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2021_Redox_for_review - Biology

2021_Redox_for_review - Biology


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Reduction-Oxidation Reactions

In General Biology, most of the reduction/oxidation reactions (redox) that we discuss occur in metabolic pathways (connected sets of biochemical reactions). Here, the cell breaks down the compounds it consumes into smaller parts and then reassembles these and other molecules into larger macromolecules. For these reasons, it is important to develop at least an intuitive understanding and appreciation for redox reactions in biology.

Most students of biology will also study reduction and oxidation reactions in their chemistry courses; these kinds of reactions are important well beyond biology. Regardless of the order in which students are introduced to this concept (chemistry first or biology first), most will find the topic presented in very different ways in chemistry and biology. That can be confusing.

Chemists often introduce the concepts of oxidation and reduction using the concept of oxidation states. See this link for more information: . Chemists usually ask students to apply a set of rules (see link) to determine the oxidation states of individual atoms in the molecules involved in a chemical reaction. The chemistry formalism defines oxidation as an increase in oxidation state and reduction as a decrease in oxidation state.

However, biologists don’t typically think about or teach redox reactions in this way. Why? We suspect it’s because most of the redox reactions encountered in biology involve a change in oxidation state that comes about trough a transfer of electron(s) between molecules. Biologists, therefore, typically define reduction as a gain of electrons and oxidation as a loss of electrons. We note that the biological electron-exchange view of redox reactions is entirely consistent with the more general definition associated strictly with changes in oxidation states. The electron-exchange model does not, however, explain redox reactions that do not involve a transfer of electrons, which sometimes occur in the context of a chemistry class. The biologist's view of redox chemistry has the advantage (in the context of biology) of being relatively easy to create a mental picture for. There are no lists of rules to remember of much inspection of molecular structure involved in developing at least a basic conceptual picture of the topic. We simply imagine an exchange between two parties - one molecule handing off one or more electrons to a partner who accepts them.

Since this is a biology reading for a biology class we approach redox from the “gain/loss of electrons” conceptualization. If you have already taken a chemistry class and this topic seems to be presented a little different in your biology course, remember that at its core, you are learning the same thing. Biologists just adapted what you learned in chemistry to make more intuitive sense in the context of biology. If you haven’t learned about redox, yet don’t worry. If you can understand what we are trying to do here, when you cover this concept in chemistry class you will be a few steps ahead. You will just need to work to generalize your thinking a little bit.

Let's start with some generic reactions

Transferring electrons between two compounds results in one of these compounds losing an electron and one compound gaining an electron. For example, look at the figure below. If we use the energy story rubric to look at the overall reaction, we can compare the before and after characteristics of the reactants and products. What happens to the matter (stuff) before and after the reaction? Compound A starts as neutral and becomes positively charged. Compound B starts as neutral and becomes negatively charged. Because electrons are negatively charged, we can explain this reaction with the movement of an electron from Compound A to B. That is consistent with the changes in charge. Compound A loses an electron (becoming positively charged), and we say that A has become oxidized. For biologists, oxidationis associated with the loss of electron(s). B gains the electron (becoming negatively charged), and we say that B has become reduced. Reductionis associated with the gain of electrons. We also know, since a reaction occurred (something happened), that energy must have been transferred and/or reorganized in this process and we'll consider this shortly.

Figure 1. Generic redox reaction with half-reactions

Attribution: Mary O. Aina

To reiterate: When an electron(s) is lost, or a molecule is oxidized, the electron(s) must then pass to another molecule. We say that the molecule gaining the electron becomes reduced. Together these paired electron gain-loss reactions are known as an oxidation-reduction reaction (also called a redox reaction).

This idea of paired half-reactions is critical to the biological concept of redox. Electrons don’t drop out of the universe for “free” to reduce a molecule nor do they jump off a molecule into the ether. Donated electrons MUST come from a donor molecule and be transferred to some other acceptor molecule. For example, in the figure above the electron the reduces molecule B in half-reaction 2 must come from a donor - it just doesn't appear from nowhere! Likewise, the electron that leaves A in half-reaction 1 above must "land" on another molecule - it doesn't just disappear from the universe.

Therefore, oxidation and reduction reactions must ALWAYS be paired. We’ll examine this idea in more detail below when we discuss the idea of “half-reactions”.

  • A tip to help you remember: The mnemonic LEO says GER (Lose Electrons = Oxidation and Gain Electrons = Reduction) can help you remember the biological definitions of oxidation and reduction.

Figure 2. A figure for the mnemonic "LEO the lion says GER." LEO: Loss of Electrons = Oxidation. GER: Gain of Electrons = Reduction

Attribution: Kamali Sripathi

• The vocabulary of redox can be confusing: Students studying redox chemistry can often become confused by the vocabulary used to describe the reactions. Terms like oxidation/oxidant and reduction/reductant look and sound very similar but mean distinctly different things. An electron donor is also sometimes called a reductant because it is the compound that causes the reduction (gain of electrons) of another compound (the oxidant). In other words, the reductant is donating it’s electrons to the oxidant which is gaining those electrons. Conversely, the electron acceptor is called the oxidant because it is the compound that is causing the oxidation (loss of electrons) of the other compound. Again, this simply means the oxidant is gaining electrons from the reductant who is donating those electrons. Confused yet?

Yet another way to think about definitions is to remember that describing a compound as reduced/oxidized is describing the state that the compound itself is in, whereas labeling a compound as a reductant/oxidant describes how the compound can act, to either reduce or oxidize another compound. Keep in mind that the term reductant is also synonymous with reducing agent and oxidant is also synonymous with oxidizing agent. The chemists who developed this vocabulary need to be brought up on charges of "willful thickheadedness" at science trial and then be forced to explain to the rest of us why they needed to be so deliberately obtuse.

The confusing language of redox: quick summary

1. A compound can be described as “reduced” - term used to describe the compound's state

2. A compound can be a “reductant” - term used to describe a compound's capability (it can reduce something else). The synonymous term "reducing agent" can be used to describe the same capability (the term "agent" refers to the thing that can "do something" - in this case reduce another molecule).

3. A compound can be an “oxidant” - term used to describe a compound's capability (it can oxidize something else). The synonymous term "oxidizing agent" can be used to describe the same capability (the term "agent" refers to the thing that can "do something" - in this case oxidize another molecule).

4. A compound can “become reduced” or "become oxidized"- term used to describe the transition to a new state

Since all of these terms are used in biology, in General Biology we expect you to become familiar with this terminology. Try to learn it and use it as soon as possible - we will use the terms frequently and will not have the time to define terms each time.

Knowledge Check Quiz

The Half Reaction

Here we introduce the concept of the half reaction. We can think each half reaction as a description of what happens to one of the two molecules (i.e. the donor or the acceptor) involved in a "full" redox reaction. A "full" redox reaction requires two half reactions. In the example below, half reaction #1 depicts the molecule AH losing two electrons and a proton and in the process becoming A+. This reaction depicts the oxidation of AH. Half reaction #2 depicts the molecule B+ gaining two electrons and a proton to become BH. This reaction depicts the reduction of B+. Each of these two half reactions is conceptual and neither can happen on its own. The electrons lost in half reaction #1 MUST go somewhere, they can't just disappear. Likewise, the electrons gained in half reaction #2 must come from something. They too just can not appear out of nowhere.

One can imagine that there might be different molecules that can serve as potential acceptors (the place for the electrons to go) for the electrons lost in half reaction #1. Likewise, there might be many potential reduced molecules that can serve as the electron donors (the source of electrons) for half reaction #2. In the example below, we show what happens (the reaction) when molecule AH is the donor of electrons for molecule B+. When we put the donor and acceptor half reactions together, we get a "full" redox reaction. In the figure below we call that reaction "Reaction #1". When this happens, we say that the two half reactions coupled.

Figure 3. Generic redox reaction where compound AH is being oxidized by compound B+. Each half reaction represents a single species or compound to either lose or gain electrons (and a subsequent proton as shown in the figure above). In half reaction #1 AH loses a proton and 2 electrons: in the second half reaction, B+ gains 2 electrons and a proton. In this example HA is oxidized to A+ while B+ is reduced to BH.

Using this idea, we can theoretically couple and think about any two half reactions, one half reaction serving as the electron donor for the other half reaction that accepts the donated electrons. For instance, using the example above, we could consider coupling the reduction of B+ that happens in half reaction 2 with another half reaction describing the oxidation of the molecule NADH. In that case, the NADH would be the electron donor for B+. Likewise you could couple the oxidation of AH that happens in half reaction #1 with a half reaction describing the the reduction of hypothetical molecule Z+. You can mix-and-match half reactions together as you please provided one half is describing the oxidation of a compound (it's donating electrons) and the reduction of another compound (it's accepting the donated electrons).

  • A note on how we write full reactions versus half reactions: In the example above, when we write Reaction #1 as an equation, the 2 electrons and the H+ that are explicitly described in the underlying half reactions, are not explicitly included in the text of the full reaction. In the reaction above you must infer that an exchange of electrons happens. This can be observed by trying to balance charges between each reactant and its corresponding product. Reactant AH becomes product A+. In this case, you can infer that some movement of electrons must have taken place. To balance the charges on this compound (make the sum of charges on each side of the equation equal) you need to add 2 electrons to the right side of the equation, one to account for the "+" charge on A+ and a second to go with the H+ that was also lost. The other reactant B+ is converted to BH. It must therefore gain 2 electrons to balance charges, one for B+ and a second for the additional H+ that was added. Together this information leads you to conclude that the most likely thing to have happened is that two electrons were exchanged between AH and B+.

  • This will also be the case for most redox reactions in biology. Fortunately, in most cases, either the context of the reaction, the presence of chemical groups often engaged in redox (e.g. metal ions), or the presence of commonly used electron carriers (e.g. NAD+/NADH, FAD+/FADH2, ferredoxin, etc.) will alert you that the reaction is of class "redox". You will be expected to learn to recognize some of these common molecules.

Reduction Potential

By convention, we quantitatively characterize redox reactions using an measure called reduction potentials. The reduction potential attempts to quantitatively describe the “ability” of a compound or molecule to gain or lose electrons. The specific value of the reduction potential is determined experimentally, but for the purpose of this course we assume that the reader will accept that the values in provided tables are reasonably correct. We can anthropomorphize the reduction potential by saying that it is related to the strength with which a compound can “attract” or “pull” or “capture” electrons. Not surprisingly this is is related to but not identical to electronegativity.

What is this intrinsic property to attract electrons?

Different compounds, based on their structure and atomic composition have intrinsic and distinct "attractions" for electrons. This quality leads each molecule to have its own standard reduction potential or E0. The reduction potential is a relative quantity (relative to some “standard” reaction). If a test compound has a stronger "attraction" to electrons than the standard (if the two competed, the test compound would "take" electrons from the standard compound), we say that the test compound has a positive reduction potential. The magnitude of the difference in E0’ between any two compounds (including the standard) is proportional to how much more or less the compounds "want" electrons. The relative strength of the reduction potential is measured and reported in units of Volts (V) (sometimes written as electron volts or eV) or milliVolts (mV). The reference compound in most redox towers is H2.


Possible NB Discussion Point

Rephrase for yourself: How do you describe or think about the difference between the concept of electronegativity and red/ox potential?


Redox student misconception alert: The standard redox potential for a compound reports how strongly a substance wants to hold onto an electron in comparison to hydrogen. Since both redox potential and electronegativity are both discussed as measurements for how strongly something "wants" an electron, they are sometimes conflated or confused for one another. However, they are not the same. While the electronegativity of atoms in a molecule may influence its redox potential, it is not the only factor that does. You don't need to worry about how this works. For now, try to keep them as different and distinct ideas in your mind. The physical relationship between these two concepts is well beyond the scope of this general biology class.

The Redox Tower

All kinds of compounds can take part in redox reactions. Scientists have developed a graphical tool, the redox tower, to tabulate redox half reactions based on their E0' values. This tool can help predict the direction of electron flow between potential electron donors and acceptors and how much free energy change might be expected to change in a specific reaction. By convention, all half reactions in the table are written in the direction of reduction for each compound listed.

In the biology context, the electron tower usually ranks a variety of common compounds (their half reactions) from most negative E0' (compounds that readily get rid of electrons), to the most positive E0' (compounds most likely to accept electrons). The tower below lists the number of electrons that are transferred in each reaction. For example, the reduction of NAD+ to NADH involves two electrons, written in the table as 2e-.

oxidized form

reduced form

n (electrons)

Eo´ (volts)

PS1* (ox)

PS1* (red)

-

-1.20

Acetate + CO2

pyruvate

2

-0.7

ferredoxin (ox) version 1

ferredoxin (red) version 1

1

-0.7

succinate + CO2 + 2H+

a-ketoglutarate + H2O

2

-0.67

PSII* (ox)

PSII* (red)

-

-0.67

P840* (ox)

PS840* (red)

-

-0.67

acetate

acetaldehyde

2

-0.6

glycerate-3-P

glyceraldehyde-3-P + H2O

2

-0.55

O2

O2-

1

-0.45

ferredoxin (ox) version 2

ferredoxin (red) version 2

1

-0.43

CO2

glucose

24

-0.43

CO2

formate

2

-0.42

2H+

H2

2

-0.42 (at [H+] = 10-7; pH=7)

Note: at [H+] = 1; pH=0 the Eo' for hydrogen is ZERO. You will see this in chemistry class.

α-ketoglutarate + CO2 + 2H+

isocitrate

2

-0.38

acetoacetate

b-hydroxybutyrate

2

-0.35

Cystine

cysteine

2

-0.34

Pyruvate + CO2

malate

2

-0.33

NAD+ + 2H+

NADH + H+

2

-0.32

NADP+ + 2H+

NADPH + H+

2

-0.32

Complex I FMN (enzyme bound)

FMNH2

2

-0.3

Lipoic acid, (ox)

Lipoic acid, (red)

2

-0.29

1,3 bisphosphoglycerate + 2H+

glyceraldehyde-3-P + Pi

2

-0.29

Glutathione, (ox)

Glutathione, (red)

2

-0.23

FAD+ (free) + 2H+

FADH2

2

-0.22

Acetaldehyde + 2H+

ethanol

2

-0.2

Pyruvate + 2H+

lactate

2

-0.19

Oxalacetate + 2H+

malate

2

-0.17

α-ketoglutarate + NH4+

glutamate

2

-0.14

FAD+ + 2H+ (bound)

FADH2 (bound)

2

0.003-0.09

Methylene blue, (ox)

Methylene blue, (red)

2

0.01

Fumarate + 2H+

succinate

2

0.03

CoQ (Ubiquinone - UQ + H+)

UQH.

1

0.031

UQ + 2H+

UQH2

2

0.06

Dehydroascorbic acid

ascorbic acid

2

0.06

Plastoquinone; (ox)

Plastoquinone; (red)

-

0.08

Ubiquinone; (ox)

Ubiquinone; (red)

2

0.1

Complex III Cytochrome b2; Fe3+

Cytochrome b2; Fe2+

1

0.12

Fe3+ (pH = 7)

Fe2+ (pH = 7)

1

0.20

Complex III Cytochrome c1; Fe3+

Cytochrome c1; Fe2+

1

0.22

Cytochrome c; Fe3+

Cytochrome c; Fe2+

1

0.25

Complex IV Cytochrome a; Fe3+

Cytochrome a; Fe2+

1

0.29

1/2 O2 + H2O

H2O2

2

0.3

P840GS (ox)

PS840GS (red)

-

0.33

Complex IV Cytochrome a3; Fe3+

Cytochrome a3; Fe2+

1

0.35

Ferricyanide

ferrocyanide

2

0.36

Cytochrome f; Fe3+

Cytochrome f; Fe2+

1

0.37

PSIGS (ox)

PSIGS (red)

.

0.37

Nitrate

nitrite

1

0.42

Fe3+ (pH = 2)

Fe2+ (pH = 2)

1

0.77

1/2 O2 + 2H+

H2O

2

0.816

PSIIGS (ox)

PSIIGS (red)

-

1.10

* Excited State, after absorbing a photon of light

GS Ground State, state prior to absorbing a photon of light

PS1: Oxygenic photosystem I

P840: Bacterial reaction center containing bacteriochlorophyll (anoxygenic)

PSII: Oxygenic photosystem II

Table 1. Common redox tower used in Bis2A. By convention the tower half reactions are written with the oxidized form of the compound on the left and the reduced form on the right. Compounds that make good electron donors have highly negative reduction potentials. Compounds such as Glucose and Hydrogen gas are excellent electron donors. By contrast compounds that make excellent electron acceptors, such as Oxygen and Nitrite are excellent electron acceptors.

Video on electron tower

For a short video on how to use the electron tower in redox problems click here or below. This video was made by Dr. Easlon for Bis2A students. (This is quite informative.)

What is the relationship between ΔE0' and ΔG?

How do we know if any given redox reaction (the specific combination of two half reactions) is energetically spontaneous or not (exergonic or endergonic)? Moreover, how can we determine what the quantitative change in free energy is for a specific redox reaction? The answer lies in the difference in the reduction potentials of the two compounds. The difference in the reduction potential for the reaction (E0'), can be calculated by taking the difference between the E0' for the oxidant (the compound getting the electrons and causing the oxidation of the other compound) and the reductant (the compound losing the electrons). In our generic example below, AH is the reductant and B+ is the oxidant. Electrons are moving from AH to B+. Using the E0' of -0.32 for the reductant and +0.82 for the oxidant the total change in E0' or E0' is 1.14 eV.

Figure 4. Generic red/ox reaction with half reactions written with reduction potential (E0') of the two half reactions indicated.

∆E0' between oxidant and reductant can tell us about the spontaneity of a proposed electron transfer. Intuitively, if electrons are proposed to move from a compound that "wants" electrons less to a compound that "wants" electrons more (i.e. a move from a compound with a lower E0'to a compound with a higher E0', the reaction will be energetically spontaneous). If the electrons are proposed to move from a compound that "wants" electrons more to a compound that "wants" electrons less (i.e. a move from a compound with a higher E0'to a compound with a lower E0', the reaction will be energetically non-spontaneous). Because of the way biological/biochemical redox tables are ordered (small E0' on top and larger E0' on the bottom) transfers of electrons from donors higher on the table to acceptors lower on the table will be spontaneous.

It is also possible to quantify the amount of free energy change associated with a specific redox reaction. The relationship is given by the Nernst equation:

Figure 5. The Nernst equation relates free energy of a redox reaction to the difference in reduction potential between the reduced products of the reaction and oxidized reactant.
Attribution: Marc T. Facciotti

Where:

  • n is the number of moles of electrons transferred
  • F is the Faraday constant of 96.485 kJ/V. Sometimes it is given in units of kcal/V which is 23.062 kcal/V, which is the amount of energy (in kJ or kcal) released when one mole of electrons passes through a potential drop of 1 volt

Note that the signs of ∆E and ∆G are opposite one another. When ∆E is positive, ∆G will be negative. When ∆E is negative, ∆G will be positive.

Alternative View of Some Common Confusing Issues in Basic Redox Chemistry for Biology

This reading tries to break down some of the more challenging topics that students encounter when studying redox chemistry in General Biology. This reading is not a substitute for your main reading but rather a complement to it that revisits some of the same topics through a different lens.

Finding ΔE

Students often struggle with finding the ∆E for a given redox reaction. One of the main barriers to developing this skill seems to be associated with developing a picture of the redox reaction itself. From the context of most biological redox reactions it is useful to imagine/picture a redox reaction as a simple exchange of electrons between two molecules, an electron donor and an electron acceptor that accepts electrons from the donor.

An analogy with kiwi fruit: To help build this mental picture we offer an analogy. Two people are standing next to one another. At the start, one person is holding a kiwi fruit in their hand and the second person's hands are empty. In this reaction, person 1 gives the kiwi to person 2. At the end of the reaction, person 2 is holding a kiwi and person 1 is not. We can write this exchange of kiwi fruit in the form of a chemical reaction:

person 1(kiwi) + person 2() <-> person 1() + person 2(kiwi).

start/initial state <-> final/end state

If we read this "reaction" from left to right, person 1 is a kiwi donor and person 2 is a kiwi acceptor. We can extend this analogy a little by proposing that person 1 and person 2 have different desire and ability to grab and hold kiwi fruit - we'll call that property "kiwi-potential". We can then propose a situation where person 1 and person 2 compete for a kiwi. Let's propose that person 2 has a higher "kiwi-potential" than person 1 - that is, person 2 has a stronger desire and ability to grab and hold kiwi than person 1.

If we set up a competition where person 1 starts with the kiwi and person 2 competes for it, we should expect that after some time the kiwi will be exchanged to person 2 and stay there most often. At the end of the reaction the kiwi will be with person 2. Due to the difference in "kiwi-potential" between person 1 and person 2, we can say that the spontaneous direction of kiwi flow is from person 1 to person 2. If we ever observed the kiwi flow from person 2 to person 1 we could probably conclude that person 1 required some extra help/energy to make that happen - flow from person 2 to person 1 would be non-spontaneous.

Let's call the "kiwi-potential", Kp. In our analogy, Kpperson 1 < Kpperson 2. We can calculate ∆Kp, the difference in Kp between the two people, and that will tell us something about how likely we can expect to see kiwi exchange hands between these two people. The bigger the difference in Kp the more likely the kiwi will move from the person who has a lower Kp to the person who has the higher Kp.

By definition, to calculate ∆Kp we obtain the solution to ∆Kp = Kpfinal/end - Kpinitial/start. Since the kiwi is with person 2 at the end of the reaction and it starts with person 1 at the beginning of the reaction we would calculate ∆Kp = Kpperson 2 - Kpperson 1.

Doing it with electrons instead of kiwi fruit: To find ∆E for a redox reaction we can translate this analogy to the molecular space. Instead of people, we have two molecules. Instead of a kiwi, we have electrons. Different molecules have different inherent abilities to grab and hold electrons and this can be measured by the value E. If two molecules exchange one or more electrons we can imagine that electrons will flow spontaneously from a molecule with lower E0 to one with a higher E0. We can write a familiar reaction with those substitutions.

molecule 1(electron) + molecule 2() <-> person 1() + molecule 2(electron).

start/initial state <-> final/end state

To find ΔE0, you solve for ΔE0 = E0-final/end - E0-initial/start. Alternatively, you can think of it as ΔE0 = E0-acceptor - E0-donor.

When evaluating a redox reaction for ∆E you therefore need to:

  1. First, find which of the reactants is the electron donor. The donor can also be associated with the initial state because it is the molecule that initially (before the start of the reaction) has the electron(s) to donate. This will always be one of the reactants and will be the molecule that gets oxidized (i.e. the molecule that loses electrons).

  2. Second, find which of the reactants is the electron acceptor. This will also always be a reactant and will be the molecule that becomes reduced by the reaction (i.e. gains electrons). This molecule can be also associated with the final state since, in its reduced form, it is the molecule that has the electrons at the end of the reaction.

  3. Third, calculate ΔE0 = E0-acceptor - E0-donor or if you prefer, ΔE0 = E0-final/end - E0-initial/start.

In the example above, we can examine the reactants and determine that NAD+ is the oxidized form of the electron carrier - it can, therefore, not be the donor. This means that H2 must act as the electron donor in this reactant. During the reaction electrons flow onto NAD+ from the donor H2 creating the reduced product NADH and oxidized product H+. To calculate ∆E0 we say that at the start of the reaction the exchanged electrons are on the donor H2. We say that at the end of the reaction the electrons are found on NADH. Calculating ∆E0 requires us to evaluate the difference:

E0-acceptor - E0-donor

or equivalently,

E0-final/end - E0-initial/start.

Using a redox table to find E0 values for the start and end molecules shows us that NAD+/NADH has an E0 of -0.30 while H+/H2 has an E0 of -0.42.

Therefore, ΔE0 = (-0.30) - (-0.42) = 0.12 V.

We can see intuitively that this reaction is spontaneous: electrons are flowing from a molecule that "wants" electrons less (E0 of H+/H2 = -0.42) to a molecule that wants them more (E0 of NAD+/NADH = -0.30).

Reading Different Looking Redox Towers

Novice students of redox chemistry will all undoubtedly run across different ways of representing a redox tower. These different representations may look different but contain the same information. Without explanation, however, reading these tables - when they look different - can be confusing. We will compare and contrast different common forms of redox towers.

Redox Tower: Type 1

Figure 1. A Generic redox tower with oxidized/reduced couple listed with its reduction potential (E0') .

Attribution: Caidon Iwuagwu

In this type of redox tower, the oxidized and reduced forms of a molecule are separated by a slash. There is a line drawn from each half-reaction to its redox potential E0 reported on the vertical axis.

Redox Tower: Type 2

Electron Acceptor

Electron Donor

E0(eV)

CO2 + 24e-

glucose

- 0.43

2H+ + 2e-

H2

- 0.42

CO2 + 6e-

methanol

- 0.38

NAD+ + 2e-

NADH

- 0.32

CO2 + 8e-

acetate

- 0.28

S0 + 2e-

H2S

- 0.28

SO42- + 8e-

H2S

- 0.22

Pyruvate + 2e-

lactate

- 0.19

S4O62- + 2e-

S2O32-

+ 0.024

Fumarate + 2e-

succinate

+ 0.03

Cytochrome box + 1e-

Cytochrome bred

+ 0.035

Ubiquinoneox + 2e-

Ubiquinonered

+ 0.11

Fe3+ + 1e- → (pH 7)

Fe2+

+ 0.2

Cytochrome cox + 1e-

Cytochrome cred

+ 0.25

Cytochrome aox + 1e-

Cytochrome ared

+ 0.39

NO3- + 2e-

NO2-

+ 0.42

NO3- + 5e-

1/2 N2

+ 0.74

Fe3+ + 1e- → (pH 2)

Fe2+

+ 0.77

1/2 O2 + 2e-

H2O

+ 0.82

In this type of redox tower, each row consists of a half-reaction. The oxidized form of a molecule is shown in the first column, the reduced form of the molecule is shown in the second column. Finally, the E0 value of the molecule is listed in the third column from the left. The number of electrons transferred to reduce the oxidized form of the molecule is shown in column 1. While the format of the table looks different from Type 1 tower, both contain the exact same information.

Redox Tower: Type 3

oxidized form

reduced form

n (electrons)

Eo´ (volts)

CO2

glucose

24

-0.43

2H+

H2

2

-0.42 (at [H+] = 10-7; pH=7)

Note: at [H+] = 1; pH=0 the Eo' for hydrogen is ZERO. You will see this in chemistry class.

CO2

methanol

6

-0.38

NAD+ + 2H+

NADH + H+

2

-0.32

CO2

acetate

8

-0.28

S0

H2S

2

-0.28

SO42-

H2S

8

-0.22

Pyruvate + 2H+

lactate

2

-0.19

S4O62-S4O62-20.024

Fumarate

succinate

2

0.03

Cytochrome boxCytochrome bred10.035

Ubiquinone; (ox)

Ubiquinone; (red)

2

0.1

Fe3+ (pH = 7)

Fe2+ (pH = 7)

1

0.20

Cytochrome c; Fe3+

Cytochrome c; Fe2+

1

0.25

Cytochrome a

Cytochrome a

1

0.39

Nitrate

nitrite

2

0.42

Nitrate

1/2 N2

5

0.74

Fe3+ (pH = 2)

Fe2+ (pH = 2)

1

0.77

1/2 O2 + 2H+

H2O

2

0.816

In this redox tower, the oxidized form of a molecule is in the leftmost column, its reduced form is in the second column from the left, the number of electrons transferred is in the third column from the left, and the E0 is in the far right column.

Again, all of these towers contain the exact same information and are used in an identical manner.

Special note: If you have studied redox chemistry in a formal chemistry course, you might notice two key differences between the towers you use in a biology setting and those used by chemists.

1. In chemistry, the redox towers are flipped relative to those in biology: In chemistry, the molecules with the most positive E0 are listed starting at the top of the table and the compounds with the most negative E0 are listed at the bottom. In bioloigal redox tables molecules with the largest E0 are listed at the bottom while those with the smallest E0 are listed starting at the top. The biology orientation has the advantage of making it easy to picture electrons spontaneously falling down the table from molecules that "want" the electrons less (lower E0) to molecules that "want" electrons more (higher E0).

2. In chemistry, the redox potential for hydrogen (H+/H2) is listed as 0. This is because (a) redox potentials for chemistry are measured under a set of non-biologically relevant standard conditions and (b) hydrogen is being used as the common standard redox potential against which all other redox potentials are measured. In biology, the redox potential for hydrogen (H+/H2) is listed as -0.42. This difference between the chemistry and biology tables comes about because the redox potential for (H+/H2) in biology is measured at a physiological pH of 7.0.

Familiarize yourself with how to read and interpret all three types of redox towers!

Chemistry and Biology Teach Redox Differently

For students who have been taught redox chemistry in a formal chemistry course, biological lessons in redox can sometimes seem like they're talking about something completely different. Not surprisingly, chemists tend to teach the most proper and universally applicable approach to evaluating redox reaction. This approach consists of using a set of rules to formally evaluate whether atoms in a molecule have undergone a change in oxidation state. Meanwhile, biologists tend to approach the discussion of redox reactions by thinking about electron transfers between molecules. It turns out that the approach biologists take is not as rigorous as how chemists approach redox reactions and can sometimes not identify bona fide redox reactions that wouldn't be missed using the chemist's approach. However, since the vast majority of biological redox reactions do involve a transfer of electron (and therefore change in redox states).

Let's look at a specific example to see the differences in approach.

The Chemistry Approach (oxidation numbers):

To evaluate/solve redox reactions in CHEMISTRY, we use the concept of oxidation states/numbers (we will just be saying oxidation number here). The oxidation number of an element refers to how the electrons are shared between atoms in a chemical compound and they tell us about the movement of the electrons in the redox reaction. There are specific rules to assigning oxidation numbers, we will not be going over all of them since they will not be applicable to the redox reactions you will see in General Biology, but here are a few:

  1. A single element has an oxidation number of 0

  2. Fluorine ALWAYS has an oxidation number of -1

  3. Hydrogen has an oxidation number of +1 with nonmetals and -1 with metals.

etc.

For more on calculating oxidation numbers see:

So to find which elements are reduced/oxidized when given a redox reaction, you must track the change of the oxidation numbers between the reactants and the products. Here is an example:

In the unbalanced reaction NO3-+ FADH2⟶ NO2-+ FAD+

  1. Using the rules, we observe that in NO3-, the oxidation number of Nitrogen is +5. In NO2-, the oxidation number of Nitrogen is +3. So because +5 ⟶ +3, N is reduced in this reaction.

  2. We could conduct a similar calculation for key atoms on FAD+ and FADH2 to discover that FADH2 is oxidized in the reaction.

The Biology/Biochemistry Approach (electron flow):

To evaluate/solve redox reactions in biology/biochemistry, we typically do not use or assign oxidation numbers to evaluate redox reactions. Rather we follow the exchange of electrons between molecules. Fortunately, biology tends to reuse a limited number of electron carriers and redox towers tell us which form of a compound is reduced or oxidized. As mentioned above, the basic approach to redox in biology is to define oxidation as: the loss of electrons. Reduction is defined as: the gain of electrons. Here is an example:

NO3-+ FADH2⟶ NO2-+ FAD+

Here we examine the reactants and immediately spot the common electron carrier FADH2, the reduced form of the electron carrier. In the products we observe the oxidized form of the electron carrier FAD+. We conclude that FADH2 lost electrons (became oxidized) in the reaction. Since the electrons had to go somewhere they were likely accepted by NO3- which then became reduced to NO2-. In this case the biologist's model arrives at the same conclusion as the chemist's approach through a more intuitive approach that doesn't require memorizing numerous rules and how to apply them.

In our General Biology class, we take the biology/biochemistry approach to redox. You will not need to know how to calculate redox states in this course.

DISCLAIMER: DO NOT WORRY IF YOU HAVE NOT TAKEN CHEMISTRY YET !! WE WILL NOT BE USING THE CHEMISTRY APPROACH WHEN IT COMES TO REDOX REACTIONS IN OUR CLASS. THE PURPOSE OF THIS IS JUST TO DISTINGUISH AND HOPEFULLY CLARIFY THE TWO APPROACHES FOR STUDENTS THAT MAY HAVE ALREADY TAKEN A CHEMISTRY COURSE!!


Extracellular miRNAs in redox signaling: health, disease and potential therapies

Extracellular microRNAs (miRNAs) have emerged as important mediators of cell-to-cell communication and intertissue crosstalk. MiRNAs are produced by virtually all types of eukaryotic cells and can be selectively packaged and released to the extracellular medium, where they may reach distal cells to regulate gene expression cell non-autonomously. By doing so, miRNAs participate in integrative physiology. Oxidative stress affects miRNA expression, while miRNAs control redox signaling. Disruption in miRNA expression, processing or release to the extracellular compartment are associated with aging and a number of chronic diseases, such as obesity, type 2 diabetes, neurodegenerative diseases and cancer, all of them being conditions related to oxidative stress. Here we discuss the interplay between redox balance and miRNA function and secretion as a determinant of health and disease states, reviewing the findings that support this notion and highlighting novel and yet understudied venues of research in the field.

Keywords: Aging Cancer Cardiovascular Disease Exosomes Extracellular vesicles Metabolic syndrome Neurodegenerative diseases Oxidative stress RNA therapeutics Redox biology miRNA.


The Aging Liver: Redox Biology and Liver Regeneration

Significance: During aging, excessive production of reactive species in the liver leads to redox imbalance with consequent oxidative damage and impaired organ homeostasis. Nevertheless, slight amounts of reactive species may modulate several transcription factors, acting as second messengers and regulating specific signaling pathways. These redox-dependent alterations may impact the age-associated decline in liver regeneration. Recent Advances: In the last few decades, relevant findings related to redox alterations in the aging liver were investigated. Consistently, recent research broadened understanding of redox modifications and signaling related to liver regeneration. Other than reporting the effect of oxidative stress, epigenetic and post-translational modifications, as well as modulation of specific redox-sensitive cellular signaling, were described. Among them, the present review focuses on Wnt/β-catenin, the nuclear factor (erythroid-derived 2)-like 2 (NRF2), members of the Forkhead box O (FoxO) family, and the p53 tumor suppressor. Critical Issues: Even though alteration in redox homeostasis occurs both in aging and in impaired liver regeneration, the associative mechanisms are not clearly defined. Of note, antioxidants are not effective in slowing hepatic senescence, and do not clearly improve liver repopulation after hepatectomy or transplant in humans. Future Directions: Further investigations are needed to define mutual redox-dependent molecular pathways involved both in aging and in the decline of liver regeneration. Preclinical studies aimed at the characterization of these pathways would define possible therapeutic targets for human trials.

Keywords: aging liver hepatic progenitors liver repopulation redox homeostasis.


Oxidative eustress: On constant alert for redox homeostasis

In the open metabolic system, redox-related signaling requires continuous monitoring and fine-tuning of the steady-state redox set point. The ongoing oxidative metabolism is a persistent challenge, denoted as oxidative eustress, which operates within a physiological range that has been called the 'Homeodynamic Space', the 'Goldilocks Zone' or the 'Golden Mean'. Spatiotemporal control of redox signaling is achieved by compartmentalized generation and removal of oxidants. The cellular landscape of H2O2, the major redox signaling molecule, is characterized by orders-of-magnitude concentration differences between organelles. This concentration pattern is mirrored by the pattern of oxidatively modified proteins, exemplified by S-glutathionylated proteins. The review presents the conceptual background for short-term (non-transcriptional) and longer-term (transcriptional/translational) homeostatic mechanisms of stress and stress responses. The redox set point is a variable moving target value, modulated by circadian rhythm and by external influence, summarily denoted as exposome, which includes nutrition and lifestyle factors. Emerging fields of cell-specific and tissue-specific redox regulation in physiological settings are briefly presented, including new insight into the role of oxidative eustress in embryonal development and lifespan, skeletal muscle and exercise, sleep-wake rhythm, and the function of the nervous system with aspects leading to psychobiology.

Keywords: Homeodynamics Hydrogen peroxide Oxidative stress Redox biology Redox landscape Steady-state.


Redox Biology

  • Nutrition
  • Polyphenolics
  • Cancer
  • Metabolism
  • Cardiovascular
  • Diabetes
  • Inflammation
  • Aging
  • Neuroscience
  • Cell and Molecular Biology
  • Cell Signaling
  • Bioenergetics

Redox Biology will also consider research articles focused in chemical or biochemical mechanisms of redox biology, if these include data demonstrating effects in physiologically relevant models. Studies of uncharacterized complex mixtures of natural products are not a suitable area of focus for the journal. Studies using commercial, undefined kits as the sole or primary assay for redox related changes which are not validated using other methods will not be considered for publication.

Redox Biology introduces Graphical Redox Reviews. These reviews will summarize schematically key concepts, established and novel ideas in redox biology which will be accessible to a broad audience and freely downloaded as PowerPoint slides for use in teaching or conference presentations.

Graphical Redox Reviews ideally should have a format of 3-5 color schemes with a title, short introductory paragraph, extended figure legends, and include up to 20 selected citations.


Chronic kidney disease (CKD) is a global, public health burden with increased incidence and prevalence, worldwide. Since patients at all stages of CKD experience significant cardiovascular morbidity and mortality and/or decreased quality of life, CKD is now regarded as a major global health priority. Atherosclerosis is highly prevalent in CKD it occurs even at the early stages, progresses along with disease deterioration, and is further exacerbated in end-stage kidney disease. This heavy cardiovascular burden in the CKD population cannot be solely explained by traditional risk factors.

Over the last decade, oxidative stress has emerged as a novel risk factor for cardiovascular morbidity and mortality in CKD patients. Oxidative stress is progressively enhanced in CKD and triggers atherosclerosis, through direct damage to the vascular endothelium. Endothelial dysfunction, the very basis of atherosclerosis is also evident at early CKD stages and is a novel, independent risk factor for morbidity and mortality in these patients. Therefore, understanding the cellular and molecular mechanisms underlying the pathogenesis of oxidative stress and endothelial dysfunction and their interplay in uremic patients may lead to the discovery of strategies to prevent or even treat various oxidative stress-mediated complications, including atherosclerosis.

The aim of this Special Issue is to solicit original research, as well as review articles, that help elucidate the effect of redox biology systems, oxidative stress, and endothelial dysfunction in CKD patients (including kidney transplant recipients, end-stage kidney disease, haemodialysis and peritoneal dialysis patients). We highly encourage the submission of in vitro, in vivo, clinical studies describing the interplay between oxidative stress and endothelial dysfunction, inflammation, pathophysiological mechanisms underlying these associations, modulatory roles of antioxidant, and anti-inflammatory agents.


2021_Redox_for_review - Biology

NEET 2021 Syllabus is authorised by the Medical Council of India. The entire syllabus of NEET 2021 is prepared based on the syllabus of secondary and higher secondary education pattern. As per the announcement made by CBSE, the syllabus will be reduced from the previous one approx. 30%. Due to the challenging situation caused by the COVID-19 pandemic in India, offline classes have been hampered and for this, all the institutes had to conduct online classes for the students. This condition led to a reduction of the syllabus in this academic year. Check NEET 2021 Syllabus

However, the revised syllabus for NEET 2021 is expected to be announced in December 2020. The syllabus consists of three main subjects such as Physics, Chemistry, and Biology, and also it is structured in an organised way to give a concrete idea on the subject matter from where the questions are supposed to come in the examination.

NEET 2021 Syllabus: Expected Reduction in Physics

Class wise expected topics that could be eliminated from the syllabus of Physics are mentioned below in a table.

Topic Subtopics
Class XI
Physical World and Measurement along with Nature of Physical Laws This topic comprises the scope and anticipation of Physic, the impact of technology on the society
Motion in a straight line It covers all about position time graph along with speed and velocity
Motion’s Law This includes the instinctive concept of force, Newton’s first law of motion that defines momentum, Newton’s second law of motion that defines impulse and Newton’s third law of motion that defines forces.
System of rotational motion of particles It covers affirmation and implementation of parallel along with perpendicular axes theorem.
The universal law of gravitation This topic consists of Kepler’s law of planetary motion and stimulation caused for gravity
Mechanical properties of Solid It contains the elastic behaviour, trimmed modulus of rigidity and also Poisson’s ration that covers elastic energy
Thermodynamics and all the information regarding Thermal properties There are various topics under the section of Thermal Properties are Heat, Transfer condition of Heat, Temperature and under Thermodynamics the matters are heat engine and refrigerator
Weaves and Oscillation This topic includes basic mode and harmonics, doppler effect
Class XII
Electric Charges and their Conservation Electric Dipole Under this topic all the areas about inside and outside area of spherical shell
Current Electricity This topic defines various matters such as colour code carbon resistors and its series along with parallel combination
Magnetic effects along with moving charges This topic covers cyclotron and give basic idea about magnetic field and also includes factors controlling the strength of electromagnets and its stability
Induction, Wave and Spectrum of Electromagnet, Alternating Current This topic give concept of displacement current and in Alternative Current, there are power factors along with wattless current
Ray Optics and Optical Instruments This topic comprises reflection of light, formula of mirror, scattering of light, Brewster’s law, problem sort out capacity by microscope and astronomical telescope
Dual Nature of radiation and matter This area covers Davisson-Germer experiments
Nuclei and Atoms All the areas that fall under this topic are radioactivity, alpha, beta and gamma particles, radioactive decay law, etc.

NEET 2021 Syllabus: Expected Reduction in Chemistry

Class wise expected topics that could be eliminated from the syllabus of Chemistry are mentioned below in a table.

Topic Subtopics
Class XI
The Fundamental idea about Chemistry Under this topic all the matters that are included are the scope of the chemistry, laws of chemical combination, atomic theory of Dalton that defines the concept of atoms, elements and molecule masses.
Structure of Atom This area covers the concept of shells and subshells, idea of orbital, rules of filling electrons in orbitals that explains Aufbau principle, Pauli exclusion principles, Rule of Hund, atom’s electric features, etc.
Classification of elements and Periodicity in Properties This topic contains Modern periodic law, periodic trends in element’s properties where atomic and ionic radii, ionization enthalpy, valence along with electronegativity are included.
Chemical Bonding and Molecular Structure All the subtopics that are included in this topic are Valence electrons, ionic bond, bond parameters. Aside from Lewis structure, resonance, a brief idea about hybridization involving, hydrogen bond, etc are included.
States of Matter for Gases and Liquids This area consists of Boyle’s law, Charles’s law, Law of GAY Lussac. Apart from that Law of Avogadro, concept of kinetic energy along with molecular speeds, etc
Chemical Thermodynamics, Equilibrium, Redox Reactions and Hydrogen Under the first topic the areas are specific heat capacity and equilibrium criteria. Under the second one basic idea on Hydrolysis of salts and Henderson Equilibrium and there is all the application of redox reaction, properties and uses of hydrogen in Redox Reactions and Hydrogen
s-Block and p-Block elements Under s-Block the areas are Sodium Carbonate, Sodium Chloride, Sodium Hydroxide and Sodium Hydrogen carbonate, etc and in p-Block elements subtopics are compounds of Silicon and its uses, reaction with acids, etc
Class XII
Chemical Kinetics This topic covers the basic idea of collision theory, activation energy, Arrhenius equation, etc.
Solid state and Solution This first one comprises electrical and magnetic properties, Band theory of metals, n and p type semiconductors and the second one covers abnormal molecular mass and also Van’t Hoff factor
D and F Block Elements All the areas that are covered under this topic are chemical reactivity of lanthanides, Preparation and properties of KMnO4 and K2Cr2O7, etc
Haloalkanes and Haloarenes This topic defines Uses and environmental effects of -dichloromethane, tetrachloromethane, trichloromethane, etc. Apart from that iodoform, freons are also there.

NEET 2021 Syllabus: Expected Reduction in Biology

Class wise expected topics that could be eliminated from the syllabus of Biology are mentioned below in a table.

Topic Subtopics
Class XI
Diversity of Living Organism This topic includes the concept of a living world that covers taxonomy and systematics, various tools for the study of taxonomy. Apart from that, the plant kingdom includes Angiospermae and its features along with classification.
Structural Organisation in Animal and Plants Under this topic there are Morphology and anatomy of flowering Plants, and also anatomy, functions of structural organism of animals
Plant Physiology This topic comprises transports in Plants that defines movement of water, transpiration pull, etc, mineral nutrition that covers all the essential minerals along with the role of macro and micronutrients, etc. Apart from that, all the phases and conditions of plant growth and its development process.
Human Physiology This area defines digestion and absorption that comprises various areas such as the role of digestive enzymes along with gastrointestinal hormones, types and function of locomotion and movement. Also includes neural control and coordination
Class XII
Reproduction in Organism This topic contains characteristic features of all the organism, various ways of reproduction, etc.
Genetics and Evolution This area covers origin of life, Darwin’s contribution, principles of Hardy-Weinberg, etc
Biology and Human Welfare Under this topic there are all the strategies and its implementation of food production, ecosystems, all types of environmental problems such as air pollution, water pollution, solid waste management and the most important issues like greenhouse effects, etc

NEET 2021 Syllabus Reduction FAQ

Ques. Is there any chance to get some questions from the reduced topic from NEET 2021 Syllabus?

Ans. No, you will not get any question from any topic that is removed from NEET 2021 Syllabus. All the questions will come from the remaining topics of the syllabus.

Ques. Are the topic 𠇋iomolecules” excluded from NEET 2021 Syllabus?

Ans. This topic falls under the Biology syllabus and if the topic gets removed from NEET 2021 Syllabus then you can skip this topic.

Ques. What are the subtopics of Body Fluids and Circulation that I can keep aside for NEET 2021?

Ans. All the areas that cover this topic are the composition of blood groups, coagulation of blood, the composition of lymph and its functions, the human circulatory system that contains structure and features of heart, etc.

Ques. Shall I omit the topics that are not included in NEET 2021 Syllabus If I appear in NEET 2022?

Ans. No, if you are appearing in NEET 2021 then you can omit those topics otherwise you have to wait until some announcements are there for NEET 2022 Syllabus.

Ques. Where shall I get the revised syllabus for NEET 2021?

Ans. You will get the revised syllabus for NEET 2021 only from the official website of the institute. 


2021_Redox_for_review - Biology

All articles published by MDPI are made immediately available worldwide under an open access license. No special permission is required to reuse all or part of the article published by MDPI, including figures and tables. For articles published under an open access Creative Common CC BY license, any part of the article may be reused without permission provided that the original article is clearly cited.

Feature Papers represent the most advanced research with significant potential for high impact in the field. Feature Papers are submitted upon individual invitation or recommendation by the scientific editors and undergo peer review prior to publication.

The Feature Paper can be either an original research article, a substantial novel research study that often involves several techniques or approaches, or a comprehensive review paper with concise and precise updates on the latest progress in the field that systematically reviews the most exciting advances in scientific literature. This type of paper provides an outlook on future directions of research or possible applications.

Editor’s Choice articles are based on recommendations by the scientific editors of MDPI journals from around the world. Editors select a small number of articles recently published in the journal that they believe will be particularly interesting to authors, or important in this field. The aim is to provide a snapshot of some of the most exciting work published in the various research areas of the journal.


Ferroptosis: mechanisms, biology and role in disease

The research field of ferroptosis has seen exponential growth over the past few years, since the term was coined in 2012. This unique modality of cell death, driven by iron-dependent phospholipid peroxidation, is regulated by multiple cellular metabolic pathways, including redox homeostasis, iron handling, mitochondrial activity and metabolism of amino acids, lipids and sugars, in addition to various signalling pathways relevant to disease. Numerous organ injuries and degenerative pathologies are driven by ferroptosis. Intriguingly, therapy-resistant cancer cells, particularly those in the mesenchymal state and prone to metastasis, are exquisitely vulnerable to ferroptosis. As such, pharmacological modulation of ferroptosis, via both its induction and its inhibition, holds great potential for the treatment of drug-resistant cancers, ischaemic organ injuries and other degenerative diseases linked to extensive lipid peroxidation. In this Review, we provide a critical analysis of the current molecular mechanisms and regulatory networks of ferroptosis, the potential physiological functions of ferroptosis in tumour suppression and immune surveillance, and its pathological roles, together with a potential for therapeutic targeting. Importantly, as in all rapidly evolving research areas, challenges exist due to misconceptions and inappropriate experimental methods. This Review also aims to address these issues and to provide practical guidelines for enhancing reproducibility and reliability in studies of ferroptosis. Finally, we discuss important concepts and pressing questions that should be the focus of future ferroptosis research.

Figures

Figure 1.. An overview of ferroptosis.

Figure 1.. An overview of ferroptosis.

A schematic chart showing that ferroptosis is executed by…

Figure 2.. Ferroptosis-suppressing pathways.

Figure 2.. Ferroptosis-suppressing pathways.

(A) The canonical ferroptosis controlling axis entails uptake of cystine via…

Figure 3.. Mechanisms of phospholipid peroxidation.

Figure 3.. Mechanisms of phospholipid peroxidation.

Lipid peroxidation, the hallmark of ferroptosis, occurs in both…

Figure 4.. Metabolism and cell signaling in…

Figure 4.. Metabolism and cell signaling in ferroptosis.

The figure depicts the regulation of ferroptosis…

Figure 5.. Relationship of ferroptosis to other…

Figure 5.. Relationship of ferroptosis to other biological processes and diseases.


2021_Redox_for_review - Biology

All articles published by MDPI are made immediately available worldwide under an open access license. No special permission is required to reuse all or part of the article published by MDPI, including figures and tables. For articles published under an open access Creative Common CC BY license, any part of the article may be reused without permission provided that the original article is clearly cited.

Feature Papers represent the most advanced research with significant potential for high impact in the field. Feature Papers are submitted upon individual invitation or recommendation by the scientific editors and undergo peer review prior to publication.

The Feature Paper can be either an original research article, a substantial novel research study that often involves several techniques or approaches, or a comprehensive review paper with concise and precise updates on the latest progress in the field that systematically reviews the most exciting advances in scientific literature. This type of paper provides an outlook on future directions of research or possible applications.

Editor’s Choice articles are based on recommendations by the scientific editors of MDPI journals from around the world. Editors select a small number of articles recently published in the journal that they believe will be particularly interesting to authors, or important in this field. The aim is to provide a snapshot of some of the most exciting work published in the various research areas of the journal.


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