Hello multicellular organisms and welcome back to Biology 3012. Today we're going to take a look at chapter 4, a tour of the cell. So we know that all forms of life are made up of cells and I had said that we are multicellular organisms and what that means is that we have all kinds of cells that all work together. They all take on different structures, different functions to be able to work together as an individual organism.
So all forms of life that we look around and we can see with our naked eye, those are all multicellular organisms. So all plants, animals, and most fungi are all multicellular organisms. But out there, there is also so many even more abundant single-celled organisms.
And these are organisms that we can't see with our naked eye. They are all made up of just one individual cell. The entire organism is just one cell.
And since we can't see one cell with our naked eye, it means that we don't see all of these forms of life around us. If we look at this measurement chart, it gives us a little bit of an idea of the scale of some of these different concepts that we're going to look at today. So this... is a logarithmic scale again, so it's similar to what we learned about with the pH scale where each one of these units represents 10 times. So what that means is that if you look at the difference between 1 millimeter and 1 centimeter, 1 centimeter would be 10 times bigger than 1 millimeter because it's one unit.
Now for those measurements, for the most part, people know there are 10 millimeters and 1 centimeter, so it makes sense that 1 centimeter is 10 times larger than 1 millimeter. But when we go down here to nanometers and micrometers, we might not be as familiar with them. So this here would be saying, let's go with 1 micrometer here, also called a micron. Well, how small is a micron compared to a millimeter? Well, look on this logarithmic scale and it's 10 times 10 times 10. So a thousand times smaller.
There would be a thousand microns in one individual millimeter. So if you're talking about things that are this small, well then of course you can't see it with your bare eyes. Something that's a thousand times smaller than a millimeter. On this chart here, you can see the scale of of um measurements that we can see with our naked eye. So anything that's this large we can physically see but anything smaller than this we can't see with our naked eye.
Well then how do we know about them? Well scientists learned about them when we um with microscopes. So there's different types of microscopes.
The light microscopes were the first microscopes that were invented and The way that they work is that light passes through the organism and then there's a lens that helps to magnify that organism that's under the microscope. So magnification means it's increasing the specimen's size. But another concept with microscopes is resolving power.
And resolving power means our eyes are able to separate two objects as separate. So what I mean by that is if I put... two grains of salt on the table and you were looking and you wanted to see okay well is it two grains of salt or is it one grain of salt?
The closer they are together the harder it is for our eyes to see it as two separate grains of salt. As long as those grains of salt are at least 0.1 millimeters apart from one another, so a tenth of a millimeter apart from one another, then our eyes are able to resolve it as two objects. meaning it's able to differentiate it so that we can see that it's two individual grains of salt. Anything that's closer than that though, closer than a tenth of a millimeter, we would just see it as one object. So those two grains of salt would just look like one grain of salt.
So when, why does it, why does resolving power matter? Well, if we can't see anything as two separate objects if it's closer than one millimeter apart, All of the things that we're talking about at the cellular level, they're going to be smaller than our eyes can see and smaller than our eyes can resolve or distinguish as separate objects. So the microscopes help us to be able to learn a lot about these cells. And this is actually a great example of how science advances technology, but technology also helps to advance science. When we advance technology, well then having these better tools to be able to better study science, it allows us to learn a lot more.
So the light microscope allowed us to be able to resolve points that were 0.2 micrometers apart. So that would be 0.0002 millimeters. So much more resolving power than our bare eyes.
That allowed us to learn so much more about cells because we were able to see so much better. Well then later with the invention of the electron microscopes, we could see even more. Now all of a sudden, the resolving power was the ability to be able to see 0.2 nanometers apart, so even a thousand times smaller than what we could see with the light microscope.
And we could magnify objects up to a hundred thousand times, so we could start to learn a lot more. The pictures that are taken from under a microscope are called micrographs. So here you can see this is a picture taken from a light micrograph. Oftentimes when you're looking in the textbook, it won't say light micrograph, but next to images, you'll see something like LM. And what that means is that that photo was taken underneath a light microscope.
If you see SEM, that stands for scanning electron micrograph. So this was taken under an electron microscope, and in particular, a scanning electron microscope. which is good for being able to see the surfaces of the specimens under the microscope.
A transmission electron micrograph is a picture taken under the transmission electron microscope, and this microscope is able to transmit into past the surface of the cell and into the inside of the cell. So you're able to see those objects, those internal details of the cells. Okay, so after the invention of the microscope, which was first invented in the 1600s, scientists started to be able to look at all forms of life under the microscope.
And every time they were looking under the microscope, they said, well, that's weird. All of these things seem to have these little cells inside of them. So eventually, after a couple hundred years of scientists examining these organisms and realizing that they all have cells, eventually this led to the development of the cell theory in the mid-1800s. And what this is saying is that all living things are composed of cells, and cells, they can't just spontaneously appear.
These cells all come from earlier cells, so cells create more cells. Now all of these cells though, they don't look identical to one another. They can have different sizes, different shapes, and different functions, but eventually scientists categorized cells into a couple of major categories here. They divided them up into prokaryotic cells or eukaryotic cells. Prokaryotic cells are smaller, more simple cells, whereas eukaryotic cells are larger, more complex.
There's more little machines inside of it here that we'll take a look at. So there's a lot of details on this table here but we're going to break it down in the next couple of slides. So let's look at some of the similarities between these prokaryotic and eukaryotic cells first. Well, all of these cells are made up of a plasma membrane.
So this is just a thin barrier that's separating the inside of the cell from the outside of the cell and all cells have this. Another similarity is that they all have this fluid inside of them and that's called cytosol. And that fluid is where the rest of the components that are inside of the cell are just floating around in that fluid.
Now many students here they get a little confused and they're like, well I thought that fluid inside of a cell was called cytoplasm. Well cytosol is the actual fluid itself. The cytoplasm refers to the fluid, so the cytosol, as well as everything else that's floating around in that cytosol.
So cytoplasm is cytosol plus the organelles and all those cellular components floating around in it. Other similarities of all cells, they all have at least one chromosome. So this is the genetic material inside of the cell, and this is made up of DNA. Another similarity here is that they all have ribosomes.
Ribosomes we're going to take a look at, but these are little structures that help to build proteins. In our last chapter we talked about proteins, how crucial they are for living organisms, how they have so many different functions. Well, we have to have a way to be able to build those proteins so that they can complete all of those important functions.
So, Ribosomes are what's going to read the instructions from that DNA and be able to make the proteins. So all cells have ribosomes regardless of whether they're prokaryotic or eukaryotic cells. Well, now then let's look at some of the differences.
Well, prokaryotic cells, they're much older. They first appeared in the fossil record about three and a half billion years ago. and they evolved on Earth on their own for a long time.
Eukaryotic cells, they only appear in the fossil record about 2.1 billion years ago. So still a long time, but not nearly as long as those prokaryotic cells. The prokaryotic cells are much smaller. It's often said that they're around 10 times smaller on average than the eukaryotic cells. And they're much simpler in structure.
They don't have membrane-bound organelles. And we'll take a look at what that means, membrane-bound organelles. But eukaryotic cells have these membrane-bound organelles that are like our DNA or endoplasmic reticulum. And we'll take a look at those and what the membrane-bound component means, as well as the function of all these different membranous organelles. Some other differences here, the prokaryotic cell, it doesn't have a nucleus, it just has this kind of general region where the DNA typically congregates inside of the cell.
Whereas for eukaryotic cells, there's a nucleus that's going to actually wrap around the DNA and form a little house for that DNA. Another key difference is that prokaryotic cells typically have a rigid cell wall, Whereas eukaryotic cells do not. We will see though that different eukaryotic cells do have different structures So animal cells don't have cell walls, but plant cells do have cell walls So some variation there.
If we take a look at a prokaryotic cell This is just showing a very generalized prokaryotic cell. So again, there's so much diversity They don't all look like this, but let's look at a couple of the different structures here So we mentioned inside here we have the genetic material, the DNA, that's just contained within this nucleoid region. So it's not wrapped around here, there's no membrane here that's enclosing that DNA, it's just within held within this region inside of the cell that's called the nucleoid.
All these little dots are the ribosomes, so there are ribosomes inside of here. The yellow membrane is the plasma membrane that all forms of life have, and we'll take a look at the structure of that plasma membrane today. The green is the cell wall that prokaryotic organisms typically have.
And then outside of that, you have this orange capsule, this kind of sticky coating that some prokaryotic organisms have as well. Some other structures that we haven't talked about yet are a couple of structures for locomotion. So we'll look at a few options here for locomotion or for to allow that organism to be able to move These pili they are little they're little projections all over this the body here of this organism all over the surface of this organism and they can help with locomotion But they're also attachment structures so they can help that organism stick to a surface here the flagella are these longer projections that can undulate back and forth to be able to propel that organism through its environment. So there's lots of structures here, but again this is a much more simple type of cell as compared to the eukaryotic cell.
So then let's take a look at the eukaryotic cell and what it looks like. This is showing an idealized animal cell. So what that means is that most of our animal cells, most of the cells within our bodies, don't actually look like this.
This is showing an idealized one where it's showing all of the different organelles inside of it. Inside of our bodies, our cells are going to take on different structures because they're all going to have different functions. And we know that structure and function is closely related to one another. So the Cells that we find within our muscles are going to be completely different than the cells that we find within our skin. So these the cells are going to have different function or different structures that lead to different functions and they're going to have different quantities of the different organelles inside of them too.
And speaking of organelles, we can see here that there's lots of organelles inside of this animal cell. So we can see here that it has a nucleus, so this is a nice membranous little ball inside of the that cell that's going to house the DNA, so the DNA would be inside of here, so it's keeping that DNA, the genetic instructions, nice and safe. We can see those membrane-bound organelles, so organelles that have a membrane, so we've got the endoplasmic reticulum here that we'll talk about. lysosomes, lysosomes what we can actually only find in animal cells not in plant cells.
We've got this Golgi apparatus made up of individual Golgi bodies here. We've got the mitochondrion or plural mitochondria. We can see that all these little dots, we have lots of ribosomes here similar to what we saw in those prokaryotic organisms.
And we also have this cytoskeleton. So this is kind of like a skeleton, like our skeleton provides support to our body. This is the skeleton of that individual cell that's going to provide some structural support to that cell.
So there's a lot going on here in this generalized animal cell. In today's lecture we're going to focus on looking at the structure of each one of these organelles and then talk about the function of them as well. This slide here is showing a similar eukaryotic cell, but this is a plant cell rather than an animal cell.
So there's a couple of key differences to point out here. One, we said that the plant cells don't have lysosomes, but plant cells do have chloroplasts, which animal cells don't. We can also see here that this plant cell has a rigid cell wall around its cell membrane or its plasma membrane that you don't find in animal cells.
The last difference that I'll point out here is that you can see this large vacuole. Animal cells do have vacuoles as well, but they're not typically as large and significant as that. So let's take a look at each one of these organelles and what their structure and function is. The first structure that we're going to take a closer look at is the plasma membrane.
So remember this is the thin membrane that's separating the living cell, the inside of the cell, from the surroundings around the cell, the non-living environment. Now the plasma membrane is made up of a phospholipid bilayer. Phospholipids are an interesting molecule that has two components to it, a head and a tail. Now the interesting part about it is that the head is hydrophilic. And remember hydrophilic means water loving or it is able to mix with water.
The tail is hydrophobic. Remember hydrophobic or water fearing meaning that it's not able to mix with water. So these phospholipids they end up allying in a bilayer or a double layer and the reason they do that is so that those hydrophilic heads are directed toward the outside of the cell and the In cytosol or the cytoplasm on the inside of the cell, remember most of a living organism is made up of water. So you're going to have lots of water here. You're going to have lots of water here.
These hydrophobic tails are like, I don't want to be near the water. So the hydrophobic tails are directed toward one another in this double membrane so that they aren't attached to the water at all, aren't interacting with the water. Now you might be thinking, well, what about over here?
Over here, they're going to be interacting with the water. But no, this is just showing a cross section, just a small section of that plasma membrane. We have to remember that this would be forming a sphere. So if this was just a sphere, well, then all of these on the inside of the sphere would all, the inside of the sphere would all be made up of these hydrophilic heads.
And then if you picture the outside of that sphere would all be made up these hot of these hydrophilic heads and the little hydrophobic tails would be on the inside of that bilayer where they're protected from the water. So even if I took, if I could physically take a handful of phospholipids and I threw those into a bowl of water, over time they would actually automatically orient themselves into this bilayer and this bilayer would be in the shape of this sphere. Because that's the only way that those hydrophobic tails can all be protected from that water and those hydrophilic heads can all be interacting with the water. So it's really neat the way that they are able to do this automatically just because of the chemical properties that they have of being part hydrophilic and part hydrophobic. Okay, now the plasma membrane is often referred to as a fluid mosaic.
Well, what does that actually mean? The fluid part means that the plasma membrane is not a rigid structure. It's free moving. It's constantly moving.
All of these little hydroph, um, all these little phospholipids, they aren't, um, rigid in space. They're constantly moving around with one another. Now the mosaic part, if you think of a mosaic and artwork of all these kind of, I picture my kids mosaics that they bring home from school sometimes, where it's all these different components that are all glued onto this piece of paper.
Well, the mosaic part of a plasma membrane is referring to all kinds of proteins. that are studded within that phospholipid bilayer. So we can see here lots of different proteins and that's the mosaic part of the plasma membrane. Now these proteins are all going to have different functions and we'll take a look at those functions in chapter five. Okay well then the plasma membrane we said for plant cells on the outside of that plasma membrane we have a cell wall.
That cell wall is a rigid structure surrounding that plasma membrane, and its job is to protect the cell, maintain the shape of the cell, and it actually helps prevent the cell from absorbing too much water too. It helps to maintain turgor pressure, which we'll also talk about later in the course. These plant cell walls are made up of cellulose.
Now if you remember back to our last chapter, cellulose is just a... polysaccharide. So it's a complex carbohydrate that's made up of linking lots of glucose molecules together, and they end up forming these really rigid fibrils.
So the cellulose that's all packed in together is what's forming these rigid cell walls. Now animal cells, they don't have cell walls, but they do have something. They have what's called an extracellular matrix, which is really just kind of a big mess of collagen fibers and some other fibers as well, that their job is to help maintain the shape of that cell, but also to help these cells become sticky. So they stick all of these cells together into tissues.
Tissues, we said, are collections of cells that all work together toward a common function. So the extracellular matrix helps to... hold these cells together into the tissues.
Well, if they're working, if these cells are working together in a tissue, they have to be able to communicate with one another. And one of the ways that they communicate is through little kind of secret hallways called cell junctions. They're little hallways that are connecting these cells together. And these little hallways between the cells allows for coordinated communication. from one cell to the next within that tissue.
The next structure that we're going to take a look at is the nucleus and the ribosomes, which is the genetic control of the cell. The nucleus is referred to as the control center of the cell because it's the component that has all of the instructions for the rest of the cell. If you picture the cell as a factory, And inside of the factory, you've got all these machines, all these tools, all these people that are like, yeah, okay, we're ready, let's work. Except they're like, well, what are we making?
When are we making it? How do we make it? Well, that's where the nucleus comes into play.
The nucleus is what's going to provide the instructions to say, here's what we need, here's how we build it, this is when we need it. So it makes sure that the rest of the cell... knows how to function and is functioning properly.
So let's take a look at the structure of the nucleus. The nucleus is made up of this nuclear envelope, which is a double membrane similar to what we saw with the plasma membrane. Inside of the nucleus, on the very inside here, you can see this dense part called the nucleolus, and this is where the components of the ribosomes are made. So the ribosome is made up of a couple of subunits that we'll take a look at.
Those subunits are made here, and then they pass out through these nuclear pores into the cytoplasm, and that's where the ribosomes will stay. You also see inside of the nucleus here, all these purple strands are the chromatin. The chromatin is the loose form of DNA, and the DNA is the actual genetic instructions. So if I said, alright, here's a page of instructions and we are going to follow these instructions to be able to make a... Let's just go with an Ikea desk.
These instructions are going to tell us how to put together the Ikea desk. And then I take these instructions and I fold them in half and I fold it again and I fold it again. And I keep folding it until these instructions are like a little inch by inch square. And I say, okay, here's the instructions, make the desk. Well, the first thing you would have to do is unfold those instructions so that you can actually read the information.
Most of the time when we think of DNA, we think of DNA in this chromosome form. The chromosome form is the version of those instructions that are all wrapped together, are all folded up. The chromosome is important for the nucleus to organize its DNA before cell division, but when that DNA is tightly coiled together and wrapped up like this, you can't access the instructions. So you need to have a loose form of the DNA where you can actually get the instructions that are contained within those letters of that double helix. So chromatin is this looser form of DNA so that you can actually read those instructions.
This is where ribosomes come into play. Ribosomes are the organelle that's responsible for reading the instructions and actually making the proteins. So they're the ones that are actually making that IKEA desk from that instruction booklet. Well, the components of the ribosome we mentioned are made inside of the nucleus, but then they come out and the ribosomes are found outside of the cell, or pardon me, outside of the nucleus. They can be found floating around in the cytosol, or they can be found studded in what's called the endoplasmic reticulum that we're going to take a look at shortly.
So the ribosome's job is to read the genetic instructions and make the proteins. Well, we said that the genetic instructions are inside of the nucleus. The ribosomes are outside of the nucleus.
Well, that's a bit of a problem. If you actually can't get into the room where the instructions are held, then you can't read those instructions to actually make that IKEA desk. So you need some kind of a messenger.
The messenger is what's going to take the information from the nucleus, bring it out to the ribosomes, so that the ribosomes are able to read the instructions and make the appropriate proteins. The messenger is a form of RNA called messenger RNA. RNA or mRNA.
That mRNA is just a copy that's made of the instructions in the DNA. It creates an exact copy letter by letter of those instructions and then it's able to leave through those little pores in the nucleus out into the cytosol. So we saw the nuclei or those pores in the nucleus were important for the components of the ribosome to leave, but it's also important for the mRNA to be able to leave so that now you have the ribosomes, they can access that messenger RNA and read those instructions to be able to make the corresponding proteins.
And then we've already talked about how proteins are the main components of the cell that actually do the work of the cell. So let's say the cell needs something transported. Well, it's the protein, a particular protein that gets made that's going to be what actually transports that substance across the cell. If you are a muscle cell and you need to contract, well, it's the proteins that are actually helping that muscle cell to contract.
So it's all of these different proteins that are actually going to carry out the functions required for that cell to be able to survive. Next we're going to take a look at the endomembrane system. Endo just means internal.
So this is just the internal membrane system. So really it's just all of the membranous organelles inside of the cell. So we took a look at the nuclear envelope already.
We saw that it's made up of a membrane so it's also one of these components of the endomembrane system. But we're going to focus on these other ones here, the endoplasmic reticulum, the Golgi apparatus, lysosomes, and vacuoles. Now all of these components of the endomembrane system work closely together, so they're either all physically connected to one another, or they can be linked by little transport vesicles. These are little membranous sacs that can carry particles from one part of the endomembrane system to another. Let's take a closer look at the endoplasmic reticulum first.
So this is the main manufacturing facility of the cell, and it produces a huge variety of different molecules. You can see the location of it is surrounding that nuclear envelope, so it's actually connected to that nuclear envelope. And the endoplasmic reticulum is made up of two different components. The rough...
ER or the rough endoplasmic reticulum and the smooth ER or the smooth endoplasmic reticulum. The smooth endoplasmic reticulum, if we zoom in on it here, you can see that it looks smoother and the reason that it looks smoother is because it doesn't have these ribosomes studding its surface. So since it doesn't have ribosomes, we know that it's not making proteins, but if it's part of the manufacturing facility, it has to be making something. So the smooth ER is responsible for making lipids. So we saw with lipids things like triglycerides, also steroids, so something like cholesterol.
An interesting component of the smooth ER is that it also helps the liver to detoxify circulating drugs. So if you would look at the smooth ER in liver cells as opposed to other cells around on the body, you're going to have a larger amount of smooth ER in those liver cells so that it can help to detoxify circulating drugs. An interesting part is that the amount of smooth ER in those cells forms the basis of strengthening tolerance.
So what I mean by that is that if you think of, let's go with alcohol. If you don't ever drink alcohol, then you're going to have a smaller amount of Smoothie R in your liver cells as opposed to somebody who drinks a lot more alcohol. Well then, if you drink one beer today, then your body, as you drink beer or as you drink alcohol, your body is going to have to detoxify the alcohol. And you're going to actually have...
larger amount of smoothie R in those liver cells the more often you drink. So if you don't normally drink then you have a small amount of smoothie R. The more you drink the more often your body has to detoxify these these drugs well then the more smoothie R you're going to develop over time. So maybe originally you had a small amount of smoothie R and after one drink you would feel a little bit of a buzz.
Well then now after you've been drinking for a while that one drink isn't going to do anything because you have a larger amount of smooth ER that's going to be able to detoxify that alcohol much quicker. So now you're going to need two beer or three beer etc. The more you drink or the more often you drink I should say the more smooth ER you're going to develop which means the faster your body's going to be able to detoxify that alcohol that enters your body.
So just an interesting little kind of aside there for smooth ER. The next component of the endoplasmic reticulum is the rough ER. So rough is referring to the ribosomes, the appearance of the ribosomes when you're looking at that ER from underneath a microscope. And that the rough ER, because it has ribosomes, we know the ribosome's job is to make proteins.
So the rough ER is really focused on making proteins. And now we saw with ribosomes, they can be studded in the rough ER, or they can just be floating around in the cytosol. Well, the ribosomes that are found on that rough ER are going to be making the proteins that are transported to other organelles and eventually exported out of that cell.
So if we look at this diagram here, you can see this is just showing a little, the little end of the rough ER studded with that ribosome. that ribosome is going to read the mRNA, make the polypeptide that folds into the protein. That protein is going to get transported through a little transport vesicle.
So you actually have a little bit of the membrane here that pinches off of the rough ER forming this transport vesicle. And then that transport vesicle most often is next going to the Golgi apparatus. The Golgi apparatus is the organelle that's going to receive, refine, and store the chemical products of the cell.
So it's really modifying those proteins that are made by that endoplasmic reticulum. Now, some students say, well, Golgi apparatus, I thought it was Golgi body. Well, one of these individual plates is called a Golgi body.
The Golgi apparatus is this entire unit made up of these stacks of membranous plates. So the Golgi apparatus is going to work closely with that endoplasmic reticulum. If we look at this diagram here, you can see how that transport vesicle that we just saw butted off of that endoplasmic reticulum is going to come and merge with that Golgi body that that protein inside of it will be able to get released into the Golgi body.
It's going to get refined, and then once it's all refined and ready to go, you're going to have another transport vesicle that buds off of that Golgi body, so you have that more polished protein. Well, then that protein can then merge with the plasma membrane, or that transport vesicle can merge with the plasma membrane and release that protein outside of the cell. So that's how that protein is going to get exported out of the cell.
Now another component of the endomembrane system that we haven't covered yet are lysosomes. Lysosomes are these little membrane enclosed sacs that are full of digestive enzymes, and they're primarily found within animal cells. So the digestive enzymes inside of the lysosomes, they're able to break down large molecules.
So those large molecules that we talked about in the last chapter like proteins, we can, the lysosomes can help break those proteins down into their individual building blocks or monomers. So remember the monomer for proteins are the amino acids. The monomer for polysaccharides are the monosaccharides. Fats or triglycerides can be broken down into the glycerol and the three fatty acid tails. Nucleic acids can be broken down into their individual nucleotides.
So these digestive enzymes are going to be able to break down these large molecules into smaller molecules. So a couple of functions of lysosomes. One of them is used for digestion. So.
You can have cells that are going to engulf little nutrients in these little food vacuoles or in these little sacks. And then the lysosomes can merge with those food vacuoles to be able to break down those large molecules into its building blocks. And then the cell will be able to use those building blocks to remake whatever proteins it needs at that particular point in time. Another function here is that the...
A lysosome can merge with vacuoles that are containing dead or damaged organelles, and it can break down those organelles, again, break it down into its building blocks, and then be able to remake new functioning organelles. Now, if you think about it, these lysosomes, they're carrying digestive enzymes. Now why do you think we would need to have the digestive enzymes in these little vesicles rather than just free-floating through the cytoplasm?
Well, the cell wants to be able to control what is getting broken down. So those lysosomes keep the digestive enzymes together into one little vesicle so that it can merge with whatever it needs to be getting broken down. If those digestive enzymes were just free-floating through the cytoplasm, Well, then maybe those digestive enzymes are going to start breaking down functioning organelles. Maybe they're going to start breaking down things that you don't actually want the digestive enzymes breaking down. So by having them in these little lysosomes, it helps to control those digestive enzymes.
The last component of the endoplasmic, or the endomembrane system are vacuoles. And vacuoles are just sacs of membrane that have a variety of different sizes and functions. So we mentioned food vacuoles in our last slide.
So that would be one example of vacuoles. Over here on the right, it's showing a different category of vacuoles called contractile vacuoles. So there's certain freshwater protists that have these contractile vacuoles that can pump out extra water that that organism has absorbed.
So it just helps to... regulate the amount of water inside of that organism. Another example of a vacuole is the central vacuole, often referred to as the large central vacuole that's found in plant cells.
So this is a very versatile compartment and it makes up a huge component, often more than 50% of the volume of that plant cell, and it's going to be able to help store nutrients and water as well as pigments that are going to be able to help attract insects for pollination. It can also hold poisons that are going to help protect that plant cell from getting eaten by herbivores or animals that like to eat plants. So, again, that's a type of vacuole that's only found within plant cells. Now, all cells, regardless of whether they're plant or animal cells, they all need energy in order to survive.
And in particular, the energy that they need is called ATP, adenosine triphosphate. In chapter 5, we'll take a closer look at ATP. But for now, we're going to take a look at a couple of the organelles that are responsible for helping to produce ATP.
So plant and animal cells are going to get energy in different ways, but ultimately it's going to get converted to ATP. If we look at plants first, plants have chloroplasts. That allows them to be able to absorb energy from the sun, complete photosynthesis to convert that energy from the sun into chemical energy stored within the bonds of sugar molecules.
We'll look at photosynthesis and all the details of how this happens in our next unit in the course. Well then, after the plants have this sugar, well, they still don't have ATP. So they still need to... take that sugar energy or the energy stored within sugars and convert it to ATP. And the way that this is done is through cellular respiration by the mitochondria.
Cellular respiration, we'll also take a look at the details of cellular respiration in our next unit. So many people have the misconception that plant cells have chloroplasts, animal cells have mitochondria. Well plant cells, they have chloroplasts, but they also have mitochondria. The chloroplasts allows them to get energy from the Sun and produce chemical energy or sugars. but they still need the mitochondria to then take the energy of the sugars and convert it to ATP.
Animal cells, we don't have chloroplasts, we just have mitochondria. So we can't get energy from the sun, we have to use chemical energy. So we have to get that chemical energy in some way, often through eating, in order to get the energy that we need to complete cellular respiration and produce ATP. So let's take a closer look at the structure of chloroplasts and mitochondria.
Chloroplasts are the organelles shown here that are producing or that are producing sugars through the process of photosynthesis and that conversion that's the conversion of light energy that they absorb from the Sun to chemical energy stored within the bonds of sugars. Now again animals we don't have chloroplasts so we can't directly absorb energy from the sun, but we eat plants or we eat animals that ate plants and ultimately all of the energy from the plants came from the sun. So the living world relies on energy from the sun in order to provide energy to our ecosystems.
If we look at the structure of chloroplasts, you can see that it has two membranes, an inner membrane and an outer membrane. Inside the inner membrane there's a liquid called the stroma, and within the stroma there's these little stacks of disks called grana. These little grana are responsible for actually completing photosynthesis. So it's arranged within these disks so that it's going to increase the surface area, and that increased surface area is... is important because then there's more location for the sun to hit for the granite to be able to absorb that sun energy and actually complete photosynthesis.
And again the details of that process we'll take a look at in unit two. Mitochondria again are found in all eukaryotic cells, animal and plant cells, and they are responsible for cellular respiration which we'll take a closer look at in unit two. But that's the process that's converting the energy from sugars into ATP, which is the actual energy unit that our cells are able to use to complete work. If we look at the structure of mitochondria, similar to chloroplasts, it has two membranes, an inner membrane and an outer membrane.
And inside of that inner membrane, you have lots of folding. These different folds are called Christie. And again, this comes down to increasing the surface area. And that increased surface area means that there's more location for cellular respiration to actually happen. And that means that there's more energy that's going to be able to be produced.
On the inside of that inner membrane is the matrix. That's the fluid inside of the mitochondria. So the matrix inside of the mitochondria is kind of the equivalent of the stroma inside of the chloroplasts.
Now an interesting component of chloroplasts and mitochondria is that they each have their own DNA. So they're organelles, but they have their own DNA. And what this suggests is that they used to be their own free-living organisms. The endosymbiotic theory is saying that mitochondria and chloroplasts used to be their own organisms, but then somehow they got... engulfed by another organism and that's how they they get to be inside endo inside of another organism symbiosis a symbiotic relationship is one where both organisms benefit from the relationship so the endosymbiotic theory is that there's a symbiotic relationship between two organisms one is inside of the other but they are both benefiting from that relationship.
So my little diagram here is going to help explain that where this would be a mitochondria, but at this point in time, it's just, it's not inside of a cell. It's just its own cell. It's its own organism and it has its own DNA.
Well, this larger prokaryotic cell, it comes along and it eats that mitochondria. And normally it would engulf it and then break it down, except that when it engulfed the mitochondria, it realized, well, wait a minute, I am getting more energy now. And the mitochondria, it had a nice little home now. So both organisms benefited from this relationship.
Well, then now we have this organism. that has a mitochondria inside of it, an organelle inside of it, that's now going to have a different function. It's not going to be its own organism trying to survive. Now its function is going to be to produce energy to supply to the larger cell, and the cell in turn provides protection to that mitochondria. Well then, later on, going around.
Eats another organelle or eats another organism. And over here, this was our little chloroplast that used to be its own organism. It has its own DNA.
It was just living on its own, but then it got engulfed by this larger organism. And now that it's living inside this larger organism again, whoa, wait a minute. Instead of breaking him down, he's giving me energy.
So overall, the... Endosymbiotic theory is saying that plant and animal cells arose because of one free-living prokaryote that established residence inside of a larger host prokaryote and ultimately because they all benefited from the relationship it remained that way and that's what led to our current plant and animal cells. The final component of the cell that we have to take a look at is the cytoskeleton. The cytoskeleton is a network of fibers of all different shapes and sizes that are extending through the cytoplasm, and it's going to help maintain the shape of that cell, provides mechanical support for that cell, for that cell to be able to move, and it allows, or provides anchorage for the organelles within that cell so that the organelles aren't just kind of free-floating. that cytosol.
So the cytoskeleton is very dynamic and what that means is that it's constantly changing. It's made up of proteins and these protein fibers are constantly changing in size so that it can rearrange the cytoskeleton from inside of the cell to help reinforce locations that need to be reinforced at that time. One reason that an organism would do this is to allow that organism to be able to move.
So this is a neat organism shown over here called an amoeba. And this amoeba can move, but it doesn't have any arms or legs. It doesn't have any cilia or flagella that we'll take a look at shortly. The way that it moves is through these pseudopods or these projections of cytoplasm. And what it does is that these...
microtubules are going to be built up in the direction that the amoeba wants to move. So if the amoeba wants to move this way, well then it's building up the microtubules, adding on more protein subunits in this direction here. And as it does that, over here on the other side, it's going to break down these protein subunits and move them over here and rebuild them in this direction. So that slowly, the amoeba will move in this direction.
Well that's one way that organisms can move, but the cytoskeleton also has a more efficient way for these organisms to move, and that's by arranging microtubules on the outside of the plasma membrane of that organism. So the cytoskeleton can have flagella or cilia, that are going to be able to help that organism move more efficiently through its environment. If we look at this example here, we can see this is a human sperm cell, and we can see this long flagella on the end of it is going to undulate back and forth like a snake to be able to propel that sperm through its environment. Flagella, they're often long, whip-like, and they are often either singly or just in small groups on that organism.
Over here you can see a couple of examples of cilia. Cilia, they are typically much smaller, much more numerous, and they work together rather than singly. So the cilia are going to work together, you can kind of picture them as um, ormen on a boat.
And you have all of these people that are paddling at the same time on this ore boat to help this boat push through the water. So these cilia work in the same way. They all work together to help push this organism through its environment. Down here we can see a different type of cilia though. These ones are cilia that are lining the respiratory tract of humans and You can see here, you can imagine, I guess, that we don't really want, if it's a respiratory check, let's just look or let's consider...
These cells lining our nasal passageways, so lining the inside of our nose. Well, we don't really want our nose lining cells to move. So these cilia aren't moving our cells, but they are moving what's surrounding the cells. So the cells are all locked into place because they're all connected together in the tissue because of that extracellular matrix.
But these cilia are going to help propel mucus through that respiratory tract. The mucus lines the respiratory tract to help be able to trap dust and pathogens, but then you don't want that mucus clogging up your airways. So the cilia are going to help propel that mucus through our respiratory tract to open up our airways.
And again, the cilia here are moving in the same way where they're all going to move in a coordinated fashion to help push that mucus through. Okay, so the cytoskeleton ends off the different components of the cells that we need to look at. As always, if you have any questions about the content that we covered or about content in the textbook that we didn't cover, please feel free to shoot me off an email and I'd be happy to help you out.
If not, then happy studying!