For part two of this lecture, we're going to talk about the individual organelles and what their functions are. So for each of these, you should understand kind of what their job is and how a cell might be different depending on whether there is a lot of that particular organelle or not a lot of that particular organelle. Before we get into the specifics of the organelles, let's first talk about why organelles are so important and this slide is going to be a little bit longer of an explanation.
What this gets at is the concept of a surface area to volume ratio. Surface area is how much surface there is and volume is size. Now for a cell in order for things to function things need to move in and out of the cell, right?
For a cell to work, in other words, for a cell to have lots and lots of chemical reactions to break down larger molecules, connect smaller molecules together, lots of chemical reactions need to happen. And as a result, the cell kind of needs supplies from its environment, and it needs to get rid of waste. This is a cell's metabolism.
And increased surface area means it is easier for stuff to move in and out of the cell. If there's more space, that makes sense, right? Intuitively, more space means that there's more space for molecules to move in and out of the cell.
As cells grow larger, volume increases, the metabolic demand of that cell also. increases. So we run into a problem because volume increases can occur without proportional surface area increases. So if we look at like this cube here and we increase the volume without a compensatory increase in surface area we've dramatically reduced the surface area to volume ratio. However, if we instead increase the volume the same, but increase the surface area at the same rate, we have not changed that surface area to volume ratio.
So if we're if these two cubes here were cells, this cell would not be as metabolically active as this cell because they both have the same metabolic demand they both kind of need the same energy to do their thing but this one has a much better chance of getting the nutrients it needs expelling the waste that it doesn't need and therefore being more successful so anytime we add an organelle we add a compartment to the inside of a cell We are increasing that surface area to volume ratio. Eukaryotes, unlike prokaryotes, can be multicellular large organisms. That is because our cells have this ability to scale surface area with increased volume.
When we talk about on like the individual cell level, this goes into cell specialization, right? Nerve cells, cells in our nervous system are extremely long and thin. So you increase the volume by increasing the size of the cell, but by staying long and thin, as opposed to being like bulky and large, you maintain a high surface area.
This is why nerve cells are able to function so efficiently. and why our nervous system works so well. So any sort of like increasing in the like folds of the membrane or like increased number of channel and carrier proteins, this is all going to increase membrane permeability.
And if you're increasing membrane permeability, you are allowing the cell to perform more metabolic functions. because they can get the nutrients they need in and expel the waste that they don't need So let's talk about these different compartments or organelles within a eukaryotic cell. First is the nucleus. The nucleus is where DNA, our main genetic material, is housed. It's also where RNA is going to be produced.
An important process from taking DNA to protein is transcribing that DNA into RNA. That happens inside the nucleus. The nucleus has a... envelope surrounding it.
These are two membranes with kind of holes punched in them. These holes are called nuclear pores. This is important for getting certain proteins in and out of the nucleus, also for getting that RNA out of the nucleus into the cytoplasm for protein synthesis to occur.
The nuclear lamina is the inner surface of the nucleus. It gives it, it kind of keeps it sturdy, maintains its shape. And the nucleolus is a specialized kind of dense region in the nucleus. These are where ribosomes are produced.
So again, those ribosomes have to, once they get produced here, they need to leave the nucleus. So they use those nuclear pores. The ribosomes, these are one organelle that are found in both eukaryotes and prokaryotes.
This is where proteins are synthesized. They are structurally composed of a combination of proteins and a specific type of RNA. They have two subunits, a large subunit and a small subunit. We'll talk more about the structure of the ribosome and its function later in the semester when we get to central dogma stuff.
So like DNA to RNA to protein. But for now, just understand it's kind of basic structure and that it is. its function which is site of protein synthesis and that it is present in both prokaryotes and eukaryotes next we have mitochondria which of course are the powerhouse of the cell what that means is this is where most of the cell's usable energy is going to be produced mitochondria are a little more complex than just the powerhouse of the cell They're a pretty interesting organelle. They have their own DNA, different from the DNA in our nucleus, and they can divide independently of the nucleus in the cell as well.
This is some evidence of a concept called endosymbiotic theory. This is a theory that states mitochondria, or something like mitochondria, used to exist as their own organelle. individual prokaryotic organism, because that circular DNA is very much like the circular DNA in prokaryotes.
But somewhere along the timeline of evolution, these mitochondria-like prokaryotes formed a mutually beneficial relationship with what would become a eukaryotic cell, where, you know, the mitochondria... Got mitochondrial-like organism, got protection by being inside the eukaryote, and the eukaryote got its own little energy source, its own battery, to produce and store ATP. The membrane does have a double membrane system and increased folds in that inner membrane surface.
Increased folding is going to improve the surface area to volume ratio for the mitochondria, which is very important. because more surface area in the mitochondria means more metabolic activity in the mitochondria, which means more energy, more ATP production. So often cells that are very active in movement, like our muscle cells or growth, will have lots and lots and lots of mitochondria because they need lots and lots and lots of energy. An organelle that is similar to mitochondria but not found in animal cells are chloroplasts.
They also have circular DNA like the mitochondria, suggesting in endosymbiotic theory, maybe chloroplasts used to exist as their own organism, as prokaryotes, but over the course of evolution have become part of plant cells. The chloroplasts'main job, they're the site of photosynthesis. So this is where plant cells are going to take light energy.
photo light and transform it into chemical energy in the form of a carbohydrate. We're going to talk about why this is so important more in later modules but for now just understand that this is happening in chloroplasts. Light energy transformed into chemical energy in the form of carbohydrates. This is also where the chloroplasts are often depicted as green because this is where the pigment chlorophyll lives and this is what gives leaves their green color. This slide is dedicated to vesicles.
Vesicles are smaller organelles typically have a you know single membrane and they can fuse with the cell membrane. There are specialized types of vesicles but generally a vesicles job is to carry stuff molecules and other substances around the cell. And they also play roles in transport in and out of the cell through endocytosis and exocytosis, like we talked about in module three.
Some specialized types of vesicles that will be mentioned, paroxysomes, their job is to break down any kind of toxic material. which is a common byproduct of ATP production or energy production. If these were to accumulate in the cell, it would not be good.
So these peroxisomes are basically like the garbage men. They take away the trash and make sure the cell can function properly and remain metabolically active. Lysosomes will digest larger molecules, break down large macromolecules into smaller, more usable things for the cell.
And then vacuoles, mostly found in plants and fungi. They are storage sites. They have a really diverse role. Some of them help with reproduction in non-animal cells, very important for metabolism. And they're also really important for osmotic regulation or regulating the direction of osmosis.
So by like hanging onto or releasing lots of solute, they can can influence whether a cell takes in lots of water or releases lots of water. And finally, we have the endomembrane system. This is a series of interconnected membrane-enclosed organelles. First are the endoplasma reticulum, or ER.
There are two versions of the ER. The rough ER It's called rough because it has these little bumps on it. These little bumps are ribosomes. So while some ribosomes can exist in the cytoplasm, others will adhere and stick to the rough ER. Let's think about why that might be the case.
Well, the rough ER's main job is to modify and transport proteins. So ribosomes are where proteins are made. It makes sense for these to be close to the rough ER where those proteins are going to be modified.
The smooth ER is similar except no ribosomes and its job is nothing to do with proteins. Instead, its major job is to synthesize lipids, very important for building and maintaining our cell membrane. And it's also a storage site for calcium ions. This becomes very important when we start to learn about muscle contraction. next semester in Bio 152. But Smooth-ER, lipid synthesis, it also does some detoxification like the peroxisomes and stores calcium.
The Golgi apparatus, sometimes continuous with the rough ER, serves a, and Smooth-ER, serves a kind of similar function of modification of these proteins and these lipids. The Golgi is um Often they are to kind of send the molecule or like modify the molecule in a way that it knows to go to where it's supposed to be. Right. If it's a phospholipid, it is it goes to the golgi and the golgi kind of helps direct it to the membrane.
If it is a protein that is important for, you know, maintaining genetic material in the nucleus, then. that sort of information will happen in the Golgi apparatus before that protein is released into the cytoplasm. And then these proteins and lipids, once they get modified in a certain way, they will be housed inside of a lysosome. Like I said, some lysosomes, they are there to digest or break down macromolecules. Others are designed to kind of take material in and transport them to other parts of the cell.