Cellular processes

Cellular Processes Introduction •The blue sections summarize key information and vocabulary terms are underlined. •Now that we know the basic parts of cells, how do cells get the materials and create the energy needed to function? •In this unit, WE will discuss: – define homeostasis, or the process by which organisms maintain a stable environment – the forms of transport – the creation of energy molecules during photosynthesis, – and the release of cellular energy during cellular respiration. Concentrations & Cells • Cells have to move things in and out of their membranes on a regular basis (food, water, waste, etc.). • The concentration in a solution measures the amount of solute (stuff in it, EX sugar in tea). • Depending on the amount of concentration, the molecules will move accordingly. • Video <a target="_blank" rel="noopener noreferrer nofollow" class="link" href="http://universe-review.ca/I12-18-solution.jpg" download="true">http://universe-review.ca/I12-18-solution.jpg</a> Diffusion • In a solution, the particles are moving constantly (in a glass of water, molecules are still moving). • Molecules will always move from a highly concentrated (CROWDED) areas to the LESS crowded areas. • Diffusion is the movement of particles to reach equilibrium (the same or equal on all sides). • Sketch it! <a target="_blank" rel="noopener noreferrer nofollow" class="link" href="http://www.okc.cc.ok.us/biologylabs/Image" download="true">http://www.okc.cc.ok.us/biologylabs/Image</a> s/Cells_Membranes/diffusion.gif Osmosis • Water is one of the most important items that cells need in order to function. • Osmosis is the diffusion of water through a selectively permeable (to permeate is to pass through) membrane. • Selectively permeable means that only certain things cross in to and out of the cell. • Water will move easily, until the concentration is equal on both sides of the cell membrane. <a target="_blank" rel="noopener noreferrer nofollow" class="link" href="http://www.okc.cc.ok.us/biologylabs/I" download="true">http://www.okc.cc.ok.us/biologylabs/I</a> mages/Cells_Membranes/osmosis.gif Types of Solutions • Hypotonic solution has a higher concentration inside the cell, so molecules will travel in to the cell (grow). • An isotonic solution (or “same strength”) is one where the concentration inside the cell matches the outside of the cell – there is little to no movement. • Hypertonic solution has a higher concentration outside the cell, so molecules will move out of the cell and it will shrink. <a target="_blank" rel="noopener noreferrer nofollow" class="link" href="http://biology.unm.edu/ccouncil/Biology_124/I" download="true">http://biology.unm.edu/ccouncil/Biology_124/I</a> mages/tonicity1.jpeg • Sketch the graphics (be sure to include the arrows showing the movement of water!!!) • Use the word memory tools to rember! <a target="_blank" rel="noopener noreferrer nofollow" class="link" href="http://2.bp.blogspot.com/_10M5kCPMTYM/S-1kTAlmmrI/AAAAAAAAABc/Duc-" download="true">http://2.bp.blogspot.com/_10M5kCPMTYM/S-1kTAlmmrI/AAAAAAAAABc/Duc-</a> LSzYgyU/s1600/Bio+hypotonic,+Isotonic,+Hypertonic.gif Hyp”O”tonic Solutions – The cell gr”O”ws! Hype”R”tonic Solutions – The cell sh”R”inks! I”S”otonic Solutions – The “S”ame! Passive Transport vs Active Transport • Diffusion and osmosis are processes within the cell that occur naturally, without the need for energy (Passive). • Protein channels: large proteins within the cell membrane act as “doorways” to move molecules and substances in and out of the cell (facilitated transport) • Active transport is a process by which cells use energy to move molecules against the concentration gradient . • Simulation <a target="_blank" rel="noopener noreferrer nofollow" class="link" href="http://www.daviddarling.info/images/" download="true">http://www.daviddarling.info/images/</a> active_transport.jpg Transport Cont. • Endocytosis: the process when cells move shift the cytoplasm to surround a substance; means “cell eating” • Exocytosis: the process when cells expel (get rid of) material through their membranes • Video <a target="_blank" rel="noopener noreferrer nofollow" class="link" href="http://static.howstuffworks.com/gif/ar" download="true">http://static.howstuffworks.com/gif/ar</a> tificial-blood-8.jpg Energy • All life depends on energy, or the ability to do work, in order to function. • All cells are constantly using energy. • The original source of all energy on Earth is the sun. • Energy = in the chemical bonds of compounds such as carbohydrates (sugars) and lipids (fats). <a target="_blank" rel="noopener noreferrer nofollow" class="link" href="http://www.noaanews.noaa.gov/stories2005/images/s" download="true">http://www.noaanews.noaa.gov/stories2005/images/s</a> un-soho011905-1919z.jpg • Photosynthesis is the process by which autotrophs (plants and some forms of bacteria) convert light energy into the energy of carbohydrates (glucose). Photosynthesis <a target="blank" rel="noopener noreferrer nofollow" class="link" href="http://static.howstuffworks.com/gif/irrigationphotosynthesis.gif" download="true">http://static.howstuffworks.com/gif/irrigationphotosynthesis.gif</a> <a target="blank" rel="noopener noreferrer nofollow" class="link" href="http://www.ap.stmarys.ca/~ishort/Images/Ear" download="true">http://www.ap.stmarys.ca/~ishort/Images/Ear</a> th/Atmos/photosynthesis.jpg Photosynthesis Cont. • Remember that glucose (type of sugar) is a carbohydrate and carbohydrates are biomolecules used for energy. • Photosynthesis creates glucose using energy from the sun in chloroplasts. 6CO2 + 6H2O + Sunlight = C6H12O6 + 6 O2 (Carbon Dioxide) (Water) (Energy) (Glucose) (Oxygen) • Simulation Chloroplasts & Chlorophyll • Chlorophyll is the green pigment that absorbs light energy from the sun. <a target="blank" rel="noopener noreferrer nofollow" class="link" href="http://botit.botany.wisc.edu/images/130/F" download="true">http://botit.botany.wisc.edu/images/130/F</a> erns/Gametophyte_Images/Chloroplasts MC.low.jpg • Cellular respiration occurs in ALL CELLS, plants, animals, as well as microscopic life, while photosynthesis occurs only in some. • Cell respiration occurs in the mitochondria. • It releases energy from glucose and other food molecules in the presence of oxygen. C6H12O6 + 6O2 = 6H2O + 6CO2 + ATP (Glucose) (Oxygen) (Water) (Carbon Dioxide) (Energy) Cellular Respiration <a target="blank" rel="noopener noreferrer nofollow" class="link" href="http://www.learner.org/channel/workshops/sheddinglight" download="true">http://www.learner.org/channel/workshops/sheddinglight</a> /images/photosynthesis.gif A Comparison Photosynthesis: (Reactants) (Products) 6 CO2 + 6 H2O + Energy = C6H12O6 + 6 O2 (Carbon Dioxide) (Water) (Glucose) (Oxygen) # of C = # of H = _____ # of O = _____ Cellular Respiration: (Reactants) (Products) C6H12O6 + 6O2 = 6CO2 + 6H2O + Energy (ATP) (Glucose) (Oxygen) (Carbon Dioxide) (Water) # of C = _____ # of H = _____ # of O = _____ ATP = Energy • Energy comes in many different forms, but our bodies use chemical energy (found within the bonds of compounds) for life’s processes. • ATP is one of the most important compounds in the body because it is used to store/release energy within the cell. • Adenosine Tri Phosphate (ATP) <a target="_blank" rel="noopener noreferrer nofollow" class="link" href="http://www.ustboniface.mb.c" download="true">http://www.ustboniface.mb.c</a> a/cusb/abernier/Biologie/Mo dule1/Images/atp.jpg ATP Cont. • Think of ATP as a fully charged battery, ready to do work for the cell. • When the bonds in the compound are broken in ATP, energy is released. • Observe the simulation; breaking off one phosphate creates ADP (Adenosine Di Phosphate) and releases energy to be used in the cell. • Sketch it! <a target="blank" rel="noopener noreferrer nofollow" class="link" href="http://student.ccbcmd.edu/biotutorials/ener" download="true">http://student.ccbcmd.edu/biotutorials/ener</a> gy/images/atp.gif In Conclusion • In this unit, we learned how cells move materials in an out of cell membranes. • Remember: – Simple Diffusion = the movement of particles from high concentrations to low concentrationsLet’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.<h3 collapsed="false" seolevelmigrated="true">Overview of fuel breakdown pathways</h3>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:<figure data-type="blockquoteFigure"><div><blockquote>[\text C_6\text H{12} \text O_6] + [6\text O_2] [\rightarrow] [6\text{CO}_2] + [6\text H_2\text O] [\quad\quad\quad\Delta G = -686 : \text{kcal/mol}]</blockquote><figcaption></figcaption></div></figure>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.<img src="https://cdn.kastatic.org/ka-perseus-images/ca71797a1224ae28afe904c997dad2a2dcdcfbae.png" data-width="100%" data-align="center" alt="Structure of ATP.">Image modified from "<a target="_blank" rel="noopener noreferrer" class="link" href="http://cnx.org/contents/GFy_h8cu@10.53:mcCadNdV@7/ATP-Adenosine-Triphosphate" download="true">ATP: Adenosine triphosphate: Figure 1</a>," by OpenStax College, Biology, <a target="_blank" rel="noopener noreferrer" class="link" href="https://creativecommons.org/licenses/by/4.0/" download="true">CC BY 4.0</a>.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 ([\text H^+]) out of the matrix of the mitochondrion, forming an electrochemical gradient. When the [\text 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.<img src="https://cdn.kastatic.org/ka-perseus-images/d7bcec76c0d817d86ec09afd57f839221d692edf.png" data-width="100%" data-align="center" alt="Simplified diagram showing oxidative phosphorylation and substrate-level phosphorylation during glucose breakdown reactions. Inside the matrix of the mitochondrion, substrate-level phosphorylation takes place when a phosphate group from an intermediate of the glucose breakdown reactions is transferred to ADP, forming ATP. At the same time, electrons are transported from intermediates of the glucose breakdown reactions to the electron transport chain by electron carriers. The electrons move through the electron transport chain, pumping protons into the intermembrane space. When these protons flow back down their concentration gradient, they pass through ATP synthase, which uses the electron flow to synthesize ATP from ADP and inorganic phosphate (Pi). This process of electron transport, proton pumping, and capture of energy from the proton gradient to make ATP is called oxidative phosphorylation.">Image modified from "<a target="_blank" rel="noopener noreferrer" class="link" href="https://commons.wikimedia.org/wiki/File:Mitochondrial_electron_transport_chain%E2%80%94Etc4.svg" download="true">Etc4</a>" by Fvasconcellos (<a target="_blank" rel="noopener noreferrer" class="link" href="http://creativecommons.org/publicdomain/mark/1.0/" download="true">public domain</a>).Is that really what a mitochondrion looks like?When organic fuels like glucose are broken down using an electron transport chain, the breakdown process is known as cellular respiration.<h3 collapsed="false" seolevelmigrated="true">Electron carriers</h3>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).<img src="https://cdn.kastatic.org/ka-perseus-images/78cdd9e6cd3bbf3a25f29204a74406e0c6efe9df.png" data-width="100%" data-align="center" alt="Chemical structures of NAD+ and NADH. NADH has a hydrogen attached to one nitrogen-containing ring, whereas in NAD+ this same ring lacks a hydrogen and has a positive charge.">Image modified from"<a target="blank" rel="noopener noreferrer" class="link" href="http://cnx.org/resources/b17e8c463121bd0cc2c0f273aa9a0b2b743f7aba/Figure_07_01_01ab.jpg" download="true">Energy in living systems: Figure 1</a>," by OpenStax College, Biology (<a target="blank" rel="noopener noreferrer" class="link" href="https://creativecommons.org/licenses/by/3.0/us/" download="true">CC BY 3.0</a>).When NAD[^+] and FAD pick up electrons, they also gain one or more hydrogen atoms, switching to a slightly different form:<figure data-type="blockquoteFigure"><div><blockquote>[\text{\blue{NAD}}^+] [+] [2\text{e}^-] [+] [2 \text{\purple{H}}^+] [\rightarrow] [\text{\blue{NAD}}][\text{\purple{H}}] [+] [\text{ \purple{H}}^+]</blockquote><figcaption></figcaption></div></figure><figure data-type="blockquoteFigure"><div><blockquote>[\text{\green{FAD}}] [+] [2\text{e}^-] [+] [2 \text{\purple{ H}}^+] [\rightarrow] [\text{\green{FAD}\purple{H}}\purple 2]</blockquote><figcaption></figcaption></div></figure>And when they drop electrons off, they go neatly back to their original form:<figure data-type="blockquoteFigure"><div><blockquote>[\text{\blue{NAD}}][\text{\purple{H}}] [\rightarrow] [\text{\blue{NAD}}^+] [+] [2\text{e}^-] [+] [\text{\purple{H}}^+]</blockquote><figcaption></figcaption></div></figure><figure data-type="blockquoteFigure"><div><blockquote>[\text{\green{FAD}\purple{H}}\purple 2] [\rightarrow] [\text{\green{FAD}}] [+] [2\text{e}^-] [+] [2 \text{\purple{ H}}^+]</blockquote><figcaption></figcaption></div></figure>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.<h3 collapsed="false" seolevelmigrated="true">Redox reactions: What are they?</h3>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:<figure data-type="blockquoteFigure"><div><blockquote>[\text{Mg} + \text{Cl}_2 \rightarrow \text{Mg}^{2+} + 2 \text{Cl}^-]</blockquote><figcaption></figcaption></div></figure>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 <a target="_blank" rel="noopener noreferrer nofollow" class="link" href="https://www.khanacademy.org/science/biology/cellular-respiration-and-fermentation/intro-to-cellular-respiration/v/oxidation-and-reduction-from-biological-view" download="true">oxidation and reduction in biology</a>, 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:<figure data-type="blockquoteFigure"><div><blockquote>[2][\text{H}_2] [+] [\text{O}_2] [\rightarrow] [2][\text{H}_2\text{O}] [+] [\text{heat}]</blockquote><figcaption></figcaption></div></figure>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 [\text H] and [\text 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 [\text H] atoms are bonded to each other in [\text H_2], they share electrons equally: neither can win the tug-of-war for the electrons. The same is true for [\text O] atoms bonded to each other in [\text O_2]. However, the situation is different in the product, [\text H_2\text O]. Oxygen is much more electronegative, or electron-hungry, than hydrogen, so in an [\text O-\text H] bond in a water molecule, the electrons will be hogged by the [\text O] atom and spend more time close to it than to the [\text H].So, even though no electrons were fully gained or lost in the above reaction:<ul><li>[\text O] has more electron density after the reaction than before (was reduced)</li><li>[\text H] has less electron density than it did before (was oxidized)</li></ul>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 [\text O] and [\text H] atoms. Check out <a target="blank" rel="noopener noreferrer nofollow" class="link" href="https://www.khanacademy.org/science/biology/cellular-respiration-and-fermentation/intro-to-cellular-respiration/v/oxidation-and-reduction-from-biological-view" download="true">Sal's video</a> to see how oxidation states can be used as "bookkeeping tools" to represent shifts in electron sharing.<h3 collapsed="false" seolevelmigrated="true">What about gaining and losing [\text{H}] and [\text O] atoms?</h3>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 [\text H] and [\text O] atoms as a proxy for the transfer of electrons.In general:<ul><li>If a carbon-containing molecule gains [\text H] atoms or loses [\text O] atoms during a reaction, it’s likely been reduced (gained electrons or electron density)</li><li>On the other hand, if a carbon-containing molecule loses [\text H] atoms or gains [\text O] atoms, it’s probably been oxidized (lost electrons or electron density)</li></ul>For example, let’s go back to the reaction for glucose breakdown:<figure data-type="blockquoteFigure"><div><blockquote>[\text C_6\text H{12} \text O_6] + [6\text O_2] [\rightarrow] [6\text{CO}_2] + [6\text H_2\text O]</blockquote><figcaption></figcaption></div></figure>In glucose, carbon is associated with [\text H] atoms, while in carbon dioxide, it is not associated with any [\text H]s. So, we would predict that glucose is oxidized in this reaction. Similarly, the [\text O] atoms in [\text O_2] end up being associated with more [\text H]s 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 <a target="_blank" rel="noopener noreferrer nofollow" class="link" href="https://www.khanacademy.org/science/biology/cellular-molecular-biology/cellular-respiration/v/oxidation-and-reduction-in-cellular-respiration" download="true">redox reactions in respiration</a>.)Why does this trick work? Here is one way you can think about it, from Sal's video on <a target="blank" rel="noopener noreferrer nofollow" class="link" href="https://www.khanacademy.org/science/biology/cellular-respiration-and-fermentation/intro-to-cellular-respiration/v/oxidation-and-reduction-from-biological-view" download="true">oxidation and reduction in biology</a>:<ul><li>The atoms that [\text H] is usually bound to in organic molecules, such as [\text{C, O, N,}] and [\text P,] are more electronegative than [\text H] itself. So, if a [\text H] atom and its electron join a molecule, odds are that whatever's bonded to the new [\text{H}] is going to hog the electron and become reduced.</li><li>[\text 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.</li></ul><h3 collapsed="false" seolevelmigrated="true">What's the point of all this redox?</h3>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:<figure data-type="blockquoteFigure"><div><blockquote>[\text C_6\text H{12} \text O_6] + [6\text O_2] [\rightarrow] [6\text{CO}2] + [6\text H_2\text O] [\quad\quad\quadΔG = -686 : \text{kcal/mol}]</blockquote><figcaption></figcaption></div></figure>Which we can rewrite a bit more clearly as:<figure data-type="blockquoteFigure"><div><blockquote>[\text C_6\text H{12} \text O_6] + [6\text O_2] [\rightarrow] [6\text{CO}_2] + [6\text H_2\text O] + [\text{energy!}]</blockquote><figcaption></figcaption></div></figure>As Sal explains in his video on <a target="_blank" rel="noopener noreferrer nofollow" class="link" href="https://www.khanacademy.org/science/biology/cellular-molecular-biology/cellular-respiration/v/oxidation-and-reduction-in-cellular-respiration" download="true">redox reactions in respiration</a>, electrons are at a higher energy level when they are associated with less electronegative atoms (such as [\text C] or [\text H]) and at a lower energy level when they are associated with a more electronegative atom (such as [\text 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.<img src="https://cdn.kastatic.org/ka-perseus-images/baee747294ce2e06e8ae40ce1a371edd48695a48.png" data-width="100%" data-align="center" alt="A diagram shows the step-down flow of electrons to create ATP. Electrons are represented by a picture of a sun with an e and negative symbol in the center. The steps are shown as 4 steps. The first electron is to the right of the steps and is labeled Electrons removed from glucose. There is a large arrow pointing towards the electron on the top step of the steps image. From that electron there is a large arrow pointing towards an image labeled ATP. There is also an arrow pointing from the electron on the top step down an electron on the next, lower step. The arrow is labeled Redox reactions. From the electron on that step there is a large arrow pointing towards an image labeled ATP and another arrow pointing down to the next lower step at another electron. That arrow is labeled redox reaction. From that electron, there is a large arrow pointing towards an image labeled ATP, and another arrow pointing towards an electron on the lowest step. The electron on the lowest step has a large arrow pointing from the electron to an image labeled ATP. The electron on the lowest step also has an arrow pointing towards an electron at the base of the steps, and next to that electron is a chemical formula that says electron plus oxygen plus 2 hydrogen ions produces water.">Image modified from <a target="_blank" rel="noopener noreferrer" class="link" href="http://cnx.org/contents/FPtK1zmh@6.17:nWir-Uwu@3/Carbohydrate-Metabolism" download="true">Carbohydrate metabolism: Figure 1</a> by OpenStax College, Anatomy & Physiology, <a target="_blank" rel="noopener noreferrer" class="link" href="https://creativecommons.org/licenses/by/3.0/" download="true">CC BY 3.0</a>Wait, does the energy literally turn into ATP?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.Attribution and references<h3 collapsed="false" seolevelmigrated="true">Cellular respiration is one of the most elegant, majestic, and fascinating metabolic pathways on earth. At the same time, it’s also one of the most complicated. When I learned about it for the first time, I felt like I had tripped and fallen into a can of organic-chemistry-flavored alphabet soup!</h3>Luckily, cellular respiration is not so scary once you get to know it. Let's start by looking at cellular respiration at a high level, walking through the four major stages and tracing how they connect up to one another.<h3 collapsed="false" seolevelmigrated="true">Steps of cellular respiration</h3><img src="https://cdn.kastatic.org/ka-perseus-images/b7c6b018ad440976202bbf875e814de46387dc48.png" data-width="100%" data-align="center" alt="Overview of the steps of cellular respiration.

Glycolysis. Six-carbon glucose is converted into two pyruvates (three carbons each). ATP and NADH are made. These reactions take place in the cytosol.
Pyruvate oxidation. Pyruvate travels into the mitochondrial matrix and is converted to a two-carbon molecule bound to coenzyme A, called acetyl CoA. Carbon dioxide is released and NADH is made.
Citric acid cycle. The acetyl CoA combines with a four-carbon molecule and goes through a cycle of reactions, ultimately regenerating the four-carbon starting molecule. ATP (or, in some cases, GTP), NADH, and FADH_2 are made, and carbon dioxide is released. These reactions take place in the mitochondrial matrix.
Oxidative phosphorylation. The NADH and FADH_2 produced in other steps deposit their electrons in the electron transport chain in the inner mitochondrial membrane. As electrons move down the chain, energy is released and used to pump protons out of the matrix and into the intermembrane space, forming a gradient. The protons flow back into the matrix through an enzyme called ATP synthase, making ATP. At the end of the electron transport chain, oxygen accepts electrons and takes up protons to form water.">During cellular respiration, a glucose molecule is gradually broken down into carbon dioxide and water. Along the way, some ATP is produced directly in the reactions that transform glucose. Much more ATP, however, is produced later in a process called oxidative phosphorylation. Oxidative phosphorylation is powered by the movement of electrons through the electron transport chain, a series of proteins embedded in the inner membrane of the mitochondrion.These electrons come originally from glucose and are shuttled to the electron transport chain by electron carriers [\text{NAD}^+] and [\text{FAD}], which become [\text{NADH}] and [\text{FADH}_2] when they gain electrons. To be clear, this is what's happening in the diagram above when it says [+] [ \text{NADH}] or [+] [ \text{FADH}_2]. The molecule isn't appearing from scratch, it's just being converted to its electron-carrying form:<figure data-type="blockquoteFigure"><div><blockquote>[\text{NAD}^+] [+] [2 e^-] [+] [2 \text H^+] [\rightarrow] [\text{NADH}] [+] [\text H^+]</blockquote><figcaption></figcaption></div></figure><figure data-type="blockquoteFigure"><div><blockquote>[\text{FAD}] [+] [2e^-] [+] [2 \text H^+] [\rightarrow] [\text{FADH}_2]</blockquote><figcaption></figcaption></div></figure>To see how a glucose molecule is converted into carbon dioxide and how its energy is harvested as ATP and [\text{NADH}][/][\text{FADH}_2] in one of your body's cells, let’s walk step by step through the four stages of cellular respiration.<ol><li><a target="_blank" rel="noopener noreferrer nofollow" class="link" href="https://www.khanacademy.org/science/biology/cellular-respiration-and-fermentation/glycolysis/v/glycolysis" download="true">Glycolysis.</a> In glycolysis, glucose—a six-carbon sugar—undergoes a series of chemical transformations. In the end, it gets converted into two molecules of pyruvate, a three-carbon organic molecule. In these reactions, ATP is made, and [\text{NAD}^+] is converted to [\text{NADH}].</li><li><a target="_blank" rel="noopener noreferrer nofollow" class="link" href="https://www.khanacademy.org/science/biology/cellular-respiration-and-fermentation/pyruvate-oxidation-and-the-citric-acid-cycle/a/pyruvate-oxidation" download="true">Pyruvate oxidation.</a> Each pyruvate from glycolysis goes into the mitochondrial matrix—the innermost compartment of mitochondria. There, it’s converted into a two-carbon molecule bound to Coenzyme A, known as acetyl CoA. Carbon dioxide is released and [\text{NADH}] is generated.</li><li><a target="_blank" rel="noopener noreferrer nofollow" class="link" href="https://www.khanacademy.org/science/biology/cellular-respiration-and-fermentation/pyruvate-oxidation-and-the-citric-acid-cycle/v/krebs-citric-acid-cycle" download="true">Citric acid cycle.</a> The acetyl CoA made in the last step combines with a four-carbon molecule and goes through a cycle of reactions, ultimately regenerating the four-carbon starting molecule. ATP, [\text{NADH}], and [\text{FADH}_2] are produced, and carbon dioxide is released.</li><li><a target="_blank" rel="noopener noreferrer nofollow" class="link" href="https://www.khanacademy.org/science/biology/cellular-respiration-and-fermentation/oxidative-phosphorylation/v/oxidative-phosphorylation-and-the-electon-transport-chain" download="true">Oxidative phosphorylation.</a> The [\text{NADH}] and [\text{FADH}_2] made in other steps deposit their electrons in the electron transport chain, turning back into their "empty" forms ([\text{NAD}^+] and [\text{FAD}]). As electrons move down the chain, energy is released and used to pump protons out of the matrix, forming a gradient. Protons flow back into the matrix through an enzyme called ATP synthase, making ATP. At the end of the electron transport chain, oxygen accepts electrons and takes up protons to form water.</li></ol>Glycolysis can take place without oxygen in a process called <a target="_blank" rel="noopener noreferrer nofollow" class="link" href="https://www.khanacademy.org/science/biology/cellular-respiration-and-fermentation/variations-on-cellular-respiration/a/fermentation-and-anaerobic-respiration" download="true">fermentation</a>. The other three stages of cellular respiration—pyruvate oxidation, the citric acid cycle, and oxidative phosphorylation—require oxygen in order to occur. Only oxidative phosphorylation uses oxygen directly, but the other two stages can't run without oxidative phosphorylation.Each stage of cellular respiration is covered in more detail in other articles and videos on the site. Try watching the <a target="_blank" rel="noopener noreferrer nofollow" class="link" href="https://www.khanacademy.org/science/biology/cellular-respiration-and-fermentation/overview-of-cellular-respiration-steps/v/overview-of-cellular-respiration/" download="true">overview video</a>, or jump straight to an article on a particular stage by using the links aWhy do we need oxygen?You, like many other organisms, need oxygen to live. As you know if you’ve ever tried to hold your breath for too long, lack of oxygen can make you feel dizzy or even black out, and prolonged lack of oxygen can even cause death. But have you ever wondered why that’s the case, or what exactly your body does with all that oxygen?As it turns out, the reason you need oxygen is so your cells can use this molecule during oxidative phosphorylation, the final stage of cellular respiration. Oxidative phosphorylation is made up of two closely connected components: the electron transport chain and chemiosmosis. In the electron transport chain, electrons are passed from one molecule to another, and energy released in these electron transfers is used to form an electrochemical gradient. In chemiosmosis, the energy stored in the gradient is used to make ATP.So, where does oxygen fit into this picture? Oxygen sits at the end of the electron transport chain, where it accepts electrons and picks up protons to form water. If oxygen isn’t there to accept electrons (for instance, because a person is not breathing in enough oxygen), the electron transport chain will stop running, and ATP will no longer be produced by chemiosmosis. Without enough ATP, cells can’t carry out the reactions they need to function, and, after a long enough period of time, may even die.In this article, we'll examine oxidative phosphorylation in depth, seeing how it provides most of the ready chemical energy (ATP) used by the cells in your body.<h3 collapsed="false" seolevelmigrated="true">Overview: oxidative phosphorylation</h3><img src="https://cdn.kastatic.org/ka-perseus-images/0ff21c52cd544c8a51014fcbbc2ec144f32cd698.png" data-width="100%" data-align="center" alt="Simple diagram of the electron transport chain. The electron transport chain is a series of proteins embedded in the inner mitochondrial membrane.

In the matrix, NADH and FADH2 deposit their electrons in the chain (at the first and second complexes of the chain, respectively).

The energetically "downhill" movement of electrons through the chain causes pumping of protons into the intermembrane space by the first, third, and fourth complexes.

Finally, the electrons are passed to oxygen, which accepts them along with protons to form water.

The proton gradient produced by proton pumping during the electron transport chain is used to synthesize ATP. Protons flow down their concentration gradient into the matrix through the membrane protein ATP synthase, causing it to spin (like a water wheel) and catalyze conversion of ADP to ATP.">The electron transport chain is a series of proteins and organic molecules found in the inner membrane of the mitochondria. Electrons are passed from one member of the transport chain to another in a series of redox reactions. Energy released in these reactions is captured as a proton gradient, which is then used to make ATP in a process called chemiosmosis. Together, the electron transport chain and chemiosmosis make up oxidative phosphorylation. The key steps of this process, shown in simplified form in the diagram above, include:<ul><li>Delivery of electrons by NADH and FADH[_2]. Reduced electron carriers (NADH and FADH[_2]) from other steps of cellular respiration transfer their electrons to molecules near the beginning of the transport chain. In the process, they turn back into NAD[^+] and FAD, which can be reused in other steps of cellular respiration.</li><li>Electron transfer and proton pumping. As electrons are passed down the chain, they move from a higher to a lower energy level, releasing energy. Some of the energy is used to pump H[^+] ions, moving them out of the matrix and into the intermembrane space. This pumping establishes an electrochemical gradient.</li><li>Splitting of oxygen to form water. At the end of the electron transport chain, electrons are transferred to molecular oxygen, which splits in half and takes up H[^+] to form water.</li><li>Gradient-driven synthesis of ATP. As H[^+] ions flow down their gradient and back into the matrix, they pass through an enzyme called ATP synthase, which harnesses the flow of protons to synthesize ATP.</li></ul>We'll look more closely at both the electron transport chain and chemiosmosis in the sections below.<h3 collapsed="false" seolevelmigrated="true">The electron transport chain</h3>The electron transport chain is a collection of membrane-embedded proteins and organic molecules, most of them organized into four large complexes labeled I to IV. In eukaryotes, many copies of these molecules are found in the inner mitochondrial membrane. In prokaryotes, the electron transport chain components are found in the plasma membrane.As the electrons travel through the chain, they go from a higher to a lower energy level, moving from less electron-hungry to more electron-hungry molecules. Energy is released in these “downhill” electron transfers, and several of the protein complexes use the released energy to pump protons from the mitochondrial matrix to the intermembrane space, forming a proton gradient.Click to see a free energy diagram<img src="https://cdn.kastatic.org/ka-perseus-images/5e6c26f8f515f421608854c65437f12037296416.png" data-width="100%" data-align="center" alt="Free energy diagram showing the energetically downhill flow of electrons in the electron transport chain. Electrons move to a lower and lower free energy level as they travel through the chain. Large drops in free energy as electrons move through Complexes I, III, and IV drive proton pumping."><img src="https://cdn.kastatic.org/ka-perseus-images/62120415688fc64d57237535d282247e981bdfed.png" data-width="100%" data-align="center" alt="Image of the electron transport chain. All the components of the chain are embedded in or attached to the inner mitochondrial membrane. In the matrix, NADH deposits electrons at Complex I, turning into NAD+ and releasing a proton into the matrix. FADH2 in the matrix deposits electrons at Complex II, turning into FAD and releasing 2 H+. The electrons from Complexes I and II are passed to the small mobile carrier Q. Q transports the electrons to Complex III, which then passes them to Cytochrome C. Cytochrome C passes the electrons to Complex IV, which then passes them to oxygen in the matrix, forming water. It takes two electrons, 1/2 O2, and 2 H+ to form one water molecule. Complexes I, III, and IV use energy released as electrons move from a higher to a lower energy level to pump protons out of the matrix and into the intermembrane space, generating a proton gradient.">Image modified from "<a target="_blank" rel="noopener noreferrer" class="link" href="http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@9.85:37/Oxidative-Phosphorylation" download="true">Oxidative phosphorylation: Figure 1</a>", by OpenStax College, Biology (<a target="_blank" rel="noopener noreferrer" class="link" href="https://creativecommons.org/licenses/by/3.0/us/" download="true">CC BY 3.0</a>).All of the electrons that enter the transport chain come from NADH and FADH[_2] molecules produced during earlier stages of cellular respiration: glycolysis, pyruvate oxidation, and the citric acid cycle.<ul><li>NADH is very good at donating electrons in redox reactions (that is, its electrons are at a high energy level), so it can transfer its electrons directly to complex I, turning back into NAD[^+]. As electrons move through complex I in a series of redox reactions, energy is released, and the complex uses this energy to pump protons from the matrix into the intermembrane space.</li><li>FADH[_2] is not as good at donating electrons as NADH (that is, its electrons are at a lower energy level), so it cannot transfer its electrons to complex I. Instead, it feeds them into the transport chain through complex II, which does not pump protons across the membrane.</li></ul>Because of this "bypass," each FADH[_2] molecule causes fewer protons to be pumped (and contributes less to the proton gradient) than an NADH.More about complexes I and IIBeyond the first two complexes, electrons from NADH and FADH[_2] travel exactly the same route. Both complex I and complex II pass their electrons to a small, mobile electron carrier called ubiquinone (Q), which is reduced to form QH[_2] and travels through the membrane, delivering the electrons to complex III. As electrons move through complex III, more H[^+] ions are pumped across the membrane, and the electrons are ultimately delivered to another mobile carrier called cytochrome C (cyt C). Cyt C carries the electrons to complex IV, where a final batch of H[^+] ions is pumped across the membrane. Complex IV passes the electrons to O[_2], which splits into two oxygen atoms and accepts protons from the matrix to form water. Four electrons are required to reduce each molecule of O[_2], and two water molecules are formed in the process.More about complexes III and IVOverall, what does the electron transport chain do for the cell? It has two important functions:<ul><li>Regenerates electron carriers. NADH and FADH[_2] pass their electrons to the electron transport chain, turning back into NAD[^+] and FAD. This is important because the oxidized forms of these electron carriers are used in glycolysis and the citric acid cycle and must be available to keep these processes running.</li><li>Makes a proton gradient. The transport chain builds a proton gradient across the inner mitochondrial membrane, with a higher concentration of H[^+] in the intermembrane space and a lower concentration in the matrix. This gradient represents a stored form of energy, and, as we’ll see, it can be used to make ATP.</li></ul><h3 collapsed="false" seolevelmigrated="true">Chemiosmosis</h3>Complexes I, III, and IV of the electron transport chain are proton pumps. As electrons move energetically downhill, the complexes capture the released energy and use it to pump H[^+] ions from the matrix to the intermembrane space. This pumping forms an electrochemical gradient across the inner mitochondrial membrane. The gradient is sometimes called the proton-motive force, and you can think of it as a form of stored energy, kind of like a battery.Like many other ions, protons can't pass directly through the phospholipid bilayer of the membrane because its core is too hydrophobic. Instead, H[^+] ions can move down their concentration gradient only with the help of channel proteins that form hydrophilic tunnels across the membrane.In the inner mitochondrial membrane, H[^+] ions have just one channel available: a membrane-spanning protein known as ATP synthase. Conceptually, ATP synthase is a lot like a turbine in a hydroelectric power plant. Instead of being turned by water, it’s turned by the flow of H[^+] ions moving down their electrochemical gradient. As ATP synthase turns, it catalyzes the addition of a phosphate to ADP, capturing energy from the proton gradient as ATP.<img src="https://cdn.kastatic.org/ka-perseus-images/fe1296eb6182efc88ea25a88f0a02638a7339410.png" data-width="100%" data-align="center" alt="Overview diagram of oxidative phosphorylation. The electron transport chain and ATP synthase are embedded in the inner mitochondrial membrane. NADH and FADH2 made in the citric acid cycle (in the mitochondrial matrix) deposit their electrons into the electron transport chain at complexes I and II, respectively. This step regenerates NAD+ and FAD (the oxidized carriers) for use in the citric acid cycle. The electrons flow through the electron transport chain, causing protons to be pumped from the matrix to the intermembrane space. Eventually, the electrons are passed to oxygen, which combines with protons to form water. The proton gradient generated by proton pumping during the electron transport chain is a stored form of energy. When protons flow back down their concentration gradient (from the intermembrane space to the matrix), their only route is through ATP synthase, an enzyme embedded in the inner mitochondrial membrane. When protons flow through ATP synthase, they cause it to turn (much as water turns a water wheel), and its motion catalyzes the conversion of ADP and Pi to ATP.">Image modified from "<a target="_blank" rel="noopener noreferrer" class="link" href="http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@9.85:37/Oxidative-Phosphorylation" download="true">Oxidative phosphorylation: Figure 3</a>," by Openstax College, Biology (<a target="_blank" rel="noopener noreferrer" class="link" href="https://creativecommons.org/licenses/by/3.0/us/" download="true">CC BY 3.0</a>).This process, in which energy from a proton gradient is used to make ATP, is called chemiosmosis. More broadly, chemiosmosis can refer to any process in which energy stored in a proton gradient is used to do work. Although chemiosmosis accounts for over 80% of ATP made during glucose breakdown in cellular respiration, it’s not unique to cellular respiration. For instance, chemiosmosis is also involved in the light reactions of <a target="_blank" rel="noopener noreferrer nofollow" class="link" href="https://www.khanacademy.org/science/biology/photosynthesis-in-plants/the-light-dependent-reactions-of-photosynthesis/v/photosynthesis-light-reactions-1" download="true">photosynthesis</a>.What would happen to the energy stored in the proton gradient if it weren't used to synthesize ATP or do other cellular work? It would be released as heat, and interestingly enough, some types of cells deliberately use the proton gradient for heat generation rather than ATP synthesis. This might seem wasteful, but it's an important strategy for animals that need to keep warm. For instance, hibernating mammals (such as bears) have specialized cells known as brown fat cells. In the brown fat cells, uncoupling proteins are produced and inserted into the inner mitochondrial membrane. These proteins are simply channels that allow protons to pass from the intermembrane space to the matrix without traveling through ATP synthase. By providing an alternate route for protons to flow back into the matrix, the uncoupling proteins allow the energy of the gradient to be dissipated as heat.<h3 collapsed="false" seolevelmigrated="true">ATP yield</h3>How many ATP do we get per glucose in cellular respiration? If you look in different books, or ask different professors, you'll probably get slightly different answers. However, most current sources estimate that the maximum ATP yield for a molecule of glucose is around 30-32 ATP[^{2,3,4}]. This range is lower than previous estimates because it accounts for the necessary transport of ADP into, and ATP out of, the mitochondrion.More details<ul><li></li><li></li></ul>Where does the figure of 30-32 ATP come from? Two net ATP are made in glycolysis, and another two ATP (or energetically equivalent GTP) are made in the citric acid cycle. Beyond those four, the remaining ATP all come from oxidative phosphorylation. Based on a lot of experimental work, it appears that four H[^+] ions must flow back into the matrix through ATP synthase to power the synthesis of one ATP molecule. When electrons from NADH move through the transport chain, about 10 H[^+] ions are pumped from the matrix to the intermembrane space, so each NADH yields about 2.5 ATP. Electrons from FADH[_2], which enter the chain at a later stage, drive pumping of only 6 H[^+], leading to production of about 1.5 ATP.With this information, we can do a little inventory for the breakdown of one molecule of glucose:<table style="min-width: 75px"><colgroup><col><col><col></colgroup><tbody><tr><td colspan="1" rowspan="1"></td><td colspan="1" rowspan="1"></td><td colspan="1" rowspan="1">StageDirect products (net)Ultimate ATP yield (net)</td></tr><tr><td colspan="1" rowspan="1" style="margin: 0px; padding: 5px 10px; border: 0px; font: inherit; vertical-align: baseline; box-sizing: inherit; text-align: left; ">Glycolysis</td><td colspan="1" rowspan="1" style="margin: 0px; padding: 5px 10px; border: 0px; font: inherit; vertical-align: baseline; box-sizing: inherit; text-align: left; ">2 ATP</td><td colspan="1" rowspan="1" style="margin: 0px; padding: 5px 10px; border: 0px; font: inherit; vertical-align: baseline; box-sizing: inherit; text-align: left; ">2 ATP</td></tr><tr><td colspan="1" rowspan="1" style="margin: 0px; padding: 5px 10px; border: 0px; font: inherit; vertical-align: baseline; box-sizing: inherit; text-align: left;"></td><td colspan="1" rowspan="1" style="margin: 0px; padding: 5px 10px; border: 0px; font: inherit; vertical-align: baseline; box-sizing: inherit; text-align: left;">2 NADH</td><td colspan="1" rowspan="1" style="margin: 0px; padding: 5px 10px; border: 0px; font: inherit; vertical-align: baseline; box-sizing: inherit; text-align: left;">3-5 ATP</td></tr><tr><td colspan="1" rowspan="1" style="margin: 0px; padding: 5px 10px; border: 0px; font: inherit; vertical-align: baseline; box-sizing: inherit; text-align: left; ">Pyruvate oxidation</td><td colspan="1" rowspan="1" style="margin: 0px; padding: 5px 10px; border: 0px; font: inherit; vertical-align: baseline; box-sizing: inherit; text-align: left; ">2 NADH</td><td colspan="1" rowspan="1" style="margin: 0px; padding: 5px 10px; border: 0px; font: inherit; vertical-align: baseline; box-sizing: inherit; text-align: left; ">5 ATP</td></tr><tr><td colspan="1" rowspan="1" style="margin: 0px; padding: 5px 10px; border: 0px; font: inherit; vertical-align: baseline; box-sizing: inherit; text-align: left;">Citric acid cycle</td><td colspan="1" rowspan="1" style="margin: 0px; padding: 5px 10px; border: 0px; font: inherit; vertical-align: baseline; box-sizing: inherit; text-align: left;">2 ATP/GTP</td><td colspan="1" rowspan="1" style="margin: 0px; padding: 5px 10px; border: 0px; font: inherit; vertical-align: baseline; box-sizing: inherit; text-align: left;">2 ATP</td></tr><tr><td colspan="1" rowspan="1" style="margin: 0px; padding: 5px 10px; border: 0px; font: inherit; vertical-align: baseline; box-sizing: inherit; text-align: left; "></td><td colspan="1" rowspan="1" style="margin: 0px; padding: 5px 10px; border: 0px; font: inherit; vertical-align: baseline; box-sizing: inherit; text-align: left; ">6 NADH</td><td colspan="1" rowspan="1" style="margin: 0px; padding: 5px 10px; border: 0px; font: inherit; vertical-align: baseline; box-sizing: inherit; text-align: left; ">15 ATP</td></tr><tr><td colspan="1" rowspan="1" style="margin: 0px; padding: 5px 10px; border: 0px; font: inherit; vertical-align: baseline; box-sizing: inherit; text-align: left;"></td><td colspan="1" rowspan="1" style="margin: 0px; padding: 5px 10px; border: 0px; font: inherit; vertical-align: baseline; box-sizing: inherit; text-align: left;">2 FADH[_2]</td><td colspan="1" rowspan="1" style="margin: 0px; padding: 5px 10px; border: 0px; font: inherit; vertical-align: baseline; box-sizing: inherit; text-align: left;">3 ATP</td></tr><tr><td colspan="1" rowspan="1" style="margin: 0px; padding: 5px 10px; border: 0px; font: inherit; vertical-align: baseline; box-sizing: inherit; text-align: left; ">Total</td><td colspan="1" rowspan="1" style="margin: 0px; padding: 5px 10px; border: 0px; font: inherit; vertical-align: baseline; box-sizing: inherit; text-align: left; "></td><td colspan="1" rowspan="1" style="margin: 0px; padding: 5px 10px; border: 0px; font: inherit; vertical-align: baseline; box-sizing: inherit; text-align: left; ">30-32 ATP</td></tr></tbody></table>Click here for a diagram showing ATP production<img src="https://cdn.kastatic.org/ka-perseus-images/f270ed773086a3538b379b2747ee08234556a678.png" data-width="100%" data-align="center" alt="Overview diagram of cellular respiration showing the number of ATP and NADH/FADH2 produced at each stage.">One number in this table is still not precise: the ATP yield from NADH made in glycolysis. This is because glycolysis happens in the cytosol, and NADH can't cross the inner mitochondrial membrane to deliver its electrons to complex I. Instead, it must hand its electrons off to a molecular “shuttle system” that delivers them, through a series of steps, to the electron transport chain.<ul><li>Some cells of your body have a shuttle system that delivers electrons to the transport chain via FADH[_2]. In this case, only 3 ATP are produced for the two NADH of glycolysis.</li><li>Other cells of your body have a shuttle system that delivers the electrons via NADH, resulting in the production of 5 ATP.</li></ul>In bacteria, both glycolysis and the citric acid cycle happen in the cytosol, so no shuttle is needed and 5 ATP are produced.30-32 ATP from the breakdown of one glucose molecule is a high-end estimate, and the real yield may be lower. For instance, some intermediates from cellular respiration may be siphoned off by the cell and used in other biosynthetic pathways, reducing the number of ATP produced. Cellular respiration is a nexus for many different metabolic pathways in the cell, forming a <a target="_blank" rel="noopener noreferrer nofollow" class="link" href="https://www.khanacademy.org/science/biology/cellular-respiration-and-fermentation/variations-on-cellular-respiration/a/connections-between-cellular-respiration-and-other-pathways" download="true">network</a> that’s larger than the glucose breakdown pathways al<h3 collapsed="false" seolevelmigrated="true">keep working when you're exercising so hard that they're very low on oxygen?</h3>Both of these processes can happen thanks to alternative glucose breakdown pathways that occur when normal, oxygen-using (aerobic) cellular respiration is not possible—that is, when oxygen isn't around to act as an acceptor at the end of the <a target="_blank" rel="noopener noreferrer nofollow" class="link" href="https://www.khanacademy.org/science/biology/cellular-respiration-and-fermentation/oxidative-phosphorylation/v/oxidative-phosphorylation-and-the-electon-transport-chain" download="true">electron transport chain</a>. These fermentation pathways consist of <a target="_blank" rel="noopener noreferrer nofollow" class="link" href="https://www.khanacademy.org/science/biology/cellular-respiration-and-fermentation/glycolysis/v/glycolysis" download="true">glycolysis</a> with some extra reactions tacked on at the end. In yeast, the extra reactions make alcohol, while in your muscles, they make lactic acid.Fermentation is a widespread pathway, but it is not the only way to get energy from fuels anaerobically (in the absence of oxygen). Some living systems instead use an inorganic molecule other than [\text{O}_2], such as sulfate, as a final electron acceptor for an electron transport chain. This process, called anaerobic cellular respiration, is performed by some bacteria and archaea.In this article, we'll take a closer look at anaerobic cellular respiration and at the different types of fermentation.<h3 collapsed="false" seolevelmigrated="true">Anaerobic cellular respiration</h3>Anaerobic cellular respiration is similar to aerobic cellular respiration in that electrons extracted from a fuel molecule are passed through an electron transport chain, driving [\text{ATP}] synthesis. Some organisms use sulfate [(\text{SO}4^{2-})] as the final electron acceptor at the end ot the transport chain, while others use nitrate [(\text{NO}{3}^-)], sulfur, or one of a variety of other molecules[^1].What kinds of organisms use anaerobic cellular respiration? Some prokaryotes—bacteria and archaea—that live in low-oxygen environments rely on anaerobic respiration to break down fuels. For example, some archaea called methanogens can use carbon dioxide as a terminal electron acceptor, producing methane as a by-product. Methanogens are found in soil and in the digestive systems of ruminants, a group of animals including cows and sheep.Similarly, sulfate-reducing bacteria and Archaea use sulfate as a terminal electron acceptor, producing hydrogen sulfide [(\text H_2\text S)] as a byproduct. The image below is an aerial photograph of coastal waters, and the green patches indicate an overgrowth of sulfate-reducing bacteria.<img src="https://cdn.kastatic.org/ka-perseus-images/70a366071af1cf1ed0ccccf1b742b0c290f4eb3b.png" data-width="100%" data-align="center" alt="Aerial photograph of coastal waters with blooms of sulfate-reducing bacteria appearing as large patches of green in the water.">Image credit: "<a target="_blank" rel="noopener noreferrer" class="link" href="http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@9.85:38/Metabolism-without-Oxygen" download="true">Metabolism without oxygen: Figure 1</a>," OpenStax College, Biology, <a target="_blank" rel="noopener noreferrer" class="link" href="https://creativecommons.org/licenses/by/3.0/us/" download="true">CC BY 3.0</a>; Modification of work by NASA/Jeff Schmaltz, MODIS Land Rapid Response Team at NASA GSFC, Visible Earth Catalog of NASA images.<h3 collapsed="false" seolevelmigrated="true">Fermentation</h3>Fermentation is another anaerobic (non-oxygen-requiring) pathway for breaking down glucose, one that's performed by many types of organisms and cells. In fermentation, the only energy extraction pathway is glycolysis, with one or two extra reactions tacked on at the end.Fermentation and cellular respiration begin the same way, with glycolysis. In fermentation, however, the pyruvate made in glycolysis does not continue through oxidation and the citric acid cycle, and the electron transport chain does not run. Because the electron transport chain isn't functional, the [\text{NADH}] made in glycolysis cannot drop its electrons off there to turn back into [\text{NAD}^+]The purpose of the extra reactions in fermentation, then, is to regenerate the electron carrier [\text{NAD}^+] from the [\text{NADH}] produced in glycolysis. The extra reactions accomplish this by letting [\text{NADH}] drop its electrons off with an organic molecule (such as pyruvate, the end product of glycolysis). This drop-off allows glycolysis to keep running by ensuring a steady supply of [\text{NAD}^+].<h4 collapsed="false" seolevelmigrated="true">Lactic acid fermentation</h4>In lactic acid fermentation, [\text{NADH}] transfers its electrons directly to pyruvate, generating lactate as a byproduct. Lactate, which is just the deprotonated form of lactic acid, gives the process its name. The bacteria that make yogurt carry out lactic acid fermentation, as do the red blood cells in your body, which don’t have mitochondria and thus can’t perform cellular respiration.<img src="https://cdn.kastatic.org/ka-perseus-images/6cd39322c5448333812f01ca4e930b8af0c316c8.png" data-width="100%" data-align="center" alt="Diagram of lactic acid fermentation. Lactic acid fermentation has two steps: glycolysis and NADH regeneration.

During glycolysis, one glucose molecule is converted to two pyruvate molecules, producing two net ATP and two NADH.

During NADH regeneration, the two NADH donate electrons and hydrogen atoms to the two pyruvate molecules, producing two lactate molecules and regenerating NAD+.">Muscle cells also carry out lactic acid fermentation, though only when they have too little oxygen for aerobic respiration to continue—for instance, when you’ve been exercising very hard. It was once thought that the accumulation of lactate in muscles was responsible for soreness caused by exercise, but recent research suggests this is probably not the case.Lactic acid produced in muscle cells is transported through the bloodstream to the liver, where it’s converted back to pyruvate and processed normally in the remaining reactions of cellular respiration.<h3 collapsed="false" seolevelmigrated="true">Alcohol fermentation</h3>Another familiar fermentation process is alcohol fermentation, in which [\text{NADH}] donates its electrons to a derivative of pyruvate, producing ethanol.Going from pyruvate to ethanol is a two-step process. In the first step, a carboxyl group is removed from pyruvate and released in as carbon dioxide, producing a two-carbon molecule called acetaldehyde. In the second step, [\text{NADH}] passes its electrons to acetaldehyde, regenerating [\text{NAD}^+] and forming ethanol.<img src="https://cdn.kastatic.org/ka-perseus-images/95be5e3a235711613dca5377ed13113aa181c4a7.png" data-width="100%" data-align="center" alt="Diagram of alcohol fermentation. Alcohol fermentation has two steps: glycolysis and NADH regeneration.

During glycolysis, one glucose molecule is converted to two pyruvate molecules, producing two net ATP and two NADH.

During NADH regeneration, the two pyruvate molecules are first converted to two acetaldehyde molecules, releasing two carbon dioxide molecules in the process. The two NADH then donate electrons and hydrogen atoms to the two acetaldehyde molecules, producing two ethanol molecules and regenerating NAD+.">Alcohol fermentation by yeast produces the ethanol found in alcoholic drinks like beer and wine. However, alcohol is toxic to yeasts in large quantities (just as it is to humans), which puts an upper limit on the percentage alcohol in these drinks. Ethanol tolerance of yeast ranges from about [5] percent to [21] percent, depending on the yeast strain and environmental conditions.<h3 collapsed="false" seolevelmigrated="true">Facultative and obligate anaerobes</h3>Many bacteria and archaea are facultative anaerobes, meaning they can switch between aerobic respiration and anaerobic pathways (fermentation or anaerobic respiration) depending on the availability of oxygen. This approach allows lets them get more ATP out of their glucose molecules when oxygen is around—since aerobic cellular respiration makes more ATP than anaerobic pathways—but to keep metabolizing and stay alive when oxygen is scarce.Other bacteria and archaea are obligate anaerobes, meaning they can live and grow only in the absence of oxygen. Oxygen is toxic to these microorganisms and injures or kills them on exposure. For instance, the Clostridium bacteria that are responsible for botulism (a form of food poisoning) are obligate anaerobes[^2]. Recently, some multicellular animals have even been discovered in deep-sea sediments that are free of oxygen[^{3,4}]. <h3 collapsed="false" seolevelmigrated="true"></h3><ol><li></li></ol><h3 collapsed="false" seolevelmigrated="true"></h3>

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C6H12O6 + 6 O2 -> 6 CO2 + 6 H2O + 32 ATP (Energy)
The mitochondrion is the organelle responsible for carrying out
aerobic cellular respiration in all eukaryotic organism.
The Three Phases of Cellular Respiration:
 Glycolysis –
 occurs in cytosol outside of mitochondria
 Aerobic or anaerobic respiration
 Kreb’s Cycle (Citric Acid Cycle) –
 Occurs in the innermitochondrial matrix
 Aerobic respiration.
 Electron Transport Chain –
 Occurs across the cristae membrane.
 Aerobic respiration.

Glucose -> 2 Pyruvates (Pyruvic Acid)
2 NAD+ -> 2 NADH (reduced)
2 ATP used
4 ATP produced
Net production of 2 ATP
 Occurs in the cytosol of the cell.
 NAD+ is an electron carrier. The hydrogen
carried by NAD+ with be used later in the
electron transport chain of aerobic
respiration, or in fermentation (anaerobic
respiration)
 2 Pyruvates will be converted to Acetyl
CoA to be used in the Kreb’s Cycle of
aerobic respiration in the mitochondrion, or
will be used in fermentation (anaerobic
respiration)

2 Pyruvates (Pyruvic Acid) -----------> Acetyl CoA
 The intermediate step happens between
glycolysis and the Kreb’s Cycle of Aerobic
Respiration.
 Coenzyme A is the biological catalysts that
makes this step happen.
 Acetyl CoA will be a reactant in the Kreb’s
cycle.
Coenzyme A

1 X 2 X’s
Acetyl CoA ---> 3 CO2 6 CO2
1 ADP ---> 1 ATP 2 ATP
4 NAD ---> 4 NADH2 8 NADH2
1 FAD ---> 1 FADH2 2 FADH2

 Occurs in the innermitochondrial matrix of the
mitochondrion.
 2 turns of the cycle occur.
 Produces (reduces) electron carriers for
electron transport chain.
 The Kreb’s Cycle produces the CO2 from the
cellular respiration equation.

 Chemiosmosis/Oxidative Phosphorylation
 Occurs in through the cristae membrane of the
mitochondrion.
1. Electron carriers (NADH & FADH2) bring e-
’s to ETC.
2. Cytochrome carrier proteins pump H+
ions down concentration gradient through
ATP synthase – ADP -> ATP
3. Oxygen, O2, is the final electr
4. Produces 36 ATP

 If no oxygen is present, organisms undergo
anaerobic respiration or fermentation.
 Lactic Acid Fermentation – animals
 Alcoholic Fermentation – bacteria, yeasts, some
plants.