Cellular Respiration: Krebs Cycle, Electron Transport Chain, and Fermentation

Cellular Respiration

Krebs Cycle (Citric Acid Cycle)

The Krebs cycle, also known as the citric acid cycle, is a central metabolic pathway that completes the oxidation of glucose derivatives, generating energy carriers for the cell.

• Location: Occurs in the mitochondrial matrix.

• Main Function: Transfers energy to the electron transport chain via NADH and FADH2.

• ATP Production: Generates a small amount of ATP directly.

• Byproducts: CO2 is released as a waste product.

• Cycle Frequency: The cycle turns once per pyruvate (two turns per glucose molecule).

Key Equation:

$\text{Acetyl-CoA} + 3\ NAD^+ + FAD + GDP + P_i + 2\ H_2O \rightarrow 2\ CO_2 + 3\ NADH + 3\ H^+ + FADH_2 + GTP + CoA$

Electron Transport Chain (ETC) – Aerobic

The electron transport chain is the final stage of aerobic respiration, where most ATP is generated through oxidative phosphorylation.

• Location: Inner mitochondrial membrane (cristae).

• Main Function: Uses energy carriers (NADH and FADH2) to make ATP.

• Mechanism: Mitochondrial enzymes transfer electrons from one complex to another.

• Electron Donors: NADH and FADH2 donate electrons.

• Proton Gradient: Protons (H+) are pumped across the membrane, creating a gradient.

• ATP Synthesis: ATP synthase uses the proton gradient to form ATP (about 32–34 ATP per glucose).

• Final Electron Acceptor: Electrons are transferred to O2, forming H2O.

Key Equation:

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

Summary of Cellular Respiration

• Glycolysis: Occurs in the cytoplasm, produces 2 ATP (anaerobic).

• Krebs Cycle & ETC: Occur in mitochondria, require oxygen (aerobic), produce up to 38 ATP per glucose.

• Fermentation: Occurs when oxygen is absent, produces 2 ATP per glucose.

Fermentation

Overview

Fermentation is an anaerobic process (occurs when oxygen is low or absent) that allows cells to generate ATP by glycolysis and recycle NAD+.

• Location: Cytoplasm of cells.

• ATP Yield: Net yield is 2 ATP per glucose (from glycolysis).

• Main Purpose: Regenerates NAD+ to keep glycolysis going.

Types of Fermentation

• Lactic Acid Fermentation:

  • Pyruvate is converted to lactic acid (lactate).

  • Occurs in muscle cells during intense exercise.

  • Equation: $\text{Pyruvate} + NADH \rightarrow \text{Lactate} + NAD^+$

• Alcoholic Fermentation:

  • Pyruvate is converted to ethanol and CO2.

  • Occurs in yeast, fungi, and some bacteria.

  • Equation: $\text{Pyruvate} \rightarrow \text{Ethanol} + CO_2$

Comparison Table: Aerobic Respiration vs. Fermentation

Process	Oxygen Required?	Location	ATP Yield (per glucose)	End Products
Aerobic Respiration	Yes	Mitochondria	~38	CO2, H2O
Fermentation	No	Cytoplasm	2	Lactic acid or ethanol + CO2

Additional info: The notes reference the flow of electrons, the importance of NADH/FADH2, and the difference in ATP yield between aerobic and anaerobic pathways. These are central concepts in understanding how cells harvest chemical energy (Ch. 6 in General Biology).