Lesson 1.1: About life

OK, Penny, let's start with the simplest. Let's distinguish between living and non-living things.

Oh, that's an easy one! A person is alive, a car isn't.

Yeah, but it could become a little bit hard if you have to list all the living and non-living things every time you want to explain the difference. Besides, imagine you go into space, to an alien planet, in a galaxy far, far away, and there you find things that you have never seen before. How would you tell living and non-living things apart?

I would poke them. If it moves, then it's alive.

Though that may not be a very safe method, you are definitely on the right track, because movement is one of the main characteristics that will enable us to decide if something is or not alive. You see, there is a series of life processes which we have decided that imply that something is a living creature. 

But above all these processes, there is a general idea that could come handy to decide whether something is or not alive. If left to themselves, living things tend to grow larger, more complex and usually create more living things like themselves; on the other hand, non-living things, when left uncared, tend to rust, rot or decompose, in other words, sooner or later, they break down and become simpler. 

Alright, I get that. But didn't you say something about some life processes?

Indeed. I'm glad to see you are paying attention. So, as I said, beyond this general idea I just told you, there are some processes which when found allow us to state that something is alive. Precisely, there are seven processes (frequently summarised in the acronym Mrs Gren). Keep in mind that it may sometimes be difficult to appreciate some of these processes. But trust me on this one: if it's a living thing, it does all this stuff.

The first of these processes is nutrition.

Oh, that's just a fancy word for eating.

It is, but, you know, scientist really like fancy words. As I was saying, nutrition is the process by which living things get the material they need to live on, grow and reproduce. There are basically two ways of nutrition. On one hand, we have creatures who can take simple chemicals from the environment and, using energy (for example from the Sun), turn them into complex, high-energy food chemicals. Those would be mostly the plants. On the other hand, we have the rest of living things, who need to eat plants or other living things to obtain their food chemicals ready-made. This usually involves digesting the food, which means turning these complex chemicals into simpler ones that the body can absorb. 

So basically the animals just feed on plants and then undo what the plants have done?

Yeah, you could say so. Next process is respiration, which happens within the cells and allows the living things to break down some kinds of food chemicals and use them as a source of energy. So, as you can see, Penny, nutrition and respiration are really related. The thing is that to transform food into energy, most living things need a supply of oxygen and, besides, with this transformation, they produce a waste gas: carbon dioxide. We call breathing to the gas exchange process thanks to which living things obtain oxygen and get rid of carbon dioxide. 

Then breathing is not the same as respiration?

Not exactly. Breathing is a necessary part of the process of respiration. But breathing is just to exchange gases, and respiration consists on transforming food into energy. Shall we go ahead, Penny?

Sure! I got this.

Alright, so the next process is response (it's commonly called sensitivity, too), that is that living things react to stimuli (though not all of them react in the same way to the same stimuli), essentially because they have a much greater likelihood of survival if they can detect and respond to changes that can affect them.

One of these responses is movement, which, as I told you before, is another of the essential life processes.

Wait a sec! You said all living things do all of this stuff, and I know that plants are living creatures and they do not move.

Well, the truth is plants do move. Obviously, animals tend to move more clearly than plants, because they need to look for food and shelter and things like that. And as plants make their own food, they don't need to fool around. But, if you look carefully, you will realise that branches bend towards light, for example, and roots grow moving in search of food and water. Not to mention the opening of flowers, which is too a kind of movement.

Guy, I hate to interrupt you, but don't you think this is a bit too long for a post in a blog?

Yeah, maybe you are right there. OK, let's drop it here. We still have three more life processes to talk about, but we will save them for the next post. For the time being, I leave you here a small diagram about the life processes.

                                        

References:
Wenham, M. and Ovens, P., Understanding Primary Science, London: SAGE Publications