photosynthesis variations on a theme what do plants need? photosynthesis light reactions light h 2 o...
TRANSCRIPT
![Page 1: PHOTOSYNTHESIS Variations on a Theme WHAT DO PLANTS NEED? Photosynthesis Light reactions Light H 2 O Calvin cycle CO 2 O O C sun ground air What](https://reader030.vdocuments.us/reader030/viewer/2022032523/56649d6f5503460f94a516fc/html5/thumbnails/1.jpg)
PHOTOSYNTHESISVariations on a Theme
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WHAT DO PLANTS NEED?
• Photosynthesis• Light reactions
• Light• H2O
• Calvin cycle• CO2
O
OC sun
ground
air
What structures have plants evolved to
supply these needs?
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LEAF STRUCTURE
H2O
CO2
O2 H2O
phloem (sugar)xylem (water)
stomate guardcell
palisadeslayer
spongylayer
cuticleepidermis
O2 CO2
Transpiration
vascular bundle
Gas exchange
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CONTROLLING WATER LOSS
• Hot or dry days• Stomates close to conserve
water• Guard cells
• Gain H2O = stomates open• Loss H2O = stomates close
• Adaptation to living on land, but …
there are problems
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WHEN STOMATES CLOSE …
• Closed stomates lead to …• O2 build up from light reactions• CO2 depletion in Calvin cycle
• Causes problems in Calvin cycle
xylem (water)
phloem (sugars)
H2OO2 CO2
Houston,we have a problem …
-
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THE PROBLEM WITH RUBISCO: CO2 VS O2• RuBisCo in the Calvin cycle
• Carbon fixation molecule• Normally bonds C to RuBP• CO2 is the optimal substrate• Reduction of RuBP• Builds sugars
• When O2 concentration is high• RuBisCo bonds O to RuBP• O2 is a competitive substrate• Oxidation of RuBP• Breakdown of sugars
photosynthesis
photorespiration
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CALVIN CYCLE WHEN CO2 IS ABUNDANT
6Cunstable
intermediate
1C CO2
5CRuBP
3CPGA
ADP
ATP
3CNADP
NADPH
ADP
ATP
G3Pto make glucose
3CG3P
5C
RuBisCo
C3 plants
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CALVIN CYCLE WHEN O2 IS HIGH
5CRuBP
3C2C
to mitochondria
–––––––lost as CO2
without making ATP
photorespiration
O2
RuBisCoHey Dude,
are you highon oxygen!
It’s so sad to see a
good enzyme,go BAD!
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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 produced• ADP is not recycled
• No C6H12O6 produced• If photorespiration could be reduced, plant
would become 50% more efficient• Strong selective pressure to evolve alternative
carbon fixation systems
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REDUCING PHOTORESPIRATION
• Separate carbon fixation from Calvin cycle• C4 plants
• PHYSICALLY separate carbon fixation from Calvin cycle
• Use different cells to fix carbon vs where the Calvin cycle occurs
• Store carbon in a C4 compound
• Different enzyme captures CO2 (fix carbon)• PEP carboxylase
• Different leaf structure• CAM plants
• Separate carbon fixation from Calvin cycle by TIME OF DAY
• Fix carbon during the night• Store carbon in a C4 compound
• Perform Calvin cycle during the day
PEP = phosphoenolpyruvate
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C4 PLANTS• A better way to capture
CO2• 1st step before Calvin
cycle, fix carbon with enzyme PEP carboxylase
• Store as 4C compound• Adaptation to hot, dry
climates• Have to close stomates a
lot• Different leaf anatomy
• Sugar cane, corn, other grasses
sugar cane
corn
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C4 LEAF ANATOMY
PEP (3C) + CO2 oxaloacetate (4C)
CO2
CO2
bundlesheath
cell RuBisCo
PEPcarboxylase
stomate
C3 anatomy
O2
light reactions
C4 anatomy
• PEP carboxylase enzyme• Higher attraction for CO2 than O2
• Better than RuBisCo• Fixes CO2 into 4C compounds• Regenerates CO2 in inner cells
for RuBisCo• Keeping O2 away from RuBisCo
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COMPARATIVE ANATOMY
C3 C4
Location,location,location!
PHYSICALLY separate C fixation from Calvin cycle
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CAM (CRASSULACEAN ACID METABOLISM) PLANTS
• Adaptation to hot, dry climates• Separate carbon fixation from Calvin cycle by
TIME OF DAY• 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 inthe timing!
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CAM PLANTS
succulents
cacti
pineapple
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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
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WHY THE C3 PROBLEM?
• 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 selective pressure to evolve a better way to fix carbon & minimize photorespiration
We’ve all gotbaggage!
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It’s not easybeing green.
Any questions?