The use of energy from sunlight by photosynthesis is the basis of life on earth 2 2

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: where h is the Planck constant ( J s) and c the velocity of the light ( m s -1 ). λ is the wavelength of light. …………………..………(2.2) …………….………(2.1)

We can equate the Gibbs free energy DG with the energy of the absorbed light: We can equate the Gibbs free energy DG with the energy of the absorbed light: The introduction of numerical values of the constants h, c, and N A yields: ………………………(2.3) …….………(2.4) …….………(2.5)

where F = number of charges per mol = 96,480 Amp. s. mol -1. The introduction of this value yields: ……………….………(2.6) …………(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 left- handed 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.