Core Concepts in Microbial Metabolism and Bioenergetics

Metabolism in Microorganisms

Catabolism vs. Anabolism

Metabolism encompasses all chemical reactions in a cell, divided into catabolism (breakdown of molecules to release energy) and anabolism (synthesis of complex molecules from simpler ones, requiring energy).

• Catabolism: Degradative, energy-yielding reactions (e.g., glycolysis).

• Anabolism: Biosynthetic, energy-consuming reactions (e.g., protein synthesis).

• Example: Glucose breakdown (catabolic) vs. amino acid assembly into proteins (anabolic).

Oxidation and Reduction Reactions

Redox reactions are central to energy transfer in cells. Oxidation is the loss of electrons, while reduction is the gain of electrons.

• Oxidized molecule: Loses electrons (often hydrogen atoms).

• Reduced molecule: Gains electrons (often hydrogen atoms).

• Example: NAD+ + 2e- + 2H+ → NADH + H+

ATP Production Mechanisms

Three Mechanisms of ATP Production

Cells generate ATP through three main processes:

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP from a phosphorylated intermediate.

• Oxidative phosphorylation: ATP synthesis driven by electron transport chain and chemiosmosis.

• Photophosphorylation: ATP generation using light energy (in photosynthetic organisms).

• Equation: $\text{ADP} + \text{P}_i \rightarrow \text{ATP}$

General Function of Enzymes in Metabolism

Enzymes are biological catalysts that speed up metabolic reactions by lowering activation energy, ensuring efficient cellular processes.

• Highly specific for substrates.

• Regulated by cellular conditions.

• Example: Hexokinase catalyzes the phosphorylation of glucose in glycolysis.

Glycolysis and Alternative Pathways

Glycolysis: Reactants and Products

Glycolysis is the breakdown of glucose to pyruvate, producing ATP and NADH.

• Reactant: Glucose

• Products: 2 Pyruvate, 2 ATP (net), 2 NADH

• Equation: $\text{Glucose} + 2\text{NAD}^+ + 2\text{ADP} + 2\text{P}_i \rightarrow 2\text{Pyruvate} + 2\text{NADH} + 2\text{ATP} + 2\text{H}_2\text{O}$

Pentose Phosphate Pathway (PPP)

The Pentose Phosphate Pathway is an alternative to glycolysis, generating NADPH and ribose-5-phosphate for biosynthesis.

• Reactant: Glucose-6-phosphate

• Products: NADPH, ribose-5-phosphate, CO2

• Function: Provides reducing power and precursors for nucleotide synthesis.

Entner-Doudoroff Pathway

The Entner-Doudoroff Pathway is found in some bacteria, producing ATP, NADPH, and pyruvate from glucose.

• Reactant: Glucose

• Products: 1 ATP, 1 NADPH, 1 NADH, 2 Pyruvate

• Difference from PPP: PPP does not produce pyruvate directly; Entner-Doudoroff does.

Comparison of Glycolytic Pathways

Pathway	Main Products	Organisms
Glycolysis	2 ATP, 2 NADH, 2 Pyruvate	Most cells
Pentose Phosphate	NADPH, Ribose-5-phosphate	All cells
Entner-Doudoroff	1 ATP, 1 NADPH, 1 NADH, 2 Pyruvate	Certain bacteria

Utilization of Alternative Pathways

Cells may use PPP or Entner-Doudoroff pathways when specific biosynthetic needs arise or when glycolysis is less efficient due to environmental conditions.

• PPP: Needed for NADPH and nucleotide synthesis.

• Entner-Doudoroff: Used by some Gram-negative bacteria.

Respiration and Fermentation

Intermediate Step: Glycolysis to Krebs Cycle

Pyruvate from glycolysis is converted to acetyl-CoA, producing NADH and CO2.

• Reactant: Pyruvate

• Products: Acetyl-CoA, NADH, CO2

• Equation: $\text{Pyruvate} + \text{NAD}^+ + \text{CoA} \rightarrow \text{Acetyl-CoA} + \text{NADH} + \text{CO}_2$

Krebs Cycle (Citric Acid Cycle)

The Krebs Cycle oxidizes acetyl-CoA to CO2, generating NADH, FADH2, and ATP.

• Reactant: Acetyl-CoA

• Products (per turn): 3 NADH, 1 FADH2, 1 ATP (or GTP), 2 CO2

• Equation: $\text{Acetyl-CoA} + 3\text{NAD}^+ + \text{FAD} + \text{ADP} + \text{P}_i \rightarrow 2\text{CO}_2 + 3\text{NADH} + \text{FADH}_2 + \text{ATP} + \text{CoA}$

Electron Transport Chain (ETC)

The ETC transfers electrons from NADH and FADH2 to oxygen, generating a proton gradient for ATP synthesis.

• Reactants: NADH, FADH2, O2

• Products: ATP, H2O

• Process: Electrons move through protein complexes, pumping protons across the membrane.

Aerobic vs. Anaerobic Respiration

Aerobic respiration uses oxygen as the final electron acceptor; anaerobic respiration uses other molecules (e.g., nitrate, sulfate).

• Aerobic: Maximum ATP yield, water produced.

• Anaerobic: Lower ATP yield, alternative electron acceptors.

Chemiosmosis and ATP Synthesis

Chemiosmosis is the movement of protons across a membrane, driving ATP synthesis via ATP synthase.

• Equation: $\text{ADP} + \text{P}_i \xrightarrow{\text{ATP synthase}} \text{ATP}$

• Proton gradient is established by ETC.

Fermentation

Fermentation is an anaerobic process where organic molecules serve as electron acceptors, producing ATP by substrate-level phosphorylation.

• Types: Lactic acid fermentation, alcoholic fermentation.

• Products: Lactic acid, ethanol, CO2

• Function: Regenerates NAD+ for glycolysis.

End Products of Fermentation

Type	End Products	Function
Lactic Acid	Lactic acid	Regenerates NAD+, used in food industry
Alcoholic	Ethanol, CO2	Regenerates NAD+, brewing/baking

Photosynthesis in Microorganisms

Structures Utilized in Photosynthesis

Photosynthetic microorganisms use specialized structures such as thylakoids and chloroplasts (in eukaryotes) or chromatophores (in prokaryotes).

• Thylakoids: Membrane-bound compartments for light reactions.

• Chloroplasts: Organelles in algae and plants.

• Chromatophores: Invaginations in bacterial membranes.

Light and Dark Reactions of Photosynthesis

Photosynthesis consists of light reactions (convert light energy to chemical energy) and dark reactions (Calvin cycle, fix CO2 into organic molecules).

• Light reactions: Produce ATP and NADPH.

• Dark reactions: Use ATP and NADPH to synthesize glucose.

• Equation (overall): $6\text{CO}_2 + 6\text{H}_2\text{O} + \text{light} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

Cyclic vs. Noncyclic Photophosphorylation

Cyclic photophosphorylation involves electrons cycling back to the photosystem, producing only ATP. Noncyclic photophosphorylation transfers electrons to NADP+, producing both ATP and NADPH.

• Cyclic: Only ATP produced, electrons return to chlorophyll.

• Noncyclic: ATP and NADPH produced, electrons do not return.

Type	Products	Electron Flow
Cyclic	ATP	Returns to photosystem
Noncyclic	ATP, NADPH	Transferred to NADP+

Additional info: Some explanations and tables were expanded for clarity and completeness based on standard microbiology curriculum.