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 glucose and other molecules to produce ATP, the energy currency of the cell. This process involves a series of metabolic pathways, including glycolysis, the citric acid cycle, and the electron transport system. Each stage occurs in specific regions of the cell and involves distinct chemical reactions.

• Glycolysis: Occurs in the cytoplasm; breaks down glucose into pyruvate.

• Citric Acid Cycle (Krebs Cycle): Occurs in the inner membrane region of mitochondria; processes acetyl CoA.

• Electron Transport System: Occurs in the mitochondrial inner membrane; produces most ATP.

Citric Acid Cycle (Krebs Cycle)

The citric acid cycle is a central metabolic pathway that completes the breakdown of glucose derivatives, producing coenzymes and ATP. It occurs twice for each glucose molecule, as two acetyl groups are formed from one glucose.

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

• Citric Acid Metabolism: Carbon is removed as CO2, NAD+ is reduced to NADH, and α-ketoglutarate is formed.

• α-Ketoglutarate Metabolism: Another carbon is removed as CO2, NAD+ is reduced to NADH, ATP is produced, and succinate is formed.

• Succinate Metabolism: FAD is reduced to FADH2, forming fumarate.

• Fumarate Metabolism: NAD+ is reduced to NADH, oxaloacetate is regenerated.

Summary for both acetyl groups:

• 2 ATP produced by substrate-level phosphorylation

• 6 NADH and 2 FADH2 produced

• 4 CO2 released as waste

• 2 oxaloacetate regenerated

Coenzymes in Energy Production

Coenzymes such as NAD+ and FAD play crucial roles in cellular respiration by carrying 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.

Electron Transport System

The electron transport system (ETS) is the final stage of cellular respiration, where most ATP is produced. NADH and FADH2 deliver electrons and hydrogen ions to carrier proteins in the mitochondrial inner membrane.

• Electrons are passed along carrier proteins, losing energy.

• Energy is used to actively transport H+ ions to the outer membrane region.

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

• Process is called oxidative phosphorylation.

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

• NAD+ and FAD are recycled.

Summary: Approximately 34 ATP molecules are produced by oxidative phosphorylation.

Equation:

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

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

Overall Energy Production

Complete catabolism of glucose through glycolysis, citric acid cycle, and electron transport system yields:

• 10 NADH

• 2 FADH2

• 38 ATP (gross), 36 ATP (net gain after accounting for transport costs)

• 6 CO2 (waste)

• 6 H2O (waste)

Metabolism 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 enters glycolysis or preparatory step; 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.

Anaerobic Metabolic Pathways

Cells can produce ATP without oxygen through anaerobic respiration, such as glycolysis. In the absence of oxygen, pyruvate is converted to lactic acid, which causes muscle burning and cramping.

• Glycolysis: Anaerobic pathway; produces ATP and pyruvate.

• Lactic Acid Formation: Pyruvate is converted to lactic acid when oxygen is unavailable.

Summary Table: Energy Production from Different Sources

Energy Source	Pathway	ATP Yield	Waste Products
Glycogen	Glycolysis & Cellular Respiration	~36 ATP per glucose	CO2, H2O
Fats	Beta-oxidation & Citric Acid Cycle	~2x ATP vs. glycogen	CO2, H2O
Proteins	Deamination & Citric Acid Cycle	Similar to glycogen	CO2, H2O, Urea

Additional info: Catabolism of proteins increases during starvation, leading to muscle wasting.