Calvin or C3 Cycle or PCR
Calvin or C3 Cycle or PCR (Photosynthetic Carbon Reduction Cycle):
(Calvin or C3 Cycle )It is the basic mechanism by which CO2 is fixed (reduced) to form carbohydrates. It was proposed by Melvin Calvin. Calvin along with A.A. Benson, J. Bassham used radioactive isotope of carbon (C14) in Chlorella pyrenoidosa and Scenedesmus oblique’s to determine the sequences of dark reaction. For this work Calvin was awarded Nobel prize in 1961. To synthesize one glucose molecule Calvin cycle requires 6CO2, 18 ATP and 12 NADPH2.
Calvin cycle completes in 4 major phases(Calvin or C3 Cycle):
1. Carboxylation phase
2. Reductive phase
3. Glycolytic reversal phase (sugar formation phase)
4. Regeneration phase
1. Carboxylation phase(Calvin or C3 Cycle):
CO2 enters the leaf through stomata. In mesophyll cells, CO2combines with a phosphorylated 5-carbon sugar, called Ribulose bisphosphate (or RuBP). This reaction is catalyzed by an enzyme, called RUBISCO. The reaction results in the formation of a temporary 6 carbon compound (2-carboxy 3-keto 1,5-biphosphorbitol) Which breaks down into two molecules of 3-phosphoglyceric acid (PGA) and it is the first stable product of dark reaction (C3 Cycle).
2. Reductive Phase :
The PGA molecules are now phosphorylated by ATP molecule and reduced by NADPH2 (product of light reaction known as assimilatory power) to form 3-phospho-glyceraldehyde (PGAL).
3. Glycolytic Reversal (Formation of sugar) Phase(Calvin or C3 Cycle):
Out of two mols of 3-phosphoglyceraldehyde one mol is converted to its isomer 3-dihydroxyacetone phosphate.
4. Regeneration Phase in Calvin or C3 Cycle :
Regeneration of Ribulose-5-phosphate (Also known as Reductive Pentose Phosphate Pathway) takes place through number of biochemical steps.
C4 Cycle (HSK Pathway or Hatch Slack and Kortschak Cycle):
C4 cycle may also be referred as the di-carboxylic acid cycle or the β-carboxylation pathway or Hatch and Slack cycle or cooperative photosynthesis (Karpilov, 1970). For a long time, C3 cycle was considered to be the only photosynthetic pathway for reduction of CO2 into carbohydrates. Kortschak, Hartt and Burr (1965) reported that rapidly photosynthesizing sugarcane leaves produced a 4-C compound like aspartic acid and malic acid as a result of CO2 – fixation.
It was later supported by M. D. Hatch and C. R. Slack (1966) and they reported that a 4-C compound oxaloacetic acid (OAA) is the first stable product in CO2 reduction process. This pathway was first reported in members of family Poaceae like sugarcane, maize, sorghum, etc. (tropical grasses), but later on the other subtropical plant like Atriplex spongiosa (Salt bush), Dititaria samguinolis, Cyperus rotundus, Amaranthus etc. So, the cycle has been reported not only in the members of Graminae but also among certain members of Cyperaceae and certain dicots.
Structural Peculiarities of C4 Plants (Kranz Anatomy):
C4 plants have a characteristic leaf anatomy called Kranz anatomy (Wreath anatomy – German meaning ring or Helo anatomy). The vascular bundles in C4 plant leaves are surrounded by a layer of bundle sheath cells that contain large number of chloroplast. Dimorphic (two morphologically distinct type) chloroplasts occur in C4 plants.
In Mesopyll cell
(i) Chloroplast is small in size
(ii) Well developed grannum and less developed stroma.
(iii) Both PS-II and PS-I are present.
(iv) Non cyclic photophosphorylation takes place.
(v) ATP and NADPH2 produces.
(vi) Stroma carries PEPCO but absence of RuBisCO.
(vii) CO2 acceptor is PEPA (3C) but absence of RUBP
(viii) First stable product OAA (4C) produces.
In Bundle sheath Cell(Calvin or C3 Cycle):
(i) Size of chloroplast is large
(ii) Stroma is more developed but granna is poorly developed.
(iii) Only PS-I present but absence of PS-II
(iv) Non Cyclic photophosphorylation does not takes place.
(v) Stroma carries RuBisCO but absence of PEPCO.
(vi) CO2 acceptor RUBP (5c) is present but absence of PEPA (3C)
(vii) C3-cycle takes place and glucose synthesies.
(viii) To carry out C3-cycle both ATP and NADPH2 comes from mesophyll cell chloroplast.
Carbon dioxide from atmosphere is accepted by Phosphoenol pyruvic acid (PEPA) present in stroma of mesophyll cell chloroplast and it converts to oxaloacetic acid (OAA) in the presence of enzyme PEPCO (Phosphoenolpyruvate carboxylase). This 4-C acid (OAA) enters into the chloroplast of bundle sheath cell and there it undergoes oxidative decarboxylation yielding pyruvic acid (3C) and CO2.
The carbon dioxide released in bundle sheath cell reacts with RuBP (Ribulose 1, 5 bisphosphate) in presence of RUBISCO and carry out Calvin cycle to synthesize glucose. Pyruvic acid enters mesophyll cells and regenerates PEPA. In C4 cycle two carboxylation reactions take place.
Reactions taking place in mesophyll cells are stated below: (1st carboxylation):(Calvin or C3 Cycle)
C4 plants are better photosynthesizes. There is no photorespiration in these plants. To synthesize one glucose molecule it requires 30 ATP and 12 NADPH2.
Significance of C4 Cycle:
1. C4 plants have greater rate of carbon dioxide assimilation than C3plants because PEPCO has great affinity for CO2 and it shows no photorespiration resulting in higher production of dry matter.
2. C4 plants are better adapted to environmental stress than C3 plants.
3. Carbon dioxide fixation by C4 plants requires more ATP than C3plants for conversion of pyruvic acid to PEPA.
4. Carbon dioxide acceptor in C4 plant is PEPA and key enzyme is PEPCO.
5. They can very well grow in saline soils because of presence of C4organic acid.
Crassulacean Acid Metabolism (CAM Pathway):
It is a mechanism of photosynthesis which occurs in succulents and some other plants of dry habitats where the stomata remain closed during the daytime and open only at night. The process of photosynthesis is similar to that of C4 plants but instead of spatial separation of initial PEPcase fixation and final Rubisco fixation of CO2, the two steps occur in the same cells (in the stroma of mesophyll chloroplasts) but at different times, night and day, e.g., Sedum, Kalanchoe, Opuntia, Pineapple (Fig. 6.13). (CAM was for the first time studied and reported by Ting (1971).
Characteristics of CAM Plants:
1. Stomatal movement is scoto-active.
2. Presence of monomorphic chloroplast.
3. Stroma of chloroplast carries both PEPCO and RUBISCO.
4. Absence of Kranz anatomy.
5. It is more similar to C4 plants than C3 plants.
6. In these plants pH decreases during night and increases during day time.
Mechanism of CAM Pathway:
PHASE I. During night:
Stomata of Crassulacean plants remain open at night. Carbon dioxide is absorbed from outside. With the help of Phosphoenol pyruvate carboxylase (PEPCO) enzyme the CO2 is immediately fixed, and here the acceptor molecule is Phosphoenol pyruvate (PEP).
Malic acid is the end product of dark fixation of CO2. It is stored inside cell vacuole.
During day time the stomata in Crassulacean plants remain closed to check transpiration, but photosynthesis does take place in the presence of sun light. Malic acid moves out of the cell vacuoles. It is de-carboxylated with the help of malic enzyme. Pyruvate is produced. It is metabolized.
The CO2 thus released is again fixed through Calvin Cycle with the help of RUBP and RUBISCO. This is a unique feature of these succulent plants where they photosynthesis without wasting much of water. They perform acidification or dark fixation of CO2 during night and de-acidification during day time to release carbon dioxide for actual photosynthesis.
Ecological Significance of CAM Plants:
These plants are ecologically significant because they can reduce rate of transpiration during day time, and are well adapted to dry and hot habitats.
1. The stomata remain closed during the day and open at night when water loss is little due to prevailing low temperature.
2. CAM plants have parenchyma cells, which are large and vacuolated. These vacuoles are used for storing malic and other acids in large amounts.
3. CAM plants increase their water-use efficiency, and secondly through its enzyme PEP carboxylase, they are adapted to extreme hot climates.
4. CAM plants can also obtain a CO2 compensation point of zero at night and in this way accomplish a steeper gradient for CO2 uptake compared to C3 plants.
5. They lack a real photosynthesis during daytime and the growth rate is far lower than in all other plants (with the exception of pineapple).