IMPORTANT: Oxidation is usually EXERGONIC (energy releasing). Cells use multistep reactions to release energy from “energy-rich”, highly reduced (many hydrogen atoms) to lower-energy highly oxidized compounds.
Mechanisms of ATP Generation
Some energy released during oxidation reactions is captured within the cell when ATP is formed. P, a phosphate group is added to ADP with an input of energy to form ATP. The 2 high energy bonds that can be used to transfer energy are symbolized below as “~” → P ~ P ~ P The high energy phosphate bond that attaches the third phosphate contains the energy stored in this reaction. The addition of a phosphate group to a molecule is called Phosphorylation and because it brings energy to the reaction it increases the molecules potential energy. P ~ P + P + energy
There are 3 mechanisms of phosphorylation to generate ATP:
1. Substrate-level Phosphorylation : generates ATP with the transfer of a high-energy phosphate group from an intermediate phosphorylated compound (a substrate) directly to ADP in the cytosol. 2. Oxidative Phosphorylation : removes electrons from organic compounds and passes them through a series of electron acceptors called the Electron Transport Chain to molecules of O2. This process occurs in the inner mitochondrial membrane of cells. More on this later.
3. Photophosphorylation : Occurs in chlorophyll-containing plant cells in certain bacteria.
Carbohydrate Metabolism
During digestion of carbohydrates, both polysaccharides and disaccharides are broken down into monosaccharides: glucose, fructose, and galactose . Some fructose is converted to glucose as it is absorbed through the intestinal tract. Elsewhere, hepatocytes convert most of the remaining fructose and most of the galactose to glucose .
Carbohydrate metabolism really is glucose metabolism .
Glucose is the body’s preferred source for synthesizing ATP depending on the needs of body cells.
The following are examples of the fate of glucose.
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