Biological Classification Systems

What Is The Basis Of Classification?

The vast diversity of life on Earth necessitates a systematic way to study and organise living organisms. Biological classification is the process by which scientists group organisms into categories based on shared characteristics.

Different systems of classification have been proposed over time, using various criteria as their basis:

Early Attempts at Classification

• Aristotle (Greek philosopher) was one of the earliest to attempt a scientific basis for classification. He classified plants into herbs, shrubs, and trees. Animals were classified into those with red blood and those without red blood. This was a very simple classification based on morphological characters.

Artificial Systems of Classification

• These systems used only a few superficial morphological characters for classification, such as habit, colour, number, and shape of leaves.
• They gave equal weightage to vegetative and sexual characteristics, even though vegetative characters are often more easily affected by environmental factors.
• Example: Linnaeus's Artificial System, which classified plants based mainly on the number and arrangement of stamens and carpels (sexual characteristics).
• Drawback: Did not reflect natural relationships between organisms.

Natural Systems of Classification

• These systems were based on natural affinities among organisms.
• They considered not only external features but also internal features like anatomy, embryology, and phytochemistry.
• Example: Bentham and Hooker's System of classification for seed plants. This was a widely used natural classification system.
• Drawback: Still did not fully account for evolutionary relationships.

Phylogenetic Systems of Classification

• These are modern classification systems based on evolutionary relationships between organisms.
• They assume that organisms belonging to the same taxa have a common ancestor.
• They use information from various sources, including fossil records, morphology, anatomy, embryology, cytology, and molecular data (e.g., DNA and protein sequences).
• Example: Modern classification systems like the Five-Kingdom Classification proposed by R.H. Whittaker.

Modern Basis of Classification

Modern classification systems use a combination of various characteristics, including:

• Morphology: External features.
• Anatomy: Internal structure.
• Cytology: Study of cells (cell structure, number of chromosomes, etc.).
• Embryology: Developmental stages from embryo to adult.
• Physiology: Functioning of organs and systems.
• Ecology: Relationship with the environment.
• Molecular Biology: Comparison of DNA, RNA, and protein sequences (e.g., using techniques like DNA sequencing and electrophoresis). This provides objective evidence of evolutionary relationships.
• Fossil records: Provide evidence of extinct organisms and evolutionary history.

The transition from artificial to natural to phylogenetic systems reflects the increasing understanding of the complexity and evolutionary history of life.

Classification And Evolution

Modern biological classification is intimately linked with the concept of evolution. The systems of classification developed today aim to reflect the evolutionary history and relationships among organisms. This field is known as systematics.

Systematics and Phylogeny

• Systematics is the study of the diversification of living forms, past and present, and the relationships among living things through time.
• Modern systematics uses taxonomic classification to organise diversity but goes further to understand the evolutionary pathways that have led to this diversity.
• A key goal of systematics is to construct phylogenetic trees, which are hypotheses about the evolutionary relationships among different groups of organisms.
• Classification based on evolutionary relationships is called phylogenetic classification.

Homologous vs. Analogous Structures

Evolutionary relationships are often inferred by comparing structures in different organisms:

• Homologous Structures: Structures that have a common evolutionary origin but may have different functions. They indicate common ancestry.

  Example: The forelimbs of humans, bats, whales, and cheetahs have similar bone structure (humerus, radius, ulna, carpals, metacarpals, phalanges) indicating descent from a common ancestor, but they are adapted for different functions (grasping, flying, swimming, running).

  *(Image shows simplified bone structures of vertebrate forelimbs, highlighting similar underlying anatomy despite different external forms and functions)*

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• Analogous Structures: Structures that have different evolutionary origins but perform similar functions due to convergent evolution. They do not indicate common ancestry.

  Example: The wings of insects and birds. Both are used for flying, but they developed independently and have very different structures.

  *(Image shows external illustrations of an insect wing and a bird wing)*

Phylogenetic classification systems primarily rely on homologous structures (morphological, anatomical, or molecular) to group organisms.

The Five-Kingdom Classification (R.H. Whittaker, 1969)

This is the most widely accepted biological classification system. Whittaker proposed a five-kingdom system based on the following criteria:

1. Cell Structure: Prokaryotic or Eukaryotic.
2. Thallus Organisation: Unicellular or Multicellular.
3. Mode of Nutrition: Autotrophic (photosynthetic or chemosynthetic), Heterotrophic (saprophytic or parasitic), Holozoic.
4. Reproduction: Asexual or Sexual.
5. Phylogenetic Relationships: Evolutionary history.

The five kingdoms are:

1. Monera: Includes all prokaryotes.
2. Protista: Includes all unicellular eukaryotes.
3. Fungi: Includes eukaryotic, multicellular (except yeast), heterotrophic organisms with cell walls made of chitin.
4. Plantae: Includes eukaryotic, multicellular, autotrophic (photosynthetic) organisms with cell walls made of cellulose.
5. Animalia: Includes eukaryotic, multicellular, heterotrophic organisms without cell walls.

*(Image shows a diagram or flowchart branching from cell type (prokaryotic/eukaryotic) to cellularity (unicellular/multicellular) to mode of nutrition, leading to the five kingdoms)*

Earlier Classification Systems

• Two-Kingdom Classification (Linnaeus): Classified all organisms into Kingdom Plantae and Kingdom Animalia. Did not distinguish between prokaryotes and eukaryotes, unicellular and multicellular organisms, or photosynthetic and non-photosynthetic fungi.
• Three-Kingdom Classification (Haeckel): Added Kingdom Protista for unicellular organisms. Still didn't separate prokaryotes and eukaryotes.
• Four-Kingdom Classification (Copeland): Added Kingdom Monera for prokaryotes. Grouped all eukaryotes into Protista, Plantae, Animalia. Fungi were still included in Plantae.

The Five-Kingdom system is a more comprehensive and biologically sound classification compared to earlier systems.

Answer:

The Hierarchy Of Classification- Groups

While the previous chapter introduced the taxonomic hierarchy (Kingdom down to Species) as a system of grouping, this chapter focuses on different classification systems. However, the provided outline includes "The Hierarchy Of Classification- Groups" again. Assuming this section intends to discuss the groups within the Five Kingdom System rather than the general hierarchy which was covered, I will provide a brief overview of the kingdoms and then elaborate on Monera, Protista, and Fungi as listed in subsequent headings, as they are distinct kingdoms often covered in detail in this context. The general hierarchy (Kingdom, Phylum, etc.) was discussed in detail in the "Living World" chapter and won't be repeated here.

Overview of the Five Kingdoms

The Five Kingdom classification system groups all living organisms into five broad categories based on the criteria discussed earlier (cell structure, thallus organisation, mode of nutrition, reproduction, and phylogeny).

• Kingdom Monera: All prokaryotic organisms.
• Kingdom Protista: All unicellular eukaryotic organisms.
• Kingdom Fungi: Eukaryotic, heterotrophic organisms, mostly multicellular, with chitinous cell walls.
• Kingdom Plantae: Eukaryotic, multicellular, autotrophic organisms with cellulosic cell walls.
• Kingdom Animalia: Eukaryotic, multicellular, heterotrophic organisms without cell walls.

This system provides a framework for understanding the relationships and diversity among major groups of life.

Kingdom Monera

Kingdom Monera includes all prokaryotic organisms. They are typically unicellular.

General characteristics of Monerans:

• Cell Type: Prokaryotic. They lack a true nucleus and membrane-bound organelles (like mitochondria, chloroplasts, Golgi apparatus, endoplasmic reticulum).
• Genetic Material: Naked circular DNA (nucleoid) is present in the cytoplasm. Plasmids (extra-chromosomal circular DNA) may also be present.
• Ribosomes: 70S type (smaller than eukaryotic 80S ribosomes).
• Cell Wall: Present in most monerans, but its composition varies. In bacteria, it is made of peptidoglycan. Absent in Mycoplasma.
• Mode of Nutrition: Highly diverse. Can be Autotrophic (photosynthetic or chemosynthetic) or Heterotrophic (saprophytic or parasitic).
• Reproduction: Primarily asexual, mainly by binary fission. Sexual reproduction (transfer of genetic material) can occur via processes like conjugation, transformation, and transduction.
• Motility: Many monerans are motile, possessing flagella (bacterial flagella, different from eukaryotic flagella).

Monera is a diverse group. It is broadly classified into Archaebacteria and Eubacteria.

Archaebacteria

These are considered 'ancient bacteria'. They are known for living in some of the most extreme habitats.

• They differ from other bacteria in having a different cell wall structure (lacking peptidoglycan in most cases), which helps them survive in harsh conditions.
• Examples based on habitat:
  • Halophiles: Live in extremely salty areas (e.g., Great Salt Lake, Dead Sea).
  • Thermoacidophiles: Live in hot springs and deep-sea hydrothermal vents with high temperature and acidic pH.
  • Methanogens: Live in marshy areas and the guts of ruminant animals (like cows, buffaloes). They produce methane gas (biogas).
• Methanogens are also found in the guts of ruminants and are responsible for the production of methane from the dung of these animals.

Eubacteria

These are true bacteria. They are characterised by the presence of a rigid cell wall (peptidoglycan present) and, if motile, a flagellum.

Diversity in Eubacteria

• Bacteria shape: Eubacteria exhibit various shapes:
  • Coccus (cocci): Spherical (e.g., Streptococcus, Staphylococcus)
  • Bacillus (bacilli): Rod-shaped (e.g., Bacillus subtilis, E. coli)
  • Vibrium (vibrio): Comma-shaped (e.g., Vibrio cholerae)
  • Spirillum (spirilla): Spiral-shaped (e.g., Spirillum minus)

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• Mode of Nutrition:
  • Autotrophic Bacteria: Synthesise their own food.
    • Photosynthetic autotrophs: Use light energy (e.g., Cyanobacteria or blue-green algae - contain chlorophyll a similar to plants). Cyanobacteria are often colonial (forming filaments or colonies), can be fresh water, marine, or terrestrial. Colonies are often surrounded by a gelatinous sheath. Some can fix atmospheric nitrogen in specialised cells called heterocysts (e.g., Nostoc, Anabaena).
    • Chemosynthetic autotrophs: Oxidise various inorganic substances (like nitrates, nitrites, ammonia) and use the released energy for ATP production. They play a great role in recycling nutrients like nitrogen, phosphorus, iron, sulphur.
  • Heterotrophic Bacteria: Obtain food from other organisms or dead organic matter. They are the most abundant bacteria.
    • Saprophytic: Decomposers, living on dead organic matter.
    • Parasitic: Living on or inside other living organisms, causing diseases.

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• Reproduction:
  • Primary mode is fission (simple cell division).
  • Under unfavourable conditions, some bacteria produce spores (endospores) for survival.
  • Sexual reproduction involves primitive DNA transfer (conjugation, transformation, transduction).

*(Image shows illustrations of different bacterial shapes and a filamentous cyanobacterium with heterocysts)*

Importance of Heterotrophic Bacteria:

• They are important decomposers, helping in nutrient cycling.
• Used in the production of curd from milk.
• Used in antibiotic production.
• Fixing nitrogen in legume roots (e.g., Rhizobium).
• Many are pathogenic, causing diseases in humans, plants, and animals (e.g., cholera, typhoid, tetanus, citrus canker).

Mycoplasma:

• They are monerans but are the smallest living cells known.
• They lack a cell wall.
• They can survive without oxygen.
• Many are pathogenic to plants and animals.

Kingdom Protista

Kingdom Protista includes all unicellular eukaryotes. Members of Protista are primarily aquatic. This kingdom acts as a link between other kingdoms: Plantae, Fungi, and Animalia.

General characteristics of Protists:

• Cell Type: Eukaryotic. They have a true nucleus and membrane-bound organelles.
• Cellularity: Unicellular (though some may form colonies).
• Cell Wall: Present in some (e.g., diatoms, dinoflagellates), absent in others (e.g., Amoeba, Paramecium). The composition varies when present.
• Mode of Nutrition: Diverse. Can be Autotrophic (photosynthetic), Heterotrophic (holozoic, saprophytic), or Mixotrophic (combination of autotrophic and heterotrophic).
• Reproduction: Primarily asexual (binary fission, budding). Sexual reproduction involves cell fusion and zygote formation.
• Motility: Many are motile, using flagella or cilia or pseudopodia.

Protista is a diverse kingdom, and its members are grouped into categories like Chrysophytes, Dinoflagellates, Euglenoids, Slime Moulds, and Protozoans.

Chrysophytes

Includes diatoms and golden algae (desmids).

• Habitat: Freshwater and marine environments; planktonic (float passively in water currents).
• Microscopic.
• Photosynthetic (autotrophic).
• Cell wall: In diatoms, the cell wall forms two thin overlapping shells, which fit together like a soapbox. The walls are embedded with silica, making them indestructible.
• After death, diatoms leave behind their silica cell walls, which accumulate over millions of years to form Diatomaceous earth. This material is gritty and is used in polishing, filtration of oils and syrups.
• Diatoms are the chief 'producers' in the oceans.

*(Image shows illustrations of various shapes of diatoms and desmids)*

Dinoflagellates

• Habitat: Mostly marine, photosynthetic.
• Appearance: Vary in colour (yellow, green, brown, blue, or red) depending on the pigments in their cells.
• Cell wall: Has stiff cellulose plates on the outer surface.
• Flagella: Most have two flagella; one lies longitudinally and the other transversely in a furrow between the wall plates.
• Example: Gonyaulax (red dinoflagellate).
• Some red dinoflagellates multiply rapidly and make the sea appear red, causing red tides. Toxins released by such large numbers can kill marine animals like fish.

*(Image shows a diagram of a dinoflagellate highlighting the two flagella and cell wall plates)*

Euglenoids

• Habitat: Mostly freshwater, stagnant water.
• Cell wall: Instead of a cell wall, they have a protein-rich layer called a pellicle, which makes their body flexible.
• Flagella: Have two flagella, one short and one long.
• Pigments: Have pigments identical to those in higher plants (Chlorophyll a and b).
• Mode of Nutrition: Mixotrophic. They are photosynthetic in the presence of sunlight. When deprived of sunlight, they behave as heterotrophs by predating on other smaller organisms.
• Example: Euglena.
• Presence of a rudimentary eye spot sensitive to light.

*(Image shows a diagram of Euglena highlighting key features)*

Slime Moulds

• Mode of Nutrition: Saprophytic protists. They feed on decaying organic matter.
• Habitat: Grow on decaying twigs and leaves.
• Life cycle:
  • Under favourable conditions, they form an aggregation called plasmodium, which can grow and spread over several feet.
  • Under unfavourable conditions, the plasmodium differentiates and forms fruiting bodies bearing spores at their tips.
  • The spores possess true walls and are extremely resistant, surviving for many years even under adverse conditions.
  • The spores are dispersed by air currents.
• Example: Various types of slime moulds.

*(Image shows illustrations of the plasmodial stage and the fruiting bodies with spores)*

Protozoans

All protozoans are heterotrophs and live as predators or parasites. They are believed to be primitive relatives of animals.

Protozoans are grouped into four major types:

• Amoeboid Protozoans:
  • Habitat: Live in freshwater, sea water, or moist soil.
  • Move and capture prey by putting out false feet (pseudopodia), as in Amoeba.
  • Marine forms have silica shells on their surface.
  • Some are parasites (e.g., Entamoeba histolytica, causes amoebic dysentery in humans).

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• Flagellated Protozoans:
  • Either free-living or parasitic.
  • Have flagella.
  • Parasitic forms cause diseases (e.g., Trypanosoma, causes sleeping sickness).

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• Ciliated Protozoans:
  • Aquatic, actively moving organisms because of the presence of thousands of cilia over their body surface.
  • Have a cavity (gullet) that opens to the outside of the cell surface. The coordinated movement of rows of cilia causes the water laden with food to be steered into the gullet.
  • Example: Paramecium.

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• Sporozoans:
  • Have an infectious spore-like stage in their life cycle.
  • All are parasites.
  • Example: Plasmodium (malaria parasite), which causes malaria in humans.

*(Image shows illustrations of Amoeba with pseudopodia, Paramecium with cilia, possibly Trypanosoma with flagella, and a representation of Plasmodium life stage like sporozoite or merozoite)*

Kingdom Fungi

Kingdom Fungi is a unique kingdom of heterotrophic organisms. They show a great diversity in morphology and habitat. They are typically multicellular, except for yeast which is unicellular.

General characteristics of Fungi:

• Cell Type: Eukaryotic.
• Cell Wall: Present, made of chitin and polysaccharides.
• Mode of Nutrition: Heterotrophic.
  • Saprophytic: Feed on dead and decaying organic matter.
  • Parasitic: Live on or inside living organisms.
  • Symbiotic: Live in association with algae (lichens) or roots of higher plants (mycorrhiza).
• Body Structure: Except for unicellular yeast, fungal bodies consist of long, slender, thread-like structures called hyphae. A network of hyphae is called a mycelium.
  • Hyphae may be septate (with cross-walls dividing the hypha into cells) or aseptate/coenocytic (continuous tubes filled with multinucleated cytoplasm).
• Reproduction: Can occur by vegetative, asexual, and sexual means.
  • Vegetative reproduction: Fragmentation, fission (in yeast), budding.
  • Asexual reproduction: By spores, commonly conidia, sporangiospores, or zoospores. Spores are produced in structures called sporangia.
  • Sexual reproduction: By oospores, ascospores, and basidiospores. Sexual reproduction involves three steps:
    1. Plasmogamy: Fusion of protoplasts between two motile or non-motile gametes.
    2. Karyogamy: Fusion of two nuclei.
    3. Meiosis: Occurs in the zygote (or zygospore) to produce haploid spores.
• In some fungi, after plasmogamy, karyogamy does not follow immediately. Instead, a dikaryotic stage (n+n, with two nuclei per cell) occurs, called a dikaryon. This phase is followed by karyogamy later.

*(Image shows branched hyphae, illustrating septate and coenocytic types, and a simple diagram of a mushroom or mould with mycelium and a fruiting body)*

Kingdom Fungi is classified into various classes based on the morphology of the mycelium, mode of spore formation, and fruiting bodies.

Phycomycetes (Algal Fungi)

• Habitat: Aquatic habitats, on decaying wood in moist and damp places, as obligate parasites on plants.
• Mycelium: Aseptate and coenocytic (multinucleate cytoplasm without septa).
• Asexual reproduction: By zoospores (motile) or aplanospores (non-motile). These spores are endogenously produced in sporangium.
• Sexual reproduction: Isogamous (gametes are similar in size and shape), Anisogamous (gametes are dissimilar), or Oogamous (female gamete large and non-motile, male gamete small and motile). Gametes are produced in gametangia. The zygote develops into a thick-walled zygospore.
• Examples: Mucor, Rhizopus (the bread mould), Albugo (parasitic fungus on mustard).

*(Image shows Rhizopus with sporangiophores bearing sporangia and spreading hyphae)*

Ascomycetes (Sac Fungi)

• Habitat: Mostly multicellular (e.g., Penicillium), rarely unicellular (e.g., yeast). Saprophytic, decomposers, parasitic, or coprophilous (growing on dung).
• Mycelium: Septate and branched.
• Asexual reproduction: By conidia, produced exogenously on special mycelium called conidiophores. Conidia germinate to produce mycelium.
• Sexual reproduction: By ascospores, produced endogenously in sac-like structures called asci (singular: ascus). Asci are arranged in different types of fruiting bodies called ascocarps.
• The dikaryotic stage (n+n) is present.
• Examples: Aspergillus, Claviceps, Neurospora. Yeast ($Saccharomyces$) is also an Ascomycete. Morels and truffles are edible Ascomycetes.
• Neurospora is widely used in biochemical and genetic work.

*(Image shows Aspergillus with conidiophores and conidia, and a diagram showing asci containing ascospores, possibly arranged in an ascocarp)*

Basidiomycetes (Club Fungi)

• Habitat: Grow in soil, on logs and tree stumps, and in living plant bodies as parasites (e.g., rusts and smuts).
• Mycelium: Septate and branched.
• Asexual reproduction: Generally absent, but vegetative reproduction by fragmentation is common.
• Sexual reproduction: Sex organs are absent, but plasmogamy is brought about by fusion of two vegetative or somatic cells of different strains or genotypes. The resulting structure is dikaryotic (n+n), which gives rise to the basidium.
• Karyogamy and meiosis occur in the basidium, producing basidiospores. Basidiospores are produced exogenously on the basidium.
• Basidia are arranged in fruiting bodies called basidiocarps.
• Examples: Mushrooms, bracket fungi, puffballs. Agaricus (mushroom), Ustilago (smut), Puccinia (rust fungus).

*(Image shows a diagram of a mushroom and a microscopic view of a basidium bearing basidiospores)*

Deuteromycetes (Imperfect Fungi)

• These fungi are called Imperfect Fungi because only their asexual or vegetative phases are known. Their sexual forms (if they exist) belong to Ascomycetes or Basidiomycetes. When their sexual forms are discovered, they are moved to the appropriate class.
• Mycelium: Septate and branched.
• Asexual reproduction: Only by conidia.
• Mode of Nutrition: Mostly saprophytes or parasites, but a large number are decomposers of litter and help in mineral cycling.
• Examples: Alternaria, Colletotrichum, Trichoderma.

Kingdom Plantae

Kingdom Plantae includes all eukaryotic, multicellular autotrophs. They are characterised by the presence of a cell wall made primarily of cellulose.

General characteristics of Plants:

• Cell Type: Eukaryotic.
• Cellularity: Mostly multicellular.
• Cell Wall: Present, made of cellulose.
• Mode of Nutrition: Autotrophic, performing photosynthesis using chlorophyll and other pigments in chloroplasts. Some plants are partially or totally heterotrophic (e.g., parasitic plants like Cuscuta, insectivorous plants like Pitcher plant).
• Reproduction: Asexual (vegetative propagation) and Sexual.
• Life cycle often involves alternation of generations (a haploid gametophytic phase and a diploid sporophytic phase).

The Kingdom Plantae includes diverse groups ranging from simple algae to complex flowering plants. Based on the complexity of the plant body, presence/absence of vascular tissues, and seed formation, the kingdom is broadly classified into:

• Algae: Simple thalloid forms, mostly aquatic.
• Bryophytes: More differentiated plant body than algae, found in moist shady areas (mosses and liverworts).
• Pteridophytes: Possess vascular tissues (xylem and phloem), found in cool, damp, shady places (ferns and horsetails).
• Gymnosperms: Bear naked seeds (seeds not enclosed within an ovary/fruit).
• Angiosperms: Flowering plants, bear seeds enclosed within a fruit.

Detailed study of these groups (Algae, Bryophytes, Pteridophytes, Gymnosperms, Angiosperms) is covered in separate chapters.

Kingdom Animalia

Kingdom Animalia includes all eukaryotic, multicellular heterotrophs. They are characterised by the absence of a cell wall.

General characteristics of Animals:

• Cell Type: Eukaryotic.
• Cellularity: Multicellular.
• Cell Wall: Absent.
• Mode of Nutrition: Heterotrophic, typically by ingestion (holozoic nutrition). Food is digested in an internal cavity and absorbed.
• Motility: Most animals are motile (can move from one place to another).
• Nervous System: Typically possess a nervous system for coordination and response to stimuli.
• Reproduction: Primarily sexual reproduction, involving the formation of gametes.
• Development usually involves embryological stages.

Kingdom Animalia is classified into various phyla based on criteria such as level of organisation (cellular, tissue, organ, organ system), symmetry (asymmetrical, radial, bilateral), germ layers (diploblastic, triploblastic), coelom (body cavity), presence/absence of segmentation, notochord, etc.

• Examples of major phyla: Porifera (sponges), Coelenterata (Hydra, jellyfish), Ctenophora (comb jellies), Platyhelminthes (flatworms), Aschelminthes (roundworms), Annelida (earthworm), Arthropoda (insects, spiders, crustaceans), Mollusca (snails, oysters), Echinodermata (starfish), Hemichordata, and Chordata (vertebrates - fish, amphibians, reptiles, birds, mammals).

Detailed study of the phyla and classes within Kingdom Animalia is covered in separate chapters.

Viruses, Viroids And Lichens

While the Five-Kingdom Classification includes most life forms, there are some entities that are not classified within this system. These include Viruses, Viroids, and Lichens. This is because they do not fully fit the criteria used for classification within the five kingdoms.

Viruses

• Viruses are acellular (not made of cells).
• They are obligate intracellular parasites, meaning they can only replicate inside a living host cell, using the host's machinery.
• They lack their own metabolic machinery.
• Outside the host cell, they are inert particles.
• Viruses are considered to be on the borderline between living and non-living. They exhibit characteristics of life (reproduction, mutation) only when inside a host cell.
• Structure: A virus consists of:
  • Genetic material: Contains either RNA or DNA, but never both in a single virus. The genetic material can be single-stranded or double-stranded.
  • Protein coat (Capsid): Surrounds the genetic material. It is made up of small subunits called capsomeres. The capsid protects the nucleic acid.
  • Some viruses (like the influenza virus) also have an outer envelope derived from the host cell membrane.
• The genetic material is infectious.
• Viruses cause diseases in plants (e.g., Tobacco Mosaic Virus - TMV, showing mosaic formation, leaf rolling and curling) and animals (e.g., measles, mumps, polio, AIDS).
• Viruses that infect bacteria are called bacteriophages. They typically have double-stranded DNA.

*(Image shows diagrams of a bacteriophage (head, collar, sheath, tail fibres) and/or TMV (helical shape, RNA inside protein coat))*

Viroids

• Viroids were discovered by T.O. Diener in 1971.
• They are smaller than viruses.
• They consist of only a free RNA molecule (low molecular weight) without a protein coat.
• The RNA is circular and single-stranded.
• Viroids cause diseases, mainly in plants (e.g., Potato Spindle Tuber disease).

Lichens

• Lichens are symbiotic associations between algae and fungi.
• The algal component is called the phycobiont (autotrophic).
• The fungal component is called the mycobiont (heterotrophic).
• The phycobiont (alga) prepares food by photosynthesis for fungi.
• The mycobiont (fungus) provides shelter, water, and mineral absorption for the alga.
• The association is so close that if one saw a lichen in nature, one would think that it was a single organism.
• Lichens are important as pollution indicators. They do not grow in polluted areas (especially areas with sulphur dioxide pollution).

*(Image shows a clump of lichen with its characteristic leafy or crusty appearance on a substrate)*

These entities highlight the complexity and diversity of life (and life-like) forms, even those that do not fit neatly into standard classification systems.