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 primary energy currency of the cell. This process involves multiple stages, including glycolysis, the citric acid cycle, and the electron transport system.

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

• Citric Acid Cycle (Krebs Cycle): Occurs in the mitochondria; further oxidizes acetyl groups.

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

Citric Acid Cycle (Krebs Cycle)

Steps and Key Events

The citric acid cycle is a series of reactions that occur in the inner membrane region of the mitochondria. For each glucose molecule, the cycle occurs twice, once for each acetyl group derived from glucose catabolism.

• 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+ picks up hydrogen and electrons to become NADH, forming α-ketoglutarate.

• α-ketoglutarate is metabolized: Another carbon is removed as CO2, NAD+ becomes NADH, ADP is converted to ATP, forming succinate.

• Succinate is metabolized: FAD becomes FADH2, forming fumarate.

• Fumarate is metabolized: NAD+ becomes NADH, forming oxaloacetate, which starts the next cycle.

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

NAD+ and FAD

Coenzymes play a crucial role in transporting 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.

These coenzymes are essential for transferring energy to the final stage of ATP production.

Electron Transport System

Mechanism and ATP Production

The electron transport system (ETS) is located in the inner mitochondrial membrane. NADH and FADH2 deliver electrons and hydrogen ions to carrier proteins in the membrane.

• Electrons are passed from one carrier protein to another, losing energy in the process.

• Carrier proteins use this energy to actively transport H+ ions to the outer membrane region.

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

• This 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 produced by oxidative phosphorylation

• 6 molecules of water produced as waste

Key Equation:

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

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

Energy Production Recap

Overall Summary (Stages #1 - #4)

Complete catabolism of glucose involves over 20 chemical reactions, resulting in the following products:

• 10 NADH

• 2 FADH2

• 38 ATP (2 used for NADH transport, net gain = 36 ATP)

• 6 CO2 (waste)

• 6 H2O (waste)

Energy is released slowly, allowing for efficient ATP production.

Other Energy Sources

Catabolism of Fats, Glycogen, and Proteins

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

• Glycogen: Converted to glucose, then enters glycolysis.

• 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 produce about twice as much ATP as glycogen.

• Proteins: Broken down into amino acids. The amine group is 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.

Anaerobic Respiration

Energy Production Without Oxygen

Anaerobic respiration allows cells to produce ATP without oxygen, but only for brief periods. Glycolysis is an example of anaerobic respiration. In this pathway, pyruvate does not enter the mitochondria but is instead converted to lactic acid, which causes muscle burning and cramping.

• Glycolysis: Produces ATP anaerobically.

• Lactic Acid: Formed from pyruvate in the absence of oxygen.

Key Equation:

$\mathrm{Pyruvate} \rightarrow \mathrm{Lactic\ Acid}$

Summary Table: Energy Yield from Cellular Respiration

Stage	ATP Produced	Coenzymes Produced	Waste Products
Glycolysis	2	2 NADH	None
Citric Acid Cycle	2	6 NADH, 2 FADH2	4 CO2
Electron Transport System	34	Recycles NAD+ and FAD	6 H2O
Total	38 (Net 36)	10 NADH, 2 FADH2	6 CO2, 6 H2O

Additional info: The table summarizes the ATP yield, coenzyme production, and waste products for each stage of cellular respiration, providing a clear comparison for exam preparation.