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Chapter 4.2 – 4.3
Building sugar photosynthesis
Kevin Bleier
Milton HS, GA
Who does photosynthesis?
Overview – summary equation
sunlight, enzymes
6 CO2 + 6 H2O
C6H12O6 + 6 O2
• Once again, just a summary
• Photosynthesis a factory of enzymes just like
respiration
Overview of photosynthesis
• 2 broad steps:
1) Light reactions – trap sunlight energy,
transform into ATP / electron energy
(H2O consumed, O2 produced)
2) Calvin cycle – use ATP / electron energy to
power formation of sugar
(CO2 consumed, C6H12O6 produced)
Overview of photosynthesis
thylakoid
membrane
Overview of photosynthesis
thylakoid
membrane
Overview of photosynthesis
Light reactions – capturing light
• Evolution toward usage of visible light – the
wavelengths that enter the atmosphere the most
Multiple pigments capture light
Measuring light absorption
• Spectrophotometer
• Green light NOT
absorbed
• Blue, red light
absorbed
Multiple pigments capture light
Chlorophyll after absorbing light
• Chlorophyll energizes
electrons within it
• High energy
electrons stolen from
chlorophyll, sent to
electron transport
chain
• Water cut up to give
new low-energy e-s
to chlorophyll
Next step = electron transport chain
• Similar steps to
respiration
• High energy e-s
help ETC pump
H+ to one side
• H+ come back in
through ATP
synthase
ATP production in light reactions
Next step – another photosystem
• e-s fill a hole in another chlorophyll molecule,
where e-s have been excited again
What happens with these e-s?
• Captured by NADP+ to make NADPH
Summary of light reactions
• Purpose: generate ATP and NADPH energy
needed to power production of sugar
• Using sunlight energy to excite electrons, then
converting energy of electrons into ATP and
NADPH energy
• Electrons do NOT come from sunlight,
they come from water
Summary of light reactions
• In: H2O, ADP, NADP+
• Out: O2, ATP, NADPH
Photosynthesis: Calvin cycle
• CO2 gas fixed into
sugars already there
• ATP and NADPH
spent to convert into
other sugars
• Some sugar can leave
the cycle, others stay
to fix more CO2
Photosynthesis summary