M

300 600 900

Wavelength X (I0~9 m)

300 600 900

Wavelength X (I0~9 m)

Figure 3.72. Spectrum of relative ab-sorptance for a green plant and for a purple bacterium (based on Clayton, 1965).

The photosensitive material involved in this step is called "photosystem II". It contains the chlorophyll pigment a680 responsible for the electron transfer to the Q-molecules, where 680 is the approximate wavelength (in 10-9 m), for which the light absorptance peaks. It is believed that the initial absorption of solar radiation may be affected by any of the chlorophyll pigments present in the thylakoid and that a number of energy transfers take place before the par-

ticular pigment, which will transport an electron from H2O to Q, receives the energy (Seliger and McElroy, 1965).

The gross absorptance as a function of wavelength is shown in Fig. 3.72 for green plants and, for comparison, a purple bacterium. The green plant spectrum exhibits a peak just above 400 x 10-9 m and another one a little below 700 x 10-9 m. The bacterial spectrum is quite different with a pronounced peak in the infrared region around 900 x 10-9 m. The positions of the peaks are somewhat different for the different types of chlorophyll (a, b, c,...), and they also move if the chlorophyll is isolated from its cellular surroundings. For a given chlorophyll type, different pigments with different absorption peak positions result from incorporation into different chemical environments (notably bonding to proteins).

Returning to Fig. 3.71, the electrons are transferred from the plastoquinone to cytochrome f by a redox reaction, which transfers a corresponding number of protons (H+) across the thylakoid membrane. The electrons are further transferred to plastocyanine, which leaves them to be transported to another molecule of unknown structure, the X-protein. The energy required for this process is provided by a second photo-excitation step, in what is called "photosystem I", by means of the chlorophyll pigment a700 (spectral peak approximately at 700 x 10-9 m).

The electrons are now delivered to the outside region where they are picked up by ferredoxin, a small protein molecule situated at the outer side of the thylakoid membrane system. Ferredoxin may enter a number of different reactions, of which a very important one transfers the electrons via flavoprotein to nicotinamide-adenine dinucleotide phosphate (NADP), which reacts with protons to form NADPH2, the basis for the carbon dioxide assimilation. The protons formed by (3.36) and by the cytochrome /-plastoquinone reduction are both formed inside the membrane, but they penetrate the thylakoid membrane by a diffusion process, regulated by the reaction (3.38) described below. The NADPH2-forming reaction may then be written

which describes the possible fate of electrons and protons formed by the process (3.36), after transport by the sequence shown in Fig. 3.71 and proton diffusion through the membrane.

The transport of protons from outside the thylakoid membrane to its inside space, by means of the plastoquinone-cytochrome / redox cycle, creates an acidity (pH) gradient, which in turn furnishes the energy necessary for the phosphorylation of adenosine diphosphate (ADP) into adenosine triphosphate (ATP),

This process, which involves the action of an enzyme, stores energy in the form of ATP (4.8 x 10-20 J per molecule), which may later be used to fuel energy-demanding processes, e.g. in the chloroplast stroma or in the cytoplasm outside the chloroplast (see e.g. Douce and Joyard, 1977). It is the analogue of the energy-stocking processes taking place in any aerobic cell of plants and animals (i.e. cells using oxygen), as a result of the degradation of food (saccha-rides, lipids and proteins), and associated with expenditure of oxygen (the respiratory chain, the Krebs cycle; see e.g. Volfin, 1971). The membrane system involved in the case of food metabolism is the mitochondrion.

By means of the energy stored in ATP and the high reducing potential of NADPH2, the CO2 assimilation process may take place in the stroma of the chloroplasts independently of the presence or absence of solar radiation. The carbon atoms are incorporated into glyceraldehyde 3-phosphate, which forms the basis for synthesis of glucose and starch. The gross formula for the process leading to the synthesis of glyceraldehyde 3-phosphate in the chloroplasts (the Benson, Bassham and Calvin cycle; cf. Douce and Joyard, 1977) is

POCH2CH(OH)CHO2- + 9ADP3- + 8HPO42- + 6NADP + 9H+. (3.39)

The reactions (3.36) to (3.39) may be summarised,

3CO2 + 2H2O + HPO42- + light ^ POCH2CH(OH)CHO2- + 3O2. (3.40)

Going one step further and including the synthesis of glucose, the classical equation of photosynthesis is obtained,

6H2O + 6CO2 + 4.66 x 10-18 J ^ C6H12O6 + 6O2, (3.41)

where 4.66 x 10-18 J is the net energy to be added by solar radiation.

Efficiency of conversion

The basis for energy utilisation has traditionally been the undifferentiated biomass produced. Other possibilities would include the direct dissociation of water by the action of sunlight (photolysis), or the formation of hydrogen rather than NADPH2, after the initial oxygen formation (3.36) and electron transport to ferredoxin (Benemann and Weare, 1974; Mitsui and Kumazawa, 1977),

In both cases the problem is to control the recombination process,

so that this energy-releasing process takes place where desired and not immediately on the formation of hydrogen. The plants accomplish this by means of the thylakoid membrane and later by the chloroplast external membrane. Man-made processes may attempt to copy the membrane principle in various ways (Broda, 1975; Calvin, 1974, 1977; cf. Chapter 4), but it is not clear that present suggestions will allow separation of hydrogen and oxygen on a large scale. An advantage in this connection may lie in using the green plants to perform the first step (3.36) and transport electrons and protons (hydrogen ions) through their membranes, but to prevent the energy from being too deeply trapped in organic matter, for example, by adding a strongly reducing agent to ferredoxin, as well as suitable enzymes, in order to accomplish the reaction (3.42) at this stage.

The maximum theoretical efficiency of this process is the ratio between the heat release 9.47 x 10-19 J in (3.43) and the solar energy input required. The latter depends on the absorption properties of the plant and its chlorophyll molecules (cf. Fig. 3.72), as well as on the number of light quanta required for each molecular transformation (3.36). The energy, E, of each light quantum of wavelength X may be found from

The minimum number of quanta required to transport one electron as depicted in Fig. 3.71 is two, one of wavelength 680 x 10-9 m and the other of 700 x 10-9 m (although these need not be the quanta originally absorbed). Since (3.36) requires the transport of four electrons, the minimum requirement would be 8 quanta with a total energy of 2.3 x 10-18 J. Experimental estimates of the number of quanta needed typically give values between 8 and 10.

The efficiency of the photosynthetic process containing only the steps (3.36) and (3.42) may be written n = nx ngeom nchem , (3.44)

where nX is the fraction of the frequencies in the solar spectrum (depending on cloud cover, etc.) that is useful in the photosynthetic process, ngeom is the geometrical efficiency of passing the incoming radiation to the chlorophyll sites (depending on penetration depth in leaves, on reflectance from outer and inner surfaces and on absorption by other constituents of the leaves), and nchem is the efficiency of the photochemical reactions, the maximum value of which is given by nchem < 9.47 x 10-19 / 2.30 x 10-18 = 0.41.

This efficiency pertains to the amount of internal heat produced by (3.43). Only a part of this can be converted into useful work. This part is obtained by replacing in (3.43) the enthalpy change 9.47 x 10-19 J by the change in free energy, AG = 7.87 x 10-19 J (cf. section 4.1.1). In this way the efficiency of the photochemical reaction becomes nchem,free < °.34.

The efficiency associated with the chlorophyll absorption spectrum typically lies in the range 0.4-0.5 (Berezin and Varfolomeev, 1976), and the geometrical efficiency ngeom may be around 0.8 (for the leaf of a green plant, not including the reduction associated with the penetration of radiation through other vegetation, e.g. in a forest environment).

The overall maximum efficiency of about n ~ 0.14 found here for a hypothetical hydrogen (H2)-producing system is valid also for actual green plants which assimilate CO2. Over extended periods of time, the biomass production efficiency will have to incorporate still another factor, nresp, expressing the respiration losses associated with the life-cycle of the plant, n = n' nresp. (3.45)

The respirative energy losses emerge as heat and evaporated water, at rates depending on temperature and wind speed (Gates, 1968). The value of nresp is 0.4-0.5 for land plants and somewhat larger for aquatic plants and algae.

Actual plants may get close to the theoretical maximum efficiency, although the average plant does not. For the blue-green alga Anacystis nidulans, Goed-heer and Hammans (1975) report an energy efficiency of nchem ~ 0.30, based on 36 h of growth, including 6 h of irradiation, with a generous supply of nitrogen and other nutrients, as well as a CO2-enriched atmosphere (i.e. 73% of the maximum energy efficiency calculated above).

It is estimated that, on average, each CO2 molecule in the atmosphere becomes assimilated in a plant once every 200 years and that each O2 molecule in the atmosphere is "renewed" through a plant once every 2000 years (Seliger and McElroy, 1965).

Bacterial photosynthesis

Several bacteria use solar radiation to dissociate a compound of the general form H2X, with a net reaction scheme of the form

(Van Niel, 1941). Here (CH2O) should be understood not as free formaldehyde, but as part of a general carbohydrate compound in analogy to the more precise equation (3.40). Actually, (3.46) was proposed to be valid for both green plant and bacterial photosynthesis, but there is no detailed analogy, since the bacterial photosynthesis has been found to take place in a single step, resembling the photosystem I of the green plant two-step process. Other photo-induced reactions do take place in bacteria, connected with the ATP formation, which in this case is not a side-product of the primary photosyn-thetic process.

The compound H2X may be H2S (sulphur bacteria), ethanol C2H5OH (fermentation bacteria), etc. Most photosynthetic bacteria are capable of absorbing light in the infrared region (wavelength 800-1000 X 10-9 m). The role which the

NADP-NADPH2 cycle (3.37) and (3.39) plays for green plant photosynthesis is played by NAD (nicotinamide-adenine dinucleotide)-NADH2 for photosyn-thetic bacteria. The redox potential of NADH2 is more or less the same as that of the initial compounds, e.g. H2S, so practically no energy is stored in the process of bacterial photosynthesis (Hind and Olson, 1968).

0 0

Post a comment