science

Reverse electron flow

Reverse electron flow is a science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Reverse electron flow rather than just read about it. In short: Reverse electron flow (also known as reverse electron transport) is a mechanism in microbial metabolism. Chemolithotrophs using an electron donor with a higher redox potential than NAD(P)+/NAD(P)H, such as nitrite or sulfur compounds, must use energy to reduce NAD(P)+.

Key takeaways

Reverse electron flow belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Reverse electron flow to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Reverse electron flow from memory before moving on to harder problems.

Reference excerpt

Reverse electron flow (also known as reverse electron transport) is a mechanism in microbial metabolism. Chemolithotrophs using an electron donor with a higher redox potential than NAD(P)+/NAD(P)H, such as nitrite or sulfur compounds, must use energy to reduce NAD(P)+. This energy is supplied by consuming proton motive force to drive electrons in a reverse direction through an electron transport chain and is thus the reverse process as forward electron transport. In some cases, the energy consumed in reverse electron transport is five times greater than energy gained from the forward process. Autotrophs can use this process to supply reducing power for inorganic carbon fixation. Reverse electron transfer (RET) is the process that can occur in respiring mitochondria, when a small fraction of electrons from reduced ubiquinol is driven upstream by the membrane potential towards mitochondrial complex I. This results in reduction of oxidized pyridine nucleotide (NAD+ or NADP+). This is a reversal of the exergonic reaction of forward electron transfer in the mitochondrial complex I when electrons travel from NADH to ubiquinone.

Mechanism The term "Reverse electron transfer" is used in regard to the reversibility of the reaction performed by complex I of the mitochondrial or bacterial respiratory chain. Complex I is responsible for the oxidation of NADH generated in catabolism when in the forward reaction electrons from the nucleotide (NADH) are transferred to membrane ubiquinone and energy is saved in the form of proton-motive force. The reversibility of the electron transfer reactions at complex I was first discovered when Chance and Hollunger have shown that the addition of succinate to mitochondria in State 4 leads to an uncoupler-sensitive reduction of the intramitochondrial nucleotides (NAD(P)+). When succinate is oxidized by intact mitochondria, complex I can catalyze reverse electron transfer when electrons from ubiquinol (QH2, formed during oxidation of succinate) is driven by the proton-motive force to complex I flavin toward the nucleotide-binding site. Since the discovery of the reverse electron transfer in the 1960s it was regarded as in vitro phenomenon, until the role of RET in the development of ischemia/reperfusion injury has been recognized in the brain and heart. During ischemia substantial amount of succinate is generated in cerebral or cardiac tissue and upon reperfusion it can be oxidized by mitochondria initiating reverse electron transfer reaction. Reverse electron transfer supports the highest rate of mitochondrial Reactive Oxygen Species (ROS) production, and complex I flavin mononucleotide (FMN) has been identified as the site where one-electron reduction of oxygen takes place.

References

Worked examples

Example 1 — a first encounter with Reverse electron flow

Start with the simplest possible case. Write down what Reverse electron flow claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Reverse electron flow before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Reverse electron flow ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Reverse electron flow

In research: Reverse electron flow appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Reverse electron flow in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Reverse electron flow is common in secondary-school and first-year university syllabi. It links to neighbouring topics Metabolism, so understanding it makes those chapters shorter.
In everyday life: Look for Reverse electron flow outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Reverse electron flow in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Reverse electron flow means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Reverse electron flow out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Reverse electron flow in simple terms?

Why does Reverse electron flow matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Reverse electron flow?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Reverse electron flow.