Lesson 1.2: About life

Hey, Penny, it's been a while since our last post, and I bet you're dying to know the rest of the living processes.

Yeah..., obviously... I was kind of worried you might have forgotten... 

OK, try to conceal your excitement a little bit and let's go with the three remaining living processes.

Alright, I remember we already had nutrition, respiration, response and movement, which was actually a way of response.

Exactly, well done! The next life process is excretion, which is a fancy word for taking out of the body the waste material.

You mean the poop?

Well, yes, but not only. You see, all living things need to get rid of waste material. First of all, respiration, as we said the other day, implies producing carbon dioxide as a waste material, so releasing this carbon dioxide to the atmosphere is also a way of excretion. Just as the oxygen that the plants throw to the atmosphere during the photosynthesis is also a way excretion.

Then, of course, we have the poop, which just to sound a little bit more scientific we could call droppings. We produce these droppings as a byproduct of nutrition, and it's proper of animals, because we eat other living things, so we get some extra chemicals we cannot store, and the best way Nature has come across to allow us to take them out is poop. And pee, of course, we also have the pee (and even sweat, but that's mainly useful for humans). Plants, for their part, produce only the food chemicals they truly need, so they don't have so many things to get rid of.  

Yeah, I was kind of having some difficulties to picture a plant doing number two...

However, you will have no problem picturing plants doing our next life process: growth. In fact, plants keep growing most of their lives, whereas most animals usually have a growth period, after which our size doesn't increase. But, in any case, all living creatures grow. Growth entails mostly the overall increase in size, but in truth, it doesn't just mean becoming bigger, but also more complex and able to do a greater variety of things.

Growth requires nutrition and respiration, as for growing we need food chemicals gotten from nutrition and energy from respiration, in order to build new living material.    

Great, so this leads us to the very last life process: reproduction!

Exactly, we are nearly done here. Reproduction is the process to perpetuate life and is the way by which a living creature creates a new living creature similar to itself. Many plants and some other life forms are capable of reproducing by themselves, by what is called asexual reproduction. However, many other living beings resort to sexual reproduction, which involves two different special sex cells joining together, these cells coming usually from different parents (so two different living creatures are needed). The process of these sexual cells joining is called fertilisation.

These sex cells carry DNA from their corresponding parent, and that is why children usually resemble in some way their parents. However, the combination of the DNA of both parents ensures that offspring (i.e., the children) are never exactly like their parents, which helps to perpetuate a species and don't become extinct.

So the problem with dinosaurs was that they used asexual reproduction and that's why they disappeared?

No, not at all. Not even remotely. But don't worry yourself about dinosaurs right now, Penny. We will get there. For the time being, just verify if dinosaurs were living creatures by checking these life processes.  

                                        

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

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