We're now ready to get into the various organelles and cellular structures that you might find in cells. To make this process a little bit easier because there are a lot of organelles and a lot of cellular structures to go through, the authors of your textbook have divided up the cells and structures into four different categories based on sort of thematically where they fit. So we're going to go through each one of these categories one at a time.
So the first category is a category on genetic control. So here we're going to talk about the nucleus and ribosomes. Second category is a category on a system called the endomembrane system.
Endomembrane system. Endo means inside. and membrane is membrane. So this is a system of a series of membranes that help to produce products for the cell, to polish them, package them, and ship them to where they need to go.
Third category is a category on energy processing. Here we're going to talk about organelles like the mitochondria, the chloroplasts. We're going to throw the peroxisome in there as well. And then lastly we've got kind of a hodgepodge category for the fourth one, but this is going to include any cellular structures that have a structural role. So structural support, we're going to have movement, and then anything that's also involved in communication.
All right, so we're going to go through each one of those sections one at a time, starting with genetic control of the cell. So again, here we're going to be talking about the nucleus. The nucleus contains most of the DNA that's found in a eukaryotic cell. And say most, because it's not all. We will learn later that there are other organelles that actually have their own DNA.
And then, again, it's not good enough just to have the DNA, the genetic information of the cell, but you also need to have the means to express it. So you need to be able to make the cellular products like the proteins which are encoded in the genes. For that process you need ribosomes, right?
So ribosomes use the information in the DNA to actually make proteins. Okay so let's take a closer look at the nucleus is the most conspicuous usually organelle that's found in in cells. When you go to lab and you take a look at for example the onion skin under the microscope.
The nucleus is probably going to be the one organelle that you see inside of the the onion cells. The nucleus is a membrane-bound organelle and it actually has a very unique membrane. Most membranes, if we take a look at them, so let's draw out a membrane, most membranes, right, are a phospholipid bilayer.
So you've got your hydrophilic heads and your hydrophilic tails right on the phospholipids and you've got two of these layers right one on either side and so your membrane looks something like this right with h2o maybe outside of the cell out here and inside out there now that's for example what the cell membrane would look like now the nuclear envelope, which is what we call the membrane of the nucleus, it's unique because it is a double membrane. So instead of just having one phospholipid bilayer, it's actually got two phospholipid bilayers stacked right on top of each other. So technically it's got four individual layers of phospholipids looking like that.
You can imagine that would extend all the way around the nucleus. All right, and so this double membrane we call the nuclear envelope. And you can see that here on the right-hand side of this slide, you see the nucleus there in purple, and it has a nice double membrane there. If we take a look at the blow-up here, you can see the two phospholipid bilayers there, each on a side, right? One facing inside of the nucleus and the other one facing out towards the cytoplasm of the cell.
Now, you can imagine having a double. phospholipid bilayer. It's probably pretty hard to get materials into and out of the nucleus and yet you need access to a variety of different molecules into and out of the nucleus.
And the way that they cross that double phospholipid bilayer is through these nuclear pores. So if we look in the zoomed in little section here across that nuclear envelope, These little things that look kind of like donuts here, this guy and this guy, those are going to be the pore complexes. So essentially it's an aggregation of proteins that help to shuffle or shuttle materials into or out of the nucleus.
Membranes are also fairly floppy. We talked about this in Chapter 5. They're pretty fluid. And the membrane of the nucleus is no exception.
So if you imagine a... latex balloon, right? The latex being the membrane, it's pretty floppy and it's not going to maintain a nice rigid shape unless you fill it with something.
You usually fill a balloon with air, helium, and it'll expand and it'll give that nice rigid structure. Well the same thing kind of goes for the nucleus. You don't want that membrane to sort of collapse in on the DNA, so you have to give it that rigidity, that structure. And this is where something that's called the nuclear lamina comes into play. The nuclear lamina is essentially a layer of proteins.
just on the inside of the nuclear envelope. So you can imagine them just on the inside here. And they create kind of like a scaffold that lines the inside of the nuclear envelope and allows it to sort of maintain its round shape so it doesn't all collapse down on the DNA inside. All right, so if we're talking about the nucleus, we also have to review a little bit what exactly is inside of the nucleus. We know that the nucleus contains DNA.
We know that DNA stands for deoxyribonucleic acid, and we know that DNA molecules are organized into long chromosomes. Now, chromosome is not just made out of DNA, particularly when we're talking about eukaryotic cells, which in fact at this point in chapter 6, everything we're talking about here pertains to eukaryotic cells. We're going to say prokaryotic cells for a different chapter. So in that one chromosome, you've got a really really long DNA molecule.
And if you imagine it's like sewing thread. If I were to unwind a spool of sewing thread and just leave it on the desk, it would be tangled in a matter of minutes. And so the cells need to find a way to organize that DNA so it doesn't get tangled, it doesn't break, the cell doesn't lose any information. So the way that the cells organize their DNA is much like seamstresses will organize their threads.
They wind the thread on a spool, typically a wooden or plastic spool. But in the case of DNA... the cells are going to use protein spools. So if we take a look lower right hand corner of this slide you'll see the blue thread here is your DNA and the DNA is wound around these purple spiky looking things.
Anything purple in your textbooks again is protein. So these structures are actually aggregates of eight proteins that are called histone proteins that form a spool and that spool, that histone spool can take two lines of the DNA, and then you'll get a little stretch of empty DNA, and then you'll have two more lines around the next spool, a little stretch of empty, two more lines around the next spool. This is the way that eukaryotic DNA is organized. And then all those spools kind of condense together to form a material that's called chromatin.
And this, in fact, is what chromosomes are made out of. One single DNA molecule plus all the proteins that are used to organize it. Difference between chromatin and chromosome. I like to use a knitting analogy right so chromatin is the material that our chromosome is made out of so if you imagine yarn would be like the chromatin and then the chromosome would be a sweater right so it yarn is the material that the sweater is made up chromatin DNA plus protein is a material that a chromosome is made out apart from having DNA inside the nucleus We also have a structure that's called the nucleolus. Very close in spelling to the nucleus.
Make sure you know the difference between the two. Nucleolus is going to be sort of a dense region shown here that's a little bit darker in this purple nucleus. This is a specific part of the nucleus that's important for making a special kind of RNA. So we talked about RNAs back in chapter 4, sorry, back in chapter 5. But there are different kinds of RNAs.
The one kind of RNA that's made in the nucleolus is going to be ribosomal RNA or rRNA. rRNA is one of the two components that's used to build ribosomes. So as we'll learn in a minute, ribosomes are part protein and part this ribosomal RNA that's made in the nucleolus.
Alright, so let's talk about ribosomes for a minute. Ribosomes, remember, are not organelles. They are not membrane bound.
They're really just an aggregate of protein and ribosomal RNA. ribosomes while they seem kind of simple they're really very important to sell and that they actually create proteins that the cell needs to grow and to function now there are two different places where you can have ribosomes and this difference in location puts the two kinds of ribosomes into different categories so first of all you can have what are called free ribosomes free ribosomes are going to be ones that are floating around in the cytosol. Free ribosomes make particular kinds of proteins. Free ribosomes are going to make proteins that are going to be used inside of that particular cell. They don't go anywhere else.
So typically, let's say enzymes that are floating around inside of the cytoplasm or maybe parts of the cell's skeleton that stays on the inside. The other kind of ribosome is a bound ribosome. Bound ribosomes are going to be found attached to a membrane.
There are two places where bound ribosomes can exist. Number one, on the outside of the nuclear envelope. Number two, on the outside of another organelle that's called the endoplasmic reticulum. Let's take a couple steps back and look at this slide, right?
So we have our nucleus here in purple, and you'll notice that there's another organelle closely associated with the nucleus here in blue. That's the endoplasmic reticulum. You'll notice that the outside of The endoplasmic reticulum and the outside of the nuclear envelope, they're both spotted with these little circles.
If we look at the blow-up, those circles are in fact ribosomes attached to the outsides of these membranes. Bound ribosomes are going to make a different set of proteins. Bound ribosomes make proteins that are either destined to be exported out of the cell, like insulin, for example, is made by pancreatic cells, and the cells then dump it to the outside. so that the bloodstream can pick up the hormone and it can travel throughout the rest of your body. So, bound ribosomes can make exported proteins, or they'll make proteins that are meant to function inside of the cell's membranes, either organelle membranes or maybe even the cell membrane itself.
Ribosomes also have two parts. So, just a very simplistic view here of a ribosome. You'll notice it has a large subunit, call it the big potato, and then the small subunit, call it the small potato.