Dark Reactions

1
Dark Reactions
2
Cyclic photosphorylation generates ATP
Stromal side
Fd
PQ
PQ
P700
PSII
PSI
Thylakoid space side
3
Non-Cyclic photosphorylation generates ATP and
NADPH
NADP
Stromal side
NADPH
Q
Fd
PQ
PQ
P700
P680
PSII
PSI
H20 2H 2e- 1/2 O2
Thylakoid space side
4
Light-independent reactions(Dark Reactions)
5
The Calvin/Benson/C3 cycle

• Has 3 phases
• carbon fixation phase
• reduction of CO2 phase
• regeneration of RuBP phase

6
carbon fixation phase
7

• each CO2 molecule is attached to a five-carbon
  sugar, ribulose bisphosphate (RuBP)
• catalyzed by RuBP carboxylase or rubisco
• six-carbon intermediate splits in half to form
  two molecules of 3-phosphoglycerate

8
reduction of CO2 phase
9

• each 3-phosphoglycerate receives another
  phosphate group from ATP to form 1,3
  bisphosphoglycerate.
• pair of electrons from NADPH reduces,Gain, each
  1,3 bisphosphoglycerate to G3P.
• Electrons from NADPH change a carboxyl group to
  a carbonyl group.

10
regeneration of RuBP phase
11

• regeneration of the CO2 acceptor (RuBP), these
  five G3P molecules are rearranged to form 3 RuBP
  molecules.
• cycle must spend 3 more molecules of ATP (one per
  RuBP) to complete the cycle and prepare for the
  next.

12
Hatch Slack/C4 cycle

• Inefficient as RUBISCO can act as both a
  carboxylase and as an oxygenase.
• Oxygenase activity leads to loss of carbon that
  has already been fixed.
• Some plants have an alternative pathway where CO2
  is first fixed into C4 organic acids (C4 pathway)
  and then liberated later to undergo the
  Calvin/Benson cycle (C3 metabolism).

13

• 2 types of C4 cycle
• C3 and C4 metabolism are separated in space
  (different cells) e.g. sugar cane Characteristic
  aggregation of cells around the vascular bundles
  - Krantz morphology (C4 cycle)
• C3 and C4 metabolism are separated in time (same
  cellmesophyl) e.g. pineapple (CAM cycle).

14
Comparison of C3 C4 leaves

• C3 - note the lack of chloroplasts in the bundle
  sheath

• C4 - note the extensive chloroplasts in the
  bundle sheath

15
Separation in space (C4 cycle)
CO2
PEP (3C)
Mesophyll cell surrounding bundle sheath
OAA (4C)
ADP
ATP
Malat
Pyruvate (C3)
CO2
C3
Bundle sheath cell
sugar
Vascular tissue
16
Separation in time (CAM cycle)
CO2
PEP (3C)
In the dark
OAA (4C)
ADP
ATP
Malat
Pyruvate (C3)
CO2
C3
In the light
sugar
Mesophyl
17
Comparison of C3 C4 leaves
18
Details of C4 cycle
19
Details of CAM cycle