PinkMonkey Online Study Guide-Biology
4.5 Diversity in Photosynthetic Pathway
Various experiments and investigations regarding
the fixation of CO2 and the path of carbon during the
dark reaction have indicated that there are a number of different
pathways for CO2 fixation in green plants. These are
1. C3 Pathway (Calvin cycle)
2. C4 Pathway (H-S pathway or Hatch-Slack cycle)
3. CAM (Crassulacean acid metabolism)
1. The C3 pathway or Calvin cycle
is the main pathway and is present in most green plants. Plants
which follow the Calvin cycle are called C3 plants.
2. C4 Pathway (or Hatch-Slack pathway)
: In some plants, the first stable product, after the fixation
of CO2, is a 4-C dicarboxylic acid called oxaloacetic
acid (OAA). Such plants are called C4 plants and the
path of carbon (dark reaction) is the C4 pathway. It
was first noticed by Kortschak (1964) in the photosynthesis of sugarcane
leaves. However, details of the C4 pathway (i.e. the
first CO2 acceptor, the first stable product, the complete
carbon pathway, etc.) were worked out by Hatch and Slack (1966).
Therefore, it is known as Hatch-Slack Pathway. C4 pathway
is observed in many plants of family Gramineae (e.g. sugarcane,
maize, some other monocot and some dicot plants).
Anatomical peculiarities of C4 plants
: Most of the C4 plants have a characteristic leaf
anatomy and dimorphic chloroplasts. For example,
(1) The leaf mesophyll consists of more or less
compactly arranged cells.
(2) It is not differentiated into palisade and
spongy mesophyll as it is in C3 plants.
(3) The vascular bundles (veins) in the leaf are
surrounded by a distinct bundle sheath of radial enlarged parenchyma
(4) The chloroplasts in leaf cells are dimorphic(
i.e., of two types):
(a) Chloroplasts in the mesophyll cells are smaller and possess
(b) Chloroplasts in the bundle sheath cells are larger and without
This type of leaf anatomy in C4 plants
is described as Kranz anatomy.
Important steps in Hatch and Slack Pathway
The various steps in C4 pathway are
completed in two parts and in two different regions in the leaves.
(a) First part reactions are completed in the stroma of the chloroplasts
in mesophyll cells, and (b) Second part reactions are completed
in the stroma of chloroplasts in bundle sheath cells.
(a) Part - I (in mesophyll cells)
(i) First CO2 Fixation: In
the pathway, the first CO2 acceptor is the 3-C phosphoenol
pyruvate acid (PEP). CO2 first combines with 3-C PEP
to form 4-C OAA (oxaloacetic acid). As OAA is a dicarboxylic acid,
this is also known as the dicarboxylic acid pathway.
(ii) 4-C OAA may be converted into 4-C malic
acid or 4-C aspartic acid and transported to bundle sheath cells.
(b) Part - II (in bundle sheath cells)
(i) In the chloroplasts of the bundle sheath
cells, 4-C malic acid undergoes decarboxylation to form CO2
and 3-C pyruvic acid.
(ii) Second CO2 fixation :
The CO2 released in decarboxylation of malic acid combines
with 5-C RUDP (ribulose diphosphate) to form 2 molecules of 3-C
PGA as in the Calvin cycle. Further conversion of PGA to sugars
is the same as in the Calvin cycle.
(iii) The pyruvic acid produced in decarboxylation
of malic acid is transported back to the mesophyll cells. Here it
is converted into PEPC and again made available for the C4
These steps in the Hatch-Slack pathway in the mesophyll
and the bundle sheath cells are schematically shown in the figure
Thus, in the C4 plants, the initial
few steps are different (typical of C4 pathway). However,
later on the reactions are similar to the Calvin cycle (i.e., C3
pathway). Hence, C4 plants have both C4 and
Moreover, in C4 plants, CO2
fixation (i.e. carboxylation) occurs twice: first in the mesophyll
cells (PEP + CO2) and then again in the bundle sheath
cells (RUDP + CO2). For this reason, the C4
pathway is also called a dicarboxylation pathway.