Photosynthesis variations on the theme
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Photosynthesis: Variations on the Theme. Remember what plants need…. Photosynthesis light reactions light H 2 O Calvin cycle CO 2.  sun.  ground.  air. What structures have plants evolved to supply these needs?. xylem (water). phloem (sugar). cuticle. epidermis. palisades

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Photosynthesis: Variations on the Theme

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Photosynthesis variations on the theme

Photosynthesis:Variations on the Theme


Remember what plants need

Remember what plants need…

  • Photosynthesis

    • light reactions

      • light

      • H2O

    • Calvin cycle

      • CO2

 sun

 ground

air

What structures have plants evolved to supply these needs?


Leaf structure

xylem (water)

phloem (sugar)

cuticle

epidermis

palisades

layer

spongy

layer

stomate

guardcell

vascular bundle

Leaf Structure

O2

H2O

CO2

O2

H2O

CO2

Transpiration


A second inside a leaf

O2

CO2

O2

CO2

H2O

A second inside a leaf…

  • Gas exchange & water flow

    • CO2 in 

    • O2 out 

    • H2O out 

for Calvin cycle

waste from light reactions

for light reactions

photosynthesis

xylem (water)

phloem (sugars)

gas exchangewater loss


Controlling water loss from 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!


Closed stomates

Closed stomates

  • Closed stomates lead to…

    • O2 builds up  from light reactions

    • CO2 is depleted  in Calvin cycle

      • causes problems in Calvin Cycle

The best laidschemes ofmice and men…and plants!


Inefficiency of rubisco co 2 vs o 2

Inefficiency of Rubisco: CO2 vs O2

  • Rubisco in Calvin cycle

    • carbon fixation enzyme

      • normally bonds CtoRuBP

      • reduction ofRuBP

      • building sugars

    • when O2 concentration is high

      • Rubisco bonds OtoRuBP

      • O2 is alternative substrate

      • oxidation of RuBP

      • breakdown sugars

photosynthesis

photorespiration


Photosynthesis variations on the theme

1C

3C

CO2

RuBP

5C

6C

unstable

intermediate

ATP

ADP

3C

ATP

PGA

ADP

3C

NADPH

NADP

Calvin cycle review

Rubisco

G3P

to make

glucose

G3P


Photosynthesis variations on the theme

1C

3C

CO2

RuBP

5C

G3P

to make

glucose

3C

PGA

G3P

Calvin cycle review

Rubisco

C3 plants


Photosynthesis variations on the theme

O2

RuBP

5C

3C

2C

to mitochondria

–––––––

lost as CO2 without making ATP

Calvin cycle with O2

Rubisco

A good enzyme,gone BAD!

photorespiration


Impact of 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 systems


Reducing photorespiration

Reducing photorespiration

  • Separate carbon fixation from Calvin cycle

    • C4 plants

      • physically separate carbon fixation from actual Calvin cycle

      • different enzyme to capture CO2

        • PEP carboxylase stores carbon in 4C compounds

      • different leaf structure

    • CAM plants

      • separate carbon fixation from actual Calvin cycle by time of day

      • fix carbon (capture CO2) during night

        • store carbon in organic acids

      • perform Calvin cycle during day


C4 plants

C4 plants

  • A better way to capture CO2

    • 1st step before Calvin cycle, fix carbon with enzymePEP carboxylase

      • store as 4C compound

    • adaptation to hot, dry climates

      • have to close stomates a lot

      • different leaf anatomy

    • sugar cane, corn, other grasses…


C4 plants1

C4 Plants

corn

sugar cane


Pep carboxylase

O2

PEP carboxylase

light reactions

  • PEP carboxylase enzyme

    • higher affinity for CO2 than O2 (better than Rubisco)

    • fixes CO2 in 4C compounds

    • regenerates CO2in inner cells for Rubisco

phosphoenolpyruvate (3C) + CO2 oxaloacetate (4C)


Comparative anatomy

Location,location,

location!

Comparative anatomy

  • Separate reactions in different cells

    • light reactions

    • carbon fixation

    • Calvin cycle

C3

C4


C4 photosynthesis

CO2

O2

O2

CO2

C4 photosynthesis

Physically separated C fixation from Calvin cycle

  • Outer cells

    • light reaction & carbon fixation

    • pumps CO2 to inner cells

    • keeps O2 away from inner cells

      • away from Rubisco

  • Inner cells

    • Calvin cycle

    • glucose to veins


Cam crassulacean acid metabolism plants

CAM (Crassulacean Acid Metabolism) plants

  • Different 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 “storage” compounds

      • organic acids: malic acid, isocitric acid

    • in day, close stomates & release CO2 from “storage” compounds to Calvin cycle

      • increases concentration of CO2 in cells

    • succulents, some cacti, pineapple

It’s all inthe timing!


Cam plants

CAM plants


C4 vs cam summary

C4 vs CAM Summary

solves CO2 / O2 gas exchangevs. H2O losschallenge

C4 plants

separate 2 steps of C fixation anatomically in 2 different cells

CAM plants

separate 2 steps of C fixation temporally at2 different times


Why the c3 problem

We’ve all gotbaggage!

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 selection pressure to evolve better way to fix carbon & minimize photorespiration


Any questions

Any Questions??


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