Harvesting the Light
Harvesting the Light
Products of reactions are separated from reactants by a barrier or barriers. if this were not so we could not have any reactants—everything would already be products! In order for the grapevine to grow beyond the materials provided in the seed, the rootstock, or the cutting, it is necessary for the reactants obtained from the environment (i.e., nutrients in the soil and air) to be converted to plant material. The energy for this conversion comes from the sun, and it is the chloroplasts that take the light and, using the aforementioned materials, convert it to useful energy in the plant. So, overall, for processes to occur within the plant, a high energy species must be formed and then used. Subsequent regeneration of the high energy species can use more sunlight. The currency of energy is adenosine triphosphate (ATP). When it is used, it is converted to adenosine diphosphate (ADP) and inorganic phosphate (Pi), and in that conversion (or those conversions as more than one can be used to accomplish the same end) the barrier between reactant and product can be overcome (Figure 10.1). Additionally, for moving electrons and protons around where simple solvation (the use of—and interactions with—solvents) will not work, a cofactor (a “factor” that needs to be present in addition to an enzyme to enable the catalyzed reaction to occur) is often needed. These movements of electrons and protons are simply oxidations and reductions (see Appendix 1), and it is common to find oxidation and reduction being effected by using, as cofactors, either the oxidized or reduced forms of the phosphate ester of nicotinamide adenine dinucleotide (NADP+) to/from (NADPH) and/or the related conversion of the oxidized/reduced forms of flavin adenine dinucleotide (FAD)/(FADH2) (Figure 10.2). A cartoon representation of the chloroplast wall, with the stroma (the colorless fluid filling the chloroplast through which materials move) shown on the top and the lumen of the thylakoid body (where the light- dependent photochemistry occurs) on the bottom is provided in Figure 10.3. The working agents in the membrane are shown.
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