Lecture 2.4 The Cell and How it Works — Organelles

HAZEL SIVE: In conclusion of our first topic today, cellular chemistry, you should now be familiar with condensation and hydrolysis reactions. You should know what free energy means and be able to predict whether a reaction will proceed spontaneously or not. And you should know something about how biological pathways are put together. Let us move on then to our next topic, which is organization of the cell. Last time we talked about the cell as the building block of life, a little package of chemical reactions and macromolecules that can be used kind of like a brick to build a living organism. So let us consider this further in topic 2, cellular organization, and let us talk about the cell as some kind of building block. If you like, you can add of life next to it. So how do you get a building block? When you think about anything that is some kind of brick or some kind of package, it has some kind of surrounding material. And for the cell, this is the cell membrane. So the cell is surrounded by an important structure called the cell or plasma membrane. Can also call it the cell membrane. That is fine. And in fact, as we'll discuss, there all sorts of membranes even inside the cell. I want you to think back to the lipid section from our previous discussion where we talked about amphipathic lipids. And the plasma membrane is made of hydrolysis lipids that have got both polar and nonpolar character. And they form a bilayer as I'll show you on some slides in a moment. Now, one of the things about the plasma membrane is that it is hydrophobic. It is largely hydrophobic. And so the cell, this package of chemical reactions and products that are being made, is effectively sealed in a plastic bag. In order to get things into the cell or out of the cell, you therefore have to make some holes in this hydrophobic cell membrane to allow things in and out. And so various structures called channels or pores allow molecules through the cell membrane. Let's take a look at a couple of things here. Let's firstly talk a little about cells. There are lots of different kinds of cells. I've shown you three different kinds here-- red blood cells, spermatozoa, and neurons, each of which carry different functions. Red blood cells carry oxygen. Neurons are the information transfer system of the body. And spermatozoa carry the hereditary material of males. Each of these is surrounded by the lipid bilayer that we just discussed. And let me throw out something cool while we're thinking about cells as building blocks. You really can build with cells. They are sturdy enough to do so. Your whole body is made of microscopic cells. 10 microns-- a micron is a millionth of a meter in diameter-- all stuck together. And that's what makes our bodies. And you can build with them in really cool current ways. Here for example, a 3D printer is making an organ. You're making it mixing a polymer and various cells together and printing out a body part that can then be used in transplants. So these cells are held together by the lipid bilayer and can be used as building blocks. And let us take a little look at what the lipids look like that surround the cell. These are phospholipids. They have got a positive charge on one side of the molecule that makes them interact with water. And then they've got a really hydrophobic part that makes them not interact with water that makes them hydrophobic. These lipids spontaneously join together so that their hydrophobic ends are touching, and their hydrophilic ends are facing the water. So they make for themselves a hydrophobic internal core. And then on either side, they are facing the water which they like to interact with through their hydrophilic heads. And you can see here the bilayer. This is one layer of lipid molecules. And the top is another layer of lipid molecules that have come together in this lipid bilayer. And then as I mentioned, one has to get things through this hydrophobic lipid bilayer. And there are really two ways. Hydrophobic molecules can generally diffuse across the plasma membrane or other membranes. But hydrophilic molecules need to go through special openings in the membrane. And we've shown you here a channel or a pore that really is a hole in the membrane. It may have a way of opening and closing. But it will allow particular molecules into and out of the cell. We can schematize cells as shown in the slide. And I've written here that cells are replicating, membrane-bound factories. That's my way of thinking about what these building blocks actually are. So we've talked about the membrane bound part. What about the insides of the factory? What is inside the cell that makes it function? And what is inside the cell are a set of soft cellular structures called organelles. These are subcellular structures. And they have specific function. They may be surrounded themselves by a membrane. So they may be membrane bound. And there are lots of them. And what we're going to do in a moment is to list some of these so that you can see what's inside a cell. On this diagram, you can see again a whole bunch of different schematics depicted inside the schematic of a cell. Gives you a kind of a sense that the cell is packed with various subcellular structures each of which carry out specific functions and are really, really both absolutely essential and really fascinating. So let's make a listing that has the particular organelles and then the function of that organelle in the cell. And the first one we'll start with is the nucleus. And this is the part of the cell that carries the genes. It's where most of the hereditary material is kept. Now, on the slides, you will see that there is both a prokaryote like a bacterium that actually doesn't have a nucleus and, in fact, doesn't have a lot of these same structures that I'm listing for you on this board. And on the other side is a eukaryote, cells like human cells like our own cells. So we're going to talk pretty much about organelles that are in eukaryotes, e.g. human. The rest of the cell other than the nucleus is called the cytoplasm, everything that's non-nuclear. And the cytoplasm contains the various other organelles. The mitochondria has a number of functions. But the one that is best understood is its role in energy conversion. It's where the ready energy, the dollars and cents of being able to actually carry out chemical reactions, it's where the ready energy is produced. In plants, there is something called a chloroplast that again is involved in energy production. There is a set of organelles that are all involved in synthesizing proteins that we discussed previously. These are the endoplasmic reticulum, which always sounds like something from a horror movie-- this is involved in protein synthesis-- the ribosomes and the Golgi apparatus. And all of these are somehow involved in protein synthesis and production and figuring out what proteins are going to go where. There is the lysosome, which is involved in breaking down cellular components that are no longer needed. And then there is a set of structural components that gives the cell its shape and its integrity so it stays together and also may allow the cell to move. So we are going to list these all here as the cytoskeleton, cilia, and flagella. And all of these have got something to do with movement and in some cases with shape and structure of a cell. That is a real quick flyby of a huge amount of information. But I want you to have heard of all of these things. Here is a listing of the organelles and the things that they do. So let us take a moment for you to go and do a review question and really understand a bit about what is inside the cell.