Have you ever been asked to sweep or vacuum around your house? If not, have you ever noticed the dust that collects all around a room? Believe it or not, most of this dust comes from you and the other people you live with – as dead skin cells!
Your body is constantly producing more and more new cells while other cells die. In fact, our bodies are only made up of cells and some fluids that are trapped between them. This means that living cells make new cells – but really, what are they?
A cell is actually very complex. Some scientists spend their entire lives studying just one little part of how just one type of cell works. However, understanding what a cell is and what it does is actually quite simple. Each cell is basically a bag of water, DNA, protein and some things called organelles. The organelles inside of a cell are responsible for making energy, getting rid of waste and making proteins, among other things.
A living thing is made up of a collection of one or more cells. However, one lone cell cannot accomplish very much – take a swim, eat some lunch, find a mate – but a group of cells can get together and form a much more complicated organism. Sure, that group of cells can take swims, find mates and even eat dinner, but it can do more than one thing at one time. A plant, which is a group of cells, can capture the energy of the sun, pick up water with its roots, turn carbon dioxide into sugar and oxygen and open up its flowers all at the same time! And if you as a human are really talented, you can text message, listen to your teacher, and make googly eyes at your boy/girlfriend across the room all at the same time. Just think, if you were a single-celled amoeba, you wouldn’t even have a pocket for your phone. How wonderful it is to have more than one cell!
We say that cells are the most basic unit of structure. This is true for all living things, whether or not they have bones. For example, a jellyfish has no bones, but cells make up their arms and body. Also, a jellyfish is able to swim and sting their prey because they have special cells that perform certain functions. As humans, we have special cells that beat around 80 times every minute called heart (cardiac) cells. We also have cells that carry oxygen around our bodies called red blood cells. Our bodies need to do thousands of things, or functions, every second. For these reasons, we say that cells are also the basic unit of function in all living things.
You know that if your teacher catches you texting on your phone, that you might get a call home to your grandmother, grandfather, mom or dad. Why are those people most closely related to you, anyway? Because your cells came from their cells! All living cells come from other living cells. A famous experiment was once done on rotting meat. At the time, people believed that flies came from rotting meat – not other flies. So a scientist took two containers of rotting meat; over the first container, he put a screen that prevented flies from landing on it. He left the second container uncovered. Sure enough, flies started to land on top of the second container, and before long, the rotting meat was covered with maggots (fly babies). However, the flies never got on the meat in the first container, no maggots appeared, and no flies appeared inside the container. Flies could only come from other flies!
Since then, scientists have discovered much about cells and also about things that are not considered living cells, such as viruses. Viruses, which lack a nucleus, can infect living cells by using the cell to make more viruses, but without a living cell, a virus cannot make more copies of itself. The virus never gets any bigger or smaller, and doesn’t take any gases or other materials from its environment. Different viruses can survive in different environments, such as HIV which needs to be in body fluids like blood or semen, but no virus can make more copies of itself, by itself. This is why most scientists do not consider viruses to be alive, even though they have organic molecules like proteins.
At this point, you may be asking yourself, what are living cells actually made up of? Truthfully, you’re probably not asking yourself this question, but it would be a good question to ask. All of those organelles in the cell (like the cell membrane, DNA, ribosomes, etc.) are made up of different combinations of just a few chemical elements. It’s thought that these chemical elements were around when the Earth first cooled down 4 billion years ago: carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur. Believe it or not, almost your entire body is made up of combinations of these elements, elements that originally came from our sun!