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  • This is Chapter 25, Module 2, Glycolysis, the TCA Cycle,

  • and the Electron Transport System.

  • The learning objectives of this module

  • are one, describe the basic steps in glycolysis, the TCA

  • cycle, and the electron transport system, and two,

  • summarize the energy yields of glycolysis

  • and cellular respiration.

  • Most cells generate ATP by breaking down carbohydrates,

  • especially glucose.

  • When cells consume oxygen, they can break down glucose

  • into carbon dioxide and water.

  • And by doing so, they provide a typical body cell

  • with 36 molecules of ATP.

  • The breakdown of glucose takes place

  • in a series of small steps.

  • The first steps begin in the cytosol of the cell.

  • Here in the cytosol, glucose is broken down

  • into smaller molecules in a process called glycolysis.

  • Glycolysis does not require oxygen.

  • And so the process is said to be anaerobic,

  • which means no oxygen.

  • The molecules produced through glycolysis

  • are small enough to be absorbed by the mitochondria.

  • Once these smaller molecules are in the mitochondria,

  • they will be involved in reactions that will use oxygen

  • and are considered aerobic.

  • The reactions inside the mitochondria

  • will create most of the ATP that the cell needs to function.

  • These reactions are collectively called aerobic metabolism

  • or cellular respiration.

  • Let's start with glycolysis.

  • Glycolysis is the breakdown of glucose to pyruvic acid.

  • In this process, glucose, which is a six carbon compound,

  • will break into two three carbon compounds of pyruvic acid.

  • Pyruvic acid then loses a hydrogen atom

  • to become pyruvate.

  • During the process of glycolysis,

  • the cells must use two ATP molecules

  • to complete this process but will also create four ATP

  • through phosphorylation.

  • Since glycolysis uses two ATP but creates four ATP,

  • the net ATP production during glycolysis

  • is two ATP for every glucose molecule catabolized.

  • This amount of ATP is not enough for the cell to survive.

  • But the pyruvate molecules hold much energy

  • that will be used in the mitochondria

  • to create more ATP.

  • In addition to the ATP that are produced,

  • the process of glycolysis will lose two hydrogen atoms,

  • which will be transferred to a hydrogen carrier called NAD.

  • When the two hydrogen atoms are transferred to NAD,

  • the molecule then becomes NADH.

  • The hydrogen will be carried to the electron transport

  • system in the mitochondria.

  • We will discuss later what happens

  • to the hydrogen in the NADH molecule.

  • Glycolysis is anaerobic because it does not require oxygen

  • and it occurs in the cytosol of the cell.

  • If oxygen is present in the cell,

  • the three carbon pyruvate molecules

  • will enter the mitochondria and continue being catabolized.

  • Once in the mitochondria, each pyruvate molecule

  • will lose a carbon in a process called decarboxylation.

  • This carbon will join with oxygen

  • to become carbon dioxide.

  • The carbon dioxide will diffuse out of the cell

  • and into the bloodstream, where it will be carried to the lungs

  • and be breathed out during expiration.

  • The new molecule formed is now a two carbon molecule

  • called acetyl coenzyme A. Acetyl coenzyme A, or acetyl CoA,

  • will participate in reactions in the matrix of the mitochondria.

  • An enzyme called coenzyme A, or CoA,

  • will carry acetyl CoA into the citric acid cycle in the matrix

  • of the mitochondria.

  • Recall that mitochondria are organelles

  • with double membranes.

  • That is, the mitochondria has two layers

  • of membrane, the inner membrane and the outer membrane.

  • The space in between the inner membrane and the outer membrane

  • is called the intermembrane space.

  • The space inside the inner membrane is called the matrix.

  • The citric acid cycle will take place

  • in the matrix of the mitochondria.

  • The function of the citric acid cycle

  • is to remove hydrogen atoms from organic molecules

  • and transfer them to coenzymes.

  • These coenzymes are NAD and FAD.

  • At the start of the citric acid cycle,

  • CoA will release the two carbon acetyl CoA

  • so that it may be transferred to a four carbon molecule.

  • Together, these two molecules form a six carbon

  • compound called citric acid.

  • After releasing the acetyl CoA, the CoA molecule

  • is now free to pick up another acetyl CoA.

  • At the end of the citric acid cycle,

  • two carbon atoms will have been removed

  • to recreate the original four carbon compound.

  • These carbons join with oxygen atoms

  • to form carbon dioxide, which is a waste product of the cell

  • and which will eventually diffuse out of the cell

  • into the blood and be transported to the lungs

  • to be breathed out.

  • More importantly, during the citric acid cycle,

  • hydrogen atoms will be transferred to coenzyme NAD

  • and to another related one called FAD.

  • NAD and FAD will be called NADH and FADH2 as they both pick up

  • two hydrogen atoms each.

  • NAD is a negatively charged compound.

  • And when it picks up two hydrogen ions,

  • it becomes NADH plus H plus.

  • However, it is commonly just referred to as NADH.

  • NADH and FADH2 will carry the hydrogen over

  • to another area in the mitochondria

  • to be used in the electron transport system.

  • NADH and FADH2 release their hydrogen

  • to the cytochromes of the electron transport system.

  • Without the hydrogen, NAD and FAD

  • then return to the citric acid cycle

  • to pick up more hydrogen atoms.

  • The only immediate energy benefit of the citric acid

  • cycle is that one GTP, or guanosine triphosphate,

  • will be created from one GDP, or guanosine diphosphate.

  • GTP is equivalent to ATP because it will readily

  • transfer a phosphate group to ADP to create ATP.

  • So we can say that for each acetyl CoA,

  • one ATP is directly created by the citric acid cycle.

  • Of course, with one glucose molecule,

  • two acetyl CoA are produced.

  • So ultimately, from one molecule of glucose,

  • the citric acid cycle produces two ATP.

  • The formation of GTP from GDP in the citric acid cycle

  • is another example of substrate phosphorylation.

  • In this process, a phosphate group

  • is transferred to a suitable acceptor molecule

  • using energy from a chemical process.

  • This occurs in many reactions in the cytosol

  • where a phosphate group is transferred to an ADP

  • to produce ATP.

  • For example, the ATP produced during glycolysis

  • is generated through substrate phosphorylation.

  • Normally, however, substrate phosphorylation

  • only provides a small amount of ATP

  • and isn't enough for the cell to function.

  • Most of the ATP that the cell needs

  • will be produced during oxidative phosphorylation,

  • which is what we'll talk about next.

  • Oxidative phosphorylation is the generation

  • of ATP within the mitochondria that

  • requires both coenzymes and consumes oxygen.

  • This process produces more than 90%

  • of the ATP used by body cells.

  • The key reactions take place in the electron transport system.

  • Oxidative phosphorylation also forms water, or H2O,

  • by combining two hydrogen atoms with one oxygen atom.

  • The oxygen is provided from the atmosphere during respiration.

  • And hydrogen is found in all organic molecules.

  • So both oxygen and hydrogen are readily

  • available for this reaction.

  • The reaction of creating water releases a tremendous amount

  • of energy.

  • There is so much energy released that this type of reaction

  • is used to launch space shuttles into orbit.

  • Obviously, cells can't handle this much energy.

  • So the energy released must be gradual.

  • Oxidative phosphorylation proceeds

  • in a series of small, controlled steps

  • so that energy can be captured safely

  • and ATP can be generated in the process.

  • During these steps, molecules lose electrons in a process

  • called oxidation.

  • When one molecule loses an electron,

  • another molecule will gain it in a process called reduction.

  • When electrons are passed from one molecule to another,

  • the electron donor is oxidized.

  • And the electron recipient is reduced.

  • Oxidation and reduction are important because electrons

  • carry chemical energy.

  • In a typical oxidation-reduction reaction,

  • the reduced molecule gains energy

  • at the expense of the oxidized molecule.

  • You can remember that molecules are oxidized

  • when they lose electrons and are reduced when they gain

  • electrons by remembering the words oil rig,

  • or oxidation is loss and reduction is gain.

  • In an oxidation-reduction exchange,

  • the reduced molecule does not gain all the energy released

  • by the oxidized molecule.

  • Some energy is always released as heat.

  • The remaining energy will be used to form ATP.

  • The electron transport system starts with NADH and FADH2

  • delivering hydrogen atoms to cytochromes.

  • The cytochromes are integral and peripheral proteins

  • that are embedded in the inner mitochondrial membrane.

  • These hydrogen atoms are from the citric acid cycle

  • and were delivered by NAD and FAD

  • in the form of NADH and FADH2.

  • If you remember, hydrogen atoms, they

  • consist of one electron and one proton.

  • The electron has a negative charge.

  • And the proton has a positive charge.

  • The electron carries energy with it.

  • NADH and FADH2 will both carry two hydrogen atoms.

  • The electrons from these atoms will

  • be passed from NADH and FADH2 to one of the cytochromes embedded

  • in the inner membrane.

  • The electron will then be passed from one cytochrome

  • to the next in small steps.

  • At the last cytochrome, at of the electron transport system,

  • an oxygen atom accepts the electron

  • and will use its energy to combine the oxygen and hydrogen

  • ions to form water.

  • Note that this sequence starts with the removal of two

  • hydrogen atoms from a substrate molecule, NADH or FADH2,

  • and ends with the formation of water

  • by combining two hydrogen with one oxygen.

  • This reaction occurred in several steps.

  • Had it occurred in only one step,

  • it would've been highly explosive.

  • The coenzymes NADH and FADH2 transfer the electrons

  • to the first cytochrome in the electron transport system.

  • And the electrons continue to be passed from one cytochrome

  • to the next.

  • As they are passed along, the electrons

  • themselves release energy.

  • This energy released causes hydrogen ion pumps

  • to start working.

  • These pumps move hydrogen ions from the matrix

  • across the inner membrane and into the intermembrane space.

  • This causes a large concentration gradient

  • for hydrogen ions between the matrix

  • and the intermembrane space.

  • Following the rules of diffusion,

  • hydrogen would diffuse from the high hydrogen concentration

  • in the intermembrane space to the lower concentration

  • in the matrix.

  • Hydrogen can't cross the inner membrane

  • because they are not lipid soluble.

  • However, in the inner membrane, an integral protein

  • with a channel called ATP synthase has the ability

  • to permit hydrogen ions to diffuse into the matrix.

  • This process is called chemiosmosis.

  • And it creates a kinetic energy that will

  • be used to convert ADP to ATP.

  • Both ADP and phosphate groups are already

  • in the matrix of the mitochondria.

  • The energy from chemiosmosis will

  • be used to phosphorylate ADP, which

  • means a phosphate group will combine with ADP to form ATP.

  • Because this process uses both coenzymes and oxygen,

  • the process of making ATP through this method

  • is called oxidative phosphorylation.

  • For every NADH molecule that transfers its hydrogen

  • molecules to the electron transport system,

  • three ATP will be formed.

  • For every FADH2 molecule that transfers its hydrogen

  • molecules to the electron transport system,

  • two ATP will be formed.

  • At the end of glycolysis and cellular respiration,

  • the catabolism of one glucose molecule

  • will yield 36 ATP for the typical cell.

  • Let's summarize where these ATP come from.

  • From glycolysis, there are two ATP that are directly made.

  • And there are four ATP from the hydrogen

  • that NAD transports to the electron transport system.

  • In the citric acid cycle, there are

  • two ATP that are directly made.

  • From the conversion of pyruvate to acetyl CoA

  • and from the citric acid cycle, there

  • are eight NADH that are transported

  • through the electron transport system.

  • These eight NADH will help produce 24 ATP.

  • There are also two FADH2 from the citric acid cycle

  • that are transported to the electron transport system.

  • And they will help to produce four ATP.

  • So if we add these up, 2 plus 4 plus 2 plus 24 plus 4

  • equals 36.

  • 36 is the number of ATP that the catabolism of one glucose

  • molecule will generate.

  • 32 of these are from oxidative phosphorylation,

  • or aerobic metabolism.

  • And two of these are from anaerobic metabolism.

  • Don't forget that carbon dioxide is produced

  • in the citric acid cycle and water

  • is produced in the electron transport system.

  • This ends Chapter 25, Module 2, Glycolysis, the TCA Cycle,

  • and the Electron Transport System.

This is Chapter 25, Module 2, Glycolysis, the TCA Cycle,

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B2 中高級

第25章 模塊二 糖酵解、TCA循環和ETS (Chapter 25 Module 2 Glycolysis, TCA Cycle, and ETS)

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    Cheng-Hong Liu 發佈於 2021 年 01 月 14 日
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