Cellular Respiration and Fermentation: Pathways of Energy Harvest

Cellular Respiration and Fermentation

Overview: Life Is Work

Living cells require energy to perform their functions, which they obtain from external sources. Energy enters ecosystems as sunlight and leaves as heat, while essential chemical elements are recycled. Photosynthesis produces oxygen and organic molecules, which are used as fuel for cellular respiration in mitochondria. Cells harvest chemical energy from organic molecules to regenerate ATP, the main energy currency for cellular work. Cellular respiration consists of three main pathways: glycolysis, the citric acid cycle, and oxidative phosphorylation. Fermentation is a simpler, ancient pathway coupled to glycolysis.

Catabolic Pathways and Redox Reactions

Catabolic Pathways Yield Energy by Oxidizing Organic Fuels

• Catabolic pathways break down complex molecules, releasing stored energy.

• Electron transfer is central to these pathways, with energy released as electrons move closer to electronegative atoms.

• Fermentation partially degrades sugars without oxygen.

• Aerobic respiration uses oxygen to completely break down organic molecules, while anaerobic respiration uses other final electron acceptors.

• The overall equation for aerobic respiration (using glucose): $\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy (ATP + heat)}}$

• The catabolism of glucose is exergonic: $\Delta G = -686$ kcal/mol.

Redox Reactions in Cellular Respiration

• Redox reactions involve the transfer of electrons between reactants.

• Oxidation: Loss of electrons; Reduction: Gain of electrons.

• Example: $\mathrm{Na + Cl \rightarrow Na^+ + Cl^-}$ (Na is oxidized, Cl is reduced).

• Generalized: $\mathrm{Xe^- + Y \rightarrow X + Ye^-}$ (X is reducing agent, Y is oxidizing agent).

• Redox reactions can involve complete electron transfer or changes in electron sharing in covalent bonds.

• Electrons lose potential energy when moving toward more electronegative atoms, releasing energy for work.

Organic Fuel Molecules Are Oxidized During Cellular Respiration

• Glucose is oxidized, oxygen is reduced in a series of enzyme-catalyzed steps.

• Organic molecules rich in hydrogen are excellent fuels due to their high-energy electrons.

• Enzymes lower activation energy, allowing controlled oxidation of glucose.

Electron Carriers: NAD+ and the Electron Transport Chain

• Electrons from glucose are transferred to NAD+, forming NADH.

• NAD+ acts as an oxidizing agent; NADH stores energy for ATP synthesis.

• Electrons are transferred from NADH to oxygen via the electron transport chain (ETC), a series of proteins in the inner mitochondrial membrane.

• Overall electron flow: glucose → NADH → ETC → oxygen.

Stages of Cellular Respiration

Overview of the Three Stages

• Glycolysis (cytosol): Splits glucose into two pyruvate molecules.

• Citric Acid Cycle (mitochondrial matrix): Completes oxidation of organic fuel.

• Oxidative Phosphorylation (inner mitochondrial membrane): Includes ETC and chemiosmosis for ATP synthesis.

• Most ATP is produced by oxidative phosphorylation; some by substrate-level phosphorylation in glycolysis and the citric acid cycle.

• Maximum ATP yield per glucose: up to 32 ATP.

Glycolysis: Oxidizing Glucose to Pyruvate

• Glucose (6C) is split into two pyruvate (3C) molecules in 10 enzyme-catalyzed steps.

• Two phases:

  1. Energy investment phase: 2 ATP consumed.

  2. Energy payoff phase: 4 ATP produced (net gain 2 ATP), 2 NADH formed.

• No CO2 is released during glycolysis.

• Glycolysis occurs with or without oxygen.

Pyruvate Oxidation and the Citric Acid Cycle

• Pyruvate enters mitochondria (eukaryotes) and is converted to acetyl CoA (2C).

• Conversion involves:

  1. Release of CO2

  2. Reduction of NAD+ to NADH

  3. Attachment of CoA to form acetyl CoA

• Citric Acid Cycle (Krebs Cycle):

  • Acetyl CoA combines with oxaloacetate to form citrate.

  • Cycle releases 2 CO2 per acetyl group, generates 3 NADH, 1 FADH2, and 1 ATP (or GTP) per turn.

  • Per glucose: 6 NADH, 2 FADH2, 2 ATP (from two turns).

Oxidative Phosphorylation: Electron Transport and Chemiosmosis

• Electron Transport Chain (ETC) is a series of protein complexes (I-IV) in the inner mitochondrial membrane.

• Electrons from NADH and FADH2 pass through the chain, ultimately reducing oxygen to water.

• Energy released is used to pump H+ ions into the intermembrane space, creating a proton gradient.

• ATP synthase uses the proton-motive force to synthesize ATP from ADP and inorganic phosphate (chemiosmosis).

• Each NADH yields about 2.5 ATP; each FADH2 about 1.5 ATP.

• Total ATP yield per glucose: about 30–32 ATP (varies by cell type and shuttle systems).

Summary Table: ATP Yield from Cellular Respiration

Stage	ATP Produced (per glucose)	NADH Produced	FADH2 Produced
Glycolysis	2 (net)	2	0
Pyruvate Oxidation	0	2	0
Citric Acid Cycle	2	6	2
Oxidative Phosphorylation	~26–28	-	-
Total	~30–32	10	2

Additional info: Actual ATP yield varies due to shuttle mechanisms and use of proton-motive force for other cellular work.

Fermentation and Anaerobic Respiration

ATP Production Without Oxygen

• When oxygen is unavailable, cells can generate ATP via fermentation or anaerobic respiration.

• Anaerobic respiration uses an electron transport chain with a final electron acceptor other than oxygen (e.g., sulfate).

• Fermentation does not use an electron transport chain; glycolysis continues as long as NAD+ is regenerated.

Types of Fermentation

• Alcohol fermentation (e.g., yeast):

  1. Pyruvate → acetaldehyde + CO2

  2. Acetaldehyde reduced by NADH to ethanol (regenerates NAD+)

• Lactic acid fermentation (e.g., muscle cells, some bacteria):

  • Pyruvate reduced directly by NADH to lactate (no CO2 released)

  • Lactate can be converted back to pyruvate in the liver

Comparison of Fermentation and Respiration

Feature	Fermentation	Cellular Respiration
Final Electron Acceptor	Organic molecule (e.g., pyruvate, acetaldehyde)	O2 (aerobic) or other (anaerobic)
ATP Yield (per glucose)	2	~30–32
Electron Transport Chain	No	Yes
Location	Cytosol	Cytosol & mitochondria

Organismal Variations

• Obligate anaerobes: Only fermentation or anaerobic respiration; oxygen is toxic.

• Facultative anaerobes (e.g., yeast, muscle cells): Can use either fermentation or respiration depending on oxygen availability.

• Obligate aerobes: Require oxygen for ATP production (e.g., vertebrate brain cells).

Connections to Other Metabolic Pathways

Catabolism of Other Molecules

• Carbohydrates: Polysaccharides and disaccharides are hydrolyzed to glucose or other monosaccharides that enter glycolysis.

• Proteins: Digested to amino acids; after deamination, carbon skeletons enter glycolysis or the citric acid cycle.

• Fats: Broken down to glycerol (enters glycolysis) and fatty acids (converted to acetyl CoA via beta oxidation).

• Fats yield more than twice as much ATP per gram as carbohydrates.

Anabolic Pathways and Biosynthesis

• Intermediates from glycolysis and the citric acid cycle serve as precursors for biosynthesis (anabolism).

• Cells can synthesize amino acids, nucleotides, and fatty acids from these intermediates.

• Excess carbohydrates and proteins can be converted to fats for storage.

Regulation of Cellular Respiration

Feedback Mechanisms

• Cellular respiration is regulated by supply and demand via feedback inhibition.

• Phosphofructokinase is a key allosteric enzyme regulating glycolysis; inhibited by ATP and citrate, activated by AMP.

• Regulation ensures metabolic balance and efficient use of resources.

Evolutionary Perspective

Evolution of Glycolysis

• Glycolysis is an ancient pathway, likely present in the earliest prokaryotes before atmospheric oxygen was abundant.

• Its ubiquity and cytosolic location suggest early evolutionary origin.

Key Terms and Definitions

• ATP (Adenosine Triphosphate): Main energy currency of the cell.

• NAD+ (Nicotinamide Adenine Dinucleotide): Electron carrier; oxidized form.

• NADH: Reduced form of NAD+; carries electrons to the ETC.

• FAD/FADH2: Another electron carrier involved in the citric acid cycle and ETC.

• Substrate-level phosphorylation: Direct transfer of a phosphate group to ADP to form ATP.

• Oxidative phosphorylation: ATP synthesis powered by the ETC and chemiosmosis.

• Proton-motive force: The H+ gradient across the inner mitochondrial membrane that drives ATP synthesis.

• Beta oxidation: Catabolic process by which fatty acids are broken down to acetyl CoA.

Summary

Cellular respiration is a central metabolic pathway that efficiently extracts energy from organic molecules, primarily glucose, through a series of redox reactions. The process involves glycolysis, the citric acid cycle, and oxidative phosphorylation, with fermentation and anaerobic respiration providing alternative ATP-generating pathways in the absence of oxygen. Cellular respiration is tightly regulated and interconnected with other metabolic pathways, reflecting its evolutionary and physiological importance in all forms of life.