Biology: Biological Classification

Dr. Shivangi Shukla

08/01/2025 6:27 AM

Table of Content

Introduction

Classification is the arrangement of organisms into groups (taxa) based upon their distinctive characteristics reflecting their similarities and dissimilarities. Classification of living organisms is essential because:

· It makes the study of all living organisms easy and convenient.

· The study of one or two organisms can help us to understand distinctive features of entire group.

· It makes the study systematic.

· It helps in understanding relationships between different groups of organisms.

· It helps in studying the evolutionary relationships between organisms.

Five Kingdoms Classification

· It was RH Whittaker who proposed the five-kingdom system. These five kingdoms are categorised as follows: Animalia, Plantae, Fungi, Protista, and Monera. The thallus’s organisation, cell structure, feeding strategy, evolutionary relationship, and mode of reproduction were the basis for the classification.

Kingdom Monera

· One of the primary components of Monera is the bacterium.

· This kingdom contains both bacteria and blue-green algae, all of which are prokaryotes. They can be found in extremely harsh environments like deep oceans, snow, deserts, and hot springs. As prokaryotes without a distinct nucleus, all monerans are. They can be parasites as well as autotrophs and heterotrophs. There is a cell wall.

Archaebacteria

· There are three types of archaebacteria: methanogenic (live in marshy places), halophilic (survive in high salt concentrations), and thermoacidophilic (live in hot springs).

· Compared to other monerans, their cell wall structure is different. Methanogens aid in the production of biogas from animal excrement and are present in the intestines of several ruminants, including cows and buffalo.

Eubacteria

We refer to them as “true bacteria”. They are distinguished by having a stiff cell wall and a movable flagellum. contains blue-green algae, or cyanobacteria. Since they have chlorophyll a, they resemble plants. They are referred to as photosynthetic autotrophs for this reason.

Cyanobacteria are filamentous, unicellular algae that grow into colonies encased in a gelatinous sheath. Some have unique features called heterocysts, which aid in the fixation of nitrogen in the environment. Take Nostoc, for instance.

Chemosynthetic Autotrophs

Certain inorganic compounds, such as nitrites and nitrates, can be oxidised by these bacteria. ATP is created using the energy that is liberated during the oxidation process.

Asexual reproduction in bacteria is possible through a mechanism known as binary fission. If conditions are unfavourable, they can also proliferate through spores. Mycoplasma is included in the Monera category as well. Since they lack a cell wall, they can thrive in anaerobic environments. The smallest known living organism is Mycoplasmas.

Kingdom Protista

Eukaryotes with a single cell that belongs to the kingdom Protista. Most of them have an aquatic habitat. It is discovered that they are eukaryotic, unicellular creatures. Their nucleus is identifiable and encircled by the nuclear membrane. They can move via both flagella and cilia, breathe both aerobically and anaerobically, and reproduce both sexually and asexually.

They are separated into protozoans, euglenoids, dinoflagellates, slime moulds, and chrysophytes.

Chrysophytes

It is made up of golden algae and diatoms. Both freshwater and saltwater are home to them. Most of them are photosynthetic. Their unbreakable cell wall is a result of the silica present in it, which also contributes to the development of diatomaceous earth. They are the primary oceanic producers. Most of them are flagellates with a single cell. They resemble amoeboids.

Dinoflagellates

These organisms are marine and carry out photosynthesis. They may have a variety of colours, such as blue, red, brown, or yellow. The presence of various coloured pigments within the dinoflagellate cells is what gives the illusion of colour. Cellulose makes up the cell wall. They have two flagella; one is positioned transversely, and the other longitudinally. Among the instances are Noctiluca scintillans and Gonyaulax catenella.

Euglenoids

Freshwater organisms known as euglenoids are found in stagnant water. A layer known as a pellicle, which is rich in proteins, surrounds these organisms. They can function as both autotrophic and heterotrophic organisms, such as Euglena, and have one long and one short flagellum.

Chloroplasts are present in the majority of euglenoids so they can produce their sustenance. The paramylon (carbohydrate) food stores are present in euglenoids.

Slime moulds

They are saprophytic by nature, and in the right circumstances, they can produce plasmodiums, which are aggregation.

They have real cell walls, are quite resistant to harsh environments, and under poor circumstances, develop fruiting bodies with spores.

Protozoan like Protists

It is known that they are heterotrophs. They can exist as parasites and predators. Four categories of protozoa exist:

Freshwater, moist soil, and saltwater are home to amoeboid protozoa, which have pseudopodia that allow them to consume food particles like those found in amoeba.

Flagellate protozoa can be classified as either parasitic or free-living. Numerous parasitic illnesses are caused by this group of organisms. Trypanosoma, for instance, is the cause of sleeping sickness.

Thousands of cilia are present in ciliated protozoa. They can travel forward or backwards thanks to the cilia’s movement. It also facilitates the intake of external food. Consider Paramecium.

Kingdom Fungi

Since fungi are naturally heterotrophic, some of them can feed on live things, referred to as parasites, while others are saprophytes, meaning they feed on dead organic matter. Lichens are fungi that have the ability to coexist with other living things, including algae. We refer to these fungi as symbionts.

Mycorrhiza is the symbiotic link between fungi and the roots of higher plants, whereas lichens are the symbiotic relationship between fungi and algae.

Fungi can reproduce in a variety of ways, including fission, fragmentation, and budding. Conidia, zoospores, or sporangiospores are terms for spores that are used in asexual reproduction.

Different spores called oospores, ascospores, and basidiospores are used in sexual reproduction.

Phycomycetes, or lower fungi, reproduce asexually by producing motile spores called zoospores and immobile spores called aplanospores. The sporangium is where the spores are made. The gametes fuse to create the zygospore. The mycelium is multicellular and coenocytic (has several nuclei). For instance, Rhizopus, Albugo, and Mucor.

Ascomycetes, or SAC fungi, can live as parasites or saprophytes. Through conidia, asexual reproduction can place. Ascospores are the means of sexual reproduction; Ascomycetes, such as Penicillium, Aspergillus, Claviceps, etc., have septate and branching mycelium.

Basidiomycetes (Club Fungi): The mechanism of fragmentation is used in vegetative reproduction. There are no sexual organs. The basidium is formed by plasmogamy between two vegetative or somatic cells. To create four types of basidiospores, such as Agaricus (mushrooms), Ustilago (fire rot), etc., the basidium goes through karyogamy and meiosis.

Because deuteromycetes do not reproduce sexually at any point during their life cycle, they are sometimes referred to as defective fungi. Through conidia, sexual reproduction takes place. Some examples of septate and branching mycelium are Alternaria, Trichoderma, and so on.

A fundamental class of fungus are called zygomycetes, or conjugation fungi. Sporangiospores, which are non-motile, reproduce asexually. Consider Rhizopus.

Kingdom Plantae

Most of the plants are eukaryotic.
They contain chlorophyll.
Cells are surrounded by cell wall.
Cell walls of plant cells are comprised of cellulose.
They have an ability to grow by cell division. Growth occurs due to the presence of definite growing points or cells. In higher forms, growing areas are called meristems.
In life cycle of plant cells, the interchanges occur from the embryos and are supported by other tissues and self produce.
Plants have tissue and organ.
They obtain their energy from sun through photosynthesis.
Plants reproduce both sexually and asexually. Alternation of generation is found in plants.
They lack motility.

Kingdom Animalia

Animals are eukaryotic, multicellular and heterotrophic organisms.
They have multiple cells with mitochondria
They depend on other organisms for food.
Biologists recognize 36 phyla in the animal kingdom.
The size of animals ranges from a few celled organism like the mesozoans to animals weighing many tons like the blue whale.
The animal cell contains organelles like the nucleus, mitochondria, Golgi complex, endoplasmic reticulum, ribosomes, lysosomes, vacuoles, centrioles, and cytoskeleton.
They have tissue/organ/organ system.
Organ systems are skeletal system, muscular system, digestive system, respiratory system, circulatory system, excretory system, reproductive system, immune system and the endocrine system.
Most animals have the ability to move, they show rapid movement as compared to other organisms.

Viruses are non-cellular, microscopic infectious agents that can only replicate inside a host cell. From a biological perspective, viruses cannot be classified either a living organism or non-living. A virus can be an infectious agent which only replicates within a host organism. This is due to the fact that they possess certain defining characteristic features of living organisms and non-living entities.

In a nutshell, a virus is a non-cellular, infectious entity made up of genetic material and protein that can invade and reproduce only within the living cells of bacteria, plants and animals.

For instance, a virus cannot replicate itself outside the host cell. This is because viruses lack the required cellular machinery. Therefore, it enters and attaches itself to a specific host cell, injects its genetic material, reproduces by using the host genetic material and finally the host cell splits open, releasing the new viruses.

Viruses can also be crystallized, which no other living organisms can do. It is these factors that lead to viruses being classified in the grey area – between the living and non-living.

Viruses, viroids and lichens

· The term 'virus' has been derived from Latin, which means poison or venom or viscous fluid. They remain inactive outside a living host but become active inside the host and multiply in it.

· D.J. Ivanowsky (1892) recognised certain microbes as causal organism of the mosaic disease of tobacco. These were found to be smaller than bacteria because they passed through bacteria-proof filters.

· M.W. Beijerinek (1898) demonstrated that the extract of the infected plants of tobacco could cause infection in healthy plants and called the fluid as Contagium vivum fluidum (infectious living fluid).

· W.M. Stanley (1935) showed that viruses could be crystallised and crystals consist largely of proteins. They are inert outside their specific host cell.

· In addition to proteins, viruses also contain genetic material, that could be either RNA or DNA.

· A virus is a nucleoprotein and the genetic material is infectious. In general, viruses that infect plants have single stranded RNA and viruses that infect animals have either single or double stranded RNA or double stranded DNA.

· Bacterial viruses or bacteriophages (viruses that infect bacteria) are usually double stranded DNA viruses and contain lysozyme enzyme.

The protein coat called a capsid made of small subunits called capsomeres, protects the nucleic acid. These capsomeres arranged in helical or polyhedral geometric forms possess antigenic properties.
Viruses cause diseases like mumps, smallpox, herpes and influenza. AIDS in humans is also caused by a virus. In plants, the symptoms can be mosaic formation, leaf rolling and curling, yellowing and vein clearing, dwarfing and stunted growth.
In 1971, T.O. Diener discovered very simple smallest infectious agents called Viroids. They consist of RNA and capsid is lacking. The RNA of the viroid was of low molecular weight. They cause persistent infections.
A lichen is structurally organised entity consisting of the permanent association of a fungus and an alga. The algal component is known as phycobiont and fungal component as mycobiont, which are autotrophic and heterotrophic, respectively.
Algae prepare food for fungi and fungi provide shelter and absorb mineral nutrients and water for its partner.
Lichens reproduce both by asexual and sexual methods.

Lichens are very sensitive to SO2 and grow only in SO2 free atmosphere.