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In biology, anaerobic respiration is a way for organisms to produce usable energy, in the form

ofadenosine triphosphate, or ATP, without the involvement ofoxygen; it is respirationwithout

oxygen. This process is mainly used by prokaryotic organisms (bacteria) that live in environments

devoid of oxygen. Although oxygen is not used a final electron acceptor, the process still uses a

respiratory electron transport chain. In order for the electron transport chain to function, anexogenous final electron acceptor must be present to remove electrons from the system. In

aerobic organisms, this final electron acceptor is oxygen. Molecular oxygen is highly oxidizing

and therefore is an excellent candidate for the job. In anaerobes, the chain still functions, but

oxygen is not used as the final electron acceptor. Other less oxidizing substances such as sulfate

(SO4), nitrate (NO3), and sulfur (S) are used. Oftentimes, anaerobic organisms are obligate

anaerobes, meaning they can only respire using anaerobic compounds and will die in the

presence of oxygen.

Anaerobic respiration is not the same as fermentation, which does not use either the citric acid

cycle or the respiratory chain (electron transport chain). In anaerobic respiration, microorganisms

are donating electrons to a final electron acceptor, while in fermentation they are essentially

creating their own electron acceptor to which they can dump electrons with the purpose of

regenerating their NAD+ pool.

Oxyanions of arsenic and selenium can be used in microbial anaerobic respiration as terminal

electron acceptors. The detection of arsenate and selenate respiring bacteria in numerous

pristine and contaminated environments and their rapid appearance in enrichment culture

suggest that they are widespread and metabolically active in nature. Although the bacterial

species that have been isolated and characterized are still few in number, they are scattered

throughout the bacterial domain and include Gram-positive bacteria, beta, gamma and epsilon

Proteobacteria and the sole member of a deeply branching lineage of the bacteria, Chrysiogenes

arsenatus. The oxidation of a number of organic substrates (i.e. acetate, lactate, pyruvate,

glycerol, ethanol) or hydrogen can be coupled to the reduction of arsenate and selenate, but the

actual donor used varies from species to species. Both periplasmic and membrane-associated

arsenate and selenate reductases have been characterized. Although the number of subunits and

molecular masses differs, they all contain molybdenum. The extent of the environmental impact

on the transformation and mobilization of arsenic and selenium by microbial dissimilatory

processes is only now being fully appreciated.

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Aerobic respiration requires oxygen in order to generate energy (ATP).

Although carbohydrates, fats, and proteins can all be processed and consumed as reactant, it is

the preferred method ofpyruvate breakdown in glycolysis and requires that pyruvate enter

the mitochondrion in order to be fully oxidized by the Krebs cycle. The product of this process is

energy in the form of ATP (Adenosine triphosphate), by substrate-level

phosphorylation, NADH and FADH2

Simplified reaction:C6H12O6 (aq) + 6 O2 (g) 6 CO2 (g) + 6 H2O (l)

G = -2880 kJ per mole of C6H12O6

The negative G indicates that the reaction can happen spontaneously

The reducing potential of NADH and FADH2 is converted to more ATP through an electron

transport chain with oxygen as the "terminal electron acceptor". Most of the ATP produced by

aerobic cellular respiration is made by oxidative phosphorylation. This works by the energy

released in the consumption of pyruvate being used to create a chemiosmotic potential by

pumping protons across a membrane. This potential is then used to drive ATP synthase and

produce ATP fromADP. Biology textbooks often state that 38 ATP molecules can be made per

oxidised glucose molecule during cellular respiration (2 from glycolysis, 2 from the Krebs cycle,

and about 34 from the electron transport system).[2]

However, this maximum yield is never quite

reached due to losses (leaky membranes) as well as the cost of moving pyruvate and ADP into

the mitochondrial matrix and current estimates range around 29 to 30 ATP per glucose.[2]

Aerobic metabolism is 19 times more efficient than anaerobic metabolism (which yields 2 mol

ATP per 1 mol glucose). They share the initial pathway ofglycolysis but aerobic metabolism

continues with the Krebs cycle and oxidative phosphorylation. The post glycolytic reactions take

place in the mitochondria in eukaryotic cells, and in the cytoplasm in prokaryotic cells.

When cells cannot or do not use oxygen in ATP production, they have another pathway to make energy

(ATP). This is called Anaerobic Respiration or more commonly Fermentation.

Animals, Fungi, and Bacteria all have species with fermentation pathways. In humans it is the cause for

sore muscles after over exertion. In fungus (yeast) it causes bread to rise and beer have alcohol. Even

the bacteria in our intestines (E.Coli) ferments sugar we eat (lactose) and helps us function.

Fermentation doesn't produce ATP in humans like Aerobic respiration does. When there isn't enough

oxygen in your blood you produce only 5%-10% of the ATP you can with oxygen. Additionally, there are

waste products that must be disposed of in fermentation (lactic acid). Still, remember that 2 ATP is

better than zero ATP. While our bodies can store glucose (for example, that's what marathoners are

doing when they eat lots of pasta the night before a race), we can't store oxygen. Sometimes we can't

take in enough oxygen to keep up with our energy needs. When this happens, our muscle cells switch toanaerobic respiration -- instead of reacting with oxygen, the glucose breaks in half and forms lactic acid.

Energy is produced, but the lactic acid builds up in our muscles. This build-up makes our muscles feel

heavy and they might even cramp up.

While our bodies can store glucose (for example, that's what marathoners are doing when they eat lots

of pasta the night before a race), we can't store oxygen. Sometimes we can't take in enough oxygen to

keep up with our energy needs. When this happens, our muscle cells switch to anaerobic respiration --

instead of reacting with oxygen, the glucose breaks in half and forms lactic acid. Energy is produced, but

the lactic acid builds up in our muscles. This build-up makes our muscles feel heavy and they might even

cramp up.

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Anaerobic RespirationAerobic respiration requires oxygen. However, some organisms live in places where oxygen isnot always present. Similarly, under extreme exertion, muscle cells may run out of oxygen.

Anaerobic respiration is a form of respiration that can function without oxygen.In the absence of oxygen, organisms continue to carry out glycolysis, since glycolysis does notuse oxygen in its chemical process. But glycolysis does require NAD+. In aerobic respiration,

the electron transport chain turnsNAD

H back toNAD

+

with the aid of oxygen, therebyaverting anyNAD+ shortage and allowing glycolysis to take place. In anaerobic respiration,cells must find another way to turn NADH back toNAD+.This other way is called fermentation. Fermentations goal is not to produce additionalenergy, but merely to replenish NAD+ supplies so that glycolysis can continue churning out itsslow but steady stream of ATP. Because pyruvates are not needed in anaerobic respiration,fermentation uses them to help regenerate NAD+. While employing the pyruvates in this waydoes allow glycolysis to continue, it also results in the loss of the considerable energycontained in the pyruvate sugars.There are two principle forms of fermentation, lactic acid fermentation andalcoholicfermentation. For the SAT II Biology, remember that no matter what kind of fermentationoccurs, anaerobic respiration only produces 2 net ATP in glycolysis.

Lactic Acid Fermentation

In lactic acid fermentation, pyruvate is converted to a three-carbon compound called lacticacid:

pyruvate + NADH lactic acid + NAD+

In this reaction, the hydrogen from the NADH molecule is transferred to the pyruvatemolecule.Lactic acid fermentation is common in fungi and bacteria. Lactic acid fermentation also takesplace in human muscle cells when strenuous exercise causes temporary oxygen shortages.Since lactic acid is a toxic substance, its buildup in the muscles produces fatigue andsoreness.

Alcoholic FermentationAnother route to NAD+ produces alcohol (ethanol) as a by-product:

pyruvate + NADH ethyl alcohol + NAD+

+ CO2 Alcoholic fermentation is the source of ethyl alcohol present in wines and liquors. It also

accounts for the bubbles in bread. When yeast in bread dough runs out of oxygen, it goesthrough alcoholic fermentation, producing carbon dioxide. These carbon dioxide bubblescreate spaces in the dough and cause it to rise.Like lactic acid, the ethanol produced by alcoholic fermentation is toxic. When ethanol levelsrise to about 12 percent, the yeast dies.

Aerobic respiration as we all know uses oxygen to produce energy in every living cell. While

anaerobic respiration is the process in which oxygen is not required to produce energy. Sounds likea good deal? Unfortunately, lactic acid (Organic acid that occurs as colorless, almost odorlesscrystals or liquid, produced by certain bacteria during fermentation and by active muscle cells whenthey are exercised hard and are experiencing oxygen debt.)is produced and accumulates until themuscles cannot continue working. An accumulation of lactic acid in the muscles may cause cramp

is produced and accumulates until the muscles cannot continue working.Anaerobic respiration inhumans is less efficient than aerobic respiration at releasing energy, but releases energy faster.This explains why humans can run faster in a sprint than over longer distances. When humans stop

after a sprint, they have to continue breathing more heavily for a while. This is to take in extraoxygen in order to break down the accumulated lactic acid on top of the normal breakdown ofsugar in aerobic respiration. The body is paying back the oxygen debt(Physiological state producedby vigorous exercise, in which the lungs cannot supply all the oxygen that the muscles need. Inother words, the lungs and bloodstream, pumped by the heart, cannot supplysufficient oxygen for aerobic respiration in the muscles. In such a situation the muscles cancontinue to break down glucose to liberate energy for a short time using anaerobic

respiration.) built up during the sprint.