The Calvin cycle, also known as the light-independent reactions of photosynthesis, converts carbon dioxide into glucose using a three-stage process of carbon fixation, reduction, and regeneration. In C4 plants like maize and sugarcane, the initial product of carbon fixation is a four-carbon compound, oxaloacetate. CAM plants like cacti fix carbon dioxide at night and store it as malic acid to use during the day, avoiding photorespiration and being more water efficient.

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.