7.2.1.1 Photosynthesis and Biomass - the solar energy conversion route

Figure 7.2 Photosynthesis and Biomass

Photosynthesis is that conversion process which takes place in green plants (containing chlorophyll), which need supplies of carbon dioxide (CO₂), water (H₂O) and solar energy as inputs in order to deliver organic material, oxygen (O₂) and water (H₂O) as outputs. Simple organic substances are produced first and may then be converted into more complex organic materials, all of which constitute the gross biomass production. The organic substances are partly used for physiological processes and partly stored in the plant tissues. The quantity of stored organic material furnishes the net biomass production of a plant or of the entire vegetation.

 

Photosynthesis is just one of the possible conversion processes which can transform solar energy into biomass, but it is no ordinary process. It is possible to synthesize a similar conversion artificially (using only water, carbon dioxide and energy as inputs and with the same mass output), but not with the same efficiency. Let us analyze and compare the energy efficiency of photosynthesis and of similar artificial conversion processes.

 

 

 

For photosynthesis, the efficiency of chemical transformation is about 1.61%. In a high-tech world in which we are familiar with efficiencies approaching 99%, 1.61% seems very low! Indeed, living organisms operate biologically with processes characterized by a low energy density. But an equivalent synthetic process requires 100 times more energy for the same output of organic material! It therefore seems that photosynthesis is the most suitable route to convert solar energy into biomass, and that biomass is the most convenient form to store this converted solar energy.

Hence chlorophyll, CO₂, H₂O and solar energy are the basic requirements of the photosynthesis process, which is responsible for biomass production. The available solar energy, the local rainfall etc. set the abiotic environment, which determines the vegetation types living in that specific physical environment. There are two categories of plants, so-called C₃ type and C₄ type. The efficiency of photosynthesis is greater for C₄ type than for C₃ type plants. Consequently, in a given abiotic environment, different plants provide different biomass quantities (net biomass production). The CO₂ in the atmosphere is the essential source of Carbon for the creation of Biomass. Hence there is a balance between the absorption of CO₂ in photosynthesis and its release when Biomass energy is released (usually by combustion). Biomass is therefore a CO₂ neutral fuel and its use does not contribute to global warming.