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 biomolecules. The process involves glycolysis, the citric acid cycle (Krebs cycle), and the electron transport system, each contributing to the production of ATP, the cell's energy currency.

• Glycolysis: Occurs in the cytoplasm, breaking down glucose into pyruvate.

• Citric Acid Cycle: Takes place in the mitochondria, further oxidizing acetyl groups.

• Electron Transport System: Located in the inner mitochondrial membrane, generating most ATP via oxidative phosphorylation.

Citric Acid Cycle (Krebs Cycle)

The citric acid cycle is the third stage of cellular respiration, occurring 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 groups to the cycle, combining with oxaloacetate to form citric acid.

• Citric Acid Metabolism: Sequential removal of carbons as CO2, with NAD+ and FAD acting as coenzymes to capture high-energy electrons and hydrogen ions.

• ATP Production: Substrate-level phosphorylation yields 2 ATP per glucose.

• Coenzyme Activity: 6 NADH and 2 FADH2 are produced per glucose, carrying electrons to the electron transport system.

• Waste: 4 CO2 molecules are released.

Key Steps of the Citric Acid Cycle

• Acetyl CoA + Oxaloacetate → Citric Acid

• Citric Acid → α-Ketoglutarate (CO2 released, NADH formed)

• α-Ketoglutarate → Succinate (CO2 released, NADH and ATP formed)

• Succinate → Fumarate (FADH2 formed)

• Fumarate → Oxaloacetate (NADH formed)

Summary Table: Citric Acid Cycle Products (per glucose)

Product	Amount
ATP	2
NADH	6
FADH2	2
CO2	4

Electron Transport System (ETS)

The electron transport system is the final stage of cellular respiration, located 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, losing energy at each step.

• H+ Gradient: H+ ions are pumped to the outer membrane, creating a concentration gradient.

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

• Oxidative Phosphorylation: Process requiring oxygen; electrons and H+ combine with O2 to form water.

• ATP Yield: Approximately 34 ATP molecules produced per glucose.

Equation for ATP synthesis:

$\mathrm{ADP} + \mathrm{P_i} \rightarrow \mathrm{ATP}$

Equation for water formation:

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

Summary Table: Electron Transport System Products (per glucose)

Product	Amount
ATP	34
H2O	6

Overall Energy Production Recap

Complete catabolism of glucose involves over 20 chemical reactions, resulting in a net gain of 36 ATP molecules (after accounting for 2 ATP used to shuttle NADH from cytoplasm to mitochondria). Waste products include CO2 and H2O.

• Products: 10 NADH, 2 FADH2, 38 ATP (gross), 36 ATP (net)

• Waste: 6 CO2, 6 H2O

Other Energy Sources: Catabolism of Fats, Glycogen, and Proteins

Besides glucose, cells can metabolize fats and proteins for energy. Most energy reserves are stored as fats, followed by proteins and glycogen.

• Fats: Triglycerides break down into glycerol and fatty acids. Glycerol can enter glycolysis or be converted to pyruvate; fatty acids are converted to acetyl groups for the citric acid cycle. Fats yield about twice as much ATP as glycogen.

• Proteins: Broken into amino acids; amine groups are 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.

• Glycogen: Rapidly mobilized for energy but constitutes only about 1% of reserves.

Summary Table: Energy Reserve Distribution

Energy Source	Percentage of Reserves
Glycogen	1%
Fats	78%
Proteins	21%

Anaerobic Respiration

Anaerobic respiration allows cells to produce ATP without oxygen, typically for short periods. Glycolysis is the main anaerobic pathway, resulting in pyruvate that is converted to lactic acid instead of entering the mitochondria. Lactic acid accumulation causes muscle burning and cramping.

• Glycolysis: Anaerobic pathway producing ATP and pyruvate.

• Lactic Acid Formation: Pyruvate is converted to lactic acid in the absence of oxygen.

• Effects: Lactic acid buildup leads to muscle fatigue.

Key Terms and Definitions

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

• NAD+ (Nicotinamide Adenine Dinucleotide): Coenzyme that carries electrons and hydrogen ions.

• FAD (Flavin Adenine Dinucleotide): Coenzyme that carries electrons and hydrogen ions.

• Oxidative Phosphorylation: ATP production process requiring oxygen.

• Substrate-Level Phosphorylation: Direct ATP formation from ADP and a substrate.

Additional info: The citric acid cycle and electron transport system are central to cellular energy production, integrating the metabolism of carbohydrates, fats, and proteins. Efficient energy extraction and waste management are essential for cellular function and organismal health.