Kingdom Monera

Bacteria are the sole members of Kingdom Monera. They are the most abundant microorganisms on Earth, found almost everywhere, including extreme habitats like hot springs, deserts, snow, and deep oceans. Many also live in or on other organisms as parasites.

Monerans are characterized by their **prokaryotic cell structure**, meaning they lack a well-defined nucleus and membrane-bound organelles. Their cell walls are noncellulosic, composed of polysaccharides and amino acids.

Bacteria are grouped based on their shape:

• Spherical: **Coccus** (pl.: cocci)
• Rod-shaped: **Bacillus** (pl.: bacilli)
• Comma-shaped: **Vibrium** (pl.: vibrio)
• Spiral: **Spirillum** (pl.: spirilla)

Bacteria show extensive metabolic diversity. Some are autotrophic, synthesizing their own food (photosynthetic or chemosynthetic autotrophs). The vast majority are heterotrophic, depending on others or dead organic matter for food (saprophytic or parasitic). Bacteria primarily reproduce by **fission**. Under unfavourable conditions, they may produce spores or reproduce sexually through primitive DNA transfer.

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Archaebacteria

**Archaebacteria** are special types of bacteria that live in some of the most extreme and harsh habitats. Their survival is attributed to their distinct cell wall structure, which differs from other bacteria.

Examples of Archaebacteria:

• **Halophiles:** Live in extreme salty areas.
• **Thermoacidophiles:** Thrive in hot springs and acidic environments.
• **Methanogens:** Found in marshy areas and the gut of ruminant animals (cows, buffaloes), where they produce methane (biogas) from dung.

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Eubacteria

**Eubacteria** are the 'true bacteria', representing thousands of different species. They are characterized by a **rigid cell wall** and, if motile, the presence of a **flagellum**.

• **Cyanobacteria** (Blue-green algae): Photosynthetic autotrophs containing chlorophyll a similar to plants. They can be unicellular, colonial, or filamentous, found in freshwater, marine, or terrestrial environments. Colonies are often surrounded by a gelatinous sheath and can form blooms in polluted water bodies. Some (e.g., Nostoc, Anabaena) possess specialized cells called heterocysts for fixing atmospheric nitrogen.

• **Chemosynthetic autotrophic bacteria:** Oxidise inorganic substances (nitrates, nitrites, ammonia) to obtain energy for ATP production. They play a crucial role in nutrient recycling (nitrogen, phosphorus, iron, sulfur).
• **Heterotrophic bacteria:** Most abundant group. Many are important **decomposers**. They are involved in beneficial activities like making curd, producing antibiotics (e.g., from Streptomyces), and fixing nitrogen in legume roots. However, some are pathogens causing diseases in humans (cholera, typhoid, tetanus), crops (citrus canker), and animals.
• **Mycoplasma:** A unique group of eubacteria that completely **lack a cell wall**. They are the smallest known living cells and can survive without oxygen. Many are pathogenic in plants and animals.

Kingdom Protista

Kingdom Protista includes all **single-celled eukaryotes**. The boundaries of this kingdom are somewhat debated among biologists. Members are primarily **aquatic**. Protista forms a link connecting Kingdoms Monera (prokaryotes), Fungi, Plantae, and Animalia (multicellular eukaryotes).

Protistan cells are **eukaryotic**, possessing a well-defined nucleus and other membrane-bound organelles. Some have flagella or cilia for movement. Protists reproduce both asexually and sexually (involving cell fusion and zygote formation).

Major groups included in Protista (as per this book):

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Chrysophytes

This group includes **diatoms** and **golden algae (desmids)**. They are microscopic, found in freshwater and marine environments, and float passively in water currents (plankton). Most are **photosynthetic**. Diatom cell walls consist of two thin overlapping shells made of silica, making them indestructible. Over billions of years, their accumulation forms 'diatomaceous earth', used in polishing and filtration. Diatoms are major producers in oceans.

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Dinoflagellates

Mostly **marine and photosynthetic**. They exhibit various colours (yellow, green, brown, blue, red) due to different pigments. Their cell wall has stiff cellulose plates on the outer surface. Most have two flagella (one longitudinal, one transverse). Certain red dinoflagellates (like *Gonyaulax*) multiply rapidly, causing 'red tides'. They can release toxins that kill marine animals.

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Euglenoids

Majority are **freshwater organisms**, typically found in stagnant water. Instead of a cell wall, they have a flexible protein-rich layer called **pellicle**. They have two flagella (one short, one long). They are **photosynthetic** in sunlight but behave as **heterotrophs** by preying on smaller organisms when light is unavailable. Euglenoid pigments are identical to those of higher plants (e.g., *Euglena*).

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Slime Moulds

**Saprophytic protists**. Their body (plasmodium) moves along decaying organic matter, engulfing it. Under suitable conditions, the plasmodium forms fruiting bodies with spores at the tips. These spores have true walls, are highly resistant, and are dispersed by air currents.

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Protozoans

All protozoans are **heterotrophs**, living as predators or parasites. They are considered primitive relatives of animals. There are four major groups:

• **Amoeboid protozoans:** Live in freshwater, seawater, or moist soil. Move and capture prey using pseudopodia (false feet), like *Amoeba*. Some marine forms have silica shells. *Entamoeba* is a parasitic form.
• **Flagellated protozoans:** Free-living or parasitic, possessing flagella. Parasitic forms cause diseases (e.g., *Trypanosoma* causes sleeping sickness).
• **Ciliated protozoans:** Aquatic, actively moving due to thousands of cilia on their surface. Have a gullet cavity for food intake. *Paramoecium* is an example.

• **Sporozoans:** Diverse, characterized by an infectious spore-like stage in their life cycle. *Plasmodium* (malarial parasite), causing malaria, is the most well-known example.

Kingdom Fungi

Kingdom Fungi comprises unique **heterotrophic organisms** showing great diversity in morphology and habitat. They are found on moist bread, rotten fruits, as mushrooms, toadstools, and even causing diseases in plants (e.g., wheat rust by *Puccinia*) and animals. Some (like yeast) are unicellular and used in baking and brewing; others (like *Penicillium*) are sources of antibiotics. Fungi are cosmopolitan and prefer warm, humid places. Refrigeration prevents fungal/bacterial growth in food.

Most fungi are **filamentous**, consisting of long, slender thread-like structures called **hyphae**. The network of hyphae is the **mycelium**. Some hyphae are continuous tubes with multiple nuclei (coenocytic hyphae); others have cross walls (septae). Fungal cell walls are composed of **chitin** and polysaccharides.

Most fungi are **saprophytic**, absorbing soluble organic matter from dead substrates. Some are parasitic on living plants/animals. Others live as **symbionts**, like lichens (with algae) or mycorrhiza (with plant roots).

Reproduction in fungi:

• **Vegetative:** Fragmentation, fission (in yeast), budding (in yeast).
• **Asexual:** By spores like conidia (exogenous), sporangiospores (endogenous), or zoospores (motile).
• **Sexual:** By oospores, ascospores, or basidiospores. The sexual cycle involves:
  1. Plasmogamy (fusion of protoplasms).
  2. Karyogamy (fusion of nuclei).
  3. Meiosis in the zygote to produce haploid spores.

  In some fungi (Ascomycetes, Basidiomycetes), plasmogamy is followed by a dikaryotic stage (n+n, two nuclei per cell) before karyogamy occurs, forming a dikaryon. Later, nuclei fuse to become diploid before meiosis.

Classification of Kingdom Fungi into classes is based on mycelium morphology, spore formation, and fruiting bodies.

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Phycomycetes

Found in aquatic habitats, on decaying wood, or as obligate plant parasites. Mycelium is **aseptate and coenocytic** (multinucleate, without cross walls). Asexual reproduction by zoospores (motile) or aplanospores (non-motile), produced inside sporangia. Sexual reproduction forms zygospores by the fusion of two gametes (isogamous or anisogamous/oogamous). Examples: *Mucor*, *Rhizopus* (bread mould), *Albugo* (parasitic fungus on mustard).

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Ascomycetes

Commonly called **sac-fungi**. Mostly multicellular (e.g., *Penicillium*, *Aspergillus*), rarely unicellular (e.g., *Yeast*). Saprophytic, decomposers, parasitic, or coprophilous (grow on dung). Mycelium is **branched and septate**. Asexual spores (conidia) are produced exogenously on conidiophores. Sexual spores (ascospores) are produced endogenously in sac-like structures called asci, arranged in fruiting bodies (ascocarps). Examples: *Aspergillus*, *Claviceps*, *Neurospora*. *Neurospora* is used in genetic studies. Some (morels, truffles) are edible.

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Basidiomycetes

Common forms include **mushrooms** (*Agaricus*), bracket fungi, and puffballs. Grow in soil, on wood, or as plant parasites (rusts, smuts). Mycelium is **branched and septate**. Asexual spores are generally absent, but fragmentation is common. Sex organs are absent; plasmogamy occurs by fusion of vegetative cells. Dikaryotic stage leads to basidium. Karyogamy and meiosis in basidium produce four basidiospores exogenously. Basidiospores are produced on fruiting bodies (basidiocarps). Examples: *Agaricus* (mushroom), *Ustilago* (smut), *Puccinia* (rust fungus).

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Deuteromycetes

Commonly called **imperfect fungi** because only their asexual or vegetative phases are known. When sexual stages are discovered, they are moved to Ascomycetes or Basidiomycetes. Reproduce only by **asexual spores (conidia)**. Mycelium is **septate and branched**. Some are saprophytes or parasites; many are decomposers (especially of litter, helping in mineral cycling). Examples: *Alternaria*, *Colletotrichum*, *Trichoderma*.

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Kingdom Plantae

Kingdom Plantae includes all **eukaryotic organisms that contain chlorophyll**. These are commonly referred to as plants. The majority are autotrophic, producing food by photosynthesis. However, some members are partially heterotrophic, such as insectivorous plants (Bladderwort, Venus fly trap) that capture insects or parasitic plants (*Cuscuta*).

Plant cells have a eukaryotic structure with prominent chloroplasts (containing chlorophyll) and a cell wall primarily made of **cellulose**. Kingdom Plantae includes algae, bryophytes, pteridophytes, gymnosperms, and angiosperms.

The life cycle of plants exhibits **alternation of generations**, alternating between a diploid sporophytic phase and a haploid gametophytic phase. The relative lengths and dependence of these phases vary among different plant groups.

Kingdom Animalia

Kingdom Animalia is characterized by **heterotrophic eukaryotic organisms** that are **multicellular** and whose cells **lack cell walls**. Animals depend directly or indirectly on plants for food.

Characteristic features of animals:

• Heterotrophic mode of nutrition: They obtain food by consuming other organisms.
• Holozoic mode of nutrition: They ingest food into an internal cavity and digest it.
• Store food reserves as glycogen or fat.
• Exhibit a definite growth pattern, developing into adults with a definite shape and size.
• Show elaborate sensory and neuromotor mechanisms (especially higher forms).
• Most are capable of locomotion (movement).
• Primarily reproduce by the **sexual mode** (copulation followed by embryological development).

Viruses, Viroids, Prions And Lichens

Whittaker's Five Kingdom classification does not include certain acellular organisms like **viruses, viroids, and prions**, nor does it include **lichens**. These are briefly discussed separately.

• **Viruses:** Acellular organisms characterized by an **inert crystalline structure outside a living host cell**. They are not considered truly 'living' outside a host because they lack cellular structure and metabolic machinery. Once they infect a host cell, they take over the host's machinery to replicate themselves, often harming or killing the host. Virus means 'venom' or 'poisonous fluid'. Dmitri Ivanowsky (1892) discovered them as causal agents of tobacco mosaic disease, and M.W. Beijerinek (1898) coined the term 'virus'. W.M. Stanley (1935) showed viruses could be crystallised, composed mainly of proteins. Viruses are **obligate parasites**. They contain **genetic material** (either RNA or DNA, never both) enclosed in a protein coat called a **capsid**, made of smaller subunits called **capsomeres**. Capsomeres can be arranged helically or polyhedrally. Viruses infecting plants typically have single-stranded RNA; those infecting animals have single or double-stranded RNA or double-stranded DNA. Bacterial viruses (bacteriophages) usually have double-stranded DNA. Viral diseases include mumps, smallpox, herpes, influenza, AIDS (in humans), and mosaic formation, leaf rolling/curling, yellowing, stunted growth (in plants).

• **Viroids:** Discovered by T.O. Diener (1971). Infectious agents smaller than viruses, causing diseases like potato spindle tuber disease. They are characterized by a **free RNA** molecule (without a protein coat). Viroid RNA is of low molecular weight.
• **Prions:** Infectious neurological agents consisting of abnormally folded **proteins**. Similar in size to viruses. Cause neurological diseases like bovine spongiform encephalopathy (mad cow disease) in cattle and Creutzfeldt-Jakob disease (CJD) in humans.
• **Lichens:** Symbiotic associations between **algae** (phycobiont, autotrophic) and **fungi** (mycobiont, heterotrophic). The fungus provides shelter and absorbs water/minerals for the alga; the alga prepares food for the fungus (photosynthesis). Their association is so close that they appear as a single organism. Lichens are good **pollution indicators** as they do not grow in polluted areas (e.g., areas with sulfur dioxide pollution).

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