the carbon dioxide. The water on the product side derives from a dehydroxylation
step. This generalized reaction has two components, the light reactions of
photosystem II (PSII) and photosystem I (PSI):
(^)
(^) and the light-independent reaction:
(^) where the free energy of cleavage of high-energy phosphate bonds of ATP and the
reducing power of NADPH are used to fix and reduce CO 2 to form carbohydrate. This
reaction is mediated by the enzyme ribulose bis-phosphate carboxylase (RuBisCO).
(^) The pigment system (considered in Chapter 2) gives the phytoplankton cell access
to energy from most of the visible spectrum. An example of the relative roles of
different pigments in absorbing light is shown in Fig. 3.2a.
Fig. 3.2 (a) Fractional contribution to absorption of light by different pigments in live
Emiliania huxleyi, a coccolithophorid. Contributions to absorbance add to 100% at
each wavelength. Pigments are chlorophyll-a, chlorophyll-c, 19′
hexanoyloxyfucoxanthin, and other carotenoids. (b) Absorbance spectrum and
photosynthetic action spectrum of oxygen production per quantum in E. huxleyi. The
ratio [photosynthesis/absorbance], the relative yield, shows that absorbed quanta at all
wavelengths are roughly equally effective.
(^) (After Haxo 1985.)