Cellular Respiration & Fermentation: Mechanisms of ATP Production in Cells

Cellular Respiration & Fermentation

Overview of Cellular Respiration

Cellular respiration is the process by which cells extract energy from glucose and other high-energy compounds to produce ATP, the universal energy currency. This process occurs in both eukaryotic and prokaryotic cells and involves a series of metabolic pathways.

• ATP (Adenosine Triphosphate): The primary energy carrier in cells.

• Glucose: A six-carbon sugar that serves as the main fuel for cellular respiration.

• Major Pathways: Glycolysis, Pyruvate Oxidation, Citric Acid Cycle, Electron Transport Chain & Chemiosmosis.

Major Stages of Cellular Respiration

• Glycolysis: Occurs in the cytosol; breaks down glucose into two pyruvate molecules, producing ATP and NADH.

• Pyruvate Oxidation: Pyruvate is transported into the mitochondrial matrix (in eukaryotes) and converted to acetyl CoA by the pyruvate dehydrogenase complex, generating NADH and CO2.

• Citric Acid Cycle (Krebs Cycle): Acetyl CoA enters the cycle, resulting in the complete oxidation of carbon atoms from food, producing NADH, FADH2, ATP (or GTP), and CO2.

• Electron Transport Chain (ETC) & Chemiosmosis: NADH and FADH2 donate electrons to the ETC, which powers proton pumping and creates an electrochemical gradient. ATP synthase uses this gradient to synthesize ATP.

Structure and Function of the Mitochondrion

The mitochondrion is the site of most cellular respiration in eukaryotes. It contains an inner and outer membrane, with the matrix inside the inner membrane.

• Pyruvate Dehydrogenase: Located in the mitochondrial matrix (eukaryotes) or cytosol (prokaryotes); catalyzes pyruvate oxidation.

• Cristae: Folds of the inner membrane that increase surface area for ETC and ATP synthesis.

Pyruvate Oxidation to Acetyl CoA

Pyruvate produced from glycolysis is oxidized to acetyl CoA, releasing CO2 and reducing NAD+ to NADH.

• Reaction: $2 ext{Pyruvate} + 2 ext{NAD}^+ + 2 ext{CoA} ightarrow 2 ext{Acetyl CoA} + 2 ext{NADH} + 2 ext{CO}_2$

• Enzyme: Pyruvate dehydrogenase complex.

• Energy Capture: In NADH and thioester bond of acetyl CoA.

Other Pathways to Acetyl CoA

Acetyl CoA can be generated from carbohydrates, fats, and proteins through various catabolic pathways.

• Fatty Acid β-Oxidation: Fatty acids are broken down into acetyl CoA, producing NADH and FADH2.

• Protein Catabolism: Amino acids can be converted to intermediates that enter the citric acid cycle.

Citric Acid Cycle (Krebs Cycle)

The citric acid cycle completes the oxidation of acetyl CoA, generating high-energy electron carriers and ATP/GTP.

• Main Products per Glucose: 6 NADH, 2 FADH2, 2 ATP (or GTP), 4 CO2

• Key Steps: Acetyl CoA combines with oxaloacetate to form citrate; cycle regenerates oxaloacetate.

• Equation: $2 ext{Acetyl CoA} + 6 ext{NAD}^+ + 2 ext{FAD} + 2 ext{ADP} + 2 ext{P}_i ightarrow 4 ext{CO}_2 + 6 ext{NADH} + 2 ext{FADH}_2 + 2 ext{ATP}$

Electron Transport Chain and Chemiosmosis

The ETC uses electrons from NADH and FADH2 to pump protons across the inner mitochondrial membrane, creating a proton-motive force.

• Complexes I-IV: Sequentially transfer electrons and pump H+ into the intermembrane space.

• Oxygen: Final electron acceptor, forming water.

• Proton-Motive Force: Drives ATP synthesis via ATP synthase.

• Chemiosmosis: Process by which ATP is produced as H+ flows back into the matrix through ATP synthase.

• Equation: $NADH + H^+ + 1/2 O_2 ightarrow NAD^+ + H_2O$

ATP Yield from Glucose Oxidation

The complete oxidation of one glucose molecule yields a significant amount of ATP, with variations between eukaryotes and prokaryotes.

Catabolic Process	Products Formed	Net ATP Produced Per Molecule of Glucose
Glycolysis	2 NADH, 2 ATP, 2 pyruvate	2 ATP, 3-5 ATP (from NADH)
Pyruvate Oxidation	2 NADH, 2 acetyl CoA	5 ATP
Citric Acid Cycle	6 NADH, 2 FADH2, 2 GTP	15 ATP (NADH), 3 ATP (FADH2), 2 ATP (GTP)
Total		~30 ATP (eukaryotes), ~32 ATP (prokaryotes)

Additional info: NADH produced in the cytosol yields fewer ATP molecules than NADH produced in the matrix due to transport costs.

Fermentation: ATP Production Without Oxygen

Fermentation is an anaerobic process that allows cells to produce ATP when oxygen is absent. It regenerates NAD+ from NADH, enabling glycolysis to continue.

• Lactic Acid Fermentation: Occurs in animals; pyruvate is reduced to lactate.

• Alcohol Fermentation: Occurs in yeast; pyruvate is converted to ethanol and CO2.

• ATP Yield: Only 2 ATP per glucose, much less efficient than cellular respiration.

• Equation (Lactic Acid): $C_6H_{12}O_6 + 2 ADP + 2 P_i ightarrow 2 C_3H_6O_3 + 2 ATP$

• Equation (Alcohol): $C_6H_{12}O_6 + 2 ADP + 2 P_i ightarrow 2 C_2H_5OH + 2 CO_2 + 2 ATP$

Comparison: Cellular Respiration vs. Fermentation

Cells use fermentation only when the electron acceptor (usually oxygen) is not available. Some organisms can switch between aerobic respiration and fermentation depending on environmental conditions.

• Efficiency: Cellular respiration produces about 30 ATP per glucose; fermentation produces only 2 ATP.

• Flexibility: Facultative anaerobes can switch between pathways.

Summary Table: Cellular Respiration vs. Fermentation

Process	Oxygen Required?	ATP Yield (per glucose)	End Products
Cellular Respiration	Yes	~30-32	CO2, H2O
Fermentation	No	2	Lactate or Ethanol + CO2

Key Terms and Concepts

• Glycolysis: The breakdown of glucose to pyruvate in the cytosol.

• Pyruvate Dehydrogenase: Enzyme complex converting pyruvate to acetyl CoA.

• Citric Acid Cycle: Series of reactions that oxidize acetyl CoA to CO2 and generate electron carriers.

• Electron Transport Chain: Series of protein complexes that transfer electrons and pump protons.

• Chemiosmosis: ATP synthesis driven by proton flow through ATP synthase.

• Fermentation: Anaerobic process regenerating NAD+ for glycolysis.

Example Application

• Muscle Cells: During intense exercise, oxygen may be limited, and muscle cells switch to lactic acid fermentation, causing temporary muscle fatigue.

• Yeast: Used in baking and brewing, yeast cells perform alcohol fermentation, producing ethanol and CO2.