During Which Process is Water Produced

Identify the reactants and products of cellular respiration and where these reactions occur in a prison cell

At present that nosotros’ve learned how autotrophs like plants convert sunlight to sugars, let’due south take a wait at how all eukaryotes—which includes humans!—make apply of those sugars.

In the process of photosynthesis, plants and other photosynthetic producers create glucose, which stores energy in its chemical bonds. Then, both plants and consumers, such as animals, undergo a serial of metabolic pathways—collectively called cellular respiration. Cellular respiration extracts the energy from the bonds in glucose and converts it into a form that all living things can utilize.

Learning Objectives

  • Draw the process of glycolysis and identify its reactants and products
  • Describe the procedure of pyruvate oxidation and identify its reactants and products
  • Describe the process of the citric acid cycle (Krebs wheel) and place its reactants and products
  • Describe the respiratory chain (electron transport chain) and its role in cellular respiration

Cellular respiration is a procedure that all living things use to catechumen glucose into energy. Autotrophs (like plants) produce glucose during photosynthesis. Heterotrophs (similar humans) ingest other living things to obtain glucose. While the procedure can seem circuitous, this page takes you through the central elements of each part of cellular respiration.


is the get-go footstep in the breakdown of glucose to excerpt energy for cellular metabolism. Nearly all living organisms conduct out glycolysis as part of their metabolism. The process does not employ oxygen and is therefore
(processes that use oxygen are called aerobic). Glycolysis takes identify in the cytoplasm of both prokaryotic and eukaryotic cells. Glucose enters heterotrophic cells in two ways.

  1. Through secondary active transport in which the transport takes place confronting the glucose concentration gradient.
  2. Through a group of integral proteins called Overabundance proteins, also known as glucose transporter proteins. These transporters assistance in the facilitated diffusion of glucose.

Glycolysis begins with the vi carbon ring-shaped structure of a single glucose molecule and ends with two molecules of a three-carbon sugar chosenpyruvate(Figure 1).

Figure 1. Reactants and products of glycolysis.

Glycolysis consists of ten steps divided into two distinct halves. The commencement half of the glycolysis is likewise known equally the
energy-requiring steps. This pathway traps the glucose molecule in the jail cell and uses energy to alter it so that the six-carbon saccharide molecule can exist divide evenly into the two three-carbon molecules. The second half of glycolysis (also known as the
free energy-releasing steps) extracts energy from the molecules and stores information technology in the course of ATP and NADH, the reduced grade of NAD.

First Half of Glycolysis (Energy-Requiring Steps)

This illustration shows the steps in the first half of glycolysis. In step one, the enzyme hexokinase uses one ATP molecule in the phosphorylation of glucose. In step two, glucose-6-phosphate is rearranged to form fructose-6-phosphate by phosphoglucose isomerase. In step three, phosphofructokinase uses a second ATP molecule in the phosphorylation of the substrate, forming fructose-1,6-bisphosphate. The enzyme fructose bisphosphate aldose splits the substrate into two, forming glyceraldeyde-3-phosphate and dihydroxyacetone-phosphate. In step 4, triose phosphate isomerase converts the dihydroxyacetone-phosphate into glyceraldehyde-3-phosphate

Figure ii. The get-go one-half of glycolysis uses 2 ATP molecules in the phosphorylation of glucose, which is then split into two three-carbon molecules.

Step 1. The first step in glycolysis is catalyzed by hexokinase, an enzyme with broad specificity that catalyzes the phosphorylation of six-carbon sugars. Hexokinase phosphorylates glucose using ATP as the source of the phosphate, producing glucose-half-dozen-phosphate, a more reactive form of glucose. This reaction prevents the phosphorylated glucose molecule from continuing to interact with the Overabundance proteins, and it can no longer get out the prison cell because the negatively charged phosphate volition not allow it to cross the hydrophobic interior of the plasma membrane.

Step 2. In the second step of glycolysis, an isomerase converts glucose-half-dozen-phosphate into one of its isomers, fructose-half-dozen-phosphate. Anisomerase
is an enzyme that catalyzes the conversion of a molecule into one of its isomers. This change from phosphoglucose to phosphofructose allows the eventual carve up of the sugar into ii three-carbon molecules.

Stride iii. The 3rd step is the phosphorylation of fructose-half dozen-phosphate, catalyzed by the enzyme phosphofructokinase. A second ATP molecule donates a high-energy phosphate to fructose-vi-phosphate, producing fructose-1,6-bisphosphate. In this pathway, phosphofructokinase is a charge per unit-limiting enzyme. It is active when the concentration of ADP is high; information technology is less active when ADP levels are low and the concentration of ATP is high. Thus, if in that location is “sufficient” ATP in the system, the pathway slows down. This is a type of end product inhibition, since ATP is the end product of glucose catabolism.

Footstep 4. The newly added high-energy phosphates further destabilize fructose-1,half-dozen-bisphosphate. The quaternary stride in glycolysis employs an enzyme, aldolase, to carve 1,6-bisphosphate into two three-carbon isomers: dihydroxyacetone-phosphate and glyceraldehyde-three-phosphate.

Step 5. In the fifth step, an isomerase transforms the dihydroxyacetone-phosphate into its isomer, glyceraldehyde-3-phosphate. Thus, the pathway will continue with 2 molecules of a single isomer. At this point in the pathway, at that place is a internet investment of free energy from two ATP molecules in the breakdown of one glucose molecule.

2nd Half of Glycolysis (Energy-Releasing Steps)

And then far, glycolysis has toll the prison cell 2 ATP molecules and produced two small, iii-carbon sugar molecules. Both of these molecules will proceed through the second half of the pathway, and sufficient energy volition exist extracted to pay back the two ATP molecules used as an initial investment and produce a profit for the cell of ii additional ATP molecules and two even higher-energy NADH molecules.

This illustration shows the steps in the second half of glycolysis. In step six, the enzyme glyceraldehydes-3-phosphate dehydrogenase produces one NADH molecule and forms 1,3-bisphosphoglycerate. In step seven, the enzyme phosphoglycerate kinase removes a phosphate group from the substrate, forming one ATP molecule and 3-phosphoglycerate. In step eight, the enzyme phosphoglycerate mutase rearranges the substrate to form 2-phosphoglycerate. In step nine, the enzyme enolase rearranges the substrate to form phosphoenolpyruvate. In step ten, a phosphate group is removed from the substrate, forming one ATP molecule and pyruvate.

Effigy 3. The second half of glycolysis involves phosphorylation without ATP investment (step vi) and produces two NADH and iv ATP molecules per glucose.

Stride 6. The 6th pace in glycolysis (Figure 3) oxidizes the saccharide (glyceraldehyde-3-phosphate), extracting high-free energy electrons, which are picked up by the electron carrier NAD+, producing NADH. The sugar is then phosphorylated by the addition of a second phosphate grouping, producing 1,3-bisphosphoglycerate. Note that the 2d phosphate group does not require another ATP molecule.

Here again is a potential limiting gene for this pathway. The continuation of the reaction depends upon the availability of the oxidized grade of the electron carrier, NAD+. Thus, NADH must be continuously oxidized back into NAD+
in order to keep this pace going. If NAD+
is not bachelor, the 2d half of glycolysis slows downward or stops. If oxygen is bachelor in the system, the NADH will be oxidized readily, though indirectly, and the high-energy electrons from the hydrogen released in this process volition be used to produce ATP. In an environment without oxygen, an alternating pathway (fermentation) can provide the oxidation of NADH to NAD+.

Step vii. In the 7th step, catalyzed by phosphoglycerate kinase (an enzyme named for the reverse reaction), 1,3-bisphosphoglycerate donates a loftier-free energy phosphate to ADP, forming one molecule of ATP. (This is an instance of substrate-level phosphorylation.) A carbonyl grouping on the i,3-bisphosphoglycerate is oxidized to a carboxyl group, and 3-phosphoglycerate is formed.

Step eight. In the eighth step, the remaining phosphate group in three-phosphoglycerate moves from the third carbon to the second carbon, producing 2-phosphoglycerate (an isomer of 3-phosphoglycerate). The enzyme catalyzing this step is a mutase (a type of isomerase).

Step 9. Enolase catalyzes the ninth step. This enzyme causes ii-phosphoglycerate to lose water from its structure; this is a dehydration reaction, resulting in the formation of a double bond that increases the potential energy in the remaining phosphate bail and produces phosphoenolpyruvate (PEP).

Stride 10. The terminal step in glycolysis is catalyzed by the enzyme pyruvate kinase (the enzyme in this case is named for the reverse reaction of pyruvate’s conversion into PEP) and results in the production of a second ATP molecule by substrate-level phosphorylation and the compound pyruvic acid (or its salt form, pyruvate). Many enzymes in enzymatic pathways are named for the reverse reactions, since the enzyme can catalyze both forward and contrary reactions.

Outcomes of Glycolysis

Glycolysis starts with glucose and ends with ii pyruvate molecules, a total of four ATP molecules and two molecules of NADH. Ii ATP molecules were used in the start half of the pathway to prepare the six-carbon ring for cleavage, so the cell has a net proceeds of two ATP molecules and two NADH molecules for its use.

If the cell cannot catabolize the pyruvate molecules further, it will harvest but two ATP molecules from i molecule of glucose. Mature mammalian ruby blood cells are not capable ofaerobic respiration—the process in which organisms convert energy in the presence of oxygen—and glycolysis is their sole source of ATP. If glycolysis is interrupted, these cells lose their ability to maintain their sodium-potassium pumps, and eventually, they die.

The last step in glycolysis will not occur if pyruvate kinase, the enzyme that catalyzes the formation of pyruvate, is non available in sufficient quantities. In this state of affairs, the unabridged glycolysis pathway will proceed, but only two ATP molecules will be made in the 2nd half. Thus, pyruvate kinase is a rate-limiting enzyme for glycolysis.

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In Summary: Glycolysis

Glycolysis is the commencement pathway used in the breakup of glucose to excerpt energy. It was probably one of the earliest metabolic pathways to evolve and is used by almost all of the organisms on earth. Glycolysis consists of two parts: The first part prepares the half-dozen-carbon band of glucose for cleavage into two three-carbon sugars. ATP is invested in the process during this half to energize the separation. The second half of glycolysis extracts ATP and high-energy electrons from hydrogen atoms and attaches them to NAD+. Two ATP molecules are invested in the first one-half and 4 ATP molecules are formed by substrate phosphorylation during the 2nd half. This produces a cyberspace gain of ii ATP and two NADH molecules for the cell.

Effigy 4 shows the entire process of glycolysis in one epitome:

The metabolic pathway of glycolysis converts glucose to pyruvate by via a series of intermediate metabolites. Each chemical modification (red box) is performed by a different enzyme. Steps 1 and 3 consume ATP (blue) and steps 7 and 10 produce ATP (yellow). Since steps 6-10 occur twice per glucose molecule, this leads to a net production of energy.

Figure 4. Glycolysis

Pyruvate Oxidation

If oxygen is bachelor, aerobic respiration volition go forrad. In eukaryotic cells, the pyruvate molecules produced at the end of glycolysis are transported into mitochondria, which are the sites of cellular respiration. In that location, pyruvate volition be transformed into an acetyl group that will exist picked upwardly and activated by a carrier chemical compound called coenzyme A (CoA). The resulting compound is called
acetyl CoA. CoA is made from vitamin B5, pantothenic acid. Acetyl CoA tin be used in a diversity of means past the cell, but its major office is to evangelize the acetyl grouping derived from pyruvate to the next stage of the pathway in glucose catabolism.

Breakdown of Pyruvate

In social club for pyruvate (which is the production of glycolysis) to enter the Citric Acid Cycle (the side by side pathway in cellular respiration), it must undergo several changes. The conversion is a iii-step process (Figure 5).

This illustration shows the three-step conversion of pyruvate into acetyl CoA. In step one, a carboxyl group is removed from pyruvate, releasing carbon dioxide. In step two, a redox reaction forms acetate and NADH. In step three, the acetate is transferred coenzyme A, forming acetyl CoA.

Figure v. Upon entering the mitochondrial matrix, a multi-enzyme complex converts pyruvate into acetyl CoA. In the procedure, carbon dioxide is released and one molecule of NADH is formed.

Step one. A carboxyl group is removed from pyruvate, releasing a molecule of carbon dioxide into the surrounding medium.
The result of this step is a two-carbon hydroxyethyl group bound to the enzyme (pyruvate dehydrogenase). This is the first of the six carbons from the original glucose molecule to be removed. This pace gain twice (remember: there are
pyruvate molecules produced at the cease of glycolysis) for every molecule of glucose metabolized; thus, two of the six carbons will have been removed at the end of both steps.

Step ii. NAD+
is reduced to NADH.

The hydroxyethyl group is oxidized to an acetyl group, and the electrons are picked upwardly by NAD+, forming NADH. The loftier-energy electrons from NADH volition be used later to generate ATP.

Pace 3. An acetyl group is transferred to conenzyme A, resulting in acetyl CoA.
The enzyme-leap acetyl group is transferred to CoA, producing a molecule of acetyl CoA.

Note that during the 2d stage of glucose metabolism, whenever a carbon atom is removed, it is bound to two oxygen atoms, producing carbon dioxide, one of the major terminate products of cellular respiration.

Acetyl CoA to COii

In the presence of oxygen, acetyl CoA delivers its acetyl group to a 4-carbon molecule, oxaloacetate, to grade citrate, a six-carbon molecule with 3 carboxyl groups; this pathway volition harvest the remainder of the extractable free energy from what began as a glucose molecule. This unmarried pathway is called by different names, but nosotros will primarily call it the
Citric Acid Wheel.

In Summary: Pyruvate Oxidation

In the presence of oxygen, pyruvate is transformed into an acetyl group attached to a carrier molecule of coenzyme A. The resulting acetyl CoA tin can enter several pathways, but most often, the acetyl group is delivered to the citric acrid cycle for further catabolism. During the conversion of pyruvate into the acetyl group, a molecule of carbon dioxide and ii high-energy electrons are removed. The carbon dioxide accounts for two (conversion of two pyruvate molecules) of the six carbons of the original glucose molecule. The electrons are picked upwards past NAD+, and the NADH carries the electrons to a afterward pathway for ATP production. At this point, the glucose molecule that originally entered cellular respiration has been completely oxidized. Chemical potential energy stored within the glucose molecule has been transferred to electron carriers or has been used to synthesize a few ATPs.

Citric Acid Cycle

Like the conversion of pyruvate to acetyl CoA, the citric acid cycle takes place in the matrix of mitochondria.This single pathway is called past different names: the
citric acid cycle
(for the commencement intermediate formed—citric acid, or citrate—when acetate joins to the oxaloacetate), the
TCA cycle
(since citric acid or citrate and isocitrate are tricarboxylic acids), and the
Krebs cycle, later on Hans Krebs, who commencement identified the steps in the pathway in the 1930s in pigeon flight muscles.

Almost all of the enzymes of the citric acid cycle are soluble, with the unmarried exception of the enzyme succinate dehydrogenase, which is embedded in the inner membrane of the mitochondrion. Different glycolysis, the citric acid bike is a closed loop: The last office of the pathway regenerates the compound used in the commencement step. The eight steps of the wheel are a series of redox, aridity, hydration, and decarboxylation reactions that produce two carbon dioxide molecules, i GTP/ATP, and reduced forms of NADH and FADH2
(Figure half-dozen). This is considered an aerobic pathway because the NADH and FADH2
produced must transfer their electrons to the next pathway in the arrangement, which will utilize oxygen. If this transfer does not occur, the oxidation steps of the citric acid wheel also do not occur. Note that the citric acid cycle produces very piffling ATP straight and does not directly swallow oxygen.

This illustration shows the eight steps of the citric acid cycle. In the first step, the acetyl group from acetyl CoA is transferred to a four-carbon oxaloacetate molecule to form a six-carbon citrate molecule. In the second step, citrate is rearranged to form isocitrate. In the third step, isocitrate is oxidized to α-ketoglutarate. In the process, one NADH is formed from NAD^{+} and one carbon dioxide is released. In the fourth step, α-ketoglutarate is oxidized and CoA is added, forming succinyl CoA. In the process, another NADH is formed and another carbon dioxide is released. In the fifth step, CoA is released from succinyl CoA, forming succinate. In the process, one GTP is formed, which is later converted into ATP. In the sixth step, succinate is oxidized to fumarate, and one FAD is reduced to FADH_{2}. In the seventh step, fumarate is converted into malate. In the eighth step, malate is oxidized to oxaloacetate, and another NADH is formed.

Figure 6. In the citric acid wheel, the acetyl group from acetyl CoA is attached to a four-carbon oxaloacetate molecule to form a vi-carbon citrate molecule. Through a series of steps, citrate is oxidized, releasing 2 carbon dioxide molecules for each acetyl group fed into the cycle. In the process, three NAD+
molecules are reduced to NADH, i FAD molecule is reduced to FADH2, and one ATP or GTP (depending on the jail cell type) is produced (by substrate-level phosphorylation). Because the final production of the citric acid wheel is also the first reactant, the bike runs continuously in the presence of sufficient reactants. (credit: modification of work by “Yikrazuul”/Wikimedia Commons)

Steps in the Citric Acid Bike

Stride 1. Prior to the start of the first step, pyruvate oxidation must occur. Then, the commencement footstep of the cycle begins: This is a condensation stride, combining the two-carbon acetyl grouping with a four-carbon oxaloacetate molecule to form a six-carbon molecule of
citrate. CoA is jump to a sulfhydryl grouping (-SH) and diffuses away to somewhen combine with some other acetyl grouping. This step is irreversible because it is highly exergonic. The rate of this reaction is controlled by negative feedback and the amount of ATP available. If ATP levels increase, the charge per unit of this reaction decreases. If ATP is in short supply, the rate increases.

Footstep 2. In step two, citrate loses ane water molecule and gains another as citrate is converted into its isomer,

Step iii. In step iii, isocitrate is oxidized, producing a five-carbon molecule,
α-ketoglutarate, together with a molecule of CO2
and ii electrons, which reduce NAD+
to NADH. This step is besides regulated by negative feedback from ATP and NADH, and a positive effect of ADP.

Steps 3 and iv. Steps 3 and 4 are both oxidation and decarboxylation steps, which release electrons that reduce NAD+
to NADH and release carboxyl groups that form CO2
molecules. α-Ketoglutarate is the product of step three, and a
group is the product of step 4. CoA binds the succinyl grouping to form succinyl CoA. The enzyme that catalyzes footstep four is regulated by feedback inhibition of ATP, succinyl CoA, and NADH.

Step 5. In step five, a phosphate group is substituted for coenzyme A, and a high-energy bail is formed. This free energy is used in substrate-level phosphorylation (during the conversion of the succinyl group to succinate) to course either guanine triphosphate (GTP) or ATP. In that location are two forms of the enzyme, chosen isoenzymes, for this step, depending upon the type of beast tissue in which they are found. One course is constitute in tissues that use large amounts of ATP, such equally heart and skeletal muscle. This form produces ATP. The second course of the enzyme is constitute in tissues that take a high number of anabolic pathways, such as liver. This form produces GTP. GTP is energetically equivalent to ATP; however, its utilize is more restricted. In particular, protein synthesis primarily uses GTP.

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Step half-dozen. Stride 6 is a dehydration process that converts
fumarate. Two hydrogen atoms are transferred to FAD, producing FADH2. The free energy contained in the electrons of these atoms is insufficient to reduce NAD+
but adequate to reduce FAD. Unlike NADH, this carrier remains attached to the enzyme and transfers the electrons to the electron transport chain directly. This process is fabricated possible by the localization of the enzyme catalyzing this pace inside the inner membrane of the mitochondrion.

Step vii. Water is added to fumarate during step seven, and
is produced. The final step in the citric acid bicycle regenerates
past oxidizing malate. Another molecule of NADH is produced in the process.

Products of the Citric Acrid Cycle

2 carbon atoms come into the citric acid cycle from each acetyl group, representing four out of the half-dozen carbons of i glucose molecule. Two carbon dioxide molecules are released on each plough of the bike; however, these exercise not necessarily comprise the most recently added carbon atoms. The two acetyl carbon atoms will eventually be released on later turns of the cycle; thus, all six carbon atoms from the original glucose molecule are eventually incorporated into carbon dioxide. Each turn of the cycle forms iii NADH molecules and one FADHtwo
molecule. These carriers will connect with the last portion of aerobic respiration to produce ATP molecules. One GTP or ATP is also made in each wheel. Several of the intermediate compounds in the citric acrid cycle can be used in synthesizing non-essential amino acids; therefore, the bike is amphibolic (both catabolic and anabolic).

In Summary: Citric Acid Bicycle

The citric acid cycle is a series of redox and decarboxylation reactions that remove high-energy electrons and carbon dioxide. The electrons temporarily stored in molecules of NADH and FADH2
are used to generate ATP in a subsequent pathway. Ane molecule of either GTP or ATP is produced by substrate-level phosphorylation on each plow of the bicycle. There is no comparing of the cyclic pathway with a linear one.

Electron Transport Chain

You have simply read about two pathways in cellular respiration—glycolysis and the citric acid cycle—that generate ATP. Withal, nearly of the ATP generated during the aerobic catabolism of glucose is not generated directly from these pathways. Rather, it is derived from a process that begins with moving electrons through a series of electron transporters that undergo redox reactions:
the electron transport concatenation. This causes hydrogen ions to accumulate within the matrix space. Therefore, a concentration gradient forms in which hydrogen ions diffuse out of the matrix space by passing through ATP synthase. The current of hydrogen ions powers the catalytic action of ATP synthase, which phosphorylates ADP, producing ATP.

Electron Ship Chain

This illustration shows the electron transport chain embedded in the inner mitochondrial membrane. The electron transport chain consists of four electron complexes. Complex I oxidizes NADH to NAD^^{+} and simultaneously pumps a proton across the membrane to the inter membrane space. The two electrons released from NADH are shuttled to coenzyme Q, then to complex III, to cytochrome c, to complex IV, then to molecular oxygen. In the process, two more protons are pumped across the membrane to the intermembrane space, and molecular oxygen is reduced to form water. Complex II removes two electrons from FADH_{2}, thereby forming FAD. The electrons are shuttled to coenzyme Q, then to complex III, cytochrome c, complex I, and molecular oxygen as in the case of NADH oxidation.

Figure vii. The electron transport concatenation is a series of electron transporters embedded in the inner mitochondrial membrane that shuttles electrons from NADH and FADH2
to molecular oxygen. In the process, protons are pumped from the mitochondrial matrix to the intermembrane space, and oxygen is reduced to grade water.

The electron transport concatenation (Figure 7) is the terminal component of aerobic respiration and is the only part of glucose metabolism that uses atmospheric oxygen. Oxygen continuously diffuses into plants; in animals, it enters the trunk through the respiratory arrangement. Electron ship is a series of redox reactions that resemble a relay race or bucket brigade in that electrons are passed rapidly from one component to the side by side, to the endpoint of the concatenation where the electrons reduce molecular oxygen, producing h2o. At that place are four complexes composed of proteins, labeled I through IV in Figure vii, and the aggregation of these 4 complexes, together with associated mobile, accessory electron carriers, is called the electron transport concatenation. The electron send concatenation is present in multiple copies in the inner mitochondrial membrane of eukaryotes and the plasma membrane of prokaryotes. Notation, however, that the electron transport chain of prokaryotes may not require oxygen as some live in anaerobic weather. The mutual characteristic of all electron transport chains is the presence of a proton pump to create a proton gradient across a membrane.

Complex I

To start, two electrons are carried to the first complex aboard NADH. This complex, labeled I, is composed of flavin mononucleotide (FMN) and an fe-sulfur (Fe-S)-containing protein. FMN, which is derived from vitamin Btwo, also called riboflavin, is one of several prosthetic groups or co-factors in the electron transport chain. Aprosthetic group
is a non-poly peptide molecule required for the action of a protein. Prosthetic groups are organic or inorganic, not-peptide molecules spring to a poly peptide that facilitate its function; prosthetic groups include co-enzymes, which are the prosthetic groups of enzymes. The enzyme in complex I is NADH dehydrogenase and is a very large poly peptide, containing 45 amino acrid chains. Complex I tin pump four hydrogen ions beyond the membrane from the matrix into the intermembrane space, and information technology is in this way that the hydrogen ion gradient is established and maintained betwixt the ii compartments separated past the inner mitochondrial membrane.

Q and Complex 2

Complex Ii directly receives FADHtwo, which does not pass through circuitous I. The chemical compound connecting the beginning and 2d complexes to the third isubiquinone
(Q). The Q molecule is lipid soluble and freely moves through the hydrophobic core of the membrane. Once it is reduced, (QHii), ubiquinone delivers its electrons to the next complex in the electron transport chain. Q receives the electrons derived from NADH from complex I and the electrons derived from FADH2
from complex Ii, including succinate dehydrogenase. This enzyme and FADH2
form a small complex that delivers electrons direct to the electron transport chain, bypassing the get-go complex. Since these electrons bypass and thus do non energize the proton pump in the first circuitous, fewer ATP molecules are made from the FADHtwo
electrons. The number of ATP molecules ultimately obtained is directly proportional to the number of protons pumped across the inner mitochondrial membrane.

Complex Three

The 3rd complex is equanimous of cytochrome b, another Iron-Due south protein, Rieske heart (2Fe-2S centre), and cytochrome c proteins; this complex is too called cytochrome oxidoreductase. Cytochrome proteins accept a prosthetic group of heme. The heme molecule is similar to the heme in hemoglobin, but it carries electrons, non oxygen. As a upshot, the fe ion at its core is reduced and oxidized as it passes the electrons, fluctuating between dissimilar oxidation states: Atomic number 26+
(reduced) and Fe+
(oxidized). The heme molecules in the cytochromes have slightly dissimilar characteristics due to the effects of the different proteins binding them, giving slightly unlike characteristics to each complex. Complex III pumps protons through the membrane and passes its electrons to cytochrome c for send to the fourth complex of proteins and enzymes (cytochrome c is the acceptor of electrons from Q; nevertheless, whereas Q carries pairs of electrons, cytochrome c can accept merely one at a fourth dimension).

Circuitous IV

The fourth complex is composed of cytochrome proteins c, a, and a3. This complex contains two heme groups (one in each of the ii cytochromes, a, and a3) and 3 copper ions (a pair of CuA
and ane CuB
in cytochrome a3). The cytochromes hold an oxygen molecule very tightly between the iron and copper ions until the oxygen is completely reduced. The reduced oxygen and so picks up 2 hydrogen ions from the surrounding medium to make water (H2O). The removal of the hydrogen ions from the organisation contributes to the ion gradient used in the procedure of chemiosmosis.


In chemiosmosis, the free free energy from the series of redox reactions merely described is used to pump hydrogen ions (protons) across the membrane. The uneven distribution of H+
ions beyond the membrane establishes both concentration and electric gradients (thus, an electrochemical gradient), owing to the hydrogen ions’ positive charge and their aggregation on i side of the membrane.

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If the membrane were open to diffusion by the hydrogen ions, the ions would tend to diffuse dorsum across into the matrix, driven by their electrochemical gradient. Call up that many ions cannot diffuse through the nonpolar regions of phospholipid membranes without the aid of ion channels. Similarly, hydrogen ions in the matrix space can merely pass through the inner mitochondrial membrane through an integral membrane protein called ATP synthase (Figure eight). This complex protein acts equally a tiny generator, turned by the strength of the hydrogen ions diffusing through it, down their electrochemical gradient. The turning of parts of this molecular car facilitates the add-on of a phosphate to ADP, forming ATP, using the potential energy of the hydrogen ion slope.

Practice Question

This illustration shows an ATP synthase enzyme embedded in the inner mitochondrial membrane. ATP synthase allows protons to move from an area of high concentration in the intermembrane space to an area of low concentration in the mitochondrial matrix. The energy derived from this exergonic process is used to synthesize ATP from ADP and inorganic phosphate.

Figure eight. ATP synthase is a circuitous, molecular machine that uses a proton (H+) gradient to form ATP from ADP and inorganic phosphate (Pi). (Credit: modification of work by Klaus Hoffmeier)

Dinitrophenol (DNP) is an uncoupler that makes the inner mitochondrial membrane leaky to protons. It was used until 1938 as a weight-loss drug. What issue would you expect DNP to have on the change in pH beyond the inner mitochondrial membrane? Why practice you think this might exist an effective weight-loss drug?

Later DNP poisoning, the electron transport chain can no longer form a proton gradient, and ATP synthase can no longer make ATP. DNP is an effective diet drug considering information technology uncouples ATP synthesis; in other words, after taking it, a person obtains less energy out of the food he or she eats. Interestingly, one of the worst side effects of this drug is hyperthermia, or overheating of the body. Since ATP cannot be formed, the free energy from electron ship is lost every bit heat.

Chemiosmosis (Figure 9) is used to generate 90 pct of the ATP made during aerobic glucose catabolism; it is likewise the method used in the low-cal reactions of photosynthesis to harness the free energy of sunlight in the process of photophosphorylation. Recollect that the product of ATP using the process of chemiosmosis in mitochondria is chosen oxidative phosphorylation. The overall result of these reactions is the production of ATP from the energy of the electrons removed from hydrogen atoms. These atoms were originally part of a glucose molecule. At the terminate of the pathway, the electrons are used to reduce an oxygen molecule to oxygen ions. The extra electrons on the oxygen concenter hydrogen ions (protons) from the surrounding medium, and h2o is formed.

Practice Question

This illustration shows the electron transport chain, the ATP synthase enzyme embedded in the inner mitochondrial membrane, and the citric acid cycle occurring in the mitochondrial matrix. The citric acid cycle feeds NADH and FADH_{2} to the electron transport chain. The electron transport chain oxidizes these substrates and, in the process, pumps protons into the intermembrane space. ATP synthase allows protons to leak back into the matrix and synthesizes ATP.

Figure 9. In oxidative phosphorylation, the pH gradient formed past the electron transport chain is used past ATP synthase to form ATP.

Cyanide inhibits cytochrome c oxidase, a component of the electron transport chain. If cyanide poisoning occurs, would you await the pH of the intermembrane space to increment or subtract? What effect would cyanide have on ATP synthesis?

Later cyanide poisoning, the electron transport chain tin can no longer pump electrons into the intermembrane space. The pH of the intermembrane space would increase, the pH gradient would decrease, and ATP synthesis would cease.

ATP Yield

The number of ATP molecules generated from the catabolism of glucose varies. For example, the number of hydrogen ions that the electron send chain complexes can pump through the membrane varies between species. Another source of variance stems from the shuttle of electrons across the membranes of the mitochondria. (The NADH generated from glycolysis cannot easily enter mitochondria.) Thus, electrons are picked upward on the inside of mitochondria by either NAD+
or FAD+. As you have learned earlier, these FAD+
molecules can transport fewer ions; consequently, fewer ATP molecules are generated when FAD+
acts equally a carrier. NAD+
is used equally the electron transporter in the liver and FAD+
acts in the encephalon.

Another gene that affects the yield of ATP molecules generated from glucose is the fact that intermediate compounds in these pathways are used for other purposes. Glucose catabolism connects with the pathways that build or intermission down all other biochemical compounds in cells, and the result is somewhat messier than the platonic situations described thus far. For example, sugars other than glucose are fed into the glycolytic pathway for energy extraction. Moreover, the v-carbon sugars that form nucleic acids are made from intermediates in glycolysis. Certain nonessential amino acids can be made from intermediates of both glycolysis and the citric acid cycle. Lipids, such as cholesterol and triglycerides, are likewise fabricated from intermediates in these pathways, and both amino acids and triglycerides are cleaved down for energy through these pathways. Overall, in living systems, these pathways of glucose catabolism excerpt well-nigh 34 percent of the energy contained in glucose.

In Summary: Electron Transport Chain

The electron transport concatenation is the portion of aerobic respiration that uses free oxygen every bit the final electron acceptor of the electrons removed from the intermediate compounds in glucose catabolism. The electron send chain is equanimous of four large, multiprotein complexes embedded in the inner mitochondrial membrane and two small diffusible electron carriers shuttling electrons betwixt them. The electrons are passed through a series of redox reactions, with a modest corporeality of complimentary energy used at iii points to transport hydrogen ions across a membrane. This process contributes to the slope used in chemiosmosis. The electrons passing through the electron transport chain gradually lose energy, High-energy electrons donated to the concatenation past either NADH or FADH2
complete the chain, equally low-energy electrons reduce oxygen molecules and form water. The level of free energy of the electrons drops from well-nigh threescore kcal/mol in NADH or 45 kcal/mol in FADH2
to nearly 0 kcal/mol in water. The cease products of the electron send concatenation are water and ATP. A number of intermediate compounds of the citric acid cycle tin can be diverted into the anabolism of other biochemical molecules, such as nonessential amino acids, sugars, and lipids. These aforementioned molecules can serve as energy sources for the glucose pathways.

Let’southward Review

Cellular respiration is a drove of three unique metabolic pathways: glycolysis, the citric acid cycle, and the electron transport concatenation. Glycolysis is an anaerobic process, while the other 2 pathways are aerobic. In order to move from glycolysis to the citric acid bicycle, pyruvate molecules (the output of glycolysis) must exist oxidized in a process called pyruvate oxidation.


Glycolysis is the first pathway in cellular respiration. This pathway is anaerobic and takes place in the cytoplasm of the cell. This pathway breaks downward 1 glucose molecule and produces 2 pyruvate molecules. There are two halves of glycolysis, with five steps in each half. The first half is known as the “energy requiring” steps. This half splits glucose, and uses up two ATP. If the concentration of pyruvate kinase is loftier enough, the 2d half of glycolysis can proceed. In the second half, the “free energy releasing: steps, iv molecules of ATP and 2 NADH are released. Glycolysis has a
net proceeds
of 2 ATP molecules and 2 NADH.

Some cells (eastward.g., mature mammalian red blood cells) cannot undergo aerobic respiration, and so glycolysis is their
source of ATP. However, most cells undergo pyruvate oxidation and go on to the other pathways of cellular respiration.

Pyruvate Oxidation

In eukaryotes, pyruvate oxidation takes identify in the mitochondria. Pyruvate oxidation tin only happen if oxygen is bachelor. In this process, the pyruvate created by glycolysis is oxidized. In this oxidation process, a carboxyl group is removed from pyruvate, creating acetyl groups, which compound with coenzyme A (CoA) to form acetyl CoA. This process too releases CO2.

Citric Acid Cycle

The citric acid cycle (also known as the Krebs cycle) is the second pathway in cellular respiration, and information technology too takes place in the mitochondria. The rate of the cycle is controlled by ATP concentration. When there is more ATP bachelor, the rate slows downwardly; when at that place is less ATP the charge per unit increases. This pathway is a airtight loop: the terminal step produces the compound needed for the commencement stride.

The citric acrid bicycle is considered an aerobic pathway because the NADH and FADHtwo
it produces act as temporary electron storage compounds, transferring their electrons to the next pathway (electron ship chain), which uses atmospheric oxygen. Each turn of the citric acid bicycle provides a
cyberspace gain
of CO2, one GTP or ATP, and three NADH and 1 FADH2.

Electron Ship Chain

Nearly ATP from glucose is generated in the electron transport chain. It is the just part of cellular respiration that directly consumes oxygen; however, in some prokaryotes, this is an anaerobic pathway. In eukaryotes, this pathway takes identify in the inner mitochondrial membrane. In prokaryotes information technology occurs in the plasma membrane.

The electron transport chain is fabricated up of 4 proteins forth the membrane and a proton pump. A cofactor shuttles electrons between proteins I–III. If NAD is depleted, skip I: FADHtwo
starts on II. In chemiosmosis, a proton pump takes hydrogens from inside mitochondria to the outside; this spins the “motor” and the phosphate groups adhere to that. The movement changes from ADP to ATP, creating 90% of ATP obtained from aerobic glucose catabolism.

Allow’s Practice

Now that you’ve reviewed cellular respiration, this exercise activity will help you meet how well you know cellular respiration:

Click here for a text-but version of the activity.

Check Your Understanding

Answer the question(s) below to encounter how well you understand the topics covered in the previous department. This short quiz doesnon count toward your grade in the grade, and y’all can retake information technology an unlimited number of times.

Use this quiz to bank check your understanding and decide whether to (one) written report the previous section further or (2) move on to the adjacent section.

During Which Process is Water Produced

Source: https://courses.lumenlearning.com/suny-wmopen-biology1/chapter/cellular-respiration/