Respiration is the process by which cells oxidize a fuel, usually the simple sugar glucose, and use the energy released during this oxidation to produce ATP.
The term "metabolism" refers to the sum total of all the chemical activity that occurs within an organism. Metabolism can be further divided into two large categories, anabolism and catabolism.
Anabolism refers to those metabolic reactions associated with the synthesis of molecules, such as proteins or carbohydrates, while catabolism includes those reactions involved in the degradation of molecules. Respiration is a catabolic process.
Carbohydrates, especially the simple sugars glucose and fructose, serve as the initial substrates for the respiratory process. In plants, these substrates are produced by photosynthesis in the chloroplasts. Solar energy is used to convert carbon dioxide and water into small sugar-phosphate molecules which are then combined to form fructose.
Energy is required to form each carbon-to-carbon bond (called a covalent bond), and a specified amount of energy is stored in each covalent bond. In other words, solar energy is converted to chemical energy.
The fructose may directly enter glycolysis, the first series of reactions in cellular respiration, or it may be converted to glucose. The glucose may also enter glycolysis directly, or it may be combined with a fructose molecule to form sucrose (table sugar), which can be transported to other parts of the plant. Once sucrose reaches the target cells, it can be converted back to glucose and fructose, which can then bemetabolized via glycolysis.
Glucose can also be polymerized into large starch molecules, which can be stored. When the plant experiences an energy deficit, starches can be broken down to glucose molecules.
During respiration fuel molecules, such as glucose, undergo a series of reactions in which the molecule is oxidized into smaller molecules, and in aerobic respiration, the glucose will ultimately be degraded to carbon dioxide and water. As the molecules are degraded, the energy stored in the chemical bonds that held the glucose molecule together is released, and the cells trap this energy in the form of adenosine triphosphate (ATP) molecules.
In a sense, the oxidation of glucose as a fuel is similar to the oxidation (burning) of any other organic fuel, such as gas, fuel oil, or coal, except the biological oxidation of glucose is a stepwise process and results in the formation of ATP.
Each step requires an enzyme, an organic catalyst. There are many enzymes associated with respiration, but the three most common types are kinases, decarboxylases, and oxioreductases.
Kinases catalyze reactions associated with ATP formation or utilization; decarboxylases catalyze decarboxylation reactions which remove chemical groups called carboxyls as carbon dioxide; and oxioreductases catalyze oxidation/reduction reactions. Oxidation is the removal of electrons from a molecule, and reduction is the addition of electrons.
In general, oxidation involves the removal of two electrons and two hydrogen ions (H+) from the substrate molecule. The release of H+ into the cytosol would result in acidification of the cell; therefore, oxioreductases require the presence of coenzymes which will accept the electrons and H+.
In other words, these coenzymes are reduced as glucose is oxidized. The two most important coenzymes in respiration are nicotine adenine dinucleotide (NAD) and flavin adenine dinucleotide (FAD). The reduced forms of these coenzymes are NADH + H+ and FADH2, respectively.
Glycolysis and Fermentation
Glycolysis, the first series of reactions in the respiratory pathway, consists of nine or ten separate steps, depending on whether the initial substrate is fructose or glucose.
Because sugar molecules are not very reactive, the first two or three steps in the process use two ATP molecules to convert the six-carbon glucose or fructose to a very reactive molecule called fructose-1,6-bisphosphate. This sixcarbon compound is then broken down into the equivalent of twomolecules of a three-carbon compound called glyceraldehyde-3-phosphate (PGAL).
Each molecule of PGAL undergoes a series of reactions in which it is converted to pyruvic acid. During this conversion, one NADH + H+ is formed, and enough energy is released to produce two ATP molecules per PGAL.
In summary, glycolysis is the conversion of glucose to two molecules of pyruvic acid, two molecules of NADH + H+, and a net gain of two ATP molecules (four were produced, but two were used to initiate the process). Glycolysis occurs in the cytosol and is entirely anaerobic, meaning that it occurs in the absence of oxygen.
In anaerobic organisms, such as yeast, each molecule of pyruvic acid is decarboxylated to produce carbondioxide anda molecule calledacetaldehyde. The NADH + H+ produced during glycolysis is then used to convert the acetaldehyde to ethanol.
This anaerobic process is called fermentation. Overall, the process of fermentation results in the conversion of glucose to two molecules of carbon dioxide, two molecules of ethanol, and a net gain of two ATP molecules.
The Krebs Cycle and Oxidative Phosphorylation
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Acetyl CoA then enters a second series of reactions called the Krebs cycle, also known as the tricarboxylic acid cycle or the citric acid cycle. The two carbons from the glucose molecule combine with a four-carbon compound called oxaloactic acid to form a six-carbon compound called citric acid.
The citric acid is decarboxylated twice, and the remaining four carbon fragment is ultimately converted back to oxaloacetic acid. During this process, two molecules of carbon dioxide, three molecules of NADH + H+, one FADH2 molecule, and one ATP molecule are produced.
During glycolysis, two molecules or pyruvic acid are produced per glucose; therefore, after two turns of the Krebs cycle, glucose is converted to six molecules of carbon dioxide, six (four net) ATP molecules, ten molecules of NADH + H+, and two molecules of FADH2.
A third series of reactions, referred to as electron transport, takes place within the mitochondrial membranes. The electrons and H+ ions bound to the NADH + H+ and FADH2 are transferred to initial electron receptors and then passed through a series electron transporters, each with a lower reduction potential (the tendency to accept electrons). Several of these electron transporters are iron-sulfur containing proteins called cytochromes.
Hence, this electron transport system is sometimes referred to as the cytochrome system. The final electron acceptor in this system is oxygen. When two electrons and H+ ions are transferred to oxygen, a molecule of water is formed.
This provides the cells with a safe means of removing excess H+ ions, but more important, additional ATP is produced. As electrons are transported from NADH + H+ and FADH2 through the electron transport chain and ultimately to oxygen, energy is released.
This energy can be used to synthesize ATP from ADP (adenosine diphosphate). Since the energy is stored in the phosphate bond (ADP to ATP) and occurs only in the presence of oxygen, the production of ATP during aerobic respiration is referred to as oxidative phosphorylation.
Each mole of NADH + H+ produced within the mitochondria results in the production of three moles of ATP via the electron transport system. The NADH + H+ from glycolysis and FADH2 enter the electron transport system downstream from the NADH + H+ derived from inside the mitochondria.
As a result, each mole of these molecules produces only two moles of ATP. The total ATP production permole of glucose from electron transport and oxidative phosphorylation is thirty-two moles.
Overall, aerobic respiration uses glycolysis, the Krebs cycle, and electron transport and results in the conversion of one mole of glucose to six moles of carbon dioxide, twelve moles of water and a net of thirty-six moles of ATP.3 × 8 NADH + H+ from within the mitochondria = 24;
2 x 2 NADH + H+from glycolysis = 4;
2 × 2 FADH2 from the Krebs cycle = 4;
24 + 4 + 4 = 32.
Eachmole ofATP represents about 8,000 calories of energy; therefore the oxidation of one mole of glucose can produce a total of 288,000 calories of energy available for cellular work.2 net ATPs from glycolysis + 2 ATPs from the Krebs cycle + 32 ATPs from electron transport = 36 ATPs.