Page 166 - Algae

Photosynthesis 149

An alternative pathway for ATP production is cyclic phosphorylation, in which electrons from
PSI cycle in a closed system through the phosphorylation sites and ATP is the only product formed.
Electron arising from P 700 are transferred to ferredoxin and then to the cytochrome b 6 f complex.
Protons are pumped by this complex as electrons return to the oxidized form of reaction center
P 700 through plastocyanine. This cyclic phosphorylation takes place when NADP is unavailable
þ
to accept electrons from reduced ferredoxin because of a very high ratio of NADPH to NADP .
The electrochemical potential of the proton gradient drives the synthesis of ATP through an
ATP-synthase situated, as we have seen, anisotropically in the thylakoid membrane.

Pigment Distribution in PSII and PSI Super-Complexes of Algal Division
Absorption spectra can give us information about the spectral range in which pigment molecules
organized in the thylakoid membranes capture photons. Absorption spectra in the visible range,
from 400 to 700 nm, have been measured in vivo on photosynthetic compartments (thylakoid mem-
branes and chloroplasts) of single cells belonging to each algal division. Each spectrum represents
the envelope of the real absorption data, and is coupled to the plot of the fourth-derivative absorp-
tion spectrum. This mathematical tool allows the resolution of absorption maxima relative to the
different components of the pigment moiety characterizing the division, which cannot be detected
in the envelope spectrum because of the overlapping of their multiple spectra. These components
have been grouped and related to the following pigment classes: chlorophylls a, b, c 1 , and c 2 , caro-
tenoids, cytochromes, and phycobiliproteins. Each pigment possesses its own distinctive absorption
spectrum in the visible range, which have been decomposed in its Gaussian bands and relative
absorption maxima. By relating the absorption maxima resolved by means of fourth-derivative
analysis with the absorption maxima of the Gaussian bands of the different pigments, it is
possible to both predict the presence of a specific pigment in an alga and give an unknown alga
a plausible taxonomic framing.
Chlorophylls and cytochromes always show the same absorption maxima independently of the
algal division, though the intensity of their absorption bands may change; phycobiliproteins, caro-
tenoids, and xanthophylls show a variable distribution of their single components, which is charac-
teristic of each algal division. Figure 3.5–Figure 3.8 show the absorption spectra of the
photosynthetic compartments in all the different algal divisions. The absorption spectrum of
the tissue of a higher plant (Hedysarum coronarium) is shown for highlighting the uniformity of
the pigment distribution throughout the algal green lineage and plants.
It should be stressed that all the pigments other than chlorophyll a perform two main functions:
protection of photosynthetic assemblies from photosensitization processes (mainly carotenoids);
absorption of light at wavelengths other than those absorbed by chlorophyll a and transfer of its
energy to P 680 and P 700 .

LIGHT-INDEPENDENT REACTIONS
We have seen that NADPH and ATP are produced in the light phase of photosynthesis. The next
phase of photosynthesis involves the fixation of CO 2 into carbohydrates. Although many textbooks
state that glucose (C 6 H 12 O 6 ) is the major product of photosynthesis, the actual carbohydrate end-
products are those listed in Table 1.4 in Chapter 1 (sucrose, paramylon, starch, etc.). The fixation of
CO 2 takes place during the light independent phase using the assimilatory power of NADPH and
ATP in the chloroplast stroma (eukaryotic algae) or in the cytoplasm (prokaryotic algae). The light-
independent reactions do not occur in the dark; rather they occur simultaneously with the light reac-
tions. However, light is not directly involved. The light-independent reactions are commonly
referred to as the Calvin Benson Bassham cycle (CBB cycle) after the pioneering work of its
discoverers.
The first metabolite was a 3-carbon organic acid known as 3-phosphoglycerate (3-PG). For this
reason, the pathway of carbon fixation in algae and most plants is referred to as C3 photosynthesis.