In this video, we're going to begin our introduction to biological membranes. Now, it's important to recall from way back in our previous lesson videos, we said that in aqueous solutions, amphipathic lipids can spontaneously aggregate or clump together via the hydrophobic effect. And so if you don't remember anything about the hydrophobic effect and the ability for amphipathic lipids to spontaneously aggregate via the hydrophobic effect, then make sure to go back and check out those older lesson videos before you continue here. Now, the ability for amphipathic lipids to spontaneously aggregate via the hydrophobic effect will actually lead to the formation of 3 different types of membranes. The first are the micelles, the second are the liposomes or the vesicles, Here in the middle, we're showing you the liposome or the vesicle. Here in the middle, we're showing you the liposome or the vesicle. And then over here on the far right, we're showing you the lipid bilayer. And so starting over here on the far left with the micelle, what you'll notice is that it is a monolayer of lipids, and this is what allows it to have a hydrophobic core at its center. And so if you were to zoom in on one of these lipid molecules in the micelle, you would see that it's likely going to be a free fatty acid. And that's because free fatty acids have this triangular-shaped geometry, if you will, that makes them so suitable for the formation of micelles upon their aggregation via the hydrophobic effect. Now liposomes or vesicles, as you can see over here, they actually have a bilayer of lipids. So you can see that there's a layer of lipids here on the outside, but also here on the inside, there's a second layer. And so this is what allows liposomes or vesicles to have an aqueous core at its center instead of having a hydrophobic core at its center like micelles do. Now, liposomes or vesicles, they're generally very small and only contain a handful of dissolved molecules in the aqueous core. And if we were to zoom in on one of these lipid molecules in the liposome, you would see that it's likely going to be a free phospholipid like this one, which upon aggregation with other ones allow them to form a vesicle or bilayer because of their proper square shaped geometry that's suitable for the formation of vesicles and bilayers. And then last but not least, over here on the far right, we have the lipid bilayer, which you'll notice also has a bilayer of lipids here. And, the real difference between the lipid bilayer and the liposome or the vesicle is again that the liposome or vesicle are generally much smaller with only a handful of molecules in the aqueous core, whereas lipid bilayers are going to be much, much, much larger and they can encapsulate an entire cell and create, entire organelles. Now, notice that this lipid bilayer here has 2 different sheets or leaflets. There's this sheet or this leaflet right here, that we can call the extracellular leaflet or sheet and that's because it's on the outside of the cell and notice that this other leaflet over here, we can call the intracellular leaflet or sheet. Again, because it's on the inside here of our cell. And so, it's important to be able to distinguish between these two different leaflets. Now, one thing that I want to let you guys know here is that both glycerophospholipids and sphingophospholipids have optimal shapes or geometries that allow them to form lipid bilayers and liposomes or vesicles. And so, as we already mentioned, the micelle here is generally formed from free fatty acids which have this triangular shaped geometry. And free phospholipids, they have a square shaped geometry. And so down below here, what we're showing you is that the free fatty acids with their triangular shaped geometry, they are not suitable for forming vesicles or bilayers because they are not able to close these gaps that form, here and here and that makes them unstable as a vesicle or a lipid bilayer. And the same goes for free triacylglycerols. They tend to have a trapezoid-shaped geometry like what you see here and again, that geometry or shape will create these gaps that make it make them very unstable as they try to form vesicles or bilayers. And so really, the only, lipids that are capable of forming these, liposomes or lipid bilayers are the free phospholipids, either the glycerol phospholipids or the Sphingophospholipids. And so this here really concludes our introduction to biological membranes and as we move forward through our course, we're going to learn more and more about these biological membranes. So, I'll see you guys in our next video.
Biological Membranes - Online Tutor, Practice Problems & Exam Prep
Created using AIBiological membranes are primarily lipid bilayers that incorporate various embedded proteins, forming a fluid mosaic model. This model illustrates the dynamic nature of membranes, where phospholipids and proteins can move freely, contributing to membrane functionality. The composition of these membranes can vary significantly across different cells and organelles, affecting their properties and roles. Understanding the structure and function of membranes is crucial for grasping cellular processes, including transport mechanisms like facilitated diffusion and active transport.
Biological Membranes
Video transcript
Biological Membranes
Video transcript
So in our last lesson video, we introduced 3 different membrane structures, which were the micelles, the liposomes or the vesicles, and the lipid bilayers, and so in this video, we're going to introduce the biological membranes. Biological membranes are lipid bilayers themselves. However, they're more than just a lipid bilayer. Biological membranes are lipid bilayers with other membrane-embedded molecules as well, such as proteins, for instance. Recall from your previous biology courses that the fluid mosaic model applies to biological membranes. The fluid mosaic model is basically saying that biological membranes are both fluid and a mosaic of membrane-embedded proteins.
If we take a look at our image down below, notice over here on the far left, we're showing you a scanning electron micrograph (SEM) of a biological membrane right here. Notice that we're zooming into this specific region of the biological membrane, and we're getting this image right here. Most of the membrane molecules are indeed these phospholipids, but also embedded within the biological membrane. Notice that there's a good portion of these purple structures which are proteins. Recall that the fluid mosaic model applies to membranes like this one, where the phospholipids that we see here and the proteins and really all of the membrane-embedded molecules are fluid because they're capable of shifting around and moving to different areas within the membrane. The membrane is also a mosaic because all of these different membrane components, specifically these proteins, can make the biological membrane look like a mosaic.
To put things in perspective a little, biological membranes can actually be comprised of anywhere between 20 to 80% proteins by mass, and that is quite a lot of proteins. We already knew that biological membranes were going to be composed of mostly lipid structures because they are lipid bilayers. But maybe we did not really realize how much protein could actually be embedded within the bilayer. This goes to show that when talking about biological membranes, we cannot forget about the protein aspect of it because there can be quite a lot of proteins embedded within a biological membrane.
Another important thing to note is that membrane lipid composition can vary quite a lot from cell to cell, from sheet to sheet, and between different organelles. Clearly, different cells can have different phospholipid compositions and different proteins embedded. The sheet the sheet can also differ too. This extracellular sheet could have a different membrane composition than the intracellular sheet. Also, the membrane composition can vary between different organelles, so the mitochondria can have a different membrane composition than the nuclear membrane or than the endoplasmic reticulum membrane. This is just a general idea that's important to keep in mind.
This here concludes our review and introduction to biological membranes, and we'll be able to apply the concepts that we've learned and reviewed as we move forward in our practice problem. So, I'll see you guys there.
Membranes are a fluid mosaic of what components?
a) Proteins, cholesterol, and triacyglycerols.
b) Phospholipids, proteins, and cholesterol.
c) Phospholipids, nucleic acids, and cholesterol.
d) Eicosanoids, proteins, and phospholipids.
Which of the following lipids would likely not be involved in a lipid bilayer structure?
a) Phospholipid.
b) Cholesterol.
c) Glycolipid.
d) Sphingolipid.
e) Triacylglyceride.
f) Glycerophospholipid.
Membrane components within a lipid bilayer are held together primarily by:
a) Hydrogen bonds.
b) Covalent bonds.
c) Disulfide bonds.
d) Hydrophobic interactions.
e) Electrostatic interactions.
f) All of the above.
Here’s what students ask on this topic:
What is the fluid mosaic model of biological membranes?
The fluid mosaic model describes the structure of biological membranes. According to this model, membranes are composed of a lipid bilayer with embedded proteins. The 'fluid' part refers to the ability of lipids and proteins to move laterally within the layer, providing flexibility and dynamic nature to the membrane. The 'mosaic' part highlights the diverse array of proteins that are interspersed within the lipid bilayer, giving the membrane a mosaic-like appearance. This model is crucial for understanding membrane functionality, including transport mechanisms and cell signaling.
Created using AIHow do micelles, liposomes, and lipid bilayers differ in structure and function?
Micelles, liposomes, and lipid bilayers are all structures formed by the aggregation of amphipathic lipids via the hydrophobic effect. Micelles are spherical structures with a hydrophobic core and a monolayer of lipids, typically formed by free fatty acids. Liposomes, or vesicles, have a bilayer of lipids and an aqueous core, making them suitable for encapsulating small molecules. Lipid bilayers are larger structures that can encapsulate entire cells or organelles, with two leaflets: an extracellular and an intracellular leaflet. The primary difference lies in their size, structure, and the type of core they possess.
Created using AIWhat role do proteins play in biological membranes?
Proteins play several crucial roles in biological membranes. They contribute to the fluid mosaic model by being embedded within the lipid bilayer, allowing for dynamic movement. Proteins can function as transporters, channels, receptors, and enzymes, facilitating various cellular processes such as nutrient uptake, signal transduction, and waste removal. The composition of membrane proteins can vary significantly between different cells and organelles, affecting the membrane's properties and functions. In some membranes, proteins can constitute up to 80% of the membrane's mass, highlighting their importance.
Created using AIHow does the composition of biological membranes vary between different cells and organelles?
The composition of biological membranes can vary significantly between different cells and organelles. This variation includes differences in the types and proportions of phospholipids and proteins. For example, the mitochondrial membrane has a different lipid and protein composition compared to the nuclear membrane or the endoplasmic reticulum membrane. Even within a single cell, the extracellular and intracellular leaflets of the membrane can have different compositions. These variations affect the membrane's properties and functions, such as permeability, fluidity, and the ability to interact with specific molecules.
Created using AIWhat is the hydrophobic effect and how does it contribute to the formation of biological membranes?
The hydrophobic effect is the tendency of nonpolar molecules to aggregate in aqueous solutions to minimize their exposure to water. This effect is crucial for the formation of biological membranes. Amphipathic lipids, which have both hydrophobic and hydrophilic regions, spontaneously aggregate in water to form structures like micelles, liposomes, and lipid bilayers. In these structures, the hydrophobic tails are sequestered away from water, while the hydrophilic heads interact with the aqueous environment. This self-assembly process is fundamental to the formation and stability of biological membranes.
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