C3 cycle c4 cycle CAM cycle

1. C3 Cycle or Calvin cycle
 By Harinatha Reddy

2. Calvin cycle :
 Calvin cycle is light-independent reactions, or dark reactions, of
photosynthesis that convert carbon dioxide into glucose.

3.  The first compound that formed in Calvin cycle is
3-phosphoglycerate (PGA), one of the intermediates of glycolysis.
 The first intermediate to be identified was a three-carbon molecule.
 The plants that utilize this pathway to fix atmospheric CO2 are
referred to as C3 plants.

4. The Calvin cycle reactions can be organized into three basic stages:
 Carbon fixation,
 Reduction, and
 Regeneration.

6. Step 1: Carbon fixation.
 A CO2 molecule combines with a five-carbon acceptor molecule,
ribulose-1,5-bisphosphate (RuBP).
 This step makes a six-carbon compound (Hexose) that splits into
two molecules of a three-carbon compound, 3-phosphoglyceric
acid (3-PGA).
 This reaction is catalyzed by the enzyme Ribulose-1,5-
bisphosphate carboxylase/oxygenase, or Rubisco.

7.  In the first step of the cycle, an enzyme
nicknamed rubisco catalyzes attachment of CO2 to a five-carbon
sugar called Ribulose 1,5 bisphosphate (RuBP).
 Thus, for each CO2 enters the cycle, two 3-PGA molecules are
produced.

8. Step 2: Reduction:
 In the second stage, ATP and NADPH are used to convert the 3-
PGA molecules into molecules of a three-carbon sugar,
glyceraldehyde-3-phosphate (G3P).
 This stage gets its name because NADPH donates electrons
to, reduces, a three-carbon intermediate to make G3P.

9.  The reduction stage of the Calvin cycle, which requires ATP
and NADPH, converts 3-PGA (from the fixation stage) into a
three-carbon sugar.

10.  First, each molecule of 3-PGA receives a phosphate group from
ATP, turning into a doubly phosphorylated molecule called 1,3-
bisphosphoglycerate (and leaving behind ADP as a by-product).
 Second, the 1,3-bisphosphoglycerate molecules are reduced (gain
electrons). Each molecule receives two electrons from NADPH
and loses one of its phosphate groups, turning into a three-carbon
sugar called glyceraldehyde 3-phosphate (G3P).
 This step produces NADP+ and phosphate (Pi​) as by-products.
Reduction is a two step process:

11.  The enzyme phosphoglycerate kinase catalyses the
phosphorylation of 3-PGA by ATP (which was produced in the
light-dependent stage). 1,3-Bisphosphoglycerate (1,3BPGA,
glycerate-1,3-bisphosphate) and ADP are the products.
 The enzyme glyceraldehyde 3-phosphate dehydrogenase
catalyses the reduction of 1,3BPGA by NADPH (which is
another product of the light-dependent stage). Glyceraldehyde
3-phosphate is produced, and the NADPH itself is oxidized and
becomes NADP+.
 Again, two NADPH are utilized per CO 2 fixed.

12. Regeneration:
 Some (2 molecules ) G3P molecules go to make glucose, while
the remain (10 molecules) must be recycled to regenerate the
Ribulose 5 phosphate.
 Ribulose 5 phosphate undergoes phosphorylation in the
presence of ATP and form Ribulose 1,5 bisphosphate
(RuBP).
 Regeneration requires 6 ATP and involves a complex network
of reactions,

13.  In Calvin cycle to fix 6 carbon atoms, 18 ATP molecules and
12 NADPH undergoes hydrolysis.

14. C4 Cycle

15. Introduction:
 C4 cycle takes place in monocots such as sugarcane, maize and
sorghum .
 The initial product is 4 carbon compound (Oxaloacetate), the
process is called C4 pathway of carbon dioxide fixation.
 C4 cycle also know as Hatch Slack cycle.

18.  The chloroplasts in mesophyll cells are granal, whereas in
bundle sheath cells they are agranal.
 However, in agranal chloroplasts of bundle sheath cells grana
are absent and thylakoids are present only as stroma lamellae.

19.  In C4 grasses enlarged bundle sheath (BS) cells surround
the veins (V) and the bundle sheath cells are then surrounded
by mesophyll (M) cells.
 C4 plants shows Kranz anatomy.

20. Mesophyll chloroplast:
 In C4 the enzyme Ribulose bi phosphate carboxylase (RuBP
carboxylase) is absent. Therefore no CO2 fixation occurs.
 In C3 plants, RuBP carboxylase is abundantly present in mesophyll
chloroplast where CO2 fixation occurs.

21. Bundle sheath chloroplast:
 Grana few or absent, if present very small and poorly developed
(Chlorophyll is absent).
 High concentration of RuBP carboxylase. Therefore CO2 fixation
occurs.
 Low activity of photosystem II, hence few ATP, NADPH and O2
generated.

22.  In C4 plants (maize, sugarcane, etc.), light reactions occur in
mesophyll cells, whereas CO2 assimilation takes place in bundle
sheath cells.

23. Pyruvate kinase

24. C4 photosynthetic Carbon Cycle:
Step 1: Carboxylation:
 In C4 pathway, CO2 from the atmosphere enters through
stomata into the mesophyll cells and combines with
phosphoenol pyruvate (3-carbon compound).
 This reaction is catalysed by an enzyme known as phosphoenol
pyruvate carboxylase, With the result, a C4 acid, oxaloacetic
acid (OAA) is formed.

25. Step 2: Dehydrogenation:
 Oxaloacetic acid undergoes dehydrogenation in the presence of
NADPH produce Malate.
 This reaction catalyze by enzyme Malate dehydrogenase.
 Malate is transported to the bundle-sheath cells surrounding
a nearby vein.

26. Step 3: Decarboxylation:
 Malate undergo decarboxylation to release fixed CO2 and high
concentration of CO2 is generated near Rubisco.
 The CO2 now enters the Calvin cycle.
 During this reaction 3-carbon compound called pyruvic acid
is formed it is transported back to mesophyll cells.

27. Step 4: Phosphorylation:
 Pyruvic acid undergoes phosphorylation in the presence of ATP
and produce Phophoenol pyruvic acid.
 This reaction is catalyse by enzyme Pyruvate kinase.

28.  The C4 pathway uses more energy than the C3 pathway.
 The C3 pathway requires 18 molecules of ATP for the
synthesis of one molecule of glucose.
 whereas the C4 pathway requires 30 molecules of ATP

29. CAM Cycle:

30. Introduction:
 Some plants that are adapted to dry environments, such as cacti
and pineapples, use the Crassulacean acid metabolism (CAM)
pathway to minimize photorespiration.
 This name comes from the family of plants, the Crassulaceae, in
which scientists first discovered the pathway.
Crassulaceae Desert plants (cacti)

31.  CAM plants are typically dominant in very hot, dry areas, like
deserts.

 At night, CAM plants open their stomata, allowing CO2 to
diffuse into the leaves.

32. CO2

34. Step 1: Carboxylation:
 The CO2 reacts with Phosphoenol-pyruvate (PEP) and
form oxaloacetate (4c) by PEP carboxylase.
 Starch used in the formation of Phosphoenol-pyruvate.

35. Step 2: Dehydrogenation:
 Oxaloacetate undergo dehydrogenation in the presence of NADH
produce malate.
 These reaction is catalysed by enzyme malate (Malic)
dehydrogenase.
 The malate formed in night time stored in Vacuoles (pH
decreases) used as C02 during day time.

36. Step 3: Decarboxylation:
During day time the malate released form vacuoles ((pH
increases) and Decarboxylated in the presence of NADP+ into
CO2 and pyruvate.
This reaction is catalysed by Malate (Malic) enzyme.
The released CO2 enter into the Calvin cycle.
The released pyruvate used for the synthesis of starch.

38.  However, plant species that use CAM photosynthesis not only
avoid photorespiration, but are also very water-efficient.
 Their stomata only open at night, when humidity tends to be
higher and temperatures are cooler, both factors that reduce
water loss from leaves.

39.  The Malic acid is stored inside vacuoles until the next day.
 In the daylight, the CAM plants do not open their stomata, but
they can still photosynthesis.
 That's because the organic acids are transported out of the vacuole
and broken down to release CO2 which enters the Calvin cycle.
 This controlled release maintains a high concentration of CO2
around RUBISCO.