General Metabolic Information

ATP is our fuel

Figure 3. ATP molecule (Kenneth Todar, Figure 2. The structure of ATP)

The human body is a machine that needs energy to function. The high energy molecule ATP (adenosine triphospate) is the source for most metabolic energy. Humans, however, do not eat ATP. Instead, our bodies use chemical energy in the form of sugar, fat, and proteins to synthesize ATP in the body.

Mitochondria make ATP

Energy is synthesized in mitochondria of an eukaryotic cell. Certain cells (liver and muscle) have many mitochondria because theses cell types require and produce large amounts of energy. A mitochondrion contains two membranes. The outer membrane separates the organelle from the cell. The inner membrane surrounds a matrix compartment. In space between the two membranes, proton gradients are maintained.

Figure 4. Mitochondria are fluorescently-labeled green inside this cell. ( Douglas Kline, Nucleus, Mitochondria, and Lysosomes)

Energy storage from nutrition

Energy sources (reduced carbon compounds) such as sugar and fat are first broken into Acetyl-CoA by biochemical pathways (glycolysis and fatty acid catabolism). Acetyl-CoA is fed into the Citric Acid Cycle and oxidized to CO2 and H2O. During this process, energy is ultimately stored in NADH, GTP, and FADH2. SDH is an enzyme used in this metabolic pathway.

Figure 5. Citric Acid Cycle (Kenneth Todar, Figure 12. The tricarboxylic acid (TCA) or Kreb's cycle)

Conversion of Energy into useful Products

The electron transport chain is where energy is generated. As electrons flow through this chain of enzymes, they pump positively charged hydrogen (a proton) across a concentration gradient. When the protons flow back through the gradient, the ATP synthase makes ATP. Finally, after ATP is generated, other important energy-requiring reactions in the body can occur. After the electrons have passed through each complex in the electron transport chain, they are given to oxygen and water is created.

Figure 6. Electron Transport Chain (Miller, Overview of the Pathway)