The Calvin cycle / carbon fixation
Key things
 Every step is controlled by an enzyme!
 ATP / NADPH drive the CO2 to make an organic sugar G3P, which
occurs in the stroma of the chloroplasts.
 Rubisco is the enzyme that drives the process.
3 Phases
1. Carbon Fixation: Rubisco mediates the transfer of CO2 onto the
enzyme.
2. Reduction: ATP & NADPH are used to rearrange Rubisco into
G3P (aka PGAL), a three carbon sugar.
a. G3P- product of the Calvin cycle. It’s a three carbon sugar that
is the starting point for the synthesis of other carbohydrates.
3. Regeneration: ATP is used to reconstitute Rubisco from G3P.
You DO NOT need to copy the diagram into your notes.
http://www.phschool.com/itext/elife/site/text/chapter8/concept8.3.html
In order to get 1 G3P as a product of the Calvin cycle, three molecules of
CO2 have to be joined to three molecules of Rubisco.
This makes six molecules of G3P, one of which is a net product.
The other five G3P are used to regenerate three molecules of Rubisco.
Remember, G3P is a sugar building block.
2 G3P = 16 carbon sugars (can form polysaccharides)
Input
3 CO2
9 ATP
6 NADPH
Output
1 G3P
9 ADP + Pi
6 NADP+
(per G3P)
Rubisco: evolved in low oxygen gas concentrations and their active sites
had an affinity (natural liking) for oxygen.
PROBLEM!!!!!
Photorespiration






CO2 levels inside a leaf become too low.
Can’t make G3P
Oxygen can jump into the Calvin cycle
Mostly on hot, dry days / warm areas
Requires a lot more energy (ATP)
Plants are forced to close their stomata.
Metabolic pathway that occurs when RuBP incorporates O2 instead
of CO2 into RuBP. This is a metabolic dead end, because it uses ATP
and produces no sugar.
Most of the time it’s not a problem as long as the plant can keep open its
stomata; then photorespiration can be kept at a minimum.








C3 Leaves
No adaptations
Mostly all plants
Both stages of
photosynthesis
occur
simultaneously
O2 and CO2 are
exchanged
through stomata’s
Sugars are
transported to
vascular tissue for
transport.
G3P turns into a 3
carbon molecule.
If stomas stay
open= desiccation
Closed stomata’s=
increase O2,
decrease CO2,
increases
photorespiration =
not good.
C4 Leaves
 Carbon fixation
occurs in the
mesophyll cells;
specifically the
xylem and
phloem are
surrounded by a
thick walled,
bundle sheath of
parenchymal
cells, and is
then introduced
into the Calvin
cycle.
 CO2 is
incorporated
into 4C organic
acid, called
malic acid by an
enzyme known
as PEP
carboxylase (low
affinity for
oxygen)
 Ex- Corn





CAM Plants
Temporal
separation
(timing).
Carbon fixation
occurs during the
evening when the
stomata’s are
open.
Open at night. CO2
gets converted into
malic acid and is
stored in the
vacuoles.
During the day the
stomata’s close
and take CO2 out
of the malic acid
and add it to the
Calvin cycle to
make more sugars.
Ex- pineapple,
cacti
http://www2.mcdaniel.edu/Biology/botf99/photodark/c4.htm
Review questions will be on the board when you enter on Wednesday.