C3 Photosynthesis
Chapter 10
What you need to know!
• How photosystems convert solar energy to
chemical energy.
• How linear electron flow in the light
reactions results in the formation of ATP,
NADPH, and O2.
• How chemiosmosis generates ATP in the
light reactions.
• How the Calvin cycle uses the energy
molecules of the light reactions to produce
G3P
Photosynthesis
•
Radiation energy is transformed into
chemical bond energy in two distinct
stages:
1. Light reactions
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Occur in the thylakoid membrane
Water donates electrons to NADP+ to make
NADPH
Water is split, O2 is released
Photophosphorylation turns ADP into ATP
2. Calvin cycle
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•
Occurs in the stroma
CO2 transformed into sugar
Net Rx: 6 CO2 + 6 H2O + Light  C6H12O6 + 6O2
Big Picture
Light Reactions
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Location: thylakoid membrane
Needs: Light, H2O, NADP+, ADP, P
Makes: NADPH, O2, ATP
Includes: Linear (non-cyclical),
cyclical, & chemiosmosis
Linear (Non-cyclical) Light Rxs
• Photosystem II (P680) pigments absorb
light (photons)
• A photon excites chlorophyll which kicks an
electron e- out of the reaction center
• The excited e- is captured by the Electron
Transport Chain (ETC) between P680 and
Photosystem I (P700)
• The missing e- is replaced by splitting water
(photolysis of water):
H2O  O + 2e- + 2H+
Linear Light Rxs
• The excited e- moves down the ETC
• The e-’s energy (excited) is used to pump
H+ into the thylakoid space (creating a
concentration gradient)
• e- is deposited into P700
• P700 pigments absorb light (photons)
• A photon excites chlorophyll which kicks
an electron e- out of the reaction center
• The e- is captured by another shorter ETC
• At the end of the 2nd ETC the e- binds to
NADP+
• 2 e- and NADP+ are combined with H+ to
form NADPH
Linear Light Reactions
Cyclical Light Reactions
• Some e-’s, when kicked out of P700 do not
go down the 2nd shorter ETC
• Instead they fall back on the first ETC
between P680 and P700
• This produces less NADPH and more H+
gradient
Cyclical Light Reactions
Chemiosmosis
• This process makes ATP by using the H+
concentration gradient
• H+ concentration gradient across the
thylakoid membrane means: H+ inside the
thylakoid is high, while H+ in the stroma is
low
– On a sunny day it is 1000x’s more acidic in the
thylakoid space (pH 5 in thylakoid, pH 8 in stroma)
• ATP Synthase in the membrane functions
like a turbine: when H+s rush through ATP
Synthase (down the electrochemical
gradient) ATP Synthase turns and uses
kinetic energy to phosphorylize ADP
ADP + P  ATP
aka: Photophophorylation
Calvin Cycle
aka: light independent reactions
• Location: stroma
• Needs: CO2, ATP, NADPH
• Makes: G3P, ADP, P, NADP+
Calvin Cycle
•
Multiple enzyme pathways that uses ATP
and NADPH to reduce CO2 into C6H12O6
(glucose)
• One turn of the cycle reduces one CO2
• 3 distinct steps:
1. Carbon fixation
2. Reduction
3. Regeneration
Carbon Fixation
• First enzyme of the cycle is Rubisco
(Ribulose Bisphosphate Carboxylase)
which binds 3 CO2 to an acceptor
molecule RuBP
• Rubisco is the most famous and abundant
enzyme on earth: no other organic
molecule can chemically binding CO2
Reduction
• Several enzymes later the 3 CO2 have
been reduced to a C3 sugar called G3P
(glyceraldehyde phosphate)
– powered by 6 ATP and 6 NADPH
• G3P leaves the cycle
– 2 G3P can combine to form glucose
Regeneration
• RuBP needs to be regenerated
– powered by 3 ATP
1. Carbon Fixation
3. Regeneration
2. Reduction
Water Balance
• If water is running low, plants will close their
stomata to avoid transpiration
When stomata are closed
• CO2 is not replenished
• ADP and NADP+ are not replenished by
the Calvin Cycle
• Light Reactions run out of ADP and NADP+
• Energized e-’s fall back to the reaction
center of chlorophyll
– This can emit light (plant fluorescence)
Review
• Biology Crash Course
• http://www.youtube.com/watch?v=wEPUfJ
n0s-M
• Mr. Anderson (Bozeman)
• http://www.youtube.com/watch?v=g78utcL
QrJ4