Cellular respiration is the process where the chemical energy of food molecules such as protein, carbohydrates and glucose are converted into adenosine triphosphate (ATP).
Cellular respiration can be divided into three metabolic processes:
Glycolysis is the metabolic pathway that converts glucose into pyruvate. The energy released in this process is utilsed to form ATP.
Krebs cycle, also identified as the citric acid cycle, is a sequence of chemical reactions employed by all aerobic organisms to create energy through the oxidization of acetate derived from carbohydrates, fats and proteins into carbon dioxide and H2O.
Oxidative phosphorylation (OXPHOS) is a metabolic pathway that helps make use of energy released by the oxidation of nutrients to create ATP. Although the various forms of life here on earth use a range of different nutrients, almost all aerobic organisms carry out oxidative phosphorylation to create ATP, the molecule that supplies energy to metabolism.
All cells are capable of synthesizing ATP by means of the process of glycolysis. In numerous cells, if oxygen is not present, pyruvate is metabolized in a process known as fermentation.
Without oxygen, pyruvate (pyruvic acid) is not metabolized by cellular respiration but undergoes a process of fermentation. The pyruvate is not transported into the mitochondrion, but stays in the cytoplasm, where it is transformed to waste products that may be removed from the cell. This serves the function of oxidizing the electron carriers so that they can carry out glycolysis again and removing the extra pyruvate. Fermentation oxidizes NADH to NAD+ so it can be once again utilised in glycolysis. In the absence of oxygen, fermentation stops the build up of NADH in the cytoplasm and supplies NAD+ for glycolysis. This waste product differs depending on the organism. In skeletal muscles, the waste product is lactic acid. This type of fermentation is known as lactic acid fermentation. In yeast, the waste products are ethanol and carbon dioxide. This kind of fermentation is known as alcoholic or ethanol fermentation. The ATP produced in this process is made by substrate-level phosphorylation, which does not require any oxygen.
There are two approaches to breaking down the resulting pyruvate:
Aerobic Respiration demads oxygen and is the preferred method of pyruvate breakdown. It yields thirty-six ATP molecules, and also carbon dioxide, and water. This makes for a total gain of thirty-eight ATP molecules during cellular respiration. It occurs in cells when glucose reacts with oxygen; this takes place in the mitochondria of the cells.
Anaerobic Respiration does not require oxygen and is where the pyruvate is only partially broken down; fermentation (breaks the pyruvate down into ethanol, carbon dioxide, and water – bread, brewing etc.) and lactic acid fermentation (breaks the pyruvate down into lactic acid, carbon dioxide, and water). It happens when there is not enough oxygen for aerobic respiration
Both ethyl alcohol and lactic acid contain chemical energy that can’t be used by anaerobic respiration, making this an inefficient process. Anaerobic respiration releases a total of two ATP molecules (compare to the 38 of aerobic respiration).
Much less energy is released by anaerobic respiration than by aerobic respiration