Energy Use in Cells

Glycolysis, Krebs’s Cycle, Electron Transport, Fermentation &

Metabolism

Glycolysis

– a single molecule of glucose is enzymatically cut in half through a series of steps,

– two molecules of pyruvate are produced,– two molecules of NAD+ are reduced to two

molecules of NADH, and– a net of two molecules of ATP is produced.

Figure 6.7A

Glucose

2 Pyruvate

2 ADP

2 P2 NAD

2 NADH

2 HATP2

Figure 6.7Ca_s2Glucose

Glucose 6-phosphate

Fructose 6-phosphate

Fructose 1,6-bisphosphate

Glyceraldehyde3-phosphate (G3P)

ENERGYINVESTMENT

PHASE

PP

P P

P

P

ADP

ADP

ATP

ATP

StepSteps – A fuelmolecule is energized,using ATP.

Step A six-carbonintermediate splitsinto two three-carbonintermediates.

4

4

3

3

2

1

1

Figure 6.7Cb_s2

6 6 6

5 5 5

9

9 9

8 8

7 7

Step A redox reactiongenerates NADH.

Steps – ATP and pyruvateare produced.

ENERGYPAYOFFPHASE

1,3-Bisphospho-glycerate

3-Phospho-glycerate

2-Phospho-glycerate

Phosphoenol-pyruvate (PEP)

Pyruvate

NADH NADH

NADNAD

H H

ADP ADP

ADP ADP

ATP ATP

ATP ATP

H2O H2O

P P

P

P P

P

P

P

P

P P

P

P

P

The Krebs Cycle Releases Carbon Dioxide and Generates Energy Carriers• Pyruvate entering the mitochondrion must be

broken down into acetyl CoA before entering the Krebs Cycle

• Through a series of reactions, the Krebs cycle produces ATP, NADH, and FADH2, energy carriers that will be used in the final stage of cellular respiration

Figure 6.8

Pyruvate

Coenzyme A

Acetyl coenzyme A

NAD NADH H

CoA

CO2

3

2

1

Figure 6.9AAcetyl CoA

Citric Acid Cycle

CoA

CoA

CO22

3

3

NAD

3 H

NADH

ADPATP P

FAD

FADH2

The Krebs’s Cycle

• Remember that the citric acid cycle processes two molecules of acetyl CoA for each initial glucose.

• Thus, after two turns of the citric acid cycle, the overall yield per glucose molecule is– 2 ATP,– 6 NADH, and– 2 FADH2.

Oxidative Phosphorylation Uses Oxygen to Produce ATP in Quantity• Oxidative phosphorylation produces the

largest amount of ATP during cellular respiration

• Oxidative phosphorylation is the last stage of cellular respiration and takes place in the many folds (cristae) of the inner mitochondrial membrane

Oxidative Phosphorylation Uses Oxygen to Produce ATP in Quantity

• Energy carriers produced during the Krebs cycle donate their high-energy electrons to the electron transport chain (ETC), which releases energy as it passes electrons down the chain

• The energy released from the ETC is used to create a proton gradient

Oxidative Phosphorylation Uses Oxygen to Produce ATP in Quantity

• The movement of protons through the ATP synthase channel activates enzymes that catalyze the phosphorylation of ADP to from ATP

• Electrons that travel down the ETC are eventually accepted by O2 and H+ to make water

• Cellular respiration has a net yield of about 30 to 32 ATP molecules per molecule of glucose

Figure 6.12

NADH

FADH2

NADH FADH2NADH

orNADH

MitochondrionCYTOPLASM

Electron shuttlesacross membrane

Glycolysis

Glucose2

Pyruvate

PyruvateOxidation2 Acetyl

CoA

Citric AcidCycle

OxidativePhosphorylation

(electron transportand chemiosmosis)

Maximumper glucose:

by substrate-levelphosphorylation

by substrate-levelphosphorylation

by oxidativephosphorylation

2

2

2

2

6 2

ATP 2 about

28 ATP AboutATP32

ATP 2

Fermentation Facilitates ATP Production Through Glycolysis When Oxygen Is Absent

• Glycolysis does not require oxygen; it is an anaerobic process

• During fermentation, the pyruvate and NADH produced by glycolysis remain in the cytosol

• Postglycolytic reactions convert pyruvate and NADH into other molecules, such as alcohol or lactic acid

Fermentation Facilitates ATP Production Through Glycolysis When Oxygen Is Absent

• Fermentation by anaerobic yeasts converts pyruvate into an ethanol, releasing CO2 gas

• Lactic acids forms in severely taxed muscle cells as a result of ATP production in the absence of oxygen

Figure 6.13B

2 NAD

2 NADH

2 NAD

2 NADH

2 Ethanol

2 Pyruvate

Glucose

2 ADP

2 ATP

2 P

Gly

coly

sis

2 CO2

p. 101

Figure 6.13A

2 NAD

2 NADH

2 NAD

2 NADH

2 Lactate

2 Pyruvate

Glucose

2 ADP

2 ATP

2 P

Gly

coly

sis

Figure 6.15_1

Sugars GlycerolFatty acids Amino acids

Aminogroups

OxidativePhosphorylation

CitricAcidCycle

PyruvateOxidationAcetyl CoA

Glucose G3P Pyruvate

Glycolysis

Carbohydrates

ATP

Fats Proteins

Food

Figure 6.16_1

CarbohydratesFatsProteins

Cells, tissues, organisms

Amino acids Fatty acidsGlycerol Sugars

Aminogroups

CitricAcidCycle

PyruvateOxidationAcetyl CoA

ATP neededto drivebiosynthesis

ATP

Glucose SynthesisPyruvate G3P Glucose