Hatch & Slack Pathway or C4 Cycle or Dicarboxylic Acid Pathway or Dicarboxylation Pathway

Hatch & Slack Pathway or C4 Cycle or Dicarboxylic Acid Pathway or Dicarboxylation Pathway

Till 1965, Calvin cycle is the only pathway for CO₂ fixation. But in 1965, Kortschak, Hart and Burr made observations in sugarcane and found C₄ or dicarboxylic acid pathway. Malate and aspartate are the major labelled products. This observation was confirmed by Hatch & Slack in 1967. This alternate pathway for the fixation of CO₂ was found in several tropical and sub-tropical grasses and some dicots. C₄ cycle is discovered in more than 1000 species. Among them 300 species belong to dicots and rest of them are monocots. C₄ plants represent about 5% of

Earth’s plant biomass and 1% of its known plant species. Despite this scarcity, they account for about 30% of terrestrial carbon fixation. Increasing the proportion of C₄ plants on earth could assist biosequestration of CO₂ and represent an important climate change avoidance strategy.

C₄ pathway is completed in two phases, first phase takes place in stroma of mesophyll cells, where the CO₂ acceptor molecule is 3-Carbon compound, phospho enol pyruvate (PEP) to form 4-carbon Oxalo acetic acid (OAA). The first product is a 4-carbon and so it is named as C₄ cycle. oxalo acetic acid is a dicarboxylic acid and hence this cycle is also known as dicarboxylic acid pathway (Figure 13.20). Carbon dioxide fixation takes place in two places one in mesophyll and another in bundle sheath cell (di carboxylation pathway). It is the adaptation of tropical and sub tropical plants growing in warm and dry conditions. Fixation of CO₂ with minimal loss is due to absence of photorespiration. C₄ plants require 5 ATP and 2 NADPH + H⁺ to fix one molecule of CO₂.

1. Stage: I Mesophyll Cells

Oxaloacetic acid (OAA) is converted into malic acid or aspartic acid and is transported to the bundle sheath cells through plasmodesmata.

2. Stage: II Bundle Sheath Cells

Malic acid undergoes decarboxylation and produces a 3 carbon compound Pyruvic acid and CO₂. The released CO₂ combines with RUBP and follows the calvin cycle and finally sugar is released to the phloem. Pyruvic acid is transported to the mesophyll cells.

Activity

•               Collect the leaves of Paddy (C₃) and Sugar cane (C₄).

•               Take the cross section.

•               Observe the sections under the microscope.

•               See the difference in their anatomy (Dimorphic chloroplast and Kranz anatomy).

Kranz Anatomy: It is  the  German  term meaning a halo or wreath. In C₄ plants vascular bundles are surrounded by a layer of bundle sheath. Bundle sheath is surrounded by a ring of mesophyll cells.

The characteristic feature of C₄ plants is the presence of dimorphic chloroplast: 

Bundle sheath chloroplast: Larger chloroplast, thylakoids not arranged in granum and rich in starch. 

Mesophyll Chloroplast: Smaller chloroplast, thylakoids arranged in granum and less starch.

Table 13.4: Differences between C3 and C4 plants

3. Significance of C₄ cycle

1.           Plants having C₄ cycle are mainly of tropical and sub-tropical regions and are able to survive in environment with low CO₂ concentration.

2.           C₄ plants are partially adapted to drought conditions.

3.           Oxygen has no inhibitory effect on C₄ cycle since PEP carboxylase is insensitive to O₂.

4.     Due to absence of photorespiration, CO₂ Compensation Point for C₄ is lower than that of C₃ plants.

Differences between C₃ Plants (C₃ Cycle) and C₄ Plants (C₄ Cycle) are given in table 13.4.