Types of Cellular Respiration

There are two types of cellular respiration: aerobic and anaerobic. Aerobic respiration can only take place in the presence of oxygen. Anaerobic respiration, also called fermentation, can take place without oxygen. Cellular respiration is a series of metabolic processes that create energy in the form of adenosine triphosphate. ATP is an efficient form of stored energy that can be readily used by cells. Aerobic respiration generates the most ATP. About 36 molecules of ATP are produced in the combined processes of aerobic respiration. These processes are: glycolysis, Krebs cycle and oxidative phosphorylation.

• Red blood cells use fermentation.

Glycolysis

• Glycolysis is the first step in both aerobic and anaerobic respiration. Glycolysis occurs in the cytosol of the cell. It can occur in the presence of oxygen or in an anaerobic environment. Glycolysis does not require oxygen nor is it inhibited by oxygen. Glycolysis is a process that begins with a high-energy molecule like a sugar, protein or lipid and breaks it down into pyruvate. Pyruvate is an important intermediate molecule that fuels the next step in respiration.

  Glycolysis also results in two net ATP molecules, water, inorganic phosphate and two NADHs. NADH, nicotinamide adenine dinucleotide, is a coenzyme that is used in the remaining steps of respiration. NADH is especially important in the oxidation-reduction reactions of the electron transport chain.

Krebs Cycle

• The Krebs cycle is the second step of aerobic respiration. Krebs cycle, also called the citric acid cycle, is a series of reactions that results in just one molecule of ATP. Krebs cycle occurs in the matrix of the mitochondria, which are organelles, or individual membrane-bound structures, that are the energy powerhouses of the cell. The pyruvate and NADH molecules produced by glycolysis pass into the matrix of the mitochondrion by facilitated diffusion. Once inside the mitochondrion, pyruvate is converted into acetyl CoA. The acetyl CoA enters the Krebs cycle where it is converted into citric acid. A series of reactions follows, producing more carbon dioxide, NADH and FADH --flavin adenine dinucleotide -- another coenzyme important in the final step of aerobic respiration.

Oxidative Phosphorylation

• The final step of aerobic respiration is oxidative phosphorylation. Oxidative phosphorylation has three important parts. First, there is a series of proteins embedded in the inner mitochondrial membrane. These proteins take electrons donated from NADH or FADH2 and pass them along to the final electron acceptor. This group of proteins is sometimes simply referred to as the ETC, or electron transport chain. Second, as electrons move through the chain, protons are pumped into the inner mitochondrial space. The high concentration of protons creates the proton-motive force. Third, the protons, driven by the proton-motive force, want to travel back into the mitochondrial matrix to an area of low concentration. But they cannot simply diffuse through the membrane. Instead, they find passage through the membrane through a special protein called the ATP synthase. The energy of the protons moving through the ATP synthase drives ATP creation. Oxygen drives oxidative phosphorylation because it is the final electron acceptor in the ETC.

Fermentation

• Anaerobic respiration, or fermentation, occurs without oxygen. Fermentation reduces pyruvate formed through glycolysis to lactic acid or ethanol. The fermentation generates only two molecules of ATP. Many organisms use fermentation for their energy production. Yeast uses fermentation. Some bacteria that cannot survive in oxygenated environments use fermentation. Humans also use fermentation.

  Your red blood cells use fermentation to generate energy. This allows them to transport oxygen to all body tissues without consuming it. Fermentation also takes place in skeletal muscle fibers. Stored ATP and oxygen is quickly used up by an active muscle cell. However, these unique cells can continue respiration in the absence of oxygen. You feel the result of fermentation when the buildup of lactic acid in your muscles causes them to cramp. When your muscles are again at rest, the lactic acid is converted by your liver into glucose.

Fun Fact

• You are already familiar with the waste products of cellular respiration, though you may not realize it. Every time you exhale you are releasing carbon dioxide and water that, along with ATP, are the main products of cellular respiration. Your lungs perform an important function in the processes of cellular metabolism--they provide the surface for the gas exchange your cells depend on to complete cellular respiration. If your body cannot rid itself of carbon dioxide, your cells will be poisoned. If you cells do not receive oxygen, the functions of your body will collapse.