This document discusses the three main modes of carbon dioxide assimilation in plants: C3, C4, and CAM cycles. The C3 cycle fixes carbon through Calvin-Benson cycle in mesophyll cells throughout the leaf. Most plants use this cycle. The C4 cycle fixes carbon in mesophyll cells and transports it to bundle sheath cells, concentrating CO2 around Rubisco to promote photosynthesis. C4 plants include grasses. The CAM cycle fixes carbon at night and stores it as malic acid, releasing CO2 during the day to concentrate it around Rubisco. This allows CAM plants like cacti to keep their stomata closed in the day. C4 and CAM plants evolved to reduce photorespiration
2. Introduction
• Photosynthesis is a physiological process, in which
carbohydrate is synthesized by aerial green parts of
the plants by trapping solar energy, taking CO2 from
atmosphere & water from soil.
• Generally there are 3 modes of CO2 assimilation.
– C3 Cycle or Calvin Benson Cycle or Reductive
Pentose Phosphate Cycle
– C4 Cycle or Hatch & Slack Cycle or Dicarboxylic
Acid Cycle
– Crassulacean Acid Metabolism or CAM Cycle
3. C3 Cycle
• First product of photosynthesis is a 3-carbon
molecule.
• Most plants are C3,usually on dicot plant .
• C3 photosynthesis is a multistep process in
which the carbon from CO2 is fixed into
stable organic products, it occurs in virtually
all leaf mesophyll cells.
4. C3 Cycle(Contd…)
• Rubisco, the enzyme involved in
photosynthesis, is also the enzyme involved
in the uptake of CO2.
• Photosynthesis takes place through out the
leaf.
• Advantage of C3 :
-More efficient than C3 plants under cool and
moist conditions and under normal light
because fewer enzymes and no specialized
anatomy.
5.
6.
7. C4 Cycle
• First initial photosynthesis product is a 4-carbon
compound.
• Occurs in the more advanced plant taxa and is
especially common among monocots, such as
grasses and sedges
• Represents a biochemical and morphological
modification of C3 photosynthesis to reduce
Rubisco oxygenase activity and thereby increase
photosynthetic rate in low CO2 environments
• Photosynthesis takes place in inner cells.
8. C4 Cycle(CONTD…)
• 2 types of cells involved-
– Bundle Sheath Cell: Tightly packed around the veins
in the leaf
– Mesophyll Cell: Loosely arranged around the bundle-
sheath cells
• Uses PEP Carboxylase for the enzyme involved in
the uptake of CO2 into the plant very quickly,
and then delivers directly to Rubisco for
photosynthesis.
• The additional cost of C4 photosynthesis is the
ATP requirement associated with the
regeneration of PEP from pyruvate.
9.
10. CAM CYCLE(CONTD…)
• Refers to acid metabolism in Crassulaceae.
• These plants fix CO2 during the night, storing it as
the four-carbon acid malate.
• The CO2 is released during the day, where it is
concentrated around the enzyme RuBisCO,
increasing the efficiency of photosynthesis.
• The CAM pathway allows stomata to remain shut
during the day, reducing evapo transpiration.
• Unlike C4 pathway, CAM pathway happens in the
cells of mesophyll.
11.
12. SIGNIFICANCE OF C4 AND CAM PLANTS
C4 Plants
• C4 plants initially attach CO2 to PEP to
form the OAA using the enzyme PEP
carboxylase. This takes place in the
loosely packed cells called mesophyll
cells.
• OAA is then pumped to another set
of cells, the bundle sheath cells,
which surround the leaf vein. There,
it releases the CO 2 for use by
Rubisco.
• By concentrating CO 2 in the bundle
sheath cells, C4 plants promote the
efficient operation of the Calvin cycle
and minimize photorespiration.
• C4 plants include corn, sugar cane,
and many other tropical grasses.
CAM Plants
• CAM plants also initially attach
CO2 to PEP and form OAA.
However, instead of fixing carbon
during the day and pumping the
OAA to other cells, CAM plants fix
carbon at night and store the OAA
in large vacuoles within the cell.
• This allows them to have their
stomatas open in the cool of the
evening, avoiding water loss, and
to use the CO 2 for the Calvin-
Benson cycle during the day, when
it can be driven by the sun's energy.
• CAM plants are more common than
C4 plants and include cacti and a
wide variety of other succulent
plants.
