ENERGY MIGRATION AND THE PHOTOSYNTHETIC UNIT 1299 



tion of excitation energy from the chlorophyll molecules to the enzyme 

 ("photosynthetic unit"), or by migration to the enzyme of photochemical 

 products formed at the chlorophyll molecules, or by migration of the en- 

 zyme itself, collecting its substrate from hundreds or thousands of chloro- 

 phyll molecules, Hke a bee collects nectar from a plot of flowers. Even if 

 the phenomenon of energy migration were well established, this would not 

 prove that the other two are nonexistent or irrelevant. 



In the above discussion the analogy between "energy conduction" and electron 

 conduction in crystals was mentioned. The kind of resonance required for the diffusion 

 of energy is, however, different from that underlying the diffusion of electrons. The 

 atoms of a metal are held together by electron exchange forces (the same effect that is 

 responsible for the formation of hydrogen molecules, according to Heitler and London). 

 In molecular lattices, the resonance effects are due to excitation energy resonances, as 

 illustrated above by the example of a He2 molecule. In an H2 molecule, the degeneracy, 

 which leads to the bonding, is due to the fact that the two electrons can exchange their 

 places without a change in energy; in He2, formed from an excited He* and a normal 

 helium atom, the degeneracy is due to the fact that the excitation energy may be ex- 

 changed between the two atoms in the molecule. The first kind of resonance, extended 

 to a system of many nuclei, leads to electron conduction; the second kind of resonance 

 leads, in a similar case, to an energy conduction. 



If the electron exchange in a crystal is strong in the ground state, the crystal shows 

 metallic conduction. If only excited atoms (or molecules) easily exchange their elec- 

 trons, the crystal has insulating properties in the dark, and shows photoconductivity in 

 light. Chlorophyll in the chloroplasts is certainly not a metallic conductor; but can 

 it be a -photoelectric conductor? This could lead to a variation of the theory of the 

 photosynthetic unit in which an exchange of electrons between molecules would replace 

 the exchange of excitations. The primary effect of light on the "photosynthetic unit" 

 would then be the same as that of ultraviolet light on an alkali halide crystal: to set an 

 electron free. This electron then could diffuse through the unit until it meets a "reduc- 

 tion center" that absorbs it, the whole process being equivalent to oxidation of a chloro- 

 phyll molecule and reduction of a substrate anchored somewhere else in a "reduction 

 center." However, the similarity between the absorption spectrum of molecularly dis- 

 persed chlorophyll and that of the chlorophyll in the cell seems to preclude the possibility 

 that the excited state of chlorophyll in vivo is a "conductivity state" (which would be 

 entirely different from the excited state in vitro). Consequently this variation of the 

 "photosynthetic unit" (proposed by Katz 1949) must be rejected. 



6. Energy Transfer between Different Pigment Molecules 



Related to the problem of energy transfer between identical molecules 

 (e. g., between several chlorophyll a molecules in the hypothetical "photo- 

 synthetic unit") is that of energy transfer between different molecules, 



