Important Metabolic Pathways
The Embden-Meyerhof-Parnas (EMP) pathway, also referred to as glycolysis, is the primary pathway for many organisms to complete the catabolism of glucose. The end product of glycolysis, pyruvate, is a key to the cellular mechanisms that produce energy. Adenosine triphosphate (ATP) is the compound in which cells store energy. ATP can be produced aerobically through the citric acid cycle (also known as the Kreb cycle) and the electron transport chain or anaerobically through fermentation pathways. These pathways produce lactic acid, ethanol, and other products.
Glycolysis
Glycolysis occurs in the cytoplasm of cells and can produce two ATP per glucose molecule. The reaction below summarizes the results of glycolysis.
Pi is inorganic phosphate, ADP is adenosine diphosphate, and NADH is nicotineamide adenine dinucleotide. The pyruvate produced is further metabolized through cellular respiration (with oxygen) or fermentation (without oxygen) to produce more ATP for the cell.
Citric Acid Cycle
The citric acid cycle occurs aerobically within the mitochondria of eukaryotic cells and with the assistance of membranous enzymes in prokaryotic cells. The compounds produced by the cycle provide electrons for the electron transport chain, carbons for amino acid synthesis, energy for cellular function, and percursors for other molecules. Initially, pyruvate is converted to acetyl-coenzyme A. The resulting reaction for the cycle is displayed below.
FAD is Flavin adenine dinucleotide and GTP is guanosine triphosphate. The electrons produced are transferred to oxygen through the electron transport chain to form ATP. This process is often called oxidative phosphorylation. The overall net reaction of glucose metabolism aerobically is
Thirty-six ATP are produced per glucose molecule following glycolysis and cellular respiration. Many other pathways exist for other carbon sources such as the pentose-phosphate pathway and the Entner-Doudoroff pathway, but these will not be described here.
Lactic Acid Fermentation
Lactic acid fermentation occurs without oxygen, producing lactate from glucose while using NADH and H+ as the reducing agent. The process can be summarized by the following reaction.
Alcoholic Fermentation
Alcoholic fermentation occurs without oxygen, producing ethanol from glucose and using NADH and H+ as the reducing agent. This pathway is quite important to the brewing industry and the production of alcoholic beverages by microorganisms. The overall reaction is summarized below.
Autotrophic Metabolism
Autotrophs get their energy from carbon dioxide. The Calvin Benson Cycle is used to convert the carbon from carbon dioxide to glucose and other organic compound within the cell. Cells such as plant cells that get their energy from light fix carbon through a process called photosynthesis. The overall reaction for photosynthesis is written below.
Microorganisms and their Products
There are a variety of microorganisms that can produce useful food, nutritional supplements, and medicinal products for human and animal use. The unique growth capabilities and metabolic pathways of microorganisms influence the product possibilities. Some of the various organism, substrate, and product relationships are illustrated in the following list.
