Question

Considering your answers to Exercises 130 and 131, how can you justify the existence of proteins and nucleic acids in light of the second law of thermodynamics?

Short answer

The existence of proteins and nucleic acids, as complex biomolecules, can be justified in light of the second law of thermodynamics by considering two key aspects: energy input in biological systems and entropy increase in the surroundings. Living organisms are not isolated systems and constantly exchange energy and matter with their environment. Energy inputs from processes like photosynthesis and consumption of nutrients fuel the synthesis of complex biomolecules, leading to a local decrease in entropy within cells. However, the overall entropy of the system (cell and its environment) still increases due to energy-releasing processes, such as cellular respiration, which increase entropy in the surroundings. Thus, the formation of proteins and nucleic acids complies with the second law of thermodynamics as the net entropy change remains positive.

Step-by-step solution

Understanding the Second Law of Thermodynamics

The second law of thermodynamics states that the total entropy of an isolated system can only increase over time. Entropy is a measure of the randomness or disorder of a system. In other words, natural processes tend to result in the dispersion of energy and an increase in disorder.

Understanding Proteins and Nucleic Acids

Proteins and nucleic acids are complex biomolecules that play essential roles in living organisms. Proteins are chains of amino acids that fold into specific structures to perform a wide range of functions within cells, while nucleic acids (DNA and RNA) store and transmit genetic information. Their formation involves the organization of simpler molecules, which might appear to be in opposition to the second law of thermodynamics.

Energy Input in Biological Systems

It's important to remember that living organisms are not isolated systems. They are constantly exchanging energy and matter with their environment. For example, plants capture energy from sunlight through photosynthesis, which powers the synthesis of complex biomolecules. In turn, animals consume plants or other animals to obtain the necessary energy and nutrients. This input of energy compensates for the decrease in entropy associated with forming complex biomolecules like proteins and nucleic acids.

Entropy Increase in the Surroundings

Although the formation of proteins and nucleic acids represents a local decrease in entropy within cells, the overall entropy of the system (cell and its environment) still increases. This is due to the energy-releasing processes, such as cellular respiration, which result in an increase in entropy in the surroundings. Ultimately, the net entropy change remains positive, in accordance with the second law of thermodynamics.

Conclusion

The existence of proteins and nucleic acids can be justified within the framework of the second law of thermodynamics by considering the energy input in biological systems and the entropy increase in the surroundings. Despite their inherent complexity, the formation of these biomolecules is driven by the available energy from the environment, ensuring that the overall entropy of the system continues to increase, as stated by the second law of thermodynamics.

Key concepts

Proteins

Proteins are fascinating molecules composed of amino acid chains, which fold into precise three-dimensional structures. This folding process allows proteins to perform a multitude of functions critical to life, from acting as enzymes that catalyze chemical reactions, to providing structural support, to regulating cellular processes.
They seem to defy the natural tendency towards disorder described by the second law of thermodynamics due to their complex ordered structures.
Yet, they exist because cells are not isolated systems; they constantly interact with their environment.

• Proteins serve as the workhorses of the cell, involved in virtually every cellular activity.
• The formation of proteins involves connecting smaller molecules (amino acids), creating a highly organized structure.
• Despite this local increase in order, the overall entropy (disorder) elsewhere must increase for the process to align with the second law of thermodynamics.

In living systems, the energy required to create these complex structures is derived from outside sources, such as nutrients or sunlight.
As organisms use energy from food or sunlight, they increase disorder in their surroundings even as they create order internally, thereby adhering to the second law of thermodynamics.

Nucleic Acids

Nucleic acids, including DNA and RNA, are the molecules responsible for storing and transferring genetic information. Their formation involves linking nucleotides into long chains, resulting in orderly, information-rich structures that seem to contradict the second law of thermodynamics due to their high degree of organization.
However, this apparent contradiction is resolved when considering energy flows within biological systems.

• DNA contains the instructions necessary for the development, functioning, and reproduction of all living organisms.
• RNA plays various roles, including carrying genetic instructions from DNA to the cell's protein-making machinery.
• Forming these nucleic acids requires energy, which is typically derived from food molecules broken down during cellular processes.

When living organisms build such detailed structures, they utilize energy efficiently, which comes from metabolic processes and interactions with the environment.
Through these processes, the overall entropy of the universe still increases, satisfying the second law of thermodynamics and allowing living systems to sustain their intricate molecular structures.

Entropy in Biological Systems

Entropy refers to the measure of disorder or randomness in a system. According to the second law of thermodynamics, the total entropy of an isolated system can never decrease. In biological systems, however, organisms defy local entropy increase through careful energy management.
This energy management allows for the orderliness needed for life while adhering to the second law.

• Living organisms take in free energy from their environment and use it to maintain order and complex structures internally.
• Processes such as metabolism convert nutrients into usable energy and molecular building blocks.
• In doing so, they release waste products and heat, which increases the entropy of their surroundings.

Thus, although biological systems appear highly ordered internally, they contribute to overall disorder by consuming energy from their environment and releasing heat and other byproducts.
This increase in external disorder maintains the universal trend towards higher entropy, reflecting the foundations of the second law of thermodynamics.