Net eflciency allowing for respiration

The calculated gross energy-storage efficiency of -9% of a green plant can never be achieved in real life because all photosynthetic organisms must constantly consume a portion of their stored energy in the process of respiration to obtain the energy to stay alive. Respiration effectively reverses oxygenic photosynthesis, and so reduces the energy-storage efficiency to a net value below the gross value.
 

There are two types of respiration-dark respiration and photorespiration, the latter occurring only in the light. The rate of dark respiration in green leaves lies in the range 0.5-4.0 mg C02 drn-2 hr-l at 25 C (Zelitch, 1971). This has only a small effect on the efficiency in bright sunlight, subtracting perhaps -0.2% from the gross efficiency. Photorespiration, on the other hand, is responsible for a much more serious loss of fixed carbon. In plants using the normal C3 carbon fixation cycle, photorespiration occurs at such a rate that some 30% (at 25 C) to 40% (at 35 C) of the gross yield of photosynthesis is lost. Plants using the C4 cycle lose rather less.

The combination of dark respiration and photorespiration reduces the calculated maximum net efficiency of photosynthesis to a value between 5.3% at 35 C and 6.2% at 25 C (Bolton, 1979). This agrees well with other estimates: 5.3% (Bassham, 1976); 5.5% (Hall, 1977); 5% (Boardman and Larkum, 1975). This is the expected instantaneous maximum efficiency for a healthy leaf growing in optimal conditions. There are a number of factors, explored in the next section. that reduce time-average values well below this, although short-term values can approach 5%.

Figure 1.12 shows in another way how the upper bound of -5% on the energystorage efficiency of a green plant comes about. Nearly half (47%) of the solar energy incident on a plant is lost because it lies outside the photosynthetically active range of 40&700nm. A further 16% is lost by incomplete absorption of PAR (Photosynthetically Active Radiation) or by its absorption by components other than the chloroplast. A further 9% is lost by thermalisation-the degradation to heat of the ‘excess‘ energy of absorbed photons of wavelength below 700 nm. that is, energy above 1.77 eV, which is the threshold or ‘bandgap’ energy Us of P700. A further substantial loss of 19% arises because the synthesis of D-glucose stores only the fraction (AH/8U8) of the energy of eight thermalised P700* states. That leaves only -5% of the incident solar energy to be stored as chemical energy. This is still a formidable value; the instantaneous energy-storage capability of a green leaf in the Sun leaves most artificial molecular photoconverters of solar energy in the shade.