Chapter
3. The Dual Role of Sunlight: Energy and Information
- Electromagnetic radiation has
wave and particle properties
- Waves - we
characterize radiation by its wavelength, e.g. UV (100-400 nm), visible( 400-700 nm) and infrared radiation (700-2000
nm).
- Particles - energy
content of photons is inverse function of associated wavelength (eq. 3.2).
- Pigments are compounds that
absorb electromagnetic radiation.
- Energy absorbed is
used to excite electrons to higher energy levels.
- Excitation energy can
be transferred to an adjacent molecule (inductive resonance), lost as
heat (thermal deactivation), or the actual electron can be transferred to
an adjacent molecule (photooxidation).
- Some pigments
experience structural changes when they absorb light (e.g. cis to trans isomerization of Phytochrome).
- Absorption and action
spectra:
- Absorption spectrum
characterizes the pattern of absorption, wavelength by wavelength, of
light.
- Action spectrum
characterizes the pattern of activity, wavelength by wavelength, of
light.
- Chlorophyll is
familiar example (Figures 3.4): absorbs mostly in blue and red, and these
are the wavelengths of light most effective in photosynthesis.
- Plants synthesize a broad
range of pigments:
- Chlorophyll
(Figure 3.7 shows structure)
- Porphyrin
head(with Mg) and hydrophobic phytol tail.
- Embedded in thhylakoid membrane, bound with proteins and other
pigment molecules.
- Chlorophyll a, b,c,d are variants
characteristic of different plant, algal and bacterial taxa.
- Angiosperms cannot
synthesize chlorophyll-a in the dark: receipt of red light pulse
initiate conversion of protochlorophyll-a to chlorophyll-a.
- Carotenoids
- serve as accessory pigments during photosynthesis,
protect chlorophyll from excess photooxidation,
involved in stomatal guard cell movement, etc.
- Phytochrome - involved
in sensing day length, provides germination cues, etc.
- Flavonoids
– many types, including:
- Anthocyanins
- as floral pigments produce pink, blue and purple colors
- Flavones and flavonols - in leaves confer protection
against UV-B radiation at high altitudes.