The Hatch-Slack pathway is an alternative photosynthetic pathway to the Calvin cycle. It was discovered in C4 plants like sugarcane whose leaves produce 4-carbon dicarboxylic acids like malate and aspartate. The pathway involves two carboxylation reactions, the first in the mesophyll cell chloroplasts and the second in the bundle sheath cell chloroplasts. Malic acid is shuttled between the cell types, being decarboxylated in the bundle sheath cells to produce CO2 for the Calvin cycle. This concentrates CO2 around RuBisCO to increase photosynthetic efficiency.

1. C4 (Dicarboxylic Acid)
Pathway
Hatch- Slack Pathway

2. Introduction
• For considerable period of time, the Calvin cycle was thought to be
the only photosynthetic reaction sequence operating in higher plants
and algae.
• But in 1965, Kortschak, Hartt and Burr reported that 4-C containing
dicarboxylic acids, malate and aspartate were the major labelled
products when sugar cane leaves were allowed to photosynthesize.

3. • Hence, further studies were carried out by different scientists and led to the
establishment of yet another CO2 reduction pathway which is called Hatch-
Slack Pathway.
• As C-4 dicarboxylic acids are the earliest products, it is also called C4
dicarboxylic acid pathway.
• Besides sugarcane leaves, this pathway has been found to operate in many
plant species of the family Gramineae eg; maize, sorghum which are
grouped together as tropical grasses.
• They are all known as C4 plants.

4. Anatomical features of leaves of C4 plants
• In the leaves of these plants, the vascular bundles are surrounded by
bundle sheath of larger parenchymatous cells which in turn are surrounded
by smaller mesophyll cells.
• Moreover, the chloroplast in cells of bundle sheath are larger and usually
lack grana (agranal)
• Whereas, the chloroplast in mesophyll cells are smaller and always contain
grana.

5. Kranz Anatomy
• The anatomy of C4 plants is also called Kranz anatomy.
• The word Kranz means “wreath” or “ring”.
• Here, the mesophyll cells are clustered around the bundle-sheath cells in a
ring-like fashion.
• The number of chloroplasts in the bundle-sheath cells is more than that in
the mesophyll cells.
• This is found in C4 grasses such as maize and a few dicots.

6. Kranz Anatomy

7. Phospho Pyruvate
Dikinase
Malic
dehydogenase

8. Steps in C4 pathway
• This pathway involves two carboxylation reactions, one taking in the
chloroplasts of mesophyll cells and another in chloroplasts of bundle sheath
cells.
Step: 1
• The first step involves carboxylation of phosphoenol pyruvic acid (PEP) in
chloroplast of mesophyll cells to form C-4 dicarboxylic acid, Oxaloacetic acid
(OAA). This reaction is catalysed by phosphoenol pyruvate carboxylase.

9. Step: 2
• Oxalic acetic acid equilibriates with another C4 dicarboxylic acids, aspartic
acid and malic acid in the presence of enzymes transaminase and NADP+
specific malate dehydrogenase respectively.
Step: 3
• From chloroplasts of mesophyll cells the malic acid is transferred to the
chloroplasts of bundle sheath cells where it is decarboxylated to form CO2
and pyruvic acid in the presence of NADP+ specific malic enzyme.

10. Step: 4
• Now, the second carboxylation occurs in chloroplast of bundle sheath cells.
Ribulose-1,5-biphosphate (RuBP) accepts CO2 produced in step 3 in the
presence of RuBisCo and ultimately yield 3-Phosphoglyceric acid and is
utilized in the formation of hexose monophosphates.
Step: 5
• The pyruvic acid produced in step 3 is transferred to chloroplasts of
mesophyll cells where it is phosphorylated to regenerate phosphoenol
pyruvic acid.