1 Lecture 17 Oct 10, 2005 Photosynthesis II. Calvin Cycle Slide
2 2 Lecture Outline 1. 1.The Calvin Cycle fixes carbon makes
reduced carbon compounds 2. 2.Reactions of the Calvin Cycle
anabolic pathway input of NADPH + H +, input of ATP 3. 3.Regulation
of the Calvin Cycle 4. 4.The problem with oxygen Photorespiration
5. 5.Tricks some plants use to limit photorespiration - C4 anatomy,
C4 metabolism division of labor - CAM plants, the difference is
night and day Slide 3 3 DARK REACTIONS energy utilization The
Calvin Cycle Slide 4 4 The purpose of the Carbon-fixation (Calvin
Cycle) Reactions CO 2 + NADPH + H + + ATP C 6 H 12 O 6 + NADP + +
ADP + P i carbohydrate Note: synthesis of carbohydrate from CO 2 is
favorable only because coupled to very favorable reactions NADPH to
NADP + and ATP to ADP + P i energy released is greater than it
costs to make carbohydrate Slide 5 5 The Calvin cycle has three
phases Carbon fixation Reduction (energy input, reducing equiv
input) Regeneration of the CO 2 acceptor (energy input priming
step) Slide 6 6 Overview of Carbon-Fixation Reactions CO 2 5 Carbon
Sugar with 2 phosphates on it 3 Carbon Aldehyde with one phosphate
on it R-C-OH = O R-C-H Higher Energy Compound = O Reduction + 3
Carbon Acid with one phosphate on it 2 ATP ADP + P i NADPH NADP +
ATP ADP + P i 5 Carbon Sugar with one phosphate on it 20/24 6
Carbon Sugar - Glucose with NO phosphate on it 4/24 12 24 12 2
Slide 7 7 Carried out by the enzyme rubisco (ribulose 1,5
bisphosphate carboxylase oxygenase) Carbon Fixation Do need to know
this enzyme Key regulatory enzyme Slide 8 8 Acid -COOH Aldehyde
-C=O -C=O H PrimingStep Input of energyRegenerate What started
withCarbonFixationReduction Rubisco Slide 9 9 pH 8 / pH7 Regulation
of Rubisco 1 st Enzyme in Calvin Cycle Substrate/Product
Substrate/Product availability NADPHATP Allosterically regulated by
NADPH and ATP Narrow pH optimum Very Narrow pH optimum Enzyme must
be in reduced form pH 8 or above, inactive at 7 Slide 10 10
Integration of Light-Dependent and Light-Independent Reactions They
generally occur AT THE SAME TIME O2O2 CO 2 H2OH2O Light Light
reaction Calvin cycle NADP + ADP ATP NADPH + P 1 RuBP
3-Phosphoglycerate Amino acids Fatty acids Starch (storage) Sucrose
(export) G3P Photosystem II Electron transport chain Photosystem I
Chloroplast Figure 10.21 Slide 11 11 PhotoRespiration - the OXYGEN
PROBLEM Oxygen Oxygen is a competing substrate for the 1st enzyme
in the C 3 cycle (Rubisco) O2O2O2O2 5 Carbon Sugar with 2
phosphates on it Acid 3 Carbon Acid with one phosphate on it +
2Acid 2 Carbon Acid with one phosphate on it Energy Wasted With NO
synthesis of glucose CO 2 5 Carbon Sugar with 2 phosphates on it
Acid 3 Carbon Acid with one phosphate on it + Acid 3 Carbon Acid
with one phosphate on it Net increase in material make glucose with
extra Slide 12 12 Some Plants Deal with this problem by a DIVISION
OF LABOR BETWEEN CELLS C 4 Plants Mesophyll cells perform usual
noncyclic Light-Dependent Reactions make oxygen, ATP and NADPH Do
NOT perform the C 3 (Calvin cycle) reactions Bundle Sheath cells
perform UNusual cyclic Light-Dependent Reactions a lot of ATP but
very little NADPH and very little O 2 and very little O 2 PERFORM
the usual C 3 (Calvin Cycle) reactions Slide 13 13 C 4 leaf anatomy
and the C 4 pathway CO 2 Mesophyll cell cell Bundle- sheathcell
Vein (vascular tissue) Photosynthetic cells of C 4 plant leaf
Stomata Mesophyll cell C 4 leaf anatomy PEP carboxylase
Oxaloacetate (4 C) PEP (3 C) Malate (4 C) ADP ATP Bundle- Sheath
cell CO 2 Pyruate (3 C) CALVIN CYCLE Sugar Vascular tissue Figure
10.19 CO 2 Mesophyll Produce NADPH and ATP PEP carboxylase
insensitive to O 2 Malate brings across reducing equivalents
NADPHused NADPHregenerated Cyclic e- flow Little O 2 Malate (4 C )
Oxaloacetate (4 C) Slide 14 14 CO 2 CH 3 C=O \ O-P i PEP PEP OH /
C=O CH 2 C=O C=O + P i \ OHOxaloacetate 4C acid 4C acid OH / C=O CH
2 H--H H-C-O-H C=O \ OH malate malate 4C acid 4C acid CH 3 C=O \
O-Hpyruvate ATP ADP NADPH+ H + NADP + PEP carboxylase carries
carriesReducingequivalents Mesophyll Cell C4 Metabolism Needs ATP
and NAPDPH + H + Non-cyclic electron flow Slide 15 15 OH / C=O CH 2
C=O \ OHOxaloacetate 4C acid 4C acid OH / C=O CH 2 H--H H-C-O-H C=O
\ OH malate malate 4C acid 4C acidNADPH+ H + NADP + carries
carriesReducingequivalents Bundle Sheath - Cell C4 Metabolism ATP
Needs ATP Only Cyclic electron flow Very low O 2 CO 2 CH 3 C=O \
O-Hpyruvate ATP CalvinCycle glucose ATP NADPH Slide 16 16 Mesophyll
cells provide a means for bundle sheath cells to acquire NADPH + H
+ reducing power Mesophyll cells provide carbon dioxide to bundle
sheath cells at higher concentration than in air Bundle Sheath
cells not making oxygen, so very little competitor with C 3
reactions Costs more energy to do business this way but has the
advantage when CO 2 is limiting (when stomates are closed - like on
hot days) Who cares as long as the sun is shining ? ATP is not
limiting Slide 17 17 CAM Plants Cacti, pineapple Open their stomata
only at night, too hot during day survive very adverse (dry)
conditions NIGHT Perform PEP carboxylase reaction at night (CO 2
assimilation) accumulate malate to high concentration in central
vacuole use sugar oxidation/catabolism to power (NADH and ATP)
carbon fixation DAY Perform light reactions during the day mostly
cyclic e - flow to produce ATP (low O 2 ) decarboxylate malate to
yield CO 2 and NADPH + H + perform C3 reactions (Calvin Cycle) to
produce sugars and starch Slide 18 18 Spatial separation of steps.
In C 4 plants, carbon fixation and the Calvin cycle occur in
different types of cells. (a) Temporal separation of steps. In CAM
plants, carbon fixation and the Calvin cycle occur in the same
cells at different times. (b) Pineapple Sugarcane Bundle- sheath
cell Mesophyll Cell Organic acid CALVIN CYCLE Sugar CO 2 Organic
acid CALVIN CYCLE Sugar C4C4 CAM CO 2 incorporated into four-carbon
organic acids (carbon fixation) Night Day 1 2 Organic acids release
CO 2 to Calvin cycle Figure 10.20 SunlightPowersBothphases
SugarOxidationPowersOnePhase GenerallySlowgrowing Slide 19
19Summary 1. 1.Photosynthetic light reactions produce ATP and
reducing potential NADPH + H + ATP 2. Dark reactions use ATP and
reducing potential to synthesize carbohydrates reductionacid -
powers reduction of 3-carbon acid aldehyde to 3-carbon aldehyde - -
powers regeneration of starting material 5-carbon di-phosphate
(priming step for CO 2 fixation) tightly 3. Rubisco enzyme
regulated tightly by allosteric modulators pH, and reducing status
of stroma O 2 4. O 2 interferes with carbon fixation by Rubisco
enzyme 5. Metabolic tricks to avoid photorespiration C4
metabolismCAM metabolism - C4 metabolism - CAM metabolism
