Kingdom Classification of Living Organisms
Chronology of Classifications
Living things were initially classified into two kingdoms: Animalia and Plantae. However, with scientific advancements, it became apparent that some organisms could not be assigned to either of these two kingdoms. Haeckel suggested the establishment of the kingdom Protista to group all unicellular organisms whose characteristics did not fit either plants or animals. This included bacteria, which were later separated into another kingdom named Monera, encompassing single-celled prokaryotic organisms.
Whittaker proposed a five-kingdom classification based on cell type, cell number, and the type of nutrition. Organisms were divided into: Animalia, Plantae, Fungi, Protista, and Monera. The kingdom Fungi included mushrooms, which were separated from the Plantae kingdom.
Lynn Margulis and Karlene V. Schwartz maintained the five-kingdom system with some conceptual changes, renaming the kingdoms as: Monera, Protoctista, Fungi, Plantae, and Animalia.
Current Trends
The discovery of new species, genomic sequencing, and modern molecular biology techniques have allowed for a deeper understanding of evolutionary relationships. A current classification proposed by Cavalier-Smith suggests the existence of two superkingdoms encompassing six kingdoms. The superkingdom Prokaryota would include the kingdom Bacteria, and the superkingdom Eukaryota would be subdivided into five kingdoms: Protozoa, Chromista, Fungi, Plantae, and Animalia. The kingdom Chromista includes some organisms formerly classified within the Protoctista and Fungi kingdoms.
The Monera Kingdom
This kingdom includes unicellular prokaryotic organisms, representing the most primitive group. These organisms are believed to have evolved from a common ancestor and are known as bacteria. Around 2,000 million years ago (mya), cyanobacteria were responsible for increasing atmospheric oxygen through photosynthesis.
Characteristics:
- Are unicellular prokaryotes. Their genetic material is dispersed in the cytoplasm.
- Do not have cytoplasmic organelles except ribosomes.
- Most have a cell wall made of murein.
- Bacteria carry out diverse metabolic and chemical reactions.
Classification of Monera:
According to Carl Woese, Monera integrates two major domains: Archaea (archaebacteria) and Bacteria.
- Archaebacteria: Live in extreme environments such as salt lakes, acidic soils, and high-temperature sites. They lack murein in their cell walls, and their cell membrane lipids are formed by branched fatty acids. They are thought to be similar to the first life forms.
- Bacteria: Are typical bacteria and the best known. Most play a role in various aspects, including health. Their diversity can be grouped according to their structure:
- Gram-negative: Includes the genus Rhizobium, which fixes atmospheric nitrogen in symbiosis with the roots of leguminous plants; cyanobacteria, which perform photosynthesis like algae; and enterobacteria, some of which live in the intestinal microbial flora.
- Gram-positive: Includes streptococci and staphylococci, which are pathogenic bacteria that cause infections in the mouth, respiratory tract, and skin. Also includes lactic acid bacteria, used to produce derivatives such as yogurt, and actinomycetes, which produce antibiotics.
The Protoctista Kingdom
This kingdom comprises a wide range of primarily aquatic organisms, with different levels of organization, reproduction types, nutrition methods, and life cycles. They are simple, unicellular or multicellular organisms with eukaryotic cell organization. Includes autotrophs such as algae and heterotrophs such as protozoa.
Classification:
- Autotrophic Protoctists: Includes all algae, whether unicellular, colonial, or multicellular. They live in aquatic environments and perform photosynthesis, contributing significantly to oxygen production. They reproduce both asexually and sexually. Based on structural complexity:
- Microalgae: Can be unicellular or form colonies. They are important constituents of marine phytoplankton.
- Macroalgae: Predominantly found in seas, some notable for their large size, but with low cell differentiation. According to pigment type, they are distinguished as chlorophytes, rhodophytes, and phaeophytes.
- Heterotrophic Protoctists: Includes several divisions that share heterotrophic nutrition.
- Molds: Includes Myxomycetes, which form plasmodia, and Oomycetes (water molds), whose cell walls contain cellulose. Both produce spores.
- Protozoa: Are unicellular, ingest food by phagocytosis, and have mechanisms for movement. These include amoebae (move by pseudopodia), flagellates (with one or more flagella), ciliates (with many cilia), and Sporozoa (which are parasitic).
The Fungi Kingdom
This kingdom has about 80,000 described species.
- Have a thallophytic organization with little tissue differentiation and division of labor. The thallus structure is formed by tubular filaments called hyphae; a set of hyphae is called mycelium.
- Their cells have cell walls made of chitin and lack flagella.
- Store glycogen as an energy reserve.
- Are heterotrophic and feed by absorption. Sexual reproduction occurs by the union of sexual organs.
Classification:
- Zygomycetes: With a well-developed mycelium. Most are microscopic and known as molds. Some are saprophytes, and others are parasites.
- Ascomycetes: With septate hyphae. Sexual spores (ascospores) are formed by meiosis within a specialized structure called an ascus. Ascomycetous yeasts are unicellular and carry out fermentations, used in bread, beer, and wine production. Others cause plant diseases.
- Basidiomycetes: Their hyphae are septate and produce sexual spores (basidiospores) by meiosis on a structure called a basidium. They form fruiting bodies called mushrooms.
The Plant Kingdom
Plant characteristics include:
- Have a cormophytic organization with specialized tissues and developed organs such as roots, stems, and leaves. Bryophytes, however, have a protocormophytic organization with rhizoids.
- The fundamental component of their cell wall is cellulose.
- Store starch as a reserve substance.
- Their nutrition is autotrophic by photosynthesis using chlorophyll. They are believed to have originated from green algae that colonized the terrestrial environment, adapting their structures.