How Cells Release Chemical Energy

Chapter 8

Cellular or Aerobic Respiration

Cellular respiration evolved to enable organisms to utilize energy stored in glucose.

Taps the energy found in the bonds of organic compounds (carbohydrates, lipids, proteins)

Comparison of the Main Pathways

Aerobic respiration • Aerobic metabolic pathways (using oxygen) are used

by most eukaryotic cells• Ends in the mitochondria • Aerobes use oxygen as the final electron acceptor

Fermentation• Anaerobic metabolic pathways (occur in the absence

of oxygen) are used by prokaryotes and protists in anaerobic habitats

• Anaerobes use nitrate or sulfate as the final e- acceptor

Comparison of the Main Pathways

Aerobic respiration and fermentation both begin with glycolysis, which converts one molecule of glucose into two molecules of pyruvate

After glycolysis, the two pathways diverge• Fermentation is completed in the cytoplasm,

yielding 2 ATP per glucose molecule• Aerobic respiration is completed in mitochondria,

yielding 36 ATP per glucose molecule (liberates the most energy)

Overview of Aerobic Respiration

Three sequential stages• Glycolysis• Acetyl-CoA formation and Krebs cycle• Electron transfer phosphorylation (ATP formation)

C6H12O6 (glucose) + O2 (oxygen) →

CO2 (carbon dioxide) + H2O (water)

• Coenzymes NADH and FADH2

Fig. 8-3b, p. 125

Animation: Overview of aerobic respiration

8.2 Glycolysis – Glucose Breakdown Starts

Glycolysis starts and ends in the cytoplasm of all prokaryotic and eukaryotic cells

An energy investment/input of ATP (2) starts glycolysis

Requires a continous supply of glucose and NAD+ (reduced in both glycolysis and the kreb cycle)

Glycolysis

Two ATP’s are used to split glucose (6 carbon compound) and form 2 PGAL (3 carbon compound)

Enzymes convert 2 PGAL to 2 PGA, forming 2 NADH

Four ATP are formed by substrate-level phosphorylation (net yield of 2 ATP)

Fig. 8-4a (2), p. 126

Fig. 8-4b (1), p. 127

Glycolysis ATP-Requiring Steps

glucose

A An enzyme transfers a phosphate group from ATP to glucose, forming glucose-6-phosphate.ATP

ADP B A phosphate group from a second ATP is transferred to the glucose-6- phosphate. The resulting molecule is unstable, and it splits into two three- carbon molecules. The molecules are interconvertible, so we will call them both PGAL (phosphoglyceraldehyde).

glucose-6-phosphateATP

ADP

fructose-1,6-bisphosphate (intermediate)

So far, two ATP have been invested in the reactions.

Fig. 8-4b (2), p. 127

ATP-Generating Steps

2 PGAL

C Enzymes attach a phosphate to the two PGAL, and transfer two electrons and a hydrogen ion from each PGAL to NAD+. Two PGA (phosphoglycerate) and two NADH are the result.

2 NAD+ + 2 Pi NADH

2 reduced coenzymesD Enzymes transfer a phosphate group from each PGA to ADP. Thus, two ATP have formed by substrate-level phosphorylation.2 ADP ATP

The original energy investment of two ATP has now been recovered.

2 ATP produced by substrate-level phosphorylation

2 PEP

E Enzymes transfer a phosphate group from each of two intermediates to ADP. Two more ATP have formed by substrate-level phosphorylation.

2 ADPATP

Two molecules of pyruvate form at this last reaction step.

2 ATP produced by substrate-level phosphorylation

F Summing up, glycolysis yields two NADH, two ATP (net), and two pyruvate for each glucose molecule.

2 pyruvate

Net 2 ATP + 2 NADHto second stage

2 PGA

8.3 Second Stage of Aerobic Respiration

The second stage of aerobic respiration finishes breakdown of glucose that began in glycolysis

Occurs in mitochondria (inner compartment) begans with the chemical pyruvate to continue respiration

Includes two stages: acetyl CoA formation and the Krebs cycle (each occurs twice in the breakdown of one glucose molecule)

The Krebs Cycle

Krebs cycle• A sequence of enzyme-mediated reactions that break

down 1 acetyl CoA (transition stage) into 2 CO2

• Oxaloacetate (last intermediate) is used and regenerated

• 3 NADH and 1 FADH2 are formed, 1 ATP is formed

• Substrate level phosphorylation occurs• Electrons and hydrogens are transferred to

coenzymes in both glycolysis and the Kreb cycle

• Energy, CO2, and H+ are released

• Cycle turns 2x to break down glucose

Acetyl–CoA Formation transition from glyco

A An enzyme splits a pyruvate molecule into a two-carbon acetyl group and CO2. Coenzyme A binds the acetyl group (forming acetyl–CoA). NAD+ combines with released hydrogen ions and electrons, forming NADH. NADH

pyruvate

CO2

NAD+coenzyme A

acetyl–CoA

Fig. 8-6, p. 129

B The Krebs cycle starts as one carbon atom is transferred from acetyl–CoA to oxaloacetate. Citrate forms, and coenzyme A is regenerated.

oxaloacetate

coenzyme A

citrate

KrebsCycle

FADH2

F The coenzyme FAD com-bines with hydrogen ions and electrons, forming FADH2.

FAD

NADH

G NAD+ combines with hydrogen ions and electrons, forming NADH.

NAD+

C A carbon atom is removed from an intermediate and leaves the cell as CO2. NAD+ combines with released hydrogen ions and electrons, forming NADH.

NADH

CO2

NAD+

Pyruvate’s three carbon atoms have now exited the cell, in CO2.

D A carbon atom is removed from another intermediate and leaves the cell as CO2, and another NADH forms.

NADH

CO2NAD+

ATPE One ATP forms by substrate-level phosphorylation.

ADP + Pi

H The final steps of the Krebs cycle regenerate oxaloacetate.

Stepped Art

Animation: The Krebs Cycle - details

8.4 Aerobic Respiration’s Big Energy Payoff

Many ATP’s are formed during the third and final stage of aerobic respiration (Chemiosmotic Theory = production of ATP’s)

Electron transfer phosphorylation• Occurs in mitochondria (inner membrane)• Results in attachment of phosphate to ADP to

form ATP • Generates a hydrogen concentration (build up

of hydrogen ions between 2 membranes) gradient

Electron Transfer Phosphorylation

Summary: The Energy Harvest

Typically, the breakdown of one glucose molecule yields 36 ATP• Glycolysis: 2 ATP• Acetyl CoA formation and Krebs cycle: 2 ATP• Electron transfer phosphorylation: 32 ATP

Animation: Third-stage reactions

8.5 Anaerobic Energy-Releasing Pathways

Fermentation pathways break down carbohydrates without using oxygen (Ex. Bacteria that causes botulism)

The final steps in these pathways regenerate NAD+

Fermentation Pathways

Glycolysis is the first stage of fermentation• Forms 2 pyruvate, 2 NADH, and 2 ATP

Pyruvate turns to lactic acid using e- from NADH (Lactate/Lactose Fermentation)

Pyruvate is converted to ethanol producing acetaldehyde and CO2 (Alcoholic Fermentation)

Two Pathways of Fermentation

Alcoholic fermentation• Pyruvate produce acetaldehyde and CO2 when

converted to ethanol• Acetaldehyde receives electrons and hydrogen

from NADH, forming NAD+ and ethanol

Lactate fermentation• Pyruvate receives electrons and hydrogen from

NADH, forming NAD+ and lactate (muscle cells, sour cream, etc)

Lactate Fermentation

lactate

Alcoholic Fermentation

2 CO2

acetaldehyde

Glycolysis glucose

2 NAD+

2 ATP 2 NADH

4

pyruvate

ATP

Fig. 8-9, p. 132

2 NADH

2 NAD+

ethanol

glucose Glycolysis

2 NAD+

2 ATP 2 NADH

pyruvate

4 ATP

2 NADH

2 NAD+

Stepped Art

8.6 The Twitchers

Slow-twitch muscle fibers (“red” muscles) make ATP by aerobic respiration• Have many mitochondria• Dominate in prolonged activity

Fast-twitch muscle fibers (“white” muscles) make ATP by lactate fermentation• Have few mitochondria and no myoglobin• Sustain short bursts of activity• Human muscles have a mixture of both fibers

8.7 Alternative Energy Sources in the Body

In humans and other mammals, the entrance of glucose and other organic compounds into an energy-releasing pathway depends on the kinds and proportions of carbohydrates (our main source of energy), fats and proteins in the diet

The Fate of Glucose at Mealtimeand Between Meals

When blood glucose concentration rises, the pancreas increases insulin (hormone) secretion• Cells take up glucose faster, more ATP is formed

When blood glucose concentration falls, the pancreas increases glucagon (hormone) secretion (between meals)• Stored glycogen is converted to glucose, the

brain continues to receive glucose• Triglycerides are tapped as an energy alternative.

Energy From Fats

About 78% of an adult’s energy reserves is stored in fat (mostly triglycerides)

Excess glucose(carbs) in the diet = FAT; acetyl Co A exits the Kreb Cycle and enters a pathway that makes fatty acids

Enzymes cleave fats into glycerol and fatty acids• Glycerol products enter glycolysis• Fatty acid products enter the Krebs cycle

Energy from Proteins

Enzymes split dietary proteins into amino acid subunits, which enter the bloodstream• Used to build proteins or other molecules

Excess amino acids are broken down into ammonia (NH3) and various products that can enter the Krebs cycle

Fig. 8-12, p. 135

FOOD

FATS COMPLEX CARBOHYDRATES PROTEINS

fatty acids glycerol glucose, other simple sugars amino acids

acetyl–CoA PGAL acetyl–CoA

Glycolysis

NADH pyruvate

oxaloacetate or another intermediate of the Krebs

Krebs Cycle

NADH, FADH2

Electron Transfer Phosphorylation