BIO 2, Lecture 13BIO 2, Lecture 13BIO 2, Lecture 13BIO 2, Lecture 13FIGHTING ENTROPY II:FIGHTING ENTROPY II:

RESPIRATIONRESPIRATION

• Respiration is the process whereby cells break down complex organic molecules (like starch) and convert them into ATP + heat (not 100% efficient)

• The cell then uses the ATP to do work

$10,000 bill (Starch;

Unusable)

$1 bills(ATP; Usable)

• There are two types of respiration:

• Anaerobic respiration, also called fermentation, occurs without O2

• Example: Ethanol fermentation of glucose

C6H12O6 + ADP + Pi 2 C2H5OH + 2 CO2 + ATP + heat

• Aerobic respiration relies on O2

• Is more efficient than anaerobic respiration; generates more ATP per organic molecule, loses less energy as heat

C6H12O6 + 6O2 + ADP + Pi 6CO2 + 6H2O + ATP +

heat

• Cells do three types of work: mechanical, transport, and chemical• All must be coupled to the hydrolysis of

ATP

• Overall, the coupled reactions are catabolic• An example is the creation of the amino

acid glutamine from ammonia and glutamic acid

(b) Coupled with ATP hydrolysis, a catabolic (exergonic) reaction

Ammonia displacesthe phosphate group,forming glutamine.

(a) Anabolic (endergonic) reaction

(c) Overall free-energy change

PP

GluNH3

NH2

Glu i

GluADP+

PATP+

+

Glu

ATP phosphorylatesglutamic acid,making the aminoacid less stable.

GluNH3

NH2

Glu+

Glutamicacid

GlutamineAmmonia

∆G = +3.4 kcal/mol

+2

1

Phosphate groupsRibose

Adenine

• The bonds between the phosphate groups of ATP’s tail can be broken by hydrolysis

• Energy is released from ATP when the terminal phosphate bond is broken

Inorganic phosphate

Energy

Adenosine triphosphate (ATP)

Adenosine diphosphate (ADP)

P P

P P P

P ++

H20

i

• ATP is a renewable resource that is regenerated by addition of a phosphate group to adenosine diphosphate (ADP)

• The energy to re-phosphorylate ADP comes from catabolic reactions in the cell

P iADP+

Energy fromcatabolism (exergonic,energy-releasingprocesses)

Energy for cellular work (endergonic, energy-consuming processes)

ATP +

H2OH2OATP

• Although carbohydrates, fats, and proteins can all be broken down during respiration to produce ATP, it is helpful to trace cellular respiration with the sugar glucose

• The step-wise transfer of electrons (from high energy states in complex organic molecules to lower energy states in simple organic molecules) gently releases the energy stored in glucose to regenerate ATP from ADP + P

• Chemical reactions that transfer electrons between reactants are called oxidation-reduction reactions, or redox reactions

• In oxidation, a substance loses electrons, or is oxidized

• In reduction, a substance gains electrons, or is reduced (the amount of positive charge is reduced)

becomes oxidized

(loses electron)

becomes reduced(gains

electron)

• Some redox reactions do not transfer electrons but change the electron sharing in covalent bonds, thereby changing the potential chemical energy stored in the molecules

• Reduced organic molecules carry more potential chemical energy than oxidized forms of the same molecules• Starch is a highly reduced form of water

and carbon dioxide• Therefore, breaking starch down into

water and carbon dioxide releases energy

Reactantsbecomes oxidized

becomes reduced

Products

Methane Oxygen Carbon dioxide

Water

• During cellular respiration, the fuel (such as glucose) is oxidized, and another molecule (such as O2) is reduced

• The chemical potential energy in the reactants is greater than the chemical potential energy in the products; thus energy is releasedbecomes

oxidized

becomes reduced

• Both anaerobic and aerobic respiration begin with glycolysis

– Breaks down glucose into two molecules of pyruvate) to produce 2 ATP per glucose

– Takes place in the cytoplasm– In anaerobic respiration, the process stops

here and only 2 ATP are generated per glucose

• Aerobic respiration has two additional steps that break down the pyruvate to carbon dioxide and water to produce an additional 36 ATP– The citric acid cycle (completes the

breakdown of glucose)– Oxidative phosphorylation (accounts

for most of the ATP synthesis)

• Both steps take place in the mitochondria

• Glycolysis (“splitting of sugar”) has two major phases:– Energy investment phase– Energy payoff phase

• In the energy investment phase, 2 ATP are consumed to “kick start” the process

• In the energy payoff phase, four ATP are produced, yielding a net gain of 2 ATP

Energy investment phase

Glucose

2 ADP + 2 P 2 ATP used

formed

4 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 H2OGlucose

Net

4 ATP formed – 2 ATP used 2 ATP

2 NAD+ + 4 e– + 4 H+ 2 NADH + 2 H+

• In anaerobic respiration, the process stops with the production of 2 ATP

• Pyruvate is converted to waste products (like ethanol) but no more ATP is gained

• The energy stored in NADH is likewise wasted• It is important to note, however, that

NADH carries high energy electrons and can be harvested to produce ATP if a cell has the machinery to do so ...

Substrate-levelphosphorylatio

n

ATP

Cytosol

Glucose Pyruvate

Glycolysis

Electronscarried

via NADH

Net = 2

Net = 2 NADHPotential source of

additional ATP; WASTED in aerobic

respiration

Potential source of additional ATP;

WASTED in aerobic respiration

Anaerobic Respiration

• In anaerobic respiration, the process stops with the production of 2 ATP

• Pyruvate is converted to waste products (like ethanol) but no more ATP is gained

• The energy stored in NADH is likewise wasted• It is important to note, however, that

NADH carries high energy electrons and can be harvested to produce ATP if a cell has the machinery to do so ...

• Aerobic respiration continues the process of glycolysis to breakdown pyruvate and utilize the high energy electrons stored in NADH

• Takes place in cells that have mitochondria– The citric acid cycle (breaks down pyruvate

to CO2 and H2O in the mitochondrial matrix to produce additional ATP, NADH, and FADH2)

– Oxidative phosphorylation (harvests electrons from NADH and FADH2 in the inner mitochondrial membrane and accounts for most of the ATP synthesis)

Mitochondrion

Substrate-levelphosphorylatio

n

ATP

Cytosol

Glucose Pyruvate

Glycolysis

Electronscarried

via NADH

Substrate-levelphosphorylatio

n

ATP

Electrons carried

via NADH andFADH2

Citricacidcycle

Mitochondrion

Substrate-levelphosphorylatio

n

ATP

Cytosol

Glucose Pyruvate

Glycolysis

Electronscarried

via NADH

Substrate-levelphosphorylatio

n

ATP

Electrons carried

via NADH andFADH2

Oxidativephosphorylatio

n

ATP

Citricacidcycle

Oxidativephosphorylation

:electron transport

andchemiosmosis

• The Citric Acid Cycle:

• In the presence of O2, pyruvate (generated by glycolysis) enters the mitochondrion from the cytoplasm

• Prior to the start of the cycle, pyruvate is converted to acetyl CoA, generating one molecule of NADH

• The cycle then oxidizes acetyl CoA, generating 1 ATP, 3 NADH, and 1 FADH2 per turn

Pyruvate

NAD+

NADH+ H+

Acetyl CoA

CO2

CoA

CoA

CoA

Citricacidcycle

FADH2

FAD

CO22

3

3 NAD+

+ 3 H+

ADP +

P

i

ATP

NADH

• The cycle is “fed” by acetyl-coA• In the first step, the acetyl-coA is

combined with oxaloacetate to form citrate

• The citrate is then broken down in a series of steps to produce energy (in the form of ATP, NADH, and FADH2) + CO2 (gas)

• The end product of the cycle is oxaloacetate, which can then combine with another molecule of acetyl-coA to run the cycle again ...

Acetyl CoA

CoA—SH

Citrate

H2O

IsocitrateNAD+

NADH+ H+

CO2

-Keto-glutarate

CoA—SH

CO2

NAD+

NADH+ H+

Succinyl

CoA

CoA—SH

Pi

GTP

GDP

ADP

ATP

Succinate

FAD

FADH2

Fumarate

Citric

acidcycl

e

H2O

Malate

Oxaloacetate

NADH+H

+NAD+

1

2

3

4

5

6

7

8

• Following glycolysis and the citric acid cycle, NADH and FADH2 carry most of the energy extracted from food

• These two molecules transport the high energy electrons generated by the breakdown of glucose to pyruvate (during glycolysis) and pyruvate to oxaloacetate and CO2 (during the citric acid cycle) and donate them to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation

• The electron transport chain is located in the inner membrane of the mitochondrion

• Most of the chain’s components are proteins, which exist in multi-protein complexes

• The carriers alternate reduced and oxidized states as they accept (become reduced) and donate (become oxidized) electrons down the chain

• Electrons drop in free energy as they go down the chain and are finally passed to O2 (gas), forming H2O

NADH

NAD+2FADH2

2 FAD Multiprote

incomplexes

FAD

Fe•S

FMN

Fe•S

Q

Fe•S

Cyt b

Cyt c1 Cyt

cCyt a Cyt

a3

IV

Fre

e e

nerg

y (

G)

rela

tive t

o O

2

(kcal/

mol)

50

40

30

20

10

2

(from NADHor FADH2)

0 2 H+ + 1/2 O2

H2

O

e–

e–

e–

• As electrons are transferred down the electron transport chain, the energy released at each step is used by the protein complexes to pump H+ from the mitochondrial matrix to the inter-membrane space

• H+ then moves back across the membrane, with its diffusion gradient, passing through channels in a protein complex called ATP synthase

• ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ATP from ADP and Pi

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

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

Protein complexof electroncarriers

H+

H+H+

Cyt c

Q

V

FADH2

FAD

NAD+NADH

(carrying electronsfrom food)

Electron transport chain

2 H+ + 1/2O2

H2

O

ADP +

P i

Chemiosmosis

Oxidative phosphorylation

H+

H+

ATP synthase

ATP

21

Why we breathe

O2!!

• During aerobic respiration, most energy flows in this sequence:

glucose NADH electron transport chain proton-motive force ATP

• About 40% of the energy in a glucose molecule is transferred to ATP during aerobic respiration, generating about 38 ATP• The rest is lost as heat

Maximum per glucose:

About38 ATP

+ 2 ATP+ 2 ATP + about 34 ATP

Oxidativephosphorylation:

electron transport

andchemiosmosis

Citricacidcycle

2Acet

ylCoA

Glycolysis

Glucose

2Pyruva

te

2 NADH 2 NADH 6 NADH 2 FADH2

2 FADH2

2 NADHCYTOSOL

Electron shuttlesspan membrane

or

MITOCHONDRION

Anaerobic phase

Aerobic phase

• Comparing aerobic and anaerobic respiration:

• Both processes use glycolysis to oxidize glucose and other organic fuels to pyruvate

• However, the processes 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

Proteins

Carbohydrates

Aminoacids

Sugars

Fats

Glycerol

Fattyacids

Glycolysis

Glucose

Glyceraldehyde-3-

Pyruvate

P

NH3

Acetyl CoA

Citricacidcycle

Oxidativephosphorylati

on

Respiration can use many

different fuels (not just glucose!)

Fre

e e

nerg

y,

G

Fre

e e

nerg

y,

G

(a) Uncontrolled reaction

H2O

H2 + 1/2 O2

Explosiverelease ofheat and

lightenergy

(b) Cellular respiration

Controlledrelease ofenergy forsynthesis

ofATP

2 H+ + 2 e–

2 H + 1/2 O2

(from food)

ATP

ATPATP

1/2

O2

2 H+

2 e–Ele

ctron

transp

ort

chain

H2

O

Lightenergy

ECOSYSTEM

Photosynthesis in chloroplasts

CO2 + H2O

Cellular respiration

in mitochondria

Organicmolecules

+ O2

ATP powers most cellular work

Heatenergy

ATP