Introduction
Where It Starts:
Photosynthesis
ƒ Before photosynthesis evolved, Earth’s
atmosphere had little free oxygen
Chapter 6
ƒ Oxygen released during photosynthesis
changed the atmosphere
• Favored evolution of new metabolic pathways,
including aerobic respiration
Electromagnetic Spectrum
6.3 Overview of Photosynthesis
ƒ Photosynthesis proceeds in two stages
• Light-dependent reactions
• Light-independent reactions
Summary equation:
6H2O + 6CO2
6O2 + C6H12O6
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LightDependent
Reactions
Sites of Photosynthesis: Chloroplasts
sunlight
H2O
ADP + Pi
O2
ATP
NADP+ NADPH
ƒ Light-dependent reactions occur at a muchfolded thylakoid membrane
LightIndependent
Reactions
CO2
Calvin-Benson
cycle
• Forms a single, continuous compartment inside
the stroma (chloroplast’s semifluid interior)
H2O
ƒ Light-independent reactions occur in the stroma
phosphorylated glucose
end products (e.g., sucrose, starch, cellulose)
Fig. 6.13, p.104
Sites of Photosynthesis
Sites of Photosynthesis
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Sites of Photosynthesis
Products of Light-Dependent Reactions
ƒ Typically, sunlight energy drives the formation of
ATP and NADPH
ƒ Oxygen is released from the chloroplast (and the
cell)
ATP Formation
light energy
electron transfer
chain
Photosystem II
light energy electron transfer chain
Photosystem I
NADPH
THYLAKOID
COMPARTMENT
THYLAKOID
MEMBRANE
oxygen
(diffuses away)
STROMA
ƒ In both pathways, electron flow through electron
transfer chains causes H+ to accumulate in the
thylakoid compartment
• A hydrogen ion gradient builds up across the
thylakoid membrane
ƒ H+ flows back across the membrane through
ATP synthases
• Results in formation of ATP in the stroma
Fig. 6.8b, p.99
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6.6 Light Independent Reactions:
The Sugar Factory
Calvin–Benson Cycle
ƒ Cyclic pathway makes phosphorylated glucose
ƒ Light-independent reactions proceed in the
stroma
ƒ Carbon fixation: Enzyme rubisco attaches
carbon from CO2 to RuBP to start the Calvin–
Benson cycle
Light-Independent Reactions
• Uses energy from ATP, carbon and oxygen from
CO2, and hydrogen and electrons from NADPH
ƒ Reactions use glucose to form photosynthetic
products (sucrose, starch, cellulose)
ƒ Six turns of Calvin–Benson cycle fix six carbons
required to build a glucose molecule from CO2
6.7 Adaptations:
Different Carbon-Fixing Pathways
ƒ Environments differ
• Plants have different details of sugar production
in light-independent reactions
ƒ On dry days, plants conserve water by closing
their stomata
• O2 from photosynthesis cannot escape
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6.8 A Burning Concern
Fossil Fuel Emissions
ƒ Photoautotrophs remove CO2 from atmosphere;
metabolic activity of organisms puts it back
ƒ Human activities disrupt the carbon cycle
• Add more CO2 to the atmosphere than
photoautotrophs can remove
ƒ Imbalance contributes to global warming
7.1 Overview of
Carbohydrate Breakdown Pathways
How Cells Release Chemical Energy
Chapter 7
ƒ All organisms (including photoautotrophs)
convert chemical energy of organic compounds
to chemical energy of ATP
ƒ ATP is a common energy currency that drives
metabolic reactions in cells
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Pathways of Carbohydrate Breakdown
Pathways of Carbohydrate Breakdown
ƒ Start with glycolysis in the cytoplasm
• Convert glucose and other sugars to pyruvate
ƒ Fermentation pathways
• End in cytoplasm, do not use oxygen, yield 2 ATP
per molecule of glucose
ƒ Aerobic respiration
• Ends in mitochondria, uses oxygen, yields up to
36 ATP per glucose molecule
Overview of Aerobic Respiration
Overview of Aerobic Respiration
ƒ Three main stages of aerobic respiration:
1. Glycolysis
2. Krebs cycle
3. Electron transfer phosphorylation
Summary equation:
C6H12O6 + 6O2 → 6CO2 + 6 H2O
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7.2 Glycolysis –
Glucose Breakdown Starts
ƒ Enzymes of glycolysis use two ATP to convert
one molecule of glucose to two molecules of
three-carbon pyruvate
ƒ Reactions transfer electrons and hydrogen
atoms to two NAD+ (reduces to NADH)
Products of Glycolysis
ƒ Net yield of glycolysis:
• 2 pyruvate, 2 ATP, and 2 NADH per glucose
ƒ Pyruvate may:
• Enter fermentation pathways in cytoplasm
• Enter mitochondria and be broken down further in
aerobic respiration
ƒ 4 ATP form by substrate-level phosphorylation
7.3 Second Stage of Aerobic Respiration
ƒ The second stage of aerobic respiration takes
place in the inner compartment of mitochondria
Acetyl-CoA Formation
ƒ Two pyruvates from glycolysis are converted to
two acetyl-CoA
ƒ Two CO2 leave the cell
ƒ It starts with acetyl-CoA formation and proceeds
through the Krebs cycle
ƒ Acetyl-CoA enters the Krebs cycle
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Krebs Cycle
Energy Products
ƒ Each turn of the Krebs cycle, one acetyl-CoA is
converted to two molecules of CO2
ƒ Reactions transfer electrons and hydrogen
atoms to NAD+ and FAD
• Reduced to NADH and FADH2
ƒ After two cycles
• Two pyruvates are dismantled
• Glucose molecule that entered glycolysis is fully
broken down
ƒ ATP forms by substrate-level phosphorylation
• Direct transfer of a phosphate group from a
reaction intermediate to ADP
7.4 Third Stage:
Aerobic Respiration’s Big Energy Payoff
ƒ Coenzymes deliver electrons and hydrogen ions
to electron transfer chains in the inner
mitochondrial membrane
ƒ Energy released by electrons flowing through
the transfer chains moves H+ from the inner to
the outer compartment
Fig. 7.6a, p.113
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Hydrogen Ions and Phosphorylation
The Aerobic Part of Aerobic Respiration
ƒ H+ ions accumulate in the outer compartment,
forming a gradient across the inner membrane
ƒ Oxygen combines with electrons and H+ at the
end of the transfer chains, forming water
ƒ H+ ions flow by concentration gradient back to
the inner compartment through ATP synthases
(transport proteins that drive ATP synthesis)
ƒ Overall, aerobic respiration yields up to 36 ATP
for each glucose molecule
Electron Transfer Phosphorylation
Summary: Aerobic Respiration
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Anaerobic Pathways
Alcoholic Fermentation
ƒ Lactate fermentation
• End product: Lactate
ƒ Alcoholic fermentation
• End product: Ethyl alcohol (or ethanol)
ƒ Both pathways have a net yield of 2 ATP per
glucose (from glycolysis)
Muscles and Lactate Fermentation
7.8 Life’s Unity
ƒ Photosynthesis and aerobic respiration are
interconnected on a global scale
ƒ In its organization, diversity, and continuity
through generations, life shows unity at the
bioenergetic and molecular levels
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Energy, Photosynthesis, and
Aerobic Respiration
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