Question

Recall List the three domains into which living organisms are divided, and indicate how this scheme differs from the five-kingdom classification scheme.

Short answer

The three domains are Archaea, Bacteria, and Eukarya. This system differs from the five-kingdom scheme by separating prokaryotes into two distinct domains.

Step-by-step solution

- Identify the Three Domains

List the three domains into which living organisms are divided: Archaea, Bacteria, and Eukarya.

- Understand the Domain Classification

Archaea and Bacteria consist of prokaryotic organisms (without a nucleus), while Eukarya consists of eukaryotic organisms (with a nucleus).

- Recall the Five-Kingdom Scheme

The five-kingdom classification scheme classifies living organisms into Monera, Protista, Fungi, Plantae, and Animalia.

- Compare the Two Schemes

The main difference is that the three-domain system separates prokaryotes into two groups: Archaea and Bacteria, while the five-kingdom system groups all prokaryotes into Monera. The three-domain system is based on differences in the ribosomal RNA structure of organisms.

Key concepts

Three-Domain System

The three-domain system of classifying living organisms divides life into three primary categories: Archaea, Bacteria, and Eukarya.
This system was introduced by Carl Woese in the 1970s, based on differing sequences in ribosomal RNA (rRNA).
This approach focuses on genetic differences rather than physical characteristics, making it a more accurate reflection of evolutionary relationships.

• Archaea: Prokaryotic organisms that often live in extreme environments.
• Bacteria: Also prokaryotic, but found in more common environments.
• Eukarya: Organisms with complex cells containing a nucleus.

Five-Kingdom Classification

The five-kingdom classification system, developed by Robert Whittaker in 1969, categorizes life into Monera, Protista, Fungi, Plantae, and Animalia.
This method relies more on morphological and metabolic characteristics.
Each kingdom has defining traits:

• Monera: Unicellular prokaryotes like bacteria.
• Protista: Mostly unicellular eukaryotes.
• Fungi: Eukaryotic organisms like mushrooms and molds, decomposers.
• Plantae: Photosynthetic to produce their food.
• Animalia: Multicellular eukaryotes, usually mobile.

The five-kingdom system has now largely been replaced by the three-domain system due to advances in genetic understanding.

Prokaryotic vs Eukaryotic Organisms

Organisms can be broadly divided into prokaryotes and eukaryotes.
Prokaryotes, which include organisms from the domains Archaea and Bacteria, do not have a nucleus within their cells.
Eukaryotes, forming the domain Eukarya, have cells with a well-defined nucleus encased in a membrane.

• Prokaryotes: Typically simpler, smaller cells.
• Eukaryotes: Larger, more complex cells.

Eukaryotic cells are usually multicellular, whereas prokaryotic cells often exist as single-celled organisms.

Ribosomal RNA Differences

Ribosomal RNA (rRNA) sequences are crucial for distinguishing among different domains of life.
Carl Woese's discovery of significant variances in the rRNA of Archaea, Bacteria, and Eukarya led to the creation of the three-domain system.

• Archaea: Unique rRNA sequences that differ significantly from both Bacteria and Eukarya.
• Bacteria: Distinct rRNA sequences, different from Archaea and Eukarya.
• Eukarya: rRNA is more complex and differs greatly from prokaryotic rRNA.

These rRNA differences underscore the genetic distances and evolutionary pathways of the domains.

Archaea

Archaea are a fascinating domain of life consisting of prokaryotic organisms.
They are known for thriving in extreme environments, such as hot springs, salt lakes, and deep-sea hydrothermal vents.

• Lack a nucleus.
• Have unique membrane lipids.
• Possess distinct rRNA sequences.

Despite their similarity to bacteria in cellular structure, genetic evidence shows they are more closely related to Eukarya.

Bacteria

The domain Bacteria includes prokaryotic microorganisms found in nearly every environment on Earth.
They play critical roles in processes such as nitrogen fixation, decomposition, and symbiosis with plants and animals.

• Single-celled.
• Lack a nucleus.
• Have peptidoglycan in their cell walls.

While simple in structure, bacteria are incredibly diverse and vital for ecosystem functioning.

Eukarya

Eukarya is the domain encompassing all eukaryotic life forms.
These organisms have cells with a true nucleus enclosed by a membrane and other complex organelles.
Eukarya includes a broad range of organisms, from unicellular protists to multicellular animals and plants.

• Possess a nucleus.
• Have complex cell structures.
• Include kingdoms like Protista, Fungi, Plantae, and Animalia.

The domain showcases the complexity and diversity of life, differentiating significantly from the simpler prokaryotic organisms.