Kids Research Express

Citric Acid Cycle, series of chemical reactions occurring within the cell, responsible for the final breakdown of food molecules to form carbon dioxide, water, and energy. This process, which is carried out by seven enzymes , is also known as the tricarboxylic acid cycle, or the Krebs cycle. The citric acid cycle is active in all animals and higher plants and in most bacteria. In organisms that have cells with nuclei, the cycle is contained within a membrane-bound organelle called the mitochondrion, a structure often referred to as the power plant of the cell . Discovery of the citric acid cycle is credited to Sir Hans Adolf Krebs, a British biochemist who outlined its essential steps in 1937. Before a food can enter the citric acid cycle, it must be broken down into small units called acetyl groups. Each acetyl group (CH3CO) contains only two atoms of carbon, along with hydrogen and oxygen. At the start of the cycle, an acetyl group combines with a four-carbon molecule called oxaloace

Adenosine Triphosphate (ATP), molecule found in all living organisms that is the main immediate source of usable energy for the activities of the cells. ATP is built up by the metabolism of foodstuffs in the cell in special compartments called mitochondria. Because the energy-exchanging function of ATP and the catalytic (work-boosting) function of enzymes are intimately connected, ATP is characterized as a coenzyme. The adenosine part of the molecule is made up of adenine, a nitrogen-containing compound (also one of the principal components of the gene), and ribose, a five-carbon sugar. Three phosphate units (triphosphate), each made up of one phosphorus atom and four oxygen atoms, are attached to the ribose. The two bonds between the three phosphate groups are high-energy bonds, that is, they are relatively weak and yield their energy readily when split by enzymes. With the release of the end phosphate group, 7 kilocalories (7 calories, in common usage) of energy become available for