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Introduction to cellular respiration and redox

Intro to redox in cellular respiration. Substrate-level vs. oxidative phosphorylation. Electron carriers.


Let’s imagine that you are a cell. You’ve just been given a big, juicy glucose molecule, and you’d like to convert some of the energy in this glucose molecule into a more usable form, one that you can use to power your metabolic reactions. How can you go about this? What’s the best way for you to squeeze as much energy as possible out of that glucose molecule, and to capture this energy in a handy form?
Fortunately for us, our cells – and those of other living organisms – are excellent at harvesting energy from glucose and other organic molecules, such as fats and amino acids. Here, we’ll get a high-level overview of how cells break down fuels. Then, we'll take a closer look at some of the electron transfer reactions (redox reactions) that are key to this process.

Overview of fuel breakdown pathways

The reactions that extract energy from molecules like glucose are called catabolic reactions. That means they involve breaking a larger molecule into smaller pieces. For example, when glucose is broken down in the presence of oxygen, it’s converted into six carbon dioxide molecules and six water molecules. The overall reaction for this process can be written as:
C6H12O6 + 6O2 6CO2 + 6H2O ΔG=686kcal/mol
In a cell, this overall reaction is broken down into many smaller steps. Energy contained in the bonds of glucose is released in small bursts, and some of it is captured in the form of adenosine triphosphate (ATP), a small molecule that powers reactions in the cell. Much of the energy from glucose is dissipated as heat, but enough is captured to keep the metabolism of the cell running.
_Image modified from "ATP: Adenosine triphosphate: Figure 1," by OpenStax College, Biology, CC BY 4.0._
As a glucose molecule is gradually broken down, some of the breakdowns steps release energy that is captured directly as ATP. In these steps, a phosphate group is transferred from a pathway intermediate straight to ADP, a process known as substrate-level phosphorylation.
Many more steps, however, produce ATP in an indirect way. In these steps, electrons from glucose are transferred to small molecules known as electron carriers. The electron carriers take the electrons to a group of proteins in the inner membrane of the mitochondrion, called the electron transport chain. As electrons move through the electron transport chain, they go from a higher to a lower energy level and are ultimately passed to oxygen (forming water).
As an electron passes through the electron transport chain, the energy it releases is used to pump protons (H+) out of the matrix of the mitochondrion, forming an electrochemical gradient. When the H+ flow back down their gradient, they pass through an enzyme called ATP synthase, driving synthesis of ATP. This process is known as oxidative phosphorylation. The diagram below shows examples of oxidative and substrate-level phosphorylation.
_Image modified from "Etc4" by Fvasconcellos (public domain)._
When organic fuels like glucose are broken down using an electron transport chain, the breakdown process is known as cellular respiration.

Electron carriers

Electron carriers, also called electron shuttles, are small organic molecules that play key roles in cellular respiration. Their name is a good description of their job: they pick up electrons from one molecule and drop them off with another. You can see an electron carrier shuttling electrons from the glucose breakdown reactions to the electron transport chain in the diagram above.
There are two types of electron carriers that are particularly important in cellular respiration: NAD+ (nicotinamide adenine dinucleotide, shown below) and FAD (flavin adenine dinucleotide).
_Image modified from"Energy in living systems: Figure 1," by OpenStax College, Biology (CC BY 3.0)._
When NAD+ and FAD pick up electrons, they also gain one or more hydrogen atoms, switching to a slightly different form:
NAD+ + 2e + 2H+ NADH +  H+
FAD + 2e + 2 H+ FADH2
And when they drop electrons off, they go neatly back to their original form:
NADH NAD+ + 2e + H+
FADH2 FAD + 2e + 2 H+
The reactions in which NAD+ and FAD gain or lose electrons are examples of a class of reactions called redox reactions. Let's take a closer look at what these reactions are and why they're so important in cellular respiration.

Redox reactions: What are they?

Cellular respiration involves many reactions in which electrons are passed from one molecule to another. Reactions involving electron transfers are known as oxidation-reduction reactions (or redox reactions).
You may have learned in chemistry that a redox reaction is when one molecule loses electrons and is oxidized, while another molecule gains electrons (the ones lost by the first molecule) and is reduced. Handy mnemonic: “LEO goes GER”: Lose Electrons, Oxidized; Gain Electrons, Reduced.
The formation of magnesium chloride is one example of a redox reaction that nicely matches our definition above:
In this reaction, the magnesium atom loses two electrons, so it is oxidized. These two electrons are accepted by chlorine, which is reduced.
However, as Sal points out in his video on oxidation and reduction in biology, we should really put quotes around "gains electrons" and "loses electrons" in our description of what happens to molecules in a redox reaction. That's because we can also have a reaction in which one molecule hogs electrons rather than fully gaining them or is hogged from rather than fully losing them.
What do we mean by that? To illustrate, let's use the example from Sal's video:
2H2 + O2 2H2O + heat
This reaction does not involve an obvious electron transfer, but it's still an example of a redox reaction. That's because the amount of electron density on the H and O atoms is different in the products than in the reactants.
Why that's true is not obvious, so let's break it down using the properties of atoms. When H atoms are bonded to each other in H2, they share electrons equally: neither can win the tug-of-war for the electrons. The same is true for O atoms bonded to each other in O2. However, the situation is different in the product, H2O. Oxygen is much more electronegative, or electron-hungry, than hydrogen, so in an OH bond in a water molecule, the electrons will be hogged by the O atom and spend more time close to it than to the H.
So, even though no electrons were fully gained or lost in the above reaction:
  • O has more electron density after the reaction than before (was reduced)
  • H has less electron density than it did before (was oxidized)
For you chemistry buffs out there, this change in electron hogging during the reaction can be more precisely described as a change in oxidation states of the O and H atoms. Check out Sal's video to see how oxidation states can be used as "bookkeeping tools" to represent shifts in electron sharing.

What about gaining and losing H and O atoms?

Oxidation and reduction reactions are fundamentally about the transfer and/or hogging of electrons. However, in the context of biology, there is a little trick we can often use to figure out where the electrons are going. This trick lets us use the gain or loss of H and O atoms as a proxy for the transfer of electrons.
In general:
  • If a carbon-containing molecule gains H atoms or loses O atoms during a reaction, it’s likely been reduced (gained electrons or electron density)
  • On the other hand, if a carbon-containing molecule loses H atoms or gains O atoms, it’s probably been oxidized (lost electrons or electron density)
For example, let’s go back to the reaction for glucose breakdown:
C6H12O6 + 6O2 6CO2 + 6H2O
In glucose, carbon is associated with H atoms, while in carbon dioxide, it is not associated with any Hs. So, we would predict that glucose is oxidized in this reaction. Similarly, the O atoms in O2 end up being associated with more Hs after the reaction than before, so we would predict that oxygen is reduced. (Sal confirms this from an electron transfer perspective in his video on redox reactions in respiration.)
Why does this trick work? Here is one way you can think about it, from Sal's video on oxidation and reduction in biology:
  • The atoms that H is usually bound to in organic molecules, such as C, O, N, and P, are more electronegative than H itself. So, if a H atom and its electron join a molecule, odds are that whatever's bonded to the new H is going to hog the electron and become reduced.
  • O is more electronegative than any of the other major atoms found commonly in biological molecules. If it joins a molecule, it's likely going to pull away electron density from whatever it's attached to, oxidizing it.

What's the point of all this redox?

Now that we have a better sense of what a redox reaction is, let's spend a moment thinking about the why. Why does a cell go to the trouble of ripping electrons off of glucose, transferring them to electron carriers, and passing them through an electron transport chain in a long series of redox reactions?
The basic answer is: to get energy out of that glucose molecule! Here is the glucose breakdown reaction we saw at the beginning of the article:
C6H12O6 + 6O2 6CO2 + 6H2O ΔG=686kcal/mol
Which we can rewrite a bit more clearly as:
C6H12O6 + 6O2 6CO2 + 6H2O + energy!
As Sal explains in his video on redox reactions in respiration, electrons are at a higher energy level when they are associated with less electronegative atoms (such as C or H) and at a lower energy level when they are associated with a more electronegative atom (such as O). So, in a reaction like the breakdown of glucose above, energy is released because the electrons are moving to a lower-energy, more "comfortable" state as they travel from glucose to oxygen.
The energy that's released as electrons move to a lower-energy state can be captured and used to do work. In cellular respiration, electrons from glucose move gradually through the electron transport chain towards oxygen, passing to lower and lower energy states and releasing energy at each step. The goal of cellular respiration is to capture this energy in the form of ATP.
Image modified from Carbohydrate metabolism: Figure 1 by OpenStax College, Anatomy & Physiology, CC BY 3.0
In the next articles and videos, we'll walk through cellular respiration step by step, seeing how the energy released in redox transfers is captured as ATP.

Want to join the conversation?

  • blobby green style avatar for user YAYAA
    What is the difference between nadph and nadH
    (8 votes)
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  • starky seedling style avatar for user Jocelyn
    What is the difference between NADPH and NADH
    (5 votes)
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  • blobby green style avatar for user kevinoconnell12
    Is NAD+/NADH primarily involved with glucose (entering ETC via complex1) & FAD/FADH2 primarily (exclusively?) involved with fats & amino acids (entering ETC via complex2)?
    (4 votes)
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    • aqualine ultimate style avatar for user Gayatri Srinivasa
      Not that I know of. They are both carriers for hydrogen ions (H+) and their purpose is to get those electrons/ions to the ETC where they can be used to make ATP. Since the NADHs are dropped at the first protein complex, the hydrogen ions that it brings in go through active transport in 3 proteins, making 3 ATP for every NADH. The FADH dropps off the H+s at the second protein complex, and since the H+s it brings in only go through 2 proteins, it only makes 2 ATP for every FADH molecule.

      So to answer your question, where the molecules enters, to my knowledge, has nothing to do with whether it is involved with glucose or amino acids. Could you possibly be getting it confused with something else?
      (7 votes)
  • duskpin ultimate style avatar for user Marcos Domínguez
    I still can't comprehend the notion of electrons' energy levels. I thought that it had to do with which orbital the electron was in, being the furthest away from the nucleus the ones with higher energy levels. But then, why are electrons at a higher energy level when associated with a hidrogen than when associated with an oxygen?
    (6 votes)
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  • blobby green style avatar for user sabrina4school72
    What is the purpose of Cellular Respiration??
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    • leafers ultimate style avatar for user Caleb H.
      Cellular respiration is an absolutely essential process for any cell because it's how it produces energy.

      Cells need energy for all kinds of things: moving things around, producing substances, getting nutrients, etc.

      For cells, this energy typically comes in the form of a molecule called ATP. Cellular respiration is a series of steps by which fuel molecules (such as glucose from your food) can be used to produce ATP that the cell can use.

      Does that help?
      (7 votes)
  • blobby green style avatar for user aida raihan
    i still cant understand the concept of oxidation and reduction in term of NAD+ and FAD.. when NAD+ oxidise or reduce to NADH? why ?
    (2 votes)
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    • female robot ada style avatar for user MomoTane
      NAD+ --> NADH is reduction (because it's gaining a hydrogen). It's being reduced because NAD+ is made up of carbon, hydrogen, nitrogen, oxygen and phosophorus atoms and all of these are very electronegative except for hydrogen. When the NAD+ bonds with a hydrogen the electrons are hogged by the very negative atoms like when Sal was talking about glucose. This is the same for FAD I think because it's made up primarily of those electronegative atoms. Hope that helps :)
      (3 votes)
  • blobby green style avatar for user Tori  Herrera
    When compounds lose electrons, they lose energy; when compounds gain electrons, they gain energy. Explain that statement in terms of endergonic and exergonic reactions.
    (3 votes)
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  • blobby green style avatar for user Amelie Olsen Galvan
    Do all catabolic processes occur under anaerobic conditions, while anabolic process occur under aerobic conditions, or is that just a coincidence?
    (2 votes)
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  • aqualine ultimate style avatar for user meghasyampeela15
    Can you explain how 36 ATP is forned in cellular respiration in eukaryotes?
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    • leaf red style avatar for user Hello it's me
      Actually, the amount of ATP produced in cellular respiration actually varies. It depends on the cell's efficiency and therefore fluctuates in the maximum production of ATP. So it can be any whole number of ATPs, probably 34, 36, or 38 ATPs in a eukaryotic cell. Usually, that number varies in the oxidative phosphorylation step, depending on the amount of NADH and FADH2 available for the process. NADH produces 3 ATP while FADH2 produces 2 ATP via chemiosmosis. Glycolysis produces 2 ATP and 2 NADH, Krebs Cycle produces 2 ATP, 6 NADH, and 2 FADH2. Then, you have a net total of 36 ATP. Sal explains this much better than I could :P.
      (3 votes)
  • blobby green style avatar for user Sydney Brown
    Is the action/movement of ATP synthase passive or active?
    (2 votes)
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