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Introduction to electron transport chains and respiration*# - Biology

Introduction to electron transport chains and respiration*# - Biology


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Respiration Overview

In the next few modules we start to learn about the process of respiration and the roles that electron transport chains play in this process. A definition of the word "respiration" that most people are familiar with is "the act of breathing". When

we

breath, air including molecular oxygen is brought into our lungs from outside of the body, the oxygen then becomes reduced, and waste products, including the reduced oxygen in the form of water, are exhaled. More generically, some reactant comes into the organism and then gets reduced and leaves the body as a waste product. This generic idea, in a nutshell, can be generally applied across biology and oxygen need not always be the compound that brought in, reduced, and dumped as waste. The electrons that are dumped on oxygen or other compounds more generally known as "terminal electron acceptors." The molecules from which the electrons that are dumped onto terminal electron acceptors originate, vary greatly across biology (we have looked at one possible source - the reduced carbon-based molecule glucose).

In between the original electron source and the terminal electron acceptor are a series of biochemical reactions involving at least one redox reaction. These redox reactions harvest energy for the cell by coupling exergonic redox reaction to an energy-requiring reaction in the cell. In respiration, a special set of enzymes carry out a linked series of redox reactions that ultimately transfer electrons to the terminal electron acceptor. These "chains" of redox enzymes and electron carriers are called electron transport chains (ETC). ETCs are therefore the portion of respiration that use an electron acceptor (usually brought in from outside of the cell) as the final/terminal acceptor for the electrons that were removed from the intermediate compounds in catabolism. In aerobically respiring eukaryotic cells the ETC is composed of four large, multiprotein complexes embedded in the inner mitochondrial membrane and two small diffusible electron carriers shuttling electrons between them. The electrons are passed from enzyme to enzyme through a series of redox reactions. These reactions are couple the exergonic redox transfers to the endergonic transport of hydrogen ions across the membrane. This process contributes to the creation of a transmembrane electrochemical gradient. The electrons passing through the ETC gradually lose potential energy up until the point they are deposited on the terminal electron acceptor which is typically removed as waste from the cell. When oxygen as the final electron acceptor, the free energy difference of this multistep redox process is ~ -60 kcal/mol when NADH donates electrons or 45 kcal/mol when FADH2 donates.

Introduction to redox, oxidative phosphorylation and Electron Transport Chains

In prior modules we discussed the general concept of redox reactions in biology and introduced the Electron Tower, a tool to help you understand redox chemistry and to estimate the direction and magnitude of potential energy differences for various redox couples. In later modules, substrate level phosphorylation and fermentation were discussed and we saw how exergonic redox reactions could be directly coupled by enzymes to the endergonic synthesis of ATP. These processes are hypothesized to be one of the oldest forms of energy production used by cells. In this section we discuss the next evolutionary advancement in cellular energy metabolism, oxidative phosphorylation. First and foremost, oxidative phosphorylation does not imply the use of oxygen, it can, but it does not have to use oxygen. It is called oxidative phosphorylation because it relies on redox reactions to generate a electrochemical transmembrane potential that can then be used by the cell to do work.

A quick summary of Electron Transport Chains

The ETC begins with the addition of electrons, donated from NADH, FADH2 or other reduced compounds. These electrons move through a series of electron transporters, enzymes that are embedded in a membrane, or carriers that undergo redox reactions. The free energy transferred from these exergonic redox reactions is often coupled to the endergonic movement of protons across a membrane. Since the membrane is an effective barrier to charged species, this pumping results in an unequal accumulation of protons on either side of the membrane. This in turn "polarizes" or "charges" the membrane, with a net positive (protons) on one side of the membrane and a negative charge on the other side of the membrane. The separation of charge creates an electrical potential. In addition, the accumulation of protons also causes a pH gradient known as a chemical potentialacross the membrane. Together these two gradients (electrical and chemical) are called an electro-chemical gradient.

Review: The Electron Tower

Since redox chemistry is so central to the topic we begin with a quick review of the table of reduction potential - sometimes called the "redox tower" or "electron tower". You may hear your instructors use these terms interchangeably. As we discussed in previous modules, all kinds of compounds can participate in biological redox reactions. Making sense of all of this information and ranking potential redox pairs can be confusing. A tool has been developed to rate redox half reactions based on their reduction potentials or E0' values. Whether a particular compound can act as an electron donor (reductant) or electron acceptor (oxidant) depends on what other compound it is interacting with. The redox tower ranks a variety of common compounds (their half reactions) from most negative E0', compounds that readily get rid of electrons, to the most positive E0', compounds most likely to accept electrons. The tower organizes these half reactions based on the ability of electrons to accept electrons. In addition, in many redox towers each half reaction is written by convention with the oxidized form on the left followed by the reduced form to its right. The two forms may be either separated by a slash, for example the half reaction for the reduction of NAD+ to NADH is written: NAD+/NADH + 2e-, or by separate columns. An electron tower is shown below.

Figure 1: A common biological "redox tower"

Note

Use the redox tower above as a reference guide to orient you as to the reduction potential of the various compounds in the ETC. redox reactions may be either exergonic or endergonic depending on the relative redox potentials of the donor and acceptor. Also remember there are many different ways of looking at this conceptually; this type of redox tower is just one way.

Note: Language shortcuts reappear:

In the redox table above some entries seem to be written in unconventional ways. For instance Cytochrome cox/red. There only appears to be one form listed. Why? This is another example of language shortcuts (likely because someone was too lazy to write cytochrome twice) that can be confusing - particularly to students. The notation above could be rewritten as Cytochrome cox/Cytochrome cred to indicate that the cytochrome c protein can exist in either and oxidized state Cytochrome cox or reduced state Cytochrome cred.

Review Redox Tower Video

For a short video on how to use the redox tower in redox problems click here. This video was made by Dr. Easlon for Bis2A students.

Using the redox tower: A tool to help understand electron transport chains

By convention the tower half reactions are written with the oxidized form of the compound on the left and the reduced form on the right. Notice that compounds such as glucose and hydrogen gas are excellent electron donors and have very low reduction potentials E0'. Compounds, such as oxygen and nitrite, whose half reactions have relatively high positive reduction potentials (E0') generally make good electron acceptors are found at the opposite end of the table.

Example: Menaquinone

Let's look at menaquinoneox/red. This compound sits in the middle of the redox tower with an half-reaction E0' value of -0.074 eV. Menaquinoneox can spontaneously (ΔG<0) accept electrons from reduced forms of compounds with lower half-reaction E0'. Such transfers form menaquinonered and the oxidized form of the original electron donor. In the table above, examples of compounds that could act as electron donors to menaquinone include FADH2, an E0' value of -0.22, or NADH, with an E0' value of -0.32 eV. Remember the reduced forms are on the right hand side of the red/ox pair.

Once menaquinone has been reduced, it can now spontaneously (ΔG<0) donate electrons to any compound with a higher half-reaction E0' value. Possible electron acceptors include cytochrome box with an E0' value of 0.035 eV; or ubiquinoneox with an E0' of 0.11 eV. Remember that the oxidized forms lie on the left side of the half reaction.


Introduction to electron transport chains and respiration*# - Biology

ELECTRON TRANSPORT CHAIN AND ATP SYNTHESIS

An organism that lives in the presence of oxygen can extract a deal of energy from glucose by running it through two main metabolic pathway.
1.Glycolysis
2. Cellular respiration
By the end of these pathways, glucose has been completely oxidized and the cell has gained 36 molecules of ATP – a versatile energy carrier that fuels most kind of cellular work.

Glycolysis is a central pathway for the catabolism of carbohydrates in which the six-carbon sugars are split to three-carbon compounds with subsequent release of energy used to transform ADP to ATP.
Before glucose can be converted into ATP, it has be broken down into two pyruvate molecules. This process is known as glycolysis.
In glycolysis enzyme in the cystol splits into 2 molecules of pyruvate.

Cellular respiration is the process by which the chemical energy is released and partially captured in the form of ATP.
Cellular respiration occurs in the mitochondria which is the power house of the cell.
Cellular respiration occurs in three main phases.
1.Pyruvate oxidation.
2.Citric acid cycle.
3.Respiratory chain.

Pyruvate oxidation occurs in the inner membrane of the mitochondria.
This process is a source of acetyl-CoA molecules for the citric acid cycle.
Pyruvate oxidation occurs in three easy steps.
1. The pyruvate is oxidized (it goes from 3C to 2C acetyl . CO2 is released as a result).
2.NAD+ is reduced to NADH
3. The pyruvate dehydrogenase complex attaches CoA to acetyl.
The total energy yield for this process is 2NADH's.

This is the final phase of cellular respiration in which the cell make the bulk of its ATP. Although a few ATP molecules are formed in the glycolysis and the early stage of cellular respiration.
In glycolysis and cellular respiration, a cell breaks down a molecule of glucose and use its energy to form 36 molecule of ATP, an important cellular energy source.
Most of this ATP is generated in mitochondria during the final phase of cellular respiration, during which electron transport and ATP synthesis takes place.

ELECTRON TRANSPORT CHAIN AND ATP SYNTHESIS:

ETC and ATP synthesis takes in the mitochondria.
Mitochondria have two membranes:
1.Inner membrane
2.Outer membrane
These process takes place in the inner membrane of mitochondria.
INNER MEMBRANE: Contains the components required for the electron transport and ATP synthesis.
The two membranes are seperated by an acidic intermembrane space full of H2 ions (protons).
In the cellular respiration ,glucose is converted to CO2 and its H2 atoms are stripped off and donated to the molecules NAD+ and FAD to form NADH+H+ and FADH2.


9.1. Energy in Living Organisms

Energy production within a cell involves many coordinated chemical pathways. Most of these pathways are combinations of oxidation and reduction reactions. Oxidation and reduction occur in tandem. An oxidation reaction strips an electron from an atom in a compound, and the addition of this electron to another compound is a reduction reaction. Because oxidation and reduction usually occur together, these pairs of reactions are called oxidation reduction reactions, or redox reactions.

Electrons and Energy

The removal of an electron from a molecule, oxidizing it, results in a decrease in potential energy in the oxidized compound. The electron (sometimes as part of a hydrogen atom), does not remain unbonded, however, in the cytoplasm of a cell. Rather, the electron is shifted to a second compound, reducing the second compound. The shift of an electron from one compound to another removes some potential energy from the first compound (the oxidized compound) and increases the potential energy of the second compound (the reduced compound). The transfer of electrons between molecules is important because most of the energy stored in atoms and used to fuel cell functions is in the form of high-energy electrons. The transfer of energy in the form of electrons allows the cell to transfer and use energy in an incremental fashion—in small packages rather than in a single, destructive burst. This chapter focuses on the extraction of energy from food you will see that as you track the path of the transfers, you are tracking the path of electrons moving through metabolic pathways.

Electron Carriers

In living systems, a small class of compounds functions as electron shuttles: They bind and carry high-energy electrons between compounds in pathways. The principal electron carriers we will consider are derived from the B vitamin group and are derivatives of nucleotides. These compounds can be easily reduced (that is, they accept electrons) or oxidized (they lose electrons). Nicotinamide adenine dinucleotide (NAD) (Figure 9.2) is derived from vitamin B3, niacin. NAD + is the oxidized form of the molecule NADH is the reduced form of the molecule after it has accepted two electrons and a proton (which together are the equivalent of a hydrogen atom with an extra electron).

When electrons are added to a compound, they are reduced. A compound that reduces another is called a reducing agent. In the above equation, RH is a reducing agent, and NAD + is reduced to NADH. When electrons are removed from compound, it oxidized. A compound that oxidizes another is called an oxidizing agent. In the above equation, NAD + is an oxidizing agent, and RH is oxidized to R.

Similarly, flavin adenine dinucleotide (FAD + ) is derived from vitamin B2, also called riboflavin. Its reduced form is FADH2. A second variation of NAD, NADP, contains an extra phosphate group. Both NAD + and FAD + are extensively used in energy extraction from sugars, and NADP plays an important role in anabolic reactions and photosynthesis.

ATP in Living Systems

A living cell cannot store significant amounts of free energy. Excess free energy would result in an increase of heat in the cell, which would result in excessive thermal motion that could damage and then destroy the cell. Rather, a cell must be able to handle that energy in a way that enables the cell to store energy safely and release it for use only as needed. Living cells accomplish this by using the compound adenosine triphosphate (ATP). ATP is often called the “energy currency” of the cell, and, like currency, this versatile compound can be used to fill any energy need of the cell. How? It functions similarly to a rechargeable battery.

When ATP is broken down, usually by the removal of its terminal phosphate group, energy is released. The energy is used to do work by the cell, usually by the released phosphate binding to another molecule, activating it. For example, in the mechanical work of muscle contraction, ATP supplies the energy to move the contractile muscle proteins. Recall the active transport work of the sodium-potassium pump in cell membranes. ATP alters the structure of the integral protein that functions as the pump, changing its affinity for sodium and potassium. In this way, the cell performs work, pumping ions against their electrochemical gradients.

ATP Structure and Function

At the heart of ATP is a molecule of adenosine monophosphate (AMP), which is composed of an adenine molecule bonded to a ribose molecule and to a single phosphate group (Figure 9.3). Ribose is a five-carbon sugar found in RNA, and AMP is one of the nucleotides in RNA. The addition of a second phosphate group to this core molecule results in the formation of adenosine diphosphate (ADP) the addition of a third phosphate group forms adenosine triphosphate (ATP).

The addition of a phosphate group to a molecule requires energy. Phosphate groups are negatively charged and thus repel one another when they are arranged in series, as they are in ADP and ATP. This repulsion makes the ADP and ATP molecules inherently unstable. The release of one or two phosphate groups from ATP, a process called dephosphorylation, releases energy.

ADP-ATP Cycle

Hydrolysis is the process of breaking complex macromolecules apart. During hydrolysis, water is split, or lysed, and the resulting hydrogen atom (H + ) and a hydroxyl group (OH – ) are added to the larger molecule. The hydrolysis of ATP produces ADP, together with an inorganic phosphate ion (Pi), and the release of free energy. To carry out life processes, ATP is continuously broken down into ADP, and like a rechargeable battery, ADP is continuously regenerated into ATP by the reattachment of a third phosphate group (Figure 9.4). Water, which was broken down into its hydrogen atom and hydroxyl group during ATP hydrolysis, is regenerated when a third phosphate is added to the ADP molecule, reforming ATP.

Obviously, energy must be infused into the system to regenerate ATP. Where does this energy come from? In nearly every living thing on earth, the energy comes from the metabolism of glucose. In this way, ATP is a direct link between the limited set of exergonic pathways of glucose catabolism and the multitude of endergonic pathways that power living cells.


Uncoupling of the Electron Transport Chain Compromises Mitochondrial Oxidative Phosphorylation and Exacerbates Stroke Outcomes

Objective: Mitochondrial dysfunction is known to be implicated in stroke, but the complex mechanisms of stroke have led to few stroke therapies. The present study to disrupted mitochondrial oxidative phosphorylation through a known electron transport chain (ETC) uncoupler, Carbonyl cyanide-4 (trifluoromethoxy) phenylhydrazone (FCCP). Analyzing the resulting neurological deficits as well as infarct volume could help determine the role of mitochondria in stroke outcome and determine whether uncoupling the ETC could potentially be a strategy for new stroke therapies. The objective of this study was to determine the effects of uncoupling electron flow on mitochondrial oxidative phosphorylation and stroke infarction.

Methods: Cerebral endovascular cells (CECs) were treated with various concentrations of FCCP, and bioenergetics were measured. For the stroke mouse model, FCCP (1 mg/kg, i.p) or vehicle was administered followed by 1-hour transient middle cerebral artery occlusion (tMCAO). Infarct volume was measured after a 23-hour reperfusion, and triphenyl tetrazolium chloride (TTC) staining was used to assess infarct volume.

Results: FCCP significantly decreased basal respiration, ATP turnover, maximal respiration, and spare capacity when the concentration of FCCP was greater than 1000 nM. The mice pretreated with FCCP had a significantly increased infarct volume within the cortex, striatum, and total hemisphere. Mice receiving FCCP had a significantly increased neurological deficit score compared to the vehicle.

Conclusions: FCCP compromised mitochondrial oxidative phosphorylation in CECs in a dose-dependent manner. Uncoupling the electron transport chain with FCCP prior to tMCAO exacerbated stroke infarction in mice.

Keywords: Blood-Brain Barrier (BBB) Carbonyl Cyanide-4 (trifluoromethoxy) Phenylhydrazone (FCCP) Cerebral Endovascular Cells (CECs) Electron Transport Chain (ETC) Ischemia Transient Middle Cerebral Artery Occlusion (tMCAO) Triphenyl Tetrazolium Chloride (TTC).


Cell Respiration

Adenosine triphosphate (ATP)

  • Energy carrying molecule used by cells to fuel their cellular processes
  • ATP is composed of an adenine base, ribose sugar, & 3 phosphate (PO4) groups
  • The PO4 bonds are high-energy bonds that require energy to be made & release energy when broken
  • ATP is made & used continuously by cells
  • Every minute all of an organism’s ATP is recycled
  • Phosphorylation refers to the chemical reactions that make ATP by adding Pi to ADP ADP + Pi + energy « ATP + H2O
  • Enzymes (ATP synthetase& ATPase) help break & reform these high energy PO4 bonds in a process called substrate-level phosphorylation
  • When the high-energy phosphate bond is broken, it releases energy, a free phosphate group, & adenosine diphosphate (ADP)

Enzymes in Metabolic Pathways:

  • Biological catalysts
  • Speeds up chemical reactions
  • Lowers the amount of activation energy needed by weakening existing bonds in substrates

  • Highly specific protein molecules
  • Have anarea called the active site where substrates temporarily join

enzyme substrate complex

NADH: A second energy carrying molecule in the mitochondria produces 3 ATP

FADH2: A third energy carrying molecule in the mitochondria produces 2 ATP

  • Has outer smooth, outer membrane & folded inner membrane
  • Folds are called cristae
  • Space inside cristae is called the matrix & contains DNA & ribosomes
  • Site of aerobic respiration
  • Krebs cycle takes place in matrix
  • Electron Transport Chain takes place in cristae

Cellular Respiration Overview:

C6H12O6 + 6O2 —–> 6CO2 + 6H20 + energy (heat and ATP)

  • Controlled release of energy from organic molecules (most often glucose)
  • Glucose is oxidized (loses e-) & oxygen is reduced (gains e-)
  • The carbon atoms of glucose (C6H12O6) are released as CO2
  • Generates ATP (adenosine triphosphate)
  • The energy in one glucose molecule may be used to produce 36 ATP
  • Involves a series of 3 reactions — Glycolysis, Kreb’s Cycle, & Electron Transport Chain

  • Occurs in the cytoplasm
  • Summary of the steps of Glycolysis:


Electron Transport Chain Steps Explained with Diagram

The electron transport chain is an essential metabolic pathway that produces energy by carrying out a series of redox reactions. This BiologyWise article provides a simple explanation of this pathway.

The electron transport chain is an essential metabolic pathway that produces energy by carrying out a series of redox reactions. This BiologyWise article provides a simple explanation of this pathway.

Did You Know?

One cycle of the electron transport chain yields about 30 molecules of ATP (Adenosine triphosphate) as compared to the 2 molecules produced each via glycolysis and the citric acid cycle.

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The electron transport chain is made up of a series of spatially separated enzyme complexes that transfer electrons from electron donors to electron receptors via sets of redox reactions. This is also accompanied by a transfer of protons (H + ions) across the membrane. This leads to the development of an electrochemical proton gradient across the membrane that activates the ATP synthase proton pump, thereby, driving the generation of ATP molecules (energy). The cycle ends by the absorption of electrons by oxygen molecules.

In eukaryotic organisms, the electron transport chain is found embedded in the inner membrane of the mitochondria, in bacteria it is found in the cell membrane, and in case of plant cells, it is present in the thylakoid membrane of the chloroplasts.

In chloroplasts, photons from light are used produce the proton gradient whereas, in the mitochondria and bacterial cells, the conversions occurring in the enzyme complexes, generate the proton gradient.

Overview of Electron Transport Chain

This pathway is the most efficient method of producing energy. The initial substrates for this cycle are the end products obtained from other pathways. Pyruvate, obtained from glycolysis, is taken up by the mitochondria, where it is oxidized via the Krebs/citric acid cycle. The substrates required for the pathway are NADH (nicotinamide adenine dinucleotide), succinate, and molecular oxygen.

NADH acts as the first electron donor, and gets oxidized to NAD + by enzyme complex I, accompanied by the release of a proton out of the matrix. The electron is then transported to complex II, which brings about the conversion of succinate to fumarate. Molecular oxygen (O2) acts as an electron acceptor in complex IV, and gets converted to a water molecule (H2O). Each enzyme complex carries out the transport of electrons accompanied by the release of protons in the intermembrane space.

The accumulation of protons outside the membrane gives rise to a proton gradient. This high concentration of protons initiates the process of chemiosmosis, and activates the ATP synthase complex. Chemiosmosis refers to the generation of an electrical as well as a pH potential across a membrane due to large difference in proton concentrations. The activated ATP synthase utilizes this potential, and acts as a proton pump to restore concentration balance. While pumping the proton back into the matrix, it also conducts the phosphorylation of ADP (Adenosine Diphosphate) to yield ATP molecules.

Enzyme Complexes of Electron Transport Chain

Complex I – NADH-coenzyme Q oxidoreductase
The reduced coenzyme NADH binds to this complex, and functions to reduce coenzyme Q10. This reaction donates electrons, which are then transferred through this complex using FMN (Flavin mononucleotide) and a series of Fe-S (Iron-sulpur) clusters. The transport of these electrons brings about the transfer of protons across the membrane into the intermembrane space.

Complex II – Succinate-Q oxidoreductase
This complex acts on the succinate produced by the citric acid cycle, and converts it to fumarate. This reaction is driven by the reduction and oxidation of FAD (Flavin adenine dinucleotide) along with the help of a series of Fe-S clusters. These reactions also drive the redox reactions of quinone. These sets of reactions help in transporting the electrons to the third enzyme complex.

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Complex III – Q-cytochrome c oxidoreductase
This complex oxidizes ubiquinol and also reduces two molecules of cytochrome-c. The electron is transported via these reactions onto complex IV accompanied by the release of protons.

Complex IV – ytochrome c oxidase
The received electron is received by a molecular oxygen to yield a water molecule. This conversion occurs in the presence of Copper (Cu) ions, and drives the oxidation of the reduced cytochrome-c. Protons are pumped out during the course of this reaction.

ATP Synthase
The protons produced from the initial oxidation of the NADH molecule, and their presence in the intermembrane space gives rise to a potential gradient. It is utilized by this complex to transport the protons back into the matrix. The transport itself also generates energy that is used to achieve phosphorylation of the ADP molecules to form ATP.

Any anomalies or defects in any of the components that constitute the electron transport chain leads to the development of a vast array of developmental, neurological, and physical disorders.

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Cell Respiration Study Guide B1

1. Most eukaryotic cells produce only about ___________ ATP Molecules per Glucose Molecule.
2. What is the process by which glucose is converted to pyruvic acid?

3. At the begining of aerobic respiration, pyruvic acid bonds to a molecule called _______________________ to form Acetyl CoA.
4. The breakdown of pyruvic acid in the presence of oxygen is called ___________________ _______________________.

5. With every completion of the Krebs Cycle, how many ATP Molecules are made?
6. What is the waste product of the Krebs Cycle?

7. The conversion of pyruvic acid to carbon dioxide and ethanol is called _____________________ _______________________.

8. The release of energy from food molecules in the absence of oxygen is _______________________ _____________________________.

9. What is the byproduct of the electron transport Chain?

10. How efficient is Anaerobic Respiration? __________% Aerobic Respiration?

11. What is the first pathway of cellular respiration called?

12.What is the location of Glycolysis?

13. What is the scientific unit of Energy?

14. What do you call cellular respiration in the presence of oxygen?

15. Yeast produces ___________________ and _________________ in the process known as _____________________ _______________________.

16. In cellular respiration, glycolysis proceeds the _________________.

17. In cellular respiration, more energy is transferred in the ________________ than in any other step.

18. Glucose molecules are converted into ________________ ______________ molecules in the process of glycolysis.

19. What is the location of the electron transport chain in prokaryotes?

20. The processes of glycolysis and the anaerobic pathways is called _________________.

21. What is the product of acetyl CoA and oxaloacetic acid?

22. What molecule is the electron acceptor of glycolysis?

23. The breakdown of organic compounds to produce ATP is known as __________ __________.
24. Glycolysis begins with glucose and produces _____________ ______________.

25. An important molecule generated by both lactic acid and alcoholic fermentation is ____________.

26. In the first step of aerobic respiration, pyruvic acid from glycolysis produces CO2, NADH, H+, and ____________ ______________.

27. The electron transport chain is driven by two products of the Krebs Cycle – ______________________ and ___________________________.
28. What happens to electrons as they are transported along the electron transport chain?

29. The energy efficiency of aerobic respiration (including glycolysis) is approximately ______________ ____________________.

30. Where in the mitochondria do the reactions of the Krebs cycle occur?

31. Where in the mitochondria is the electron transport chain located?

32. In alcoholic fermentation, ethyl alcohol is produced from ____________________.

33. _______________, and ________________ supply electrons and protons to the electron transport chain.

34. The fourth step of glycolysis yields four ATP molecules, but the net yield is only two ATP molecules. Explain this discrepancy.

35. Under what conditions would cells in your body undergo lactic-acid fermentation?

36. Glycolysis produces only 3.5% of the energy that would be produced if an equal quantity of glucose were completely oxidized. What has happened to the remaining energy in the glucose?

37. Explain the role of oxaloacetic acid with respect to the cyclical nature of the Krebs cycle.

38 What happens to electrons that accumulate at the end of the electron transport chain?

39. Where in the mitochondrion do protons accumulate, and what is the source of the protons?


11.7.1 Regulatory Mechanisms

A variety of mechanisms is used to control cellular respiration. Some type of control exists at each stage of glucose metabolism. Access of glucose to the cell can be regulated using the GLUT (GLUcose Transport) proteins that transport glucose (Figure 11.15). Different forms of the GLUT protein control passage of glucose into the cells of specific tissues.

Figure 11.16 GLUT4 is a glucose transporter that is stored in vesicles. A cascade of events that occurs upon insulin binding to a receptor in the plasma membrane causes GLUT4-containing vesicles to fuse with the plasma membrane so that glucose may be transported into the cell.

Some reactions are controlled by having two different enzymes—one each for the two directions of a reversible reaction. Reactions that are catalyzed by only one enzyme can go to equilibrium, stalling the reaction. In contrast, if two different enzymes (each specific for a given direction) are necessary for a reversible reaction, the opportunity to control the rate of the reaction increases, and equilibrium is not reached. A number of enzymes involved in each of the pathways—usually the first enzyme of the pathway—are controlled by allosteric regulation. The molecules most commonly used in this capacity are the nucleotides ATP, ADP, AMP, NAD+, and NADH. These allosteric regulators may increase or decrease enzyme activity, depending on the prevailing conditions.

Regulation of Glycolysis

Step 1: Hexokinase

The control of glycolysis begins with the first enzyme in the pathway, hexokinase. This enzyme catalyzes the phosphorylation of glucose, which helps to prepare the compound for cleavage in a later step. The presence of the negatively charged phosphate in the molecule also prevents the sugar from leaving the cell. When hexokinase is inhibited, glucose diffuses out of the cell and does not become a substrate for the respiration pathways in that tissue. The product of the hexokinase reaction is glucose-6-phosphate, which accumulates when a later enzyme, phosphofructokinase, is inhibited.

Step 3: Phosphofructokinase

Phosphofructokinase is the main enzyme controlled in glycolysis. High levels of ATP, citrate, or a lower pH decrease the enzyme’s activity. An increase in citrate concentration can occur because of a blockage in the citric acid cycle.

Step 10: Pyruvate kinase

The last step in glycolysis is catalyzed by pyruvate kinase. The pyruvate produced can proceed to be catabolized or converted into the amino acid alanine. If no more energy is needed and alanine is in adequate supply, the enzyme is inhibited. The enzyme’s activity is increased when fructose-1,6-bisphosphate levels increase. (Recall that fructose-1,6-bisphosphate is an intermediate in the first half of glycolysis.) The regulation of pyruvate kinase involves phosphorylation by a kinase (pyruvate kinase kinase), resulting in a less-active enzyme. Dephosphorylation by a phosphatase reactivates it. Pyruvate kinase is also regulated by ATP (a negative allosteric effect).

If more energy is needed, more pyruvate will be converted into acetyl CoA through the action of pyruvate dehydrogenase. If either acetyl groups or NADH accumulate, there is less need for the reaction and the rate decreases. Pyruvate dehydrogenase is also regulated by phosphorylation: A kinase phosphorylates it to form an inactive enzyme, and a phosphatase reactivates it. The kinase and the phosphatase are also regulated.

Regulation of the Citric Acid Cycle

The citric acid cycle is controlled through the enzymes that catalyze the reactions that make the first two molecules of NADH. These enzymes are isocitrate dehydrogenase and α-ketoglutarate dehydrogenase. When adequate ATP and NADH levels are available, the rates of these reactions decrease. When more ATP is needed, as reflected in rising ADP levels, the rate increases. α-Ketoglutarate dehydrogenase will also be affected by the levels of succinyl CoA—a subsequent intermediate in the cycle—causing a decrease in activity. A decrease in the rate of operation of the pathway at this point is not necessarily negative, as the increased levels of the α-ketoglutarate not used by the citric acid cycle can be used by the cell for amino acid (glutamate) synthesis.

Regulation of the Electron Transport Chain

Specific enzymes of the electron transport chain are unaffected by feedback inhibition, but the rate of electron transport through the pathway is affected by the levels of ADP and ATP. Greater ATP consumption by a cell is indicated by a buildup of ADP. As ATP usage decreases, the concentration of ADP decreases and ATP begins to build up in the cell. This change is the relative concentration of ADP to ATP triggers the cell to slow down the electron transport chain.

Table 11.5 Summary of feedback controls in cellular respiration.


Watch the video: Electron Transport Chain (September 2022).


Comments:

  1. Aiekin

    Yes, faster if she already left !!

  2. Whelan

    Presumably.



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