Lecture 5-Cellular Respiration

Biology 11- Lecture 5- Cellular Respiration

1

Cellular Respiration

This handout is for lecture use only and not for commercial reproduction and

distribution.

Lightenergy

ECOSYSTEM

Photosynthesisin chloroplasts

CO2 + H2O

Cellular respirationin mitochondria

Organicmolecules

+ O2

ATP powers most cellular work

Heatenergy

ATP

energy flows into an ecosystem as sunlightand leaves as heat

photosynthesisgenerates O2 and organic molecules, which are used in cellular respiration

cells use chemical energy stored in organic molecules to regenerate ATP, which powers work

Catabolic Pathways and Production of ATP

fermentation - partial degradation of sugars that occurs without O2

aerobic respiration consumes organic molecules and O2 and yields ATP

anaerobic respiration consumes compounds other than O2

Copyright 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings

includes both aerobic and anaerobic respiration but is often used to refer to aerobic respiration

although carbohydrates, fats, and proteins are all consumed as fuel, it is helpful to trace cellular respiration with the sugar glucose:

C6H12O6 + 6 O2 6 CO2 + 6 H2O + Energy (ATP + heat)


Cellular respirationNAD+= Nicotinamide adenine dinucleotide; a coenzyme occurring in most living cells and used as an oxidizing or reducing agent in various metabolic processes.

NADH= The reduced form of NAD.

Photophosphorylation =addition of a phosphate group

Reduction =addition of hydrogen or electrons or removal of oxygen

Oxidation =removal of hydrogen or electrons or addition of oxygen.


2

Oxidation of Organic Fuel Molecules

during cellular respiration, the fuel (such as glucose) is oxidized, and O2 is reduced:


becomes oxidized

becomes reduced

Reduction =addition of hydrogen or electrons or removal of oxygen Oxidation =removal of hydrogen or electrons or addition of oxygen

Glucose Oxygen

Stepwise Energy Harvest via NAD+ and the Electron Transport Chain

in cellular respiration, glucose and other organic molecules are broken down in a series of steps

electrons from organic compounds are usually first transferred to NAD+, a coenzyme

NAD+ functions as an oxidizing agent during cellular respiration

each NADH (the reduced form of NAD+) represents stored energy that is tapped to synthesize ATP


DehydrogenaseReduction of

NAD+Oxidation of NADH

2 e + 2 H+ 2 e

+ H+

NAD+ + 2[H]

NADH

+

H+

H+

Nicotinamide(oxidized form)

Nicotinamide(reduced form)

The full name for NAD+, nicotinamide adenine dinucleotide, describes its structure: the molecule consists of two nucleotidesjoined together at their phosphate groups (shown in yellow). Nicotinamide is a nitrogenous base.

In oxidation reactions, each electron (e ) travels with a proton (H+ )-thus, as a hydrogen atom.

The hydrogen atoms are not transferred directly to oxygen, but instead are usually passed first to an electron carrier, a coenzyme NAD+ .As an electron acceptor, NAD+ functions as an oxidizing agent during respiration.

Enzyme called dehydrogenase removes a pair of hydrogen atoms (2 electrons and 2 protons) from the substrate (glucose, in this example), thereby oxidizing it. The enzyme delivers the 2 electrons along with 1 proton to its coenzyme, NAD+. The other proton is released as a hydrogen ion (H+) into the surrounding solution:

The enzymatic transfer of 2 electrons and 1 proton (H+) from an organic molecule in food to NAD+ reduces the NAD+ to NADH; the second proton (H+) is released

NAD+ as an electron shuttle The Stages of Cellular Respiration

three stages:

Glycolysis (breaks down glucose into two molecules of pyruvate)

Citric acid cycle (completes the breakdown of glucose)

Oxidative phosphorylation (accounts for most of the ATP synthesis)


Mitochondrion

Substrate-levelphosphorylation

ATP

Cytosol

Glucose Pyruvate

Glycolysis

Electronscarried

via NADH


ATP

Electrons carriedvia NADH and

FADH2

Oxidativephosphorylation

ATP

Citricacidcycle

Oxidativephosphorylation:electron transport

andchemiosmosisGlycolysis, which occurs in the cytosol, begins

the degradation process by breaking glucose

into two molecules of a compound called pyruvate

Pyruvate enters the mitochondrion. where

the citric acid cycle oxidizes it to carbon dioxide.

NADH and electron carrier coenzyme called FADH transfer electrons derived from glucose to electron transport chains which are built into the inner mitochondrial membrane.

During oxidative phosphorylation, electron transportchains convert the chemical energy to a formused for ATP synthesis in the process calledchemiosmosis.


3

Mode of ATP synthesis

Oxidative Phosphorylation

Substrate-level Phosphorylation

process that generates most of the ATP (powered by redox reactions)


accounts for almost 90% of the ATP generated by cellular respiration

Oxidative Phosphorylation

Enzyme

ADP

P

Substrate

Enzyme

ATP+

Product

- forms a smaller amount of ATP in glycolysis and the citric acid cycle

Substrate-level Phosphorylation

- occurs when an enzyme transfers a phosphate group from a substrate molecule to ADP, rather than adding an inorganic phosphate to ADP as in oxidative phosphorylation

- "Substrate molecule" here refers to an organic molecule generated as an intermediate during the catabolism of glucose.

breaks down glucose into two molecules of pyruvate (splitting of sugar)

occurs in the cytoplasm and has two major phases:

energy investment phase

energy payoff phase


1. Glycolysis

Energy investment phase

Glucose

2 ADP + 2 P 2 ATP used

formed4 ATP

Energy payoff phase

4 ADP + 4 P

2 NAD+ + 4 e + 4 H+ 2 NADH + 2 H+

2 Pyruvate + 2 H2O

2 Pyruvate + 2 H2OGlucoseNet

4 ATP formed 2 ATP used 2 ATP

2 NAD+ + 4 e + 4 H+ 2 NADH + 2 H+

Glucose, a six-carbon sugar, is split into two three-carbon sugars. Smaller sugars are then oxidized and their remaining atoms rearranged to form two molecules of pyruvate. (Pyruvate is the ionized form of pyruvic acid.)

Cell actually spends ATP

Investment is repaid with

interest during the energy payoff phase, when ATP is produced by substrate-level phosphorylation

and NAD+ is reduced to NADH by electrons

released from the oxidation of glucose.

Net energy yield from glycolysis,

per glucose molecule

A Closer Look at Glycolysis.

Glucose enters the cell and is phosphorylated by the enzyme hexokinase, which transfers a phosphate group from ATP to the sugar.

Glucose-6-phosphate is converted to its isomer, fructose-6-phosphate.

Phosphofructokinase enzyme transfers a phosphate group from ATP to the sugar, investing another molecule of ATP in glycolysis. So far, 2 ATP have been used. With phosphate groups on its opposite ends, the sugar is now ready to be split in half. This is a key step for regulation of glycolysis.

Aldolase cleaves the sugar molecule into two different three carbon sugars: dihydroxyacetone phosphate and glyceraldehyde-3-phosphate. These two sugars are isomers of each other.

Isomerase catalyzes the reversible conversion between the two three-carbon sugars.

ENERGY INVESTMENT PHASE


4

ENERGY PAYOFF PHASE

This enzyme catalyzes two sequential reactions while it holds glyceraldehyde-3-phosphate in its active site. Sugar is oxidized by the transfer of electrons and H+ to NAD+, forming NADH (a redox reaction).Coefficient 2 precedes all molecules in the energy payoff phase; these steps occur after glucose has been split into two three-carbon sugars

This step produces 2 ATP, since every product after the sugarsplitting step is doubled. This ATP debt has now been repaid. Glucose has been converted to two molecules of 3-phosphoglycerate, which is not a sugar.

This enzyme relocates the remaining phosphate group, preparing the substrate for the next reaction.

This enzyme causes a double bond to form in the substrate by extracting a water molecule, yielding phosphoenolpyruvate (PEP).

The last reaction of glycolysis produces more ATP by transferring the phosphate group from PEP to ADP, a second instance of substrate level phosphorylation. Since this step occurs twice for each glucose molecule, 2 ATP are produced.

A Closer Look at Glycolysis.

ENERGY INVESTMENT PHASE

ENERGY PAYOFF PHASE

Energy investment phase

Glucose

2 ADP + 2 P 2 ATP used

formed4 ATP

Energy payoff phase

4 ADP + 4 P

2 NAD+ + 4 e + 4 H+ 2 NADH + 2 H+

2 Pyruvate + 2 H2O

2 Pyruvate + 2 H2OGlucoseNet

4 ATP formed 2 ATP used 2 ATP

2 NAD+ + 4 e + 4 H+ 2 NADH + 2 H+

Glucose, a six-carbon sugar, is split into two three-carbon sugars. Smaller sugars are then oxidized and their remaining atoms rearranged to form two molecules of pyruvate. (Pyruvate is the ionized form of pyruvic acid.)

Cell actually spends ATP

Investment is repaid with

interest during the energy payoff phase, when ATP is

produced by substrate-level phosphorylation

and NAD+ is reduced to NADH by electrons

released from the oxidation of glucose.

Net energy yield from glycolysis,

per glucose molecule

in the presence of O2, pyruvate enters the mitochondrion

pyruvate must be converted to acetyl CoA, which links the cycle to glycolysis


2. Citric acid cycle (Krebs cycle)


Conversion of pyruvate to acetyl CoA

-Pyruvate is a charged molecule, so in eukaryotic cells it must enter the mitochondnon via active transport, with the help of a transport protein. A complex of several enzymes catalyzes the three numbered steps 1. Pyruvate's carboxyl group (-COO-) is removed and given off as a molecule of CO22. The remaining two-carbon fragment is oxidized, forming a compound named acetate. An enzyme transfers the

extracted electrons to NAD+, storing energy in the form of NADH.3. coenzyme A (CoA), a sulfur containing compound is attached to the acetate by an unstable bond that makes the

acetyl group very reactive.

This molecule is now ready to feed its acetyl group into the citric acid cycle for further oxidation


5

takes place within the mitochondrial matrix

oxidizes organic fuel derived from pyruvate, generating 1 ATP, 3 NADH, and 1 FADH2 per turn


2. Citric acid cycle (Krebs cycle)Acetyl CoA

CoASH

Citrate

H2O

IsocitrateNAD+

NADH

+ H+

CO2

-Keto-glutarate

CoASH

CO2NAD+

NADH

+ H+SuccinylCoA

CoASH

P i

GTP GDP

ADP

ATP

Succinate

FAD

FADH2

Fumarate

CitricacidcycleH2O

Malate

Oxaloacetate

NADH

+H+

NAD+

1

2

3

4

5

6

7

8

has eight steps, each catalyzed by a specific enzyme

acetyl CoA joins the cycle by combining with oxaloacetate, forming citrate

the next seven steps decomposethe citrate back to oxaloacetate, making the process a cycle

Acetyl CoA

CoASH

Citrate

H2O

IsocitrateNAD+

NADH

+ H+

CO2

-Keto-glutarate

CoASH

CO2NAD+

NADH

+ H+SuccinylCoA

CoASH

P i

GTP GDP

ADP

ATP

Succinate

FAD

FADH2

Fumarate

CitricacidcycleH2O

Malate

Oxaloacetate

NADH

+H+

NAD+

1

2

3

4

5

6

7

8

Acetyl CoA adds its two-carbon acetyl group to oxaloacetate, producing

citrate

Citrate is converted to its isomer, isocitrate, by removal

of one water molecule and addition of another

Isocitrate is oxidized, reducing NAD+ to NADH.

Then the resulting compound loses a CO2molecule.

Another CO2 is lost, and the resulting compound is oxidized,

reducing NAD+ to NADH. The remaining molecule is then attached to coenzyme A by an

unstable bond.

CoA is displaced by a phosphate group, which is transferred to GDP forming GTP, a molecule with function similar to ATP that, in some cases, is used to generate ATP

Two hydrogens are transferred to FAD (flavin adenine dinucleotide),

forming FADH2 and oxidizing succinate.

Addition of a water molecule rearranges

bonds in the substrate

The substrate is oxidized, reducing

NAD+ to NADH and regenerating oxaloacetate

Pyruvate

NAD+

NADH

+ H+ Acetyl CoA

CO2

CoA

CoA

CoA

Citricacidcycle

FADH2

FAD

CO22

3

3 NAD+

+ 3 H+

ADP + P i

ATP

NADH

NADH and FADH2produced by the cycle relay electrons extracted from food to the electron transport chain

ATP is also produced


NADH and FADH2 account for most of the energy extracted from food

these two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation


3. Oxidative Phosphorylation

intermembrane space

inner mitochondrial membrane

mitochondrial matrix Electron transport and pumping of protons (H

+),which create an H+ gradient across the membrane

ATP synthesis powered by the flow of H+ back across the membrane


6

Electron Transport Chain

found in the cristae of the mitochondrion

NADH

NAD+2

FADH2

2 FAD

MultiproteincomplexesFAD

FeS

FMN

FeS

Q

FeS

Cyt b

Cyt c1

Cyt c

Cyt a

Cyt a3

IV

50

40

30

20

10 2

(from NADHor FADH2)

0 2 H+ + 1/2 O2

H2O

e

e

e

the carriers alternate reduced and oxidized states as they accept and donate electrons

electrons are passed through a number of proteins including cytochromes

electrons are finally passed to O2, forming H2O

Electron transport chain

NADH

NAD+2

FADH2

2 FAD

MultiproteincomplexesFAD

FeS

FMN

FeS

Q

FeS

Cyt b

Cyt c1

Cyt c

Cyt a

Cyt a3

IV

50

40

30

20

10 2

(from NADHor FADH2)

0 2 H+ + 1/2 O2

H2O

e

e

e

Electron transport chain

Electrons removed from glucose by NAD+, during glycolysis and the citric acid cycle, are

transferred from NADH to the first molecule of the electron transport chain in complex I. This molecule is a flavoprotein, called flavin

mononucleotide(FMN).

Flavoprotein returns to its oxidized form as it

passes electrons to an iron-sulfur protein (Fe-S in complex I)

Iron-sulfur protein then passes the

electrons to a compound called ubiquinone (Q)

Most of the remaining electron carriers between

ubiquinone and oxygen are proteins called cytochromes, each a

different protein with a slightly different electron-carrying heme group

Each oxygen atom also picks up a pair of hydrogen

ions from the aqueous solution, forming water.

Another source of electrons for the transport chain is FADH2, which adds

its electrons to the electron transport chain at complex II a lower energy level than NADH does

The last cytochrome of the chain, cyt a3, passes its

electrons to oxygen, which is very electronegative

electron transfer in the electron transport chain causes proteins to pump H+ ions from the intermembrane space to the mitochondrial matrix

H+ then moves back across the membrane, passing through channels in ATP synthase

Chemiosmosis: The Energy-Coupling Mechanism

Chemiosmosis - a process in which a proton gradient across a mitochondrial membrane and ATP synthesis pump metabolites across a membrane

- generation of ATP by the movement of hydrogen ions across a membrane during cellular respiration.

ATP synthase uses the exergonic flow of H+

to drive phosphorylation of ATP

This is an example of chemiosmosis, the use of energy in a H+ gradient to drive cellular work


-ATP synthase, the enzyme that actually makes ATP from ADP and inorganic phosphate.

- ATP synthase uses the energy of an existing ion gradient to power ATP synthesis.

the energy stored in a H+ gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis

the H+ gradient is referred to as a proton-motive force, emphasizing its capacity to do work




7

intermembrane space


mitochondrial matrix

NADH and FADH2 shuttle high-energy electrons extracted from food during glycolysis and the citric acid cycle to an electron transport chain built into the inner mitochondrial membrane. The gold arrows trace the transport of electrons, which finally pass to oxygen at the "downhill" end of the chain, forming water.

intermembrane space



Most of the electron carriers of the chain are grouped into four complexes. Two mobile carriers, ubiquinone (Q) and cytochrome c (Cyt c),move rapidly, ferrying electrons between the large complexes. As complexes I, III, and IV accept and then donate electrons, they pump protons from the mitochondrial matrix into the intermembrane space.

intermembrane space



During chemiosmosis, the protons flow back down their gradient via ATP synthase, which is built into the membrane nearby. The ATP synthase harnesses the protonmotive force to phosphorylate ADP forming ATP.

intermembrane space


mitochondrial matrix Electron transport and pumping of protons (H

+),which create an H+ gradient across the membrane

ATP synthesis powered by the flow of H+ back across the membrane

Mitochondrion


ATP

Cytosol

Glucose Pyruvate

Glycolysis

Electronscarried

via NADH


ATP

Electrons carriedvia NADH and

FADH2


ATP

Citricacidcycle

Oxidativephosphorylation:electron transport

andchemiosmosis

An Accounting of ATP Production


8

Per molecule of GlucoseSubstrate-level phosphorylation: 2 ATP from glycolysis + 2 ATP (directly GTP) from Krebs cycle

Oxidative phosphorylation2 NADH+H+ from glycolysis: 2 1.5 ATP (if glycerol phosphate shuttle transfers hydrogen atoms) or 2 2.5 ATP (malate-aspartate shuttle)2 NADH+H+ from the oxidative decarboxylation of pyruvate and 6 from Krebs cycle: 8 2.5 ATP2 FADH2 from the Krebs cycle: 2 1.5 ATP

Altogether this gives 4 + 3 (or 5) + 20 + 3 = 30 (or 32) ATP per molecule of glucose

Fermentation and anaerobic respiration

glycolysis can produce ATP with or without O2(in aerobic or anaerobic conditions)


Anaerobic respiration uses an electron transport chain with an electron acceptor other than O2, for example sulfate

Fermentation uses phosphorylation instead of an electron transport chain to generate ATP

Fermentation

consists of glycolysis plus reactions that regenerate NAD+, which can be reused by glycolysis

two common types:

alcohol fermentation

lactic acid fermentation


pyruvate is converted to ethanol in two steps, with the first releasing CO2

used in brewing, winemaking, and baking (yeast)


Alcohol Fermentation

pyruvate is reduced to NADH, forming lactate as an end product, with no release of CO2

used to make cheese and yogurt (fungi and bacteria)

used by human muscle to generate ATP when O2 is scarce


Lactic Acid Fermentation Fermentation vs Aerobic Respiration

both use glycolysis to oxidize glucose and other organic fuels to pyruvate

have different final electron acceptors: an organic molecule (such as pyruvate or acetaldehyde) in fermentation and O2 in cellular respiration

cellular respiration produces 38 ATP per glucose molecule; fermentation produces 2 ATP per glucose molecule


http://en.wikipedia.org/wiki/Substrate-level_phosphorylationhttp://en.wikipedia.org/wiki/Substrate-level_phosphorylationhttp://en.wikipedia.org/wiki/Substrate-level_phosphorylationhttp://en.wikipedia.org/wiki/Glycolysishttp://en.wikipedia.org/wiki/Krebs_cyclehttp://en.wikipedia.org/wiki/Oxidative_phosphorylationhttp://en.wikipedia.org/wiki/Oxidative_phosphorylationhttp://en.wikipedia.org/wiki/Pyruvate_decarboxylationhttp://en.wikipedia.org/wiki/Pyruvate_decarboxylation


9

Glucose

Glycolysis

Pyruvate

CYTOSOL

No O2 present:Fermentation

O2 present:

Aerobic cellularrespiration

MITOCHONDRION

Acetyl CoAEthanolor

lactateCitricacidcycle

Proteins Carbohydrates

Aminoacids

Sugars

Fats

Glycerol Fattyacids

Glycolysis

Glucose

Glyceraldehyde-3-

Pyruvate

P

NH3

Acetyl CoA

Citricacidcycle


The catabolism of various molecules from food.

- Glycolysis and the citric acid cycle connect to many other metabolic pathways.

- Carbohydrates, fats, and proteins can all be used as fuel for cellular respiration. Monomers of these molecules enter glycolysis orthe citric acid cycle at various points.

- Glycolysis and the citric acid cycle are catabolic funnels through which electrons from all kinds of organicmolecules flow on their exergonic fall to oxygen.

Documents

Lecture 5-Cellular Respiration