Cellular Respiration and Fermentation: Energy Metabolism in Cells

3. The Krebs Cycle (Citric Acid Cycle)

Once acetyl-CoA is formed, it enters the Krebs Cycle, which takes place within the mitochondrial matrix. The primary goal of the Krebs cycle is to complete the breakdown of the original glucose molecule by stripping away its remaining high-energy electrons.

As the cycle turns, it releases $CO_{2}$ as waste and produces a small amount of ATP via substrate-level phosphorylation. Most importantly, it produces several molecules of the electron carriers NADH and FADH2.

4. Oxidative Phosphorylation: The ATP Powerhouse

The final and most productive stage is oxidative phosphorylation, which occurs at the inner mitochondrial membrane. This stage is composed of two interconnected processes: the electron transport chain (ETC) and chemiosmosis.

The electron carriers NADH and FADH2 act as shuttles, transporting electrons from their high-energy states (harvested during glycolysis and the Krebs cycle) to the electron transport chain. As these electrons move through the proteins in the inner membrane, they release energy used to pump protons across the membrane, creating a gradient.

Finally, during chemiosmosis, these protons flow back into the matrix through a specialized protein called ATP synthase. This flow of protons provides the energy necessary to synthesize the vast majority of the cell's ATP.

Exam Tip: Remember that while glycolysis and the Krebs cycle produce some ATP through substrate-level phosphorylation, the "big payoff" of ATP comes from oxidative phosphorylation at the inner mitochondrial membrane.

Energy Carriers: NADH and FADH2

To understand biochemistry, you must understand the role of electron carriers. Molecules like NADH and FADH2 are similar to the cofactors used in photosynthesis (like NADPH). They function by moving between high-energy states (when they are reduced/carrying electrons) and low-energy states (when they are oxidized/releasing electrons to the ETC). Without these carriers, the energy harvested from glucose could not be delivered to the machinery that builds ATP.

Anaerobic Conditions: Fermentation

What happens when oxygen is unavailable? Without oxygen, the electron transport chain shuts down because there is no final electron acceptor to take the electrons at the end of the chain. This causes NADH to build up and NAD+ to run out.

If the cell runs out of NAD+, glycolysis will stop, and the cell will no longer be able to produce even the tiny amount of ATP it relies on for survival. To prevent this, cells undergo fermentation.

The primary purpose of fermentation is not to produce additional ATP; rather, its goal is to regenerate NAD+ from NADH so that glycolysis can continue to run and produce a net gain of 2 ATP per glucose molecule.

There are two main pathways of fermentation:

1. Lactic acid fermentation: This occurs in certain bacteria and in animal muscle cells during intense exercise when oxygen demand exceeds supply. It converts pyruvate into lactate.
2. Alcoholic fermentation: This occurs in organisms such as yeast. It converts pyruvate into ethanol and $CO_{2}$, a process used widely in baking and brewing.

Summary Table: Energy Yield and Location