Cellular Respiration: Citric Acid Cycle & Electron Transport System

Cellular Respiration: Citric Acid Cycle & Electron Transport System

Overview of Cellular Respiration

Cellular respiration is the process by which cells extract energy from nutrients, primarily glucose, to produce ATP, the energy currency of the cell. This process involves multiple stages, including glycolysis, the citric acid cycle (Krebs cycle), and the electron transport system. Each stage occurs in specific regions of the cell and utilizes various coenzymes to facilitate energy transfer.

• Key Stages: Glycolysis, Citric Acid Cycle, Electron Transport System

• Location: Mitochondria (inner membrane region)

• Major Products: ATP, CO2, H2O

Citric Acid Cycle (Krebs Cycle)

The citric acid cycle is the third stage of cellular respiration and occurs in the inner membrane region of the mitochondria. For each glucose molecule, the cycle operates twice, once for each acetyl group derived from glucose catabolism.

• Acetyl CoA: Delivers acetyl group to the cycle, combines with oxaloacetate to form citric acid.

• Metabolic Steps:

  • Citric acid is metabolized, releasing CO2 and forming α-ketoglutarate.

  • α-ketoglutarate is further metabolized, releasing CO2 and forming succinate.

  • Succinate is metabolized, producing FADH2 and forming fumarate.

  • Fumarate is metabolized, producing NADH and regenerating oxaloacetate.

• Coenzyme Activity: NAD+ and FAD collect high-energy electrons and hydrogen ions.

• ATP Production: 2 ATP molecules produced by substrate-level phosphorylation per glucose.

• Waste: 4 CO2 molecules per glucose.

Coenzymes in Energy Production

Coenzymes such as NAD+ and FAD play crucial roles in cellular respiration by transporting high-energy electrons and hydrogen ions to the electron transport system.

• NAD+: Picks up 1 hydrogen and 2 electrons to become NADH.

• FAD: Picks up 2 hydrogen and 2 electrons to become FADH2.

• Function: Carry energy to the electron transport system for ATP production.

Electron Transport System (ETS)

The electron transport system is the final stage of cellular respiration, occurring in the inner mitochondrial membrane. NADH and FADH2 deliver electrons and hydrogen ions to carrier proteins, which transfer electrons through a series of reactions, powering the active transport of H+ ions.

• Carrier Proteins: Accept and transfer electrons, energizing the transport of H+ ions.

• ATP Synthase: Specialized enzyme channels allow H+ ions to diffuse back, catalyzing ATP synthesis from ADP and inorganic phosphate (Pi).

• Oxidative Phosphorylation: Process requiring oxygen, resulting in the production of approximately 34 ATP molecules per glucose.

• Waste: Water (H2O) formed as electrons and H+ combine with oxygen.

Equation for Water Formation:

$\frac{1}{2} O_2 + 2H^+ + 2e^- \rightarrow H_2O$

Summary of ATP Production

• Glycolysis: 2 ATP

• Citric Acid Cycle: 2 ATP

• Electron Transport System: 34 ATP

• Total: 38 ATP (net gain: 36 ATP after accounting for energy used to shuttle NADH)

Catabolism of Glycogen, Fats, and Proteins

The body utilizes various energy reserves, including glycogen, fats, and proteins, to produce ATP. Fats provide the largest energy reserve, followed by proteins and glycogen.

• Glycogen: Converted to glucose, then enters glycolysis.

• Fats: Triglycerides break down into glycerol and fatty acids. Glycerol can be converted to glucose or pyruvate; fatty acids are converted to acetyl groups for the citric acid cycle.

• Proteins: Broken into amino acids; amine group removed (excreted as urea), carbon backbone enters citric acid cycle.

• Energy Yield: Fats produce about twice as much ATP as glycogen; protein catabolism increases during starvation.

Anaerobic Respiration

Anaerobic respiration allows cells to produce ATP without oxygen, typically for brief periods. Glycolysis is an example, where pyruvate is converted to lactic acid instead of entering the mitochondria.

• Pyruvate: Converted to lactic acid in the absence of oxygen.

• Lactic Acid: Causes muscle burning and cramping during intense activity.

• ATP Production: Limited compared to aerobic respiration.

Summary Table: Energy Production from Different Sources

Energy Source	Pathway	ATP Yield	Waste Products
Glycogen	Glycolysis & Citric Acid Cycle	~36 ATP per glucose	CO2, H2O
Fats	Glycerol & Fatty Acids to Citric Acid Cycle	~2x ATP vs. glycogen	CO2, H2O
Proteins	Amino Acids to Citric Acid Cycle	Similar to glycogen	CO2, H2O, Urea

Key Terms and Definitions

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

• Citric Acid Cycle (Krebs Cycle): A series of reactions that generate high-energy coenzymes and ATP.

• Electron Transport System: A chain of proteins that transfer electrons and produce ATP via oxidative phosphorylation.

• Coenzymes: Molecules like NAD+ and FAD that transport electrons and hydrogen ions.

• Oxidative Phosphorylation: ATP production that requires oxygen and involves electron transfer.

• Anaerobic Respiration: ATP production without oxygen, resulting in lactic acid formation.

Additional info: The notes expand on the lecture objectives by providing detailed explanations of each stage of cellular respiration, the role of coenzymes, and the metabolism of alternative energy sources. The summary table and definitions ensure the guide is self-contained and suitable for exam preparation.