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# Stage 2: Krebs Cycle (HL) The Krebs Cycle takes place in the mitochondrial matrix. Oxygen is needed for this aerobic process. **Key Steps:** 1. If oxygen is present, each pyruvic acid molecule enters the mitochondrion and is broken down into one molecule of carbon dioxide and a 2-carbon molecul...

# Stage 2: Krebs Cycle (HL) The Krebs Cycle takes place in the mitochondrial matrix. Oxygen is needed for this aerobic process. **Key Steps:** 1. If oxygen is present, each pyruvic acid molecule enters the mitochondrion and is broken down into one molecule of carbon dioxide and a 2-carbon molecule called Acetyl CoA. 2. Acetyl CoA enters a series of reactions called the Krebs Cycle, where it's broken down step by step, releasing carbon dioxide, hydrogen ions (protons, H+), and electrons (e-). 3. Hydrogen ions and high-energy electrons combine with the electron acceptor NAD+ to form NADH. NAD+ stands for nicotinamide adenine dinucleotide. The reaction is: NAD+ + 2e- + H+ → NADH 4. NADH then carries these high energy electrons to the electron transport system. **Products of the Krebs Cycle (FATE):** * (a) **ATP**: Breaks down to release energy, e.g., for active transport. * (b) **Carbon Dioxide**: Released into the atmosphere (also used in photosynthesis by plants). * (c) **Hydrogen Ions (H+) and Electrons**: Trapped by NAD+ to form NADH, which then moves to the electron transport system. **Diagrammatic Representation:** _(A diagram of a metabolic pathway showing the flow of molecules and energy is presented here. It shows the conversion of glucose to pyruvic acid, the Krebs Cycle, and the electron transport chain including the role of NADH and other molecules like ATP, oxygen, and carbon dioxide.)_ **Note:** The diagram shows the Krebs Cycle and its relationship to other metabolic stages, including glycolysis and the electron transport chain. It illustrates the production of ATP, carbon dioxide, NADH, etc. The overall process depicts the energy release from glucose in aerobic respiration.