What is life? When you think about this question, it is not an easy question to answer . In this day and age of rapid development in biology where new findings in every field of biology are being made, it is easy to overlook something so common and so fundamental to this question that although you may know what life is when you see it, but if you are asked to define it, you are immediately guaranteed a confused response. Why is that important? From a scientific perspective, we would need something to distinguish what is truly living from what is not without additional human biases which can sometimes cloud even the best judgments when framing a definition especially for the sake of scientific investigation but as we will see scientists especially biologists may give different definitions to the same question about what life is depending on their expertise and we will see how that question changes as new findings in biology were being made.
The first scientist to ask the question in a scientific context was not a biologist but a physicist, Erwin Schrödinger, who is well known for his contributions in quantum mechanics which gives a description of matter and energy at the most fundamental level that is correct if counterintuitive , more specifically it was Schrödinger who developed wave mechanics where subatomic particles have a wave aspect as well a particle aspect, notably the mathematical equation that bears his name and he is well known for the paradox of the cat that can exists in two different states at once until someone looks at the cat which ends up influencing the outcome. In a series of lectures that were given at the Institute for Advanced Studies at Dublin, Ireland where Schrödinger settled after the rise of Nazi Germany, and for an audience that ranged from scientists to interested layfolks as well as some dignitaries, and was eventually published into a small book called What is Life?, Schrödinger asked the question about the nature of life and that the science of physics is adequate for the task and as Schrödinger (1944) stated ” How can the events in space and time which take place within the spatial boundary of a living organism be accounted for by physics and chemistry?” (p. 3)
To answer this question is to place the problem within the context of science and indeed like physics and chemistry, the science of biology was undergoing rapid developments. For one thing, the science of genetics which was at first studied by the monk Gregor Mendel famous for his controlled experiments on pea plants and which he derived the principles of how traits or phenotypes are inherited which was published in an obscure journal was then rediscovered around 1900 and after the discovery scientists were verifying Mendel’s results as well as making new discoveries relevant to a biological problem which was outline in Charle’s Darwin groundbreaking work On the Origin of Species published in 1859 and describes that species are evolving into new species because of the mechanism, natural selection but how does natural selection work to produce species capable of surviving in changing environments? Natural selection can only work if the populations of organisms have inheritable phenotypes that are slightly different from their parents and if the differences allow for survival, then natural selection will favor the offspring for its survival into the next generation.
The problem facing Darwin is that variation can only be favored by the constant filter of natural selection but where does the variation come from? The only field of biology that can answer that question in a scientifically meaningful way is genetics which was discovered by Mendel and later it was rediscovered and since then well up until 1944 when it was discovered that it is DNA which is the carrier of the hereditary information and this was brought to the attention of Schrödinger but as you read What is Life?, there are two answers to the question. The first one, which had an immediate impact on biologists which culminated into major breakthroughs in molecular biology and that is the source of the biological information needed for the functioning of an organism and that is the gene which Schrödinger (1944) called “an aperiodic solid”(p.61) which through the science of molecular biology revealed it to be DNA with the information on how it would replicate through one generation to the next and how that information is translated into a fully viable organism.
That first question Schrödinger asked was the starting point for the discovery of the molecular nature of the gene and inspired then a young scientist named James D. Watson and later together with Francis Crick determined the double helix of DNA and ever since then molecular biology ranged from scientific discoveries in how all of life is based on a universal genetic code to new applications in medicine but the second question asked by Schrödinger interestingly did not garnered immediate attention but soon later after the breakthroughs in genetics were made, then scientists returned back to the question with new methods and new insights.
What was that second question? There is one feature of the living world that had not escaped the notice of Schrödinger and that is the fact that an organism is a highly complex system that can produce another complex system like itself or what Schrödinger (1944) called “order based on order” (p.68). Schrödinger mentioned the fact that the universe is dominated by the second law of thermodynamics or the law of increasing entropy which states that disorder always increases.
Since living things are physical systems how is it that life can continue to evolve into complex forms when such physical systems degrade into disorder? What was it about life that avoided the inevitable decay into chaos? This question was seriously asked by Schrödinger (1944) and as clearly stated ” What is the characteristic feature of life”? (p.69). By that question he meant life’s ability to propagate its order which keeps it alive, in the biological sense of that term, and how to do so in which every process is subjected to decay as mandated by the second law. How does life propagate while avoiding disorders. Since the total entropy of the universe is always increasing, on the smaller scale such as the biosphere order can increase because of the flow of energy from the sun which powers the process of photosynthesis where green plants convert the energy of visible light along with water and carbon dioxide into carbohydrates and animals consume the carbohydrates and take in oxygen while serving as food for other organisms whether predators or parasites. It is the flow of matter and energy in an organisms and wastes released that is the key to how organisms survive and propagate their order into the next generation and as Schrödinger (1944) stated ” It can only be kept aloof from it, i.e alive by continually drawing from its environment negative entropy” (p. 71). That is, while the universes increases in positive entropy, living systems by feeding and metabolizing produce negative entropy while at the same time producing disorder elsewhere.
The ability of life to avoid thermal equilibrium indicative of maximum positive entropy or more simply death is avoided when there is a flow of matter and energy organized in a useful form and life, whether large or small depends on that flux of negative entropy which allows organisms to remain in that state of negative entropy, that is highly organized in the form of a complex, metabolizing and reproducing entity, but to be in a state of negative entropy disorder of some kind or positive entropy increases elsewhere and the definition of negative entropy as part of the definition of life was later considered and in a way it was something of a brave move on Schrödinger’s part since at the time thermodynamics only studied isolated systems or systems where nothing can enter or leave while life is considered an open system which is the opposite of a closed system. The science of thermodynamics was then eventually broadened to cover those class of systems , the open systems, which as we know are the rule rather than the exception. Eventually biology incorporated not just new findings in molecular biology but greatly benefited from the science of thermodynamics that studied the properties of open systems by the name of non equilibrium thermodynamics or NET for short.
To answer the question about the nature of life, I will not only cover the usual textbook definition but it would also be a good idea to broaden the definition of life which takes into account findings from NET.
The Textbook Definition of Life
To understand the nature of life , we will first consider a list of criteria that distinguishes life from non-life and it is these criteria that I call the textbook definition since these lists are likely to be found in a biology textbook but the problem is to find a reliable to list to cover all forms of life, not just on earth but to any systems of life that may be prevalent throughout the cosmos. Without further ado, here are the lists that defined life, in a biological sense.
1. Homeostasis: What is homeostasis? That is the process by which living things keep vital physiological functions such as concentrations of amino acids in unicellular organisms or in multicellular organisms, levels of pH in the bloodstream within narrow limits. That ability is vital to the long term survival of a given organism since whatever organism there is it must be able to cope with changes in natural conditions such as variations in sunlight, temperature, the availability of certain nutrients and so on.
2. Organization: Living things are one of the most complex entities, far more complicated than even inorganic systems such as rock crystals which are composed of a few chemical elements while a living thing such as a “simple” bacterium is composed up to six main chemical elements which are carbon, hydrogen, oxygen, nitrogen, phosphorous, and sulfur along with small amounts of other elements. Out of these six main elements or biogenic elements which are chemical elements that are responsible for forming the complex polymers of life notably nucleic acids and proteins as well as lipids or fats and carbohydrates and with a genetic code that is universal for specifying how the language is DNA can be translated into the language of proteins along with a type of lipid , phospholipid which forms the boundary called the cell membrane from the rest of the environment and the cell structure is a dynamic and negative entropy state with a high degree of order, whether it is in the form of a single cell which is either a prokaryote cell or cell that is small, lacks a nucleus and organelles or specialized cell structures that perform a specialized tasks, and are usually in the form of bacteria. This is also true for unicellular eukaryotes or cells that have a nucleus where all the genetic information is stored and a variety of organelles. The level of complexity or the amount of intricacy, interconnections between various interacting elements such as the organelles in a eukaryotic cell can be taken as a measure of organization which is even more apparent in multicellular organisms such as some species of protists, fungi, plants, and animals. There is nothing random at all in the organization of living things.
3. Metabolism: These are a set of dynamic processes that allow living things, whether of cells or complex, multicellular organisms to stay alive in the form of growth, reproduction, and adaptation as well as other activities. Metabolism consists of two main chemical processes, catabolism or breakdown is in when organisms require three major nutrients, which are carbohydrates, proteins, and lipids but in order to use them efficiently, all three must be broken down into their monomers or building blocks which are glucose, amino acids, and fatty acids respectively. This is required for sources of energy to keep up with the vital biological processes such as homeostasis and reproduction and this is done with the help of special protein compounds called enzymes which speed up chemical reactions like the process of catabolism. The opposite is anabolism or the building up and that requires energy to do so. The energy for anabolism is in the form of a universal energy donor found in all life is called adenosine triphosphate or ATP for short. The breakdown of ATP releases energy needed for every form of anabolism such as protein synthesis. As ATP is broken down it must be supplied in two processes such as respiration where energy rich monomers like glucose are combined with oxygen to release energy which is stored in ATP while water, carbon dioxide, and heat are released. ATP is also produced from photosynthesis, where visible light is used to convert carbon dioxide and water into carbohydrates. Both anabolism and catabolism are related to one another.
4.Growth: This process is when a single organism such as a cell can produce two identical cells from a single cell. This too involves metabolism in the form of ATP production and the breakdown of ATP releases energy needed for cell division. There are two kinds of cell division in both prokaryotes and eukaryote cells. For prokaryotes, it is binary fission and for eukaryotes a step by step process called mitosis. Growth for multicellular organisms such as from fertilized egg into adult is even more complex but is based on mitosis for every cell type that makes up the multicellular organism.
5. Adaptation: This is central to Darwin’s theory of evolution by natural selection and this allows organisms to survive in its own native habitat which consists of populations of organisms as well as the non living components such as water, oxygen, and mineral nutrients. As I argued in my first blog post, populations evolve because reproduction whether sexual which involves the formation of a specific set of cells called gametes which have half the genotype or genetic information of a given organisms and asexual where the offspring are reproduced and are genetically identical to the parent, is an imperfect process and even in asexual reproduction, one of the offspring may end up with a set of genes or units of hereditary information that is different from the parent. Such a change is called mutation and it can occur in the level of the gene or in the chromosomes which are structures that consists of both genes and proteins found in both prokaryotes and eukaryotes. In every reproducing populations there will be changes in the phenotype or visible characteristic and in every population no two are alike and that makes the difference between survivial or death as was first noted by Darwin and later confirmed through the Modern Synthesis or combination of both evolution by natural selection and genetics, the science of heredity.
6. Response to Stimuli: To be alive in the biological sense is to be responsive to whatever it is in the environment. This could be food, mates, or dangerous predators. Even the simplest organisms, bacteria are capable of response and can either swim towards a food source and also swim away from a posion. This is carried to a high degree of sophistication for multicellular organsims such as sensory organisms such as eyes for sensing light, antennae for detecting chemicals in the air. Wherever there is a given stimuli, a response will result which is a coordinated pattern of behavior.
7. Reproduction: A well known ability of life to make more of itself. Two forms of reproduction are asexual and sexual , the former is when a single organism can produce an identical copy or a clone of the parent and the latter the formation of gametes through a process called meiosis and fertilization is when the two gametes fuse to form the individual. Reproduction not only propagates individuals of a species but is part of natural selection as was first realized by Darwin where natural selection is the mechanism that filters out useful variations of organisms in a population and this is part of the definition of adaptation so natural selection, reproduction, and adaptation go together in any biological system.
Life as a Process and NET
When looking at this list, you may notice that all six are processes, that is life is something dynamic and if we are to incorporate all these lists indicative of life or something similar we would need to subsume all the criteria into a single scientific definition. How can that be accomplished? One science that has the potential to unify all the criteria into a coherent explanation for biology has to come not from biology but from another science, physics and the field of physics that can do that is thermodynamics.
What is thermodynamics and why is it so important to the science of biology when formulating the scientific definition and how can thermodynamics accomplish that? Thermodynamics studies energy change and how systems or anything that is studied responds to the flow of energy. Energy, as we know, is that quantity that can do useful work and indeed when a cell divides into two, the cell uses energy in the form of the breakdown of that universal energy donor ATP since producing two identical cells requires energy. Energy can be converted into many forms such as visible light, a form of electromagnetic energy can either break bonds between atoms in a molecule resulting in movement along with release of other forms of energy such as the kinetic energy of the atoms as well as vibrations of small compounds of atoms rearranged into new compounds, which is usually the case in some biochemical systems. There is the first law of thermodynamics which states that energy can change into many different forms but the total amount of energy is constant. No amount of energy can be destroyed nor created out of nothing so life , small or large can only make use of a narrow form of energy such as sunlight which is used in photosynthesis to convert carbon dioxide and water into the high energy compound , carbohydrates and in respiration the chemical energy released from the metabolism of carbohydrate powers all other biological processes. Energy is transferred into one form into another in any living system so the first law is really about the quantity of energy.
What about the second law of thermodynamics? Recall that this is one of the laws of thermodynamics that is central to Schrödinger’s question on the nature of life and that is the complex organization present in organisms and their ability to pass on that organization from one generation to the next and doing so in a universe with increasing entropy which the second law says that for every process that converts energy, that energy is degraded into a useless form, which is heat and that entropy or the measure of disorder is increasing.
How was the second law formulated? Originally what would become the second law began with the study of the efficiency of steam engines during the nineteenth century. It was discovered that the efficiency of engines could only work if there was a temperature gradient that is if there were one part of the engine that had a heat source and in another part where there was a heat sink and heat flows from hot to a cold region and it is this flow of heat that powers the engine to do useful work such as turn a flywheel and as long as heat flows from a source to a sink, useful work can be obtained but what would happened if the temperatures of source and sink are the same?
If the temperatures of the heat source and sink are the same, then there is no flow of heat whatsoever and so there is nothing to power the steam engine and as a result a state is achieved known as equilibrium where nothing happens at all.
Thermodynamics abstracts any process and places it within a concept in an effort to generalize what is true of the physical universe and the topic of research for thermodynamics, which is thermal equilibrium which was formulated independently by Lord Kelvin of Scotland and Hermann von Helmholtz of Germany based on previous work by other scientists doing similar work in thermodynamics and the second law was given the name entropy for the state of thermal equilibrium by another scientist Rudolf Clausius.
Consider how equilibrium is achieved. Suppose we have a box that is an isolated system and an isolated system is where no energy and matter can enter and none can escape but there is something in the isolated system and suppose it is hydrogen gas. In one corner of that box, is a small amount of hydrogen gas and it is allowed to diffuse only inside the box. The gas will diffuse until it fills the entire volume of the box. When it has done that , it has reached that state of equilibrium where nothing else will change and it will remain that way and we say that it has reached maximum entropy.
From the study of isolated systems, at least in the abstract, any process such as a temperature gradient will even out until all parts of the same system are at the same temperature and when all of the isolated system is at the same temperature, then the entropy has increased until it reaches a maximum where no more changes can take place and it at equilibrium that entropy has increased.
I mentioned that entropy is also the measure of the disorder that occurs in a physical system and how is it that entropy and disorder are related? Recall our imaginary experiment with the box. Inside the box is a small amount of hydrogen gas which is allowed to diffuse in the box until it has filled the entire volume. We know that matter is made up of atoms and that atoms can move with kinetic energy or energy of motion in space in any direction and at various velocities. If we can see atoms of hydrogen in the box then we would know that with the energy of movement and collisions that happen in between each hydrogen atom, that the gas will expand and it expands because of the movement of each atom which continues until the gas fills up the volume.
In the first part of the experiment, the hydrogen is in a corner of the box, and all the atoms are bunched up together. I mentioned that the entropy can be low or even negative and low entropy represent less disorder or a higher degree of order so not only entropy can be positive which corresponds to higher disorder , entropy can be zero and even negative and that corresponds to higher degrees of order. As the atoms occupy more and more space, the order goes down and the amount of disorder increases. Recall that entropy increases into disorder and when you consider the atomic structure of matter, then entropy as the quantitative measure of disorder makes sense. Eventually the kinetic energy and collisions between atoms will result in the gas filling up the box which results in maximum entropy and thus equilibrium that nothing new will happen after that.
The conclusion from studies of isolated systems is that entropy increases or disorder increases until thermal equilibrium is reached and that was the conclusion reached by studies of systems in which nothing changes. At this point, you may say that if thermodynamics is the study of equilibrium where no overall change can occur then why would it be appropriate for biology?
The science of thermodynamics that you learned in high school or in college, is really about equilibrium and does not say anything about how the real world works or in other words it deals only with isolated systems and they are really the exception than the rule. Living systems are not isolated systems but are the opposite; they are open systems and in one of the defintions of life is metabolism and that can only occur if organisms are subjected to a flux of matter and energy and as long as there is that flux or flow, living things can grow, reproduce, and evolve.
Part of the genius of Schrödinger’s insight is to realize that to understand the dynamic order that is life, is to consider that a living thing is a complex systems that is open to its environments and that every organism is subjected to flows in and out. In essence of Schrödinger’s argument, the organization that is life is characterized by a higher degree of negative entropy and negative entropy can result only if there is a flow of matter in and matter out, and the matter that is not needed ends up making more disorder so at first sight, life seems to violate the second law but in reality it is not and it is the second law, paradoxically that allows something as subtle, sophisticated, and even beautiful as a butterfly with its wing shape and pattern and the reason for a butterfly and other life forms is the maintenance of negative entropy while creating more disorder elsewhere in the environment.
To broaden the definition of life is to broaden the scope of thermodynamics moving from systems to equilibrium to the properties of systems that are near equilibrium and those systems that are far from thermodynamic equilibrium. The latter is the study of non equilibrium thermodynamics or NET for short, and that science was developed in the last part of the 20th century.
NET studies systems that are far from equilibrium and those kind of systems are the ones that are real and more common than isolated systems which are rare. A system in thermodynamic equilibrium doesn’t show any change since entropy has reached maximum and going back to our example of the box, the hydrogen gas has filled the inside of the box. Suppose the box became a open system, and energy is allowed to flow inside where previously no energy was allowed to flow inside. What would happen to the gas if energy in the form of heat flows in the box?
If energy flows into the box, the gas will no longer be in equilibrium and in fact the gas will respond by forming convection patterns and all the atoms will begin to move together in concert and bulk movement will result. If energy continues to flow into the box, convection will continue and since convection is a form of organization, there will be negative entropy and wherever energy flows in, energy in the form of infrared or heat flows out since the box is now a open system and as long as there is energy flow in and out, convection, an organized process and hence a form of negative entropy will persist
Now that we have the basic gist of NET, we can then apply this to the definition of life. Life, as a process, as defined by these six definitions which are metabolism, adaptation, homeostasis, growth, response to stimuli, and reproduction. Don’t forget that living things have a high degree of organization which is both active and dynamic unlike a crystal which has organization with the atoms all bonded together in a static pattern and with a repeated pattern whereas life has a pattern of organization in both space and time like the crystal but is constantly changing and with a higher degree of negative entropy that is the result of metabolism in which in the case of photoautrophic organisms such as green plants, which carry out photosynthesis, carbohydrates in the form of sugars, starches, and cellulose, all of which are organized polymers and high in chemical energy which is used by plant cells and the plants are consumed by animals in various levels up the food chain where the energy is utilized for synthesis of biological monomers like amino acids, the building blocks of proteins, fatty acids, the building blocks of lipids including the most important lipid, phospholipid which is the building block of cell membranes, and nucleotides , the building blocks of DNA and RNA. Organization increases in degree in the form of metabolism, homeostasis, growth, and adaptation. That sophisticated system of negative entropy known as life can result and is inevitable and has been that way ever since its origin about 3.5 billions of years ago and continue to this day since earth is an open system and even though we may not know exactly how life originated on the earth but once it did, it continue to get more and more complex evolving into new forms that can capture energy in the form of sunlight and chemical energy in the form of minerals and organic compounds.
A system like life can maintain its degree of negative entropy as long as there is a flow of matter in the form of nutrients and energy in the form of visible light as well as chemical energy flow from deep sea thermal vents, and as open systems as useful energy comes in, wastes flow out and with waste flowing out, disorder increases and if we consider life as an opens system and its environment negative entropy can increase but with the environment taken into consideration, the entropy increases much more and the total entropy increases. Life, as an open system, doesn’t violate the second law, as it is sometimes erroneously claimed but it is because of the second law that organized biological systems can result. You can say that life is possible because of the second law and that is the basis of Schrödinger’s insight.
You can then see that when we include NET which studies systems far from equilibrium, the textbook definition of life subsumes easily into the science of NET and life can be defined as a process that works against the second law and with a high degree of sophistication made possible with the flow of energy.
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