Biomass as a Source of Energy

The peculiar feature of plants is that they possess various photosynthetic pigments in thylakoids present in cells either in free state or in chloroplasts. The photosynthetic pigments are chlorophyll a, chlorophyll b, chlorophyll c, xanthophylls and carotenoids. The presence of these pigments varies with group of the plants. In prokaryotic photoautotrophs where chloroplasts lack, photosynthetic pigments are present in thylakoids. The different pigments absorb light of different wavelength. During photosynthesis in plant cells, in the presence of chlorophyll CO₂ is converted into complex carbohydrates with the evolution of oxygen. The reaction is shown below:

	light
CO2+H2O		(CH2O)n+O2
	chlorophyll

During photosynthesis, solar energy is trapped into light harvesting molecules in the chloroplasts by reduction of CO₂ into carbohydrates, fats and proteins. Radiant energy stored in plant is known as primary production which later on creates plant biomass or biomaterial. The rate of storage of photosynthetic products is known as 'primary productivity'. The energy remaining as organic matter (after respiration) is called as net primary productivity (NPP). N.P.P. is expressed as KCal or g/m²/yr. NPP accumulates over time as plant biomass. It is expressed as dry weight of organic matter per unit area (g/m²). Biomass differs from production, which is the rate of organic matter production by photosynthesis. N.P.P. and biomass of some world ecosystems are given in Table 19.2. The biomass is all forms of matter derived from biological activities and present on the surface of soil or at different depth of vast body of water, lakes, rivers, sea and ocean.

Biomass includes wood, crops, herbaceous plants, residues from agricultural and forest products, manure, fresh water and marine plants and microorganisms as well. Besides plant material, biomass also includes all animal waste, manure, etc. because in essence, the latter are basically plant based (Khashoo, 1988). Different types of biomass of various sources are given in Table 19.3.

Table 19.2. Net primary productivity and plant biomass of world ecosystems (source: Whittakar and Likens, 1973).
The route of photosynthesis differs in certain group of plants. In C₃ plants, where the first photosynthetic product is 3 carbon compound, CO₂ combines with a 5-carbon compound (ribulose biphosphate, RuBP) to form phosphoglyceric acid (a 3 carbon compound). However, in C₄ Plants, where the first photosynthetic product is a 4 carbon compound, CO₂ combines with phosphoenolpyruvate (a 3 carbon compound), instead of RuBP and produces oxaloacetic acid, a 4 carbon compound.

Moreover, photosynthetic efficiency affects the accumulation of biomass as it depends on plant efficiency and light intensity. We are fortunate enough to have abundant sun-shine in our country. Total energy received in India is about 60 x l0¹³ MWH, with 250-300 days of useful sun shine per year in most part of the country (Anonymous, 1981).

Table 19.3. Biomass as the source of energy.

Net primary productivity and biomass accumulation are high due to the high rate of CO₂ assimilation and low rate of photo-respiration between 25-30°C. Photo-respiration accounts for 50 per cent reduction in efficiency of the process, as it has a rapid rate of photo-respiration and low rate of CO₂ fixation.

Photo-respiration differs from the normal respiration. The later takes place in dark and is associated with oxidation of compounds produced during photosynthesis, which occurs in peroxisomes. Photo-respiratory CO₂ arises from glycolate pathway, not from glycolysis. In this pathway, 4 molecules of glycolates are converted into one molecule of glucose and 2 molecules of CO₂; CO₂ is released when glycine is converted into serine. Glycolate is oxidized to glycolate by enzyme glycolate oxidase.

In recent years, attempts have been made to convert C₃ plants into C₄ ones by introduction of characteristics of C₄ plants in C₃ ones. Regulation of photo-respiration by chemical or genetical manipulation offers a possibility of increasing crop-productivity. Some metabolic inhibitors are reported which catalyze photo-respiration. In 1974, Zelitsch at the Connecticut Agricultural Research Station, New Haven (USA) demonstrated that an analogue of glycolate and glyoxalate, inhibited glycolate synthesis and photo-respiration by 50 per cent, and increased 14 CO₂ assimilation by 50 per cent. In 1973, Peter Carlson at the Brookhaven National Laboratory obtained the mutants of tobacco cells by growing them on nutrient medium supplemented with sulphoxymine. The mutant cells displayed a low photorespiration rate.