1. II. The light reactions of photosynthesis
Objectives are to understand:
Variation in energy of different forms of light
Light absorption by photosynthetic pigments
Energy transduction - conversion of light energy
to chemical energy as ATP and NADPH
PP07010.jpg

2. Why did we calculate the energy of a photon or mole of photons?
This is the energy that is absorbed by plants and used to power photosynthesis!
It is the energy of each photon, the quantum energy,that that slams into the photosynthetic pigments and excites - raises the energy state - of electrons.
We need to know the input of energy to understand the energetics of photosynthesis.
e.g. How efficient is photosynthesis?
efficiency = energy output/energy input

3. Light absorption by photosynthetic pigments

4. Fig 7.15

5. grana
lamellae
Fig 7.16

6. Light absorption by photosynthetic pigments
Much of the light energy reaching Earth’s surface is in the visible
portion of the EMR spectrum.
Chlorophyll absorbs strongly in this region of the spectrum.
Fig.7.3
PP07030.jpg

7. Photosynthetic pigments of higher plants
chlorophylls (a & b) carotenoids
Fig. 7.6

8. The interaction of photosynthetic pigments Antennae transfer
light energy
to reaction centers
PP07191.jpg

9. 3) Energy transduction - conversion of light energy
to chemical energy as ATP and NADPH
a. Absorption and action spectra
b. Key experiments in understanding the light reactions
c. The “Z scheme” of electron transfer and energy capture.
d. Putting it all together - organization of the light harvesting antennas and photochemical reaction centers.

10. a. absorption and action spectra
How light absorption characteristics are measured.
Measuring an absorption spectrum using
a spectrophotometer
PP07040.jpg
Fig (blue or green or red, etc.)

12. 2 = chl a 3 = chl b 5 = beta carotene
Fig. 7.7
High
2 = chl a 3 = chl b 5 = beta carotene
Low
PP07070.jpg

13. Action spectra describe the relationship of the effect (e. g
Action spectra describe the relationship of the effect (e.g. O2 production) of light absorption to wavelength.
PP07080.jpg

14. An early action spectrum using a bioassay. Engelmann, 1800s Fig. 7.9
PP07090.jpg
Fig. 7.9

15. Pond scum or a
beautiful green alga?

16. b. Key experiments in understanding the light reactions
Emerson-Arnold expt. 1932
O2 production depended
on amount of light.
Highest efficiency
at low light - “quantum yield”
O2 production saturated
at high light.
One O2 was produced per
2500 chlorophyll
molecules.
Fig. 7.11

17. “Quantum yield” is the term given to describe
the maximum yield of O2 per photons absorbed by the leaf (or extracted chloroplast preparations).
It is equal to the slope of the photosynthetic light response curve at low light levels.
The quantum yield is an efficiency term:
Efficiency = output (O2 production)
input (light absorbed)

18. The “red drop” experiments - Emerson again.
Observation: Quantum yield dropped off sharply beyond 680nm.
Why was efficiency reduced greatly in the “far red” portion of
the spectrum (beyond 680nm or so)?
PP07120.jpg

19. Red drop experiments suggested that the energy in light particles beyond the red portion of the spectrum was insufficient to drive photosynthesis.

20. What could explain this behavior?
Emerson “enhancement effect”
Far red and red light separately gave same rate of O2 production.
Both given together gave much greater O2 production.
What could explain this behavior?
Fig. 7.13
PP07130.jpg

21. Emerson’s “enhancement effect” experiments suggested
the existence of two interacting photosystems with
different wavelength optima.

23. c. The “Z scheme” of electron transfer and energy capture.
PP07141.jpg
Fig. 7.14