No worries, I am used to such interactions. Dr. Hunt actually helped me to focus my attention on certain important issues, for which I am immensely grateful.
Chemical reactions in cells are accelerated due to enzymes. Morowitz fell within the metabolism-first camp, so he believed enzymes came later in the game. A nice book on his ideas is The Origin and Nature of Life on Earth (ONLE). Here is a helpful quote to understand his thinking:
The common feature of ionized air in a lightning bolt and wall storms in a hurricane is that both create channels to transport currents of matter and energy between two reservoirs at different potential. For lightning the potential is voltage and the current is charge, and for convective weather the potential is temperature and the current is heat. Without lightning or hurricanes, charge or heat could still move by diffusion, but the resistance to their motion through near-equilibrium states is much greater and the transport much slower than through the channel state. We understand well how voltage or temperature differences can drive these non-equilibrium channels to form and stabilize them under perturbations, and we have ways to predict the main features of the channel states from the properties of the systems in which they arise.
Whereas weather is a diffuse phenomenon primarily involving mass transport and physical state change, life creates transport channels in the chemical domain, employing the more concentrated energy flows associated with molecular re-arrangements.
The basic idea is that life started in a non-equilibrium state where the flow of energy and mass through the system caused certain chemical pathways to self-organize similarly to the formation of a funnel cloud. His model, like all others, faces four major challenges:
- Accessing a constant supply of high-energy reactants or other energy source.
- Ensuring the right reactions move forward while blocking deleterious ones.
- Coupling that source of energy to the energetically unfavorable reactions.
- Localizing the essential molecules while blocking out the others.
In relation to the first issue, he proposed several possible sources of energy, but the problem is that raw energy would have driven the system toward greater entropy. Specifically, it would have caused such damage as breaking apart macromolecules. Morowitz comments (ONLE p. 558):
For driven non-equilibrium systems, the situation is far worse. In addition to the constant thermal disruption of microscopic order, the same random reactions by which order is assembled stands ready to degrade it away. Unless a driven system is continually self-amplifying, it cannot even persist. It is as if, in addition to handling the customers, the watchmakers were bedeviled continually by gremlins that disassembled any module not kept in hand.
In relation to the second issue, the metabolism-first model has been severely criticized due to the implausibility of maintaining only life-friendly target reactions without highly specific enzymes. Leslie Orgel provided one of the most comprehensive critiques:
Almost all proposals of hypothetical metabolic cycles have recognized that each of the steps involved must occur rapidly enough for the cycle to be useful in the time available for its operation. It is always assumed that this condition is met, but in no case have persuasive supporting arguments been presented. Why should one believe that an ensemble of minerals that are capable of catalyzing each of the many steps of the reverse citric acid cycle was present anywhere on the primitive Earth, or that the cycle mysteriously organized itself topographically on a metal sulfide surface? The lack of a supporting background in chemistry is even more evident in proposals that metabolic cycles can evolve to “life-like” complexity. The most serious challenge to proponents of metabolic cycle theories—the problems presented by the lack of specificity of most nonenzymatic catalysts—has, in general, not been appreciated. If it has, it has been ignored.
Morowitz also recognized this problem:
James Trefil, Harold J. Morowitz, and Eric Smith, The Origin of Life, American Scientist
Networks of synthetic pathways that are recursive and self-catalyzing are widely known in organic chemistry, but they are notorious for generating a mass of side products, which may disrupt the reaction system or simply dilute the reactants, preventing them from accumulating within a pathway. The important feature necessary for chemical selection in such a network, which remains to be demonstrated, is feedback-driven self-pruning of side reactions, resulting in a limited suite of pathways capable of concentrating reagents as metabolism does.
However, the really serious problem is related to issue 3: directing the energy from some source (e.g. proton gradient) toward driving multiple target reactions which are energetically unfavorable. The solution must involve the continuous production of some energy currency molecule (e.g. ATP) through some sort of molecular/chemical engine, and it must include a host of complex enzymes to couple the reactions. Only enzymes could properly target and couple the right reactions and avoid others.