BIOLOGICAL CLASSIFICATION-I

Biological classification, or taxonomy, categorises and organises living organisms based on their shared characteristics and evolutionary relationships. It helps us understand the diversity of life on Earth and facilitates the study of the natural world.

• Aristotle was the first to attempt a scientific basis of classification.
• He groups Plants into: – trees, shrubs and herbs based on simple morphological characters.
• Animals into red blood and those they do not have.
• Linnaeus’ two kingdom classification groups living organisms into Plantae and Animalia
• The two-kingdom system did not distinguish between the eukaryotes and prokaryotes, unicellular and multicellular organisms, photosynthetic (green algae) and non-photosynthetic (fungi) organisms.
• Five-kingdom classification (R.H. Whittaker (1969)) divides living organisms into five groups: – Monera, Protista, Fungi, Plantae and Animalia.
• Basis of classification:
  • Cell structure, Body organisation, Mode of nutrition, ReproductionPhylogenetic relationships.
• Objections to five-kingdom classification –
  • Monera and Protista are heterogeneous groups, including photosynthetic and non-photosynthetic organisms.
  
  
  • The phylogenetic relationship of the lower organism in the kingdom system is not well established.
  
  
  • A distinction between unicellular and multicellular organisms is not possible in algae. Due to this, the kingdom Protista has not included unicellular green algae such as Chlamydomonas, Volvox, etc.
  
  
  • Viruses have not been included in this system of classification. Archaebacteria differ from other bacteria in structure, composition and physiology.

KINGDOM MONERA

• Bacteria belong to the Kingdom of Monera and are the most abundant microorganisms.
• They are everywhere, including extreme habitats such as hot springs, deserts, snow, and deep oceans.
• Bacteria can live as parasites inside or on other organisms.
• Bacteria are grouped based on their shape: cocci, bacilli, vibrio, and spirilla. (Figure 2)
• Bacteria have a simple structure but exhibit complex behaviour and extensive metabolic diversity.
• Some bacteria can synthesise their food from inorganic substrates, while most are heterotrophs and depend on other organisms or dead organic matter for food.

Archaebacteria

• Archaebacteria can live in extreme habitats such as salty areas, hot springs, and marshy areas.
• They have a different cell wall structure that allows them to survive in these harsh conditions.
• Methanogens are a type of archaebacteria present in the gut of ruminant animals.
• Methanogens are responsible for producing methane (biogas) from the dung of these animals.

Eubacteria

• There are thousands of different eubacteria or ‘true bacteria’ with rigid cell walls and flagella.
• Cyanobacteria are photosynthetic autotrophs, unicellular, colonial or filamentous, and often surrounded by a gelatinous sheath.
• Some cyanobacteria can fix atmospheric nitrogen in specialised cells called heterocysts, e.g., Nostoc and Anabaena.
• Chemosynthetic autotrophic bacteria oxidise various inorganic substances and greatly recycle nutrients like nitrogen, phosphorus, iron, and sulphur.
• Heterotrophic bacteria are the most abundant in nature and are important decomposers.
• Many bacteria significantly impact human affairs, such as producing curd, antibiotics, fixing nitrogen in legume roots, etc.
• Some bacteria are pathogens causing damage to human beings, crops, farm animals, and pets. Examples include cholera, typhoid, tetanus, and citrus canker.
• Bacteria reproduce mainly by fission and sometimes produce spores under unfavourable conditions.
• Some bacteria reproduce by a primitive type of DNA transfer from one bacterium to the other.
• Mycoplasma are organisms that completely lack a cell wall, are the smallest living cells known, and can survive without oxygen. Many mycoplasmas are pathogenic in animals and plants.

KINGDOM PROTIST

• All single-celled eukaryotes belong to Protista.
• Boundaries of this kingdom are not well defined.
• Classification varies among biologists.
• Includes Chrysophytes, Dinoflagellates, Euglenoids, Slime moulds, and Protozoans.
• Primarily aquatic organisms.
• Acts as a link between plant, animal, and fungal kingdoms.
• Eukaryotic cells with defined nucleus and organelles.
• Some have flagella or cilia.
• Reproduction involves asexual and sexual processes.

Chrysophytes (Diatoms and Golden Algae{desmids})

• Found in fresh water and marine environments.
• Microscopic and planktonic (float passively in water currents).
• Mostly photosynthetic.
• Diatoms have indestructible silica cell walls.
• Diatomaceous earth (deposits of the silica cell wall) is used in the polishing and filtration of oil and syrup.
• Diatoms are primary producers in oceans.

Dinoflagellates

• Mainly marine and photosynthetic.
• Cell wall with stiff cellulose plates.
• Typically possess two flagella.
• Some, like red dinoflagellates (e.g., Gonyaulax), cause red tides and release toxins harmful to marine life.

Red tide: - rapid multiplication of red dinoflagellate makes the sea appear red.  Dinoflagellate Appear yellow, green, brown, blue or red depending on the main pigments present in their cells.

Euglenoids

• Predominantly found in stagnant freshwater.
• Lack a cell wall but have a flexible protein-rich layer (pellicle).
• Two flagella (short and long).
• Photosynthetic in sunlight, act as heterotrophs in the absence of sunlight.
• E.g., Euglena

Slime Moulds

• Saprophytic protists.
• Move along decaying twigs and leaves engulfing organic material.
• Form aggregations called plasmodium. (Can grow and spread over several feet)
• Under adverse conditions, differentiate into fruiting bodies with spores.
• Extremely resistant spores dispersed by air currents.

Protozoans

• Heterotrophic, live as predators or parasites.
• Considered primitive relatives of animals.
• Amoeboid protozoans:
  • Habitat: Freshwater, seawater, or moist soil.
  
  
  • Locomotion and feeding: Use pseudopodia (false feet) like Amoeba for movement and prey capture.
  
  
  • Special features: Marine forms have silica shells on their surfaceParasitic
  
  
  • examples: Entamoeba and others.
• Flagellated protozoans:
  • Lifestyle: Free-living or parasitic.
  
  
  • Unique feature: Possess flagella.
  
  
  • Disease association: Parasitic forms can cause diseases like sleeping sickness.
  
  
  • Example: Trypanosoma.
• Ciliated protozoans:
  • Habitat: Aquatic environments
  
  
  • Locomotion: Actively move due to thousands of cilia.
  
  
  • Feeding mechanism: Have a gullet that opens to the cell surface, with coordinated cilia movement steering water laden with food into the gullet.
  
  
  • Example: Paramecium.
• Sporozoans:
  • Description: A diverse group with an infectious spore-like stage in their life cycle.
  
  
  • Notable example: Plasmodium, the malarial parasite.
  
  
  • Disease association: Plasmodium causes malaria, a disease significantly impacting human populations.

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