Cellular Respiration and Fermentation

Main Catabolic Pathways for Energy Production

Cells utilize catabolic pathways to break down molecules and release energy. The two primary pathways are cellular respiration and fermentation.

• Cellular Respiration: A process that uses oxygen to completely break down glucose, producing ATP, water, and carbon dioxide.

• Fermentation: An anaerobic process (does not require oxygen) that partially breaks down glucose, yielding less ATP and producing byproducts such as lactic acid or ethanol.

• Main Difference: Cellular respiration occurs in the presence of oxygen, while fermentation occurs in its absence.

Oxidation and Reduction in Cellular Respiration

Energy transfer in cells involves redox reactions (oxidation-reduction reactions).

• Oxidation: Loss of electrons from a molecule.

• Reduction: Gain of electrons by a molecule.

• Mnemonic: "LEO goes GER" (Lose Electrons = Oxidation, Gain Electrons = Reduction).

• Electron Transfer: Electrons are transferred down an electron transport chain, driving ATP synthesis.

NAD+ and Its Role in Cellular Respiration

NAD+ (Nicotinamide adenine dinucleotide) is a key electron carrier in cellular respiration.

• Function: Accepts electrons during glycolysis and the citric acid cycle, becoming NADH.

• Importance: NADH carries electrons to the electron transport chain, where ATP is produced.

• Abbreviation: NAD stands for Nicotinamide Adenine Dinucleotide.

Electron Transport Chain (ETC)

The electron transport chain is a series of protein complexes in the inner mitochondrial membrane.

• Role: Transfers electrons from NADH and FADH2 to oxygen, forming water.

• ATP Production: The flow of electrons drives the synthesis of ATP via chemiosmosis.

Steps of Cellular Respiration

Cellular respiration consists of three main stages:

• Glycolysis: Occurs in the cytoplasm; breaks glucose into pyruvate.

• Citric Acid Cycle (Krebs Cycle): Occurs in the mitochondrial matrix; oxidizes acetyl-CoA to CO2 and generates NADH and FADH2.

• Electron Transport Chain and Oxidative Phosphorylation: Occurs in the inner mitochondrial membrane; produces most of the cell's ATP.

ATP Production in Cellular Respiration

ATP is produced at different stages of cellular respiration.

• Glycolysis: Net gain of 2 ATP per glucose molecule.

• Citric Acid Cycle: 2 ATP per glucose molecule (1 per cycle).

• Oxidative Phosphorylation: About 26-28 ATP per glucose molecule.

• Total Yield: Maximum of about 30-32 ATP per glucose molecule.

Glycolysis

Glycolysis is the first step in cellular respiration, occurring in the cytoplasm.

• Process: Glucose (6 carbons) is split into two molecules of pyruvate (3 carbons each).

• ATP Yield: Net gain of 2 ATP per glucose.

• NADH Production: 2 NADH per glucose.

Conversion of Pyruvate

Before entering the citric acid cycle, pyruvate must be converted to acetyl-CoA.

• Process: Pyruvate is oxidized, releasing CO2 and generating NADH.

• Acetyl-CoA: Enters the citric acid cycle.

Citric Acid Cycle (Krebs Cycle)

The citric acid cycle completes the breakdown of glucose, generating electron carriers.

• Products per cycle: 1 ATP, 3 NADH, 1 FADH2 (per acetyl-CoA).

• Per glucose: 2 ATP, 6 NADH, 2 FADH2.

Electron Carriers and the Electron Transport Chain

NADH and FADH2 donate electrons to the electron transport chain.

• Electron Flow: Electrons move through protein complexes, pumping protons (H+) across the membrane.

• ATP Synthesis: Proton gradient drives ATP synthase to produce ATP.

Chemiosmosis

Chemiosmosis is the process by which ATP is produced using the energy of a proton gradient.

• Coupling: Electron transport and ATP synthesis are coupled via the movement of H+ ions.

• ATP Synthase: Enzyme that synthesizes ATP as protons flow back into the mitochondrial matrix.

ATP Yield from Electron Carriers

ATP yield depends on the number of NADH and FADH2 molecules produced.

• From NADH: Each NADH yields approximately 2.5-3 ATP.

• From FADH2: Each FADH2 yields approximately 1.5-2 ATP.

• Example Calculation:

Fermentation: Lactic Acid vs. Alcoholic

Fermentation allows cells to produce ATP without oxygen.

• Lactic Acid Fermentation: Pyruvate is reduced to lactic acid; occurs in muscle cells.

• Alcoholic Fermentation: Pyruvate is converted to ethanol and CO2; occurs in yeast.

• Purpose: Regenerates NAD+ so glycolysis can continue.

Summary Table: ATP Yield from Cellular Respiration

Additional info:

• Some details, such as the exact ATP yield per NADH and FADH2, may vary depending on cell type and conditions.

• "Chemiosmosis" is a key concept in understanding how the electron transport chain is coupled to ATP synthesis.