Cellular Respiration and Fermentation

A gradient is present, with a significant concentration of hydrogens (H^{+}) on one side and very few (or none) on the other side of a membrane. This proton gradient is typically established by active pumping of protons across the membrane, often by protein complexes within an electron transport chain (ETC).
The membrane in question is impervious to charged molecules, creating a challenge for these molecules to cross back into the area of lower concentration. This impermeability is crucial for maintaining the potential energy stored in the electrochemical gradient. For oxidative phosphorylation, this typically refers to the inner mitochondrial membrane in eukaryotes.

Charged molecules, specifically protons (H^{+}), can only re-enter the area of lower concentration through an enzyme known as ATP synthase.
ATP synthase plays a crucial role in the production of ATP by facilitating the passage of protons across the membrane, down their electrochemical gradient. This process, known as chemiosmosis, powers the enzyme's rotary motor to catalyze the synthesis of ATP from ADP and inorganic phosphate.

When protons pass through ATP synthase, ADP is converted back into ATP. This occurs by a process called chemiosmosis, driven by the energy released as electrons are passed down the electron transport chain.
This process is characterized by the addition of inorganic phosphates within the cytoplasm to ADP, resulting in ATP production. The energy for this comes indirectly from the oxidation of electron carriers like NADH and FADH_{2}.
Oxidative phosphorylation typically occurs concurrently with the conversion of oxygen into water (H_2O) during cellular respiration, as oxygen serves as the final electron acceptor in the electron transport chain.

Substrate-level phosphorylation is defined as the process through which a phosphate group is transferred directly from a high-energy substrate molecule to ADP to form ATP. This is a direct enzymatic transfer.
This differs from oxidative phosphorylation where the phosphate groups are added from inorganic sources rather than another molecule and is driven by an electrochemical gradient. Examples occur during glycolysis (e.g., conversion of phosphoenolpyruvate to pyruvate) and the Krebs cycle.

Anaerobic respiration utilizes an alternative final electron acceptor instead of oxygen, such as nitrate (NO3^{-}), sulfate (SO4^{2-}), or fumarate. The efficiency and output of ATP depend on the nature of this acceptor, generally yielding less ATP than aerobic respiration.
Aerobic respiration can yield a maximum of 38 ATP molecules from the complete oxidation of one glucose molecule, typically using oxygen as the electron acceptor and involving glycolysis, the Krebs cycle, and oxidative phosphorylation.
Fermentation, being a type of anaerobic respiration, is less efficient, only generating 2 ATP per glucose molecule, which occurs exclusively during glycolysis. Its primary purpose is to regenerate NAD^{+} for glycolysis to continue.

Fermentation is detailed as being a process occurring entirely in the cytoplasm. Both eukaryotes (e.g., in muscle cells during intense exercise) and prokaryotes (e.g., bacteria and yeast) undergo fermentation.
The process starts with glycolysis, which produces 4 ATP but requires 2 ATP to initiate, leading to a net gain of 2 ATP.

At the end of the fermentation process, it is crucial to address the accumulated products:
- NADH must be oxidized back to NAD^{+} to ensure glycolysis can continue functioning and produce ATP. Without NAD^{+} regeneration, glycolysis would halt.
- Pyruvate, resulting from glycolysis, must also be reduced to prevent accumulation, which could inhibit the process. The reduction of pyruvate uses the electrons from NADH, thereby oxidizing NADH back to NAD^{+} and allowing glycolysis to proceed.
The oxidation of NADH and reduction of pyruvate are essential processes in fermentation, allowing for the recycling of NAD^{+} and the conversion of pyruvate into other compounds, depending on the type of fermentation:
- In lactic acid fermentation, pyruvate is directly reduced by NADH to form lactate (lactic acid), regenerating NAD^{+}.
- In alcoholic fermentation, pyruvate is first decarboxylated to acetaldehyde, which is then reduced by NADH to form ethanol, also regenerating NAD^{+}.