carbon reactions of photosynthesis

1. Carbon reactions of
photosynthesis

2. Agenda
1. The Calvin cycle
2. C4 Metabolism
3. Crassulacean acid metabolism (CAM)

3. The Calvin cycle
• Reduction of CO2 to carbohydrate.
The photosynthetic carbon reduction cycle originally described for C3 species
(the Calvin cycle, or reductive pentose phosphate [RPP] cycle).
(a series of experiments by Melvin Calvin and his colleagues in the 1950s)
The Calvin Cycle Has Three Stages:
• Carboxylation,
• Reduction,
• Regeneration

4. An outline of Calvin cycle
In the Calvin cycle,
- CO2 and water from the environment are enzymatically
combined with a five-carbon acceptor molecule to generate
two molecules of a three-carbon intermediate (3-
phosphoglycerate).
- This intermediate (3-phosphoglycerate) is reduced to
carbohydrate by use of the ATP and NADPH generated in
Light reactions.
- The cycle is completed by regeneration of the five-carbon
acceptor (ribulose-1,5-bisphosphate, abbreviated RuBP).

5. Three stages of the Calvin cycle
1. Carboxylation of the CO2
acceptor molecule – ribulose-
1,5-bisphosphate, forming two
molecules of 3-
phosphoglycerate (the first
stable intermediate of the Calvin
cycle).
2. Reduction of 3-
phosphoglycerate (an acid) to
gyceraldehyde-3-phosphate (a
carbohydrate).
• 3. Regeneration of the CO2
acceptor ribulose-1,5-
bisphosphate from
glyceraldehyde-3-
phosphate.

6. Enzyme Rubisco
• The Carboxylation of Ribulose Bisphosphate (1-stage) is catalyzed by the
Enzyme Rubisco.
For the first time, RuBisCO was
isolated and purified in 1955.
Rubisco are found in large
quantities in leaves and are the
main fraction of chloroplast
protein.
The enzyme turns into an active
state when the chloroplasts are
illuminated.
All 8 dimers of large chains and 8
small chains are combined into a
single complex weighing 540,000
Da.

7. Dual activity: carboxylase/oxygenase activity
Rubisco has a dual function:
1) Can catalyze not only the
carboxylation reaction of the
Calvin cycle: RBP (ribulose-1,5-
bisphosphate) + CO2 → 2
phosphoglycerate.
2) Rubisco is able to react with O2,
carrying out an oxygenase reaction,
and phosphoglycolic acid is formed:
RBP + O2 → phosphoglycolate.
Then, Phosphoglycolic acid
decomposes through a series of
transformations with the release of
CO2.

8. O2 competes
with CO2 for the
common
substrate
ribulose-1,5-
bisphosphate.
This property
limits net CO2
fixation.

9. 1. Carboxylation
CO2 is added to
carbon 2 of ribulose-
1,5-bisphosphate,
yielding an
unstable, enzyme-
bound intermediate,
which is hydrolyzed
to yield two
molecules of the
stable product 3-
phosphoglycerate.

10. 2. Reduction Step
3-phosphoglycerate formed in the carboxylation stage
undergoes two modifications:
- 3-phosphoglycerate is phosphorylated to 1,3-
bisphosphoglycerate by use of the ATP (generated in
the light reactions).
- 1,3-bisphosphoglycerate is reduced to
glyceraldehyde-3-phosphate by use of the NADPH
(also generated by the light reactions).
Triose Phosphates are formed in the Reduction step.

13. 3- step. Regeneration of Ribulose-1,5-Bisphosphate
the CO2 acceptor, ribulose-1,5-bisphosphate should be constantly regenerated.
• To prevent depletion of Calvin cycle intermediates, three molecules of ribulose-
1,5-bisphosphate (15 carbons total) are formed by reactions.
Reactions:
1. One molecule of glyceraldehyde-3-phosphate is converted to dihydroxyacetone-
3-phosphate (reaction 4)
2. Dihydroxyacetone-3-phosphate then undergoes condensation with a second
molecule of glyceraldehyde-3-phosphate, gives fructose-1,6-bisphosphate (reaction
5).
3. Fructose-1,6-bisphosphate is hydrolyzed to fructose-6-phosphate (reaction 6).
4. Transketolase transfers glycolaldehyde from fructose-6-phosphate to 3-PGA with
the formation of erythrose-4-phosphate and xylulose-5-phosphate and the
glyceraldehyde-3-phosphate (reaction 7).

14. 5. Erythrose-4-phosphate then combines with a fourth molecule of triose
phosphate (dihydroxyacetone-3-phosphate) to yield the seven-carbon sugar
sedoheptulose-1,7-bisphosphate (reaction 8).
6. Sedoheptulose-1,7-bisphosphate is then hydrolyzed to give sedoheptulose-
7-phosphate (reaction 9).
7. Sedoheptulose-7-phosphate donates a two-carbon unit to the fifth (and last)
molecule of glyceraldehyde-3-phosphate and produces ribose-5-phosphate
and xylulose-5-phosphate (reaction 10).
8. The two molecules of xylulose-5-phosphate are converted to two molecules
of ribulose-5-phosphate (reaction 11a). The third molecule of ribulose-5-
phosphate is formed from ribose-5-phosphate (reaction 11b).
9. Ribulose-5-phosphate is phosphorylated with ATP, regenerating the three
needed molecules of ribulose-1,5-bisphosphate (initial CO2 acceptor)
(reaction 12).

15. Net: 6 CO2 + 11 H2O + 12 NADPH + 18 ATP →
Fructose-6-phosphate + 12 NADP+ + 6 H+ + 18 ADP + 17 Pi

16. C4 cycle
• Plants with C4 metabolism are often found in hot environments.
• There are differences in leaf anatomy between plants that have
a C4 carbon cycle (called C4 plants) and those that
photosynthesize via the Calvin cycle (C3 plants).
• A cross section of a typical C3 leaf reveals one major cell type
that has chloroplasts, the mesophyll.
• In contrast, a typical C4 leaf has two distinct chloroplast-
containing cell types: mesophyll and bundle sheath (or Kranz,
German for “wreath” –корона) cells. Operation of the C4 cycle
requires the cooperative effort of both cell types.

17. A) A C4 monocot, Saccharum
officinarum (sugarcane)
C3 monocot, Poa sp. (a grass)

18. In general,
plants with the C4 pathway, there are
two types of cells and chloroplasts:
1) small granular plastids in the cells
of leaf mesophyll;
2) large plastids, often devoid of
granules, in the bundle sheath
sells surrounding the vascular
bundles.
The bundle sheath cells have
thickened cell walls, contain a large
number of chloroplasts and
mitochondria, are located around the
vascular bundles in 1 or 2 layers.

19. • M.D.Hatch and C.R.Slack
established that the C4 acids
malate and aspartate
(containing 4 carbon atoms) are
the first stable, detectable
intermediates of photosynthesis in
leaves of sugarcane and that
carbon atom 4 of malate
subsequently becomes carbon
atom 1 of 3-phosphoglycerate.
The primary carboxylation in these
leaves is catalyzed by PEP
(phosphoenylpyruvate)
carboxylase (not by RubisCo).
• Carbon is transferred from malate
to 3-phosphoglycerate.

20. Stages of the C4 cycle
• The basic C4 cycle consists of four stages:
1. Fixation of CO2 by the carboxylation of phosphoenolpyruvate in the
mesophyll cells to form a C4 acid (malate and/or aspartate)
2. Transport of the C4 acids (malate and/or aspartate) to the bundle sheath
cells.
3. Decarboxylation of the C4 acids within the bundle sheath cells and
generation of CO2, which is then fixed and reduced by RubisCo and
converted to carbohydrate via the Calvin cycle.
4. Transport of the C3 acid (pyruvate or alanine) that is formed by the
decarboxylation step back to the mesophyll cell and regeneration of
phosphoenolpyruvate (the CO2 acceptor).

22. The C4 photosynthetic pathway

23. C4 photosynthetic pathway is a CO2-Concentrating
mechanism
• The concentration of CO2 in bundle sheath cells has an energy cost.
Cost of concentrating CO2 within the bundle sheath cell = 2 ATP per CO2.
Because of this higher energy demand,
 C4 plants photosynthesizing under non-photorespiratory
conditions (high CO2 and low O2) require more quanta of
light per CO2.
 The quantum requirement of C4 plants remains relatively
constant under different environmental conditions.

24. In hot, dry climates, the C4 Cycle reduces
photorespiration and water loss
Two features of the C4 cycle overcome the effects of higher
temperature on photosynthesis:
1) First, the affinity of PEP carboxylase for its substrate, HCO3–, is
sufficiently high that the enzyme is saturated by HCO3 – in
equlibrium with air levels of CO2 (around 300 μmol/L).
• This high activity of PEP carboxylase enables C4 plants to reduce
the stomatal aperture (to conserve water while fixing CO2).
2) The suppression of photorespiration resulting from the concentration
of CO2 in bundle sheath cells.
• These features enable C4 plants to photosynthesize more efficiently at
high temperatures than C3 plants.

25. C4 cycle has a number of advantages
1. Photorespiration process is inhibited.
the Rubisco (Enzyme) has carboxylase or oxygenase function depends on
the content of 02 and CO2.
• Photorespiration requires an increased concentration of O2.
• In chloroplasts of the bundle sheath cells, the concentration of 02 is
lowered, since only cyclic phosphorylation occurs in them, in which water
does not decompose and 02 is not released.
• At the same time, the concentration of CO2 is increased in the bundle
sheath cells.
• This inhibit the Photorespiration process in bundle sheath cells and
therefore C4-type plants are characterized by a very low CO2 loss as a
result.

26. 2. Lower optimum temperature for photosynthesis C4 plants
• C4 plants are found in hot environments, they receive additional benefits
in terms of photosynthesis productivity.
• The optimum temperature for photosynthesis in C3 plants is 20-25° C,
while in C4 plants it is 30-45°C.
3. Higher Light saturation of photosynthesis.
Light saturation net photosynthetic flux in C4-plants also occurs at
higher values of light intensity than C3-plants.
• So, in plants of the C3 path, the intensity of photosynthesis ceases to
increase at 50% of full sunlight, while this does not occur in the C4
forms.
• Such features of C4 plants explain the high f / s intensity at elevated
temperature and illumination.

27. 4. Bundle sheath cells are located very close to vascular bundles (tissues).
In C4-path plants - the formation of products of Calvin cycle occurs in chloroplasts
located directly near the vascular bundles.
• This contributes to the better outflow of assimilates and, as a result, increases the
intensity of photosynthesis.
5) Higher CO2 assimilation by C4 plants.
If C3 plants’ CO2 assimilation rate in full sunlight is 1-50 mg/dm2h, and C4 plants
assimilate at a rate of 40-80 mg/dm2-h.
Maize, sorghum, sugarcane are some of the most productive crops.
Thus, the photosynthesis intensity:
• in maize (C4 plant) is 85 mg CO2/dm2-h,
• in sorghum (C4 plant) is 55 mg CO2 / dm2-h,
• in wheat (C3 plant) only 31 mg CO2/dm2-h.
The high potential productivity of C4 plants can be fully realized in full sunlight
and high temperature.

29. Crassulacean acid metabolism (CAM)
Another mechanism for concentrating CO2 at the site of rubisco is found in
crassulacean acid metabolism (CAM).
• CAM is not restricted to the family Crassulaceae (Crassula, Kalanchoe,
Sedum); it is found in numerous angiosperm families.
Cacti and euphorbias are CAM plants, as well as pineapple, vanilla.
 The CAM mechanism enables plants to improve water use efficiency.
CAM plant loses 50-100 g of water per 1 gram of CO2 gained,
 250 to 300 g of water lost in C4 plants;
 400 to 500 g of water lost in C3 plants.

30. • The CAM mechanism is similar in many respects to the C4 cycle.
In C4 plants, formation of the C4 acids in the mesophyll is spatially
separated from Calvin cycle in the bundle sheath (C4 acids should be
decarboxylated resulting CO2 and CO2 is refixed).
• In CAM plants, formation of the C4 acids is both temporally and spatially
separated.
- At night, CO2 is captured by PEP carboxylase in the cytosol, and
- the malate that forms from the oxaloacetate product is stored in the
vacuole.
Similarity of the CAM mechanism to the C4 cycle
A CO2 uptake is temporally separated from photosynthetic reactions:
- CO2 uptake and fixation take place at night,
- decarboxylation and refixation of the internally released CO2 occur
during the day.

31. Plants with CAM pathway grow slowly
• The implementation of photosynthesis in this way
allows plants to save water as much as possible and
to support the photosynthesis process in conditions of
acute water scarcity.
• However, the CAM pathway cannot provide high
plant productivity; therefore, these plants grow
slowly and cannot compete with C3 and C4 plants
under less extreme conditions.