ABSTRACT
Life on Earth is sustained by constant energy input from the sun. Few of us need to be reminded of this, but reviewing some of the numbers and basic principles by which this energy is converted to biologically useful forms is a vital part of any introduction to biological physics. From an astronomical point of view, sunlight hits the Earth, warming it to about 290 K, and the Earth then reradiates just as much power, but in a wavelength region characteristic of that lower-temperature blackbody. Higher-energy ultraviolet (UV) photons can do severe damage to biological systems: cells must be prepared to repair UV-induced damage. The momentum of a photon can be calculated just as the de Broglie wavelength is calculated from a particle’s momentum. Momentum is normally not an important conserved quantity in biological systems because the photon absorber is always embedded in water or another closely packed medium whose atoms can take up any excess momentum.
