Laddas ned direkt. This book includes articles relating to presentations given in a variety of forms lectures, posters, contributions to round tables, software presentations at the 5th International Biothermokinetics Meeting held in Bordeaux-Bombannes, September , The fact that not just lectures were considered for these proceedings reflects the aims of BTK meetings to instigate discussion, promote scientific cooperation and confront as many different ideas as possible with each other at best heretical ones.
BTK conferences have expanded more and more; participants came to the meeting from 20 countries. It was therefore necessary to hold the round tables in parallel sessions. It is difficult to have an unbiased feeling of what should be selected as the salient features of the meeting.
As the name suggests, Biothermokinetics embraces thermodynamic and kinetic approaches to experimental and theoretical investigations of biological processes, in particular at the cellular level. This "classical" point of view is mainly represented in the chapter "Thermodynamics and Kinetics of Transport Processes and Biological Energy Transduction".
Passar bra ihop. Ladda ned. The energy content of the system is thereby partitioned into the entropic term, which is related to the random thermal motion molecular chaos of the molecules that is somehow not available for work, and tends to disappear at absolute zero temperature, and the free energy, which is somehow available for work. But as there need be no entropy generated in adiabatic processes - which occur frequently in living systems see below - the division into available and nonavailable energy cannot be absolute: in other words, the energy associated with a molecule simply cannot be partitioned into the two categories a priori.
The second law of thermodynamics is a statistical law which applies to a system consisting of a large number of particles. Thus, each cell contains only one or two molecules of each sequence of DNA in the nucleus. Similarly, it takes no more than several molecules of a hormone to bind to specific receptors in the cell membrane in order to initiate a cascade of biochemical reactions that alter the characteristics of the whole cell.
Does that mean the second law cannot be applied to living systems? This difficulty is related to the problem of Maxwell's demon 5 - an hypothetical intelligent being who can open a microscopic trapdoor between two compartments of a container of gas at equilibrium in order to let fast molecules through in one direction, and the slow ones in the other, so that work can then be extracted from the system. It became evident in the s that something like a Maxwell's demon could be achieved with little more than a trapdoor that opens in one-direction only and requires a threshold amount of energy activation energy to open it.
This is realizable in solid-state devices such as diodes and transistors that act as rectifiers 5. Similar situations are associated with biological membranes, which play a major role in structuring biological systems. It has recently been demonstrated that weak alternating electric fields can drive unidirectional active transport by this enzyme without ATP being broken down.
In other words, the energy from the electric field is directly transduced into transport work by means of the membrane-bound enzyme. Moreover, randomly fluctuating electric fields are also effective, precisely as if Maxwell's demon were involved in making good use of the fluctuations 6! Of course, there is no real violation of the second law, for rectifiers and biological membranes are both non-equilibrium structures which can store energy. The problem of Maxwell's demon is generally considered as having been 'solved' by Szilard, and later, Brillouin 2 , who showed that the demon would require information about the molecules, in which case, the energy involved in obtaining information would be greater than that gained and so the second law remains inviolate.
Perhaps, what they have failed to take account of is that the so-called information is already supplied by the special structure or organization of the system in which energy is stored. Biological membranes, in particular, are excitable structures poised for relaying and amplifying weak signals into the cell. An organism is nothing if not organized heterogeneity, with nested dynamic structures over all space-time scales.
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There is no homogeneity, no static phase held at any level. Even a single cell has its characteristic shape and anatomy, all parts of which are in constant activity; its electrical potentials and mechanical properties similarly, are subject to cyclic and non-cyclic changes as it responds to and counteracts environmental fluctuations.
Spatially, the cell is partitioned into numerous compartments by cellular membrane stacks and organelles, each with its own 'steady states' of processes that can respond directly to external stimuli and relay signals to other compartments of the cell. Within each compartment, microdomains can be separately energized to give local circuits, and single enzyme proteins, or complexes of two or more proteins function as 'molecular machines' which can cycle autonomously without immediate reference to its surroundings.
In other words, the steady 'state' is not a state at all but a conglomeration of processes which are spatiotemporally organized, ie, it has a deep space-time structure, and cannot be represented as an instantaneous state or even a configuration of states 7. The spatial extent of processes, similarly, span at least ten orders of magnitude from 10 m for intramolecular interactions to metres for nerve conduction and the general coordination of movements in larger animals.
The processes are also catenated in both time and space: the extremely rapid transient flows very short-lived pulses of chemicals or of energy triggered on receiving specific signals, are propagated to longer and longer time domains of minutes, hours, days, and so on via interlocking processes which ultimately straddle generations.
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The processes, rather than constituting the system's 'memory' as we might think, are actually projections into the future at every stage. They determine how the system responds and develops in times to come. Typically, multiple series of activities are initiated from the focus of excitation. While the array of changes in the positive direction is propagating, a series of negative feedback processes is also spreading, which has the effect of dampening the changes.
It is necessary to think of all these processes cascading in parallel in many dimensions of space and time. In case of disturbances which have no special significance for the body, homeostasis is restored sooner or later as the disturbance passes. On the other hand, if the disturbance or signal is significant enough, a series of irreversible events brings the organism to a new 'steady state' by developing or differentiating new tissues. The organism may even act to alter its environment appropriately 8. The secret of 'negentropy' lies undoubtedly in this intricate space-time organization.
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But how can one describe it in terms of the second law? As living systems consist of nested space-time compartments of various sizes, all the way down to microdomains and molecular machines, then at the very least, this implies that if thermodynamics were to apply to living systems, it must apply to individual molecules as much as to ensembles of molecules.
Such is the physiologist Colin McClare's contention 9. In order to formulate the second law of thermodynamics so that it applies to single molecules, McClare introduces the important notion of a characteristic time interval, t , within which a system reaches equilibrium at temperature q. The energies contained in the system can be partitioned into stored energies versus thermal energies. Thermal energies are those that exchange with each other and reach equilibrium in a time less than t so technically they give the so-called Boltzmann distribution characterized by the temperature q.
Stored energies are those that remain in a non-equilibrium distribution for a time greater than t , either as characterized by a higher temperature, or such that states of higher energy are more populated than states of lower energy. So, stored energy is any form which does not thermalize, or degrade into heat in the interval t. Stored energy is not the same as free energy, as the latter concept does not involve any notion of time.
Stored energy is hence a more precise concept. McClare goes on to restate the second law as follows: useful work is only done by a molecular system when one form of stored energy is converted into another. In other words, thermalized energy is unavailable for work and it is impossible to convert thermalized energy into stored energy.
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The above restatement of the second law is unnecessarily restrictive, and possibly untrue, for thermal energy can be directed or channelled to do useful work in a cooperative system, as in the case of enzymes embedded in a membrane 7 , which can undergo correlated motions. Thermalized energy from burning coal or petrol is routinely used to run machines such as generators and motor cars which is why they are so inefficient and polluting. A more adequate restatement of the second law, which can apply to single molecules as well as ensembles of molecules, I suggest, might be as follows 8,10 :.
Useful work can be done by molecules by a direct transfer of stored energy, and thermalized energy cannot be converted into stored energy.
The second half of the statement accounts for entropic decay as is usual in real processes both inside and outside the living system. The first half, however, is new and significant for biology. The major consequence of McClare's ideas arises from the explicit introduction of time, and hence time-structure. For there are now two quite distinct ways of doing useful work, not only slowly according to conventional thermodynamic theory, but also quickly - both of which are reversible and at maximum efficiency as no entropy is generated. But the attention to time-structure makes much more precise what the limiting conditions are.
Let us take the slow process first. A slow process is one that occurs at or near equilibrium. According to classical thermodynamics, a process occuring at or near equilibrium is reversible, and is the most efficient in terms of generating the maximum amount of work and the least amount of entropy. By taking explicit account of characteristic time, a reversible thermodynamic process merely needs to be slow enough for all thermally-exchanging energies to equilibrate, ie, slower than t , which can in reality be a very short period of time, for processes that have short time constants.
Thus, for a process that takes place in 10 s, a microsecond 10 -6 s is an eternity! So high efficiencies of energy conversion can still be attained in thermodynamic processes which occur quite rapidly, provided that equilibration is fast enough. This may be where spatial partitioning and the establishment of microdomains is crucial for restricting the volume within which equilibration occurs, thus reducing the equilibration time. This means that local equilibrium may be achieved at least for some biochemical reactions in the living system. We begin to see that thermodynamic equilibrium itself is a subtle concept, depending on the level of resolution of time and space.
At the other extreme, there can also be a process occurring so quickly that it, too, is reversible.
In other words, provided the exchanging energies are not thermal energies in the first place, but remain stored, then the process is limited only by the speed of light. Resonant energy transfer between molecules is an example of a fast process. It occurs typically in 10 s, whereas the molecular vibrations themselves die down, or thermalize, in 10 -9 s to 10 1 s.
Does resonant energy transfer occur in the living system? McClare 9 suggests it occurs in muscle contraction, where it has been shown that the energy released in the hydrolysis of ATP is almost completely converted into mechanical energy in a molecular machine which can cycle autonomously without equilibration with its environment.
Ultrafast, possibly resonant energy transfer processes are also operating in photosynthesis. There, the first step is the separation of positive and negative charges in the chlorophyll molecules of the reaction centre, which has been identified 11 to be a readily reversible reaction that takes place in less than 10 s. McClare's ideas have been taken up and developed by Gonda and Gray 12 , Blumenfeld 13 , and more recently, Welch and Kell 14 , among many others, particularly in the notion of nonequilibrium, 'quantum molecular energy machines'.
These ideas imply that the living system may use both means of efficient energy transfer: slow and quick reactions, always with respect to the relaxation time, which is itself a variable according to the processes and the spatial extents involved. In other words, it satisfies both quasi-equilibrium and far from equilibrium conditions where entropy production is minimum.
This insight is offered by taking into account the space-time structure of living systems explicitly.