Absorption spectrum Graph plotting light absorption vs. wavelength Figure 10.8 White light Refracting prism Chlorophyll solution Photoelectric tube Galvanometer Slit moves to pass light of selected wavelength Green light The high transmittance (low absorption) reading indicates that chlorophyll absorbs very little green light. The low transmittance (high absorption) reading chlorophyll absorbs most blue light. Blue light 1 2 3 4 0 100 0 100
Absorption spectra of 3 types of pigments in chloroplasts Three different experiments helped reveal which wavelengths of light are photosynthetically important. The results are shown below. EXPERIMENT RESULTS Absorption of light by chloroplast pigments Chlorophyll a (a) Absorption spectra. The three curves show the wavelengths of light best absorbed by three types of chloroplast pigments. Wavelength of light (nm) Chlorophyll b Carotenoids Figure 10.9
Action spectrum of a pigment Profiles relative effectiveness of different wavelengths of radiation in driving photosynthesis Rate of photosynthesis (measured by O 2 release) Action spectrum.(b)
Action spectrum for photosynthesis first demonstrated by Theodor W. Engelmann 400 500 600 700 Aerobic bacteria Filament of alga Engelmann‘s experiment.(c)
Chlorophyll a (blues) Main photosynthetic pigment Chlorophyll b (yellows) Accessory pigment C CH CH 2 C C C C C C N N C H 3 C C C C C C C C C N C C C C N Mg H H 3 C H C CH 2 CH 3 H CH 3 C H H CH 2 CH 2 CH 2 H CH 3 C O O O O O CH 3 CH 3 CHO in chlorophyll a in chlorophyll b Porphyrin ring: Light-absorbing “head” of molecule note magnesium atom at center Hydrocarbon tail: interacts with hydrophobic regions of proteins inside thylakoid membranes of chloroplasts: H atoms not shown Figure 10.10
Other accessory pigments Absorb different wavelengths of light & pass the energy to chlorophyll a Carotenoids – yellows & oranges
Excitation of Chlorophyll by Light When a pigment absorbs light Goes from ground state to excited state, which is unstable Excited state Energy of election Heat Photon (fluorescence) Chlorophyll molecule Ground state Photon e – Figure 10.11 A
You've reached the end of your free preview.
Want to read all 46 pages?