Download The use of energy from sunlight by photosynthesis is the basis of life

2
The use of energy from sunlight
by photosynthesis is the basis of
life on earth
2.1 How did photosynthesis start?
Figure 2.1
Life on
earth
involves a
CO2 cycle.
2.2 Pigments capture energy from
sunlight
Photon energy
The energy of the photon is proportional to its frequency v:
…………….………(2.1)
where h is the Planck constant (6.6. 10-34 J s) and c the velocity of
the light (3 . 108 m s-1). λ is the wavelength of light.
…………………..………(2.2)
We can equate the Gibbs free energy DG with the energy of the
absorbed light:
………………………(2.3)
The introduction of numerical values of the constants
h, c, and NA yields:
…….………(2.4)
…….………(2.5)
……………….………(2.6)
where F = number of charges per mol = 96,480 Amp . s . mol-1. The
introduction of this value yields:
…………(2.7)
Figure 2.2 Spectrum of the electromagnetic radiation.
The section shows the visible spectrum
Figure 2.3
Absorption
spectrum of
chlorophyll-a
(chl-a) and
chlorophyll-b
(chl-b) and of the
xanthophyll
lutein dissolved
in acetone. The
intensity of
the sun’s
radiation at
different
wavelengths is
given as a
comparison.
Figure 2.4 Structural formula of chlorophyll-a.
In chlorophyll-b the methyl group in ring b is replaced by a formyl
group (red). The phytol side chain gives chlorophyll a lipid character
2.3 Light absorption excites
the chlorophyll molecule
Figure 2.5 Resonance structures of chlorophyll-a. In the region marked red, the
double bonds are not localized; the ∏ electrons are distributed over the entire
conjugated system. The formyl residue of chlorophyll-b attracts electrons and
thusaffects the ∏ electrons of the conjugated system.
Figure 2.6
This simplified
scheme shows
only the
excitation
states of
the two main
absorbing
maxima of the
chlorophylls.
The second
excitation state
shown here is
in reality the
third singlet.
Figure 2.7
Fluorescent
light generally
has a longer
wavelength
than excitation
light.
2.4 An antenna is required to
capture light
Figure 2.8 Photons are collected by an antenna and their energy is
transferred to the reaction center. In this scheme the squares
represent chlorophyll molecules. The excitons conducted to the
reaction center cause a charge separation
Figure 2.9 Structural formula of a carotene (β - carotene) and of two
xanthophylls (lutein and violaxanthin). Due to the conjugated
isoprenoid chain,these molecules absorb light and also have lipid
character.
Figure 2.10 Basic scheme of an antenna.
Figure 2.11 Sterical arrangement of the LHC-IIb monomer in the thylakoid
membrane, viewed from the side. Three α-helices of the protein span the
membrane. Chlorophyll-a (black) and chlorophyll-b (red) are oriented almost
perpendicularly to the membrane surface. Two lutein molecules (black) in the center
of the complex act as an internal cross brace.
Figure 2.12
The LHC-II-trimer
viewed from above
from the stroma
side. Within each
monomer the
central pair of
helices form a lefthanded supercoil,
which is surrounded
by chlorophyll
molecules. The chl-b
molecules (red) are
positioned at the
side of the
monomers.
Figure 2.13 Scheme of the arrangement of the light harvesting complexes in the
antenna of photosystem II in a plant viewed from above (after Thornber); a means
LHC-IIa and so on. The inner antenna complexes are linked by
LHC-IIa and LHC-IIc monomers to the core complex. The function of the LHC-IId and
LHC-IIe monomers is not entirely known.
Figure 2.14 Scheme of a side view of the structure of a phycobilisome.
The units shown consist of three α- and three β-subunits each, (After
Bryanth.)
Figure 2.15 Structural formula of the biliproteins present in the
phycobilisomes, phycocyanin (black), and phycoerythrin (difference
from phycocyanin shown in red).
Figure 2.16
Absorption
spectra of the
phycobiliproteins
phycoerythrin,
phycocyanin, and
allophycocyanin
and, for the sake
of comparisn,
also of
chlorophyll-a.