Question

What cell wall polysaccharide common in Bacteria is absent from Archaea? What is unusual about S-layers compared to other cell walls of prokaryotes? What types of cell walls are found in Archaea? (Section 2.12)

Short answer

Bacteria have peptidoglycan. S-layers in Archaea are protein or glycoprotein layers. Archaea have pseudopeptidoglycan, polysaccharides, proteins, or glycoproteins.

Step-by-step solution

Identify the common bacterial cell wall polysaccharide

Identify the polysaccharide found in the cell walls of Bacteria. This polysaccharide is peptidoglycan, which is unique to Bacteria and absent from Archaea.

Determine the unusual characteristics of S-layers

The S-layers (surface layers) in Archaea are unusual because they consist of protein or glycoprotein and serve as the main structural component of the cell wall, which is not the case for Bacteria whose main structural component is peptidoglycan.

Identify cell wall types in Archaea

Archaeal cell walls do not contain peptidoglycan. Instead, they may have pseudopeptidoglycan, polysaccharides, proteins, or glycoproteins. S-layers can also be present in many Archaea.

Key concepts

peptidoglycan

Peptidoglycan is a crucial component of bacterial cell walls. It provides structural strength and rigidity to the bacterial cell, preventing it from bursting in a hypotonic environment. Peptidoglycan is composed of long chains of alternating sugars, N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), which are cross-linked by short peptide chains. This creates a mesh-like layer that is strong yet flexible.

In contrast, Archaea do not have peptidoglycan in their cell walls. This is a significant difference between Bacteria and Archaea. Instead, Archaea use other substances, like pseudopeptidoglycan, proteins, or glycoproteins, to build their cell walls.

S-layers

S-layers, or surface layers, are a unique feature of many Archaeal cell walls. They are made up of proteins or glycoproteins and can serve as the main structural component of the cell wall. This is unusual because, in Bacteria, the primary structural component is typically peptidoglycan.

S-layers offer several advantages:

• They provide protection against environmental stress, such as extreme temperatures or pH levels.
• They allow for selective permeability, controlling the movement of molecules into and out of the cell.
• They play a role in cell adhesion and surface recognition, which is important for interactions with the environment and other cells.

These S-layers form a crystalline array on the cell surface, offering both durability and flexibility.

pseudopeptidoglycan

In some Archaea, the cell wall contains a substance known as pseudopeptidoglycan, or pseudomurein. Although it is similar in function to bacterial peptidoglycan, it has a different chemical composition.

Pseudopeptidoglycan is composed of different sugar molecules, with N-acetyltalosaminuronic acid (NAT) replacing N-acetylmuramic acid (NAM). The sugar chains are still cross-linked by peptide bonds, but the structure does not include the same amino acids found in bacterial peptidoglycan.

This distinction helps Archaea remain resilient in harsh environments, as pseudopeptidoglycan can provide stability and protection without the vulnerability associated with bacterial cell walls.

glycoproteins

Glycoproteins are another key component of Archaeal cell walls. These molecules consist of proteins with carbohydrate groups attached. They play several roles in the structure and function of the cell wall.

Some Archaea have cell walls composed entirely of glycoproteins, providing a protective layer around the cell. Glycoproteins can help maintain the integrity of the cell wall under extreme conditions, such as high salinity or temperature.

Glycoproteins also aid in cell signaling and interactions with other cells and the environment. This is crucial for survival, as it allows Archaea to adapt to changing conditions and effectively compete for resources.

Overall, while Bacteria use peptidoglycan, Archaea have evolved various strategies—including S-layers, pseudopeptidoglycan, and glycoproteins—to build robust and adaptable cell walls.