1. C3, C4, and CAM plants all have the same goal,
to make carbohydrates.
What happens to the triose-phosphates made in the Calvin cycle?
Used to synthesize starch for storage in chloroplast.
Exported from chloroplast for sucrose synthesis in the cytosol.
How is starch vs. sucrose synthesis regulated?
Why is it regulated?

2. cytosol Triose phosphates produced in the Calvin cycle can
be used for starch or sucrose synthesis.
chloroplast
Calvin
cycle
Starch
Triose-P
Sucrose
cytosol

3. Starch is synthesized in the chloroplast.
Fig. 8.15

4. Starch vs. sucrose synthesis is regulated by level of cytosolic Pi as it affects triose-P export from chloroplast.
When [Pi] is high,
triose-P is exported
in exchange for Pi
& used to synthesize
sucrose.
If [Pi] is low, then
triose-P is retained
in chloroplast and
used to synthesize
starch.
Fig. 8.14

5. More on the ecological aspects of photosynthesis (Ch. 9)
Stomatal conductance
light, temperature, relative humidity, Y, [CO2]
carbon isotope discrimination
Light
Leaf movements
Sun and shade leaves - anatomical and photosynthetic properties.
Temperature
Leaf energy balance
C3 vs. C4
quantum yield differences
Atmospheric CO2
History of atmospheric CO2
Current trend of rising CO2
Implications for C3 & C4 photosynthesis

6. Review: Stomatal aperture regulates the conductance of the diffusion pathway for CO2 entering the leaf and H2O leaving the leaf.
PP04100.jpg
Fig. 4.10

7. What factors influence stomatal conductance?
Environmental cues Effect on stomatal cond.
1. light increases as light increases
2. relative humidity increases as r.h. increases
3. temperature increases as temp. increases
Internal cues
1. leaf water potential decreases as Y decreases
2. internal [CO2] decreases as [CO2] increases
3. hormonal control decreases as [ABA] increases
(abscisic acid, ABA)
All these cues ultimately influence the turgor pressure of the guard cell, which in turn causes the opening or closing of the stomatal pore.

8. light Fig. 9.6 Light and leaf movements
Light affects photosynthesis and leaf temperature
Solar tracking allows
leaves to increase
light absorption compared
to a fixed orientation.
Diaheliotropic leaves
Fig. 9.6

9. Some plants change
leaf angle to reduce
light absorption.
Paraheliotropic leaves
Why?

10. What happens to the light that strikes a leaf?
Absorbed
85 to 90% of the PAR, 60% of total energy
2. Reflected
0 to 8% of the PAR
3. Transmitted (passes through leaf)
0 to 8% of PAR
Fig. 9.2

11. Absorption is high for PAR and decreases greatly
at longer wavelengths.
Fig. 9.3
PP09030.jpg

12. Light level attenuates (decreases) with depth in a plant canopy because each layer of leaves absorbs light.
Fig. 9.7
PP09070.jpg

13. Leaf anatomy responds
to light level.
Which is the “sun”
Leaf and which is the
“shade” leaf?
Sun leaf
Shade leaf

14. Sun leaves Shade leaves
Thicker, Thinner, fewer
more cell layers cell layers
More Rubisco Less Rubisco
per unit chlorophyll per chlorophyll
Less chlorophyll More chl per
per reaction center reaction center
Light acclimation (phenotypic plasticity) vs. light adaptation.

15. Physiological differences of sun and shade leaves
Anatomical and biochemical differences between sun and shade leaves determine photosynthetic properties.
Physiological differences of sun and shade leaves
Fig. 9.9
Sun leaf vs. shade leaf
Sun leaf has:
Higher max. photo. rate
Higher light sat’n level
Higher light compensation point
PP09090.jpg

16. Fig. 9.10 Acclimation to growth light level - same pattern as sun vs.
shade species
differences.
PP09100.jpg

17. Light and leaf temperature. Heat loads on leaves in the sun are large.
How do leaves prevent overheating?
PP09140.jpg

18. Mechanisms of heat dissipation by leaves
Leaves can lose heat in three main ways:
1. Emission of radiation
2. Conduction/convection
3. Evaporation
Each term can be included as
part of an “energy balance”
equation.
PP09140.jpg

19. Leaf energy balance At constant temperature: Energy In = Energy Out
(Radiation absorbed + Conduction/Convection + Condensation)
= (Radiation emitted + Conduction/Convection
loss + Evaporation)

20. Leaf temperature and photosynthesis
Which is
C3 and C4? C4 C3

21. Why does the quantum yield of C3 plants decrease with
increasing temperature? Why is the quantum yield of C4
plants insensitive to temperature?
Fig 9.23
PP09230.jpg

22. Photosynthetic responses to CO2
History of atmospheric CO2
Fig. 9.16
PP09161.jpg

23. Current trend of rising CO2
Fig. 9.16
The Mauna Loa
CO2 record
PP09162.jpg

24. Photosynthetic response to CO2 of C3 & C4 plants
PP09201.jpg

25. PP09202.jpg

26. Photosynthetic response to temperature
Fig. 9.22
CO2-temperature
interaction in a C3
plant.
Why does the temp.
for maximum phot.
increase at elevated
CO2?
PP09220.jpg

28. 2. Second approach
Henry’s Law: concentration of a gas dissolved in water is proportional to the gas partial pressure (or [gas] at same total pressure) above the water.
Changing the gas partial pressure produces a proportional change in dissolved concentration.
Examples
If dissolved concentration is 11.68µM at 345ppm CO2, then the dissolved concentration is 2 x if gas concentration is 2 X 345ppm.
At 250ppm CO2, dissolved CO2 is x 250/345 = 8.46 µM.