Also known as the Krebs Cycle or TCA Cycle (tricarboxylic acid cycle), the Citric Acid Cycle is common to all aerobic metabolisms, and takes place generally in the mitochodrial matrix. It helps in generating energy for cellular processes through a series of chemical reactions that break down large nutrient molecules and produces carbon dioxide, as well as other important compounds. These include amino acids, and the coenzyme NADH (Nicotinamide adenine dinucleotide) which then acts as a reducing agent, giving up an electron…particularly in the oxidative phosphorylation pathway…which allows the creation of another coenzyme, ATP (adenasine triphosphate), the “molecular unit of currency” for intracellular energy exchange.
Research has shown that this pathway can be influenced by acupuncture.
More details:
Wikipedia:
https://en.wikipedia.org/wiki/Citric_acid_cycle#Major_metabolic_pathways_converging_on_the_TCA_cycle
Khan Academy:
http://www.youtube.com/watch?v=juM2ROSLWfw