Glycolysis Krebs Cycle and Electron Transport

8/8/2019 Glycolysis Krebs Cycle and Electron Transport

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Much of the energy in a cell is trapped in the bonds of ATP molecule more specifically in the

phosphate group (p)

The addition of P to chemical compound is called phosphorylation. Organisms use three different

mechanisms to generate ATP from ADP

A Substrate level phosporylation involves a high energy P directly transferred from a

phosphorylated compound ( a substrate) to ADP Generally the P has acquired its energy from a

previous reaction in which the substrate its self was oxidized.

Glycolysis The oxidation of glucose to to pyruvic acid, is usually the first stage in carbohydrate

catabolism

Glycolysis is also called the embden Meyerhof pathway. Glycolsis means the splitting of sugar and

this is exactly what happens

The enzymes of glycoysis split glucose a 6 carbon sugar into two three carbon sugars. These sugars

are then oxidized releasing there energy and their atoms are rearranged to form two molecules of

pyruvic acid

During glycolsis NAD+ is reduced to NADH. And there is net production of two ATP molecules by

substrate level phosphorylation

Glycolsis does not require oxygen.

This pathway is a series of ten chemical reactions each catalyzed by a different enzyme.

To summarize the process, glycolysis consists of two basic stages, a preparatory stage and and

energy conserving stage.

y First two molecules of ATP are used as a six carbon glucose molecule is phosphorylatedrestructured and split into two three carbon compounds: glycealdehyde-3 phosphate (GP)

and dihydroxy acetone phosphate (DHAP) DHAP is readily converted to GP. The conversion


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of DHAP into GP me

ns that from this point on in glycolysis two molec

les of GP are fe

into

the remaining chemical reactions

y In theenergyconservation stage The two threecarbon molec les are oxidized inseveralsteps to tow molec les of pyruvic acid. In these reactions two molecules of NAD+ are

reduced to NADH and four molecules of ATP are formed bysubstrate level phosphorylation

y


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The krebscycle also called the tricarboxylic acid or citric acid cycle is a series of biochemical

reactions in which large amounts of potential energystored in acetyl CoA is released step bystep by

a series of oxidation and reductions in the form ofelectrons to electron carrier co enzymeschiefly

NAD+. The pyruvic acid derivates are oxidized thecoenzymes are reduced.

Pyruvic acid generated from glycolsiscannot enter the krebscycle directly. It must lose one molecule

of CO2 in a processcalled decarboylation. The two carbon group called an acetyl group attaches to a

coenzyme in a high energy bond. Resulting in thecompound known as acetyl coA. During this

reaction pyruvic acid is oxidized and nad+ is reduced to NADH

As oxidation produces two molecules of pyruvic acid for every glucose molecule two molecules of

CO2 are produced

As acetyl coA enters the krebscycle theco enzyme is released and the acetyl group attached to a

four carbon oxaloacetic acid to form a six carbon citric acid. Thesynthesis of thiscompound re

uires

energy which is provided by the removal of the high energy Co enzyme bond. The formation ofcitric

acid is the first step in the krebscycle

Decarboxylation is a major component of the krebscycle. All threecarbon atoms in ther pyruvic acid

areeventually released as CO2 in the krebscycle. This represents theconversion of all 6 atoms

contained in the original glucose molecule into CO2

Another major step is oxidation and reduction. Hyrogen atoms are picked up by theco enzymes

NAD+ and FAD to form NADH and FADH2


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If we look at the krebscycle as a whole wecan see that

For every two molecules of acetyl coA that enter cycle four molecules of CO2 are liberated by

decarboxlyation, 6 moleucles of NADH are made and two molecules of FADH2.

And two molecules of ATP are generated bysubstrate level phosphorylation.


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The CO2 iseventually released as CO2 gas through respiration.

Most of theenergycontained in glucose is now contained in theco enzymes FADH2 and NADH

In the next series of reactions, a series of reductions transfers theenergystored in these to ATP in a

processcalled theelectron transport chain.

It consists of a series ofcarrier molecules that arecapable of oxidation and reduction. As the

electrons are passed through thechain there occurs a stepwise release ofenergy which is used todrive thechemiosmotic production of ATP.

The final oxidation is irreversible

In eukaryoticcell it takes place in inner membranes of the mitochondria, in prokaryoti cells it takes

place within the plasma membrane in thecristae

There are threeclasses of molecules involved in theelectron transport chain

The first being flavoproteins, these are molecules derived from riboflavin(b12) and arecapable of

carrying out both oxidation and reduction reactions. An important conenzyme is flavin

mononeculeotide

Thesecond important molecule involved is thecytochromes. These are protein which contain an

iron heme group capable ofexsisting in both fe2+ and fe3+states.

Thecytochromes involved in theelectron transport chain are. CYT B CYT C CYTC1 CYTA and CYT3


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The third group of molecule is a small non protein carrier known asco enzyme Q

Theelectron transport chain contained in bacteria aresomewhat diverse, even in a single bacteria

several different types ofelectron transport chain might exist.

The first step in theelectron transport chain is the transport ofelectrons from the high energy

coenzyme NADH to falvin mononeculotide the first carrier in thechain

This passage involves the transport of a hydrogen atom with two electrons to FMN which then picks

up an additional H+ from thesurrounding aqueoussolution.

As a result NADH is oxidized to NAH+ AND FMN IS reduced to FMNH2+.

In thesecond step FMNH2+ passed to hydrogens to the oppositeside of the mitochondrial

membrane. And passes two electrons to Q

As a result FMNH2+ is reduced to fmn and Q picks up additional h2+ ions from thesurround aqueous

solution and passes them to the other side of the membrane

The next step involves thecytochromes.. Electrons are passed from Q to B to C to C1 to A to A3

The last cytochrome passes itselectrons to molecular O2 which becomes negativelycharged and

picks up a proton from the aqeoussolution to form H20

An important thing to note is the presences ofcarrierssuch as FMN nd Q that can accept both

electrons and protons and carriers that can only accept electrons.

The result of this is a build of protons on oneside of the membrane. Just as water behind a dam

storesenergy that can be used to provideelectricity, this build up of protons providesenergy for the

generation of ATP by thechemiosmotic mechanism


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A energeticelectrons pass down theelectron transport chian some of thecarriers in thechian

actively pump protons across the membrane. These arecalled proton pumps

The phospolipid membrane is usually impermeable to protons, so this one directional pumping

establishes a proton gradient. In addition to theconcentration gradient a charge gradient is also

established. The resulting electrochemical gradient has potential energy, called the proton motive

force.

The protons with the higher concentration gradient can diffuse across the membrane only through

special channels that contain an enzymecalled ATP synthases. The resultant energy is used to

synthesize ATP from ADP and P

The inner mitochondrial membranecontains theelectron transporter carriers and the ATP synthase,

whereas in most prokaryoticcells the plasma memebrane does. AN electron transport chian also

operates in photophoysporlation and operates in the thylokoid membrane ofcynobacteria and in

eukaryoticchloroplasts.


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A summary of aerobic resperation.

Theelectron transport chain regenerates the FAD and NAD needed in glycolysis. Thevarious

transfers in theelectron transport chain generate about 34 molecules of ATP from each molecule of

glucose oxidized

Approximately threeeach from each of the ten molecules of NADH(30) and two molecules for each

of the two molecules of FADH2(4) . To arrive at the total number of34.

In aerobic respiration among prokaryotes a total of38 molecules of ATP can be generated from one

molecule of glucose. Four of these are from substrate level phosphorylation

Aerobic respiration amoung eukaryotes produces a total of only36 ATP. There are fewer ATPS than

in prokaryotes becauseenergy is lost when theelectrons areshuttled across the mitochondrial

membrane.

This is thesummary of aerobic respiration of a prokaryote

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Glycolysis Krebs Cycle and Electron Transport