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 adenosine triphosphate (ATP), the main energy currency of the cell. This process involves a series of metabolic pathways that occur in the cytoplasm and mitochondria of eukaryotic cells. The main stages include glycolysis, the citric acid cycle (Krebs cycle), and the electron transport system.

The Citric Acid Cycle (Krebs Cycle)

Location and General Function

The citric acid cycle, also known as the Krebs cycle, occurs in the inner membrane region of the mitochondria. It is the central metabolic pathway that completes the oxidation of organic molecules, generating electron carriers for the next stage of cellular respiration.

• Occurs twice per glucose molecule (once for each acetyl group derived from glucose).

• Main function: To harvest high-energy electrons from carbon fuels.

Steps of the Citric Acid Cycle

1. Acetyl CoA delivers an acetyl group (2 carbons) to oxaloacetate (4 carbons), forming citric acid (6 carbons).

2. Citric acid is metabolized: One carbon is removed as CO2; NAD+ is reduced to NADH; the molecule becomes α-ketoglutarate (5 carbons).

3. α-Ketoglutarate is metabolized: Another carbon is removed as CO2; NAD+ is reduced to NADH; ADP is phosphorylated to ATP; the molecule becomes succinate (4 carbons).

4. Succinate is metabolized: FAD is reduced to FADH2; the molecule becomes fumarate (4 carbons).

5. Fumarate is metabolized: NAD+ is reduced to NADH; the molecule is converted back to oxaloacetate, ready to start the cycle again.

Key Products per glucose (2 cycles):

• 2 ATP (by substrate-level phosphorylation)

• 6 NADH

• 2 FADH2

• 4 CO2 (waste)

Role of Coenzymes

• NAD+ (Nicotinamide adenine dinucleotide): Accepts electrons and hydrogen ions to become NADH, which carries energy to the electron transport system.

• FAD (Flavin adenine dinucleotide): Accepts electrons and hydrogen ions to become FADH2, also transporting energy to the electron transport system.

Electron Transport System (ETS)

Location and Function

The electron transport system is located in the inner mitochondrial membrane (cristae). It uses the high-energy electrons carried by NADH and FADH2 to generate a proton gradient, which drives the synthesis of ATP.

Mechanism of ATP Production

• NADH and FADH2 donate electrons to carrier proteins in the inner mitochondrial membrane.

• As electrons move through the chain, energy is used to pump H+ ions into the intermembrane space, creating a proton gradient.

• H+ ions flow back into the mitochondrial matrix through ATP synthase, catalyzing the conversion of ADP and inorganic phosphate (Pi) into ATP.

• This process is called oxidative phosphorylation because it requires oxygen and involves the addition of a phosphate group to ADP.

• Oxygen acts as the final electron acceptor, combining with electrons and H+ to form water (H2O).

Equation for water formation:

$1/2\, O_2 + 2H^+ + 2e^- \rightarrow H_2O$

ATP Yield and Summary

• Approximately 34 ATP molecules are produced by oxidative phosphorylation per glucose molecule.

• NAD+ and FAD are regenerated and reused in earlier stages.

• 6 molecules of water are produced as waste.

Overall Energy Production from Glucose

Complete catabolism of one glucose molecule through glycolysis, the citric acid cycle, and the electron transport system yields a net gain of about 36 ATP molecules (after accounting for the cost of shuttling electrons into the mitochondria).

• 10 NADH

• 2 FADH2

• 6 CO2 (waste)

• 6 H2O (waste)

ATP Production Table:

Stage	ATP Produced
Glycolysis	2
Citric Acid Cycle	2
Electron Transport System	34
Total	38
ATP used for NADH transport	-2
Net Gain	36

Alternative Energy Sources: Fats and Proteins

Catabolism of Fats

• Fats are the largest energy reserve in the body (about 78%).

• Triglycerides are broken down into glycerol and fatty acids.

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

• Fatty acids are converted into acetyl groups, which enter the citric acid cycle.

• Fat metabolism produces about twice as much ATP as glycogen metabolism.

Catabolism of Proteins

• Proteins are broken down into amino acids.

• The amine group (NH2) is removed and converted to urea, which is excreted in urine.

• The remaining 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

Anaerobic respiration allows cells to produce ATP in the absence of oxygen, but only for short periods. Glycolysis is the main anaerobic pathway in humans.

• During anaerobic respiration, pyruvate does not enter the mitochondria.

• Instead, pyruvate is converted into lactic acid, which causes muscle burning and cramping.

• Only 2 ATP are produced per glucose molecule during anaerobic glycolysis.

Summary Table: Aerobic vs. Anaerobic Respiration

Pathway	Oxygen Required?	ATP Yield (per glucose)	End Products
Aerobic Respiration	Yes	~36	CO2, H2O
Anaerobic Respiration	No	2	Lactic acid

Key Terms and Concepts

• ATP (Adenosine Triphosphate): The main energy carrier in cells.

• Substrate-level phosphorylation: Direct formation of ATP in glycolysis and the citric acid cycle.

• Oxidative phosphorylation: ATP formation powered by the electron transport system and proton gradient.

• Coenzymes: Molecules like NAD+ and FAD that carry electrons and hydrogen ions.

• Glycolysis: The breakdown of glucose to pyruvate in the cytoplasm.

• Citric Acid Cycle: Series of reactions in the mitochondria that oxidize acetyl groups to CO2 and generate NADH and FADH2.

• Electron Transport System: Chain of proteins in the mitochondrial membrane that produce ATP using electrons from NADH and FADH2.