What is Life?

The smallest unit of life is a cell. The human body (and other complex multicellular organisms) have millions, if not billions of different types of cells: skin cells act as a barrier between you and the environment, neurons (nerve cells) fire electrical impulses that tell you how to move and remember, various blood cells to carry oxygen and keep disease at bay, and so many more (even some we haven’t actually discovered yet!). But at its core, isn’t a cell just a group of molecules and chemical reactions? So what really makes life different from, well, the not-life!?

A diagram showing different types of cell in the human body: Stem Cells, Bone Cells, Blood Cells,Muscle Cells, Fat Cells, Skin Cells, Nerve Cells, Endothelial Cells, Sex Cells, Pancreatic Cells, Cancer Cells

At school, you were probably taught that for something to be alive, it must do MRS GREN: move, respire, sense, grow, reproduce, excretion and nutrition (I may or may not have had to look it up..oops!)

But life: its a tad more complicated.

What does life do?

Let’s break down MRS GREN:

Movement and Sensitivity can be lumped under one heading: Response. In almost all multicellular organisms (including Humans), this is controlled by the nervous system: a huge network of specialised cells that sense and react to outside stimuli.

Respiration and Nutrition can be summed up by the process of metabolism. Metabolism is the collective name for all the chemical reactions that take place in a cell, vital to release energy (catabolism) and build complex molecules out of simpler ones (anabolism). In my eyes, this is what life is based on: if it doesn’t metabolise, then surely it isn’t alive? Then again, there are many inorganic chemical reactions that can take place (many is an understatement, chemical reactions happen absolutely EVERYWHERE!)

Then you have Homeostasis. I don’t think this is really covered by MRS GREN (let me know in the comments if I’ve missed something!) I guess excretion could come under homeostasis, as it is all about control: homeostasis is the regulation of a cell’s internal environment, without being affected by external conditions. An often cited example is temperature: the enzymes needed for metabolism require certain conditions to work best at, namely temperature and pH. For humans, our optimum temperature is around 37 degrees C (97 Fahrenheit for my American readers). If it drops too far below this, our body simply won’t have enough energy to carry out vital processes and you enter hypothermia. Hyperthermia happens when your internal temperature gets too hot, resulting in enzyme denaturing. So its really important metabolism for there to be homeostasis. One question I had when writing this was, “what about cold-blooded animals?” Ectotherms lack the internal systems needed for thermal regulation and instead rely on the temperature of its environment to keep warm. It is for this reason that desert animals hide in the sand when it gets too hot, reptiles bask in the sun, and why hibernation is so important for many cold-blooded animals. But that doesn’t mean they aren’t alive, because they have adapted behaviourally to regulate their temperatures, and still regulate other factors such as pH and mineral levels.

All animals Grow even single-celled organisms! They can enlarge over time, just like us, but rather than making new cells, they simply expand!

Reproduction is also important, as it allows for evolution and natural selection to take place, as well as the only way to ensure a species’ survival. In most single-celled organisms, this is asexual, such as in mitosis, where the cell splits in two, but in more complex life it requires two ‘parents’.

Sometimes, you will see MRS GREN as MRS GREEN. This extra E stands for evolution and is really important. It links in with reproduction because genetic information is passed down through generations. In humans, this genetic code is DNA, and in some bacteria and *viruses* it is RNA. Random mutations in DNA are the reason for evolution, along with natural selection.

What isn’t alive?

It isn’t just cold-blooded animals that ‘defy’ these rules: hybrid animals such as mules (donkey/horse cross) and ligers (tiger/lion cross) cannot reproduce within the ‘species’… but I’m pretty sure that they can be considered alive!

One hotly contested topic regarding ‘what is life’ is the virus. A virus cannot reproduce without a host and is relatively inactive other than at the times it can reproduce. It essentially consists of genetic material and a few enzymes encapsulated by a protein coat. More like a bunch of chemicals than anything. But according to the NASA definition below, it’s alive.

Some more food for thought: if you took out a single cell from the human body, it wouldn’t really be alive. I mean, it can’t respire without oxygen or glucose, and it can’t access either (or reproduce) by itself… yet as a whole organism, we are most definitely alive!

Additionally, as we enter an ever more digital age, questions around Artificial Intelligence arise.

If the MRS GREN parameters aren’t always true, then how can we define life?

Defining Life

Google is not very helpful in this situation:

NASA has a much broader definition of life: “A self-sustaining chemical system capable of Darwinian evolution.” This will be useful for NASA’s quest to find extra-terrestrial life, but it has its pitfuls. The main point of the definition in my eyes is that it has to be “capable of Darwinian Evolution” This is an important point, because otherwise, fire could be classed as ‘alive’ (it consumes oxygen and fuel,‘excretes’ ash and carbon dioxide, it grows, and can ‘reproduce’by starting other fires). But if you have a spacecraft or rover looking for life, and even if it can see something that looks alive, it is unlikely to see evolution, as it takes such a long time to occur.

When looking for life elsewhere, NASA often looks for atmospheric biomarkers/ biosignatures. These are chemicals/elements found in the atmosphere or on the surface of planets/moons. For example, if both oxygen and methane are present, then it suggests that there is a constant supply of both molecules because otherwise they would react and disappear. One supplier could be life! Unfortunately, there could be other reasons behind the supply, such as volcanic activity. Alternatively, you could look for anti-biosignatures, which would tell you that there is likely no life there. For example, hydrogen or carbon monoxide, which are consumed by microbial life. In addition, the presence of water and other organic molecules, or simple CHNOPS/SPONCH elements, would be a marker saying life could exist here. Perhaps a definition of life could be made on a chemical basis, but it wouldn’t account for weird life forms very unlike our own.

And then we are back to physics. Is there a perfect definition we can pluck from the realm of physics? Perhaps not perfect, but Erwin Schrodinger (that’s right, the cat guy) suggested that life can be described by thermodynamics and entropy. I mentioned this in my ‘What is a Human’ post a few weeks back’. It suggests that organisms are structures with low entropy, at the cost of increasing entropy in its surroundings. This is because they dissipate energy, in the form of heat and metabolic waste. To be perfectly honest, I don’t really understand this concept, and as far as I know, it has the same downfalls as MRS GREN: inorganic entities like fire also fit into this description. And it isn’t really an easy to understand definition of life.

Many of the problems we have in defining life comes down to the fact that we only have one example: Carbon-based Earth-life! Even on one planet alone, it is so varied and complex, and we hardly understand much about ourselves yet, let alone every species on the planet! And so it begs the question: do we really need a definition of life?

Let me know what you think in the comments!


If you can’t trust an atom… trust in science!

☆it’s like magic, but it’s true whether you believe in it or not!☆

See you next time!


Human Cells, with DNA, Blood cells, surface skin cells, bone cell, columnar epithelial and goblet cells, neuron, smooth muscle cells, neural cells, cardiac muscle, nerve cell, reproductive cells, sperm, ovum, lymphocyte, monocyte, neutrophil, eosinophil. Vector illustration cartoon.

Edit: Obviously there is so much more I could talk about here, especially the historical and philosophical aspects, but there is only so much time in the day, and there’s always room for part two!


4 thoughts on “What is Life?”

  1. On Schrodinger, he wrote a book about life named “What Is Life? The Physical
    Aspect of the Living Cell”. There’s an interesting point raised about the paradox of life being low entropy versus the idea of a heat death with maximum entropy which links quite well. I agree with you – low entropy systems does not equal life, but life cannot exist without being a low entropy system itself.


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