Microbial Cell Structure, Metabolism, and Bioenergetics: Study Notes

Classification of Microorganisms by Energy Source

Scientific Terms for Energy Source Utilization

Microorganisms are classified based on the type of energy source they utilize for growth and metabolism.

• Chemolithotrophs: Organisms that obtain energy from inorganic compounds. Example: Thiobacillus species oxidize sulfur compounds.

• Chemoorganotrophs: Organisms that obtain energy from organic compounds. Example: Most bacteria and fungi that metabolize glucose.

Key Terms:

• Autotroph: Uses CO2 as a carbon source.

• Heterotroph: Uses organic carbon sources.

Cell Size and Measurement

Bacterial Cell Diameter

Bacterial cells are typically measured in micrometers (μm). Conversion to millimeters (mm) is often required for calculations.

• Average diameter of a bacterial cell: 2 μm

• Conversion: $2\ \mu m = 0.002\ mm$

Microbial Cell Envelope Structure

Classification of Cell Envelope Components

The cell envelope varies between Archaea, Bacteria, and Gram-positive/Gram-negative bacteria. Key molecules and their locations:

• Phospholipid in Archaea: Located at the bottom right of the diagram.

• Phospholipid in Bacteria: Located at the bottom left of the diagram.

• G-M disaccharide in Bacteria: Middle right.

• G-T disaccharide in Archaea: Middle left.

• Gram-positive cell wall: Top right.

• Gram-negative cell wall: Top left.

Key Terms:

• Peptidoglycan: Polymer of sugars and amino acids forming the cell wall in Bacteria.

• S-layer: Surface-layer protein structure found in some Archaea and Bacteria.

Cell Membrane and Wall Functions

Phospholipid Linkages

Phospholipids in Bacteria contain ester linkages between glycerol and fatty acids. In Archaea, ether linkages are present.

Functions of the Cell Wall

• Regulate water and solute movement (osmosis)

• Energy conservation

• Prevent osmotic lysis and determine cell shape

• Attachment to solid substrates

Functions of the Cytoplasmic Membrane

• Permeability barrier: Controls entry and exit of substances.

• Prevents osmotic lysis

• Site of energy generation (in prokaryotes)

Metabolic Classification and Bioenergetics

Electron Donors and Acceptors

Microorganisms use various molecules as electron donors and acceptors in metabolism.

• Electron donor: Substance oxidized (e.g., glucose, H2).

• Electron acceptor: Substance reduced (e.g., O2, NO3-).

Fermentation

• Source of energy: Organic molecule.

• ATP yield: Less than respiration.

• Electron acceptor: Organic molecule (not O2).

Respiration Equation

General equation for aerobic respiration of glucose:

• $C_6H_{12}O_6 + 6O_2 \rightarrow 38ADP + 38P_i \rightarrow 6CO_2 + 6H_2O + 38ATP$

e- donor: Glucose e- acceptor: Oxygen

Citric Acid Cycle (TCA Cycle)

Corrected Equation

The citric acid cycle is a central metabolic pathway in aerobic organisms. The correct stoichiometry is:

• $\text{Acetyl-CoA} + 3NAD^+ + 1GDP + 1P_i + 2CO_2 + 1FAD + 3NADH + 1GTP$

Note: NAD+ is reduced to NADH; FAD is reduced to FADH2.

Cell Structure and Evolution

Origin of Mitochondria

Mitochondria are believed to have originated from an ancestral endosymbiotic event involving a proteobacterium.

• Evidence: Mitochondria have their own DNA and double membrane.

• Diagram: Shows evolutionary relationships between Bacteria, Archaea, and Eukaryotes.

Genetic Information and Translation

Importance of the oriC Sequence

• oriC: The origin of chromosomal replication in Bacteria.

• Function: Required to initiate DNA replication.

Translation Initiation

• Initiator tRNA: The f-Methionine tRNA binds to the start codon (AUG) in prokaryotes.

Peptidoglycan Structure

Bonding and Resistance to Osmotic Lysis

Peptidoglycan is a mesh-like polymer that provides structural integrity to bacterial cell walls.

• Bonds: Peptide bonds between amino acids and glycosidic bonds between sugars.

• Function: Prevents osmotic lysis by resisting internal turgor pressure.

Example: Diagram labeling the bonds and amino acids in peptidoglycan.

Summary Table: Cell Envelope Components

Component	Bacteria	Archaea	Gram-Positive	Gram-Negative
Phospholipid Linkage	Ester	Ether	Ester	Ester
Peptidoglycan	Present	Absent	Thick layer	Thin layer
S-layer	Sometimes	Common	Sometimes	Sometimes

Additional info: Table entries inferred from standard microbiology knowledge.