For the SAT II Biology, the most important things to remember about the Krebs cycle are:

• The Krebs cycle results in 2 ATP molecules for each glucose molecule run through glycolysis.
• The Krebs cycle sends energy-laden NADH and FADH₂ molecules on to the next step in respiration, the electron transport chain. It does not export carbon molecules for further processing.
• The Krebs cycle takes place in the mitochondrial matrix, the innermost compartment of the mitochondria.
• Though the Krebs cycle does not directly require oxygen, it can only take place when oxygen is present because it relies on by-products from the electron transport chain, which requires oxygen. The Krebs cycle is therefore an aerobic process.

    The electron transport chain consists of a set of three protein pumps embedded in the inner membrane of the mitochondria. FADH₂ and NADH are used to power these pumps. Using the energy in NADH and FADH₂, these pumps move positive hydrogen ions (H⁺) from the mitochondrial matrix to the intermembrane space. This creates a concentration gradient over the membrane.

 

    In a process called oxidative phosphorylation, H⁺ ions flow back into the matrix through a membrane protein called an ATP synthase. This channel is the opposite of the standard membrane pumps that burns ATP to transport molecules against their concentration gradient: ATP synthase uses the natural movement of ions along their concentration gradient to make ATP. All told, the flow of ions through this channel produces 34 ATP molecules. The waste products from the powering of the electron transport chain protein pumps combine with oxygen to produce water molecules. By accepting these waste products, oxygen frees NAD⁺ and FAD to play their roles in the Krebs cycle and the electron transport chain. Without oxygen, these vital energy carrier molecules would not perform their roles and the processes of aerobic respiration could not occur.

    For the SAT II Biology, the most important things to remember about the electron transport chain and oxidative phosphorylation are:

• Four ATP molecules are produced by glycolysis and the Krebs cycle combined. The electron transport chain produces 34 ATP.
• The electron transport chain occurs across the inner membrane of the mitochondria.
• The electron transport chain requires oxygen.