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).