Hatch and Slack Cycle
Or
C4 Pathway (Dicarboxylic Acid Pathway)
In several plants like sugarcane, maize, Euphorbia, Amaranthus, Sorghum, Portulaca, and Chenopodium first stable products for initial CO2 fixation, in dark reactions are oxaloacetic acid, aspartic acid and malic acid. They are also known as C4 plants because all are four carbon compounds. During dark phase CO2 combines with phosphoenol pyruvic acid (PEP) to form oxaloacetic acid in mesophyll chloroplasts. It is further reduced to malic acid or animated as aspartic acid. Malic acid is transferred to chloroplasts in bundle sheath, where liberation CO2 comes in contact with RuBP to undergo Calvin cycle. Pyruvic acid which is formed from malic acid again enters into mesophyll chloroplasts to form PRP to complete the Hatch-Slack Cycle. It is mainly exhibited in tropical and subtropical grasses and some dicotyledons.
PEP + CO2 + H2O ———> Oxalo-acetic acid + H3PO4
[in the presence of PEP carboxylase]
Oxaloacetiuc acid is reduced to malic acid or aminated to form aspartic acid.
Oxaloacetic acid + NADPH ———> Malic acid + NADP+
[dehydrogenase]
Oxaloacetic acid + NH3 + NADPH ———> Aspartic Acid NADP+ + H2O
Now the malic acid or aspartic acid is transported to bundle sheath. Here they are decarboxylated (or deaminated in the case of aspartic acid) to form pyruvate and CO2.
Malic acid + NADP+ ———> Pyruvate + Co2 + NADPH
CO2 is again fixed inside the bundle sheath cells through Calvin cycle. RuBP of Calvin cycle is called secondary acceptor of CO2 in C4 plants. Pyruvate is sent back to mesophyll cells. Here, it is changed to PEP. Energy for this is provided by ATP which is converted into AMP.
Pyruvate + ATP ———> PEP + AMP + H3PO4
Energy requirement: Formation of AMP to ATP requires double the energy than energization of ADP to ATP. Therefore, actual requirement of energy is equal to two molecules of ATP. This energy is in addition to 3 ATP required for fixation of one molecule of CO2 through Calvin cycle. Therefore, C4 plants consume 5 ATP molecules per molecule of CO2 fixed instead of 3 ATP molecules for C3 plants. For the formation of a glucose molecule, C4 plants require 30 ATP while C3 plants utilize only 18 ATP. Thus in C4 pathway additional requirement of 12 ATP molecules can be explained.
6 PEP + 6 RuBP + 6 CO2 + 30 ATP + 12 NADPH ———> 6 PEP + 6 RuBP + C6H12O6 + 30 ADP + 30 H3PO4 + 12NADP+
Significance
(i) C4 plants are more efficient in picking up CO2 even when it is found in low concentration because of the high affinity of PEP.
(ii) Concentric arrangement of mesophyll cells produce a smaller area in relation to volume for better utilization of available water. In this way, they are able to reduce the intensity of solar radiations.
(iii) They consume more energy. Their requirement is of 2 more ATP molecules per molecule of CO2 fixed. However in tropics, where these plants grow, plenty of energy is available.
(iv) They can tolerate excess of saline conditions.
Another interesting feature is that the plants undergoing Hatch-Slack pathway show Kranz anatomy. In a leaf, vascular bundle is surrounded by bundle sheath and mesophyll cells. Both are filled with chloroplasts. Chloroplasts in the cells of bundle sheath lack grana while mesophyll chloroplasts are normal as in C3 plants. Bundle sheath cells contain Calvin cycle enzymes. Due to high concentration of CO2 in bundle sheath cells, RuBP carboxylase works only for Calvin cycle and not for photorespiration. Bundle sheath cells send the sugar they male into phloem, which transports in throughout the plant.