Biological Classification

1. Chapter at a Glance

Since the time of Aristotle, biologists have tried to group living organisms in a sensible way. Aristotle divided plants into trees, shrubs and herbs, and animals into those with red blood and those without. Linnaeus later used a two-kingdom system (Plantae and Animalia). That system did not separate prokaryotes from eukaryotes, unicellular from multicellular organisms, or photosynthetic (green algae) from non-photosynthetic (fungi) organisms. In 1969, R.H. Whittaker proposed the five-kingdom classification: Monera, Protista, Fungi, Plantae and Animalia. This chapter describes the first three kingdoms in detail, briefly introduces Plantae and Animalia, and ends with acellular forms — viruses, viroids and prions — and the symbiotic lichens. NEET asks two to three questions from this chapter every year, mostly on characteristic examples, fungal classes, protist groups and viruses.

2. Five-Kingdom Classification

Whittaker used five main criteria: cell structure (prokaryotic or eukaryotic), body organisation (unicellular or multicellular), mode of nutrition (autotrophic or heterotrophic), reproduction and phylogenetic relationships. The three-kingdom system had separated Protista from plants and animals; the four-kingdom system added Monera. The five-kingdom system also separated Fungi from plants, because fungi have chitin cell walls and are heterotrophic.

3. Kingdom Monera

Bacteria are the sole members of kingdom Monera. They are the most abundant micro-organisms and occur almost everywhere — hot springs, deserts, snow, deep oceans — and many live as parasites in or on other organisms. Bacteria are grouped by shape into four types:

• Coccus (plural cocci) — spherical
• Bacillus (plural bacilli) — rod-shaped
• Vibrio (plural vibrios) — comma-shaped
• Spirillum (plural spirilla) — spiral

Although their structure is simple, bacteria are very complex in behaviour and show the most extensive metabolic diversity of any group. Some are autotrophic: they synthesise their own food from inorganic substrates, either by photosynthesis or by chemosynthesis. The majority are heterotrophic, depending on other organisms or dead organic matter for food.

Archaebacteria

These bacteria are special because they live in some of the harshest habitats: extreme salty areas (halophiles), hot springs (thermoacidophiles) and marshy areas (methanogens). Archaebacteria differ from other bacteria in having a different cell wall structure, which is responsible for their survival in extreme conditions. Methanogens are present in the gut of several ruminant animals such as cows and buffaloes, and they produce methane (biogas) from the dung of these animals.

Eubacteria

• Eubacteria ("true bacteria") are characterised by a rigid cell wall and, if motile, a flagellum.
• Cyanobacteria (blue-green algae) have chlorophyll a similar to green plants and are photosynthetic autotrophs. They are unicellular, colonial or filamentous, freshwater, marine or terrestrial. Colonies are generally surrounded by a gelatinous sheath. They often form blooms in polluted water bodies. Some, like Nostoc and Anabaena, can fix atmospheric nitrogen in specialised cells called heterocysts.
• Chemosynthetic autotrophic bacteria oxidise various inorganic substances such as nitrates, nitrites and ammonia and use the released energy for ATP production. They play a great role in recycling nutrients like nitrogen, phosphorus, iron and sulphur.
• Heterotrophic bacteria are the most abundant in nature. The majority are important decomposers. Many have a significant impact on human affairs: they help make curd from milk, produce antibiotics and fix nitrogen in legume roots. Some are pathogens, causing damage to humans, crops, farm animals and pets — cholera, typhoid, tetanus and citrus canker are well-known bacterial diseases.

Bacteria reproduce mainly by fission. Sometimes, under unfavourable conditions, they produce spores. They also reproduce by a sort of sexual reproduction, by adopting a primitive type of DNA transfer from one bacterium to the other.

The mycoplasma are organisms that completely lack a cell wall. They are the smallest living cells known and can survive without oxygen. Many mycoplasma are pathogenic in animals and plants.

4. Kingdom Protista

All single-celled eukaryotes are placed under Protista, but the boundaries of this kingdom are not well defined. What is "a photosynthetic protistan" to one biologist may be "a plant" to another. Members of Protista are primarily aquatic. This kingdom forms a link with the others dealing with plants, animals and fungi. Being eukaryotes, the protistan cell has a well-defined nucleus and other membrane-bound organelles. Some have flagella or cilia. Protists reproduce asexually and sexually by a process involving cell fusion and zygote formation.

5. Kingdom Fungi

Fungi form a unique kingdom of heterotrophic organisms and show great diversity in morphology and habitat. You have seen fungi on moist bread and rotten fruits. The common mushroom and toadstools are fungi. White spots on mustard leaves are due to a parasitic fungus. Some unicellular fungi, e.g. yeast, are used to make bread and beer. Other fungi cause diseases in plants and animals; wheat rust is caused by Puccinia. Some are the source of antibiotics, e.g. Penicillium. Fungi are cosmopolitan and occur in air, water, soil and on animals and plants. They prefer to grow in warm and humid places — which is why we keep food in the refrigerator.

• With the exception of yeasts (unicellular), fungi are filamentous. Their bodies consist of long, slender, thread-like structures called hyphae. The network of hyphae is called mycelium.
• Some hyphae are continuous tubes filled with multinucleated cytoplasm — these are called coenocytic hyphae. Others have septae or cross walls in their hyphae.
• Cell walls of fungi are composed of chitin and polysaccharides.
• Most fungi are heterotrophic and absorb soluble organic matter from dead substrates — saprophytes. Those depending on living plants and animals are parasites. They can also live as symbionts — in association with algae as lichens and with roots of higher plants as mycorrhiza.

Reproduction in fungi

• Vegetative: fragmentation, fission and budding.
• Asexual: by spores called conidia, sporangiospores or zoospores.
• Sexual: by oospores, ascospores and basidiospores. The various spores are produced in distinct structures called fruiting bodies.

The sexual cycle involves three steps:

1. Fusion of protoplasms between two motile or non-motile gametes — called plasmogamy.
2. Fusion of two nuclei — called karyogamy.
3. Meiosis in the zygote, resulting in haploid spores.

When a fungus reproduces sexually, two haploid hyphae of compatible mating types come together and fuse. In some fungi, the fusion of two haploid cells immediately results in diploid cells (2n). In other fungi (ascomycetes and basidiomycetes), an intervening dikaryotic stage (n + n, two nuclei per cell) occurs; such a condition is called a dikaryon and the phase is called the dikaryophase of the fungus. Later, the parental nuclei fuse and the cells become diploid. The fungi form fruiting bodies in which reduction division occurs, leading to the formation of haploid spores.

Fungi are classified into four classes on the basis of the morphology of the mycelium, the mode of spore formation and the fruiting bodies.

6. Kingdom Plantae and Kingdom Animalia

Kingdom Plantae includes all eukaryotic, chlorophyll-containing organisms commonly called plants. A few members are partially heterotrophic, such as the insectivorous plants (bladderwort, Venus fly trap) and parasites (Cuscuta). Plant cells have a eukaryotic structure with prominent chloroplasts and a cell wall mainly made of cellulose. Plantae includes algae, bryophytes, pteridophytes, gymnosperms and angiosperms. The life cycle of plants has two distinct phases — the diploid sporophytic and the haploid gametophytic — that alternate with each other. The lengths of the haploid and diploid phases, and whether they are free-living or dependent on others, vary among different groups. This phenomenon is called alternation of generations (studied in the next chapter).

Kingdom Animalia is characterised by heterotrophic eukaryotic organisms that are multicellular and whose cells lack cell walls. They directly or indirectly depend on plants for food. They digest their food in an internal cavity and store food reserves as glycogen or fat. Their mode of nutrition is holozoic — by ingestion of food. They follow a definite growth pattern and grow into adults with a definite shape and size. Higher forms show elaborate sensory and neuromotor mechanisms. Most are capable of locomotion. Sexual reproduction is by copulation of male and female, followed by embryological development.

7. Viruses, Viroids, Prions and Lichens

In the five-kingdom classification, some acellular organisms like viruses and viroids, and lichens, are not mentioned.

Viruses

• Viruses did not find a place in classification because they are not considered truly "living", if we understand living as organisms that have a cell structure. Viruses are non-cellular organisms characterised by having an inert crystalline structure outside the living cell.
• Once they infect a cell, they take over the machinery of the host cell to replicate themselves, killing the host. Viruses are therefore obligate parasites.
• The name virus, meaning venom or poisonous fluid, was given by Pasteur. D.J. Ivanowsky (1892) recognised certain microbes as the 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. Beijerinck (1898) demonstrated that the extract of infected tobacco plants could cause infection in healthy plants and called the fluid Contagium vivum fluidum (infectious living fluid).
• W.M. Stanley (1935) showed that viruses could be crystallised and that the crystals consist largely of proteins. They are inert outside their specific host cell.
• In addition to proteins, viruses contain genetic material, which could be either RNA or DNA. No virus contains both RNA and 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.
• The protein coat called capsid, made of small subunits called capsomeres, protects the nucleic acid. These capsomeres are arranged in helical or polyhedral geometric forms.
• Viruses cause diseases like mumps, small pox, herpes and influenza. AIDS in humans is also caused by a virus. In plants, symptoms include mosaic formation, leaf rolling and curling, yellowing and vein clearing, dwarfing and stunted growth.

Viroids and prions

• In 1971, T.O. Diener discovered a new infectious agent that was smaller than viruses and caused potato spindle tuber disease. It was found to be a free RNA; it lacked the protein coat found in viruses, hence the name viroid. The RNA of the viroid was of low molecular weight.
• Prions are infectious agents consisting of abnormally folded protein. They are similar in size to viruses. Prions cause bovine spongiform encephalopathy (BSE), commonly called mad cow disease, in cattle, and its analogous variant Creutzfeldt–Jakob disease (CJD) in humans.

Lichens

Lichens are symbiotic associations — mutually useful associations — between algae and fungi. The algal component is known as the phycobiont and the fungal component as the mycobiont; these are autotrophic and heterotrophic, respectively. Algae prepare food for fungi, and fungi provide shelter and absorb mineral nutrients and water for their partner. So close is their association that if one saw a lichen in nature, one would never imagine that it had two different organisms within it. Lichens are very good pollution indicators — they do not grow in polluted areas.

8. Master Summary Table

9. Mind Map

10. Tips and Tricks

11. Important One-liners

• The five-kingdom classification was proposed by R.H. Whittaker (1969).
• Methanogens in the rumen of cattle produce methane.
• Heterocysts of Nostoc and Anabaena fix atmospheric nitrogen.
• Mycoplasma are the smallest living cells and lack a cell wall.
• Diatoms are the chief producers in the oceans.
• Gonyaulax causes red tides.
• Euglena has a pellicle instead of a cell wall.
• Fungal cell walls contain chitin.
• Albugo is a parasitic fungus on mustard.
• Neurospora is used in biochemical and genetic studies.
• Deuteromycetes are called imperfect fungi.
• Prions cause mad cow disease and CJD.

12. NEET-pattern Questions with Detailed Answers

These questions are original and based on concepts that NEET UG asks again and again.

13. Quick Revision Summary