lecture: variations in photosynthesis part 2 AWS
LECTURE: VARIATIONS IN PHOTOSYNTHESIS PART 2 KIRSTIN BROWN
Lecture: VARIATIONS IN PHOTOSYNTHESIS part 2 Outline: 1. C4 plants o Review of Calvin cycle o Review of photorespiration o C4 photosynthesis
Calvin Cycle Review
Calvin Cycle Light dependent reactions o Two photosystems: ➢ Photosystem II o Absorbs light energy, transfers electron
from H2O through ETC – proton gradient ➢ Photosystem I o Absorbs light energy, transfers electron from PSI to reduce NADP+ to NADPH NADP+
NADPH
NR Fd
Chloroplast Stroma
*e-
PSII e-
Thylakoid Lumen
PSII: photosystem II PQ: plastoquinone PC: plastocyanin PSI: photosystem I Fd: ferredoxin NR: NADP reductase
PQ
H2O
cytochrome complex
H+
ATP
ADP
*e-
PSI e-
PC
H+
O2 + H+ H+
H+
H+
H+ H+ H+ H+ H+
Calvin Cycle NADPH and ATP used to make organic molecules in light independent reactions o The Calvin Cycle (carbon fixation) ➢ Occurs in the stroma of the chloroplast ➢ Produces Glyceraldehyde-3-phosphate
(G3P) from CO2
➢ Ribulose Bisphosphate Carboxylase
(Rubisco) is the carbon fixing enzyme
CO2 + 1,5-Bisphosphate 2 3-phosphoglycerate ➢ For every 3 molecules of CO2, 1 molecule of
G3P is produced, 9 ATP are used, and 6 NADPH are used
Calvin Cycle Review 6 CO2
1. Carbon Fixation Rubisco
x6
6 RuBP
6 ADP 6 ATP
12 3-PGA
3. Regeneration 12 ATP 12 ADP
10 G3P
12 NADPH 12 NADP+
2. Reduction 2 G3P
Leaf Structures Where is photosynthesis occurring? o Guard cells open and close pores called stomata
depending on environmental conditions
➢ Light intensity ➢ Humidity ➢ CO2 concentrations
Leaf Structures Where is photosynthesis occurring? o Stomata allow CO2 to enter and O2 to leave ➢ Also leads to water loss through transpiration ➢ Under hot, dry conditions – stomata close
➢ Prevents water loss, but leads to build up of O2
Photorespiration Why is build up of O2 a problem? o Rubisco, the enzyme that normally fixes CO2,
can also use O2 as a substrate 6 O2
Rubisco
6 RuBP
x6 6 phosphoglycolate
6 3-PGA
Photorespiration Why is build up of O2 a problem? o The 3 molecules of phosphoglycolate go
through a salvage pathway – half of the 3-PGA is recovered 6 O2
Rubisco
6 RuBP
x6 6 phosphoglycolate
Not enough C to regenerate 6 RuBP
C3 Plants
6 3-PGA 3 CO2 3 3-PGA
9 G3P
Not G3P exits Calvin cycle to make glucose
C4 Photosynthesis Physically separate light and dark reactions o The light-dependent reactions occur in the
mesophyll
o The Calvin cycle occurs in bundle-sheath cells
C4 Photosynthesis Physically separate light and dark reactions o The light-dependent reactions occur in the
mesophyll
o The Calvin cycle occurs in bundle-sheath cells
o No O2 build up near Rubsico
NADP+
NADPH
NR Fd
Chloroplast Stroma
*e-
PSII e-
Thylakoid Lumen
PSII: photosystem II PQ: plastoquinone PC: plastocyanin PSI: photosystem I Fd: ferredoxin NR: NADP reductase
PQ
H2O
cytochrome complex
H+
ATP
ADP
*e-
PSI e-
PC
H+
O2 + H+ H+
H+
H+
H+ H+ H+ H+ H+
C4 Photosynthesis Physically separate light and dark reactions o CO2 fixation occurs in mesophyll by PEP
carboxylase to produce oxaloacetate
o Oxaloacetate is converted to malate and
transported to the bundle sheath cells
o Malate is broken down to release CO2, which
then feeds into the Calvin cycle
o A 3-carbon molecule, pyruvate, is returned to
the mesophyll and converted back to PEP Mesophyll cell PEP
CO2
+
Bundle sheath cell
oxaloacetate
Calvin Cycle
PEP Carboxylase Malate
ADP ATP Malate
CO2 Pyruvate
Pyruvate
C4 Photosynthesis Examples of C4 plants o Usually exist in hot, dry conditions
Lecture: VARIATIONS IN PHOTOSYNTHESIS part 2 Outline: 1. C4 plants o Review of Calvin cycle o Review of photorespiration o C4 photosynthesis
Calvin Cycle Review
Calvin Cycle Light dependent reactions o Two photosystems: ➢ Photosystem II o Absorbs light energy, transfers electron
from H2O through ETC – proton gradient ➢ Photosystem I o Absorbs light energy, transfers electron from PSI to reduce NADP+ to NADPH NADP+
NADPH
NR Fd
Chloroplast Stroma
*e-
PSII e-
Thylakoid Lumen
PSII: photosystem II PQ: plastoquinone PC: plastocyanin PSI: photosystem I Fd: ferredoxin NR: NADP reductase
PQ
H2O
cytochrome complex
H+
ATP
ADP
*e-
PSI e-
PC
H+
O2 + H+ H+
H+
H+
H+ H+ H+ H+ H+
Calvin Cycle NADPH and ATP used to make organic molecules in light independent reactions o The Calvin Cycle (carbon fixation) ➢ Occurs in the stroma of the chloroplast ➢ Produces Glyceraldehyde-3-phosphate
(G3P) from CO2
➢ Ribulose Bisphosphate Carboxylase
(Rubisco) is the carbon fixing enzyme
CO2 + 1,5-Bisphosphate 2 3-phosphoglycerate ➢ For every 3 molecules of CO2, 1 molecule of
G3P is produced, 9 ATP are used, and 6 NADPH are used
Calvin Cycle Review 6 CO2
1. Carbon Fixation Rubisco
x6
6 RuBP
6 ADP 6 ATP
12 3-PGA
3. Regeneration 12 ATP 12 ADP
10 G3P
12 NADPH 12 NADP+
2. Reduction 2 G3P
Leaf Structures Where is photosynthesis occurring? o Guard cells open and close pores called stomata
depending on environmental conditions
➢ Light intensity ➢ Humidity ➢ CO2 concentrations
Leaf Structures Where is photosynthesis occurring? o Stomata allow CO2 to enter and O2 to leave ➢ Also leads to water loss through transpiration ➢ Under hot, dry conditions – stomata close
➢ Prevents water loss, but leads to build up of O2
Photorespiration Why is build up of O2 a problem? o Rubisco, the enzyme that normally fixes CO2,
can also use O2 as a substrate 6 O2
Rubisco
6 RuBP
x6 6 phosphoglycolate
6 3-PGA
Photorespiration Why is build up of O2 a problem? o The 3 molecules of phosphoglycolate go
through a salvage pathway – half of the 3-PGA is recovered 6 O2
Rubisco
6 RuBP
x6 6 phosphoglycolate
Not enough C to regenerate 6 RuBP
C3 Plants
6 3-PGA 3 CO2 3 3-PGA
9 G3P
Not G3P exits Calvin cycle to make glucose
C4 Photosynthesis Physically separate light and dark reactions o The light-dependent reactions occur in the
mesophyll
o The Calvin cycle occurs in bundle-sheath cells
C4 Photosynthesis Physically separate light and dark reactions o The light-dependent reactions occur in the
mesophyll
o The Calvin cycle occurs in bundle-sheath cells
o No O2 build up near Rubsico
NADP+
NADPH
NR Fd
Chloroplast Stroma
*e-
PSII e-
Thylakoid Lumen
PSII: photosystem II PQ: plastoquinone PC: plastocyanin PSI: photosystem I Fd: ferredoxin NR: NADP reductase
PQ
H2O
cytochrome complex
H+
ATP
ADP
*e-
PSI e-
PC
H+
O2 + H+ H+
H+
H+
H+ H+ H+ H+ H+
C4 Photosynthesis Physically separate light and dark reactions o CO2 fixation occurs in mesophyll by PEP
carboxylase to produce oxaloacetate
o Oxaloacetate is converted to malate and
transported to the bundle sheath cells
o Malate is broken down to release CO2, which
then feeds into the Calvin cycle
o A 3-carbon molecule, pyruvate, is returned to
the mesophyll and converted back to PEP Mesophyll cell PEP
CO2
+
Bundle sheath cell
oxaloacetate
Calvin Cycle
PEP Carboxylase Malate
ADP ATP Malate
CO2 Pyruvate
Pyruvate
C4 Photosynthesis Examples of C4 plants o Usually exist in hot, dry conditions
