Where It Starts:
Photosynthesis
Introduction
 Before photosynthesis evolved, Earth’s
atmosphere had little free oxygen
 Oxygen released during photosynthesis
changed the atmosphere
• Favored evolution of new metabolic pathways,
including aerobic respiration
Electromagnetic Spectrum
Overview of Photosynthesis
 Photosynthesis proceeds in two stages
• Light-dependent reactions
• Light-independent reactions
Summary equation:
6H2O + 6CO2
6O2 + C6H12O6
LightDependent
Reactions
sunlight
H2O
ADP + Pi
O2
ATP
NADP+ NADPH
LightIndependent
Reactions
CO2
Calvin-Benson
cycle
H2O
phosphorylated glucose
end products (e.g., sucrose, starch, cellulose)
Fig. 6.13, p.104
Sites of Photosynthesis: Chloroplasts
 Light-dependent reactions occur at a muchfolded thylakoid membrane
• Forms a single, continuous compartment inside
the stroma (chloroplast’s semifluid interior)
 Light-independent reactions occur in the stroma
Sites of Photosynthesis
Sites of Photosynthesis
Sites of Photosynthesis
Products of Light-Dependent Reactions
 Typically, sunlight energy drives the formation of
ATP and NADPH
 Oxygen is released from the chloroplast (and the
cell)
light energy
electron transfer
chain
Photosystem II
light energy electron transfer chain
Photosystem I
NADPH
THYLAKOID
COMPARTMENT
THYLAKOID
MEMBRANE
oxygen
(diffuses away)
STROMA
Fig. 6.8b, p.99
ATP Formation
 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
 H+ flows back across the membrane through
ATP synthases
• Results in formation of ATP in the stroma
Light Independent Reactions:
The Sugar Factory
 Light-independent reactions proceed in the
stroma
 Carbon fixation: Enzyme rubisco attaches
carbon from CO2 to RuBP to start the Calvin–
Benson cycle
Calvin–Benson Cycle
 Cyclic pathway makes phosphorylated glucose
• Uses energy from ATP, carbon and oxygen from
CO2, and hydrogen and electrons from NADPH
 Reactions use glucose to form photosynthetic
products (sucrose, starch, cellulose)
 Six turns of Calvin–Benson cycle fix six carbons
required to build a glucose molecule from CO2
Light-Independent Reactions
Adaptations:
Different Carbon-Fixing Pathways
 Environments differ
• Plants have different details of sugar production
in light-independent reactions
 On dry days, plants conserve water by closing
their stomata
• O2 from photosynthesis cannot escape
A Burning Concern
 Photoautotrophs remove CO2 from atmosphere;
metabolic activity of organisms puts it back
 Human activities disrupt the carbon cycle
• Add more CO2 to the atmosphere than
photoautotrophs can remove
 Imbalance contributes to global warming
Fossil Fuel Emissions
How Cells Release Chemical Energy
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
 Start with glycolysis in the cytoplasm
• Convert glucose and other sugars to pyruvate
 Fermentation pathways
• End in cytoplasm, do not use oxygen, yield 2 ATP
per molecule of glucose
 Aerobic respiration
• Ends in mitochondria, uses oxygen, yields up to
36 ATP per glucose molecule
Pathways of Carbohydrate Breakdown
Overview of Aerobic Respiration
 Three main stages of aerobic respiration:
1. Glycolysis
2. Krebs cycle
3. Electron transfer chain
Summary equation:
C6H12O6 + 6O2 → 6CO2 + 6 H2O
Overview of Aerobic Respiration
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
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
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
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
Fig. 7.6a, p.113
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 Chain
Summary: Aerobic Respiration
Anaerobic Pathways
 Lactic acid fermentation
• End product: Lactic acid (lactate)
 Alcoholic fermentation
• End product: Ethyl alcohol (or ethanol)
 Both pathways have a net yield of 2 ATP per
glucose (from glycolysis)
Alcoholic Fermentation
Muscles and Lactate Fermentation
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