Characteristics of eukaryotic cells

- [Voiceover] All living things are made of cells. And humans and all multicellular organisms are made of a type of cell called eukaryotic cells. But what is it that makes a cell eukaryotic? To answer that question, let's look at the two major types of cells. On the left is a prokaryotic cell, and on the right is a diagram of a eukaryotic cell. So let's define eukaryotic cells as how they're different than prokaryotic cells. So usually, eukaryotic cells are much larger than prokaryotes, and they're also, eukaryotic cells are also found in multicellular organisms, although there are single cellular eukaryotes such as certain protists. But what are the defining characteristics that make a cell eukaryotic? The most important thing that eukaryotic cells possess that prokaryotes don't is the idea of compartmentalization. This is kind of a big word, but essentially, what it means is that the cell is divided into different compartments. And we can see some of these compartments drawn here on the right in our eukaryotic cell, whereas our prokaryotic cell doesn't appear to have many different compartments. But why is it that a eukaryotic cell has lots of different compartments within it? Well, I kind of think of it as rooms in a house. So if you have lots of different rooms in a house, each room can do something different. So you could have a living room, and a kitchen, and a dining room, and a bathroom, and a bedroom. And they all have a different function. Prokaryotic cells then would be like a one-room house where everything is in a single room. And you could see that this single room has to be able to do everything. So the compartmentalization of the eukaryotic cell into different compartments or kind of rooms allows each different compartment to have a different function. And this is really important in the complex needs of the cells. The name for these compartments in the eukaryotic cell is organelles. Just like your body has different organs that have different functions, your heart pumps blood, your lungs take in oxygen, different organelles in the cell have different functions. And these organelles are separated from other parts of the cell by membranes. And the name "eukaryotic" comes from one of these organelles. The word "eu" means "well" in Greek, and "karyote" means "kernel" or "nut." So when scientists originally looked at eukaryotic cells, they saw a very well-formed circular object in the center that looked kind of like a nut. And so, they called these cells "eukaryotes" because they had a well-formed nut in the center of the cell. This nut is what we now know as the nucleus. The nucleus is a circular membrane-bound organelle that contains all of the genetic material in the cell. So all of your chromosomes are inside the nucleus. This separates them from the outside of the cell. The nucleus is the defining organelle of eukaryotic cells. In contrast, prokaryotic cells have their genetic material as well as everything else in the cell just kind of floating around all in one big space. And so, in prokaryotes, to divide and become new cells, all they have to do is make two copies of everything, and then split down the middle in a process called binary fission. But in eukaryotes, we can see that because the cell has all of these different compartments or organelles, there is a more complex way to divide into a new cell. And the process that eukaryotic cells use to divide is called mitosis. So these are the defining characteristics of eukaryotic cells that sets them apart from prokaryotic cells. They're compartmentalized into different membrane-bound organelles, the most important of which is called the nucleus, which is responsible for the name "eukaryotic." And finally, they divide by the process of mitosis. Let's go through some of the other important organelles within eukaryotic cells. So first, let's go back to the nucleus. And the nucleus, I kind of think of as the control center of the cell because that's where all of the DNA is, and where DNA is transcribed into mRNA. Probably the next most important organelle in the body are called mitochondria. Mitochondria, you can think of as the cell's power plant because mitochondria is the site of cellular respiration where glucose is converted into ATP to provide the energy for everywhere else in the cell. The next organelle we'll look at is called the endoplasmic reticulum. And so surrounding the nucleus are dozens and dozens of folds of membranes in this really complex pattern. But the unique thing about it, inside these folded membranes is it's all continuous, so it all encloses one single compartment, even though the membrane is folded back and forth on itself lots and lots of times. And the endoplasmic reticulum is primarily the site of protein synthesis. mRNA, which is transcribed from DNA in the nucleus, is translated at the endoplasmic reticulum. So I think of the endoplasmic reticulum as the factory of the cell where proteins are produced. After these proteins are produced, they are sent to another organelle that also has kind of a lot of folded membrane structure, but it's not a continuous compartment like the endoplasmic reticulum. And this organelle is called the Golgi apparatus. And the Golgi apparatus is kind of like the mail room of the cell. So proteins that are produced in the endoplasmic reticulum are sent to the Golgi apparatus, and the Golgi apparatus is able to send those proteins to other parts of the cell, depending on where that protein needs to go. So if the protein needs to be secreted by the cell, the Golgi apparatus sends it to the outer membrane of the cell and releases it. If it needs to go to a different organelle, the Golgi apparatus can send those proteins, as well as other biological macromolecules, to those sites as well. The last two organelles to mention are the lysosome and the peroxisome. And cells can have multiple ones of each of these, but I mentioned them together because they both have a similar structure. They're kind of a spherical organelle enclosed by a single plasma membrane. And the environment inside the lysosomes and the peroxisomes is very different from the environment outside in the cell's cytosol. And so the lysosomes and peroxisomes are kind of like recycling centers in the cell. That's because when cellular components are no longer functional and need to be broken down, they're trafficked to the lysosome where they are broken down into their more basic components that can be reused to rebuild new proteins or other macromolecules. The peroxisome has a similar function to the lysosome, but it's a little bit different because in the peroxisome is the site where reactive oxygen species like peroxides are reduced into nontoxic forms. So these are the main organelles of the eukaryotic cell, which we remember is defined by the compartmentalization of the cell into different membrane-bound organelles.