Lecture 7 Outline (Ch. 10)
I.
Light reactions
II.
Calvin cycle
III. C4 and CAM plants
VI. Summary
Light Reaction Review
H2O
Light
NADP+
ADP
+ P
i
Light
Reactions
ATP
NADPH
Chloroplast
O2
Calvin
Cycle
Photosynthesis – light absorption
Pigments held by proteins in thylakoid membranes
• light energy absorbed
by pigments, transferred
• to reaction center
- two special chlorophyll a
- 1° electron acceptor
TWO Photosystems (PS)
Photosynthesis –
Photosystems II & I
• PSII: P680, more powerful
PS, longer ETC, e- replaced
from splitting water
• PS I: P700, lower energy
PS, short ETC NADPH
made, e- replaced from PSII
Photosynthesis – Photosystems
Photosystem II
Photosystem I
If chloroplasts thylakoids are isolated in
solution, when light is shined on them, the pH
of the solution goes up (more alkaline). Why?
Light
No Light
pH = 6.5
pH = 8
Calvin cycle (Light-independent Reaction)
CO2 + H2O + light energy
C6H12O6 + O2
H2O
CO2
Calvin cycle:
• regenerative
Light
NADP+
ADP
+ Pi
RuBP 3-Phosphoglycerate
Calvin
Cycle
• anabolic
ATP
• CO2 in, sugar out
NADPH
G3P
Starch
(storage)
Chloroplast
• during daylight
O2
Glucose
(export)
Carbon fixation
• 3 stages of
Calvin-cycle: • 5-C: ribulose bisphosphate (RuBP) links to CO
2
Catalyzing enzyme: Rubisco
• 6-C unstable –
split  2(3-C)
Reduction
• 3 stages of
Calvin-cycle:
• uses ATP
• 3-C molecules reduced
• e- from NADPH
• reduced 3-C: G3P
one leaves, 5 continue
• 3 stages of
Calvin-cycle:
Regeneration of C-acceptor
• still 5 G3P  converted to 3 RuBP
• multiple steps
• uses ATP
• every 3 cycles:
1 G3P made
3 RuBP regenerated
Requires THREE turns
of the whole cycle!!!
Calvin Cycle
In thinking about photosynthesis:
Are the compounds listed
here used or produced in:
Glucose
O2
CO2
H2O
ATP
ADP + Pi
NADPH
NADP+
Photosystem I
Photosystem II
The Calvin
cycle
Self-Check
_____
_____
_____
“Photo” = Light Reactions
_____
_____
_____
_____
“Synthesis” = Calvin cycle
_____
_____
Self-Check
Calvin (C3) Cycle:
_____
energy from
light dependent
reactions
1
?
?
_____
Where?
_____
_____
2
3
_____
_____
_____
?
______
What about plants in HOT/DRY climates?
Photorespiration –
Rubisco can fix O2
instead of CO2 when
levels of CO2 are low
What would
cause CO2
levels to be
low in plant
tissues?
What about plants in HOT/DRY climates?
C4 plants – use PEP carboxylase to fix CO2 into oxaloacetate
High affinity for CO2, none for O2
In bundle-sheath cells, split into pyruvate
+ CO2 – then proceed with Calvin cycle
In mesophyll cells
What about plants in HOT/DRY climates?
CAM
plants –
fix CO2 at
night,
perform
the rest
of Calvin
cycle in
the day
Photosynthesis – summary
• light reaction: Light energy + H2O IN
O2, NADPH, ATP OUT
Thylakoids
• light-independent:
CO2, NADPH, ATP IN
G3P (sugar), RuBP OUT
Stroma
KNOW THIS FIGURE! 