Cellular Respiration: Citric Acid Cycle & Electron Transport System

Cellular Respiration: Citric Acid Cycle & Electron Transport System

Overview of Cellular Respiration

Cellular respiration is a multi-stage process by which cells extract energy from glucose and other organic molecules. The process involves glycolysis, the citric acid cycle (Krebs cycle), and the electron transport system, ultimately producing ATP, the cell's main energy currency.

• Citric Acid Cycle (Krebs Cycle): Completes the breakdown of glucose derivatives, generating electron carriers.

• Electron Transport System: Uses electrons from carriers to produce a large amount of ATP.

• Other Energy Sources: Fats and proteins can also be catabolized to produce ATP.

• Anaerobic Respiration: Allows ATP production without oxygen, though less efficiently.

Citric Acid Cycle (Krebs Cycle)

Location and Sequence of Events

The citric acid cycle occurs in the inner membrane region of the mitochondria. For each glucose molecule, the cycle runs twice (once for each acetyl group derived from glucose catabolism).

• Entry: Acetyl CoA delivers an acetyl group, which combines with oxaloacetate to form citric acid.

• Decarboxylation: Carbons are removed as CO2 during the cycle.

• Electron Carriers: NAD+ and FAD pick up hydrogen ions and electrons, forming NADH and FADH2.

• ATP Production: A small amount of ATP is produced by substrate-level phosphorylation.

• Cycle Continuation: The cycle regenerates oxaloacetate to begin again.

Key Steps and Products (Per Glucose)

• 6 NADH (from 6 NAD+)

• 2 FADH2 (from 2 FAD)

• 2 ATP (substrate-level phosphorylation)

• 4 CO2 (waste)

Note: These totals are for both acetyl groups from one glucose molecule.

Role of Coenzymes

• NAD+ (Nicotinamide adenine dinucleotide): Accepts hydrogen ions and electrons, becoming NADH.

• FAD (Flavin adenine dinucleotide): Accepts two hydrogen ions and two electrons, becoming FADH2.

• Both coenzymes transport high-energy electrons to the electron transport system.

Electron Transport System (ETS)

Mechanism and ATP Production

The electron transport system is located in the inner mitochondrial membrane (cristae). NADH and FADH2 donate electrons to carrier proteins, which pass them along a chain, releasing energy used to pump H+ ions across the membrane.

• Proton Gradient: H+ ions accumulate in the outer mitochondrial compartment.

• ATP Synthase: H+ ions flow back through ATP synthase, catalyzing the formation of ATP from ADP and inorganic phosphate (Pi).

• Oxygen: Final electron acceptor, combines with H+ and electrons to form water.

Equation for Oxidative Phosphorylation:

$\text{ADP} + \text{Pi} \xrightarrow{\text{ATP synthase}} \text{ATP}$

$\frac{1}{2} \text{O}_2 + 2\text{H}^+ + 2\text{e}^- \rightarrow \text{H}_2\text{O}$

Summary of ATP Yield

• Approximately 34 ATP molecules produced by oxidative phosphorylation.

• 6 molecules of water formed as waste.

• NAD+ and FAD are regenerated for reuse.

Overall Energy Production from Glucose

• Total ATP: 38 ATP produced, but 2 are used for shuttling, resulting in a net gain of 36 ATP per glucose molecule.

• Waste Products: 6 CO2 and 6 H2O.

• Efficiency: Energy is released gradually, allowing for efficient ATP production.

Catabolism of Fats, Glycogen, and Proteins

Alternative Energy Sources

When glucose is scarce, cells can metabolize fats and proteins to generate ATP. Most energy reserves in the body are stored as fat.

• Glycogen: About 1% of energy reserves; broken down to glucose for glycolysis.

• Fats: About 78% of energy reserves; triglycerides are broken into glycerol and fatty acids. Glycerol enters glycolysis or the preparatory step, while fatty acids are converted to acetyl groups for the citric acid cycle. Fats yield about twice as much ATP as glycogen.

• Proteins: About 21% of energy reserves; broken into amino acids. The amine group is removed (excreted as urea), and the carbon backbone enters the citric acid cycle at various points. Protein catabolism increases during starvation, leading to muscle wasting.

Anaerobic Respiration

ATP Production Without Oxygen

Cells can produce ATP anaerobically for short periods. Glycolysis is an anaerobic process; in the absence of oxygen, pyruvate is converted to lactic acid instead of entering the mitochondria. This process is less efficient and leads to lactic acid buildup, causing muscle fatigue and cramping.

• Glycolysis: Occurs in the cytoplasm, does not require oxygen.

• Lactic Acid Formation: Pyruvate is reduced to lactic acid, regenerating NAD+ for continued glycolysis.

• Limitation: Only a small amount of ATP is produced anaerobically.

Summary Table: ATP Yield from Cellular Respiration

Stage	ATP Produced	Key Products	Location
Glycolysis	2	2 NADH, 2 Pyruvate	Cytoplasm
Citric Acid Cycle	2	6 NADH, 2 FADH2, 4 CO2	Mitochondrial Matrix
Electron Transport System	34	6 H2O	Inner Mitochondrial Membrane
Total	38 (Net 36)	-	-

Additional info: The above summary integrates the catabolism of carbohydrates, fats, and proteins, and highlights the importance of coenzymes and the efficiency of aerobic versus anaerobic pathways in cellular energy production.