RESPIRATION Aerobic respiration is the process of producing

RESPIRATION

• Aerobic respiration is the process of producing cellular energy involving oxygen. Cells break down food in the mitochondria in a long, multistep process that produces roughly 36 ATP. The first step in is glycolysis, the second is the citric acid cycle and the third is the electron transport system. • Anaerobic respiration occurs when the amount of oxygen available is too low to support the process of aerobic respiration. There are two main types of anaerobic respiration, alcoholic fermentation and lactic acid fermentation.

Anaerobic respiration is respiration using electron acceptors other than molecular oxygen (O 2). In aerobic organisms undergoing respiration, electrons are shuttled to an electron transport chain, and the final electron acceptor is oxygen. It produces lactic acid, rather than carbon dioxide and water. In aerobic organisms undergoing respiration, electrons are shuttled to an electron transport chain, and the final electron acceptor is oxygen. Molecular oxygen is a highly oxidizing agent and, therefore, is an excellent electron acceptor. In anaerobes other less-oxidizing substances such as sulphate (SO 42−), nitrate (NO 3−), sulphur (S), or fumarate are used. These terminal electron acceptors have smaller reduction potentials than O 2, meaning that less energy is released per oxidized molecule. Therefore, generally speaking, anaerobic respiration is less efficient than aerobic.

• Anaerobic respiration takes place in the cytoplasm of cells. Indeed, most cells that use anaerobic respiration are bacteria or archaea, which don’t have specialized organelles • Both aerobic and anaerobic respiration involve chemical reactions which take place in the cell to produce energy, which is needed for active processes. Aerobic respiration takes place in the mitochondria and requires oxygen and glucose, and produces carbon dioxide, water, and energy.

Cellular respiration (both aerobic and anaerobic) • utilizes highly reduced chemical compounds such as NADH and FADH 2 (for example produced during glycolysis and the citric acid cycle) to establish an electrochemical gradient (often a proton gradient) across a membrane, resulting in an electrical potential or ion concentration difference across the membrane. The reduced chemical compounds are oxidized by a series of respiratory integra membrane proteins with sequentially increasing reduction potentials with the final electron acceptor being oxygen (in aerobic respiration) or another chemical substance (in anaerobic respiration). A proton motive force drives protons down the gradient (across the membrane) through the proton channel of ATP synthase. The resulting current drives ATP synthesis from ADP and inorganic phosphate.

Fermentation, • in contrast, does not utilize an electrochemical gradient. Fermentation instead only uses substrate-level phosphorylation to produce ATP. The electron acceptor NAD+ is regenerated from NADH formed in oxidative steps of the fermentation pathway by the reduction of oxidized compounds. These oxidized compounds are often formed during the fermentation pathway itself, but may also be external. For example, in homofermentative lactic acid bacteria, NADH formed during the oxidation of glyceraldehyde-3 phosphateis oxidized back to NAD+ by the reduction of pyruvate to lactic acid at a later stage in the pathway. In yeast, acetaldehyde is reduced to ethanol to regenerate NAD+. The two processes thus generate ATP in very different ways, and the terms should not be treated as synonyms.

• Along with photosynthesis and aerobic respiration, fermentation is a way of extracting energy from molecules, but it is the only one common to all bacteria and eukaryotes. It is therefore considered the oldest metabolic pathway, suitable for an environment that does not yet have oxygen. Yeast, a form of fungus, occurs in almost any environment capable of supporting microbes, from the skins of fruits to the guts of insects and mammals and the deep ocean, and they harvest sugar-rich materials to produce ethanol and carbon dioxide.

Types of Anaerobic Respiration • Lactic acid fermentation – In this type of anaerobic respiration, glucose is split into two molecules of lactic acid to produce two ATP. • C 6 H 12 O 6 (glucose)+ 2 ADP + 2 phosphate → 2 lactic acid + 2 ATP • Alcoholic fermentation – In this type of anaerobic respiration, glucose is split into ethanol, or ethyl alcohol. This process also produces two ATP per sugar molecule • C 6 H 12 O 6 (glucose) + 2 ADP + 2 phosphate → 2 C 2 H 5 OH (ethanol) + 2 CO 2 + 2 ATP • Other types of fermentation – Other types of fermentation are performed by some bacteria and archaea. These include proprionic acid fermentation, butyric acid fermentation, solvent fermentation, mixed acid fermentation, butanediol fermentation, Stickland fermentation, acetogenesis, and methanogenesis.

Examples of Anaerobic Respiration • • Sore Muscles and Lactic Acid During intense exercise, our muscles use oxygen to produce ATP faster than we can supply it. When this happens, muscle cells can perform glycolysis faster than they can supply oxygen to the mitochondrial electron transport chain. The result is that lactic acid fermentation occurs within our cells – and after prolonged exercise, the built-up lactic acid can make our muscles sore! • • Yeasts and Alcoholic Drinks Alcoholic drinks such as wine and whiskey are typically produced by bottling yeasts – which perform alcoholic fermentation – with a solution of sugar and other flavoring compounds. Yeasts can use complex carbohydrates including those found in potatoes, grapes, corn, and many other grains, as sources of sugar. Putting the yeast and its fuel source in an airtight bottle ensures that there will not be enough oxygen around to interfere with the anaerobic respiration that produces the alcohol!Alcohol is actually toxic to the yeasts that produce it – when alcohol concentrations become high enough, the yeast will begin to die.

• Swiss Cheese and Propionic Acid • Propionic acid fermentation gives Swiss cheese its distinctive flavor. The holes in Swiss cheese are actually made by bubbles of carbon dioxide gas released as a waste product of a bacteria that uses propionic acid fermentation. • • Vinegar and Acetogenesis • Bacteria that perform acetogenesis are responsible for the making of vinegar, which consists mainly of acetic acid. Vinegar actually requires two fermentation processes, because the bacteria that make acetic acid require alcohol as fuel! •

• Glycolysis is the process by which glucose is broken down anaerobically into incompletely oxidized compounds like pyruvate, a process which is usually coupled to the synthesis of 2 ATP and 2 NADH for every one glucose molecule processed. • ED pathway occurs only in prokaryotes and it uses 6 phosphogluconate dehydratase and 2 -keto-3 deoxyphosphogluconate aldolase to create pyruvate from glucose. ED pathway produces only one ATP per glucose—half as much as the EMP pathway. .

• The pentose phosphate pathway (PPP; also called the phosphogluconate pathway and the hexose monophosphate shunt) is a process that breaks down glucose-6 -phosphate into NADPH and pentoses (5 -carbon sugars) for use in downstream biological processes. . During this process two molecules of NADP+are reduced to NADPH. The pathway is especially important in red blood cells (erythrocytes).

• There are two distinct phases in the pathway. The first is the oxidative phase, in which NADPH is generated, and the second is the non-oxidative synthesis of 5 -carbon sugars. For most organisms, the pentose phosphate pathway takes place in the cytosol; in plants, most steps take place in plastid. • Similar to glycolysis, the pentose phosphate pathway appears to have a very ancient evolutionary origin. The reactions of this pathway are mostly enzyme-catalyzed in modern cells, however, they also occur non-enzymatically under conditions that replicate those of the Archean ocean, and are catalyzed by metal ions, particularly ferrous ions (Fe(II)). This suggests that the origins of the pathway could date back to the prebiotic world.