Photosynthesis, thermodynamics

These membranes mimic natural photosynthesis except that the electrons are directed to form hydrogen. Several sensitizers and catalysts are needed to complete the cycle, but progress is being made. Various siagle-stage schemes, ia which hydrogen and oxygen are produced separately, have been studied, and the thermodynamic feasibiHty of the chemistry has been experimentally demonstrated. 

Why Do We Need to Know This Material The topics described in this chapter may one day unlock a virtually inexhaustible supply of clean energy supplied daily by the Sun. The key is electrochemistry, the study of the interaction of electricity and chemical reactions. The transfer of electrons from one species to another is one of the fundamental processes underlying life, photosynthesis, fuel cells, and the refining of metals. An understanding of how electrons are transferred helps us to design ways to use chemical reactions to generate electricity and to use electricity to bring about chemical reactions. Electrochemical measurements also allow us to determine the values of thermodynamic quantities. 

We cover each of these types of examples in separate chapters of this book, but there is a clear connection as well. In all of these examples, the main factor that maintains thermodynamic disequilibrium is the living biosphere. Without the biosphere, some abiotic photochemical reactions would proceed, as would reactions associated with volcanism. But without the continuous production of oxygen in photosynthesis, various oxidation processes (e.g., with reduced organic matter at the Earth s surface, reduced sulfur or iron compounds in rocks and sediments) would consume free O2 and move the atmosphere towards thermodynamic equilibrium. The present-day chemical functioning of the planet is thus intimately tied to the biosphere. 

The solid state and the surface chemistry of some of the solid Fe-phases impart to these oxides and sulfides the ability to catalyze redox reactions. Surface complexes and the solid phases themselves acting as semiconductors can participate in photoredox reactions, where light energy is used to drive a thermodynamically unfavorable reaction (heterogeneous photosynthesis) or to catalyze a thermodynamically favorable reaction (heterogeneous photocatalysis). 

The reduced compounds produced from the oxidation of organic matter are thermodynamically imstable in the presence of O2. Their ensuing oxidation by O2 regenerates the oxidized species (NO3, SO4 , and CO2) needed to reinitiate the cycle. Water is also regenerated and, hence, is made available to participate in photosynthesis. 

The most famous example of nature manipulating thermodynamics is probably the photosynthesis reaction, without which most life forms would not be present on the Earth. Thermodynamically, CO2 + H2O are much more stable than carbohydrate + O2. However, nature can convert CO2 + H2O to carbohydrate + O2 through the photosynthesis reaction ... 

The first step (Reaction 1-120) produces the highly reactive O radical, which can either recombine to form O2, or react with O2 to form O3. Somehow a significant fraction (more than the equilibrium fraction) goes to O3, often with the help of molecules such as N2. Hence, the photochemical production of ozone is another example of nature manipulating thermodynamics and kinetics to produce something that "should not be there," similar to photosynthesis. 

The reasons for isotope discrimination are isotope effects which are caused by both kinetic and thermodynamic factors. Especially the kinetic isotope effect during primary C02-fixation in photosynthesis is relevant for the source-specific discrimination of compounds from C3 and C4 plants. 

The natural cycles of the bioelements carbon, oxygen, hydrogen, nitrogen and sulphur) are subjected to various discrimination effects, such as thermodynamic isotope effects during water evaporation and condensation or isotope equilibration between water and CO2. On the other hand, the processes of photosynthesis and secondary plant metabolism are characterised by kinetic isotope effects, caused by defined enzyme-catalysed reactions [46]. 

Photosynthesis occurs only in plants, algae, and some bacteria, but all forms of life are dependent on its products. In photosynthesis, electromagnetic energy from the sun is used as the driving force for a thermodynamically unfavorable chemical reaction, the synthesis of carbohydrates from C02 and H20 (Equation E9.1). 

Our earth is limited but not closed. Nearly 100% energy source of living creatures depends on photosynthesis, including the fossil fuels that are the main energy resource of human life. This is the important point for our future that our earth is open to the universe, more correctly to the sun. If the earth is closed no animals and plants can exist. The first law of thermodynamics designates that the... 

In natural waters organisms and their abiotic environment are interrelated and interact upon each other. Such ecological systems are never in equilibrium because of the continuous input of solar energy (photosynthesis) necessary to maintain life. Free energy concepts can only describe the thermodynamically stable state and characterize the direction and extent of processes that are approaching equilibrium. Discrepancies between predicted equilibrium calculations and the available data of the real systems give valuable insight into those cases where chemical reactions are not understood sufficiently, where nonequilibrium conditions prevail, or where the analytical data are not sufficiently accurate or specific. Such discrepancies thus provide an incentive for future research and the development of more refined models. 

---