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 nutrients, primarily glucose, to produce ATP. The process involves glycolysis, the citric acid cycle (Krebs cycle), and the electron transport system. This section focuses on the citric acid cycle and electron transport system, as well as the metabolism of fats and proteins, and anaerobic pathways.

Citric Acid Cycle (Krebs Cycle)

Location and General Function

• Occurs in: The inner membrane region of the mitochondria.

• Main function: Completes the breakdown of acetyl groups derived from glucose, generating electron carriers and ATP.

• Cycle repetition: For each glucose molecule, the cycle runs twice (once for each acetyl group).

Steps of the Citric Acid Cycle

• Acetyl CoA delivers acetyl group to the cycle, combining with oxaloacetate to form citric acid.

• Citric acid is metabolized: Carbon is removed as CO2; NAD+ is reduced to NADH; the molecule becomes α-ketoglutarate.

• α-Ketoglutarate is metabolized: Another carbon is removed as CO2; NAD+ is reduced to NADH; ADP is converted to ATP; the molecule becomes succinate.

• Succinate is metabolized: FAD is reduced to FADH2; the molecule becomes fumarate.

• Fumarate is metabolized: NAD+ is reduced to NADH; the molecule becomes oxaloacetate, which restarts the cycle.

Note: All steps above are for one acetyl group; double the totals for one glucose molecule.

Coenzyme Roles

• 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.

Summary Table: Citric Acid Cycle Products (per glucose)

Product	Amount Produced
ATP	2
NADH	6
FADH2	2
CO2	4
Oxaloacetate	2 (recycled)

Electron Transport System (ETS)

Mechanism and ATP Production

• Location: Inner mitochondrial membrane (cristae).

• NADH and FADH2: Donate electrons to carrier proteins in the membrane.

• Electron transfer: Electrons move through a series of proteins, losing energy at each step.

• Proton gradient: Energy from electrons is used to pump H+ ions to the outer membrane region, creating a gradient.

• ATP synthesis: 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.

Key Equation: Oxidative Phosphorylation

$\text{ADP} + \text{P}_i \xrightarrow{\text{ATP synthase}} \text{ATP}$

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

Summary Table: Electron Transport System Products

Product	Amount Produced
ATP	34
H2O	6
NAD+ & FAD	Recycled

Overall Summary of Cellular Respiration (Stages 1-4)

• Glucose is fully catabolized to CO2 and H2O.

• Total ATP yield: 38 ATP (gross), 36 ATP (net, accounting for transport costs).

• Waste products: 6 CO2, 6 H2O.

• Energy is released gradually for efficiency.

Metabolism of Glycogen, Fats, and Proteins

Alternative Energy Sources

• Glycogen: Short-term energy reserve (~1%).

• Fats: Main energy reserve (~78%), yield about twice as much ATP as glycogen.

• Proteins: Secondary energy reserve (~21%), used during starvation.

Fat Catabolism

• Triglycerides are broken into glycerol and fatty acids.

• Glycerol can be converted to glucose (glycolysis) or pyruvate (preparatory step).

• Fatty acids are converted to acetyl groups, entering the citric acid cycle.

• Fat catabolism provides energy from adipose tissue stores.

Protein Catabolism

• Proteins are broken into amino acids.

• Amine group (NH2) is removed, forming urea (excreted in urine).

• 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

• Occurs when oxygen is unavailable (e.g., intense exercise).

• Glycolysis is the main anaerobic pathway, producing ATP and pyruvate.

• Pyruvate is converted to lactic acid instead of entering mitochondria.

• Lactic acid buildup causes muscle burning and cramping.

References

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