Chapter 19, Problem 1RQ

Summary Introduction

To review:

The distinctive structures of archaeal cell membrane and envelope.

Introduction:

Archaea are unicellular microorganisms that are similar to bacteria. Archaea lack true nucleus and hence, are considered prokaryotic. As their molecular characteristics are distinct, they are different from other prokaryotes such as bacteria. Archaea live in extreme conditions such as salt lakes, hot spring, and are even found in the gut of ruminant animals. They have specialized systems that help them tolerate such extreme conditions.

Explanation of Solution

Archaeal signatures are the distinctive traits of Archaea. These traits include components of the cell membrane and certain metabolic pathways. Some of the distinctive features of Archaea are described as follows:

Isoprenoid membranes: The membrane lipids of Archaea are entirely different from bacteria and eukaryotes. Archaeal lipids remain stable in extreme temperature and acidity. The distinctive structure of archaeal lipids that distinguishes them from bacteria and eukaryotes are mentioned below:

1. L-glycerol: Instead of D (dextrorotatory)-glycerol, Archaea have L (levorotatory)-glycerol. The chiral forms show similar stability but they require different enzymes for their biochemical activities.

2. Ether linkage: The side chains and glycerol units are linked by ether linkage instead of ester links. Ether bonds have higher stability as compared to ester bonds.

3. Isoprenoid chains: Archaeal lipids have branched methyl chains at every fourth carbon that arise by the condensation of isoprene units. These condensed chains are called isoprenoids.

4. Cross-linking lipids: Cross-linkage is formed by covalent bonds either between adjacent lipids or between the lipids present on the opposite side of the membrane. A tetramer is formed at the cross-linking of the two pairs of lipid chains.

5. Cyclopentane rings: In some Archaea, cyclopentane rings are formed and they provide stability to membranes at high temperatures.