with the natural environment. Four principles form the foundation of modern biology are cell theory, evolution, genetics, and homeostasis. In this article, we shall study the characteristics of life.

Growth and Change:

All living organisms have the ability to grow and change. An increase in mass and an increase in the number of individuals are two characteristics of the growth. Multicellular organisms grow by cell division. A seed under the right conditions will sprout and form a seedling that will grow into a larger plant.  

Even the smallest bacteria grow by binary fission. The growth is also required for the persistence of the species. The growth of plants takes place throughout life and at a specific portion of the body but the growth in the animal is time-bound and overall. After some period, the growth in animals occurs by cell division of certain tissues to replace the lost cells. In unicellular organisms, the growth is by the increase in the mass.

Nonliving objects like mountains, boulders and sand dunes also grow but this growth is due to the accumulation of substance on their surface. Thus both the living and non-living grow. Hence growth cannot be considered as characteristic of life.

Reproduction:

All living organisms (multicellular and unicellular) have the ability to reproduce. Living things make more organisms like themselves. If a species does not reproduce the next generation, the species will go extinct. Reproduction is the process of producing the next generation. Reproduction may be a sexual or asexual process. Sexual reproduction involves two parents and the fusion of gametes, haploid sex cells from each parent. Sexual reproduction produces offspring that are genetically unique and increases genetic variation within a species. Asexual reproduction involves only one parent. It occurs without a fusion of gametes and produces offspring that are all genetically identical to the parent. Genetic variation is not possible in asexual reproduction.

Many organisms like mules, sterile worker bee, warblers, infertile human couples, etc. do not reproduce. Thus reproduction cannot be considered as a characteristic feature of living organisms.

Cellular Organization:

All living organisms, whether made up of one cell or many cells, have some degree of organization. A cell is the smallest unit that can perform all life’s processes. Some organisms, like bacteria, are made up of one cell and are called unicellular organisms. Other organisms, such as humans or higher-level plants, are made up of multiple cells and are called multicellular organisms.

Complex multicellular organisms at the highest level, the organism is made up of organ systems, or groups of specialized parts that carry out a certain function in the organism. For example, the digestive system of humans. Organ systems are made up of organs. For example, the digestive system is made of organs like mouth, esophagus, stomach, liver, gall bladder, small intestine, large intestine, etc. Organs are structures that carry out specialized jobs within an organ system. Thus in the digestive system, the stomach performs the function of churning the food and add acid to it. All organs are made up of tissues. Tissues are groups of cells that have similar abilities and that allow the organ to function. Tissues are made up of cells. A cell is covered by a membrane, contains all genetic information necessary for replication, and be able to carry out all cell functions. Within each cell are organelles. Organelles are tiny structures that carry out functions necessary for the cell to stay alive. Organelles are made up of biological molecules, the chemical compounds that provide physical structure and that bring about movement, energy use, and other cellular functions. All biological molecules are made up of atoms. Atoms are the simplest particle of an element that retains all the properties of a certain element.

Beyond the organism level, organisms form populations which make up parts of an ecosystem. Different ecosystems collectively form the biosphere. Thus the cellular organization is a defining feature of living organisms.

Metabolism:

Metabolism is essentially a collection of chemical reactions occurring within the body (or cell). In body two activities are continuously taking place anabolic activities (making up) and catabolic activities (breaking up). All living organisms are made up of chemical substances. These chemical substances belong to different classes like carbohydrates, lipids, proteins, etc. Collectively they are called biomolecules. During anabolic activities, the food material is digested, absorbed and assimilated in the body. In catabolic activities, the stored substances are broken down by hydrolysis or oxidation to produce energy in the form of ATP which is required for doing regular activities by the body. Metabolism includes processes such as protein synthesis, chemical digestion, cell division, or energy transformation.

Metabolism is observed in all living organisms. Hence metabolism is a defining feature of all living beings.

Maintain Homeostasis:

All living things, from single cells to entire organisms, have mechanisms that allow them to maintain stable internal conditions despite changes in their external environment. This process is called homeostasis and is an important characteristic of all living organisms. By this process, the body temperature, sugar level in the body is maintained at a constant level. Multicellular organisms usually have more than one way of maintaining important aspects of their internal environment.

Without these mechanisms, organisms can die. For example, a cell’s water content is closely controlled by the taking in or releasing water. A cell that takes in too much water will rupture and die. A cell that doesn’t get enough water will also shrivel and die. It is a vital characteristic of life. If it is disturbed, it will result in diseases and if not controlled can threaten the life of the organism.

Responding to the Environment:

All living organisms respond to their environment. Living things know what is going on around them (consciousness) and respond to the changes in the environment. The response may be physical, chemical or biological. Human beings are only animals with self-consciousness. When touch me not plant is touched its leaves close. The Venus flytrap traps insects.

The stem of the plant moves in the direction of light and above the ground. (positively phototropic and negatively geotropic. The Root grows towards the soil and away from light (positively geotropic and negatively phototropic).

Heredity:

Heredity means that our genetic information can be passed from one generation to another. This way characteristics are transferred from one generation to the other.

Adaptation:

An adaptation refers to the process of becoming adjusted to an environment. Adaptations may include structural, physiological, or behavioral traits that improve an organism’s likelihood of survival.

Conclusion: Characteristics of Life of Living Organisms?

Thus the main characteristics of life (living organisms) are the self-replicating, evolving and self-regulating interactive systems that can respond to external stimuli.

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