How Cells Release Chemical Energy
Chapter 7
Biology Concepts and Applications, Eight Edition, by Starr, Evers, Starr. Brooks/Cole, Cengage Learning 2011.
7.1 Overview of Carbohydrate Breakdown Pathways
All organisms (including photoautotrophs) convert chemical energy of organic compounds to chemical energy of ATP
ATP is a common energy currency that drives metabolic reactions in cells
Pathways of Carbohydrate Breakdown
Photoautotrophs Photosynthetic autotrophs• Produce sugar
Fermentation pathways anaerobic pathway• End in cytoplasm, do not use oxygen, yield 2 ATP
per molecule of glucose Aerobic respiration oxygen-requiring pathway
that breaks down carbohydrates to produce ATP• Ends in mitochondria, uses oxygen, yields up to
36 ATP per glucose molecule
• **Occurs in the presence of OXYGEN**
Pathways of Carbohydrate Breakdown
Overview of Aerobic Respiration
Three main stages of aerobic respiration:1. Glycolysis in the cytoplasm
•Convert glucose and other sugars to (2) pyruvate and (2) ATP
•Pyruvate is 3-carbon end product of glycolysis
2. Krebs cycle
3. Electron transfer phosphorylation
Summary equation:
C6H12O6 + 6O2 → 6CO2 + 6 H2O
Overview of Aerobic Respiration
Key Concepts: ENERGY FROM CARBOHYDRATE BREAKDOWN
All organisms produce ATP by degradative pathways that extract chemical energy from glucose and other organic compounds
Aerobic respiration yields the most ATP from each glucose molecule
In eukaryotes, aerobic respiration is completed inside mitochondria
7.3 Glycolysis – Glucose Breakdown Starts
Enzymes of glycolysis use two ATP to convert one molecule of glucose to two molecules of three-carbon pyruvate
Reactions transfer electrons and hydrogen atoms to two NAD+ (reduces to NADH)
4 ATP form by substrate-level phosphorylation• Transfers a phosphate group directly from a
substrate to ADP to form ATP
Products of Glycolysis
Net yield of glycolysis:• 2 pyruvate, 2 ATP, and 2 NADH per glucose
Pyruvate may: • Enter fermentation pathways in cytoplasm
• Enter mitochondria and be broken down further in aerobic respiration
Glycolysis
Glycolysis
ATP
ATP
glucose
ADP
ADP
PP
glucose–6–phosphate
fructose–1,6–bisphosphate
DHAP
Fig. 7.4c1, p.111
Glycolysis
ENERGY REQUIRING PHASE
ATP
2 ADP
2 NAD+ + 2 Pi
2 PGA
NADH
2 PGAL
Fig. 7.4c2, p.111
ATP
2 pyruvate
2 PEP
2 ADP
to second stage Net 2 ATP + 2 NADH
2 ATP producedby substrate-levelphosphorylation
2 ATP producedby substrate-levelphosphorylation
2 reduced coenzymes
ENERGY PRODUCINGPHASE
Key Concepts: GLYCOLYSIS
Glycolysis is the first stage of aerobic respiration and of anaerobic routes (fermentation pathways)
As enzymes break down glucose to pyruvate, the coenzyme NAD+ picks up electrons and hydrogen atoms
Net energy yield is two ATP
7.4 Second Stage of Aerobic Respiration
The second stage of aerobic respiration takes place in the inner compartment of mitochondria
It starts with acetyl-CoA formation and proceeds through the Krebs cycle• Kreb cycle cyclic pathway that, along with
acetyl-CoA formation, breaks down pyruvate to carbon dioxide
Second Stage of Aerobic Respiration
Acetyl-CoA Formation
Two pyruvates from glycolysis are converted to two acetyl-CoA
Two CO2 leave the cell
Acetyl-CoA enters the Krebs cycle
Krebs Cycle
Each turn of the Krebs cycle, one acetyl-CoA is converted to two molecules of CO2
After two cycles• Two pyruvates are dismantled
• Glucose molecule that entered glycolysis is fully broken down
Energy Products
Reactions transfer electrons and hydrogen atoms to NAD+ and FAD• Reduced to NADH and FADH2
ATP forms by substrate-level phosphorylation• Direct transfer of a phosphate group from a
reaction intermediate to ADP
Net Results
Second stage of aerobic respiration results in• Six CO2, two ATP, eight NADH, and two FADH2
for every two pyruvates
Adding the yield from glycolysis, the total is• Twelve reduced coenzymes and four ATP for
each glucose molecule
Coenzymes deliver electrons and hydrogen to the third stage of reactions
Second Stage Reactions
NADH
NADH
FADH2
ATP
KrebsCycle
Fig. 7.6a, p.113
KrebsCycle
NADH
NADH
coenzyme A
coenzyme A
acetyl–CoA
oxaloacetate citrate
pyruvate
Acetyl–CoAFormation
CO2
CO2
NAD+
NAD+
NAD+
FAD
NAD+
CO2
ADP + Pi
Fig. 7.6b, p.113
7.5 Third Stage:Aerobic Respiration’s Big Energy Payoff
Coenzymes deliver electrons and hydrogen ions to electron transfer chains in the inner mitochondrial membrane
Energy released by electrons flowing through the transfer chains moves H+ from the inner to the outer compartment
Hydrogen Ions and Phosphorylation
H+ ions accumulate in the outer compartment, forming a gradient across the inner membrane
H+ ions flow by concentration gradient back to the inner compartment through ATP synthases (transport proteins that drive ATP synthesis)
The Aerobic Part of Aerobic Respiration
Oxygen combines with electrons and H+ at the end of the transfer chains, forming water
Overall, aerobic respiration yields up to 36 ATP for each glucose molecule
Electron Transfer Phosphorylation
Fig. 7.7a, p.114
NADHFADH2
ATP
Fig. 7.7b, p.114
ADP + Pi
H+
H+
H+
H+
H+H+
H+H+H+
H+ H+ H+
INNERMITOCHONDRIAL
MEMBRANE
OUTERCOMPARTMENT
INNERCOMPARTMENT
H2O
1/2 O2
H+
Key Concepts:HOW AEROBIC RESPIRATION ENDS
In the Krebs cycle (and a few steps before)• Pyruvate is broken down to carbon dioxide
• Coenzymes pick up electrons and hydrogen atoms
In electron transfer phosphorylation• Coenzymes deliver electrons to transfer chains
that set up conditions for ATP formation
Oxygen accepts electrons at end of chains
Fig. 7.8, p.115
glucose
Glycolysis
2 pyruvate
KrebsCycle
Electron TransferPhosphorylation
(2 net)
2 NADH
ATP
2 FADH2
6 NADH
2 acetyl-CoA
32
ADP + Pi
H+
ATP
2 NADH
2 NADH
ATP
ATP
H+ H+ H+ H+
2 NAD+
2 CO2
4 CO2
2
oxygen
INNER MITOCHONDRIALCOMPARTMENT
OUTER MITOCHONDRIALCOMPARTMENT
CYTOPLASM
water
2
7.6 Anaerobic Energy-Releasing Pathways
Different fermentation pathways begin with glycolysis and end in the cytoplasm• Do not use oxygen or electron transfer chains
• Final steps do not produce ATP; only regenerate oxidized NAD+ required for glycolysis to continue
Anaerobic Pathways Lactate fermentation
• End product: ATP & Lactate
• Bacteria break down lactose in milk produce buttermilk, cheese, and yogurt
• Yeast preserve pickles, cored beef, and sauerkraut Alcoholic fermentation
• End product: ATP & Ethyl alcohol (or ethanol)
• Yeast to make bread dough rises as CO2 forms bubbles
Both pathways have a net yield of 2 ATP per glucose (from glycolysis) and NAD+
Fig. 7.9a, p.116
Fig. 7.9b, p.116
Fig. 7.9c, p.116
glucoseGlycolysis
2
pyruvate
NADH
2 NAD+
2ATP
ATP4
NADH
2 NAD+
2
LactateFermentation
lactate
Alcoholic Fermentation
Animal Skeletal Muscle
Red fibers (legs of chicken)• A lot of mitochondria and Myoglobin (stores oxygen)
• Produce ATP by aerobic respiration
• Sustain prolonged activity (marathon runs) White fibers (wings of chicken)
• Few mitochondria and no myoglobin can not carry out a lot of aerobic respiration
• Most ATP produced by lactate fermentation•ATP produced quick by not for long
• Short strenuous activity (Sprinting and weight lifting) Humans Mixed fibers
Muscles and Lactate Fermentation
Key Concepts: HOW ANAEROBIC PATHWAYS END
Fermentation pathways start with glycolysis
Substances other than oxygen are the final electron acceptor
Compared with aerobic respiration, net yield of ATP is small
7.7 Alternative Energy Sources in the Body
In humans and other mammals, foods enter aerobic respiration at various steps• Simple sugars from carbohydrates
• Glycerol and fatty acids from fats
• Carbon backbones of amino acids from proteins
Disposition of Organic Compounds
Alternative Energy Sources
Fig. 7.12a, p.119
FOOD
fats COMPLEX CARBOHYDRATES PROTEINS
glucose, other simple sugars amino acids
Glycolysis
glycerolfatty acids
pyruvate
acetyl-coA
NADH
KrebsCycle
NADH, FADH2
PGALacetyl-coA
oxaloacetateor anotherintermediateof the Krebs
Electron TransferPhosphorylation
Key Concepts: OTHER METABOLIC PATHWAYS
Molecules other than glucose are common energy sources
Different pathways convert lipids and proteins to substances that may enter glycolysis or the Krebs cycle
Life’s Unity
Photosynthesis and aerobic respiration are interconnected on a global scale
In its organization, diversity, and continuity through generations, life shows unity at the bioenergetic and molecular levels
Energy, Photosynthesis, and Aerobic Respiration
Key Concepts: PERSPECTIVE AT UNIT’S END
Life shows unity in its molecular and cellular organization and in its dependence on a one-way flow of energy
Animation: Alternative energy sources
Animation: Fermentation pathways
Animation: Functional zones in mitochondria
Animation: Glycolysis
Animation: Overview of aerobic respiration
Animation: The Krebs Cycle - details
Animation: Third-stage reactions
Animation: Where pathways start and finish