HW 3-5 Introduction to Light Absorbance Spectra

Figure 10.10 Excitation of isolated chlorophyll by light Link to fluorescence of chlorophyll Isolated chloroplast before excitation Isolated chloroplast after excitation

The Photosynthetic process evolved in bacteria and plants to utilize VISIBLE light, one of the most common range of energies emitted by our Sun.

Electrons Make Quantum Leaps between Energy Levels when Atoms absorb Light

Electrons Make Quantum Leaps between Energy Levels when Atoms absorb Light Link to Phet Laser quantum

Absorption Spectra of a Molecule is much more complex than a single type of atom due to numerous overlapping quantized electronic transitions of valence electrons. (Many different vibrational and rotational motions possible in a big molecule)

Which absorption spectrum represents hemoglobin Which absorption spectrum represents hemoglobin? Which represents chlorophyll? I) II) Chlorophyll appears Green Hemoglobin is Red

Chlorophyll inside a leaf absorbs RED and BLUE light and reflects (transmits) GREEN Light

Molecules that have alternating single and double bonds (resonance delocalization of pi bonding e-’s) tend to absorb visible and ultraviolet light; absorption of low energy visible light is due to transitions of weakly bonded pi bonding electrons from ground to excited states. PABA (sunscreen) Chlorophyll a

Pi bonding electrons are on the average farther from nuclei and therefore more weakly attracted than sigma bonding electrons

Structures of Common Photosynthetic Pigments Chlorophyll a is the most abundant pigment is most plants. Separation of photosynthetic pigments by paper chromatography

Figure 10.9 Location and structure of chlorophyll molecules in plants Chlorophyll a and Cholorphyll b are the most important light absorbing molecules in photosynthesis

Absorption Spectrum vs Action Spectrum

IB and AP Bio Exams Why is the absorbance spectrum of chlorophyll not the same as the action spectrum?

Key conclusion: Chlorophyll a and Chlorophyll b are responsible for a significant fraction of light absorption in photosynthesis but not all. Presence of additional pigments accounts for the difference.

Link to action and absorbance Why do plants or algae have multiple pigments?

Do all wavelengths of light penetrate equally through a forest canopy or down to the depths in water?

Survival Advantage if you live near the ground in the forest or underwater to be able to utilize many different pigments in order to utilize as many different wavelengths as possible Do all wavelengths of light penetrate equally through a forest canopy or down to the depths in water?