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Building Sugars• What products came from the
light-dependent reactions?– ATP and NADPH
• Light-independent reactions use ATP and NADPH to build sugars from CO2
– ATP energy source– NADPH high energy electrons
and H+
• Where do these reactions happen?– Stroma of the chloroplasts
• Who mapped it out?– Melvin Calvin (called Calvin
Cycle)
The Calvin Cycle: Part 1• Where do we also find G3P?
– Half-way point in glycolysis– Can start glycolysis from here
Phase 1: Carbon Fixation1) CO2 is fixed to 1,5-
bisphosphate (RuBP) by the enzyme RuBP carboxylase/oxygenase (rubisco) creating a 6-C sugar
2) 6-C is broken into two 3-phosphateglycerates (3PGA)
CO2+ATP+NADPHADP+Pi+NADP++glyceraldehyde-3-phosphate(G3P)
The Calvin Cycle: Part 2Phase 2: Reduction3) NADPH and ATP supply the energy, electrons, and H+ needed to change 3PGA into G3P• How many G3P do we have?
– 2• How many do we need to make
glucose?– 2
• Why do plants NOT just make sugar with the 2 G3P?– Need to regenerate RuBP or cycle
shuts down• Where have we seen this before?
– Oxaloacetate in the Krebs cycle
The Calvin Cycle: Part 3Phase 3: Regeneration4) G3P molecules will be used to build RuBP molecules • 3 turns of the cycle will produce 6
G3P but 5 will be used to place the RuBP used; the left over G3P is used to build sugars
3 Turns:- 3 CO2 and 3 RuBP (5-C) 18 C
- 6 G3Ps produced- 5 G3Ps 15 C 3 RuBP- Last G3P builds a sugar• How many turns are needed to
make glucose?– 6! That is really complex
• How many ATP are used to make glucose?– 18 ATP (3 ATP/Turn; 6 Turns)
Calvin Cycle: Regulation• What is used to regulate the
cycle?– ATP and NADPH levels
• What enzyme do they effect?– Rubisco
• Rubisco is found in all photosynthetic organisms and up to 50% of all proteins in a leaf; most abundant protein on Earth (6kg/person)
• What does the name RuBP carboxylase/oxygenase tell you about the enzyme?– Reacts with both CO2 and O2
– Competitively inhibit each other
The Problem with Gas Exchange• How does CO2 and O2 get
in and out of a plant?– Stromata under the
leaves• What else can escape
through a stromata that the plant needs?– H2O
• What problems must plants in dry, hot climates deal with?– The plant needs to take
in CO2 and release O2, but the open stromata will also release H2O
H2O out
Photorespiration• What happens if O2 builds in
the plant?– RuBP is inhibited
• RuBP, when bound to O2, will break down into 3PGA and Phosphogylcolate (2-C)
• Phosphogylcolate is dephospholated and gylcolate (toxin) is broken down to CO2
• Photorespiration plants use O2 to make CO2 (and H2O)
• Why is this inefficient?– Lose RuPB and only get one
3PGA
C4 Pathway• Plants that use 3PGA in their
Calvin cycle are C3 pathway plants
• C4 pathway CO2 is fixed to phosphoenolpyruvate (PEP) to make oxaloacetate by PEP carboxylase
• Where is oxaloacetate used?– Starting point of Krebs; can
make ATP from this• NADPH reduces oxaloacetate
to malate (Krebs) and then pyruvate and releases CO2
• Pyruvate can use ATP to recycle PEP
C4 Pathway• C4 plants separate the C4 cycle from the
Calvin cycle– C4 mesophyll cells– Calvin bundle sheath cells
• Malate diffuse through the mesophyll to the bundle sheath to be broken into CO2 and pyruvate
• Pyruvate diffuses back to be recycled to PEP
• Why is C4 better than photorespiration?– Creates CO2 without using RuPB and
lowering Calvin cycle output– PEP carboxylase is very efficient do
stromata do not have to be open long• Why is it not used by all plant then?
– Extra amount of ATP is needed to recycle PEP
CAM Pathway• CAM Crassulacean Acid
Metabolism– Water storing plants in desert areas
• Starts the same as C4:– CO2 fixed to oxaloacetate by PEP
carboxylase and malate releases CO2 for the Calvin cycle
• In CAM all these reactions occur in the same cells (bundle sheath cells) but at different times– Calvin cycle Day– Carbon fixation Night
• Night:– Stromata open– Malate is made and stored in the
large vacuole• Day:
– Stromata stay closed– CO2 is released from malate