The document summarizes the Hatch-Slack or C4 pathway, an alternative to the Calvin cycle that fixes carbon and minimizes photorespiration. The pathway was discovered in 1966 and involves two cell types - mesophyll and bundle sheath cells. Carbon dioxide is initially fixed by the mesophyll cells into a four-carbon acid like malate or aspartate, which is then transported to the bundle sheath cells. There, the four-carbon acid is decarboxylated, releasing carbon dioxide to enter the Calvin cycle. The three-carbon acid byproduct is returned to the mesophyll cells, completing the five-stage cycle. The document further describes the three main types of C4 cycles that differ in the

1. St. Xavier’s College, Mahuadanr (Plant Physiology)
Dr. Emasushan Minj (Assistant Professor) 1
Hatch-Slack Pathway (C4 Cycle)
 It is alternate type of carbon fixation have evolved that minimize
photorespiration and optimize the Calvin cycle.
 It is also known as C4 Cycle.
 This cycle was discovered by Hatch and Slack, in Australia (1966).
 Initially CO2 is fixed by four carbon acid.
 These plants are adapted to arid climate condition and able to give higher
photosynthetic output even at high temperature and high light intensity.
 It is present in both monocots and dicots.
 They are particularly common in Gramineae (corn, sugarcane),
Chenopodiaceae (Atriplex) and Cyperaceae families.
 In C4 plants there are two distinct types of photosynthetic cells:
a. Bundle sheath cells
b. Mesophyll cells
The basic C4 cycle completes in five stages:
1. Fixation of CO2 by the carboxylation of phosphoenolpyruvate in the
mesophyll cell to form a C4 acid (Oxaloacetatic acid).
2. Oxaloacetate then converts into other C4 acids such as malate or aspartate.
3. Transport of the C4 acids to the bundle sheath cells.
4. Decarboxylation of the C4 acids within the bundle sheath cells and
generation of CO2, which enter in the C3 Cycle.
5. Transport of the C3 acid that is formed by the decarboxylation step back to
the mesophyll cell.

2. St. Xavier’s College, Mahuadanr (Plant Physiology)
Dr. Emasushan Minj (Assistant Professor) 2
The C4 cycle transports different C4 acids from mesophyll to bundle sheath
cells, employs different enzymes to decarboxylate the C4 acids in the bundle
sheath cells and transports diferrent C3 acids from bundle sheath cells to
mesophyll cells.
Based on these differences, there are three different types of C4 cycles-
i. NADP+ -malic enzyme type (NADP-ME)
ii. NAD+-malic enzyme type (NAD-ME)
iii. PEP Carboxykinase type (PEPCK)
NADP+ -malic enzyme type decarboxylating enzyme transport C4 acid (Malate)
from mesophyll to bundle of sheath cells and C3 acid (pyruvate) transported from
bundle sheath cell to mesophyll.

3. St. Xavier’s College, Mahuadanr (Plant Physiology)
Dr. Emasushan Minj (Assistant Professor) 3
NAD+-malic enzyme type decarboxylating enzyme transport C4 acid
(Aspartate) from mesophyll to bundle of sheath cells and C3 acid (Alanine)
transported from bundle sheath cell to mesophyll.
PEP Carboxykinase type decarboxylating enzyme transport C4 acid
(Aspartate/Malate) from mesophyll to bundle of sheath cells and C3 acid
(Phosphoenolpyruvate) transported from bundle sheath cell to mesophyll.