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CELLULAR RESPIRATION: CELLULAR RESPIRATION: HARVESTING CHEMICAL HARVESTING CHEMICAL

ENERGYENERGY

Section B: The Process of Cellular RespirationSection B: The Process of Cellular Respiration

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• Aerobic respirationAerobic respiration is relate to the structure is relate to the structure

of a mitochondrion.of a mitochondrion.

• GlycolysisGlycolysis and and KrebsKrebs cycle synthesis 10% of cycle synthesis 10% of

ATP.ATP.

• Electron transportElectron transport chain synthesis 90% of chain synthesis 90% of

ATP.ATP.

ObjectivesObjectives

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Section B: The Process of Cellular RespirationSection B: The Process of Cellular Respiration

Respiration involves three stages:Respiration involves three stages:

1.1. GlycolysisGlycolysis harvests chemical energy by oxidizing glucose to harvests chemical energy by oxidizing glucose to

pyruvate and produces about 5% of ATP (pyruvate and produces about 5% of ATP (in cytoplasm))

2.2. Krebs cycleKrebs cycle completes the energy-yielding oxidation of organic completes the energy-yielding oxidation of organic

molecules molecules and produces about 5% of ATPand produces about 5% of ATP ((in mitochondrial matrix))

3.3. EElectron transportlectron transport chain to synthesis chain to synthesis ATP ATP and produces about and produces about

90% of ATP90% of ATP ((inner mitochondrial membrane).

Cellular respiration generates many ATP molecules. For Cellular respiration generates many ATP molecules. For each sugar molecule it oxidizes (each sugar molecule it oxidizes (38 ATP molecule38 ATP molecule))

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• Respiration occurs in three metabolic Respiration occurs in three metabolic stages: stages: glycolysis, the Krebs cycleglycolysis, the Krebs cycle, , and the and the electron transport chainelectron transport chain and and oxidative phosphorylation.oxidative phosphorylation.

Respiration involves glycolysis, the Krebs cycle, Respiration involves glycolysis, the Krebs cycle, and electron transportand electron transport

Fig. 9.6, Page 160Fig. 9.6, Page 160

• Glycolysis occurs in the Glycolysis occurs in the cytoplasmcytoplasm..– It begins catabolism by breaking It begins catabolism by breaking

glucoseglucose into two molecules of into two molecules of pyruvatepyruvate..

• The Krebs cycle occurs in the The Krebs cycle occurs in the mitochondrial matrixmitochondrial matrix..– It degrades pyruvate to carbon dioxide.It degrades pyruvate to carbon dioxide.

• Several steps in glycolysis and the Krebs cycle transfer electrons Several steps in glycolysis and the Krebs cycle transfer electrons from substrates to from substrates to NADNAD++, forming , forming NADNADHH..

• NADNADHH passes these electrons to the electron transport chain. passes these electrons to the electron transport chain.

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I- Substrate-level phosphorylation:I- Substrate-level phosphorylation:• Some ATP is generated in Some ATP is generated in glycolysisglycolysis and in and in Krebs cycle Krebs cycle by by

Substrate-level phosphorylationSubstrate-level phosphorylation. . phosphate group is transferred phosphate group is transferred from an organic molecule (the substrate) to ADP, from an organic molecule (the substrate) to ADP, forming forming 10% 10% ATP ATP (4 ATP).(4 ATP).

• Ultimately Ultimately 38 ATP38 ATP are produced per are produced per mole of glucose that is degraded to mole of glucose that is degraded to carbon carbon COCO22 and and HH22OO by respiration. by respiration.

II- Oxidative phosphorylationII- Oxidative phosphorylation::

• As electrons passed along the chain, As electrons passed along the chain, their energy stored in the mitochondrion their energy stored in the mitochondrion in a form that can be used to synthesize in a form that can be used to synthesize the rest the rest 90% of the ATP90% of the ATP ATPATP (34 ATP).(34 ATP).

• via via Oxidative phosphorylationOxidative phosphorylation..

PhosphorylationPhosphorylation

• During glycolysis, glucose (During glycolysis, glucose (a six carbon-sugar) is split into two molecules ) is split into two molecules (each is (each is three-carbon sugarthree-carbon sugar).).

• These smaller sugars are oxidized and rearranged to form two molecules of These smaller sugars are oxidized and rearranged to form two molecules of pyruvatepyruvate..

• Each of the 10 steps in glycolysis is catalyzed by a specific enzyme.Each of the 10 steps in glycolysis is catalyzed by a specific enzyme.• These steps can be divided into two phases: These steps can be divided into two phases:

1)- Energy investment phase:1)- Energy investment phase: طاقة طاقة إستهالك إستهالك ATP is consumed to provides activation ATP is consumed to provides activation energy by phosphorylating glucose energy by phosphorylating glucose (this requires 2 ATP per glucose). (this requires 2 ATP per glucose).

2)- Energy payoff phase:2)- Energy payoff phase: طاقة طاقة إنتاج إنتاج ATP is produced by substrate-level ATP is produced by substrate-level phosphorylation and NADphosphorylation and NAD++ is reduced to is reduced to NADNADHH..

• 4 ATP4 ATP and and 2NADH2NADH are produced per are produced per glucoseglucose..

• Thus, the net yield from glycolysis is Thus, the net yield from glycolysis is 2 ATP2 ATP and and 2 NADH2 NADH per glucose. per glucose.

• Oxygen is not required for glycolysisOxygen is not required for glycolysis

1- Glycolysis1- Glycolysis ( (splitting glucosesplitting glucose): harvests chemical ): harvests chemical energy by oxidizing glucose to 2-pyruvateenergy by oxidizing glucose to 2-pyruvate

Summary of Glycolysis (Summary of Glycolysis (Splitting of glucoseSplitting of glucose) )

It is the process of breaking a It is the process of breaking a glucoseglucose into 2 into 2 Pyruvates.Pyruvates.

It is a source for some It is a source for some ATPATP & & NADNADHH and occurs in the and occurs in the CYTOSOLCYTOSOL (cytoplasm). (cytoplasm).

1)- Glucose is phosphorylated twice by adding 2 P coming from 2 ATP (substrate-level-phosphorylation).

A)- Energy investment phaseA)- Energy investment phase

It has two phasesIt has two phases

2)- Thus, Glucose (6-CC) splits into two small sugar molecules (each with 3-C).

The net yield of this process is the formation of 2 NADH, 2 ATP and 2 pyruvate2 pyruvate molecules.

B)- Energy pay-off phaseB)- Energy pay-off phase

4ATP are formed by adding 4P to 4ADP molecules.

It is the process of producing some of the remaining energy (ATP) It is the process of producing some of the remaining energy (ATP)

fromfrom the the Pyruvate Pyruvate molecules. It occurs mainly in molecules. It occurs mainly in mitochondrial mitochondrial matrixmatrix if oxygen is presentif oxygen is present..

It is the main source for preparing most of the cellular NADIt is the main source for preparing most of the cellular NADHH (storing (storing energy molecule), energy molecule), and for producing some more of the cellularand for producing some more of the cellular ATPATP. .

It includes two cycles : It includes two cycles :

Pre-Krebs cycle Pre-Krebs cycle التحضيرية التحضيرية المرحلة Pre-Krebs cycle Pre-Krebs cycle المرحلة التحضيرية التحضيرية المرحلة المرحلة

Krebs cycleKrebs cycleKrebs cycleKrebs cycle

22. The Krebs cycle. The Krebs cycle completes the energy-yielding completes the energy-yielding oxidation of organic molecules (in mitochondrial matrix):oxidation of organic molecules (in mitochondrial matrix):

A)- Pre-Krebs cycleA)- Pre-Krebs cycle

Pyruvate is converted into acetyle-CoAacetyle-CoA in the presence of O2 through 3 steps

a)- a)- C=OC=O-- group of pyruvate is released as group of pyruvate is released as COCO22..

b)- The remaining two-b)- The remaining two-CC fragments are oxidized ( fragments are oxidized (releasing releasing ee--) into) into

acetateacetate and the resulting and the resulting ee-- transform transform NADNAD++ into into NADNADHH..

c)- The coenzyme-A (c)- The coenzyme-A (CoACoA) transform acetate compound into) transform acetate compound into acetyle- acetyle-CoACoA, which will be ready for Krebs Cycle for further, which will be ready for Krebs Cycle for further oxidation. oxidation.

2 NAD2 NAD++ 2 NAD2 NADHH ++ HH++

2Pyruvate2Pyruvate 2Acetyle-2Acetyle-CoACoACoACoA

+ CO+ CO22 + H + H22OO

Krebs CycleKrebs Cycle

B)- Krebs cycleB)- Krebs cycle

It has eight steps starting with 2 It has eight steps starting with 2 acetyle-acetyle-CoACoA compunds. They are summarized compunds. They are summarized as in Fig. 9.12:as in Fig. 9.12:

2 ATP2 ATP

6 NAD6 NADHH

2 FAD2 FADHH22

OutputOutput

FFlavin lavin AAdenine denine DDinucleotideinucleotide

Thus, the outcome of the two cycles is Thus, the outcome of the two cycles is (for the 2 Acetyle-CoA molecules):(for the 2 Acetyle-CoA molecules):

Pre-Krebs Pre-Krebs CycleCycle

• This cycle begins when acetate from each acetyl-This cycle begins when acetate from each acetyl-CoA combines with oxaloacetate (4 C atoms) to CoA combines with oxaloacetate (4 C atoms) to form citrate (citric acid).form citrate (citric acid).

• Ultimately, the oxaloacetate is recycled and the Ultimately, the oxaloacetate is recycled and the acetate is broken down to COacetate is broken down to CO22..

• Each cycle produces one ATP by substrate-level Each cycle produces one ATP by substrate-level phosphorylation, phosphorylation, three NADHthree NADH, and , and one FADHone FADH22

(another electron carrier) (another electron carrier) per acetyl CoAper acetyl CoA..

• If If OO22 is present, is present, pyruvatepyruvate enters the mitochondrion where enzymes of enters the mitochondrion where enzymes of

the Krebs cycle complete the oxidation of this organic fuel to COthe Krebs cycle complete the oxidation of this organic fuel to CO22..

22. The Krebs cycle. The Krebs cycle completes the energy-yielding completes the energy-yielding oxidation of organic molecules (oxidation of organic molecules (in mitochondrial matrixin mitochondrial matrix):):

• As pyruvate enters the mitochondrion which modifies As pyruvate enters the mitochondrion which modifies pyruvatepyruvate to to acetyl-CoAacetyl-CoA which enters the Krebs cycle in the matrix. which enters the Krebs cycle in the matrix.– A carboxyl group is removed as COA carboxyl group is removed as CO22..

– A pair of electrons is A pair of electrons is transferred from the transferred from the remaining two-carbon remaining two-carbon fragments to NADfragments to NAD++ to form to form NADH. NADH.

– The oxidized fragment, The oxidized fragment, acetate, combines with acetate, combines with coenzyme A to form coenzyme A to form acetyl-CoAacetyl-CoA..