Cellular Respiration and Fermentation

Overview of Cellular Respiration

Cellular respiration is a series of metabolic pathways that convert the chemical energy stored in organic molecules into ATP, the energy currency of the cell. This process occurs in both plant and animal cells, primarily within the mitochondria, and involves the breakdown of glucose and other organic fuels. The energy released is used to synthesize ATP, while some is lost as heat.

• ATP (Adenosine Triphosphate): The main energy carrier in cells, used to drive most cellular work.

• Energy Flow: Energy enters ecosystems as light, is captured by photosynthesis, and is released as heat during cellular respiration.

• Chemical Recycling: Photosynthesis and cellular respiration are complementary; the products of one are the reactants of the other.

• General Equation for Cellular Respiration:

Catabolic Pathways and ATP Production

Catabolic pathways break down complex molecules, releasing stored energy. This energy is harvested through redox reactions, where electrons are transferred from food molecules to electron carriers, ultimately generating ATP.

• Fermentation: Partial degradation of sugars without oxygen.

• Aerobic Respiration: Uses oxygen as the final electron acceptor, yielding the most ATP.

• Anaerobic Respiration: Uses electron acceptors other than oxygen.

Redox Reactions in Cellular Respiration

Oxidation and Reduction

Redox (reduction-oxidation) reactions involve the transfer of electrons between molecules. The molecule that loses electrons is oxidized, while the molecule that gains electrons is reduced.

• Reducing Agent: Donates electrons and becomes oxidized.

• Oxidizing Agent: Accepts electrons and becomes reduced.

Electron Carriers: NAD+ and FAD

During cellular respiration, electrons from organic molecules are usually transferred to electron carriers such as NAD+ (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide). These carriers shuttle electrons to the electron transport chain.

• NAD+ + 2e- + 2H+ → NADH + H+

• FAD + 2e- + 2H+ → FADH2

Stages of Cellular Respiration

1. Glycolysis

Glycolysis is the first stage of cellular respiration and occurs in the cytoplasm. It splits one molecule of glucose (6 carbons) into two molecules of pyruvate (3 carbons each), producing a small amount of ATP and NADH.

• Energy Investment Phase: 2 ATP are used to phosphorylate glucose and its intermediates.

• Energy Payoff Phase: 4 ATP are produced (net gain of 2 ATP), and 2 NAD+ are reduced to 2 NADH.

• Net Reaction:

2. Pyruvate Oxidation

In the presence of oxygen, pyruvate enters the mitochondrion (in eukaryotes) and is converted to acetyl CoA. This step links glycolysis to the citric acid cycle.

• Decarboxylation: Pyruvate loses one carbon as CO2.

• Reduction: NAD+ is reduced to NADH.

• Formation of Acetyl CoA: The remaining two-carbon fragment is attached to coenzyme A.

3. Citric Acid Cycle (Krebs Cycle)

The citric acid cycle completes the breakdown of glucose by oxidizing acetyl CoA to CO2. It occurs in the mitochondrial matrix and generates ATP, NADH, and FADH2.

• Per Turn (per acetyl CoA): 1 ATP, 3 NADH, 1 FADH2, 2 CO2

• Cycle Runs Twice Per Glucose: Because each glucose yields 2 acetyl CoA.

• Main Steps: Acetyl group combines with oxaloacetate to form citrate; citrate is progressively oxidized, regenerating oxaloacetate.

4. Oxidative Phosphorylation (Electron Transport Chain & Chemiosmosis)

Oxidative phosphorylation is the final stage, where most ATP is produced. NADH and FADH2 donate electrons to the electron transport chain (ETC), which is embedded in the inner mitochondrial membrane. The energy released pumps protons (H+) across the membrane, creating a proton gradient. ATP synthase uses this gradient to synthesize ATP from ADP and Pi in a process called chemiosmosis.

• Electron Transport Chain: Series of protein complexes that transfer electrons to O2, forming H2O.

• ATP Yield: Up to 32 ATP per glucose molecule.

• Proton-Motive Force: The electrochemical gradient of H+ drives ATP synthesis.

Fermentation and Anaerobic Respiration

Fermentation

Fermentation is an anaerobic process that allows glycolysis to continue in the absence of oxygen by regenerating NAD+ from NADH. It produces less ATP than aerobic respiration.

• Alcohol Fermentation: Pyruvate is converted to ethanol and CO2; NAD+ is regenerated.

• Lactic Acid Fermentation: Pyruvate is reduced to lactate; NAD+ is regenerated without CO2 release.

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

Anaerobic Respiration

Some organisms use an electron transport chain with a final electron acceptor other than oxygen (e.g., sulfate ion). This process is less efficient than aerobic respiration but more efficient than fermentation.

Summary Table: Comparison of Energy-Yielding Pathways

Key Terms and Concepts

• Substrate-Level Phosphorylation: Direct transfer of a phosphate group to ADP from a substrate.

• Oxidative Phosphorylation: ATP synthesis powered by the electron transport chain and chemiosmosis.

• ATP Synthase: Enzyme that synthesizes ATP using the proton gradient.

• Proton-Motive Force: The force generated by the transmembrane proton gradient.

Summary of ATP Production

• Glycolysis: 2 ATP (net), 2 NADH

• Pyruvate Oxidation: 2 NADH

• Citric Acid Cycle: 2 ATP, 6 NADH, 2 FADH2

• Oxidative Phosphorylation: ~26-28 ATP

• Total: Up to 32 ATP per glucose

Additional info: The actual ATP yield varies due to the coupling efficiency of the electron transport chain, the use of the proton-motive force for other cellular work, and the shuttle systems that transfer electrons from cytosolic NADH into mitochondria.