Chapter 1 Energy conversion in higher plants and algae

Abstract

The chapter discusses energy conversion in higher plants and algae. Energy conversion in oxygenic photosynthesis of higher plants and algae is the process that converts the energy of electromagnetic radiation, in the visible region of the solar spectrum, into chemical energy in the form of nicotinamide adenine dinucleotide phosphate (NADPH) and adenosine triphosphate (ATP), which are subsequently utilized by a sequence of enzymatic reactions to convert CO 2 into organic molecules. The photosynthetic apparatus of green plants and cyanobacteria oxidizes water and transfers electrons to NADP, with a net gain in electrochemical potential of 1.13 eV (at pH 7), utilizing the energy of two light quanta per electron. The complete system is contained in the chloroplasts, and is localized within the thylakoid membranes, with the exception of the electron carrier ferredoxin, which is in solution in the stroma, and serves to transfer electrons from the reducing end of photosystem I (PS I) to a membrane-bound flavoprotein, which then reduces NADP, and of the copper protein plastocyanin (PC, the electron donor to PS I), which is in solution in the internal phase of thylakoids. The effect of cations in inducing thylakoid appression and the correlated phenomena is independent of the cation present, but only depends on its charge. Grana formation and the segregation, even “moderate,” of PS I from PS II has important effects on the partition of excitation energy between the photosystems. As the formation of grana can be observed reversibly in isolated thylakoids upon addition of cations, the additional problem arises of discriminating the effect of cations per se, if any, from that of membrane stacking and the segregation of the Chl-protein complexes in different regions of the membranes.