3 Important Synthetic Reaction of Photosynthesis (With Diagram)

1. The Calvin Cycle:

The elucidation of the sequence of chemical reactions that result in the incorporation of carbon dioxide into sugars and starches relied heavily on the use of radio­active isotopes.

Using ¹⁴C-labeled carbon dioxide, it was possible to add ¹⁴CO₂ at known times to an ac­tively photosynthesizing system, halt the process a short time later, and then identify the compounds into which the labeled carbon dioxide became incorpo­rated.

Identification of the ¹⁴C-containing intermedi­ates was carried out using combined paper -chroma­tography and autoradiography.

Ruben and co-workers first showed that the active form of CO₂ in the chloroplast was carbonic acid. M. Calvin and co-workers established the sequence of reactions that follow the formation of carbonic acid and its entry into the chloroplast. They added ¹⁴CO₂ to cultures of the alga Chlorella and allowed the cells to photosynthesize for given periods of time (usually be­tween 2 and 60 seconds).

The Chlorella cells were then killed and the soluble cell components were extracted and concentrated. The extracts containing radioactive carbon were chromatographed on paper, and the spots containing radioactivity were identified. In 1961, Calvin received the Nobel Prize for this most impor­tant series of experiments.

When photosynthesis in the presence of ¹⁴CO₂ was allowed to proceed for only 2 seconds, the major la­beled compound identified was 3-phosphoglyceric acid (PGA). After 7 seconds, sugar phosphates and diphos­phates were found in addition to PGA. A 60-second exposure to ¹⁴CO₂ produced labeled phosphoenol- pyruvic acid (PEP), carboxylic acids, and amino acids. Using many different time intervals, the entire se­quence of reactions was uncovered, and it was found that many of the steps were the reverse of those in the glycolytic pathway (Fig. 17-16).

In the chloroplast stroma, CO₂ in the form of carb­onic acid reacts with the sugar ribulose diphosphate (RuDP) to form an unstable six-carbon compound that immediately splits to form two molecules of 3- phosphoglyceric acid. The enzyme catalyzing this re­action is ribulose-1, 5-diphosphate carboxylase and the radioactive carbon of ¹⁴CO₂ is incorporated into the carboxyl group of PGA.

PGA is then reduced to 3-phosphoglyceraldehyde (PGAL) in two steps. First, each PGA is phosphoryla- ted by ATP and then reduced by NADPH. The ATP and NADPH were produced by photochemical reac­tions in the grana lamellae. Thus, for each molecule of C0₂ fixed and converted to PGAL, two ATP and two NADPH molecules are required. These reactions are catalyzed by a kinase and a dehydrogenase.

Some of the PGAL is isomerized by triose phosphate isomerase to form dihydroxyacetone phosphate (DHAP). The enzyme aldolase then condenses PGAL and DHAP to produce fructose-1, 6-diphosphate (FDP). Fructose diphosphatase splits off the phos­phate group at the first carbon atom, producing fructose-6-phosphate (F6P). F6P may then be con­verted to fructose, glucose, or starch.

F6P and PGAL are also used for the resynthesis of RuDP (Fig. 17-17). F6P and PGAL are converted to erythrose-4-phosphate (E4P) and xylulose-5- phosphate (X5P). An aldolase then catalyzes the con­densation of E4P and DHAP to form sedoheptulose- 1, 7-diphosphate (SDP), which is then converted to sedoheptulose-7-phosphate (S7P). S7P and PGAL also react to form ribose-5-pbosphate (R5P) and X5P. The R5P is then isomerized to form ribulose-5-phosphate (Ru5P). Ru5P is also formed from X5P. Finally, ATP is used to phosphorylate Ru5P, thereby forming RuDP.

For each C0₂ fixed, one RuDP, two ATP, and two NADPH are consumed and one phosphate sugar is produced. In actuality, all the RuDP is resynthesized. Fixation of every three molecules of C0₂ results in the formation of six PGAL. Five of these are recycled to replenish the pool of RuDP. The extra PGAL is the photosynthetic product and is used for sugar and starch synthesis. Figure 17-18 summarizes all of the steps and the requisite numbers of molecules partici­pating in the dark reactions; the pathway is known as the Calvin cycle.

Because additional ATP is consumed in the forma­tion of RuDP, a total of nine ATP and six NADPH are required for the fixation of three CO₂ molecules. Since six CO₂ molecules are required to produce one six- carbon sugar molecule, 18 ATP molecules are con­sumed in the fixation (or three ATP per CO₂).

It has been estimated that about 5 x 10¹⁶ grams of carbon are fixed annually by photosynthesis and this corresponds to storage of 20.1 x 10¹⁷ kJ or 4.8 x 10¹⁷ kcal of energy. Because about 28.0 x 10²¹ J or 6.7 x 10²¹ kcal of light energy fall on the earth each year, photo­synthesis traps a mere 0.0072%.

2. C₄ Photosynthesis (Hatch-Slack Pathway):

The C₃ or Calvin cycle is not the only metabolic path­way found in plants for fixing CO₂. M. D. Hatch and C. R. Slack showed that in some plants, CO₂ is also fixed into four-carbon compounds. This mechanism (which operates in conjunction with the Calvin cycle in the leaves of these plants) has since come to be known as the Hatch-Slack or C₄ pathway.

Plants that employ the Hatch-Slack pathway (e.g., corn, sugar cane, and certain grasses) are characteristically found in arid environments and possess an unusual leaf anatomy. In the leaves of plants employing only the Calvin cycle, the C₃ reactions occur in both the pali­sade mesophyll and spongy mesophyll layers. How­ever, in plants that can carry out the C₄ reactions, the Calvin cycle occurs only in the layer of bundle sheath cells that surrounds the veins of the leaf.

The C₄ reac­tions take place in the mesophyll layers that lie imme­diately under the upper and lower epidermis. In addi­tion to their unusual leaf anatomy, plants using the Hatch-Slack pathway are characterized by high pho­tosynthetic and growth rates, low photorespiration rates, and low rates of water loss through the stomata of the leaves.

As in the Calvin cycle, the sequence of reactions that constitutes the C₄ pathway (and the relationship of these reactions to the Calvin cycle) was worked out using C-labeled carbon dioxide. The reactions are summarized in Figure 17-19. Carbon dioxide passes from the air surrounding the leaves through the epi­dermis and into the mesophyll cells.

Here the CO₂ con­denses with phosphoenolpyruvate (PEP, a three- carbon compound) to form oxaloacetate (a four-carbon compound). The reaction is catalyzed by the enzyme phosphoenolpyruvate carboxylase. In some plant spe­cies, the oxaloacetate is then converted to malate, whereas in others it is converted to aspartate. The malate (or aspartate) then passes into the bundle sheath cells, where it is decarboxylated within the cells’ chloroplasts.

As seen in Figure 17-19, the CO₂ that is released contains the same carbon atom that entered the mesophyll cells. This carbon dioxide then enters the Calvin cycle reactions of the bundle sheath cells’ chloroplasts, as described earlier. The three- carbon compounds formed after decarboxylating ma­late or aspartate are transported back into the mesophyll where they are reconverted to PEP.

The sugars that accumulate from the Calvin cycle reac­tions are temporarily converted to starch under active photosynthetic conditions. At night or during dark­ness or dim light, the starches are converted back to sugars and transported out of the leaf by the vascular tissue of the veins (vascular bundles).

The presence of the C₄ pathway may seem like a needless addition to the Calvin cycle system. However, there is evidence indicating that the mesophyll is able to build up a high concentration of fixed CO₂ by this method, which could provide evolutionary advantages. In addition, the rapid and efficient fixing and storing of CO₂ as four-carbon acids decreases the leaf’s need to have a large number of stomata (openings in the leaf epidermis that allow CO₂ and other gases to dif­fuse into the leaf). Open stomata, though allowing the passage of CO₂ into the plant, also allow water to es­cape from the plant—a disadvantage to plants in arid climates!

3. Crassulacean Acid Metabolism:

Crassulacean acid metabolism (CAM) is a special form of metabolism associated with photosynthesis that is carried out by members of the plant family Crassulaceae (succulent herbs such as Sedum). Plants using this form of metabolism have closed stomata during the daylight hours and therefore cannot absorb suffi­cient CO₂ for photosynthesis. But during the night (dark) hours, the stomata open and the leaf cells can fix CO₂ in the dark by combining it with PEP to form oxaloacetate.

The oxaloacetate is converted into ma­late for storage. During the following daylight hours, the malate is decarboxylated and the CO₂ is utilized in Calvin cycle reactions. The three-carbon pyruvate re­maining after decarboxylation is converted first into PEP and then into phosphoglyceric acid and is also utilized in the Calvin cycle. The PEP required for the dark fixation of CO₂ is derived from some of the starch produced from the Calvin cycle products (Fig. 17-20).