The krebs cycle is a part of a series of reactions that help the cell produce energy in the form o f ATP from glucose molecules. The krebs cycle is also sometimes referred to as the citric acid cycle. The krebs cycle takes place in the mitochondria of the cell and breaks down pyruvic acid from gylcolysis into carbon dioxide and ATP molecules. The ATP molecules are used for cellular energy and the carbon dioxide molecules are released. In the process electrons are picked up by NAD+ and FAD which are electron carrier molecules. The electrons are transferred to the electron transport system for another reaction to extract even more ATP molecules.
Source: Concepts in Biology 12th edition. Copywright 2007 McGraw-Hill.