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The laws of thermodynamics are a set of four statements that describe how energy is transformed and how it is transmitted from one system to another or between a system and its environment. These laws are of immense importance for science, since they help us understand the reason why many of the phenomena that we see every day occur.
In this sense, no phenomenon is more special and impressive than life itself, and it does not escape the laws of thermodynamics. Next, we will explore how these laws apply to biological systems, and how they have helped us understand everything from the simplest processes, such as passive diffusion through a membrane, to the complex machinery that allows us to transform our food into energy to maintain life. life.
The laws of thermodynamics are four:
- Zero law.
- The first law of thermodynamics.
- The second law of thermodynamics.
- The third law of thermodynamics.
However, of the four laws, the zeroth law is relatively trivial and the third law has few direct applications in biology, so in this article we only cover the zeroth law and the third law superficially.
Thermodynamic systems in biology
To fully understand thermodynamics in general, one must begin by understanding what a thermodynamic system is. This refers to the portion of the universe that we are studying. The rest of the universe that is not part of the system is called the environment.
Depending on the characteristics of their walls or the border between the system and the environment, the systems can be isolated, closed or open. Biological systems are generally open systems that allow the passage of both energy and matter from the environment to the system and vice versa.
the zero law
The zeroth law has to do with thermal equilibrium, that is, the condition in which two bodies that are in thermal contact do not exchange heat with each other. This law can be stated as follows:
Two systems in thermal equilibrium with a third are also in thermal equilibrium with each other.
This is illustrated in the following figure. If systems A and B are in thermal equilibrium and systems B and C are also in thermal equilibrium, then systems A and C must be in thermal equilibrium.
Application of the zeroth law in biological systems
As we have just observed, the zeroth law allows us to establish when two systems are in thermal equilibrium. We apply this law without realizing it every time we take a temperature measurement with a thermometer.
For example, if we leave the thermometer in contact with the inside of our mouth (which is a biological system), thermal equilibrium will eventually be reached between the glass of the thermometer and the mouth. However, when reading the temperature thanks to the mercury inside, we assume that the mercury will also be in thermal equilibrium with the mouth, despite not being in direct contact with it.
However, since the mercury is in contact with and is in thermal equilibrium with the glass, and the glass is in thermal equilibrium with the mouth, then the zeroth law states that the mercury must also be in thermal equilibrium with the mouth.
The first law of thermodynamics
The first law is the law of conservation of energy. This states that the energy in the universe is constant. It is neither created nor destroyed, it is only transformed . This means that no process can ever occur within any system (whether biological or not) in which the system gains energy of some kind without the environment losing it.
This law has a very simple mathematical form which is:
where U represents the internal energy of the system, q is the amount of heat that enters the system, and w is the amount of work the system transmits to the surroundings. In some cases, the work is written with a positive sign, but it is replaced by the work that the environment does on the system; in any case, the meaning of both equations is exactly the same.
Application of the first law in biological systems
It is very easy to understand the application of the first law to biological systems of any size, from a small bacterium, to a human being, to a giant sequoia. It is simply a balance of energy.
Example of the application of the first law in biological systems
We can see our food as sources of energy, the “calories” we eat. Body fat, which is one of the ways the body stores energy, represents the internal energy level, while w, the work the system does, is exercise. Seen this way, the first law gives us a very simple explanation to understand why we get fat. Whenever we eat food, that is, calories, if we do not burn them by exercising to return them to the environment, then these are going to be stored in the form of internal energy, that is, in the form of body fat.
Anyone who wants to lose weight must make sure that q (what they eat) is less than w (the energy they spend exercising and developing their vital functions).
This law allows us to clearly establish which processes are possible and which are impossible. Losing weight by eating more calories than we burn is simply impossible, no matter how much they want to convince us of it.
The second law of thermodynamics
The second law states that, in any natural or spontaneous process, part of the internal energy is always lost in the form of heat. This explains why a ball that is released from a certain height and is allowed to bounce each time it reaches a lower height, until it ends up at rest on the ground.
If we go by the first law, the potential energy that was originally stored in the ball had to have gone somewhere. The second law establishes that this energy is dissipated in the form of heat towards the surroundings.
Application of the second law in biological systems
The second law has many implications for biology and biological systems. However, to understand how it applies to this branch of science, we must first understand the concepts of entropy and Gibbs free energy, and how they relate to the second law.
entropy
Whenever you talk about the Second Law, you talk about entropy, a physical concept represented by the letter S. Entropy was originally discovered as a state function whose change during a thermodynamic process is a measure of the amount of heat dissipated during this process. However, a scientist named Ludwig Boltzmann discovered that entropy is actually a measure of the disorder of a system.
Through various mathematical manipulations, it was concluded that the second law could be stated in terms of the entropy change of the universe (ΔS U ) as follows:
Every natural or spontaneous process necessarily implies an increase in the entropy of the universe .
That is to say, that entropy and the Second Law provide us with a tool to predict when a process will be spontaneous and when it will not. Furthermore, it gives us an explanation about the trend of all processes in the universe since the Big Bang . We could say that everything that happens in the universe today is aimed at dissipating in the form of heat all the energy that was released during the formation of the universe.
Gibbs free energy
On a practical level, the second law is applied to biological systems by means of another state function called the Gibbs free energy, represented by the letter G. As its name indicates, this consists of the maximum amount of energy that a system is free. to use to do a job other than expansion. This is particularly relevant in biology and biochemistry, as it includes work on processes such as diffusion across membranes (whether active or passive), all enzyme-catalyzed reactions, electrochemical processes (including action potentials in neurons and muscle cells), etc.
The importance of the Gibbs energy is that, under the normal conditions in which life and biological processes occur, the change in the Gibbs free energy, that is, ΔG, is directly related to the change in the entropy of the universe. (ΔS U ), in such a way that if we know the sign of ΔG, then we can infer the sign of ΔS U , so we can use it as a criterion of spontaneity for chemical reactions and other processes that occur within the cells of our body.
The spontaneity criteria are summarized in the following table:
| sign of ΔG | sign of ΔS U | spontaneity of the process |
| ΔG > 0 (positive) | ΔS U < 0 (negative) | spontaneous process |
| ΔG < 0 (negative) | ΔS U > 0 (positive) | non-spontaneous process |
| ΔG = 0 | ΔS U = 0 | System in thermodynamic equilibrium |
The coupling of biochemical reactions
Processes that have a negative free energy change and are therefore spontaneous release energy and are therefore called exergonic or exothermic processes. On the other hand, those with a negative ΔG are not spontaneous, they absorb energy and are called endergonic or endothermic.
Simply put, spontaneous processes release energy naturally, while non-spontaneous processes cannot occur spontaneously unless the free energy required for them to occur is provided. This means that a spontaneous reaction can be used to provide the energy needed for a non-spontaneous reaction to occur.
To understand this better, let’s imagine a car that is at the base of a mountain. It would be very rare to see him spontaneously climb the mountain with the engine off and without any help. However, when you start the engine, the combustion of gasoline or the flow of electricity spontaneously release large amounts of energy, energy that is used to turn the wheels and propel the car up. In this way, a spontaneous process was coupled with a non-spontaneous one.
Example of the application of the second law in biological systems
The most important example of the application of this law to biological systems is the use of ATP as an energy source to drive most of the biochemical reactions that keep life going.
The hydrolysis of ATP is a strongly exothermic process (as is the combustion of gasoline in the previous example). Enzymes inside cells use this and other spontaneous hydrolysis reactions to release the energy they need to drive other biochemical reactions essential to life, such as protein and nucleic acid biosynthesis.
The third law of thermodynamics
The third law (or third principle) states that any system tends to lose entropy as the temperature decreases, and that it reaches that minimum at absolute zero. For the case of perfect monatomic crystalline solids, the entropy at absolute zero is zero.
This law allows us to understand entropy as an absolute scale, and also allows us to determine the value of the absolute entropy of any substance in any set of temperature and pressure conditions.
Application of the third law in biological systems
The usefulness of this law is that it allows us to have a direct measure of the true level of disorder of different chemical substances under different conditions, and greatly facilitates the theoretical calculation of entropy variations (and by extension, free energy). de Gibbs) for any chemical reaction, including biochemical reactions that occur in biological systems.
References
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Khan Academy. (2017). The laws of thermodynamics (article). Retrieved from https://es.khanacademy.org/science/ap-biology/cellular-energetics/cellular-energy/a/the-laws-of-thermodynamics
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