Cellular Respiration: Citric Acid Cycle & Electron Transport System

Cellular Respiration

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 several 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 transfer energy.

Citric Acid Cycle (Krebs Cycle)

Location and Sequence of Steps

The citric acid cycle occurs in the inner membrane region of the mitochondria. For each glucose molecule, the cycle operates twice, once for each acetyl group formed from glucose catabolism. The cycle is essential for the complete oxidation of acetyl groups, resulting in the production of ATP, NADH, FADH2, and CO2.

• Step 1: Acetyl CoA delivers an acetyl group, which combines with oxaloacetate to form citric acid.

• Step 2: Citric acid is metabolized; a carbon is removed as CO2, and NAD+ is reduced to NADH. The molecule becomes α-ketoglutarate.

• Step 3: α-ketoglutarate is metabolized; another carbon is removed as CO2, NAD+ is reduced to NADH, ADP is converted to ATP, and the molecule becomes succinate.

• Step 4: Succinate is metabolized; FAD is reduced to FADH2, and the molecule becomes fumarate.

• Step 5: Fumarate is metabolized; NAD+ is reduced to NADH, and the molecule becomes oxaloacetate, which starts the next cycle.

Coenzyme FAD (Flavin adenine dinucleotide) becomes FADH2 by picking up two high-energy H+ ions and two electrons, transporting them to the electron transport system.

Summary of Citric Acid Cycle

• ATP: 2 molecules produced by substrate-level phosphorylation (for both acetyl groups).

• Coenzyme Activity: 6 NAD+ become 6 NADH; 2 FAD become 2 FADH2.

• Waste: 4 CO2 molecules produced; 2 oxaloacetate molecules recycled.

At the end of the citric acid cycle, glucose is fully catabolized, and most energy is stored in NADH and FADH2.

Electron Transport System (ETS)

Mechanism and ATP Production

The electron transport system is located in the inner mitochondrial membrane (cristae). NADH and FADH2 deliver hydrogen ions and electrons to carrier proteins in the membrane. Electrons are passed along a chain of proteins, losing energy at each step, which is used to actively transport H+ ions to the outer membrane region.

• High H+ Concentration: H+ ions accumulate in the outer membrane region.

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

• Oxidative Phosphorylation: This process requires oxygen and results in the addition of a phosphate group to ADP.

• Water Formation: Spent H+ ions and electrons combine with oxygen to form water.

• Coenzyme Recycling: NAD+ and FAD are regenerated for reuse.

$\text{ADP} + \text{Pi} \rightarrow \text{ATP}$

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

Summary of Electron Transport System

• ATP: Approximately 34 molecules produced by oxidative phosphorylation.

• Coenzyme Activity: NADH and FADH2 drop off hydrogen and electrons, becoming NAD+ and FAD again.

• Waste: 6 molecules of water produced.

Energy Production Recap

Overall ATP Yield and Waste Products

Cellular respiration involves over 20 chemical reactions to fully catabolize glucose. The net gain of ATP is 36 molecules, accounting for the energy used to shuttle NADH from the cytoplasm to the mitochondria. Waste products include CO2 and H2O.

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

• Waste: 6 CO2, 6 H2O

• Efficiency: Energy is released slowly for optimal efficiency.

Metabolism of Glycogen, Fats, and Proteins

Alternative Energy Sources

While glycogen is a minor energy reserve, fats and proteins provide significant energy. After glycogen is depleted, the body metabolizes fats and proteins to produce ATP.

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

• Proteins: Proteins are broken into amino acids. The amine group is removed and excreted as urea, while the carbon backbone enters the citric acid cycle. Protein catabolism increases during starvation, leading to muscle wasting.

Anaerobic Respiration

Energy Production Without Oxygen

Anaerobic respiration allows cells to produce ATP without oxygen, typically for short periods. Glycolysis is an example, where pyruvate does not enter the mitochondria but is converted to lactic acid, causing muscle burning and cramping.

• ATP Yield: Only 2 ATP molecules are produced per glucose molecule.

• Lactic Acid: Accumulates in muscle tissue when oxygen is unavailable.

References

• Johnson, M.D. (2017). Human biology: Concepts and current issues (8th ed). Pearson Education Inc.