ABSTRACT
Photosynthesis is the making (synthesis) of organic structures and chemical energy stores by the action of solar radiation (photo). It is by far the most important renewable energy process, because living organisms are made from material fixed by photosynthesis, and our activities rely on oxygen in which the solar energy is mostly stored. For instance, human metabolism continuously releases about 150W per person from food. Thus, both the materials and the energy for all life are made available in gases circulating in the Earth’s atmosphere, namely carbon dioxide and oxygen. Sadly, despite photosynthesis being a physically induced process and the driving function of natural engineering, the subject is missing from most physics and engineering texts. This chapter tries to rectify this omission by describing a cheap process that provides abundant stored energy – an engineer’s dream, but a natural phenomenon. The continuous photosynthetic output flux on the Earth is about 09×
1014W (i.e. about 15 kW per person; the power output of 100 000 large nuclear power stations). This chapter discovers how the process occurs within molecules and cells, and how eventually it may be utilised at these levels. Energy supply from plant and animal materials, biomass, is discussed in Chapter 11. Solar radiation incident on green plants and other photosynthetic organisms relates to two main effects: (1) temperature control for chemical reactions to proceed, especially in leaves, and (2) photo excitation of electrons for the production of oxygen and carbon structural material. It is so important to maintain leaf temperature in the correct range that some solar radiation is reflected or transmitted, rather than absorbed (this is why leaves are seldom black). The energy processes in photosynthesis depend on the photons (energy packets) of the solar radiation, labelled ‘h ’, where h is Planck’s constant and is the frequency of the radiation. The organic material produced is mainly carbohydrate, with carbon in a medium position of oxidation and reduction (e.g. glucose, C6H12O6). If this (dry) material is burnt in oxygen, the heat released is about 16MJkg−1 (4.8 eV per carbon atom, 460 kJ per mole of carbon). The fixation of one carbon atom from
atmospheric CO2 to carbohydrate proceeds by a series of stages in green plants, including algae:
1 Reactions in light, in which photons produce protons from H2O, with O2 as an important by-product, and electrons are excited in two stages to produce strong reducing chemicals.
