Gavin Howe
The second law of thermodynamics states that the entropy of an isolated system always increases over time. This means that when energy is transferred from one form to another, the entropy increases as a result. Living organisms are not closed systems, and therefore the second law of thermodynamics does not apply to them in the truest sense. However, living organisms can gain or lose energy from their external environment, making them open systems. The laws of thermodynamics are important unifying principles of biology and govern the chemical processes in all biological organisms.
What You'll Learn
Living organisms are open systems
The ability of living organisms to obtain and utilize external energy is essential for their growth and development, and maintenance of their highly ordered state. Cells, for example, are highly ordered and have low entropy. However, the processes performed to maintain this order result in an increase in entropy in the organism's surroundings.
Living organisms, as open systems, are not subject to the law of entropy in the same way that closed systems are. While they still experience increases in entropy, they have the informational capability to maintain their structure and functionality, demonstrating that the second law of thermodynamics does not apply directly to living systems.
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Entropy and the second law
The second law of thermodynamics is a physical law based on the universal empirical observation of heat and energy interconversions. It establishes the concept of entropy as a physical property of a thermodynamic system. Entropy is the measure of disorder in a closed system. The second law states that the entropy of an isolated system left to spontaneous evolution cannot decrease – it tends towards a state of thermodynamic equilibrium where entropy is highest. This accounts for the irreversibility of natural processes.
The second law applies to closed systems, and the only known closed system that exists is the entire universe. Living systems are not closed systems, as they can gain or lose energy from the external environment, and are therefore open systems. As such, the second law of thermodynamics does not apply directly to living systems.
Living systems require a constant energy input to maintain their highly ordered state. Cells, for example, are highly ordered and have low entropy. However, the processes performed to maintain this order result in an increase in entropy in the cell's/organism's surroundings. The transfer of energy causes entropy in the universe to increase.
Living organisms contain information in their DNA that allows the system to obtain energy from outside the system. This energy is used to overcome entropy – the spontaneous breakdown of organisation within the cells and systems within the living organism.
The laws of thermodynamics are important unifying principles of biology. They govern the chemical processes (metabolism) in all biological organisms. The first law of thermodynamics, also known as the law of conservation of energy, states that energy can neither be created nor destroyed. It may change from one form to another, but the energy in a closed system remains constant.
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Energy transfer in biological systems
The laws of thermodynamics are important unifying principles of biology, governing the chemical processes (metabolism) in all biological organisms. The First Law of Thermodynamics, also known as the law of conservation of energy, states that energy can neither be created nor destroyed. In a closed system, such as the universe, this energy is not consumed but transformed from one form to another.
The Second Law of Thermodynamics states that when energy is transferred, there will be less energy available at the end of the transfer process than at the beginning. Due to entropy, which is the measure of disorder in a closed system, all of the available energy will not be useful to the organism. Entropy increases as energy is transferred.
Living systems require constant energy input to maintain their highly ordered state. Cells, for example, are highly ordered and have low entropy. In the process of maintaining this order, some energy is lost to the surroundings or transformed. So while cells are ordered, the processes performed to maintain that order result in an increase in entropy in the cell's/organism's surroundings. The transfer of energy causes entropy in the universe to increase.
Photosynthesis is a process common to all photoautotrophic organisms, in which light is absorbed by chlorophyll and linked to the production of ATP. The light-dependent reaction involves chlorophyll absorbing light, leading to photoionisation, with some of the energy from the released electrons conserved in the production of ATP and reduced NADP. The light-independent reaction uses reduced NADP from the light-dependent reaction to form a simple sugar.
Respiration is a process common to all organisms, in which various substances are used as respiratory substrates. The hydrolysis of these substrates is linked to the production of ATP. In both respiration and photosynthesis, ATP production occurs when protons diffuse down an electrochemical gradient through molecules of the enzyme ATP synthase, embedded in the membranes of cellular organelles.
In communities, the biological molecules produced by photosynthesis are consumed by other organisms, including animals, bacteria, and fungi. Some of these are used as respiratory substrates by these consumers.
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Living organisms and closed systems
Living organisms are not closed systems and, therefore, are not directly relevant to the second law of thermodynamics. The second law of thermodynamics states that the entropy of a closed system will always increase over time and can never be a negative value. The only known closed system that exists is the entire universe.
Living systems cannot be closed systems or they would not be living. Human organisms, for example, gain energy from external sources such as the sun or other organisms. They also lose energy in the form of heat. In addition, they gain mass through food, which comes from an outside source. Therefore, matter and energy enter and exit the system.
Living organisms are open systems that require a constant energy input to maintain their highly ordered state. Cells, for example, are highly ordered and have low entropy. However, the processes performed to maintain that order result in an increase in entropy in the cell's/organism's surroundings.
The ability of living organisms to grow and increase in complexity, as well as to form correlations with their environment in the form of adaptation and memory, is not opposed to the second law. Under some definitions, an increase in entropy can also result in an increase in complexity. For a finite system interacting with finite reservoirs, an increase in entropy is equivalent to an increase in correlations between the system and the reservoirs.
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The second law and life on Earth
The second law of thermodynamics is a physical law based on the observation that heat flows spontaneously from hotter to colder regions of matter. It establishes the concept of entropy, which is a measure of disorder in a closed system. The second law predicts the direction of natural processes, stating that they run only in one direction and are not reversible.
Living organisms do not violate the second law of thermodynamics. Human organisms, for example, are not a closed system, with energy input and output, and therefore are not directly relevant to the second law. Living systems can gain or lose energy from their external environment, making them open systems.
Living organisms require constant energy input to maintain their highly ordered state, and this process results in an increase in entropy or disorder in their surroundings. In the case of plants, they achieve a non-equilibrium thermodynamic state by alternating between phases of solar energy consumption through photosynthesis and subsequent biochemical reactions, which result in the synthesis and then release of energy from adenosine triphosphate (ATP). Animals, on the other hand, must consume plants or other animals for energy, and the higher an organism is on the food chain, the less available energy it receives.
Thus, life on Earth is dependent on the continuous input of energy, and while living things have the informational capability to maintain an open system, they are still subject to the overall increase in entropy in their surroundings and the universe as a whole.
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Frequently asked questions
The Second Law of Thermodynamics is a physical law based on universal observation concerning heat and energy interconversions. It establishes the concept of entropy as a physical property of a thermodynamic system. It states that heat always flows spontaneously from hotter to colder regions of matter and that not all heat can be converted into work in a cyclic process.
Living organisms are not closed systems, and therefore, they can gain or lose energy from their external environment. This makes them open systems. The Second Law of Thermodynamics applies to closed systems, so it does not directly apply to living organisms. However, living systems require constant energy input to maintain their highly ordered state, and the processes performed to maintain this order result in an increase in entropy in the organism's surroundings.
No, living organisms do not violate this law. While they can gain or lose energy, they are still subject to the effects of entropy. Living things die when the disorder in their systems increases to the point where the system can no longer function.
The laws of thermodynamics are important unifying principles in biology, governing the chemical processes (metabolism) in all biological organisms. The transfer of energy in biological processes is not 100% efficient, and the loss of energy to the environment results in an increase in disorder or entropy.
