Aerobic Cellular Respiration and Fermentation: Pathways of Energy Production

Cellular Respiration

Overview of Aerobic Cellular Respiration

Cellular respiration is a series of metabolic processes by which cells extract energy from glucose and other organic molecules. In the presence of oxygen, this process is called aerobic respiration and involves the complete oxidation of glucose to carbon dioxide and water, producing ATP as the main energy currency of the cell.

• Overall Equation:

$\mathrm{C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy (ATP + heat)}}$

• Redox Reactions: Glucose is oxidized (loses electrons), and oxygen is reduced (gains electrons).

• Electron Carriers: Electrons are transferred from glucose to NADH and FADH2, then to the electron transport chain (ETC), and finally to oxygen.

• Energy Release: The transfer of electrons to oxygen is exergonic, releasing energy to drive ATP synthesis.

Stages of Aerobic Cellular Respiration

Aerobic respiration occurs in four main stages:

1. Glycolysis

2. Formation of Acetyl CoA (Pyruvate Oxidation)

3. The Citric Acid Cycle (Krebs Cycle)

4. Electron Transport Chain and Oxidative Phosphorylation

Glycolysis

Process and Products

Glycolysis is the enzymatic breakdown of one glucose molecule (6 carbons) into two molecules of pyruvate (3 carbons each). This process occurs in the cytosol and does not require oxygen.

• Location: Cytosol

• Enzymes: Each step is catalyzed by a specific enzyme.

• Net Results (per glucose):

  • 2 Pyruvate + 2 H2O

  • 2 NADH + 2 H+

  • 2 ATP (net gain)

Pyruvate is then transported into the mitochondrion for further oxidation.

Formation of Acetyl CoA (Pyruvate Oxidation)

Conversion and Products

Each pyruvate molecule is converted into acetyl CoA in the mitochondrial matrix. This step links glycolysis to the citric acid cycle.

• For each pyruvate (2 per glucose):

  • 1 Acetyl CoA

  • 1 NADH

  • 1 CO2

• For one glucose: 2 Acetyl CoA, 2 NADH, 2 CO2

The Citric Acid Cycle (Krebs Cycle)

Cycle Steps and Yield

The citric acid cycle completes the oxidation of glucose by breaking down acetyl CoA into CO2 and transferring electrons to NAD+ and FAD.

• First Step: Acetyl CoA (2C) combines with oxaloacetate (4C) to form citrate (6C), releasing CoA-SH.

• Location: Mitochondrial matrix

• Enzymes: Each of the eight steps is catalyzed by a different enzyme.

• Products per Acetyl CoA:

  • 3 NADH

  • 1 FADH2

  • 1 ATP (by substrate-level phosphorylation)

  • 2 CO2

  • 1 Oxaloacetate (regenerated)

• For one glucose (2 Acetyl CoA): Multiply all yields by 2.

Electron Transport Chain and Oxidative Phosphorylation

Electron Transport Chain (ETC)

The ETC is a series of protein complexes embedded in the inner mitochondrial membrane (cristae). Electrons from NADH and FADH2 are transferred through these complexes, ultimately reducing oxygen to water.

• Electron Carriers: Most are proteins called cytochromes containing iron atoms.

• Directionality: Each carrier has a higher affinity for electrons than the previous one.

• Final Electron Acceptor: Oxygen, which combines with H+ to form water.

Chemiosmosis and ATP Synthesis

As electrons move through the ETC, energy is used to pump H+ ions from the matrix into the intermembrane space, creating a proton gradient (proton motive force). H+ flows back into the matrix through ATP synthase, driving ATP production.

• ATP Yield:

  • Each NADH: enough for 2.5 ATP

  • Each FADH2: enough for 1.5 ATP

  • Total ATP from one glucose: about 32 ATP

Summary Table: ATP Yield from Aerobic Respiration (per glucose)

Stage	ATP Produced (net)	NADH Produced	FADH2 Produced
Glycolysis	2	2	0
Pyruvate Oxidation	0	2	0
Citric Acid Cycle	2	6	2
Oxidative Phosphorylation (from NADH & FADH2)	~28	-	-
Total	~32	10	2

Additional info: Actual ATP yield may vary depending on cell type and shuttle mechanisms.

Fermentation

Anaerobic Pathways

When oxygen is not available, cells can generate ATP through fermentation. Fermentation consists of glycolysis followed by reactions that regenerate NAD+, allowing glycolysis to continue.

• Location: Cytosol

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

Types of Fermentation

• Lactic Acid Fermentation (e.g., in human muscle cells):

  • Pyruvate accepts electrons from NADH, forming lactate (lactic acid) and regenerating NAD+.

• Alcohol Fermentation (e.g., in yeast):

  • Pyruvate is converted to acetaldehyde (releasing CO2), which then accepts electrons from NADH to form ethanol, regenerating NAD+.

Regulation of Cellular Respiration

Feedback Mechanisms

Cellular respiration is tightly regulated by feedback mechanisms to match the cell's energy needs. The main control point is the enzyme phosphofructokinase in glycolysis, which is allosterically inhibited by high levels of ATP and citrate, and activated by AMP.

• High ATP or citrate: Inhibits phosphofructokinase, slowing glycolysis and respiration.

• High AMP: Activates phosphofructokinase, increasing glycolysis and respiration.

Additional info: This regulation ensures efficient use of resources and prevents excess ATP production.