Calvin Cycle Organisms capture and store free energy for use in
biological processes
Slide 2
Where does the Calvin Cycle take place? Stroma of the
chloroplast the fluid filled area outside of the thylakoid
membrane
Slide 3
How does CO 2 enter the Calvin Cycle? CO2 enters through the
stomata microscopic pores in leaves Once in the leaf the CO2
diffuses into mesophyll cells where it can enter the chloroplast
Within the chloroplast carbon fixation takes place
Slide 4
Fig. 10-3a 5 m Mesophyll cell Stomata CO 2 O2O2 Chloroplast
Mesophyll Vein Leaf cross section
Slide 5
What occurs during carbon fixation? Carbon dioxide joins a
five-carbon molecule called ribulose bisphophate (RuBP) This
reactions is catalyzed by RuBP carboxylase, aka Ribisco Ribisco the
most abundant enzyme in nature This enzyme often takes up 50% of
the total chloroplast protein content Ribisco is a slow only
catalyzing 3 molecules of substrate per second (compared to 1,000
per second) Unstable 6 carbon compound is formed which splits to
form 2 three carbon molecules of PGA (phosphoglycerate)
Slide 6
How is PGA turned into sugar? Each molecule of PGA is
systematically reduced by enzyme action. NADPH provides the
hydrogen atoms and ATP provides the energy for these reactions to
occur. (NADPH and ATP from Light Reactions) PGAL
(phosphoglyceraldehyde), also called G3P
(glyceraldehyde-3-phosphate) is the final product of the Calvin
Cycle G3P can be exported to the cytoplasm and combined to form
fructose-6-phosphate and glucose 1-phosphate. Fructose and glucose
can join to form sucrose
Slide 7
How does the Calvin Cycle get back to 5-C RuBP? For every 3
molecules of carbon dioxide fixed, 6 molecules of G3P are formed
Only 1 of the G3P exits the cycle The other five G3P (3C) molecules
are used to regenerate 3 molecules of RuPB (5C) using ATP from the
Light Reactions
Slide 8
Fig. 10-18-3 Ribulose bisphosphate (RuBP) 3-Phosphoglycerate
Short-lived intermediate Phase 1: Carbon fixation (Entering one at
a time) Rubisco Input CO 2 P 3 6 3 3 P P P P ATP 6 6 ADP P P 6
1,3-Bisphosphoglycerate 6 P P 6 6 6 NADP + NADPH i Phase 2:
Reduction Glyceraldehyde-3-phosphate (G3P) 1 P Output G3P (a sugar)
Glucose and other organic compounds Calvin Cycle 3 3 ADP ATP 5 P
Phase 3: Regeneration of the CO 2 acceptor (RuBP) G3P
Slide 9
Alternative Carbon Fixation Mechanisms Organisms use feedback
mechanisms to maintain their internal environments and respond to
external environmental changes
Slide 10
Why do plants need alternative mechanisms for carbon fixation?
Dehydration is a problem for plants, sometimes requiring trade-offs
with other metabolic processes, especially photosynthesis On hot,
dry days, plants close stomata, which conserves H 2 O but also
limits photosynthesis The closing of stomata reduces access to CO 2
and causes O 2 to build up These conditions favor a seemingly
wasteful process called photorespiration
Slide 11
What is photorespiration? In most plants (C 3 plants), initial
fixation of CO 2, via rubisco, forms a three-carbon compound In
photorespiration, rubisco adds O 2 instead of CO 2 in the Calvin
cycle Photorespiration consumes O 2 and organic fuel and releases
CO 2 without producing ATP or sugar
Slide 12
How do C4 plants avoid photorespiration? C 4 plants minimize
the cost of photorespiration by incorporating CO 2 into four-carbon
compounds in mesophyll cells This step requires the enzyme PEP
carboxylase PEP carboxylase has a higher affinity for CO 2 than
rubisco does; it can fix CO 2 even when CO 2 concentrations are low
These four-carbon compounds are exported to bundle-sheath cells,
where they release CO 2 that is then used in the Calvin cycle
Slide 13
Fig. 10-19 C 4 leaf anatomy Mesophyll cell Photosynthetic cells
of C 4 plant leaf Bundle- sheath cell Vein (vascular tissue) Stoma
The C 4 pathway Mesophyll cell CO 2 PEP carboxylase Oxaloacetate
(4C) Malate (4C) PEP (3C) ADP ATP Pyruvate (3C) CO 2 Bundle- sheath
cell Calvin Cycle Sugar Vascular tissue
Slide 14
How do CAM plants avoid photorespiration? Some plants,
including succulents, use crassulacean acid metabolism (CAM) to fix
carbon CAM plants open their stomata at night, incorporating CO 2
into organic acids Stomata close during the day, and CO 2 is
released from organic acids and used in the Calvin cycle
Slide 15
Fig. 10-20 CO 2 Sugarcane Mesophyll cell CO 2 C4C4 Bundle-
sheath cell Organic acids release CO 2 to Calvin cycle CO 2
incorporated into four-carbon organic acids (carbon fixation)
Pineapple Night Day CAM Sugar Calvin Cycle Calvin Cycle Organic
acid (a) Spatial separation of steps (b) Temporal separation of
steps CO 2 1 2
Slide 16
Review The energy entering chloroplasts as sunlight gets stored
as chemical energy in organic compounds Sugar made in the
chloroplasts supplies chemical energy and carbon skeletons to
synthesize the organic molecules of cells Plants store excess sugar
as starch in structures such as roots, tubers, seeds, and fruits In
addition to food production, photosynthesis produces the O 2 in our
atmosphere
Slide 17
Fig. 10-21 Light Reactions: Photosystem II Electron transport
chain Photosystem I Electron transport chain CO 2 NADP + ADP P i +
RuBP 3-Phosphoglycerate Calvin Cycle G3P ATP NADPH Starch (storage)
Sucrose (export) Chloroplast Light H2OH2O O2O2