|Renewable biological systems for alternative sustainable energy production. (FAO Agricultural Services Bulletin - 128) (1997)|
|Chapter 2 - Energy conversion by photosynthetic organisms|
|2.2 Photosynthesis mechanisms|
Biological energy conversions can be categorized into two groups: i) photosynthesis (the process whereby solar energy is fixed to yield energy useful to organisms and industry), and ii) biomass conversion (the product of photosynthesis) into energy. Photosynthesis occurs in plants, algae and photosynthetic bacteria, while biomass conversion reactions often occur in non-photosynthetic microorganisms. This Chapter focuses on photosynthetic processes.
Photosynthesis is often regarded as a CO2 anabolic reaction, whereby glucose is formed from CO2 and water. CO2 anabolism is an energy-consuming reaction in that it utilizes chemical energy produced by photosynthesis. In its narrowest sense, photosynthesis can be regarded as a process whereby energy is supplied for CO2 anabolism. In a broader sense, photosynthesis, including CO2 anabolism, can be divided into several steps: i) photoelectric charge isolation using photon energy (conversion to electrical energy), ii) fixation of electrical energy in the form of chemical energy (ATP synthesis), and iii) chemical reactions involving ATP (fixation of CO2, and hydrogen production).
The supply of energy for CO2 anabolism is common to all photosynthetic organisms which exhibit photosynthesis. Energy conversion, ATP synthesis and the production of both CO2 and hydrogen on the other hand, are not unique to photosynthetic organisms, but occur in all types of microorganisms, and are in fact similar to the respiratory processes which occur in mitochondria of higher organisms.
Two types of photosynthesis are distinguishable on the basis of source of the electrons used as energy carriers. In plants such as green algae, and cyanobacteria (blue-green algae), water is the electron source, while in photosynthetic bacteria, organic or sulfur compounds provide electron sources.
Photosynthetic mechanisms which occur within plant photosynthetic membranes are schematically presented in Figure 2-1. Two Photosystem II water molecules are initially decomposed by four incident photons, to yield one oxygen molecule and four excited electrons. Excited electron energy is subsequently utilized in ATP synthesis. Unlike in the case of ordinary chemical reactions, ATP synthesis cannot be stoichiometrically analyzed (2). The ratio of excited photons to ATP produced is still a somewhat debatable issue. Although it has generally been thought that two photons give rise to the formation of two ATP molecules, some researchers claim that three photons are involved (3). Furthermore, other researchers have suggested a loose coupling between proton transport and ATP synthesis (4, 5):
4 photons + 2H2O + 2(or more)ADP = 2(or more)ATP + 4H+ + 4e-+O2
Subsequent to their energy release in ATP production, photosystem II electrons are transported to photosystem I, where they are again excited to a higher energy level, allowing them to be utilized for NADP reduction. NADP serves both as an electron carrier and an oxidizing and reducing agent in vivo. Two photons are utilized per molecule of NADP reduced:
4 photons + 4e- + 2NADP + 4H+ = 2NADPH.
4 photons + 2(or more)ADP = 2(or more)ATP.
CO2 + 3ATP + 2NADPH = CH2O + 3ADP + 2NADP.
CO2 + 10 photons + H2O = CH2O + 1/2O2.
Experimental data indicates that between 8 and 12 photons are required for fixation of one molecule of CO2. Since the energy equivalent of one photon (700 nm) is approximately 170 kJ/E, and the change in free energy during the fixation of CO2 is approximately 450 U/mol, the energy efficiency of this process for monochromatic light of a wavelength of 700 nm is estimated to be approximately 21-33%. However, owing to the quantum nature of photosynthetic reactions, energy efficiency decreases if light of shorter wavelengths (i.e. higher quantum energy) is used. Additionally, energy losses, energy requirements for plant growth, and the distribution of solar energy wavelengths need to be considered.
Plant photosynthesis takes place only in the presence of visible light (400-700 nm). However, solar light contains both visible and infrared components. Since visible light accounts for about 45% of all solar energy, the maximum achievable energy efficiency for CO2 fixation using solar radiation is approximately 13%.
Bacterial photosynthesis is thought to be a relatively old form of photosynthesis. It incorporates the use of either organic or sulfur compounds as electron donors in photosystem I (Figure 2-2). Unlike in the case of plant photosynthesis, cyclic photophosphorylation takes place in bacterial photosynthesis, i.e. electrons are repeatedly excited in a cyclic manner, with ATP being generated in each cycle. Photosynthetic bacteria are also capable of reducing electron carriers such as NAD, via a linear reaction similar to the electron transmission which occurs during plant photosynthesis (Figure 2-2).
CO2-fixing reactions do not produce energy during bacterial photosynthesis (i.e equimolar amounts of organic compounds are produced through decomposition of organic compounds), except when sulfur compounds serve as electron carriers. The energy conversion efficiency for this type of photosynthesis is more fully described in Chapter 5.
2 photons + 1 (or more)ADP = 1 (or more)ATP.
CH2O + H2O = 4e- + 4H+ + CO2.
The structure of the photosynthetic reaction center (RC), involved in the early steps of photosynthesis, has been elucidated for certain photosynthetic bacteria (Fig. 2-3). Such chlorophyll- containing bacteria which include Rhodopseudomonas viridis and Rhodobacter sphaeroides, show similarities with respect to the arrangement of chlorophyll, and the three-dimensional structures of major portions of the proteins possessing that pigment. Such structural similarities between photosynthetic bacteria, seem to suggest the acquisition of an optimal structure by these bacteria, over a long evolutionary period.
Pigments such as bacteriochlorophyll are also present within the RC. Photoelectric charge isolation takes place within dimers of these bacteriochlorphyll pigments, resulting in the release of high-energy electrons, via the action of bacteriochlorophyll monomers such as bacteriopheophytin, quinone A, and quinone B. These high-energy electrons are subsequently conjugated with proton transportation in the cytochrome b/c1 complex.
A noteworthy feature of the RC function is that photon involvement in photoelectric charge isolation, resembles that which occurs in photo-semiconductors. These RC centres can thus be regarded as molecular elements produced by nature. The fact that photoelectric charge isolation is observed in protein molecules will greatly influence future research relevant to molecular elements and solar batteries.