Glucose Metabolism: Fermentation, Glycolysis, Krebs, and Electron Transport

Definition: An anaerobic process in which NADH is converted back to NAD+ while pyruvate is reduced to a waste byproduct (lactic acid or ethanol) to be eliminated from the cell.
ATP yield: Does not produce additional ATP; glycolysis remains the energy source that can run on regenerated NAD+. Some microbes can proceed with respiration to gain more energy.
Key role: Regenerates NAD+ so glycolysis can continue in the absence of oxygen.

Primary pathway for glucose catabolism in both anaerobic and aerobic conditions; 10 steps; energy investment phase uses 2 ATP.
Reactants: glucose, 2 NAD+, 2 ADP, 2 Pi (inorganic phosphate).
Products after glycolysis: 2 pyruvate, 2 NADH, 4 ATP (gross); net ATP = $2$ because 2 ATP were invested.
Key equation (as stated):
$\text{Glucose} + 2\,\text{NAD}^+ + 2\,\text{ADP} + 2\,\text{P}_i \rightarrow 2\,\text{Pyruvate} + 2\,\text{NADH} + 2\,\text{ATP} + 2\,\text{H}^+.$
NAD+/NADH cycling: To continue glycolysis, NADH must be reoxidized to NAD+; this occurs via fermentation or respiration.

Krebs cycle is the central aerobic pathway that processes the energy trapped in pyruvate-derived acetyl groups.
Pyruvate is converted to acetyl-CoA (one acetyl-CoA per pyruvate), entering the TCA cycle with 2 acetyl-CoA per glucose.
Products (per glucose): CO₂, NADH, and FADH₂; immediate ATP yield
Direct ATP yield from TCA: $2$ ATP total (one per acetyl-CoA).
Purpose: Generate reduced electron carriers (NADH and FADH₂) to fuel the Electron Transport System.

Location: Inner mitochondrial membrane.
Mechanism: Electrons from NADH and FADH₂ are transferred through a chain of electron carriers, pumping protons to create a proton motive force.
ATP synthesis: Proton motive force drives ATP synthase to produce ATP; up to $34$ ATP can be generated from the electron transport system per glucose under aerobic conditions.
Note: Anaerobic respiration yields fewer ATP than aerobic respiration.