Variations in Photosynthesis
Lecture 9
Fall 2008
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
– Loss of water
vapor from the
leaf through
diffusion and
evaporation
• Plants need to
balance loss of
water to
atmosphere with
uptake of water
from soil
2
Water Loss from Leaves
Occurs through stomata
(Path)
1. Water vapor in leaf
spaces diffuses out of
stomata
• Leaf air spaces are
moist, air is dryer
• Driving force:
Concentration gradient
of water vapor inside vs.
outside
2. Liquid water in leaf
evaporates to replace
water vapor that
diffused out
Blue dots = water vapor
3
The role of stomata
Gas Exchange
• CO2 enters, O2 leaves via open
stomata
Water Balance
• Majority of water loss through
stomata
• Stomata typically open during the
day and closed at night
• Plant can control whether stomata
are open or closed
– Can close stomata during the
day to prevent water loss
4
Balance between photosynthesis and
water loss
• Stomata need to be open for gas exchange
– No incoming CO2, then low input for Calvin
cycle
• Stomata need to be closed to prevent excessive
water loss
– If water in the soil is limited, and
– The day is hot or dry
5
Balance between photosynthesis and water
loss
C3
• Uses a 3-carbon sugar as substrate in
Calvin cycle (3-phosphoglycerate)
• Stomata open during day, closed at night
• Not typically found in dry (arid)
environments
• In hot temperatures, must close stomata
– Photosynthesis reduced, so productivity
reduced
6
7
Photorespiration
• A metabolic pathway that consumes O2
and ATP, releases CO2 and reduces
photosynthetic output
– ~50% in some crop plants
– Produces no ATP
– Produces no sugar
– Releases CO2 rather than fixing it
Photorespiration
8
• Rubisco (RuBP carboxylase) adds CO2 to
ribulose bisphosphate (RuBP)
• Rubisco can also bind O2
• O2 and CO2 compete for binding at active site
• As concentrations of CO2 decline, O2 gets added
to Calvin cycle instead
Fig. 10.18
Photorespiration
• Results of ribulose
bisphosphate & O2
– 3-phosphoglycerate (3
carbon)
– Phosphoglycolate (2
carbon)
• Phosphoglycolate not
used in Calvin cycle
• Carbon in
phosphoglycolate must
be salvaged
– Needed for regeneration
of RuBP
Fig. 10.18
9
10
Photorespiration
• Salvage pathway involves chloroplasts,
mitochondria and peroxisomes
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
• 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
11
12
Alternative Pathways: C4
C4
• First carbon fixation produces a 4-carbon
sugar (oxaloacetate)
• Can do photosynthesis with less CO2
coming into leaf
• Can close stomata during day, without
losing much productivity
• Plants found in hot/dry environments
– E.g., many grasses, sugar cane, corn
Alternative Pathways: C4
• Two types of cells
– Mesophyll cells
• Carbon fixation
(C4 pathway)
– Bundle-sheath cells
• Calvin cycle in chloroplasts
See Fig. 10.19
• Spatial separation
• Keeps high CO2 levels in bundle-sheath cells
cycle
13
14
Alternative Pathways: C4
In mesophyll cells
• C4 Pathway
• CO2 fixed to PEP
(phosphoenolpyruvate)
– Requires PEP carboxylase
– Forms oxaloacetate (4 carbon)
• PEP carboxylase
– No affinity for O2
– Higher affinity for CO2 than
rubisco has
– Able to add CO2 to PEP even
in low CO2 concentrations
Fig. 10.19
15
Alternative Pathways: C4
• Oxaloacetate converted to
malate or aspartate
• Exported from mesophyll cell
to bundle sheath cell
– Plasmodesmata
In bundle-sheath cells
• Malate broken down into
pyruvate and CO2
• CO2 to Calvin cycle
• Pyruvate to mesophyll cells
– Regenerates to PEP
– Requires ATP
Fig. 10.19
17
Cyclic Electron Flow
• Uses cyclic electron flow to generate extra ATP
• Occurs in bundle-sheath cells
– Thylakoids only have PS1 (and cytochrome
complex)
Fig. 10.15
18
Alternative Pathways: C4
• PS2 primarily in well developed
granum
• PS1 and ATP synthase
primarily in unstacked areas of
membrane
• Cytochrome complexes
distributed evenly
Fig. 10.17
19
Alternative Pathways: C4
20
Cyclic Electron Flow
• Linear electron flow produces 6 ATP & 6 NADPH
– Calvin cycle requires 9 ATP
• Cyclic electron flow produces extra ATP
– No water split
– No O2 produced
– No NADPH
Fig. 10.15
16
Alternative Pathways: C4
• Besides the 4-carbon
product, what else
must travel between
the mesophyll cells
and the bundle
sheath cells?
21
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)
22
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
23
Alternative Pathways: CAM
• Malic acid leaves vacuole
• Broken down to form
pyruvate and CO2
• CO2 to Calvin cycle
• Pyruvate converted to
starch and stored
24
Alternative Pathways: CAM
Fig. 10.20
24
Trade Offs
Why aren’t all plants C4 or CAM?
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
26
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