Physicists originally summarized the first and second rules of thermodynamics using experimental data. These principles control and penetrate all part of existence, not just physics but also biology.
The First Law of Thermodynamics in Biology
Energy may only be changed from one form to another or moved between various components of a system according to the first law of thermodynamics that also referred to as the law of energy conservation. This idea also guides energy flow in biological systems: energy in living things comes from the sun and moves from plants to animals. Hydrogen fuses into helium in the sun center to emit photons under tremendous pressure and temperature. Like a huge nuclear reactor, the sun gives life on Earth endless energy.
It is an endergonic process to synthesise organic molecules from carbon dioxide and water. Because reactants have far lower potential energy than products, the process is not spontaneous in thermodynamics. Carbohydrate synthesis requires sun energy input.
Photosynthesis is the most widely used way on Earth to absorb sun energy. It takes place in the chloroplasts of plants or algae. Various wavelengths of light are absorbed by chlorophyll and other auxiliary pigments including carotenoids and xanthophylls. They move the energy to reaction centers to break up water into protons and oxygen. Some energy in proton enters into high-energy molecules like NADPH, while another part is stored in a proton gradient to create ATP, similar to the process in mitochondria. The Calvin cycle turns these high-energy molecules and carbon dioxide into triose. Some bacteria synthesize organic chemicals without using chlorophyll but rather proteins that absorb green light, which give them a purple hue.
The Second Law of Thermodynamics, Entropy, and Life
Entropy changes often accompanies energy flow. It is a fundamental thermodynamic concept that measures the degree of disorder or randomness in a system. Low entropy systems are more organized and have fewer microstates than high entropy systems, which have more microstates and are more chaotic. Entropy in an isolated system always rises or stays constant according to the second rule of thermodynamics. It seems that chaotic situations are the tendency of natural processes. For instance, prolonged close contact between two different metals will cause them to diffuse into one another. The disorder is generated at their interface.
However, entropy seems to diminish in life that develops from simple disordered forms to complex and ordered forms. A simple fertilised egg grows into a fully developed creature. Life evolves from the simplest molecules to single-celled organisms and then to complex multicellular organisms during evolution. Some thought the second rule of thermodynamics did not apply to life because of this seeming contradiction. This idea is wrong. Because life is an open system that exchanges matter and energy with its environment, the idea of growing entropy only applies to isolated or adiabatic systems. The external environment must be included to create an isolated system inorder to use the second rule of thermodynamics.
Now, let's consider photosynthesis. In addition to reactants and products, the photons must also be considered. A plant absorbs at least 8-10 photons to create one glucose molecule. Entropy change in photosynthesis consists of two parts. Entropy is reduced when glucose and oxygen are produced from carbon dioxide and water. Another part is entropy increase. When high-energy photons are captured by plants, only part of the energy
is transferred to organic compounds, while the rest is dissipated as
infrared radiation to raise the environment entropy. The total system entropy will be raised by the combination of these two components.
Life Far from Equilibrium: Positive Entropy, Negative Entropy, Dissipative Structures
Life is a system far from thermodynamic equilibrium. Such systems demand more sophisticated methodologies for investigation. Ilya Prigogine founded non-equilibrium thermodynamics in the 1970s. Systems far from balance will exchange energy and matter with their environment to go from chaos to order when external disturbances surpass a specific threshold. We call these ordered structures far from equilibrium condition dissipative structures.
Living things have two components to their entropy change: ΔS₁ and ΔS₂. The positive entropy produced by irreversible internal biological processes including respiration, organic matter breakdown, and material diffusion is denoted as ΔS₁. Entropy shift brought on by matter and energy interaction between an organism and its surroundings is represented by ΔS₂. The development of a young person into a mature person is one of the more organized life processes when ΔS₁ + ΔS₂ < 0. ΔS₁ + ΔS₂ = 0 keeps an organism steady and order in maturity. The organism becomes more chaotic and disorderly when ΔS₁ + ΔS₂ > 0. It is common in old or sick ones. Death of life will occur in equilibrium at its highest entropy. Living things have to make sure ΔS₁ + ΔS₂ ≤ 0, which implies that ΔS₂ has to be negative.
Consuming low-entropy foods and eliminating high-entropy trash help to attain this negative entropy. For example, when animals eat carbohydrates, proteins and lipids, metabolism produces high-entropy products such as waste, CO₂, H₂O and urea. Synthesizing proteins from chaotic amino acids, the order maintained by immune system and dispersing heat outward is another way to reduce the entropy.