What makes a good electron acceptor

These levels correspond to successively more positive redox potentials, or to successively decreased potential differences relative to the terminal electron acceptor. Individual bacteria use multiple electron transport chains, often simultaneously.

Bacteria can use a number of different electron donors, a number of different dehydrogenases, a number of different oxidases and reductases, and a number of different electron acceptors. For example, E. A common feature of all electron transport chains is the presence of a proton pump to create a transmembrane proton gradient.

Bacterial electron transport chains may contain as many as three proton pumps, like mitochondria, or they may contain only one or two. They always contain at least one proton pump. In the present day biosphere, the most common electron donors are organic molecules. Organisms that use organic molecules as an energy source are called organotrophs.

Organotrophs animals, fungi, protists and phototrophs plants and algae constitute the vast majority of all familiar life forms. Some prokaryotes can use inorganic matter as an energy source. Inorganic electron donors include hydrogen, carbon monoxide, ammonia, nitrite, sulfur, sulfide, and ferrous iron. Electron flow in these organisms is similar to those in electron transport, ending in oxygen or nitrate, except that in ferric iron-reducing organisms the final enzyme in this system is a ferric iron reductase.

Since some ferric iron-reducing bacteria e. Organic compounds may also be used as electron acceptors in anaerobic respiration. Learning Objectives Describe various types of electron acceptors and donors including: nitrate, sulfate, hydrgoen, carbon dioxide and ferric iron. Key Points Both inorganic and organic compounds may be used as electron acceptors in anaerobic respiration. Organic compounds include DMSO. These molecules have a lower reduction potential than oxygen.

The Basics of Redox : In every redox reaction you have two halves: reduction and oxidation. An electrochemical gradient represents one of the many interchangeable forms of potential energy through which energy may be conserved. In biological processes, the direction an ion moves by diffusion or active transport across a membrane is determined by the electrochemical gradient. In the mitochondria and chloroplasts, proton gradients are used to generate a chemiosmotic potential that is also known as a proton motive force.

This potential energy is used for the synthesis of ATP by phosphorylation. An electrochemical gradient has two components. First, the electrical component is caused by a charge difference across the lipid membrane. Second, a chemical component is caused by a differential concentration of ions across the membrane. The electrochemical potential difference between the two sides of the membrane in mitochondria, chloroplasts, bacteria, and other membranous compartments that engage in active transport involving proton pumps, is at times called a chemiosmotic potential or proton motive force.

In respiring bacteria under physiological conditions, ATP synthase, in general, runs in the opposite direction, creating ATP while using the proton motive force created by the electron transport chain as a source of energy.

The overall process of creating energy in this fashion is termed oxidative phosphorylation. The same process takes place in the mitochondria, where ATP synthase is located in the inner mitochondrial membrane, so that F1 part sticks into the mitochondrial matrix where ATP synthesis takes place. Cellular respiration both aerobic and anaerobic utilizes highly reduced species such as NADH and FADH2 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 species are oxidized by a series of respiratory integral membrane proteins with sequentially increasing reduction potentials, the final electron acceptor being oxygen in aerobic respiration or another species in anaerobic respiration. The membrane in question is the inner mitochondrial membrane in eukaryotes and the cell membrane in prokaryotes.

A proton motive force or pmf drives protons down the gradient across the membrane through the proton channel of ATP synthase. Proton reduction is important for setting up electrochemical gradients for anaerobic respiration. For example, in denitrification, protons are transported across the membrane by the initial NADH reductase, quinones, and nitrous oxide reductase to produce the electrochemical gradient critical for respiration.

In organisms that use hydrogen as an energy source, hydrogen is oxidized by a membrane-bound hydrogenase causing proton pumping via electron transfer to various quinones and cytochromes.

Sulfur oxidation is a two step process that occurs because energetically sulfide is a better electron donor than inorganic sulfur or thiosulfate, allowing for a greater number of protons to be translocated across the membrane. In contrast, fermentation does not utilize an electrochemical gradient. Instead, it only uses substrate-level phosphorylation to produce ATP. These oxidized compounds are often formed during the fermentation pathway itself, but may also be external.

In yeast, acetaldehyde is reduced to ethanol. Anoxic hydrocarbon oxidation can be used to degrade toxic hydrocarbons, such as crude oil, in anaerobic environments.

Hydrocarbons are organic compounds consisting entirely of hydrogen and carbon. The majority of hydrocarbons occur naturally in crude oil, where decomposed organic matter provides an abundance of carbon and hydrogen.

The combustion of hydrocarbons is the primary energy source for current civilizations. Crude oil contains aromatic compounds that are toxic to most forms of life. Their release into the environment by human spills and natural seepages can have detrimental effects. Marine environments are especially vulnerable. Despite its toxicity, a considerable fraction of crude oil entering marine systems is eliminated by the hydrocarbon-degrading activities of microbial communities.

Although it was once thought that hydrocarbon compounds could only be degraded in the presence of oxygen, the discovery of anaerobic hydrocarbon-degrading bacteria and pathways show that the anaerobic degradation of hydrocarbons occurs naturally. Contaminated soil : Microbes may be used to degrade toxic hydrocarbons in anaerobic environments.

The facultative denitrifying proteobacteria Aromatoleum aromaticum strain EbN1 was the first to be determined as an anaerobic hydrocarbon degrader, using toluene or ethylbenzene as substrates. Some sulfate-reducing bacteria can reduce hydrocarbons such as benzene, toluene, ethylbenzene, and xylene, and have been used to clean up contaminated soils.

The genome of the iron-reducing and hydrocarbon degrading species Geobacter metallireducens was recently determined. Anaerobic oxidation of methane AOM is a microbial process that occurs in anoxic marine sediments. It is believed that AOM is mediated by a syntrophic aggregation of methanotrophic archaea and sulfate-reducing bacteria, although the exact mechanisms of this syntrophic relationship are still poorly understood.

AOM is considered to be a very important process in reducing the emission of methane a greenhouse gas from the ocean into the atmosphere. Recent investigations have shown that some syntrophic pairings are able to oxidize methane with nitrate instead of sulfate. Privacy Policy. Skip to main content.

Microbial Metabolism. Search for:. Anaerobic Respiration. Electron Donors and Acceptors in Anaerobic Respiration In anaerobic respiration, a molecule other than oxygen is used as the terminal electron acceptor in the electron transport chain.

Learning Objectives Describe various types of electron acceptors and donors including: nitrate, sulfate, hydrgoen, carbon dioxide and ferric iron. Key Takeaways Key Points Both inorganic and organic compounds may be used as electron acceptors in anaerobic respiration. Organic compounds include DMSO. These molecules have a lower reduction potential than oxygen.

Therefore, less energy is formed per molecule of glucose in anaerobic versus aerobic conditions. The reduction of certain inorganic compounds by anaerobic microbes is often ecologically significant. Key Terms anaerobic : Without oxygen; especially of an environment or organism.

Nitrate Reduction and Denitrification Denitrification is a type of anaerobic respiration that uses nitrate as an electron acceptor. Learning Objectives Outline the processes of nitrate reduction and denitrification and the organisms that utilize it.

Generally, several species of bacteria are involved in the complete reduction of nitrate to molecular nitrogen, and more than one enzymatic pathway has been identified in the reduction process. Complete denitrification is an environmentally significant process as some intermediates of denitrification nitric oxide and nitrous oxide are significant greenhouse gases that react with sunlight and ozone to produce nitric acid, a component of acid rain.

Key Terms electron acceptor : An electron acceptor is a chemical entity that accepts electrons transferred to it from another compound. It is an oxidizing agent that, by virtue of its accepting electrons, is itself reduced in the process. Sulfate and Sulfur Reduction Sulfate reduction is a type of anaerobic respiration that utilizes sulfate as a terminal electron acceptor in the electron transport chain.

Learning Objectives Outline the process of sulfate and sulfur reduction including its various purposes. Key Takeaways Key Points Sulfate reduction is a vital mechanism for bacteria and archaea living in oxygen-depleted, sulfate-rich environments. Sulfate reducers may be organotrophic, using carbon compounds, such as lactate and pyruvate as electron donors, or lithotrophic, and use hydrogen gas H 2 as an electron donor.