HMM1414 Chapter 7

8/9/2019 HMM1414 Chapter 7

1/32

HMM/SCM1414_Biology 1

CHAPTER 7

RESPIRATION

Living cells require energy from outside

sources to perform tasks.

Energy enters most ecosystems as

sunlight and leaves as heat.(Figure 9.2, Campbell, page 160)

Photosynthesisgenerates O2 and organic

molecules that mitochondria of eukaryotes

use as fuel for cellular respiration.

Cells harvestchemical energy stored in

organic molecules and use it to regenerate

ATP, the molecule that drives most cellular

work.

3 pathways of respiration: glycolysis,

citric acid cycle, and oxidative

phosphorylation.

7.1 ATP (Adenosine Triphosphate) Immediate source of energy that drives

most cellular work.

Cells manage their energy resources to

do this work by energy coupling - use ofan exergonicprocess to drive anendergonic one.

1

8/9/2019 HMM1414 Chapter 7

2/32


Endergonic Reaction - reaction thatends with a net gainin energy

Products have more energy than

reactants, e.g., photosynthesis

Exergonic Reaction - reactionthat ends with a net lossin energy

Reactants have more energy than

products, e.g., cellular respiration

Structure and hydrolysis of ATP

ATP = Nucleotide with unstable

phosphate bonds that cell hydrolyzes for

energy to drive endergonic reactions.

Consists ofadenine, ribose, & chain ofthree phosphate groups.

2

8/9/2019 HMM1414 Chapter 7

3/32


Unstable bonds between phosphate

groups can be hydrolyzed in an exergonic

reaction.

When terminal phosphate bond is

hydrolyzed, a phosphate group is removed

producing ADP (adenosine diphosphate).

ATP + H2O ADP + Pi Under standard lab conditions, reaction

releases -31 kJ/mol (-7.3 kcal/mol). In living cell, reaction releases -55 kJ/mol

(-13 kcal/mol) - 77% more than under

standard conditions.

Terminal phosphate bonds of ATP are

unstable, so:

Products of hydrolysis reaction are morestable than reactants.

Hydrolysis of phosphate bonds is thus

exergonic as system shifts to a more

stable state.

How ATP performs work Exergonic hydrolysis of

ATP is coupled with endergonic processes

by transferring a phosphate group to

another molecule.

3

8/9/2019 HMM1414 Chapter 7

4/32


Phosphate transfer is enzymatically

controlled.

Molecule receiving phosphate

(phosphorylated or activated

intermediate) becomes more reactive.

Example,

conversion of glutamic acid (Glu) to

glutamine (Gln):

Glu + NH3Gln

G = +14.2 kJ/mol (+3.4 kcal/mol)(endergonic)

Two step process of energy coupling

with ATP hydrolysis:

1. Hydrolysis of ATP and phosphorylation ofglutamic acid.

Glu + ATP Glu-(Pi) + ADPUnstable

phosphorylatedintermediate

2. Replacement of the phosphate with thereactant ammonia.

Glu-(Pi) + NH3 Gln + (Pi)Overall G:

4

8/9/2019 HMM1414 Chapter 7

5/32


Glu + NH3Gln G = +14.2 kJ/molATP ADP + Pi G = -31.0 kJ/mol

Net G = -16.8 kJ/mol (Overall process is exergonic)

5

8/9/2019 HMM1414 Chapter 7

6/32


The regeneration of ATP

ATP is continually regenerated by cell.

Process is rapid (107 molecules used and

regenerated/sec/cell).

Reaction is endergonic.

ADP + Pi ATP

G = + 31 kJ/mol (+7.3 kcal/mol)

Energy to drive endergonic regeneration

of ATP comes from exergonic process of

cellular respiration.

6

8/9/2019 HMM1414 Chapter 7

7/32


7.2 Aerobic RespirationPreview of cellular respiration(See Figure 9.6, Campbell, page 164)

Stages of respiration: glycolysis, citric

acid cycle, and electron transport chain

and oxidative phosphorylation.

Glycolysis - in cytoplasm. Glucose broken down into two molecules

of pyruvate.

Citric acid cycle - in mitochondrialmatrix.

Completes breakdown of glucose by

oxidizing a derivative of pyruvate to CO2.

Several steps in glycolysis and citric acid

cycle are redox reactions - dehydrogenase

enzymes transfer electrons from substrates

to NAD+, forming NADH.

NADH passes electrons to electron

transport chain (ETC). Electrons then move from molecule to

molecule until they combine with molecular

O2 and H+ to form water.

7

8/9/2019 HMM1414 Chapter 7

8/32


As they are passed along chain, energy

carried by electrons is transformed in

mitochondrion into a form that can be used

to synthesize ATP via oxidativephosphorylation. Inner membrane of mitochondrion is site

of electron transport and chemiosmosis,

processes that together constitute

oxidative phosphorylation.

Oxidative phosphorylation produces

almost 90% of ATP generated by

respiration.

Some ATP is formed directly during

glycolysis and citric acid cycle by

substrate-level phosphorylation. Enzyme transfers phosphate group from

an organic substrate to ADP, forming

ATP. (See Figure 9.7, Campbell, page 164)

For each molecule of glucose degraded to

CO2 & H2O by respiration, cell makes up to

38 ATP, each with 7.3 kcal/mol of free

energy.

Respiration uses small steps in

respiratory pathway to break large

denomination of energy contained in

glucose into the small change of ATP.

8

8/9/2019 HMM1414 Chapter 7

9/32


Quantity of energy in ATP is more

appropriate for level of work required in

cell.

7.2.1 Glycolysis(See Figure 9.9, Campbell, page 166 - 167)

Glucose is split into two 3C sugars.

3C sugars oxidized and rearranged to

form two molecules of pyruvate, ionizedform of pyruvic acid.

Two phases of glycolysis:(Figure 9.8, Campbell, page 165)

1. Energy investment phase

Cell invests ATP to provide activation

energy by phosphorylating glucose.

Requires 2 ATP per glucose.

2. Energy payoff phase

ATP produced by substrate-levelphosphorylation.

NAD+ reduced to NADH by electronsreleased by oxidation of glucose.

Net yield from glycolysis is 2 ATP and 2

NADH per glucose.

No CO2 is produced during glycolysis.

Glycolysis can occur in presence or

absence of O2.

9

8/9/2019 HMM1414 Chapter 7

10/32


The Glycolytic Pathway(Figure 9.9, Campbell, page 166 167)

http://www.db.uth.tmc.edu/faculty/alevine/1521_2000/glydetail.htm

10

8/9/2019 HMM1414 Chapter 7

11/32


11

8/9/2019 HMM1414 Chapter 7

12/32


Overall reaction showing allreactants

and products resulting from glycolysis.

Glucose + 2ATP +2Pi + 4ADP + 2NAD

+2 Pyruvate + 2ADP +2ATP + 2NADH + 2H+ +

2H2O

Equation showing netreaction of

glycolytic pathway.

Glucose + 2Pi + 2ADP+ 2NAD+

2 Pyruvate + 2ATP +2NADH + 2H+ + 2H2O

7.2.2 Pyruvate oxidation(Figure 9.10, Campbell, page 168)

More than three-quarters of original

energy in glucose is still present in the two

molecules of pyruvate.

If O2 is present, pyruvate enters

mitochondrion where enzymes of citric acid

cycle complete its oxidation to CO2.

After pyruvate enters mitochondrion via

active transport, it is converted to acetyl

coenzyme A (acetyl CoA).

12

8/9/2019 HMM1414 Chapter 7

13/32


This is accomplished by multienzyme

complex that catalyzes three reactions:

1. Carboxyl group is removed as CO2.2. Remaining 2C fragment is oxidized to

acetate. An enzyme transfers the pair of

electrons to NAD+ to form NADH.

3. Acetate combines with coenzyme A toform the very reactive molecule acetyl

CoA.

Acetyl CoA is now ready to feed its acetyl

group into the citric acid cycle for further

oxidation.

7.2.3 The Krebs cycle/ Citric Acid Cycle(Figure 9.12, Campbell, page 169)

Cycle oxidizes organic fuel derived from

pyruvate.

Acetyl group of acetyl CoA joins cycle by

combining with oxaloacetate, forming

citrate.

Citrate regeneraterd back (via several

steps) to OAA.

Three CO2 molecules are released,

including the one released during the

conversion of pyruvate to acetyl CoA.

13

8/9/2019 HMM1414 Chapter 7

14/32


Cycle generates one ATP per turn by

substrate-level phosphorylation.

GTP molecule is formed by substrate-

level phosphorylation.

GTP used to synthesize an ATP, the only

ATP generated directly by cycle.

Most of chemical energy is transferred to

NAD+ and FAD during redox reactions.

Reduced coenzymes NADH and FADH2then transfer high-energy electrons to

electron transport chain.

For every acetyl CoA, each cycle

produces:

(i) 1 ATP by substrate-level

phosphorylation(ii) 3 NADH, and

(iii) 1 FADH2

14

8/9/2019 HMM1414 Chapter 7

15/32


http://chemistry.gsu.edu/glactone/PDB/Proteins/Krebs/Krebs.html

Summary:

Acetyl CoA + 3 NAD+

+ FAD + ADP + Pi

+2H2O

2 CO2 + CoA-SH +

3NADH + 3H+ +

FADH2 + ATP

15

8/9/2019 HMM1414 Chapter 7

16/32


16

8/9/2019 HMM1414 Chapter 7

17/32


7.2.4 Electron transport chain (oxidative

phosphorylation)

Only 4 of 38 ATP produced by respiration

of glucose are produced by substrate-level

phosphorylation:

Glycolysis 2 ATP.

Citric acid cycle 2 ATP.

NADH and FADH2 account for the majority

of energy extracted from food.

These reduced coenzymes link glycolysis

and citric acid cycle to oxidative

phosphorylation, which uses energy

released by ETC to power ATP synthesis.

The Pathway of Electron Transport(Figure 9.13, Campbell, page 171)

ETC is a collection of molecules

embedded in cristae.

Most components of ETC are proteins

bound to prosthetic groups. Electrons drop in free energy as they

pass down ETC.

17

8/9/2019 HMM1414 Chapter 7

18/32


During electron transport along ETC,

electron carriers alternate between

reduced and oxidized states as they accept

and donate electrons.

Each component of chain becomes

reduced when it accepts electrons from

its uphill neighbor, which is less

electronegative.

It then returns to its oxidized form as itpasses electrons to its more

electronegative downhill neighbor.

Electrons carried by NADH are

transferred to the first molecule in ETC, a

flavoprotein.

Electrons continue along chain thatincludes several cytochrome proteins andone lipid carrier.

Prosthetic group of each cytochrome is a

heme group with an iron atom that

accepts and donates electrons.

Last cytochrome of chain, cyt a3, passesits electrons to oxygen, which is very

electronegative.

Each oxygen atom also picks up a pair of

H+ from aqueous solution to form water.

18

8/9/2019 HMM1414 Chapter 7

19/32


For every two electron carriers (four

electrons), one O2 molecule is reduced to

two molecules of water.

Electrons carried by FADH2 have lower

free energy and are added at a lower

energy level than those carried by NADH.

ETC provides about one-third less energy

for ATP synthesis when electron donor is

FADH2 rather than NADH. ETC generates no ATP directly.

Its function is to break the large free

energy drop from food to oxygen into a

series of smaller steps that release energy

in manageable amounts.

How does the mitochondrion couple

electron transport and energy release to

ATP synthesis?

Chemiosmosis.

Chemiosmosis: Energy-CouplingMechanism(Figure 9.14, Campbell, page 171)

1. NADH delivers two electrons and two

protons to the first protein complex (I) in

cytochrome system located in cristae.

19

8/9/2019 HMM1414 Chapter 7

20/32


2. Complex uses energy released from

electrons to actively pump H+ from matrix

to inter-membrane space.

3. Process is done two more time times.

Electrons are passed through two more

protein complexes (III & IV) which

transport two more H+ across membrane.

4. After passing through three protein

complexes, electrons combine with oneoxygen atom and two H+ to form water.

5. This transport across membrane produces

a concentration gradient with more H+ on

one side of membrane than the other. The

H+ gradients that results is referred to as

a proton-motive force. This gradient isused to make ATP.

6. Cristae membrane is very impermeable to

H+ except through a special protein called

ATP synthase. As protons pass through

this protein, energy is obtained to make

ATP from ADP & Pi.7. FADH2 delivers its electron via protein

complex II and so results in fewer

electrons being into the inter-membrane

space.

20

8/9/2019 HMM1414 Chapter 7

21/32


Oxidative phosphorylation = Electrontransport + chemiosmosis.

21
http://en.wikipedia.org/wiki/Image:Etc3.png

8/9/2019 HMM1414 Chapter 7

22/32


7.2.5 Calculations of Total ATPProduction by Cellular Respiration

Products generated when cellularrespiration oxidizes a molecule of glucose

to 6 CO2 molecules:(Figure 9.16, Campbell, page 173)

Conversions

NADH in cytoplasm produces 2 or 3 ATPby oxidative phosphorylation depending on

shuttle system used to transport electrons

from cytosol into mitochondrion:

If electrons are passed to FAD, e.g. brain

cells, 2 ATP are produced.

If electrons are passed to NAD+, e.g. liver

cells & heart cells, 3 ATP are produced.

NADH in mitochondria produces 3 ATP.

FADH2 adds its electrons to the electron

transport system at a lower level than

NADH, so it produces 2 ATP.

22
http://faculty.clintoncc.suny.edu/faculty/Michael.Gregory/files/Bio%20101/Bio%20101%20Lectures/Energy/energy.htm#NAD+http://faculty.clintoncc.suny.edu/faculty/Michael.Gregory/files/Bio%20101/Bio%20101%20Lectures/Energy/energy.htm#ATP%20(adenosine%20triphosphate)http://faculty.clintoncc.suny.edu/faculty/Michael.Gregory/files/Bio%20101/Bio%20101%20Lectures/Energy/energy.htm#FADhttp://faculty.clintoncc.suny.edu/faculty/Michael.Gregory/files/Bio%20101/Bio%20101%20Lectures/Energy/energy.htm#FADhttp://faculty.clintoncc.suny.edu/faculty/Michael.Gregory/files/Bio%20101/Bio%20101%20Lectures/Energy/energy.htm#NAD+http://faculty.clintoncc.suny.edu/faculty/Michael.Gregory/files/Bio%20101/Bio%20101%20Lectures/Energy/energy.htm#ATP%20(adenosine%20triphosphate)http://faculty.clintoncc.suny.edu/faculty/Michael.Gregory/files/Bio%20101/Bio%20101%20Lectures/Energy/energy.htm#FAD

8/9/2019 HMM1414 Chapter 7

23/32


Glycolysis

Substrate-level phosphorylation = 2 ATP

2 NADH = 2 x 2 ATP = 4 ATP; or= 2 x 3 ATP = 6 ATP

Formation of Acetyl CoA

2 NADH = 2 x 3 ATP = 6ATP

Krebs Cycle

6 NADH = 6 x 3 ATP = 18 ATP

2 FADH2 = 2 x 2 ATP = 4 ATP

Substrate-level phosphorylation = 2 ATP

Total Yield

Glycolysis = 2 ATP

Aerobic respiration = 34 or 36 ATP

5. Summary

Pathway Substrate-level

phosphorylation

Oxidative

phosphorylation

Total

ATP

Glycolysi

s

2 ATP 2 NADH = 4-6 ATP 6 - 8

Coa 2 NADH = 6 ATP 6

Krebs

cycle

2 ATP 6 NADH = 18 ATP

2 FADH2 = 4 ATP

24

23

8/9/2019 HMM1414 Chapter 7

24/32


Total 4 ATP 32 34 ATP 36-38

How efficient is respiration in generating

ATP?

Complete oxidation of glucose releases

686 kcal/mol.

Phosphorylation of ADP to form ATP

requires at least 7.3 kcal/mol.

Efficiency of respiration

= 7.3 kcal/mol x 38 ATP/glucose x 100%

686 kcal/mol glucose

= 40%.

60% of energy from glucose lost as

heat.

Some of that heat is used to maintainour high body temperature (37C).

Cellular respiration is remarkably

efficient in energy conversion.

24

8/9/2019 HMM1414 Chapter 7

25/32


7.3 Anaerobic respiration Without oxygen, oxidative

phosphorylation ceases. Through fermentation, some cells can

oxidize organic fuel and generate ATP

without use of O2.

Anaerobic catabolism of sugars can

occur by fermentation. Fermentation generate ATP from glucose

by substrate-level phosphorylation as long

as there is a supply of NAD+ to accept

electrons.

If NAD+ pool is exhausted, glycolysis

shuts down. Under aerobic conditions, NADH transfers

its electrons to ETC, recycling NAD+.

Under anaerobic conditions, various

fermentation pathways generate ATP by

glycolysis and recycle NAD+ by transferring

electrons from NADH to pyruvate orderivatives of pyruvate.

25

8/9/2019 HMM1414 Chapter 7

26/32


7.3.1 Ethanol fermentation(Figure 9.17 (a), Campbell, page 173)

In alcohol fermentation, pyruvate isconverted to ethanol in two steps.

1.Pyruvate converted to acetaldehyde (2C),

by removal of CO2.

2.Acetaldehyde reduced by NADH to

ethanol.

Alcohol fermentation by yeast - brewingand winemaking.

7.3.2 Lactic fermentation(Figure 9.17 (b), Campbell, page 173)

Pyruvate is reduced directly by NADH toform lactate without release of CO2.

Lactic acid fermentation by some fungi

and bacteria is used to make cheese and

yogurt.

Human muscle cells switch from aerobic

respiration to lactic acid fermentation togenerate ATP when O2 is scarce.

The waste product, lactate, may cause

muscle fatigue, but ultimately it is

converted back to pyruvate in the liver.

26

8/9/2019 HMM1414 Chapter 7

27/32


Fermentation and Cellular RespirationCompared Similarities both use

1. Glycolysis to oxidize sugars to

pyruvate with a net production of 2 ATP

by substrate-level phosphorylation.

2. NAD+ as an oxidizing agent to

accept electrons from food duringglycolysis.

Difference

1. Mechanism for oxidizing NADH to NAD+.

Fermentation - electrons of NADH are

passed to an organic molecule toregenerate NAD+.

Respiration - electrons of NADH are

ultimately passed to O2, generating

ATP by oxidative phosphorylation.

2. ATP generated per molecule of glucose.

Aerobic respiration: 36 - 38 ATP.

Anaerobic respiration: 2 ATP.

27

8/9/2019 HMM1414 Chapter 7

28/32


Facultative Anaerobes Makes ATP by aerobic respiration ifO2 is

present but can switch to fermentation inabsence ofO2.

Example: Yeast and many bacteria &

human muscle cells (at a cellular level).

Aerobic conditions: pyruvate is converted

to acetyl CoA and oxidation continues in

the citric acid cycle.

Anaerobic conditions: pyruvate serves as

an electron acceptor to recycle NAD+.

Evolutionary Significance of Glycolysis Oldest bacterial fossils are more than 3.5billion years old, appearing long before

appreciable quantities of O2 accumulated in

atmosphere.

Therefore, first prokaryotes may have

generated ATP exclusively from

glycolysis. The fact that glycolysis is the most

widespread metabolic pathway and occurs

in cytosol without membrane-enclosed

organelles suggests that glycolysis evolved

early in history of life.

28
http://en.wikipedia.org/wiki/Adenosine_triphosphatehttp://en.wikipedia.org/wiki/Aerobic_respirationhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Fermentation_(biochemistry)http://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Adenosine_triphosphatehttp://en.wikipedia.org/wiki/Aerobic_respirationhttp://en.wikipedia.org/wiki/Oxygenhttp://en.wikipedia.org/wiki/Fermentation_(biochemistry)http://en.wikipedia.org/wiki/Oxygen

8/9/2019 HMM1414 Chapter 7

29/32

8/9/2019 HMM1414 Chapter 7

30/32


Fats must be digested to glycerol and

fatty acids.

Glycerol can be converted to G3P, an

intermediate of glycolysis.

Energy-rich fatty acids are split into 2C

fragments via beta oxidation. These molecules enter citric acid cycle

as acetyl CoA.

Biosynthesis (Anabolic Pathways) Intermediaries in glycolysis and citric

acid cycle can be diverted to anabolic

pathways.

Example, human cell can synthesize

about half the 20 different amino acids bymodifying compounds from citric acid

cycle.

Glucose can be synthesized from

pyruvate,

Fatty acids from acetyl CoA.

Glycolysis and citric acid cycle functionas metabolic interchanges that enable cells

to convert one kind of molecule to another.

Example, excess carbohydrates and

proteins can be converted to fats through

intermediaries of glycolysis and citric

acid cycle.

30

8/9/2019 HMM1414 Chapter 7

31/32


Feedback mechanisms control cellularrespiration Basic principles of supply and demandregulate metabolic economy.

If cell has excess of a certain amino acid,

it uses feedback inhibition to prevent

intermediates from citric acid cycle being

synthesized to that amino acid.

Rate of catabolism also regulated by ATP

level in cell.

If ATP levels drop, catabolism speeds up

to produce more ATP.

Control of catabolism is based mainly on

regulating activity of enzymes at strategic

points in catabolic pathway.

One strategic point occurs in 3rd step of

glycolysis, catalyzed by allosteric enzyme,

phosphofructokinase.

Catalyzes earliest step that irreversiblycommits the substrate to glycolysis.

Inhibited by ATP and stimulated by AMP

(derived from ADP).

When ATP levels are high, inhibition of

this enzyme slows glycolysis.

31

8/9/2019 HMM1414 Chapter 7

32/32


As ATP levels drop and ADP and AMP

levels rise, enzyme becomes active

again and glycolysis speeds up.

Citrate is also an inhibitor of

phosphofructokinase.

Synchronizes rate of glycolysis and citric

acid cycle.

If intermediaries from citric acid cycle

are diverted to other uses (e.g., amino acidsynthesis), glycolysis speeds up to replace

these molecules.

Date post:	29-May-2018
Category:	Documents
Upload:	luqman-al-bashir-fauzi
View:	216 times
Download:	0 times

HMM1414 Chapter 7

Documents