What Is Life?

I have a friend who is keenly aware that he is dying. Lovingly, courageously, magnificently he is preparing for the moment when life as he has experienced it will cease to be. I was with him recently, and I left filled with admiration for his inner state, which is becoming ever deeper, more still, and more contented as the days go by. Being present with someone dear to us who is approaching death is a profound experience. It is a very direct reminder that death ultimately comes to each one of us, to all living beings.

Many spiritual traditions, no matter what ideas they may have about an afterlife, urge us to remember on a daily basis the inescapable fact of bodily death, and to use it to guide our lives. Biology, equally, reminds us that death is inevitable, and that we encounter it directly every day. To take just one example, whenever we eat we cause the lives of other organisms—be they animals, plants, fungi, or anything else—to come to an end. The energy that is contained in the molecules of their bodies is what sustains our own life.

What is the life that we work so hard to sustain? The answer is not at all simple, even from a strictly scientific point of view, so let us approach it in steps. For example, we can ask “When does life begin?” In a non-scientific conversation, even this relatively well-defined question, a stepping-stone to the bigger question of what life actually is, can sometimes seem difficult and contentious. To a biologist, however, it has a clear and direct answer. Life does not begin in each succeeding generation, it continues from the previous one. Let me explain.

Some organisms reproduce primarily asexually—bacteria, for example. A bacterial cell duplicates its DNA and then divides into two. Each cell receives a full complement of DNA and roughly half of the other constituents contained in the mother cell. The mother cell was alive, the two daughter cells are alive. Life has continued without interruption.

 Marchantia gemmae B

The life of multicellular organisms is similarly continuous. In this photograph, for example, we see part of a common liverwort called Marchantia (liverworts are a small group of plants related to mosses). Like many ancient plants, it reproduces both sexually and asexually. Here we see a group of asexual reproductive structures called gemmae cups, within each one of which is a set of tiny, disc-shaped gemmae. When rain comes along and drops fall into the cups, they spray out the gemmae. (You can see an arc of gemmae expelled from the largest cup.) If a gemma falls or drifts onto a suitable patch of ground it sprouts, and a new Marchantia plant is established. Thus, the picture shows us part of the whole sequence by which the life of one generation is linked to the life of the next: the plant’s body forms gemmae cups, which form gemmae, which are dispersed by rain and start the next generation. The cells of every structure in the process are alive.

The principle that life is continuous applies equally well to organisms that reproduce only sexually, including human beings. The mother produces eggs, the father sperm. In the natural course of events only living eggs can be fertilized, and only living sperm can do the job. The fertilized egg starts to divide, each new cell gets the same full set of chromosomes and genes, and a new organism starts to form. At the cellular level there has been no interruption of life. Life from life. In this sense, our lives are continuous with the lives of some of the first organisms our Earth ever saw, billions of years ago. It is a magnificent understanding!

Important as this understanding is, however, it skirts the central question. Saying that bacterial cells are alive, that eggs and sperm are alive, and that life is continuous from one generation to the next does not tell us what makes those cells be alive, what makes them different from cells that have been killed (like bacteria that are killed by antibiotics) or that have died a natural death (like cells in our gut wall that die by the millions every day). What does biology, or science more generally, tell us about the state of being alive?

This question is a thorny one, and over the years various approaches to it have been explored. I will offer you only one of those approaches, because it seems to me to be the one that is most widely accepted in scientific circles at the present time.

The first basic foundation of the current biological understanding of life is that it is an energy-consuming process. In Nature’s Depths we have repeatedly seen that all living systems use energy. On Earth, the vast majority of this energy comes from the Sun, converted by photosynthesis from the electromagnetic form in which it reaches us via sunlight into the chemical form that living cells require, starting with the biosynthesis of sugar. Living cells absolutely need this chemical energy. If you starve them, they will die. This is what happens during a heart attack or an ischemic stroke. A blockage in an artery deprives heart cells or brain cells of oxygen. Without oxygen the cells cannot obtain energy from the metabolism of sugar or other nutrients, and they die. Their death is marked by the fact that they cannot metabolize ever again, no matter how much oxygen is later provided. A sufficiently long interruption in the process that supplies chemical energy to the cells kills them.

The second foundation is that all unambiguously living things on Earth are composed of cells. Some, like bacteria, archaea, yeasts, and many protists, are unicellular—the entire organism is just a single cell. Others, like most fungi, plants, and animals, are multicellular and are, as the term implies, made up of multiple cells. All living systems we know of on Earth are made up of cells, whether one or many.

What is a cell? We have circled around this question on Nature’s Depths because cells are difficult or impossible to see on a walk in the woods—a clear view requires a microscope or other special optics. Here is a simplified description. A cell is a tiny sac separated from its environment by a membrane made up of lipids and proteins. The material inside the membrane, the cytoplasm, contains a variety of microscopic structures, and has a chemical composition that is different from that of the cell’s environment.

The largest of the internal structures is the nucleus, enclosing the chromosomes on which the genes made of DNA are located. There is an important exception, however. While bacteria and archaea have chromosomes and genes made of DNA like all other organisms do, these chromosomes are free in the cytoplasm rather than being enclosed within nuclei. That’s one of the major reasons for separating bacteria and archaea from protists, fungi, plants, and animals, all of whose cells have well-defined nuclei.

Metabolic energy is used to build the intracellular structures and cell membranes, and to sustain the chemical differences across membranes. Energy is also required to replace degraded molecules, to generate electrical signals, to produce cellular movement (including muscle contraction), to synthesize new DNA (so that the next generation of cells can be formed), and so forth. As cells die, their structures begin to show marked abnormalities. A living cell, then, is one with an intact structure, and the capability of using energy to carry out cellular functions.

The third foundational property of living things is that they reproduce, and do so with great fidelity. Both single cells and whole organisms reproduce. As we saw earlier, life does not spring forth anew with each generation, but continues without interruption from the previous one. This is true whether we are talking of unicellular organisms, or single cells within multicellular organisms, or multicellular organisms as a whole.

What exactly is reproduction? If you reproduce a CD, you make a new CD that is just like the old one—fidelity to the original is implied. The same is true of living cells—daughter cells are much like their mother cells. Changes do occur, of course, and we know a great deal about how this happens. For example, during sexual reproduction there is a reassortment of the chromosomes that carry the maternal and paternal genes, which is the biggest reason why children resemble their parents but are not perfect copies of either of them. Moreover, mutations—changes in the DNA—occur on a regular basis. If they happen while eggs or sperm are being made, their effects are revealed in the offspring. If they happen during the development of a single individual, they may be innocent or they may contribute to the establishment of a cancer. And nowadays we also hear a great deal about epigenetics, a suite of mechanisms that can be environmentally influenced and that affect the functioning of genes in a way that can be transmitted from one cell generation to the next even while the genes themselves remain unaltered, not indefinitely but across several generations.

In sum: Living systems are utterly dependent on the utilization of metabolic energy; they are composed of cells that have bounding membranes and other characteristic structures; and they reproduce with great albeit not perfect fidelity. These three attributes continue uninterrupted from one generation to the next. To a biologist, this is a compelling description of life. A cell that does not metabolize and whose structure is not sufficiently intact is not alive, and all cells come from pre-existing living cells.

Not all cells, however, are capable of taking their turn in giving rise to the next cellular generation. In multicellular organisms, the majority of cells ultimately become so specialized, both in their structure and in their function, that they abandon the ability to reproduce. Examples include nerve cells with their intricate branching patterns, muscle cells with an elaborate contractile apparatus, fat cells filled with bulging fat droplets, sperm cells with whiplash tails, and so forth. When tissues containing such terminally differentiated cells are called upon to form new cells in order to recover from damage or to increase their function, they do so by the division of reserve cells that have not fully differentiated. These are the famous stem cells about which so much has been written in the popular press in recent years.

I would like finally to link this focus on the life of single cells to another personal experience at the human level. Decades ago Yvonne and I were privileged to be with her mother as she drew her last breath at the age of 93. Grammy, as she was known in the family, was cared for at Cabrini Hospital in Seattle. Cabrini was operated by the Missionary Sisters of the Sacred Heart, founded by Mother Francesca Xavier Cabrini who in 1946 was declared a saint by Pope Pius XII. The staff at Cabrini were wonderful, and when Grammy’s kidneys started to fail and it was clear that her time was approaching, they left us alone to keep her company and sing to her with no interruptions.

For a long time Grammy looked like she was simply resting, but she did not respond to us or speak, most likely because the nerve cells on which consciousness is dependent had already started to go silent. After some hours, her breathing became shallower and shallower, and finally ceased. We can imagine that the brain cells whose activity generates the cycle of breathing in and breathing out—that traditional and evocative sign of a living body—were also stilled. With breathing at a standstill, no cells anywhere in the body were getting fresh oxygen. Soon they, too, would die, never again to have the attributes of life that I have described for you.

With her breath stopped, and her heart stopped, and no sign of any activity in her brain, Grammy was gone. But not completely. The descendants of many of her genes still survive in Yvonne, and in Yvonne’s brother, and in our two daughters, and the brother’s children, and in all the grandchildren. They do their work in these new bodies. If the grandchildren in turn have children, some of Grammy’s genes will be represented for at least one more generation. Life on Earth is truly a marvelous process, and continuity is one of its greatest hallmarks!


Posted in: Rumination