What are archaea?

Bacteria and archaea are two different prokaryotic domains that, along with eukarya, make up the three domains of life. Archaeal organisms can be found all over the biosphere. Many genera are extremophiles, meaning they thrive in extreme environments such as extremely high temperatures, high salinity, or pH extremes. Bacterial and archaeal cells are characterized by the absence of a nucleus and membrane-bound organelles.

The structures of archaeal cells and their functions

Archaea lack a nucleus and other membrane-bound organelles in their cells. Archaea and bacteria have equivalent cell structures in general, but archaea differ in cell constituents and organization. The archaeal cell is composed of external cellular structures, cell envelopes, and cytoplasm.

External cellular structures

Pilus

Pili is a group of short, narrow protein fibers that protrude from the cell surface. The rigid fibers serve as structural support for specific adhesive molecules known as adhesins, which are attached at the apex. As a result, the purpose of pili is to attach cells to various surfaces, resulting in the formation of biofilms. The pili play a role in conjugation in the organism that has an F+ plasmid. This is the exchange of genetic material between prokaryotic cells, and pili have a central cavity to facilitate this. The conjugation pilus, also known as the sex pilus, has receptor sites that recognize recipient cells and allow them to receive the donor's genetic material.

Flagella

Flagella are long filaments that protrude from the cell's surface. Flagella are responsible for motility. Numerous archaeal species can locomote through fluids or over moist surfaces in the form of flagellar or gliding motility. Depending on the species, one or more flagella may be attached across the cell surface at various locations.

Archaea flagellum is made of polymerized flagellin with a hook attached. The cell is propelled through a liquid medium by this rotating structure with switches. In the composition and process of assembly, the archaeal flagellum differs from the bacterial flagellum. It is generally formed from several kinds of flagellins and is glycosylated. It is slender (10-15 nm) compared to the bacterial flagellum (18-24 nm). Archaea do not have microtubules or dynein in their flagella (eukaryotes do have microtubules and dynein in their flagella structure).

The cell envelope

The cell envelope is a specialized structure surrounding the cell cytoplasm and forms the cell wall and membrane. The cell wall is relatively permeable to the movement of molecules. The cell membrane controls the transport of metabolic products and nutrients.

Cell wall

The archaeal cell wall is a moderately rigid structure that protects the cell from the environment as well as from internal cellular stresses. Archaeal cell walls can be spherical, rod, spiral, lobed, rectangular, or irregular-shaped, depending on the organism's environment. In some archaeal species, the cell wall is absent. Except for methane bacteria, which have pseudopeptidoglycan in their cell wall, archaeal cell walls differ from bacterial cell walls in their chemical makeup and absence of peptidoglycans.

The archaeal cell wall is made up of proteinaceous S-layers and is called pseudopeptidoglycan. The archaeal S-layer is made up of a variety of structural proteins and glycoproteins. The proteins are anchored into the plasma membrane and form a two-dimensional crystalline arrangement with a smooth surface. A few S-layers are made up of two distinct S-layer proteins. Even though archaea do not have peptidoglycan, they do have pseudomurein, a constituent with a similar chemical structure. S-layers may perform a variety of functions in various prokaryotic cells, including assisting the cell in coping with osmotic pressure and helping to interact with the host by pathogenic prokaryotes.

Glycocalyx

Outside of the cell wall, prokaryotes have additional cell envelope structures such as glycocalyces and S-layers. The glycocalyx is the outer sugar coat that surrounds the cell wall. The two kinds of glycocalyx are capsules and slime layers. A capsule is a tightly arranged layer found outside of the cell wall, which is formed of proteins or polysaccharides. A slime layer is loosely attached to the cell wall. It is formed of glycoproteins, glycolipids, and polysaccharides. The glycocalyx acts as a barrier between the cell and its surroundings. Glycocalyx promotes cell adhesion to surfaces, assisting in biofilm formation.

Plasma membrane

The plasma membrane is the thin, semi-permeable membrane that separtes the cells internal environment from the external environment, thereby provide protection to the cells. The plasma membrane of archaea is distinguished by several features that differ from other domains.

1. The glycerol linkage between the head of the phospholipid and the side chain exhibits chirality which is in the L-isomeric form (in bacteria and eukaryotes D-isomeric form is present).
2. The presence of an ether linkage between the glycerol and the side chain (ester-linked lipids observed in bacteria and eukaryotes).
3. Branching isoprenoid side chains (unbranched fatty acid side chains in bacteria and eukaryotes).
4. Plasma membranes are monolayers. The isoprene chains of one phospholipid connect with the isoprene chains of another phospholipid on the other side of the membrane (bacteria, eukaryotes have only lipid bilayers, which keep the two sides of the membrane apart).

The cell cytoplasm and internal structures

Cytoplasm

The cytoplasm is the region where the majority of growth and metabolism takes place. It is surrounded by the cell membrane. The cytoplasm is made up of the cytosol, which is a semifluid mass of amino acids, proteins, sugars, salts, nucleotides, ions, and vitamins that are all dissolved in water, as well as several subcellular compartments or structures, each with a particular function.

Nucleoid

The nucleoid is the region where genetic information is stored. In archaeal cells, the chromosome segment exists as a diffuse mass known as the nucleoid. The nucleoid is not enveloped by a membrane; rather, it is a cytoplasmic subcompartment devoid of deoxyribonucleic acid (DNA) aggregates and ribosomes. With some exceptions, each cell has a single chromosome that forms a closed loop of DNA and protein.

Plasmids

In addition to a nucleoid, several archaeal cells possess smaller DNA molecules known as plasmids. Plasmids contain extraneous genetic information. These extra-chromosomal DNA molecules occur in the form of closed loops with five to 100 genes. A cell may contain one or more plasmids, each of which may consist of similar or dissimilar genes.

Ribosomes

The ribosome is one of the universal cellular components. When observed through an electron microscope, the cell cytoplasm contains thousands of these spherical-shaped particles, giving it a granular appearance. There are free ribosomes and cell membrane-bounded ribosomes. Ribosomes are made up of proteins and ribosomal RNA (rRNA) and consist of a small (30S) and a large subunit (50S). The two subunits combine to form a 70S functional ribosome, which is required for protein synthesis. Free ribosomes generate soluble proteins that are used in the cell, whereas membrane-bounded ribosomes generate proteins for the cell membrane and secretion.

Context and Applications

This topic is significant in the exams at school, graduate, and post-graduate levels, especially for

• Bachelors in Zoology/Botany
• Masters in Zoology/Botany

Practice Problems

Question 1: How the archaeal cell wall differs from the cell walls of other prokaryotes?

1.  Lack of a peptidoglycan layer

2.  Lack of S-layer

3.  Lack of ribosome

4.  None of the above

Answer: Option 1 is correct.

Explanation: Archaea is the domain of unicellular prokaryotes with cell walls that do not consist of peptidoglycan and are composed of S-layers.

Question 2: Which of the following is the example of additional cell envelope structures exterior to the cell wall of prokaryotes?

1.  Pili

2. Glycocalyx

3. Flagella

4. None of the above            

Answer: Option 2 is correct.

Explanation: Outside of the cell wall, prokaryotes have additional envelope structures such as glycocalyces and S-layers. The glycocalyx is located outside the cell wall in most of the prokaryotes. In some prokaryotes, the S-layer is found inside the cell wall, and in archaea, the S-layer is the cell wall itself.

Question 3: Which of the following structures is involved in conjugation?

1.  Flagellum

2.  Pilus

3.  Ribosome

4.  None of the above

Answer: Option 2 is correct.

Explanation: Pili are protein fibers that extend from the cell surface and play a role in horizontal gene transfer known as conjugation. This is the exchange of genetic material between cells, and pili have a central cavity to facilitate this.

Question 4: Which of the following is true for the plasma membrane of archaea?

1.   Unbranched side chain of fatty acids

2.   Branching isoprenoid side chains

3.   Ester-linked lipids

4.   None of the above

Answer: Option 2 is correct.

Explanation: The archaeal plasma membrane shows ether-linkage between the glycerol and the side chain (ester-linked lipids observed in bacteria and eukaryotes) and branching isoprenoid side chains (unbranched side chains of fatty acids in bacteria and eukaryotes).

Question 5: The extrachromosomal DNA molecules are known as _____.

1.   Nucleoid

2.   Plasmid

3.   Ribosomes

4.   None of the above

Answer: Option 2 is correct.

Explanation: In addition to a nucleoid, several archaeal cells possess smaller DNA molecules known as plasmids. Plasmids contain extraneous genetic information.