The second law of thermodynamics is a fundamental law of physics that describes the tendency of things to become disordered everywhere. It states that the degree of disorder always rises in the universe and in any isolated system (a collection of matter that is completely isolated from the rest of the universe). Because this law has such profound implications for people's day-to-day lives, we are going to restate it several times.

The second law can be understood in terms of probability, for example, by asserting that systems will naturally shift toward the configurations that have the highest likelihood and saying that systems will naturally shift toward those configurations that have the highest probability. Imagine a box containing 100 coins, all of which are laying with their heads facing up. If a series of unfortunate events causes the box to become unsettled, the coins' arrangement will likely change to one in which there are an equal number of heads and tails. The explanation is simple: while there are an infinite number of possible arrangements of the individual coins in the combination that can produce a result with a probability of 50/50, there is only one possible arrangement that can maintain all of the coins in their original, upright positions. Because a 50-50 split is the most likely outcome, we refer to the situation as being more "disordered." For the same reason, it is a common occurrence for one's living area to gradually become more chaotic without conscious effort: this natural tendency requires continual labor to reverse.

The entropy of a system is a useful metric for determining the degree of disorder that exists inside that system; the higher the degree of disorder, the higher the entropy. The second law of thermodynamics can also be stated in a different form, which suggests that systems will naturally move toward more entropic configurations. This is an alternative method to state the law.

It would appear that living cells go against the second rule of thermodynamics because they bring order into existence by continuing to exist, multiplying, and making more complex organisms. How in the world is that even a possibility? The solution lies in the fact that a cell is not an independent system; rather, it obtains its source of energy from its surrounding environment in the form of food, photons from the sun, or even, in the case of some chemosynthetic bacteria, simply from inorganic substances. After that, it makes order within itself by utilizing this energy in a specific way. During the chemical processes that are responsible for the formation of order, the cell converts some of the energy it needs into heat. The heat escapes from the cell and is discharged into its surroundings, causing disruption in those areas. Therefore, in accordance with the second law of thermodynamics, there is an increase in the entropy of the cell's surrounds as well as the cell itself.

For the purpose of gaining an understanding of the rules that regulate the transformation of energy, picture a cell that is encircled by a sea of matter, which represents the rest of the universe. As it grows and continues to function, the cell works to maintain its own internal order. But at the same time as it is producing molecules and assembling them into cellular structures, it is also continuously producing heat energy. Heat is the most disordered form of energy, which occurs when molecules are moving around in an unpredictable manner.

According to the first law of thermodynamics, energy cannot be created or destroyed; yet, according to the second law, it can be converted from one form to another. The first law of thermodynamics dictates that the total amount of energy must remain the same at all times. However, the chemical reactions that are taking place inside the cell will cause variations in the amount of energy that is present in its various forms. For example, when an animal cell consumes food, a portion of the energy that is held in the chemical bonds between the molecules of the food (referred to as "chemical-bond energy") is turned into the random thermal motion of molecules. This process is known as thermonuclear fusion (heat energy).

If the processes that establish molecular order and the reactions that generate heat within the cell are not directly connected to one another, then the cell will not be able to benefit from the heat energy that is released by the cell. The intimate linkage of the creation of heat to an increase in order marks a significant distinction between the metabolism of a cell and the inefficient burning of fuel that occurs in a fire. For the time being, it is sufficient to understand that cells must directly link the "controlled burning" of food molecules to the creation of biological order in order to build and maintain an island of order in a universe that is prone to chaos. This is necessary in order for cells to build and maintain an island of order in a universe that is prone to chaos.

Whether it is the sugars that a plant has photosynthesized as food for itself or the combination of large and tiny molecules that an animal has consumed, all animal and plant cells are powered by the energy that is stored in the chemical bonds of organic molecules. This can be either the sugars that a plant has photosynthesized as food for itself or the combination of large and tiny molecules that an animal has consumed. In order to maintain their existence, advance their development, and produce offspring, organisms need to get this energy in a form that can be utilized. In order to liberate energy, the molecules of food must first go through the process of gradual oxidation, often known as controlled burning.

Because there is a significant amount of oxygen present in the atmosphere of the Earth, the forms of carbon and hydrogen that are the most energetically stable are CO2 and H2O, respectively. Therefore, a cell is able to extract energy from sugars and other organic molecules by allowing the carbon and hydrogen atoms in those molecules to interact with oxygen to form carbon dioxide and water, respectively. This process is known as aerobic respiration. The process in question is referred to as aerobic respiration.

The processes of photosynthesis and respiration are mutually supportive of one another. This suggests that the interactions that take place between plants and animals are not strictly in a one-way route. On this planet, plants, animals, and microorganisms have coexisted for such a long time that many of them have made major contributions to each other's environments. This is because of the extended period of time during which they have done so. Burning organic substances is done by aerobic respiration, which makes use of the oxygen that was created during photosynthesis. Yesterday, CO2 molecules were released into the atmosphere as a result of the respiration of an animal, as well as the respiration of a fungus or bacterium that was decomposing dead organic matter. These CO2 molecules are now being converted into organic molecules through the process of photosynthesis, which occurs in a green leaf. As a consequence of this, we are able to understand that the utilization of carbon results in the formation of a vast cycle that has an effect on the biosphere, which is comprised of everything that is alive on Earth.

The oxidation of organic molecules does not take place in a single step within the cell, similar to what takes place when organic matter is burned in a fire. During the course of metabolism, these molecules are involved in a vast array of chemical processes. The vast majority of these events are mediated by enzymes, and very rarely do they entail the direct addition of oxygen. Before delving into the specifics of some of these chemical processes and the roles they play, we first discuss what is meant by the phrase "oxidation process."

The term "oxidation" can refer to a wider range of reactions than only the addition of oxygen atoms because it describes any process in which electrons are transferred from one atom to another. In this context, the term "oxidation" refers to the process of losing electrons, whereas "reduction," which is the opposite of "oxidation," refers to the process of adding electrons. Therefore, an atom of chlorine is said to be reduced if it gains an electron, becoming Cl-, whereas an atom of iron, Fe2+, is said to be oxidized if it loses an electron, becoming Fe3+. Since the total number of electrons remains the same during the course of a chemical reaction and there is neither a gain nor a loss of electrons, oxidation and reduction must always take place simultaneously. For instance, when one molecule acquires an electron through the process of reduction, the corresponding electron on another molecule is lost (oxidation). When, for example, a sugar molecule is oxidized to produce CO2 and water, the oxygen molecules that are involved in the creation of H2O gain electrons in the process and are said to have been "reduced."

The terms "oxidation" and "reduction" still apply even when there is just a partial movement of electrons between atoms that are joined by a covalent bond. This is because oxidation and reduction both include the movement of electrons. When a polar covalent bond is formed between a carbon atom and an atom that has a high affinity for electrons like oxygen, chlorine, or sulfur, the carbon atom is forced to give up more electrons than it receives in exchange for those electrons. Because the positive charge of the carbon nucleus is now somewhat greater than the negative charge of its electrons, the atom is said to be oxidized once it acquires a fraction of a positive charge. This phenomenon is also known as oxidation. On the other hand, a carbon atom that is part of a C-H bond is said to be reduced when it has a few electrons more than it need to perform its function.