II. The light reactions of photosynthesis

Objectives are to understand:

1) Variation in energy of different forms of light

2) Light absorption by photosynthetic pigments

3) Energy transduction - conversion of light energyto chemical energy as ATP and NADPH

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

Light absorption by photosynthetic pigments

Fig 7.15

Fig 7.16

grana lamellae

Fig.7.3

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.

Photosynthetic pigments of higher plants1. chlorophylls (a & b) 2. carotenoids

Fig. 7.6

The interaction of photosynthetic pigmentsAntennae transfer light energy to reaction centers

3) Energy transduction - conversion of light energyto 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.

Fig. 7.4 (blue or green or red, etc.)

How light absorption characteristics are measured.

Measuring an absorption spectrum using a spectrophotometer

a. absorption and action spectra

Fig. 7.7High

Low

2 = chl a3 = chl b5 = beta carotene

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

Fig. 7.9

An early actionspectrum usinga bioassay.

Engelmann, 1800s

Pond scum or a beautiful green alga?

b. Key experiments in understanding the light reactionsEmerson-Arnold expt. 1932

Fig. 7.11

O2 production dependedon amount of light.

Highest efficiencyat low light - “quantum yield”

O2 production saturatedat high light.

One O2 was produced per 2500 chlorophyllmolecules.

“Quantum yield” is the term given to describethe 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)

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)?

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

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

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

c. The “Z scheme” of electron transfer and energy capture.

Fig. 7.14