Evolution of Classification Systems in Biology

Classification Systems for Living Organisms

• Classification systems have undergone several changes over time, with the understanding of what groups or organisms to include in the kingdoms of life also changing.
• The five kingdoms are: Monera, Protista, Fungi, Plantae, and Animalia.

Characteristics of the Five Kingdoms

• Monera: prokaryotic cells, non-cellulosic cell walls, absent nuclear membrane, cellular organization, autotrophic and heterotrophic modes of nutrition.
• Protista: eukaryotic cells, present cell walls, present nuclear membrane, cellular organization, autotrophic and heterotrophic modes of nutrition.
• Fungi: eukaryotic cells, present cell walls with chitin, present nuclear membrane, multicellular or loose tissue organization, heterotrophic mode of nutrition.
• Plantae: eukaryotic cells, present cell walls with cellulose, present nuclear membrane, tissue/organ organization, autotrophic mode of nutrition.
• Animalia: eukaryotic cells, absent cell walls, present nuclear membrane, tissue/organ system organization, heterotrophic mode of nutrition.

Dinoflagellates

• They are mostly marine and photosynthetic, with stiff cellulose plates on the outer surface of their cell walls.
• They have two flagella, one lying longitudinally and the other transversely in a furrow between the wall plates.
• They can undergo rapid multiplication, making the sea appear red (red tides), and releasing toxins that can kill other marine animals.

Euglenoids

• They are mostly freshwater organisms found in stagnant water, with a protein-rich layer called a pellicle instead of a cell wall.
• They have two flagella, a short and a long one, and are photosynthetic in the presence of sunlight.
• They can behave like heterotrophs by predating on other smaller organisms when deprived of sunlight.

Slime Moulds

• They are saprophytic protists that move along decaying twigs and leaves, engulfing organic material.
• Under suitable conditions, they form an aggregation called a plasmodium, which can grow and spread over several feet.
• During unfavourable conditions, the plasmodium differentiates and forms fruiting bodies bearing spores at their tips.

Protozoans

• They are all heterotrophs and live as predators or parasites.
• They are believed to be primitive relatives of animals.
• There are four major groups of protozoans: amoeboid, flagellated, ciliated, and sporozoans.

Kingdom Fungi

• They are a unique kingdom of heterotrophic organisms, showing a great diversity in morphology and habitat.
• They are composed of long, slender thread-like structures called hyphae, which form a network called mycelium.
• Their cell walls are composed of chitin and polysaccharides, and they are mostly heterotrophic and absorb soluble organic matter from dead substrates.
• They can reproduce by vegetative means, asexual reproduction, and sexual reproduction.

Exercises

• Classification systems have undergone changes over time due to the evolution of new understanding of the characteristics of different organisms.
• Two economically important uses of heterotrophic bacteria are:
  • Decomposing organic matter and recycling nutrients.
  • Producing antibiotics and other medicinal products.
• Two economically important uses of archaebacteria are:
  • Decomposing organic matter in extreme environments.
  • Producing biofuels and other industrial products.
• The nature of cell walls in diatoms is that they are made of silica (silicon dioxide).
• Algal bloom and red tides refer to the rapid growth and multiplication of algae and dinoflagellates, respectively, often causing harm to the environment and other organisms.
• Viroids are different from viruses in that they are single-stranded RNA molecules that replicate and cause disease in plants, whereas viruses are composed of DNA or RNA and a protein coat.
• The four major groups of protozoans are amoeboid, flagellated, ciliated, and sporozoans.
• Plants are autotrophic, but some plants can be partially heterotrophic, such as Indian pipe plant and Venus flytrap.
• Phycobiont and mycobiont refer to the algal and fungal components, respectively, of a lichen.