Cellular Respiration & Fermentation: Processes, Pathways, and Energy Production

Exergonic Breakdown of Organic Molecules

The breakdown of molecules releases stored energy.

Glucose oxidation

Releases energy to produce ATP.

Fermentation

Partial breakdown of sugars without oxygen.

Yeast performing alcohol fermentation

To make bread rise.

Aerobic Respiration

Consumes organic molecules and oxygen, producing ATP.

Humans using oxygen

To fully oxidize glucose during exercise.

Anaerobic Respiration

Uses an electron acceptor other than oxygen (e.g., nitrate, sulfate).

Some bacteria in deep-sea vents

Using sulfate as the final electron acceptor.

Cellular Respiration Overview

C6H12O6 + 6O2 → 6CO2 + 6H2O + Energy (ATP + heat)

A muscle cell oxidizing glucose

To generate ATP for contraction.

Redox Reactions

Transfer of electrons between reactants.

Oxidation

Substance loses electrons.

Glucose is oxidized

To CO₂.

Reduction

Substance gains electrons.

Oxygen is reduced

To H₂O in the electron transport chain.

Energy Harvest via NAD+

Electrons from glucose transferred to NAD+, forming NADH.

NADH

Carries electrons to the electron transport chain.

In glycolysis

NAD+ is reduced to NADH when glyceraldehyde-3-phosphate is oxidized.

Glycolysis

Breaks glucose into 2 pyruvate molecules.

Glycolysis location

Occurs in the cytoplasm.

Glycolysis phases

Has two phases: Energy Investment and Energy Payoff.

Glycolysis oxygen requirement

Works with or without oxygen.

In human muscle cells

Glycolysis provides ATP during sprinting when oxygen is limited.

Pyruvate Oxidation

Pyruvate → Acetyl CoA.

Pyruvate Oxidation Products

Produces CO₂ and NADH.

Citric Acid Cycle

Completes glucose breakdown to CO₂.

Citric Acid Cycle Products

Produces ATP, NADH, and FADH₂.

Citric Acid Cycle Yield

One cycle yields 3 NADH, 1 FADH₂, and 1 ATP from a single Acetyl CoA.

Oxidative Phosphorylation

Accounts for most ATP.

Oxidative Phosphorylation Mechanism

Powered by redox reactions.

Electron Transport Chain (ETC)

NADH & FADH₂ donate electrons.

Final Electron Acceptor

In mitochondria, oxygen is the final electron acceptor → H₂O.

Chemiosmosis

H+ gradient drives ATP synthase.

ATP Synthase Yield

ATP synthase producing ~28 ATP in mitochondria.

Energy Flow in ATP Production

Glucose → NADH/FADH₂ → ETC → Proton-motive force → ATP.

ATP Yield

~32 ATP (34% efficiency).

ATP Yield Variability

Yield depends on electron shuttle systems and proton gradient use.

Example of Anaerobic Respiration

Methanogens reduce CO₂ to methane in oxygen-free environments.

Example of Lactic Acid Fermentation

Muscle cells performing lactic acid fermentation during intense exercise.

Alcohol Fermentation

Pyruvate → ethanol + CO₂; regenerates NAD+.

Example of Alcohol Fermentation

Brewing beer and wine.

Lactic Acid Fermentation

Pyruvate → lactate; regenerates NAD+.

Obligate Anaerobes

Cannot survive with oxygen.

Example of Obligate Anaerobes

Clostridium botulinum.

Facultative Anaerobes

Can switch between fermentation and respiration.

Example of Facultative Anaerobes

Yeast or E. coli.

Metabolic Connections

Glycolysis and the Citric Acid Cycle are hubs for metabolism.

Fatty Acids in Metabolism

Fatty acids broken into Acetyl CoA feed into the citric acid cycle.

Amino Acids in Metabolism

Amino acids can be deaminated and funneled into glycolysis or the citric acid cycle.