Problem 3 - Campbell Biology

Question

Which of the following statements is not true? \begin{equation}\begin{array}{l}{\text { (A) Archaea and bacteria have different membrane lipids. }} \\ {\text { (B) The cell walls of archaea lack peptidoglycan. }} \\ {\text { (C) Only bacteria have histones associated with DNA. }} \\ {\text { (D) Only some archaea use } \mathrm{CO}_{2} \text { to oxidize } \mathrm{H}_{2} \text { , releasing methane. }}\end{array}\end{equation}

Step-by-Step Solution

Answer

Statement C is not true.

Step 1: Analyze Statement A

Statement A claims that archaea and bacteria have different membrane lipids. This is true because archaea have ether-linked lipids while bacteria have ester-linked lipids.

Step 2: Analyze Statement B

Statement B states that the cell walls of archaea lack peptidoglycan. This is true because while the cell walls of bacteria contain peptidoglycan, archaea have cell walls composed of other polysaccharides and proteins.

Step 3: Analyze Statement C

Statement C claims that only bacteria have histones associated with DNA. This is false because some archaea also have histones that help in DNA packaging similar to eukaryotes.

Step 4: Analyze Statement D

Statement D mentions that only some archaea use CO₂ to oxidize H₂, releasing methane. This is true as methanogenesis is a unique metabolic process found only in some archaea.

Key Concepts

membrane lipids

Membrane lipids are crucial components of all living cells, forming the structure of the cell membrane.
Both archaea and bacteria have unique types of membrane lipids that differentiate them.

• Archaea have ether-linked lipids: These lipids have a glycerol backbone connected to fatty acids via ether bonds. Ether bonds are more stable than ester bonds, allowing archaea to survive in extreme environments.
• Bacteria have ester-linked lipids: In bacterial membranes, the glycerol backbone is connected to fatty acids via ester bonds. These are more common in moderate and diverse environments.

This structural difference is essential for the function and stability of their cell membranes, especially under varying environmental conditions. Understanding the difference in membrane lipids helps in the study of microbial physiology and adaptation.

peptidoglycan

Peptidoglycan is a polymer that forms a protective layer in the cell walls of bacteria.
It is a critical component for maintaining the shape and integrity of bacterial cells.

• Bacterial cell walls: In bacteria, peptidoglycan provides structural support and prevents osmotic lysis. It is composed of sugar derivatives and amino acids, forming a mesh-like layer.
• Archaeal cell walls: Archaeal cell walls lack peptidoglycan. Instead, they may contain pseudo-peptidoglycan, proteinaceous layers, or polysaccharide layers. These structures function similarly to peptidoglycan but are distinct in their composition.

This difference is significant for antibiotic targeting. Many antibiotics, like penicillin, target peptidoglycan synthesis, which is why such drugs are effective against bacteria but not archaea.

histones

Histones are proteins associated with the packaging of DNA into chromosomes.
They play a crucial role in gene regulation and chromatin structure.

• Eukaryotic cells: In eukaryotes, histones are well-known for their role in winding DNA into nucleosomes, which facilitate efficient packaging and regulation of DNA.
• Bacteria: Bacteria typically do not have histones. Their DNA is organized differently, usually involving proteins that are not histone-like.
• Archaea: Some archaea possess histones, which resemble eukaryotic histones. These archaeal histones also help in packaging DNA, similar to their function in eukaryotes.

This similarity between archaea and eukaryotes highlights a closer evolutionary relationship, suggesting that they share a common ancestor distinct from bacteria.

methanogenesis

Methanogenesis is a unique metabolic process found only in some archaea.
It involves the production of methane (CH₄) as a metabolic by-product.

• Process: In methanogenesis, archaea use carbon dioxide (CO₂) to oxidize hydrogen (H₂), producing methane and water. This process occurs in anaerobic (oxygen-free) environments.
• Significance: Methanogenesis is important in various ecological contexts, such as in the digestive tracts of ruminants (like cows) and in anaerobic sediments.
• Archaeal uniqueness: This process is exclusive to some archaea, making them distinct from bacteria. It is of particular interest in studies of early life and bioenergy applications.

Understanding methanogenesis is essential for fields like environmental science and renewable energy, as methane is a potent greenhouse gas and a potential biofuel.