Transcript C 4 Photosynthesis

Other Types of Photosynthesis
C4 Photosynthesis and CAM Photosynthesis
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Calvin Cycle Reactions:
Carbon Dioxide Fixation
• CO2 is attached to 5-carbon RuBP molecule
– Result in a 6-carbon molecule
– This splits into two 3-carbon molecules
(3PG)
– Reaction accelerated by RuBP
Carboxylase (Rubisco)
• CO2 now “fixed” because it is part of a
carbohydrate
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Calvin Cycle Reactions:
Carbon Dioxide Reduction
• 3PG reduced to BPG
• BPG then reduced to G3P
• Utilizes NADPH and some
ATP produced in light
reactions
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Calvin Cycle Reactions:
Regeneration of RuBP
• RuBP used in CO2 fixation must be
replaced
• Every three turns of Calvin Cycle,
– Five G3P (a 3-carbon molecule)
used To remake three RuBP (a 5carbon molecule)
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The Calvin Cycle:
Fixation of CO2
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Importance of Calvin Cycle
• G3P (glyceraldehyde-3-phosphate) can be
converted to many other molecules
• The hydrocarbon skeleton of G3P can form
– Fatty acids and glycerol to make plant oils
– Glucose phosphate (simple sugar)
– Fructose (which with glucose = sucrose)
– Starch and cellulose
– Amino acids
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Other Types of Photosynthesis
C4 Photosynthesis and CAM Photosynthesis
Most plants are C3 plants
•In C3 plants, the Calvin cycle
fixes CO2 directly; the first
molecule following CO2 fixation
is 3PG.
•In hot weather, stomata close
to save water; CO2
concentration decreases in
leaves; O2 increases.
•O2 combines with RuBP
instead of CO2
•This is called photorespiration
since oxygen is taken up and
CO2 is produced; this produces
less 3PG.
C4 Photosynthesis
•In a C3 plant, mesophyll cells contain well-formed chloroplasts,
arranged in parallel layers.
•In C4 plants, bundle sheath cells as well as the mesophyll cells contain
chloroplasts.
•In C4 leaf, mesophyll cells are arranged concentrically around the
bundle sheath cells.
C4 Photosynthesis
•Remember C3 plants use RuBP carboxylase to fix CO2 to
RuBP in mesophyll; the first detected molecule is 3PG.
•C4 plants use the enzyme PEP carboxylase (PEPCase) to fix
CO2 to PEP (phosphoenolpyruvate, a C3 molecule); the end
product is oxaloacetate (a C4 molecule).
•In C4 plants, CO2 is taken up in mesophyll cells and malate, a
reduced form of oxaloacetate, is pumped into the
bundle-sheath cells; here CO2 enters Calvin cycle.
•In hot, dry climates, net photosynthetic rate of C4 plants
(e.g., corn) is 2–3 times that of C3 plants.
•Photorespiration does not occur in C4 leaves because PEP
does not combine with O2; even when stomata are closed,
CO2 is delivered to the Calvin cycle in bundle sheath cells.
•C4 plants have advantage over C3 plants in hot and dry
weather because photorespiration does not occur; e.g.,
bluegrass (C3) dominates lawns in early summer, whereas
crabgrass (C4) takes over in the hot midsummer.
CAM Photosynthesis
•CAM (crassulacean-acid metabolism) plants form a
C4 molecule at night when stomata can open without
loss of water; found in many succulent desert plants
including the family Crassulaceae.
•At night, CAM plants use PEPCase to fix CO2 by
forming C4 molecule stored in large vacuoles in
mesophyll.
•C4 formed at night is broken down to CO2 during the
day and enters the Calvin cycle which now has NADPH
and ATP available to it from the light-dependent
reactions.
•CAM plants open stomata only at night, allowing CO2
to enter photosynthesizing tissues; during the day,
stomata are closed to conserve water but now CO2
cannot enter photosynthesizing tissues.
•Photosynthesis in a CAM plant is minimal, due to
limited amount of CO2 fixed at night; but this does
allow CAM plants to live under stressful conditions.