24. C4 Cycle
As Blackman illustrated the existence of a dark reaction.
This reaction was referred to as Blackman’s reaction. There
are two types of cyclic reactions occurring in the dark
reaction –
Calvin cycle or C3 cycle
Hatch and Slack pathway or C4 cycle
25. The 4-carbon oxaloacetic acid is the first stable
compound of the Hatch and Slack cycle, hence is
called the C4 cycle.
This pathway is a common sight in several grasses,
maize, sugarcane, amaranthus, sorghum. The C4
plants depict a different kind of leaf anatomy (Kranz
anatomy).
26. The chloroplasts are dimorphic and in the leaves, vascular bundles are
wrapped by a bundle sheath of larger parenchymatous cells. Such
bundle sheath cells possess chloroplasts, which are larger, containing
starch grains, lacking grana while the chloroplasts in the mesophyll cells
always possess grana and are smaller. The bundle sheath cells appear as
a wreath or a ring while cells are larger. This characteristic leaf anatomy
of the C4 plants is referred to as Kranz Anatomy.
Kranz Anatomy
27. The C4 Cycle depicts two carboxylation reactions occurring in the
chloroplasts of the mesophyll cells and others in the chloroplast of
the bundle sheath cells. The Hatch and Slack Cycle involves four
steps –
Carboxylation
Breakdown
Splitting
Phosphorylation
28. Carboxylation
Occurs in the chloroplasts of the mesophyll cells. A 3-carbon
compound, Phosphoenolpyruvate, collects carbon dioxide
and in the presence of water, transforms to 4 carbon
oxaloacetate. The enzyme phosphoenolpyruvate carboxylase
catalyzes the reaction.
29. Breakdown
Readily, oxaloacetate disintegrates into 4 carbon malate
and aspartate. The enzyme involved in the reaction is
transaminase and malate dehydrogenase. The compounds
formed diffuse into the sheath cells from the mesophyll
cells.
31. Phosphorylation
The pyruvate molecules are moved to the chloroplasts of the
mesophyll cells wherein, in the presence of ATP, it is phosphorylated
for the regeneration of phosphoenolpyruvate. Pyruvate
phosphokinase catalyzes the reaction and phosphoenolpyruvate is
regenerated.
34. CAM Photosynthesis
CAM pathway is adapted in plants to perform photosynthesis
under stress. The CAM pathway reduces photorespiration.
In CAM plants stomata are open at night and they absorb carbon
dioxide at night to reduce water loss during the daytime.
35. The process has the following steps:
1.The first step in carbon dioxide fixation is the combination of
CO2 with PEP (phosphoenolpyruvate) to form 4 carbon
oxaloacetate (same as C4 plants) in the chloroplast of mesophyll
cells. The reaction is catalysed by PEPcarboxylase. This occurs at
night.
2.Oxaloacetate is converted to malate and other C4 acids. Malate is
stored in vacuoles at night.
3.During the day time, stomata remain closed, so there is no gas
exchange. Malate is transported out of the vacuole and CO2 is
released by the process of decarboxylation.
4.This CO2 finally enters the Calvin cycle and carbon dioxide
fixation completes. The CO2 which gets accumulated around
RuBisCO increases the efficiency of the photosynthesis process
and minimizes photorespiration.
36. CAM Plants Examples
CAM plants are mostly xerophytic. CAM pathway is also present in some
aquatic plants such as Hydrilla, Vallisneria, etc. In aquatic plants, the CAM
pathway occurs due to scarcity of CO2. Carbon dioxide supply is limited due to
slower diffusion in water. Aquatic CAM plants absorb CO2 at night when there
is less competition from other photosynthetic plants.
Some of the common examples of CAM plants
Orchids, Cacti, Aloe, Pineapple, Agave, Moringa, Some species
of Euphorbia and Bromelioideae, etc.
