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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