## Photosynthesis

The basic equation of photosynthesis is

where hv is a photon energy and (CH2O) is a fragment of carbohydrate molecule, releasing 470 kJ mol"1 of energy (that equals an increase in enthalpy, AH). Since the change of free energy is equal to AG = 504 kJ mol"1, and GA = AH— TAS, the change of entropy, AS, is equal to (470 — 504)/273 = 116JK—1mol—1 (T = 293 K), that is, photosynthesis is an antientropic process.

Efficiency of photosynthesis is defined in different ways. Its theoretically maximal value is the ratio of AG to the total energy of eight photons (Eph = 1470 kJ mol"1), which are necessary to get one molecule of O2, ^max = 504/1470 = 34%. On the other hand, since the 'useful' work, which can be performed by photosynthesis, is 'exergy', Ex = - TAS = 34kJmol"1, then efficiency is defined as ^ex = Ex/Eph = 34/1470 = 2.3%.

theoretical value, 34%, and what is more, this efficiency should not exceed the 'exergic' efficiency, 2.3%.

From the thermodynamic point of view, a 'green leaf' is a heat machine with photosynthesis as the working process, and molecules of chlorophyll, adenosine tripho-sphate (ATP) etc., transferring energy of photons into leaves as the working body. Efficiency of the heat machine is ^af = (Teaf - T^/Tlef, where Tkaf and Tair are the mean daily temperatures of leaves and surrounding air; since the reaction of photosynthesis is exogenous, the leaf is warmed, Tleaf> Tair. Under summer conditions in temperate forest, T~ 5 °C and Tair ~ 20 °C on average; therefore, ^leaf = 5/298 = 1.7%.

It is known that about 98-99% of solar energy, reaching the Earth's surface, is reflected from leaves and other surfaces and absorbed by other molecules, which convert it to heat. Thus, vegetation is available to catch about 1-2% of incident solar energy, that is, these numbers constitute its efficiency.

The rate at which plants convert PAR (or inorganic chemical energy) to the chemical energy of organic matter is named gross primary production (productivity) (GPP). This value (as well as biomass) is often reported in grams or metric tons of either dry weight or carbon (the latter is about one-half of the first). Since enthalpy of 1 of carbon is equal to ~42kJg"\ then production and biomass can be also reported in joules.

Fifteen to sixty percent of the energy assimilated by plants immediately is spent in cellular respiration, when carbohydrates, proteins, and fats are broken down, or oxidized, to provide energy (in the form of ATP) for the cell's metabolic needs. The residual (40-85%) is stored in biomass as net primary production (NPP). The highest annual NPP, 2000gCm"2yr"1, occurs in swamps, marshes, and tropical rainforests; the lowest, 20gCm"2yr"1, occurs in deserts. The mean NPP for terrestrial ecosystem is about 400 gC m"2 yr"1. Among aquatic ecosystems, the highest NPP, 2000 gC m"2 yr"1, occurs in estuaries; the mean NPP in the ocean is 75gCm"2yr"1, so that the ocean is a desert (see Table 1). 