The most common set of catabolic reactions in animals can be separated into three main stages. In the first, large organic molecules such as proteins, polysaccharides or lipids are digested into their smaller components outside cells. Next, these smaller molecules are taken up by cells and converted to yet smaller molecules, usually acetyl coenzyme A (acetyl-CoA), which releases some energy. Finally, the acetyl group on the CoA is oxidized to water and carbon dioxide in the citric acid cycle and electron transport chain, releasing the energy that is stored by reducing the coenzyme nicotinamide adenine dinucleotide (NAD+) into NADH.
Metabolism involves a vast array of chemical reactions that involve the transfer of functional groups. Cells use a small set of metabolic intermediates to carry chemical groups between different reactions.
Cell metabolism explains series of chemical reactions e.g., the utilization of nutrients by cells. The nutrients penetrate the cell, which transforms them and uses them in the form of heat, energy, proteins and reserves. The waste products created by these transformations such as urea and carbon dioxide are evacuated. These group-transfer intermediates are called coenzymes.
Each class of group-transfer reaction is carried out by a particular coenzyme, which is the substrate for a set of enzymes that produce it, and a set of enzymes that consume it. These coenzymes are therefore continuously being made, consumed and then recycled.
Adenosine triphosphate (ATP), the universal energy currency of cells is an important coenzyme used to transfer chemical energy between different chemical reactions. ATP acts as a bridge between catabolism and anabolism, with catabolic reactions generating ATP and anabolic reactions consuming it.
Thus, the metabolism has got its complexity, its efficiency, its integration, and its responsiveness to subtle changes. The concepts of metabolism that we learn in this chapter will help us to understand how matter and energy flow during life's processes and how that flow is regulated.