Control of Cell Size: Study with Video Lessons, Practice Problems & Examples

Cell size is regulated by three key extracellular factors: mitogens, growth factors, and survival factors. Mitogens stimulate cell division by removing inhibitors of the cell cycle, while growth factors promote protein synthesis, leading to increased cell size. Survival factors enhance cell survival by suppressing apoptosis. Understanding the distinction between cell growth and cell proliferation is crucial; growth refers to the size of a single cell, whereas proliferation involves the division of multiple cells. Key intracellular proteins, such as the E2F transcription factor, play significant roles in these processes, particularly in cancer development.

Cell Size Control

Hi. In this video, we're going to be talking about control of cell size. So first, let's talk about the extracellular factors. There are three extracellular factors that play a role in cell division, cell size, and cell survival. And all of these things, right, I know I mentioned size, but division and survival also relate back to size. Right? Because, in order to divide, the cell has to be larger. In order to survive, it can't be too small. So all of these factors play a role in making sure the cell is the right size. So mitogens are extracellular factors that stimulate cell division, and they do this by removing negative control, so things that are going to block the cell cycle progression. So they remove these inhibitors of the cell cycle. And usually, they do this through G1 and S phase CDKs. So mitogen is super important, right? Because they're stimulating cell division, and they're stimulating cells to grow. Then you have the growth factors. And these are really responsible for stimulating cell growth, sort of independent of anything else, because they promote protein synthesis. So if you're creating more proteins, then you're going to have more proteins, you're going to have more organelles, and you're going to have more proteins in the membrane. It's going to have to keep growing and growing. And if it also works by inhibiting degradation. So not only creating more proteins, but you're not degrading them as fast. And so, growth factors really stimulate cell growth, independent of other things like division or survival or whatever. And then, finally, you have survival factors, and these are going to stimulate cell survival, and they do this by suppressing apoptosis, which, if you remember what apoptosis is, that's just regulated cell death. And we'll go over that a lot more in another video. But so those are the three factors that really control cell survival, growth, and division. But it's super important to understand the difference between each of the three and not confuse them. So you're like, okay, well, they're all, you know, pretty much stimulating the same thing, but they're not, because each one is activating a specific way. So mitogens are removing these inhibitors of the cell cycle, growth factors are promoting protein synthesis, and survival factors are suppressing or blocking regulated cell death. And so, when we talk about cell growth, it's also really important to understand that doesn't mean cell proliferation. So what does cell proliferation mean? Right. So that's going to be cells dividing a lot. So there's going to be many cells. If you start out with 2 cells and after an hour, you end up with 32, that's a lot of cell proliferation. But cell growth is actually referring to one cell and its size. So that is a super important differentiation. So here's how a growth factor stimulated cell growth. So here's a growth factor. It binds to something in the plasma membrane, which stimulates a variety of different proteins sort of downstream, which will trigger the cell to grow. And all of these factors work very similarly, but this is just an example. So when the cell needs to grow, these growth factors bind and they stimulate a variety of intracellular proteins. So let's talk about a few of these intracellular proteins that control cell size in the cell cycle. So, mitogens, like I said before, really work by stimulating these G1 CDKs that enter the cell into the growth phase of the cell cycle. So if mitogens weren't present during the cell cycle, then what would happen is the cell would enter G0, and it would pretty much stay there. So without mitogens, the cells aren't going to grow and they're not going to divide. So, mitogens work by activating these intracellular G1 CDKs. A second pathway is through the E2F transcription factor. And so, this is activated by mitogens, it's activated by growth factors. Here are a couple of examples, you don't necessarily need to know those, but if you just see them in your book, that's what they are. And so, E2F transcription factor, so it's obviously doing something to transcribe genes. And so, the genes that it is activating are promoting actually entry into S phase. So that's a super important one. Actually entry into S phase. So, that's a super important one, right? Because when the cell has finally reached its size, it doesn't need to keep growing. It needs to stop growing, and it needs to move on through the cell cycle. And so, the E2F transcription factor is super important to kind of recognize when the cell has actually gotten to its size and promoting that movement to the next step. You're actually going to probably be hearing a lot about this pathway, and I'm just going to briefly mention. And the reason I'm telling you this about this protein is, 1, because you're going to see it a lot in your book and you need to know what it does, but 2, is because this protein is actually mutated in a lot of cancers, and you can imagine, right? Because if this protein is responsible for inhibiting movement through the cell cycle and stopping cell growth, then if it's mutated and it doesn't work, then the transcription factor is going to just go through the cell cycle as many times as it wants even if the cell isn't running. So that's a really important one, especially in cancer. Then you have the RAS MAPK signaling pathway, which we've talked about previously a little while back. And this works very similar. It activates transcription factors that support growth. And then, finally, the DNA damage repair pathway, and this pathway can also pause the cell cycle, and result in cell growth while the cell is trying to repair its DNA. And this pathway, which we've talked about before, is the PI3K-AKT pathway of signaling. So here's this example that I said I was going to talk about, which is the E2F and the retinoblastoma. So here, we have some type of cyclin and some type of CDK, which are working together and they're like, hey, we want to promote the cell cycle. So what they do is they actually promote E2F. And so when E2F is now activated and it's ready to go, it will promote the cell cycle. But when RB comes in and binds to it, this regulating this one, is mutated, so the RB is mutated, then this E2F is just free to promote the cell cycle and promote cell division and cell growth just whenever it wants to without inhibition, which you can very easily see how this results in cancer. So with that, let's now turn the page.

Which of the following factors is not responsible for controlling cell division, size, or survival?

• A. Growth factors
• B. Survival factors
• C. Integrins
• D. Mitogens

Which of the following proteins is a transcription factor that activates genes to promote entrance into S phase?

• A. Mitogens
• B. RAS-MAPK
• C. PI3K-AKT
• D. E2F

Here’s what students ask on this topic:

What are the key extracellular factors that control cell size?

The key extracellular factors that control cell size are mitogens, growth factors, and survival factors. Mitogens stimulate cell division by removing inhibitors of the cell cycle, primarily through G1 and S-phase CDKs. Growth factors promote cell growth by enhancing protein synthesis and inhibiting protein degradation, leading to an increase in cell size. Survival factors enhance cell survival by suppressing apoptosis, which is the regulated process of cell death. Each of these factors plays a distinct role in ensuring that cells grow, divide, and survive appropriately.

How do mitogens influence cell division and size?

Mitogens influence cell division and size by removing inhibitors of the cell cycle, thereby promoting cell cycle progression. They primarily act through G1 and S-phase CDKs (cyclin-dependent kinases). By activating these CDKs, mitogens help the cell transition from the G1 phase to the S phase, where DNA replication occurs. This process is crucial for cell division, as cells need to grow to a certain size before they can divide. Without mitogens, cells would enter a quiescent state (G0 phase) and not proceed with division or growth.

What is the difference between cell growth and cell proliferation?

Cell growth refers to the increase in size of a single cell, primarily through the synthesis of proteins and organelles. It is driven by growth factors that promote protein synthesis and inhibit protein degradation. On the other hand, cell proliferation refers to the process of cell division, resulting in an increase in the number of cells. This involves the entire cell cycle, including DNA replication and mitosis. While cell growth focuses on the enlargement of individual cells, cell proliferation is about the multiplication of cells.

How do growth factors stimulate cell growth?

Growth factors stimulate cell growth by binding to receptors on the cell membrane, which activates intracellular signaling pathways. These pathways promote protein synthesis and inhibit protein degradation. As a result, the cell produces more proteins and organelles, leading to an increase in cell size. Growth factors work independently of cell division and survival, focusing solely on enlarging the cell. This process is crucial for cells to reach the appropriate size before they can proceed with division or other functions.

What role does the E2F transcription factor play in cell size control?

The E2F transcription factor plays a crucial role in cell size control by regulating the expression of genes necessary for the cell cycle's S phase. Activated by mitogens and growth factors, E2F promotes the transcription of genes required for DNA replication. This ensures that once a cell has reached its appropriate size, it can proceed to the S phase and continue through the cell cycle. E2F is also significant in cancer development, as mutations in its regulatory pathways can lead to uncontrolled cell growth and division.