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 (Krebs cycle), and the electron transport system. Each stage occurs in specific regions of the cell and involves distinct biochemical reactions.

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

• Citric Acid Cycle: Occurs in the inner membrane region of mitochondria; further catabolizes acetyl groups.

• 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, once for each acetyl group formed.

• 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 at each step.

• 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 produced by oxidative phosphorylation

• 6 molecules of water produced as waste

Overall Energy Production

Complete catabolism of one glucose molecule involves over 20 chemical reactions and yields:

• 10 NADH

• 2 FADH2

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

• 6 CO2 and 6 H2O as waste

Metabolism of Glycogen, Fats, and Proteins

Cells can utilize other macromolecules for energy production, especially when glucose reserves are low.

• Glycogen: Rapidly converted to glucose for glycolysis; constitutes only 1% of energy reserves.

• Fats: Main energy reserve (78%); 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. Fats produce about twice as much ATP as glycogen.

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

Anaerobic Respiration

Anaerobic respiration allows cells to produce ATP without oxygen, but only for brief periods. Glycolysis is the main anaerobic pathway, producing pyruvate. 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.

Key Equations

• ATP Synthesis (Oxidative Phosphorylation):

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

• Water Formation:

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

Summary Table: Energy Production from Different Sources

Source	Pathway	ATP Yield	Waste Products
Glucose (Glycogen)	Glycolysis, Citric Acid Cycle, ETS	~36 ATP	CO2, H2O
Fats	Glycerol (Glycolysis), Fatty Acids (Citric Acid Cycle)	~2x ATP vs. glycogen	CO2, H2O
Proteins	Amino Acids (Citric Acid Cycle)	Similar to glycogen	CO2, H2O, Urea

Additional info: Fats are the primary energy reserve in the body, followed by proteins and glycogen. Catabolism of proteins is a last resort and can lead to muscle wasting during starvation.