Birgit Woelker, PhD

Chapter 6

How Cells Harvest Chemical Energy

INTRODUCTION TO CELLULAR RESPIRATION

Photosynthesis and cellular respiration provide energy for life

•  Cellular respiration makes ATP and consumes O2

–  During the oxidation of glucose to CO2 and H2O

CO2

H2O

Glucose

O2

ATP

ECOSYSTEM

Sunlight energy

Photosynthesis in chloroplasts

Cellular respiration in mitochondria

(for cellular work)

Heat energy

+! +!

Cellular respiration makes ATP and consumes O2 during the oxidation of glucose to CO2 and H2O

Photosynthesis uses solar energy to produce glucose and O2 from CO2 and H2O

Breathing supplies oxygen to our cells and removes carbon dioxide

•  Breathing provides for the exchange of O2 and CO2

–  Between an organism and its environment

CO2

CO2

O2

O2 Bloodstream

Muscle cells carrying out Cellular Respiratin

Breathing

Glucose + O2

CO2 + H2O + ATP

Lungs

Cellular respiration banks energy in ATP molecules

•  Cellular respiration breaks down glucose molecules

–  And banks their energy in ATP

C6H12O6 CO2 6 H2O ATPs

Glucose Oxygen gas Carbon dioxide

6

Water Energy

O2 6 + + +

Figure 6.3

The human body uses energy from ATP for all its activities

•  ATP powers almost all cellular and body activities

Table 6.4

Cells tap energy from electrons “falling” from organic fuels to oxygen

•  Electrons lose potential energy

–  During their transfer from organic compounds to oxygen

•  NADH passes electrons to an electron transport chain

•  As electrons “fall” from carrier to carrier and finally to O2

•  Energy is released in small quantities

NAD+

H+

NADH Energy released and available

for making 2

2

O2

2

H2O

! 2 1

ATP

•  When glucose is converted to carbon dioxide

–  It loses hydrogen atoms, which are added to oxygen, producing water

C6H12O6 6 O2 6 CO2 6 H2O

Loss of hydrogen atoms (oxidation)

Gain of hydrogen atoms (reduction)

Energy

(ATP) Glucose

+ + +

Figure 6.5A

•  Dehydrogenase removes electrons (in hydrogen atoms) from fuel molecules (oxidation)

–  And transfers them to NAD+ (reduction)

Figure 6.5B

O H H O 2H

Reduction

Dehydrogenase

(carries 2 electrons)

NAD+ 2H

2H+! 2e-!

NADH H+!

Oxidation

+

+

+

+

NADH

NADH FADH2

GLYCOLYSIS Glucose Pyruvate CITRIC ACID

CYCLE

OXIDATIVE PHOSPHORYLATION

(Electron Transport and Chemiosmosis)

Substrate-level phosphorylation

Oxidative phosphorylation

Mitochondrion

and

High-energy electrons carried by NADH

ATP ATP ATP

CO2 CO2

Cytoplasm

Substrate-level phosphorylation

Cellular respiration occurs in three main stages •  Stage 1: Glycolysis

–  Occurs in the cytoplasm

–  Breaks down glucose into pyruvate, producing a small amount of ATP

•  Stage 2: The citric acid cycle

–  Takes place in the mitochondria

–  Completes the breakdown of glucose, producing a small amount of ATP

–  Supplies the third stage of cellular respiration with electrons

•  Stage 3: Oxidative phosphorylation

–  Occurs in the mitochondria

–  Uses the energy released by “falling” electrons to pump H+ across a membrane

–  Harnesses the energy of the H+ gradient through chemiosmosis, producing ATP

Glycolysis harvests chemical energy by oxidizing glucose to pyruvate

•  In glycolysis, ATP is used to prime a glucose molecule

–  Which is split into two molecules of pyruvate

NAD+ NADH H+

Glucose 2 Pyruvate

ATP 2P 2 ADP

2 2

2

2

+

+

Figure 6.7A

•  Glycolysis produces ATP by substrate-level phosphorylation

–  In which a phosphate group is transferred from an organic molecule to ADP

Enzyme

Adenosine

Organic molecule (substrate)

ADP ATP

P

P P P

P

Figure 6.7B

•  In the first phase of glycolysis

–  ATP is used to energize a glucose molecule, which is then split in two

ATP

Glucose PREPARATORY PHASE (energy investment)

ADP

Step

Glucose-6-phosphate

Fructose-6-phosphate

P

P

Fructose-1,6-diphosphate

ATP

ADP

P P

Steps – A fuel molecule is energized, using ATP.

Step A six-carbon intermediate splits into two three-carbon intermediates.

1

2

3

4 4

1 3

Figure 6.7C Pyruvate

ATP

ADP

ATP

ADP

P

ATP ATP

ADP ADP

P

2-Phosphoglycerate

P

H2O H2O

Phosphoenolpyruvate (PEP)

Steps – ATP and pyruvate are produced.

P 3 -Phosphoglycerate

P

P

9 9

6 6

7 7

8 8

6 9 Step A redox reaction generates NADH.

P

NADH NADH P

P P P P

P+H+ +H+

ENERGY PAYOFF PHASE

Glyceraldehyde-3-phosphate (G3P)

1,3 -Diphosphoglycerate

P

5

6 9

5 5

6 6

7 7

8 8

9 9

NAD + NAD +

•  In the second phase of glycolysis

–  ATP, NADH, and pyruvate are formed

Figure 6.7C

CO2

Pyruvate

NAD+ NADH + H+!

CoA Acetyl CoA

(acetyl coenzyme A)

Coenzyme A

Figure 6.8

Pyruvate is chemically groomed for the citric acid cycle

•  Prior to the citric acid cycle

–  Enzymes process pyruvate, releasing CO2 and producing NADH and acetyl CoA

1

2

3

The citric acid cycle completes the oxidation of organic fuel, generating many NADH and FADH2 molecules

•  In the citric acid cycle

–  The two-carbon acetyl part of acetyl CoA is oxidized

CoA CoA

CO2

NAD+

NADH FAD

FADH2

ATP P

CITRIC ACID CYCLE

ADP +

3

3

+!3 H+

Acetyl CoA

2

Figure 6.9A

and Steps and

CITRIC ACID CYCLE

Oxaloacetate

CoA

CoA

2 carbons enter cycle

Acetyl CoA

Citrate

leaves cycle

+ H+!

NAD+

NADH

CO2

Alpha-ketoglutarate

leaves cycle CO2

ADP P

NAD+ NADH + H+!

ATP

+!

Succinate

FAD

FADH2

Malate

+ H+!

NAD+

NADH

Step

Acetyl CoA stokes the furnace.

Steps

NADH, ATP, and CO2 are generated during redox reactions.

Redox reactions generate FADH2 and NADH.

Figure 6.9B

•  For each turn of the cycle

–  Two CO2 molecules are released

–  The energy yield is one ATP, three NADH, and one FADH2

2

2

1

1

3

3

4

4

5

5

Most ATP production occurs by oxidative phosphorylation

•  Electrons from NADH and FADH2

–  Travel down the electron transport chain to oxygen, which picks up H+ to form water

•  Energy released by the redox reactions

–  Is used to pump H+ into the space between the mitochondrial membranes

•  In chemiosmosis, the H+ diffuses back through the inner membrane through ATP synthase complexes

–  Driving the synthesis of ATP

Intermembrane space

Inner mitochondrial membrane

Mitochondrial matrix

Protein complex

Electron flow

Electron carrier

NADH NAD+

FADH2 FAD

H2O ATP ADP

ATP synthase

H+ H+ H+ H+

H+ H+

H+

H+ H+

H+

H+

H+

H+

H+

+ P

O2

Electron Transport Chain Chemiosmosis

.

OXIDATIVE PHOSPHORYLATION

+ 2 12

Figure 6.10

Q+

Cyt c

Certain poisons interrupt critical events in cellular respiration

•  Various poisons

–  Block the movement of electrons

–  Block the flow of H+ through ATP synthase

–  Allow H+ to leak through the membrane

H+

H+

H+

H+

H+

H+ H+ H+ H+

H+

H+

H+ H+

O2

H2O P ATP

NADH NAD+

FADH2 FAD

Rotenone Cyanide, carbon monoxide

Oligomycin

DNP

ATP Synthase

+! 2

ADP +!

Electron Transport Chain Chemiosmosis

1

2

Figure 6.11

Each molecule of glucose yields ~38 molecules of ATP

NADH NADH

NADH NADH FADH2

Cytoplasm

Electron shuttle across membrane Mitochondrion

GLYCOLYSIS

Glucose Pyruvate

by substrate-level phosphorylation

by substrate-level phosphorylation

by oxidative phosphorylation

OXIDATIVE PHOSPHORYLATION

(Electron Transport and Chemiosmosis)

2 Acetyl CoA

CITRIC ACID CYCLE

+ 2 ATP + 2 ATP + about 34 ATP

Maximum per glucose: About 38 ATP

2

2 6 2

2 2 (or 2 FADH2)

Fermentation is an anaerobic alternative to cellular respiration

•  Under anaerobic conditions, many kinds of cells

–  Can use glycolysis alone to produce small amounts of ATP

•  In lactic acid fermentation NADH is oxidized to NAD+ as pyruvate is reduced to lactate

2 Lactate

NAD+ NADH NADH NAD+ 2 2 2 2

2 ATP 2 ADP + 2 2 Pyruvate

GLYCOLYSIS

P

Glucose

•  In alcohol fermentation NADH is oxidized to NAD+ while converting pyruvate to CO2 and ethanol

NAD+ NADH NADH NAD+ 2 2 2 2

GLYCOLYSIS

2 ADP + 2 P ATP

Glucose 2 Pyruvate

released CO2

2 Ethanol

2 2

INTERCONNECTIONS BETWEEN MOLECULAR BREAKDOWN AND SYNTHESIS

•  Cells use many kinds of organic molecules as fuel for cellular respiration

•  Carbohydrates, fats, and proteins can be converted to molecules that enter glycolysis or the citric acid cycle

OXIDATIVE PHOSPHORYLATION

(Electron Transport and Chemiosmosis)

Food, such as peanuts

Carbohydrates Fats Proteins

Sugars Glycerol Fatty acids Amino acids

Amino groups

Glucose G3P Pyruvate Acetyl CoA

CITRIC ACID

CYCLE

ATP

GLYCOLYSIS

Cells use many kinds of organic molecules as fuel for cellular respiration

Food molecules provide raw materials for biosynthesis

ATP needed to drive biosynthesis

ATP

CITRIC ACID

CYCLE

GLUCOSE SYNTHESIS Acetyl CoA Pyruvate G3P Glucose

Amino groups

Amino acids Fatty acids

Glycerol Sugars

Carbohydrates Fats Proteins

Cells, tissues, organisms

• Cells use some food molecules and intermediates from glycolysis and the citric acid cycle as raw materials

• This process of biosynthesis consumes ATP

The fuel for respiration ultimately comes from photosynthesis

•  All organisms

–  Can harvest energy from organic molecules

•  Plants, but not animals

–  Can also make these molecules from inorganic sources by the process of photosynthesis