Krebs Cycle

The Krebs Cycle takes place in the matrix of mitochondria in eukaryotes or in the cytoplasm of aerobic prokaryotes. During this process, the Acetyl CoA molecules (remnants from glucose) are oxidized, transferring their energy to electron carriers NAD+ and FAD, which become NADH and FADH2.

Two ATP molecules per glucose are produced through substrate level phosphorylation, where phosphate groups transfer directly from succinyl-CoA to ADP. By the end of the Krebs Cycle, all carbon atoms from the original glucose are released as carbon dioxide gas.

You don't need to memorize every step of this complicated cycle, but remember three key points: Acetyl CoA is oxidized with energy transferred to electron carriers, carbon atoms are released as CO2, and ATP is created via substrate level phosphorylation.

💡 Think of the Krebs Cycle as an energy transfer station - it takes the energy from food molecules and packages it into electron carriers (like NADH) that can power the next phase of respiration.

Electron Transport Chain Basics

The Electron Transport Chain (ETC) occurs on the cristae (folds) of the inner mitochondrial membrane in eukaryotes or on the cell membrane folds in aerobic prokaryotes. These folds increase surface area, allowing for greater ATP production.

The ETC transfers energy from electron carriers (NADH and FADH2) through a series of reactions that establish a hydrogen ion $H+$ gradient across membranes. This process oxidizes the electron carriers formed during glycolysis and the Krebs Cycle to generate large amounts of ATP.

The chain itself consists of proteins called cytochromes that are highly conserved across all organisms. The process begins when NADH is oxidized by the first cytochrome (FMN), and electrons move through the chain from less electronegative to more electronegative locations.

As electrons move through three proton pumps, their energy powers the pumping of hydrogen ions from the matrix into the intermembrane space, creating a concentration gradient that stores potential energy. The larger this gradient, the more potential energy is available.

ATP Production and Oxygen's Role

ATP synthase is an enzyme embedded in the inner membrane that allows protons to flow back into the matrix. As protons move through this channel, their kinetic energy is harnessed to add a phosphate group to ADP, creating ATP. This process, called chemiosmosis, produces most of the ATP created during cellular respiration.

Each NADH molecule powers the pumping of three protons, ultimately providing energy for three ATP molecules. FADH2, which enters the chain later, only provides energy for two ATP molecules. In total, the electron transport chain can produce up to 34 ATP molecules per glucose molecule.

Oxygen plays a crucial role as the final electron acceptor in the chain. Because of its high electronegativity, oxygen pulls electrons from the final cytochrome, preventing a buildup of negative charge that would shut down respiration. The negatively charged oxygen then combines with hydrogen ions to form water.

🔑 Without oxygen as the final electron acceptor, the entire electron transport chain would back up and stop working - this is why we need to breathe oxygen to sustain life!