If you are reading this, you must be alive; the device you are reading it on (computer, tablet, phone) is not alive. The science of living things is a specialised subject called biology. You might then expect that we can define what makes something alive.
Usually living things are defined to have several characteristics which can be summarised in a list like the one below.
- They are made up of cells.
- They respond to stimuli.
- They use energy.
- They grow and develop.
- They can reproduce.
Let’s look at each of these characteristics in turn.
What is a cell? We could think of it as the smallest part of a living thing that can have independent existence. Then using the first characteristic to define living things would create a circular argument. (A living thing is something made of cells; a cell is the smallest part of a living thing). The best that anyone seems to be able to do is to describe a typical cell – it is surrounded by a cell membrane and contains a nucleus, and so on. But red blood cells don’t have a nucleus. So maybe a cell is just a little object surrounded by a membrane – a layer of special materials called lipids. But we can make little objects like this, called lipid vesicles, and nobody would claim that they are alive.
A common example of response to stimuli (the second characteristic) is plants growing towards light. However, electrons will leave the surface of some metals when a light shines on them. This is called the photoelectric effect. Nobody believes that these metal surfaces are alive because they respond to light.
A block of ice uses energy (called latent heat) to turn into water – it isn’t alive!
A baby grows and develops into an adult human. A snow flake grows and develops into its final form as it falls towards the ground.
Living things certainly reproduce to create the next generation. But you can make a mould of a metal object, pour molten metal into the mould and get an exact copy. Suppose the important point about reproduction is that living things can reproduce unaided – unlike the metal object where someone has to make the mould and pour the molten metal. But, if reproduction is supposed to be unaided, does this mean that people who seek medical intervention because they are infertile are not alive? I guess you could argue that human beings are capable of reproduction and, since infertile people are human beings, they must be alive. But this seems to mean that the list of characteristics does not apply to every living individual thing.
It is often claimed that living things have all five of the characteristics listed above. Let’s think about hinnies. A hinny is the child of a female donkey and a male horse. (The child of a male donkey and a female horse is called a mule). Hinnies are infertile. So they can’t reproduce. But nobody would claim that they are not alive – they look and behave very much like donkeys. (Although, there do appear to be two recorded instances of a hinny breeding with a donkey).
What about viruses? Most people would think that someone who studies viruses is a biologist. Is a virus a living thing? It isn’t made up of conventional cells, although it contains some of the materials that are contained in cells. It can’t reproduce independently – it has to infect an animal or plant cell and take over its machinery to make new viruses, using information provided by the virus itself to make them. It’s not clear whether viruses are living or non-living objects.
We all know that there is a different between trees and butterflies, that are alive, and rocks, that are not. The problem arises when we try to define exactly what that difference is. Viruses seem to be somewhere between what we think of as living things and non-living things.
So it seems that there is no clear boundary between living and non-living things. And so it is very difficult to answer the simple question – what is life?
Related posts
16.5 Exponential growth: bacteria
16.1 Drug safety
Follow-up posts
16.41 Physics, chemistry and biology
16.46 The placebo effect
16.47 Fats
18.27 Diffusion through membranes, osmosis and dialysis
23.5 Biological classification
Thinking about Science with David Hukins 
In his charmingly quirky monologue :What is Life?” first written in 1944, Edwin Schrödinger pointed out that living things have another characteristic; negative entropy. Whereas the natural inclination of all (inanimate) things in the universe is to move towards disorder, living things characteristically take dispersed substances and incorporate them into organized structures. From a physicist’s point of view, it is another interesting way to characterise “life”
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I am very grateful to Peter Purslow for reminding me that one characteristic of living things is that they decrease entropy. What does this mean? In simple terms, it means that they create patterns. They join small molecules together to make bigger molecules and organise these molecules into cells. They put cells together to make roots, flowers etc. (plants) or limbs, organs (like the heart) etc. (animals). In non-living systems, patterns tend to spontaneously become disrupted – their entropy increases. This spontaneous increase in entropy is sometimes called the “second law of thermodynamics”.
Let’s consider a very simple pattern – a solution of salt separated from pure water by a cellulose membrane. This pattern will degrade naturally because water will pass spontaneously into the salt solution to dilute it. So the difference between the solution and the water is less marked. This process is called “osmosis” and occurs spontaneously to increase the entropy of the system.
In order to decrease entropy (creating patterns) living things need to use energy. Plants make their food by using energy from sunlight, in a process called “photosynthesis”. Animals gain their energy by eating plants or by eating animals that eat plants. Both animals and plants can use their food to make complicated patterns. When the food runs out, the patterns that they make become disrupted (increase in entropy) so that life is replaced by death and decay.
However, the use of energy to decrease entropy is not unique to living things. In many parts of the world, solar energy is used to remove water from sea-water by reversing the process of osmosis – a process called “reverse osmosis”. In this process water is separated from sea-water leaving behind a much more concentrated solution of salt. So like the other characteristics that I listed in post 16.18, decreasing entropy appears to be a characteristic of living things but can also be exhibited by non-living systems.
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