Cellular Respiration: Redox Reactions, Glycolysis, Citric Acid Cycle, Oxidative Phosphorylation, and Fermentation

Cellular Respiration and Energy Production

Redox Reactions in Catabolic Pathways

Cellular respiration is a series of metabolic processes that convert biochemical energy from nutrients into adenosine triphosphate (ATP), releasing waste products. The process relies heavily on redox (reduction-oxidation) reactions to extract energy from organic molecules.

• Redox Reactions: Involve the transfer of electrons from one molecule (the reductant) to another (the oxidant).

• Oxidation: Loss of electrons from a substance.

• Reduction: Gain of electrons by a substance.

• Catabolic Pathways: Break down complex molecules (like glucose) into simpler ones, releasing energy.

• Energy Yield: As organic fuels are oxidized, electrons are transferred to electron carriers (such as NAD+), which ultimately transfer electrons to oxygen, forming water and releasing energy used to synthesize ATP.

• Example: During cellular respiration, glucose is oxidized to carbon dioxide, and oxygen is reduced to water.

Glycolysis: The First Stage of Glucose Oxidation

Pathway and Cellular Location

Glycolysis is the initial metabolic pathway of glucose catabolism, occurring in the cytosol of the cell. It breaks down one molecule of glucose into two molecules of pyruvate.

• Location: Cytoplasm (cytosol) of both prokaryotic and eukaryotic cells.

• Process: Consists of 10 enzyme-catalyzed steps divided into two phases: the energy investment phase and the energy payoff phase.

• Net Products: 2 pyruvate, 2 ATP (net), and 2 NADH per glucose molecule.

• Equation:

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

• Example: Glycolysis is the universal pathway for glucose catabolism, present in nearly all living organisms.

Oxidation of Pyruvate and the Citric Acid Cycle

Pyruvate Oxidation and Citric Acid Cycle Location

After glycolysis, pyruvate is transported into the mitochondrion (in eukaryotes), where it is further oxidized. The citric acid cycle (Krebs cycle) completes the oxidation of organic molecules.

• Pyruvate Oxidation: Each pyruvate is converted to acetyl-CoA, producing NADH and releasing CO2.

• Location: Mitochondrial matrix (in eukaryotes); cytoplasm in prokaryotes.

• Chemical Equation for Pyruvate Oxidation:

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

• Citric Acid Cycle: Acetyl-CoA enters the cycle, which generates NADH, FADH2, ATP (or GTP), and CO2.

• Location: Mitochondrial matrix.

Events of the Citric Acid Cycle

• Main Purpose: Complete the oxidation of acetyl groups, transferring electrons to NAD+ and FAD.

• Products per Acetyl-CoA: 3 NADH, 1 FADH2, 1 ATP (or GTP), and 2 CO2.

• Cycle Steps: Includes citrate formation, isomerization, decarboxylation, and regeneration of oxaloacetate.

• Example: The citric acid cycle is amphibolic, serving both catabolic and anabolic roles.

Oxidative Phosphorylation and ATP Yield

Steps and ATP Accounting

Oxidative phosphorylation is the final stage of cellular respiration, consisting of the electron transport chain (ETC) and chemiosmosis.

• Electron Transport Chain (ETC): Series of protein complexes in the inner mitochondrial membrane that transfer electrons from NADH and FADH2 to oxygen.

• Proton Gradient: Electron transfer drives protons across the membrane, creating an electrochemical gradient.

• Chemiosmosis: ATP synthase uses the proton gradient to synthesize ATP from ADP and inorganic phosphate.

• Total ATP Yield: Up to 32-34 ATP molecules per glucose (varies by cell type and shuttle systems).

• Equation for Oxidative Phosphorylation:

$\text{NADH} + \text{FADH}_2 + \text{O}_2 + \text{ADP} + \text{P}_i \rightarrow \text{NAD}^+ + \text{FAD} + \text{H}_2\text{O} + \text{ATP}$

• Example: Cyanide poisoning inhibits the ETC, preventing ATP production.

ATP Yield Table

The following table summarizes the approximate ATP yield per glucose molecule during cellular respiration:

Stage	ATP Produced (per glucose)
Glycolysis	2 (net)
Citric Acid Cycle	2
Oxidative Phosphorylation	~28
Total	~32

Fermentation: Anaerobic ATP Production

Types and Mechanisms

Fermentation allows cells to produce ATP without oxygen by regenerating NAD+ for glycolysis.

• Lactic Acid Fermentation: Pyruvate is reduced to lactate, regenerating NAD+. Occurs in muscle cells under anaerobic conditions.

• Alcohol Fermentation: Pyruvate is converted to ethanol and CO2, regenerating NAD+. Common in yeast and some bacteria.

• ATP Yield: Only 2 ATP per glucose (from glycolysis).

• Example: Yogurt production (lactic acid fermentation), brewing (alcohol fermentation).

Interactions with Other Metabolic Pathways

Integration of Glycolysis and Citric Acid Cycle

Glycolysis and the citric acid cycle are central hubs that interact with various other metabolic pathways, allowing the cell to adapt to different nutrient sources and biosynthetic needs.

• Catabolic Interactions: Other carbohydrates, fats, and proteins can enter glycolysis or the citric acid cycle at various points after conversion to intermediates.

• Anabolic Interactions: Intermediates from these pathways can be diverted for biosynthesis of amino acids, nucleotides, and lipids.

• Example: Fatty acids are broken down via beta-oxidation to acetyl-CoA, which enters the citric acid cycle.

Additional info: These interactions ensure metabolic flexibility and efficient use of available resources.