Microorganisms in food

8. How cells obtain energy from food

Cells require a constant supply of energy to stay alive. This energy is derived from the chemical bond energy in food molecules, which thereby serve as fuel for cells.

Sugars are particularly important fuel molecules. In a first step all carbohydrates (as well as proteins and fats to a certain degree) are broken down into glucose, which is then in a chain of reactions called glycolysis converted into into two molecules of pyruvate. During pyruvate formation, two types of energy-carrying molecules are produced—ATP and NADH. Some of the steps in this process are triggered by certain enzymes.

The steps following glycolysis depend on the absence or presence of oxygen as well as the presence of either yeasts or lactic acid producing bacteria.

In the presence of Oxygen and yeast the pyruvate produced during glycolysis is rapidly transported into the mitochondria, where it undergoes alcoholic fermentation. In the so-called citric-acid or Krebs cycle, the pyruvate is first converted into CO₂ plus acetyl CoA, which is then further converted into products, which are excreted from the cell—for example, into ethanol and CO₂ used in brewing and breadmaking. This process yields a considerable amount of energy-carrying molecules, like ATP, as well as GTP, NADH, NADPH and FADH₂

Alcoholic fermentation: C₆H₁₂O₆ --> 2C₂H₅OH + 2CO₂  (Glucose --> Ethanol + Carbondioxide)

In the absence of oxygen, pyrovate will be turned into lactic acid with the help of lactic acid forming bacteria. These anaerobic energy-yielding pathways are called lactic acid fermentation, and yields the NAD⁺ that is needed to fuel glycolisis.

Lactic acid fermentation: C₆H₁₂O₆ --> 2CH₃-CH(OH)-COOH (Glucose --> Lactic acid)

In a last step the chemical energy is released from the energy-carrying molecules NADH and FADH₂, which transfer the electrons that they have gained to specialized electron acceptors in the cell membrane. The electrons released in this process are used to pump H⁺ ions (protons) across the membrane thereby generating a gradient of H⁺ ions between the inside and the outside of the cell. This gradient serves as a source of energy, being tapped like a battery to drive a variety of energy-requiring reactions. The most prominent of these reactions is the generation of ATP by the phosphorylation of ADP.

Source: Alberts B, Johnson A, Lewis J, et al., Molecular Biology of the Cell. 4th ed, New York: Garland Science; 2002

I have put together the animation below to illustrate the different steps involved in the process of cells acquiring energy from food.