Source: Images and Video Created by Amanda Soderlind
In this lesson today, we are going to discuss the cell theory, as well as basic characteristics of all cells. So we're going to start by discussing the cell theory. And a theory is a scientific explanation for something that has been thoroughly tested.
The cell theory is actually a theory that explains characteristics of cells-- how cells behavior. And it's the work of several scientists. And they came up with these three points that they believe explain characteristics of cells. These points were come up with through various types of testing and experiments and observations.
The first part of the cell theory states that all organisms are made up of cells. If you'll remember, an organism is a living thing. So anything that is living is made up of cells. That's our first part to the cell theory.
Number two states that cells are the smallest units of life. So the smallest organism is going to be made up of one cell. If it's not made up of cells, it can't be living. So a cell, therefore, is the smallest unit of life. You can probably think of many unicellular organisms that are just made up of one cell, and they are the smallest living things.
All cells arise from preexisting cells. So that's part three of our cell theory. Basically, what this is saying is, cells don't just spontaneously appear. New cells are made from preexisting cells, through different processes they use, for example, meiosis or mitosis.
Those are ways that new cells could be made. So these are the three parts to the cell theory that explain characteristics of cells. All cells, whether they're prokaryotic or eukaryotic-- which we'll get to in just a second-- have three main characteristics in common. And those characteristics are that they all have a plasma membrane.
A plasma membrane is just something that will enclose all of the cell parts. And we'll discuss what a plasma membrane looks like a little bit more detail in just a few minutes when we discuss these two cells down here.
The next characteristic that they all have in common is that they have DNA. DNA is just genetic information. So all cells contain some sort of genetic information that allows the cell to function and reproduce.
And the third thing they all have in common is that they all have cytoplasm. Cytoplasm is basically just like a jelly-like fluid that fills the inside of the cell. It protects the cell and kind of acts as a medium for all the organelles to the in.
We're going to take a look at two different types of cells. The two main categories of cells are prokaryotic and eukaryotic cells. So we're going to start with an example of a eukaryotic cell.
Eukaryotic cells are cells where the DNA is contained in a nucleus. So we're going to look at an example of a human cell. This might be an example of one of our body cells. Our cells are all eukaryotic, because they all contain a nucleus.
Another characteristic of eukaryotic cells, as we said, all cells have a plasma membrane. So that's the outer layer of the cell that encloses all of the cell organelles. In other future lessons, you're going to get into more detail about characteristics of the plasma membrane.
But just to kind of give you a little introduction to that, the plasma membrane of our cells is made up of a lipid bilayer which is composed of two layers of phospholipids. That lipid bilayer is what makes up the plasma membrane of our cells. And that helps to control what goes into and what can come out of the cell.
It also contains DNA. So within the nucleus is where our DNA is contained. And cytoplasm-- all this free space within the cell is the cytoplasm.
Something else you might want to know about eukaryotic cells is that, generally, they are more complex than prokaryotic cells. And they contain a lot more organelles. So here we have an example of a mitochondria, which makes energy for the cell. We have some ribosomes, some other cell organelles you might be familiar with, endoplasmic reticulum, Golgi apparatus, et cetera. So they have several organelles contained within that plasma membrane.
Eukaryotic cells, like I said, are more complex than prokaryotic cells. And they're generally a little bit bigger in size. So if we take a look at prokaryotic cells, here's an example of a prokaryotic cell. And we're going to use a bacterial cell as an example. So we're going to pretend this is some type of bacteria.
Prokaryotic cells do contain DNA-- as we said all cells contain DNA. But the difference between a prokaryotic cell and a eukaryotic cell is that in a prokaryotic cell, the DNA is not contained in a nucleus.
You'll notice, there's no nucleus surrounding the DNA of this bacteria. It's just kind of free floating in the cell. But it does still have the plasma membrane, which is the outer layer of the cell. And I'll go ahead and label the DNA here. And it does still have cytoplasm.
You'll notice in this prokaryotic cell, there are some ribosomes in there as well, which help to make proteins for the cell. So again, prokaryotic cells are a little bit less complex than eukaryotic cells-- generally a little bit smaller. Bacteria would be an example. And many prokaryotic cells have something like this, which is called flagella. Basically, that's just a means of movement for the cell, which is something eukaryotic cells you generally won't find.
This has been an overview of the cell theory, as well as some basic characteristics of prokaryotic and eukaryotic cells.