Five Kingdoms of Life: Characteristics and Classification

Posted on Dec 18, 2024 in Biology

Monera: Prokaryotes

Prokaryotes are single-celled organisms with a very simple structure. Bacteria are prokaryotes and come from more primitive groups with similar body structures. Their evolution traces back to a common ancestor, an anaerobic prokaryote.

• Cyanobacteria: Responsible for the increase of oxygen in the atmosphere thanks to their photosynthetic capacity.

Prokaryotes are classified into two major domains:

• Archaea: These bacteria live outdoors, have no murein cell wall, and their lipids are formed by branched fatty acids. They are most similar to the first organisms that appeared on Earth.
• Bacteria: Bacteria are grouped according to the structure and composition of their cell wall:
  • Gram-negative: Do not stain. Examples include the genus Rhizobium and some that live in the human gut’s microbial flora (e.g., Escherichia coli and Salmonella).
  • Gram-positive: Stain. In humans, examples include streptococci and staphylococci. In the food industry, acid-lactic bacteria are used.

Protoctista Kingdom

This kingdom includes a wide variety of organisms that are difficult to characterize. They can be unicellular or multicellular, with eukaryotic cellular organization, and can be autotrophic or heterotrophic. They are classified according to their type of nutrition:

• Autotrophs: All algae are unicellular, colonial, or multicellular, living in aquatic environments. They conduct photosynthesis, provide oxygen, and reproduce both asexually and sexually. Based on their structural complexity, they are divided into:
  • Microalgae: Unicellular or form colonies that are more or less stable. They are major contributors to marine phytoplankton, such as dinoflagellates and diatoms.
  • Macroalgae: Dominate the seas, some notable for their large size. Cell differentiation is low. Examples include chlorophytes (green algae), rhodophytes (red algae), and phaeophytes (brown algae).
• Heterotrophs: This group includes several divisions that have heterotrophic nutrition in common:
  • Protoctist Molds: Includes the Myxomycetes and the Oomycetes, whose cells have walls of cellulose. They produce spores and were once considered inferior fungi.
  • Protozoa: Unicellular organisms that ingest their food by phagocytosis. They have different mechanisms of transport, such as amoebas that move by pseudopodia, ciliates that move by cilia, and sporozoans, which are mostly parasites.

Fungi Kingdom

Fungi have a thallophyte organization, with a thallus structure formed by hyphae (mycelium). Their cells have cell walls made of chitin and lack flagella. They store glycogen and are heterotrophic. Reproduction is sexual, by spores. Fungi are classified as:

• Zygomycetes: Have well-developed mycelium and produce zygospores (resting spores with thick walls). Most are molds, some are saprophytes, and others are parasites.
• Ascomycetes: Have hyphae that form ascospores (sexual spores produced by meiosis). Yeasts are unicellular ascomycetes that produce fermentations. Others cause serious diseases in plants.
• Basidiomycetes: Produce basidiospores (sexual spores produced by meiosis) and form fruiting bodies, which are mushrooms.

Plant Kingdom

Plants have a cormophyte organization, except for bryophytes, which have a protocormophyte organization. The key component of their cell wall is cellulose. They store starch and are autotrophic, performing photosynthesis through chlorophyll. They are classified into:

• Non-vascular Plants: Called bryophytes, they lack conducting tissues. The most important group is mosses.
• Seedless Vascular Plants: Called pteridophytes, they have conducting tissues and were constitutive of the dominant vegetation on Earth. The most important group is ferns.
• Vascular Plants with Seeds: Called spermatophytes, they have seeds as dispersal structures that contain nutrients. This group is divided into:
  • Angiosperms: Plants with seeds protected in a fruit. They have perfect flowers that house the sexual organs. According to their number of seeds, they are classified as:
    • Monocots: Have a single cotyledon, fascicular root, elongated leaves, and the number of flower pieces is in multiples of 3.
    • Dicots: Have 2 cotyledons, taproot, leaves varied in shape, and the number of floral parts is in multiples of 4 or 5.
  • Gymnosperms: Plants with naked seeds and flowers grouped in sexual organs. They have no calyx or corolla.

Animal Kingdom

Animals are multicellular, eukaryotic, lack cell walls, and have tissue organization. They have bilateral or radial symmetry, store food as glycogen or fat, have heterotrophic nutrition, and generally reproduce sexually. They are capable of displacement. They are classified into:

Animals Without Skeletons

• Porifera: Known as sponges, they have pores and a larger hole called an osculum. They are very simple animals, sedentary, and live in aquatic environments.
• Cnidaria: Known as coelenterates, they have tentacles with cnidoblasts. They are aquatic animals that can form colonies. Some live fixed to the substrate (polyps), and others are free-living (jellyfish).
• Platyhelminthes: Also called flatworms, their bodies are elongated, and they have a very simple nervous system. Some are free-living (planarians), and others are parasites (tapeworms).
• Nematodes: They are worm-like with a cylindrical body but are not segmented. Some are free-living, and others are parasites.
• Annelida: Have an elongated worm shape. Some organs are repeated, and they can live in water or on land.
• Mollusca: Their bodies are divided into several parts. They live in aquatic environments or very wet land. They include gastropods (snails), bivalves (mussels), and cephalopods (cuttlefish).

Animals with External Skeletons

• Arthropods: They have jointed appendages like antennae, legs, or mouthparts. Their body is surrounded by an exoskeleton of chitin. They live in terrestrial and aquatic environments. They are the largest group of invertebrates.
  • Crustaceans: Their body is divided into cephalothorax and abdomen. They have two pairs of antennae and five pairs of legs and are aquatic.
  • Insects: Their body is divided into head, thorax, and abdomen. They have one pair of antennae, three pairs of legs, and many have two pairs of wings. They live in all environments.
  • Arachnids: Their body is divided into cephalothorax and abdomen. They lack antennae and have four pairs of legs and two pairs of exclusive appendages: chelicerae and pedipalps. They live in terrestrial environments.
  • Myriapoda: Their body is divided into a head with a pair of antennae and a trunk with many rings. Each ring has one or two pairs of legs. They are terrestrial.

Animals with Internal Skeletons

• Echinoderms: Their body has radial symmetry under the skin and a skeleton of calcareous plates. They have an ambulacral apparatus (tubing and feet through which water circulates and is involved in locomotion). They live at sea.
• Chordata: They are the most evolved, with a dorsal skeletal axis in the notochord and a primitive vertebral column.
  • Agnatha: Elongated and cylindrical fish without scales or jaws.
  • Chondrichthyes: Called cartilaginous fish, the scales that cover their skin are denticles. Their tail fin has unequal lobes. They live in water.
  • Osteichthyes: Called bony fish, their skin is covered with arranged scales. Their caudal fin has equal lobes. They live in water.
  • Amphibians: Have bare skin with mucous glands. Some adults have a tail, while others do not. Part of their life cycle is on land, and another part is in the water.
  • Reptiles: Their skin is covered with scales attached to each other. They are oviparous and live on land.
  • Birds: Their skin is covered with feathers, except for the legs. They have a horny beak and are oviparous.
  • Mammals: Their skin is covered with hair. Females have mammary glands, they have a highly developed nervous system, and they are viviparous.