What Is Life?

Photo by Johannes Plenio

In this article, we will learn about:

1.

• How you can determine if something is living: MR GREEN

• Cells

1

---

What I mean by this question is, what is a living thing? What makes something living as opposed to non living? Living things or life forms are called organisms and they come in a huge number of varieties. We have animals, plants, fungi (like mushrooms, for example) and even organisms which are too small to see with the naked eye (we'll get to them later).

In order to determine if something is living, you need to investigate it. If it has qualities or abilities that only organisms have, then it is alive. Have a think about what would make something living as opposed to non living or how can you tell if it's alive?

By Tim De Groot

Imagine you traveled into space and landed upon a distant planet. You find something on the ground. How would you determine if it's a living thing?

Maybe it looks like a rock, but it's bright blue, which is not a colour you would usually see in a rock. Perhaps you saw it move or react to sunlight? Take movement for example. You might expect organisms to move. After all, just look at any animal: it'll walk on legs, or fly by the use of its wings, or crawl or swim. So movement is a good characteristic to look for in an organism (we'll get to how other things like plants move in a moment). So if it's moving, does that mean it's alive? What about wind? You could say wind moves, so is wind alive? What about something man made, like a car? We can see that while movement is a good ability to look for when we're determining what makes something alive, it can't be the only one. We need more characteristics in order to settle this problem of whether our thing on another planet is alive or not.

Mr Green

There are seven characteristics that all organisms have. If the thing on our distant planet has all seven of these, then we can know that it's alive. It's important to note that if the thing does not have these it may well have been alive once upon a time, but is no longer. Therefore, these characteristics will not help us find out if the thing we're looking at was once alive. But for now, when we consider it a living thing, we mean in contrast to it not ever being alive or, in other words, an inanimate object. The characteristics are listed within an acronym, which helps us remember what they are. So Mr Green stands for:

M ovement

R espiration

G rowth

R eproduction

E xcitability

E xcretion

N utrition

Let's go through each one at a time.

Movement

We already took a brief look at how animals move, and you've seen it before yourself. But what about plants and fungi? Well, they're usually fixed in one place by their roots, so they don't get up and walk or crawl around anywhere. However, they can and do move their bodies. They sort of sway in one direction; like if you stood still and moved your hips.

Respiration

This is the ability of organisms to generate or make energy using oxygen and food. It is not the same thing as breathing, although that brings in oxygen to use for respiration. We'll look into this more in our dedicated article on respiration.

Growth

This one is fairly straightforward. Organisms grow bigger over time.

Reproduction

It would be nice if my car could make lots of little cars that are able to grow up and become big cars. Then I'll have lots of cars that I can drive! Sadly, it can't.

Reproduction is the ability for something to make more of itself. Without the ability to reproduce, there would only be a few organisms at some point in time and unless they lived forever, there would be no more of them once they died out. Reproduction allows a species, or type of organism, to survive. We will die one day, but due to our children and their children and so on being born, the human species will continue to live on, even though individual humans die.

For more on reproduction, check out its dedicated article, once it's made.

Excitability

Admittedly, we don't see this word used much. In fact, it's barely used at all other than in this acronym. Excitability is when organisms respond to stimuli. A stimulus (the singular form of the word) is something that an organism reacts to via its senses. For example, our eyes react to light, or really anything we can see in the presence of light; our noses respond to smell; our ears hear sounds and we can feel things through touch.

Excretion

This is the removal of waste products from the body. You know what some of these are and I don't want to bring them up right now. The substances in the air we breathe also get excreted. As I mentioned with respiration, you take in oxygen into your body when you breathe in, but oxygen isn't the only element that's in air. Please see the article on air, when it becomes available. But simply put, the rest of the air doesn't hang around in your body for long and gets excreted.

Waste products are either harmful or they're normal, harmless substances that damage us in high quantities.

Nutrition

Organisms need to eat in order to survive. Some of what they eat comes from other organisms. Animals eat plants and other animals. Plants make their own food, using sunlight. Fungi eat dead things, such as leaves that are rotting away on the ground after falling off trees. Organisms also eat minerals. Not that they chew on rocks to get them but they ingest (another word for eat) minerals in small amounts and these may or not come from other organisms.

So after examining our mysterious thing on the planet, we can see how many of these characteristics it has. If it exhibits all of them, we can safely say it's an organism.

Having said this though, there is one special component of organisms that they all have. Every organism consists of one or more cells.

Cells, the building blocks of life

Back in 1665, an English scientist named Robert Hooke looked at a slice of cork (which is bark from an oak tree) under a microscope which magnified it 30 times (or in other words, it looked 30 times bigger). What he discovered was that the cork was made up of individual 'boxes' which he called cells. He thought it was unique to cork. He didn't realise that this was a major discovery and that in fact these cells are part of every organism.

At the same time lived a Dutch scientist named Anton van Leeuwenhoek, who made magnifying lenses that could see things 300 times larger. He used one to look at drops of pond water and found tiny living things moving around - organisms with only a single cell, known as micro-organisms or microbes. Whenever you see the word 'micro', it means it's something very small. Finally, in 1839, cells were acknowledged as the universal unit of life by two German scientists, Matthias Schleiden and Theodor Schwann.

By the way, you don't need to remember the names of these scientists or these dates just to learn the science. However, it does illustrate how scientists make discoveries and how they collaborate or work together to determine what really exists in nature and how things work.

So back to cells. They are the building blocks of organisms and are too small to see with the naked eye; requiring a microscope (like the one on the left) to get a good look at them. A microbe has only one cell and therefore the whole organism is the cell. Animals, plants and fungi are multicellular: they are made up of many cells. Since there are different parts to them, they have many varying kinds of cells that do different jobs. We can compare them to Lego bricks and other Lego parts. They join together and the different ones vary in the way they look and their function. For example, you have the blocks to build the walls of a house and you have the window blocks. So for example with cells, you have ones that make up the eye and they work together to bring in light. The cells in hands and fingers allow for movement so they can grab and hold things. The cells of a tongue allow it to move and crush food and also to taste it.

The articles dedicated to cells will give you a better understanding. Stay tuned!

*This image is made by applying fluorescent (glowing) substances onto cells. You wouldn't see this just by looking through a microscope!

So there you have it. Now you know how to identify organisms in outer space! Also, you understand a little bit about how organisms are made up (their structure) and how they function.