Photosynthesis in green plants

Preeminent in importance among the macro-cyclic complexes of Group 2 elements are the chlorophylls, which are modified porphyrin complexes of Mg. These compounds are vital to the process of photosynthesis in green plants (see Panel). Magnesium and Ca are also intimately... 

Photosynthesis in green plants occurs in two basic processes. In the dark (the Calvin cycle) carbon dioxide is reduced by a strong reducing agent, the reduced form of nicotinamide adeninedinucleotide phosphate, NADPH2, with the help of energy obtained from the conversion of ATP to ADP ... 

To begin our discussion of a mechanistic model for photosynthesis, let us return to Eq. (12.1b) for the overall process of photosynthesis in green plants ... 

To make the two systems directly analogous, Van Niel<19> suggested that the reaction for photosynthesis in green plants should be written as... 

As a model of photosynthesis in green plants, platinized chlorophyll a dihydrate polycrystals were used.173 Illumination of Pt-chlorophyll in the presence of C02 and water gave formic acid by the reaction... 

The chlorophyll molecule (309) is involved in initiating photosynthesis in green plants and contains magnesium coordinated to a partially reduced porphyrin (namely, a chlorin derivative). Life relies ultimately on the unique redox and electron transfer abilities of the chlorophylls which are necessary for the conversion of light to chemical energy. Chlorophyll mainly absorbs light from the far red region of the spectrum... 

Photosynthesis in green plants involves the oxidation of water to dioxygen, followed by reduction of carbon dioxide to yield glucose. That is, the latter process involves the synthesis of glucose from C02 and H20 via a photo-initiated reaction. 

Much interest has recently been shown in artificial photosynthesis. Photosynthesis is a system for conversion or accumulation of energy. It is also interesting that some reactions occur simultaneously and continuously. Fujishima et al. [338] pointed out that a photocatalytic system resembles the process of photosynthesis in green plants. They described that there are three important parts of the overall process of photosynthesis (1) oxygen generation by the photolysis of water, (2) photophosphorylation, which accumulates energy, and (3) the Calvin cycle, which takes in and reduces carbon dioxide. The two reactions, reduction of C02 and generation of 02 from water, can occur simultaneously and continuously by a sonophotocatalytic reaction. 

However, unlike photosynthesis in green plants, the titanium oxide photocatalyst does not absorb visible light and, therefore, it can make use of only 3-4% of solar photons that reach the Earth. Therefore, to address such enormous tasks, photocatalytic systems which are able to operate effectively and efficiently not only under ultraviolet (UV) but also under sunlight must be established. To this end, it is vital to design and develop unique titanium oxide photocatalysts which can absorb and operate with high efficiency under solar and/or visible-light irradiation [9-16]. 

When titanium oxides are irradiated with UV light that is greater than the band-gap energy of the catalyst (about X < 380 nm), electrons (e ) and holes (h+) are produced in the conduction and valence bands, respectively. These electrons and holes have a high reductive potential and oxidative potential, respectively, which, together, cause catalytic reactions on the surfaces namely photocatalytic reactions are induced. Because of its similarity with the mechanism observed with photosynthesis in green plants, photocatalysis may also be referred to as artificial photosynthesis [1-4]. As will be introduced in a later section, there are no limits to the possibilities and applications of titanium oxide photocatalysts as environmentally harmonious catalysts and/or sustainable green chemical systems. ... 

If a scale of electrochemical redox potentials is considered, it is obvious that water is not an easily oxidizable species nor is carbon dioxide easily reduced. The energy requirement of photosynthesis in green plants can be met only by the cooperation of two excited chlorophyll molecules. The first one gives a part of its excitation energy to the second one (Figure 5.4). 

An extraordinary variety of reactions of organic compounds are known to occur under the influence of visible and ultraviolet light. Some of these, such as the photochemical halogenation of alkanes and photosynthesis in green plants, already have been discussed (see Sections 4-4D and 20-9). It is notour purpose here to review organic photochemistry in detail — rather, we shall mention a few types of important photochemical reactions and show how these can be explained by the principles discussed in the preceding section. 

In a measurement of the quantu efficiency of photosynthesis in green plants, it was found that 8 quanta of red light at 685nm were needed to evolve one molecule of 02. The average energy storage in the photosynthetic process is 469... 

First, it s pretty clear where natural polymers originate. When organisms die, the polymers in them eventually break down and are returned to the earth. The cellulose in dead trees and plants is hydrolyzed by microorganisms, the products metabolized and returned to the atmosphere as C02 and water. Photosynthesis in green plants converts this C02 into new cellulose in living trees and plants, and the cycle continues. 

In this equation, (CH2O) represents carbohydrate, primarily sucrose and starch. The mechanism of photosynthesis is complex and requires the interplay of many proteins and small molecules. Photosynthesis in green plants takes place in chloroplasts (Figure 19.1). The energy of light captured by pigment molecules, called chlorophylls, in chloroplasts is... 

Photosynthesis in green plants is mediated by two kinds of membrane-bound, light-sensitive complexes—photosystem I (PS I) and photosystem II (PS II). Photosystem I typically includes 13 polypeptide chains, more than 60 chlorophyll molecules, a quinone (vitamin Kj), and three 4Fe-4S clusters. The total molecular mass is more than 800 kd. 

All free energy used by biological systems originates from solar energy stored by photosynthesis in green plants, algae, or photosynthetic bacteria. The... 

The major pigments responsible for photosynthesis in green plants are ... 

Recently a number of covalently linked porphyrin-quinone systems such as IS (Malaga et al., 1984) or 16 (Joran et al., 1984) have been synthesized in order to investigate the dependence of electron-transfer reactions on the separation and mutual orientation of donor and acceptor. These systems are also models of the electron transfer between chlorophyll a and a quinone molecule, which is the essential charge separation step in photosynthesis in green plants. (Cf. Section 7.6.1.) Photoinduced electron transfer in supra-molecular systems for artificial photosynthesis has recently been summarized (Wasielewski, 1992). 

Chloroplast Site of photosynthesis in green plants and algae has its own DNA... 

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