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Photorespiration & Variations in
Photosynthesis
Lecture 15
Spring 2014
Variation in Photosynthesis
Three types of photosynthesis
• C3
• C4
• CAM
Why do plants need three different types of
photosynthesis?
1
Water Balance in Plants
Transpiration & role of
stomata
2
Balance between photosynthesis and water
loss
C3
• 3-carbon sugar (3-phosphoglycerate) in
Calvin cycle
• Day/Night cycle
• In hot temperatures, must close stomata
3
Rubisco
• ribulose
bisphosphate
(RuBP)
carboxylase
• Rubisco has
affinity for both
CO2 and O2
Fig. 10.19
4
Photorespiration
• A metabolic pathway that consumes O2 and ATP,
releases CO2 and reduces photosynthetic output
5
Why Photorespiration?
• Rubisco evolved in bacteria before O2 in
atmosphere
– Enzyme had affinity for both
• Modification of active site to reduce access to
oxygen may also reduce the access to CO2
• C3 plants compensate by increasing the
concentration of rubisco.
– Half of the protein in the chloroplast is rubisco
• PR may be protective in terms of preventing
damage from excessive light
6
7
Alternative Pathways: C4
C4
• First carbon fixation produces a 4-carbon
sugar (oxaloacetate)
• Can close stomata during day
• hot/dry environments
– E.g., many grasses, sugar cane, corn
Alternative Pathways: C4
• Two types of cells
– Mesophyll cells
• Carbon fixation
– Bundle-sheath cells
• Calvin cycle in chloroplasts
See Fig. 10.19
• Keeps high CO2 levels in bundle-sheath cells
8
9
Alternative Pathways: C4
In mesophyll cells
• CO2 fixed to PEP
(phosphoenolpyruvate)
• PEP carboxylase
– No affinity for O2
Fig. 10.19
10
Cyclic Electron Flow
• Linear electron flow produces 6 ATP & 6 NADPH
– Calvin cycle requires 9 ATP
• Uses cyclic electron flow to generate extra ATP
• Occurs in bundle-sheath cells
Fig. 10.15
11
Alternative Pathways: CAM
CAM (Crassulacean acid metabolism)
• Stomata open during night
• CO2 stored as 4-carbon compound to be
used the next day
• Stomata can be closed during the day
• Plants found in hot/dry environments
– Cactus, pineapple, many orchids
– (some are members of plant family
Crassulaceae)
12
Alternative Pathways: CAM
• Temporal separation of
carbon fixation and Calvin
cycle
• Takes in CO2 at night
• Uses C4 pathway to fix
CO2 into organic acids
– malate
• Stored in vacuole
– Malic acid
– Active transport
13
Alternative Pathways: CAM
• Malic acid leaves vacuole
• Broken down to form
pyruvate and CO2
• CO2 to Calvin cycle
• Pyruvate converted to
starch and stored
14
Alternative Pathways: CAM
Fig. 10.20
15
Trade Offs
Type of CO2
fixation
Energy to fix
one CO2
C3
3 ATP
Water
Transpired per
CO2 fixed
400 – 500 g
C4
5 ATP
250-300 g
CAM
6.5 ATP
50-100 g
16
Importance of Photosynthesis
• Provides the carbon
compounds for most
organisms on the planet
• Changed Earth’s
atmosphere to the current
one
• Important part of the CO2
cycle – Moderates
temperature on the planet