Different Pathway of Photosynthesis or Alternate Choices of Pathways of Dark Reactions of Photosynthesis

 They are more than one pathway of dark reactions of CO2fixation leading to the synthesis of carbohydrate. Following are the three types of pathways, which are well established now.

1. Calvin cycle (C3 plants)

2. Hatch and Slack cycle (C4 plants)

3. CAM cycle (CAM plants)

1. C3 CYCLE (or) CALVIN CYCLE

This dark reaction process of photosynthesis has been named variously such as -Calvin Cycle, Bassham and Calvin Cycle, Bassham and Calvin Cycle, Blackman reaction, Carbon assimilation, Path of carbon in photosynthesis, Reductive Pentose Phosphate Cycle.

Calvin cycle consists of two important steps :

i. Synthesis of carbohydrate

ii. Regeneration of RuDP

Sequences of reaction taking place in Calvin (C3) cycle are furnished below :

i. Synthesis of Carbohydrate

1. CO2 is first accepted by RuDP and forms an unstable 6-carbon compound from which two molecules of phosphoglyceric acid (PGA) are formed. The reaction is regulated by an enzyme called carboxydismutase or RuDP carboxylase (Rubisco).

2. The phosphoglyceraldehyde molecule is converted into dihydroxyacetone phosphate in presence of the enzyme triose phosphate isomerase.

3. The phosphoglyceraldehyde molecule is converted into dihydroxyacetone phosphate in presence of the enzyme triose phosphate isomerase.

4. Phosphoglyderaldehyde and dihydroxyacetone phosphate (one molecule each) unite to form fructose-1,6-diphosphate with the regulation of the aldolase enzyme.

5. From fructose-1,6-diphosphate, different types of compounds are synthesized converted into glucose or starch.

In this Calvin Cycle, only one molecule of CO2 is utilized at a time; therefore, the cycle has to run 6 times totally, for carbohydrate synthesis using CO2, and H2O. Hence, the regeneration of RuDP is essential to carry on the processes of photosynthesis.

ii. Regeneration of Ribulose diphosphate
Regeneration of ribulose diphosphate is essential to carry on the process of photosynthesis. The sequential reactions leading to the regeneration of RuDP would be as follows:

1. The so formed fructose-6-phosphate and phosphoglyceraldehyde combine and break into4-carbon compound (erythrose-4-phosphate) and 5-carbon compound (xylulose-5-phosphate) in presence of enzyme transketolase.

2. Erythrose-4-phosphate combines with a molecule of dihydroxyacetone phosphate to form sedoheptulose-17-diphosphate in presence of enzyme aldolase.

3. From sedoheptulose-1,7-diphosphate, one phosphate is removed in presence of enzyme phosphatase to form sedoheptulose-7- diphosphate.

4. Sedoheptulose-7-diphosphate and phosphoglyceraldehyde combine in presence of transketolase and produce one molecule each of xylulose-5-phosphate and ribose-5-phosphate.

5. Both these compounds convert into ribulose-5-phosphate in presence of enzyme phosphopeptide isomerase. Thus, ribulose-5-phosphate is formed.

6. Ribulose-5-phosphate is converted into ribulose-1,5-diphosphate mediated by phosphopentokinase utilizing ATP molecule coming from photophosphorylation. The ATP is converted into ADP.

Schematic representation of the Calvin (C3) cycle 

Simplified schematic representation of Calvin (C3 ) cycle

Three Phases of Calvin Cycle

It is evident that Calvin Cycle consists of the following three phases, viz.,

i. Carboxylative phase (Reaction No.1 of Carbohydrate synthesis)

ii. Reductive phase (Reaction Nos.2-5 of Carbohydrate synthesis)

iii. Regenerative phase (Reaction Nos.1-6 of Regeneration of RuDP)

Enzymes involved in Calvin Cycle

In the first carboxylative phase, only one enzyme, viz., carboxydismutase is involved which catalyzes the carboxylation of ribulose 1,5-diphosphate to form PGA.

In the second reductive phase, three enzymes are involved in reducing PGA to triose phosphate and they are :

i. Phosphoglyceryl kinase

ii. Triosephosphate dehydrogenase, and

iii. Triosephosphate isomerase.

In the regenerative phase, seven enzymes participate and they are :

i. Aldolase

ii. Phosphatase

iii. Transketolase

iv. Transaldolase

v. Phosphoribose isomerase

vi. Phosphopentose epimerase, and

vii. Phosphorubulokinase.

Why named as C3 cycle or C3 Plant?

Because the first visible product of the Calvin Cycle is 3-Phosphoglyceric acid (PGA), Which is a 3- carbon compound, the Calvin cycle is popularly known as C3 Cycle. The plants, which possess the C3 pathway or Calvin Cycle are called C3 plants. Most of the higher plants possess a C3 pathway for the fixation of CO2 in photosynthesis, Examples are rice, wheat, barley, oats, rye, most of the pulses and oilseed crops, etc.

2. CYCLE (or) HATCH AND SLACK CYCLE

For many years, the Calvin cycle as described earlier was thought to be the only photosynthetic reaction operating in higher plants. But, Kortschak et al. (1965) reported that 4-carbon compounds, dicarboxylic acids, malate, and aspartate were the major labeled products when sugarcane leaves were to photosynthesize for shorter periods using 14CO2. Later on, Hatch and Slack (1966) of Australia also confirmed this in several plants. They have proposed an alternate pathway, which is called as C4 Cycle or Hatch-Slack

Pathway or Di-carboxylic Acid Cycle (C4), because 4-carbon containing dicarboxylic acids are the earliest products after carboxylation in this pathway.

Examples of C4 Plants

Examples of C4 plants are sugarcane, maize, sorghum, pearl millet, Amarantus, panicum maximum, Chloris, Atriplex, Digitaria, and Cyperus. The C4 cycle has also been reported in some members of the families, Cyperaceae, and certain dicots belonging to Amaranthaceae, Chenopodiaceae, Compositae, Euphorbiaceae, Portulacaceae.

Structural Peculiarities of C4 Plants:

1. Presence of bundle sheath cells containing chloroplasts.

2. Bundle sheath cells are radially arranged around a vascular bundle.

3. Bundle sheath cells lack grana in their chloroplasts.

4. Bundle sheath cells are arranged in one or more layers consisting of large thick-walled cylindrical cells, around the vascular bundle, a characteristic feature of C4 plants.

5. Bundle sheath cells remain surrounded by one or more wreath-like layers of mesophyll cells. This anatomical arrangement is called as Kranz type (Kranz=wreath, a German term). (Kranz: Wreath of mesophyll cells around bundle sheath cells)

6. Mesophyll cells have well-developed grana. (Thus, the C4 pathway constitutes an example of dimorphism of chloroplasts) But, some C4 plants (bermudagrass)have grana in chloroplasts of bundle sheath cells).

7. The ratio of PS I: PS II activity is three times higher than the bundle sheath cells.

8. The mesophyll cells are almost three times more active in a non-cyclic electron transport system than that of bundle sheath cells.

9. For cyclic electron transport, both the cells are equally efficient.

10. Clear-cut categorization of enzymes is found in the C4 cycle.

Most of the PEP carboxylase occurs in mesophyll cells. While most ribulose-1,5-diphosphate carboxylase and malic enzymes are found in bundle sheath cells. The C4 cycle is also referred to as the dicarboxylic acid cycle or the β -Carboxylation pathway or Hatch-Slack cycle or Co-operative Photosynthesis (Karpilov, 1970).

In this Cycle, the characteristic point is the primary carboxylation reaction and the phosphoenolpyruvate (PEP) is found to be a CO2 acceptor molecule.

Site of Occurrence of C4 Pathway

The C4 pathway is known to operate in two types of cells :

i. Chloroplasts of mesophyll cells.

ii. Chloroplasts of bundle sheath cells.

The Hatch-Slack (C4) pathway is schematically represented in Fig.19.

i. Reactions in the chloroplasts of mesophyll cells

1. Here, CO2 is reduced by the carboxylation of PEP to form oxaloacetate (OAA). This reaction requires a molecule of water and releases a molecule of phosphoric acid as a by-product. The enzyme, phosphoenolpyruvate carboxylase (PEPCase) mediates this carboxylation reaction.

2. OAA is readily converted into malate or aspartate depending upon species. Malate is derived from OAA by reduction with NADPH2 in the presence of the enzyme, malic dehydrogenase.

Malate is then transported to the chloroplasts of bundle sheath cells.

ii. Reactions in the chloroplasts of bundle sheath cells

1. Here, Malate is decarboxylated by NADP specific malate enzyme to produce pyruvate and CO2. This CO2 is used in the C3 cycle for carboxylation of ribulose-1,5-diphosphate in the presence of enzyme, RuBP carboxylase (RuBP Case) and produces phosphoglycerate, the first stable product of Calvin Cycle( C3 ) of photosynthesis,

2. It follows Calvin Cycle in further steps to produce starch and to regenerate ribulose

1,5-diphosphate.

3. Simultaneously, Pyruvate is transported back to the chloroplast of mesophyll cells, where it is reconverted into the phosphoenolpyruvate by utilizing ATP generated during the light phase in the presence of enzyme pyruvate phosphate dikinase. The ATP is converted into AMP. Since the conversion results in the form of AMP, the requirement to regenerate ATP from AMP is 2 ATP. This is how 12 additional ATP molecules are needed in the C4 pathway.

Physiological significance of C4 plants

1. Presence of C4 pathway offers adaptation mechanism to xerophytic plants.

2. Plant can photosynthesize even with a very low concentration of CO2 (up to 10ppm) in the atmosphere.

3. Therefore, partial closure of stomata (due to xerophytic conditions) would not affect photosynthesis much.

Thus, the plants can adapt to grow at low water content, high temperature, and bright light intensities. Hence, this cycle is best suited to the crops grown in dry climates in both tropics and subtropics.

4. In the C4 pathway, photorespiration is absent; hence, the photosynthetic rate remains higher.

5. C4 plants are about twice as efficient as C3 plants in converting solar energy into carbohydrates or dry matter.

Categories of C4 plants

Chollet and Ogren (1975) recognized three types of C4 plants. They are :

a. In the first category, CO2 is initially fixed by phosphoenolpyruvate, and oxaloacetate is formed. `

The malate produced from it will be transported to bundle sheath cells. Example: Sugarcane and maize.

b. Second group includes plants such as Panicum maximum and Chloris gayana, in which case it is aspartate, rather than malate, transported to bundle sheath cells. There it is transmitted to oxaloacetate, which is converted into pyruvate and CO2 by PEP carboxykinase.

c. In the third case, the aspartate produced in mesophyll cells is transported to bundle sheath cells where it is transmitted and reduced to malate. The malate is then decarboxylated to form pyruvate and CO2. The example includes Atriplex spongiosa.