1.2. How life generates order[[Student version, December 8, 2002]] 9
Figure 1.2: (Diagram.) (a)Energy budget of Earth’s biosphere. Most of the incident high-quality energy gets
degraded to thermal energy and radiated into space, but some gets captured and used to create the order we see in
life. (b)What plants do with energy: High-quality solar energy is partly used to upgrade low-energy molecules to
high-energy molecules, and the ordered structures they form; the rest is released in thermal form. (c)What animals
do with energy: The high-quality energy in food molecules is partly used to do mechanical work and create ordered
structures; the rest is released in thermal form.
this energy is just reflected into space. The rest leaves when the Earth radiates it away as thermal
energy to the rest of the Universe. Thus Earth constantly accepts energy from the Sun, a very hot
body, and exports it as radiation at its own surface temperature. On a dead rock like the Moon,
this is the whole story. But, as depicted symbolically in Figure 1.2b,c, there is a more interesting
possibility.
Suppose that the incoming energy flux is of higher “quality” than the outgoing flux, and hence
represents a netflow of orderinto the Earth (Chapter 6 will sharpen this statement). Then we can
imagine some enterprising middleman inserting itself in the middle of this process andskimming
off some of the incoming flux of order,using it to create more and better middlemen! Looking only
at the middle layer, it wouldseemas though order were magically increasing. That is,
The flow of energy through a system can leave behind increased order. (1.6)This is life’s big trick. The middle zone is our biosphere; we are the middlemen.^3 Green plants
ingest a high-quality form of energy (sunlight), passing it through their bodies to exit as thermal
(^3) Asecond, largely independent, biosphere exists in hot ocean vents, fuelled not by the Sun but by high-energy
chemicals escaping from inside the Earth.