Cellular Respiration in DETAIL

H. Biology

The Stages of Cellular Respiration

• Respiration is a cumulative process of 3 metabolic stages1. Glycolysis2. Kreb’s Cycle (The citric acid cycle)3. Electron Transport Chain (Oxidative phosphorylation)

Stage #1: Glycolysis Glycolysis produces energy by oxidizing glucose

pyruvateGlycolysis

Means “splitting of sugar”Breaks down glucose into pyruvateOccurs in the cytoplasm of the cell1 glucose breaks down 4 ATP made

+ 2 pyruvate molecules(net gain of 2 ATP…NOT 4 ATP)

GlycolysisMain Goal of Glycolysis is to turn glucose into two pyruvate: - Series of 10 steps - Produces a net gain of 2 ATP and 2 NADH (e- carriers) - From here it can go to the Krebs cycle (aerobic respiration) or to Fermentation (anaerobic) - Glycolysis is anaerobic - Occurs in the cytoplasm

Overall: Glucose → 2 Pyruvate; net gain 2 ATP and 2 NADH

Intermediate step

Pyruvate (made in the cytosol via glycolysis) diffuses into the mitochondria. As it diffuses is, it produces one molecule of CO2 loses a carbon (goes from 3C to 2C). This new 2C molecule is acetyl CoA. Acetyl CoA is what enters into the Krebs cycle.

Glycolysis

NET Glucose 2 pyruvate (pyruvic acid)

+ 2 H2O 4 ATP formed – 2 ATP used 2 ATP GAIN

substrate-level phosphorylation used 2 NAD+ + 4e- + 4H+ 2 NADH + 2H+

**Glycolysis can proceed WITHOUT O2

• Glycolysis consists of two major phases1. Energy

investment phase (endergonic= uses 2 ATP)

2. Energy payoff phase

(exogonic = makes 4 ATP)

Glycolysis Citricacidcycle

Oxidativephosphorylation

ATP ATP ATP

2 ATP

4 ATP

used

formed

1 Glucose

2 ADP + 2 P

4 ADP + 4 P

2 NAD+ + 4 e- + 4 H +

2 NADH

+ 2 H+

2 Pyruvate + 2 H2O

Energy investment phase

Energy payoff phase

Glucose 2 Pyruvate + 2 H2O

4 ATP formed – 2 ATP used 2 ATP

2 NAD+ + 4 e– + 4 H + 2 NADH

+ 2 H+

Stage #2: The Kreb’s Cycle

• The citric acid cycle–Takes place in the matrix of the

mitochondrion

**NEEDS O2 TO PROCEED (unlike glycolysis)

• Stage #1 ½ : The Citric Acid Cycle• Before the citric acid cycle can begin– Pyruvate must first be converted to acetyl CoA, which links

the citric acid cycle to glycolysisCYTOSOL MITOCHONDRIO

N

NADH + H+NAD+

2

31

CO2

Coenzyme A

(a vitamin)Pyruvate

Acetyle CoA

S CoA

C

CH3

O

Transport protein

O–

O

O

C

C

CH3

Figure 9.10

Uses active

transport

Diffuses out of cell

Acetyl group= unstable

The Kreb’s Cycle

• Also called the “Citric Acid cycle”

• NAD+ and FAD (both are coenzymes) = electron “carriers”; proton acceptors– They are reduced and carry e-’s from

Citric cycle to ETC–Dehydrogenase catalyzes hydrogen

transfer reaction

NAD+ and FAD

Oxidized Form Reduced Form NAD+ NADH (2 e-, 1 H)

FAD FADH2 (4 e-, 2 H)

THINK: FADH2 come into play in the 2nd stage of cellular respiration; it is also the 2nd electron carrier

An overview of the citric acid cycle

(this occurs for EACH pyruvate molecule)

ATP

2 CO2

3 NAD+

3 NADH+ 3 H+

ADP + P i

FAD

FADH2

Citricacidcycle

CoA

CoA Acetyle CoA

NADH+ 3 H+

CoA

CO2

Pyruvate (from glycolysis,2 molecules per glucose)

ATP ATP ATP

Glycolysis Citricacidcycle

Oxidativephosphorylation

Figure 9.11

Krebs/ citric acid cycleMain Function of the Krebs → to make electron carriers (NADH and FADH2) to send to the ETC

Series of 8 steps; Occurs in the mitochondrial matrix

So…1 glucose produces: 2 ATP6 NADH2 FADH2

(remember: 1 glucose = 2 pyruvates)

Acetyl CoA (2C) enters the Krebs and combines with another molecule (4C) to form citric acid (hence citric acid cycle)

Electron Carriers

Krebs →Makes 1 ATP, 3 NADH, and 1 FADH2 per turn

You do NOT need to memorize

this!

Figure 9.12

Acetyl CoA

NADH

Oxaloacetate

CitrateMalate

Fumarate

SuccinateSuccinyl

CoA

a-Ketoglutarate

Isocitrate

Citricacidcycle

S CoA

CoA SH

NADH

NADH

FADH2

FAD

GTP GDP

NAD+

ADP

P i

NAD+

CO2

CO2

CoA SH

CoA SH

CoAS

H2O

+ H+

+ H+ H2O

C

CH3

O

O C COO–

CH2

COO–

COO–

CH2

HO C COO–

CH2

COO–

COO–

COO–

CH2

HC COO–

HO CHCOO–

CH

CH2

COO–

HO

COO–

CH

HC

COO–

COO–

CH2

CH2

COO–

COO–

CH2

CH2

C O

COO–

CH2

CH2

C O

COO–

1

2

3

4

5

6

7

8

Glycolysis Oxidativephosphorylation

NAD+

+ H+

ATP

Citricacidcycle

Figure 9.12

Kreb’s Cycle Summary• pyruvate Acetyl-CoA + 1 NADH • Each turn of cycle uses 1 pyruvate–So… 1 glucose molecule produces 2 turns

of Kreb’s cycle• 1 turn of cycle yields 4 NADH, 1 ATP,

and 1 FADH2 and 3 CO2 (as waste product) Remember to multiply by 2…why?

Stage #3: Oxidative Phosphorylation (Electron Transport Chain (ETC) + Chemiosmosis)

• Chemiosmosis couples electron transport to ATP synthesis

• NADH and FADH2

–Donate e-s to ETC, which powers ATP synthesis using oxidative phosphorylation

**OCUURS IN CRISTAE (folds of inner membrane)

The Pathway of Electron Transport

• In the ETC…–e-s from NADH and FADH2 lose

energy in several steps**NEEDS O2 TO PROCEED

(unlike glycolysis)

Electron transport chain (ETC)

Occurs on the inner membrane of the mitochondria; Energy from NADH and FADH2 power ATP synthesis

The ETC is a series of proteins throughout the membrane; the electrons lose energy every time they get passed down the chain…

OXYGEN IS THE FINAL ELECTRON ACCEPTOR!!! → oxygen combines with the electrons and H+ to make WATER

Main Goal of the ETC → • It creates a proton gradient that powers chemiosmosis which creates ATP through ATP Synthase

•NADH and FADH2 drop off electrons to the ETC• As the electrons get passed down the chain, they lose energy•The ETC uses that energy (from the electrons) and pumps the protons OUT of the matrix into the intermembrane space• This creates a concentration gradientThe protons then diffuse back INTO the matrix through the ATP synthase (chemiosmosis)-This creates a TON of ATP either 32 or 34 ATPs

• Final e- acceptor after the electrons go down the ETC is OXYGEN (from the atmosphere)• It combines with e- and H+ to make the final product = WATER

Electron transport chain (ETC)

ETC Characteristics

• Lots of proteins (cytochromes) in cristae increases surface area– 1000’s (many) copies of ETC

• ETC carries e-’s from NADH/FADH2 O2

• O2 = pulls e-s “down” ETC due to electronegativity (high affinity for e-s)

• Chemiosmosis and the electron transport chain

Oxidativephosphorylation.

electron transportand chemiosmosis

Glycolysis

ATP ATP ATP

InnerMitochondrialmembrane

H+

H+H+

H+

H+

ATPP i

Protein complexof electron carriers

Cyt c

I

II

III

IV

(Carrying electronsfrom, food)

NADH+

FADH2

NAD+

FAD+ 2 H+ + 1/2 O2

H2O

ADP +

Electron transport chainElectron transport and pumping of protons (H+),

which create an H+ gradient across the membrane

ChemiosmosisATP synthesis powered by the flowOf H+ back across the membrane

ATPsynthase

Q

Oxidative phosphorylation

Intermembranespace

Innermitochondrialmembrane

Mitochondrialmatrix

Figure 9.15

http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120071/bio11.swf::Electron%20Transport%20System%20and%20ATP%20Synthesis

Protin motive force is used!

• At the end of the chain– Electrons are passed to oxygen, forming water–O2 = final e- acceptor

• NAD delivers e- higher than FAD NAD provides 50% more ATP

What happens at the end of the ETC chain?

ETC is a Proton (H+) Pump• Uses the energy from “falling” e-s

(exergonic flow) to pump H+’s from matrix to outer compartment

• A H+ (proton) gradient forms inside mitochondria

• ETC does NOT make ATP directly but provides stage for CHEMIOSOMOSIS to occur

RECALL…Chemiosmosis

• Is an energy-coupling mechanism that uses energy in the form of a H+ gradient across a membrane to drive cellular work• Uses ATP synthase• Makes ~90% of ATP in Cell Resp. • Proposed by Peter Mitchell (1961)

Chemiosmosis: The Energy-Coupling Mechanism

• ATP synthase– Is the enzyme

that actually makes ATP

INTERMEMBRANE SPACE

H+

H+

H+

H+

H+

H+ H+

H+

P i

+ADP

ATP

A rotor within the membrane spins clockwise whenH+ flows past it down the H+

gradient.

A protein anchoredin the membraneholds the knobstationary.

A rod (or “stalk”)extending into the rotor/ knob alsospins, activatingcatalytic sites inthe knob.

Three catalytic sites in the stationary knobjoin inorganic Phosphate to ADPto make ATP.

MITOCHONDRIAL MATRIXFigure 9.14

http://www.sigmaaldrich.com/life-science/metabolomics/learning-center/metabolic-pathways/atp-synthase.html

• There are three main processes in this metabolic enterprise

Electron shuttlesspan membrane

CYTOSOL 2 NADH

2 FADH2

2 NADH 6 NADH 2 FADH22 NADH

Glycolysis

Glucose2

Pyruvate

2AcetylCoA

Citricacidcycle

Oxidativephosphorylation:electron transport

andchemiosmosis

MITOCHONDRION

by substrate-levelphosphorylation

by substrate-levelphosphorylation

by oxidative phosphorylation

Maximum per glucose:

About36 or 38 ATP

+ 2 ATP + about 32 or 34 ATP

or

Figure 9.16

+ 2 ATP

• About 40% of the energy in a glucose molecule–Is transferred to ATP during

cellular respiration, making ~36- 38 ATP

Overall (Aerobic Respiration) FADH2 NADH CO2 ATP Total ATP

Gained

Glycolysis2 4 2 (net)

Pyruvate Acetyl-

CoA

2

Kreb’sCycle

2 6 6 2 2

ETC~32-34

ATP GRAND TOTAL = ~36-38 ATP per 1 GLUCOSE

Cellular Respiration overviewATP Summary →Glycolysis – 2 ATPKrebs – 2 ATPETC – 32/34 ATP