this presentation explains all the phases of dark reaction. it includes c3 cycle, c4 cycle, CAM cycle, photorespiration.

1. Photosynthesis- Dark reaction
BY: Dr. Sunita Sangwan
Assistant Professor, Botany
Dept. of Higher Education, Haryana

2. Introduction
 This is the second step in the mechanism of
photosynthesis. The chemical processes of photosynthesis
occurring independent of light is called dark reaction.
 It takes place in the stroma of chloroplast. The dark
reaction is purely enzymatic and it is slower than the light
reaction. The dark reactions occur also in the presence of
light. In dark reaction, the sugars are synthesized from
CO2.
 The energy poor CO2 is fixed to energy rich carbohydrates
using the energy rich compound, ATP and the assimilatory
power, NADPH2 of light reaction.

3. Cont…..
 The process is called carbon fixation or carbon
assimilation. Since Blackman demonstrated the
existence of dark reaction, the reaction is also called as
Blackman’s reaction. In dark reaction two types of
cyclic reactions occur
1. Calvin cycle or C3 cycle
2. Hatch and Slack pathway or C4 cycle

4. Calvin cycle
 Given by Melvin Calvin & they also awarded Nobel
prize for it.

5. Calvin Cycle
1. Carboxylation
4. Regeneration

7. Summary of Calvin cycle

8. Reaction of Calvin cycle

9. C4 Cycle
 Given by M.D Hatch & Slack in 1966. they establish the another
pathway for CO2 reduction called Hatch & Slack pathway.
 Since the first stable product in this cycle is a 4-carbon dicarboxylic
acid, it is also called C4 Dicarboxylic acid pathway.
1. Concentric arrangement of mesophyll tissues around the vascular
bundles
2. Bundle sheath cells are larger in size than the outer mesophyll
cells
3. Presence of chloroplast in mesophyll & bundle sheath cells
4. Dimorphic chloroplast
5. Chloroplast of mesophyll cell lack RUBISCO activity
Kranz Anatomy – a special feature of C4 plant

11. Significance of Kranz anatomy
 Concentric arrangements of tissue provides a small
surface area in relation to volume. This reduces the
transpiring surface.
 It also helps the plants to survive in higher irradiation
 C4 plants accumulate excess of cations from the soil
which are balanced by the anions produced during C4
carboxylation.

12. The basic C4 cycle consists of four stages:
The C4 Cycle Concentrates CO2 in Bundle Sheath Cells
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 to the bundle sheath cells
3. Decarboxylation of the C4 acids within the bundle sheath cells
and generation of CO2, which is then reduced 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 the CO2 acceptor phosphoenolpyruvate.

13. The basic C4 cycle consists of four stages

14. Significance of C4 pathway
 C4 plants can perform normal photosynthesis even in low CO2 conc.
And high intensities of light.
 They are able to trap CO2 from outside (air) and from inside
(produced by respiration) because of high efficiency of PEP
carboxylase. Hence they are able to perform photosynthesis even
when stomata are closed.
 Effect of water stress in C4 plants is minimum as bundle sheath cells
are close to water supply, i.e. vascular bundles
Thus C4 plants possess better adaptability to adverse environmental
conditions (low CO2 conc., high light intensity, water stress etc.) and
possess higher photosynthetic ability than C3 plants

15. Various mechanism for C4 pathways
C4 pathway
NADP+- Me- Type
NAD+- Me- Type
PCK- Me- Type

16. NADP+- Me- Type C4 pathway
This occurs in the C4 plants posses NADP+ Malic enzyme for decarboxylation.
Ex. Zea mays, Saccharum officinarum, Euphorbia hirta etc. these plants have
centrifugal chloroplast & grana are absent.

17. NAD+- Me- Type C4 pathway
This occurs in the C4 plants posses NAD+ Malic enzyme for decarboxylation.
Ex. Atriplex, Portulaca, Cynodon, Amaranthus etc. these plants have centripetal
chloroplast & grana stacks are present

18. PCK+- Me- Type C4 pathway

19. PCK- Me- Type C4 pathway
Difference in PCK- Me- Type to NAD+- Me- type
1. Mitochondria is not involved in this cycle
2. In bundle sheath cells, oxalo acetic acid (OAA) is directly decarboxylated without
converting into malic acid.

20. Crassulacean Acid Metabolism (CAM)
 Dark fixation of CO2
 A third mechanism for concentrating CO2 at the site of rubisco is
found in crassulacean acid metabolism (CAM). Despite its name,
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,
and agave.
 The CAM mechanism enables plants to improve water use
efficiency. Typically, a CAM plant loses 50 to 100 g of water for
every gram of CO2 gained, compared with values of 250 to 300 g
and 400 to 500 g for C4 and C3 plants respectively (see Chapter 4).
Thus, CAM plants have a competitive advantage in dry
environments.

21. Steps of CAM pathway
1. Acidification
2. Decarboxylation
3. Synthesis of carbohydrates

22. CAM

23. CAM
 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 decarboxylation of the C4 acids and from
refixation of the resulting CO2 by the Calvin cycle in the bundle
sheath. 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. During the day, the stored malate is transported to the
chloroplast and decarboxylated by NADP-malic enzyme, the
released CO2 is fixed by the Calvin cycle, and the NADPH is used
for converting the decarboxylated triose phosphate product to
starch.

24. Significance of CAM cycle
 CAM cycle possess a survival value to CAM plants which are
characterized to grow in xeric conditions. as stomata closed
during day to conserve water to protect the plant from
desiccation, CAM proves to be advantageous to them in
following ways:
1. By receiving and storing CO2 during night by acidification
2. By providing CO2 supply during the day to perform
photosynthesis even when stomata remain closed.

25. Photorespiration
An important property of rubisco is its ability to catalyze both the
carboxylation and the oxygenation of RuBP.
Oxygenation is the primary reaction in a process known as
photorespiration.
Because photosynthesis and photorespiration work in diametrically
opposite directions, photorespiration results in loss of CO2 from
cells that are simultaneously fixing CO2 by the Calvin cycle.
CO2 and O2 compete for reaction with ribulose-1,5-bisphosphate
because carboxylation and oxygenation occur within the same active
site of the enzyme.
Offered equal concentrations of CO2 and O2 in a test tube,
angiosperm rubiscos fix CO2 about 80 times faster than they
oxygenate.

26. Conti….
However, an aqueous solution in equilibrium with air at 25°C has a
CO2:O2 ratio of 0.0416 . At these concentrations, carboxylation in air
outruns oxygenation by a scant three to one.
As the temperature increases, the concentration of CO2 in a solution in
equilibrium with air decreases more than the concentration of O2 does
Consequently, the concentration ratio of CO2 to O2 decreases as the
temperature rises. As a result of this property, photorespiration
(oxygenation) increases relative to photosynthesis (carboxylation) as the
temperature rises.
This effect is enhanced by the kinetic properties of rubisco, which also
result in a relative increase in oxygenation at higher temperatures.
Overall, then, increasing temperatures progressively tilt the balance away
from the Calvin cycle and toward the oxidative photosynthetic carbon
cycle
The C2 oxidative photosynthetic carbon cycle acts as a scavenger operation
to recover fixed carbon lost during photorespiration by the oxygenase
reaction of rubisco

29. You tube video links for these topics
 https://youtu.be/vaQOjWiAab4 Photosynthesis Dark phase- Calvin ( C3)
cycle
 https://youtu.be/B78EWR4kCLU Photosynthesis Dark phase- C4 cycle
 https://youtu.be/ep5beU2VVdI Photosynthesis Dark phase- CAM cycle
 https://youtu.be/sFQDPx2UyPw Photosynthesis Dark phase- Photorespiration
or C2 cycle

30. References
 Taiz, L. and Zeiger, E. (2010) Plant Physiology. 5th Edition, Sinauer
Associates, Inc., Sunderland.
 Hopkins, W.G. and Hüner, N.P. (2004) Introduction to Plant Physiology. 3rd
Edition, John Wiley & Sons, Inc, Hoboken.