Transcript Plant Adaptations: C3 and C4 plants

Plant Adaptations: C3 and C4
plants
Leaf anatomy
Remember The Needs of Plants!
Plants need to take in:
water (from soil)
nutrients (from soil)
CO2 (from atmosphere)
Plants need to release:
water vapor (through leaves)
O2 (through leaves)
Controlling water loss from leaves
• Hot or dry days
– stomates close to conserve water
– guard cells
• gain H2O = stomates open
• lose H2O = stomates close
– adaptation to
living on land,
but…
– creates PROBLEMS!
When stomates close…
• Closed stomates lead to…
– O2 build up  from light reactions
– CO2 is depleted  in Calvin cycle
• causes problems in Calvin Cycle
Inefficiency of RuBisCo: CO2 vs O2
• RuBisCo in Calvin cycle
– carbon fixation enzyme
•
•
•
•
normally bonds C to RuBP
CO2 is the optimal substrate
reduction of RuBP
building sugars
– when O2 concentration is high
•
•
•
•
photosynthesis
RuBisCo bonds O to RuBP
O2 is a competitive substrate
oxidation of RuBP
breakdown sugars
photorespiration
Calvin cycle when CO2 is abundant
1C
ATP
RuBP
CO2
5C RuBisCo
ADP
G3P
to make
glucose
6C
unstable
intermediate
5C
G3P 3C
C3 plants
3C
PGA
NADPH
ATP
NADP
ADP
3C
Calvin cycle when O2 is high
to
mitochondria
–––––––
lost as CO2
without
making ATP
O2
RuBP
Hey Dude,
are you high
on oxygen!
It’s so
sad to see a
good enzyme,
go BAD!
5C RuBisCo
2C
3C
photorespiration
Photorespiration
• In most plants initial fixation of CO2 occurs via rubisco
and results in a three-carbon compound, 3phosphoglycerate.
• When their stomata are closed on a hot, dry day, CO2
levels drop as CO2 is consumed in the Calvin cycle.
• At the same time, O2 levels rise as the light reaction
converts light to chemical energy.
• While rubisco normally accepts CO2, when the O2/CO2
ratio increases (on a hot, dry day with closed stomata),
rubisco can add O2 to RuBP.
• When rubisco adds O2 to RuBP, RuBP splits into a
three-carbon piece and a two-carbon piece in a process
called photorespiration.
– The two-carbon fragment is exported from the chloroplast
and degraded to CO2 by mitochondria and peroxisomes.
– Unlike normal respiration, this process produces no ATP, nor
additional organic molecules.
• Photorespiration decreases photosynthetic output by
taking organic material from the Calvin cycle.
• Photorespiration can drain away as much as 50% of the
carbon fixed by the Calvin cycle on a hot, dry day.
• Certain plant species have evolved alternate modes of
carbon fixation to minimize photorespiration.
Impact of Photorespiration
• Oxidation of RuBP
– short circuit of Calvin cycle
– loss of carbons to CO2
• can lose 50% of carbons fixed by Calvin cycle
– reduces production of photosynthesis
• no ATP (energy) produced
• no C6H12O6 (food) produced
– if photorespiration could be reduced, plant
would become 50% more efficient
• strong selection pressure to evolve alternative carbon
fixation systems
Reducing photorespiration
• Separate carbon fixation from Calvin cycle
– C4 plants
• PHYSICALLY separate carbon fixation from Calvin cycle
– different cells fix carbon vs. where Calvin cycle occurs (different
leaf structure)
– PEP carboxylase
– CAM plants
• TEMPORALLY separate carbon fixation from Calvin cycle
• fix carbon during night, Calvin cycle during day
Working of the C4
• After entering through stomata, CO2 diffuses into a
mesophyll cell.
• Instead the CO2 is inserted into a 3-carbon
compound (C3) called phosphoenolpyruvic acid
(PEP) forming the 4-carbon compound oxaloacetic
acid (C4).
• Oxaloacetic acid is converted into malic acid or
aspartic acid (both have 4 carbons), which is
transported (by plasmodesmata) into a bundle
sheath cell.
– Bundle sheath cells are deep in the leaf so
atmospheric oxygen cannot diffuse easily to them;
• These features keep oxygen levels low.
• Here the 4-carbon compound is broken down into
carbon dioxide, which enters the Calvin cycle to form
sugars and starch.
Comparative anatomy
Location,
location,location!
C3
C4
PHYSICALLY separate C fixation from Calvin cycle
AP Biology
C4 Leaf
Biochemistry
Up Close:
Photosynthesis
across 2
different cells.
CAM (Crassulacean Acid Metabolism) plants
 Adaptation to hot, dry climates

separate carbon fixation from Calvin cycle by TIME
 close stomates during day
 open stomates during night


at night: open stomates & fix carbon
in 4C “storage” compounds
in day: release CO2 from 4C acids
to Calvin cycle
 increases concentration of CO2 in cells

succulents, some cacti, pineapple
It’s all in
the timing!
CAM plants
cacti
succulents
pineapple
CAM Plant Biochemistry:
Photosynthesis at 2 times of day
C4 vs CAM Summary
solves CO2 / O2 gas exchange vs. H2O loss challenge
C4 plants
separate 2 steps
of C fixation
anatomically in 2
different cells
CAM plants
separate 2 steps
of C fixation
temporally =
2 different times
night vs. day
Why the C3 problem?
We’ve all got
baggage!
• Possibly evolutionary baggage
– Rubisco evolved in high CO2 atmosphere
• there wasn’t strong selection against active site of
Rubisco accepting both CO2 & O2
• Today it makes a difference
– 21% O2 vs. 0.03% CO2
– photorespiration can drain away 50% of carbon
fixed by Calvin cycle on a hot, dry day
– strong selection pressure to evolve better way
to fix carbon & minimize photorespiration
Supporting a biosphere
• On global scale,
photosynthesis is the
most important process
for the continuation of life on Earth
– each year photosynthesis…
• captures 121 billion tons of CO2
• synthesizes 160 billion tons of carbohydrate
– heterotrophs are dependent on plants as food
source for fuel & raw materials