Energy-Generating Pathways & Fermentation

Energy-Generating Pathways – Core Overview

• All cells must continually regenerate ATP to power biochemical reactions.
• Three canonical metabolic routes supply ATP:
  • Cellular Respiration – electron flow begins with reduced cofactors (NADH, FADH$_2$) and ends on an external (non-organic) electron acceptor via an Electron Transport Chain (ETC).
  • Occurs in distinct aerobic and anaerobic variants.
  • Fermentation – redox-balance pathway independent of an ETC; electrons ultimately placed on an organic molecule generated inside the cell (frequently an alcohol or an acid).
  • Photophosphorylation and other specialized energy systems exist (not featured in transcript) but share the core goal: re-forming \text{ATP} from \text{ADP} + \text{P_i}.

Cellular Respiration – Detailed Scheme

• Initiation: Glycolysis converts glucose → 2 pyruvate + net 2\,\text{ATP} + 2\,\text{NADH}.
• Preparatory Step (link reaction): Pyruvate → Acetyl-CoA + \text{CO}_2 (generates additional NADH).
• Krebs (Citric Acid) Cycle: Complete oxidation of Acetyl-CoA; yields \text{CO}2, more NADH, FADH$2$, and substrate-level \text{ATP/GTP}.
• Electron Transport Chain (ETC):
  • NADH/FADH$_2$ donate electrons.
  • Proton gradient established across a membrane (plasma membrane in prokaryotes, mitochondrial inner membrane in eukaryotes).
  • ATP synthase uses proton-motive force to generate the bulk of ATP (oxidative phosphorylation).

Aerobic Respiration

• Final electron acceptor: \text{O}2 ➝ reduced to \text{H}2\text{O}.
• Typical ATP yield (textbook): \approx 36–38\,\text{ATP} per glucose (exact number depends on shuttle efficiency, P/O ratios, organism).
• Transcript descriptor: “Lots of ATP.”

Anaerobic Respiration

• Final electron acceptor: Inorganic molecules other than oxygen (e.g.
  \text{NO}3^-, \text{SO}4^{2-}, \text{CO}_3^{2-}, \text{Fe}^{3+}).
• ATP yield: Lower than aerobic because alternative acceptors have less-positive redox potentials ➝ smaller proton gradient ➝ fewer oxidative phosphorylation events.
• Transcript descriptor: “Some ATP made.”

Fermentation – Mechanistic Framework

• Key Features
  • Can operate in the presence or absence of oxygen; ETC not required.
  • Sole ATP source = glycolysis (substrate-level phosphorylation).
  • To recycle \text{NAD}^+, electrons from NADH are transferred to an internal organic molecule derived from pyruvate.
• ATP yield: Small (net 2\,\text{ATP} per glucose). Transcript: “Small amounts of ATP.”

Common Fermentation Pathways & Major End-Products

• Alcohol (Ethanolic) Fermentation
  • End-products: Ethanol + \text{CO}_2.
  • Organisms: Saccharomyces cerevisiae, Candida spp.
  • Commercial items: Beer, wine, bread (dough rising from \text{CO}_2).
• Acetone–Butanol–Ethanol (ABE) Fermentation
  • End-products: Acetone, butanol, ethanol, \text{CO}_2.
  • Microbe: Clostridium acetobutylicum.
  • Products: Commercial solvents, gasoline alternatives.
• 2,3-Butanediol Fermentation
  • End-products: Acetoin, 2,3-butanediol, formic acid, lactic acid, \text{CO}2, \text{H}2.
  • Microbes: Klebsiella, Enterobacter.
  • Application: Chardonnay wine bouquet, industrial chemistry.
• Butyric Acid Fermentation
  • End-products: Butyric acid, \text{CO}2, \text{H}2.
  • Microbe: Clostridium butyricum.
  • Product: Butter flavor.
• Lactic Acid Fermentation
  • End-products: Lactic acid only (homo-lactic) or mixture with ethanol/CO$_2$ (hetero-lactic).
  • Microbes: Streptococcus, Lactobacillus, E. coli, Propionibacterium.
  • Foods: Yogurt, sauerkraut, mustard, Swiss cheese.

Human & Medical Relevance

• Muscle anaerobiosis: During intense exercise, oxygen limitation triggers lactic acid fermentation in muscle cells ➝ \text{lactate} accumulation; allows temporary ATP supply without O$_2$.
• Pathogenesis: Some fermentative bacteria (e.g.
  Clostridium perfringens) generate gas gangrene—rapid tissue destruction via gas & acid buildup.

Prokaryotic vs. Eukaryotic Spatial Organization

• Glycolysis
  • Location for both cell types: Cytoplasm.
• Krebs Cycle
  • Prokaryotes: Cytoplasm.
  • Eukaryotes: Mitochondrial matrix.
• ETC / Chemiosmosis
  • Prokaryotes: Plasma membrane with periplasmic space acting like the inter-membrane space.
  • Eukaryotes: Inner mitochondrial membrane; protons accumulate in the inter-membrane space.
• Anaerobic ETC ( prokaryotes)
  • Same membrane site; alternative terminal reductases (nitrate reductase, sulfate reductase, etc.).
• Fermentation
  • Entirely cytoplasmic in both cell types.

Comparative Energetics Summary

• Transcript flow chart:
  • Aerobic respiration: Glycolysis → Pyruvate → Krebs Cycle → ETC (O$_2$ as acceptor) → "lots" of ATP.
  • Anaerobic respiration: Glycolysis → Pyruvate → Krebs Cycle → ETC (inorganic acceptor) → “some” ATP.
  • Fermentation: Glycolysis → Pyruvate → Organic end-product (no ETC) → “small amounts” of ATP.
• Quantitative frame (classical values; not explicitly in transcript):
  \text{ATP}{\text{aerobic}} \approx 36!\text{–}!38 > \text{ATP}{\text{anaerobic}} \approx 2!\text{–}!30 > \text{ATP}_{\text{fermentation}} = 2
  (Ranges depend on organism & acceptor).

Key Definitions & Concepts

• \text{ATP}: universal energy currency; hydrolysis releases \approx 30.5\,\text{kJ mol}^{-1}.
• Substrate-level phosphorylation: Direct production of ATP by transfer of a phosphate group to ADP.
• Oxidative phosphorylation: ATP synthesis powered by electrochemical proton gradient generated by ETC.
• Redox balance: Maintaining oxidized cofactors (NAD$^+$, FAD) is essential for pathway continuity; fermentation offers an ETC-independent recycling method.
• Final/terminal electron acceptor: Molecule that receives electrons at the end of a respiratory chain or fermentation.

Practical / Ethical / Economic Connections

• Food technology: Controlled fermentation underpins global industries (bakery, dairy, brewing, winemaking, probiotics).
• Renewable energy: ABE fermentation produces bio-butanol – candidate gasoline substitute with higher energy density than ethanol.
• Health & Medicine: Understanding anaerobic metabolism aids in treating ischemic injuries, lactic acidosis, and combating anaerobic bacterial infections (e.g.
  gas gangrene).
• Ecological impact: Anaerobic respiration in soils & sediments participates in nitrogen and sulfur cycles (denitrification, sulfate reduction ➝ \text{H}_2\text{S} production).

Equations & Numerical Recap

• Net glycolytic reaction (embden–meyerhof pathway):
  \text{Glucose} + 2\,\text{ADP} + 2\,\text{P_i} + 2\,\text{NAD}^+ \rightarrow 2\,\text{Pyruvate} + 2\,\text{ATP} + 2\,\text{NADH} + 2\,\text{H}^+
• Lactic acid fermentation (homolactic):
  \text{Pyruvate} + \text{NADH} + \text{H}^+ \rightarrow \text{Lactate} + \text{NAD}^+
• Alcohol fermentation (yeast):
  \text{Pyruvate} \xrightarrow{\text{Decarboxylase}} \text{Acetaldehyde} + \text{CO}_2
  \text{Acetaldehyde} + \text{NADH} + \text{H}^+ \xrightarrow{\text{Dehydrogenase}} \text{Ethanol} + \text{NAD}^+
• Proton-motive force relation: \Delta G = -nF\Delta\Psi - 2.303 nRT\Delta pH (drives ATP synthase).