Physical Properties of Biological Membranes - Online Tutor, Practice Problems & Exam Prep

The physical properties of biological membranes are influenced by lipid diffusion, which includes lateral diffusion (rapid, uncatalyzed side-to-side movement) and transverse diffusion (slow, catalyzed flip-flop movement). Enzymes like flippase and floppase, which require ATP, facilitate lipid movement between membrane layers, while scramblase operates without ATP. The transition temperature (T_m) determines membrane fluidity, with higher temperatures leading to increased permeability. Factors affecting T_m include the length and saturation of fatty acid chains, alongside the presence of cholesterol and proteins, which collectively regulate membrane dynamics.

Physical Properties of Biological Membranes

In this video, we're going to begin our lesson on the physical properties of biological membranes, and we're going to start off by talking about lateral and transverse diffusion. There are two main types of lipid diffusion that describe the fluid-like motion of lipids within a bilayer.

The first type of lipid diffusion is lateral diffusion. Lateral diffusion, as its name implies, is an uncatalyzed lateral movement of lipids. Lateral, which means side to side, involves lipids always remaining along the same sheet of the lipid bilayer. Although this is an uncatalyzed process, it's still an extremely fast process. If we take a look at our image on the left hand side, notice that we're showing you lateral diffusion. We're focusing specifically on this phospholipid right here. This phospholipid is capable of diffusing both to the left and to the right, and here, we're showing it diffused to the right to this position. The side-to-side diffusion along the same bilayer sheet is an incredibly fast process, even though it's an uncatalyzed process.

The second type of lipid diffusion is transverse diffusion, otherwise known as flip-flop diffusion. Transverse or flip-flop diffusion is a catalyzed process, unlike the previous lateral diffusion, which was an uncatalyzed process. This process transfers lipids across to the opposite sheet of the lipid bilayer instead of keeping the lipid on the same sheet as in lateral diffusion. If there is not an enzyme present, then transverse diffusion is going to be an extremely slow process, and it could be so slow that it could take days without an enzyme. This slow rate, without an enzyme, allows the inner and outer sheets of a biological membrane to maintain different lipid compositions. If we look at the right side of our image below, notice that we're showing you transverse or flip-flop diffusion. Focus on this phospholipid molecule right here, and notice that in transverse diffusion, it's diffusing across to the opposite bilayer sheet, and ends up in this position on the opposite bilayer sheet. Without an enzyme, as mentioned, it could take days, making it an incredibly slow process if no enzyme is involved.

Moving forward, we're going to talk about what specific enzymes allow transverse or flip-flop diffusion to occur at a significant rate. I'll see you guys in our next video.

Physical Properties of Biological Membranes

So from our last lesson video, we already know that transverse diffusion is an extremely slow process. And so in order for transverse diffusion to occur at a reasonably fast rate, it's going to require an enzyme to catalyze the process. In this video, we're going to talk about the enzymes that catalyze transverse diffusion. There are three main types of membrane-embedded enzymes that catalyze transverse diffusion. We've got those three enzymes numbered down below. It's important to note that the first two enzymes require energy in the form of ATP to catalyze transverse diffusion. Whereas the last enzyme does not require ATP, so no ATP is required to catalyze transverse diffusion.

The very first enzyme on this list is going to be the flippase. The flippase, as its name implies, will flip very specific lipids from the outer sheet of the membrane to the inner sheet of the biological membrane. What helps me remember that flippases will flip lipids from the outer sheet to the inner sheet and not vice versa, is that I correspond the "i" in flip with the "i" in inner. This reminds me that the lipid's final position is going to be in the inner sheet. This helps remind me that flippases flip lipids from the outer to the inner sheet of the biological membrane.

The next enzyme we have here is the opposite of the flippase, which is the floppase. The floppase, as its name implies, is going to flop lipids from the inner sheet to the outer sheet of the biological membrane. What helps me remember that floppases will flop lipids from the inner to the outer sheet is that I correspond the "o" in floppase with the "o" in outer. This reminds me that the lipid's final position is going to be in the outer sheet. Thus, floppases will flop lipids from the inner to the outer sheet.

The last enzyme on our list is the scramblase. The scramblase, as its name implies, is going to scramble the lipids in either direction. It scrambles them essentially in both directions across the lipid bilayer, specifically down the concentration gradient. Because the lipids are moving down their concentration gradients, it does not require ATP for them to flow down the concentration gradients.

If we take a look at our image down below, notice that we've got the three different enzymes here. On the far left, we have the flippase, which is going to flip the phospholipid from the outer sheet to the inner sheet of the biological membrane. Note that the outer sheet is labeled as this top sheet here, and the inner sheet is labeled as this bottom sheet down here. Notice that the flippase does require ATP, energy in the form of ATP, to catalyze this process. The next one that we have is the floppase, which is going to do the opposite of the flippase and flop the phospholipid from the inner sheet to the outer sheet, which is the final position. Again, it's going to use energy in the form of ATP to catalyze that process. Last, but not least, we have the scramblase here. The scramblase is going to flip and flop the phospholipids in both directions. It is going to transport lipids down their concentration gradients, which is why no ATP is required for this process.

If you think about this for a moment, notice that both the flippase and the floppase are going to help create diversity between these two different sheets of the biological membrane. Whereas, the scramblase is really going to eliminate the diversity between these two different sheets. By controlling the expression of what specific enzymes are going to be in the membrane, a cell can control the composition and the diversity between its outer and inner biological membrane sheets. This concludes our introduction to the enzymes that catalyze transverse diffusion, and we'll be able to get some practice applying these concepts as we move forward. I'll see you guys in our next video.

Physical Properties of Biological Membranes

In this video, we're going to introduce the transition temperature of lipid bilayers. The transition temperature is also commonly referred to as the melting temperature and is abbreviated as the TM. This is the specific temperature where a membrane will either lose or gain its fluidity depending on whether the temperature is above or below the transition or melting temperature. At temperatures above the TM, membranes are going to transition from having a thick gel-like viscosity to having a very fluid like viscosity. It's important to note that the more fluid like the membrane is, the more permeable the membrane will also be. The more permeable the membrane, the easier it is for substances to cross or penetrate through the membrane.

If we take a look at our image down here on the left-hand side, notice that we're showing you this graph where we have the membrane's fluidity on the y-axis and the temperature on the x-axis increasing from left to right. Notice that we have the specific transition or melting temperature marked here as the TM, which corresponds with a very unique position on this graph, where the membrane is either losing or gaining its fluidity, again depending on if the temperature is above or below the TM. At temperatures below the TM on this part of the x-axis, notice that the membrane takes on a very gel-like, viscous, and rigid structure as we see here. This gel-like structure is going to be less permeable. Substances that used to be able to easily cross the membrane are going to have a much harder time crossing the membrane if it is gel-like and less permeable. Of course, at temperatures above the TM, over here on this part of the x-axis, notice that it corresponds with a membrane fluidity that is very fluid-like as the ones that we see here. Notice that the membranes are much more separated and much more permeable, and so this will allow substances to cross through the membrane much easier.

Now, it's important to note that these membrane transition temperatures, like the ones we see right here, are primarily dictated by the same exact two factors that affect the fatty acid's melting point. Recall, we talked about a fatty acid's melting point much earlier in our lesson. Down below right here in this box, notice that we have the two primary factors that affect the temperature, the melting or transition temperature. These two primary factors are the same two factors that affect the fatty acid's melting point. The first is the length of the chains of the phospholipids. The longer those hydrocarbon chains are, the higher the transition temperature or the melting temperature will be. The second primary factor is going to be the degree of saturation or the amount of double bonds present in the hydrocarbon chains of the phospholipids. The more double bonds that are present, the lower the melting or transition temperature will be.

It's also important to recall that when it comes to a biological membrane, there are other molecules that can also affect the fluidity of the membrane, such as proteins or cholesterol, which are also embedded in the membrane. We cannot forget about these other molecules as well that can also help to affect the fluidity and the permeability of the membrane. Cells are actually able to control the fluidity of their membrane and the permeability of their membrane by affecting these two primary factors as well as by affecting the other molecules that are embedded in the membrane.

This here concludes our introduction to the transition temperature of lipid bilayers, and we'll be able to apply the concepts that we've learned here in our next video. So, I'll see you guys there.

The fluidity of a bilayer is generally increased by: 

a) The binding of water to the fatty acyl side chains. 

b) An increase in fatty acid chain length. 

c) An increase in the number of double bonds in the fatty acid hydrocarbon chains. 

d) A decrease in temperature.

• A. The binding of water to the fatty acyl side chains.
• B. An increase in fatty acid chain length.
• C. An increase in the number of double bonds in the fatty acid hydrocarbon chains.
• D. A decrease in temperature.

Which of the following would increase the transition temperature of a membrane? 

a) A decrease in the fatty acid tail length. 

b) An increase in the number of double bonds in the fatty acid chains. 

c) Loose packing of fatty acid tails. 

d) Free fatty acids in the environment. 

e) None of the above would increase the transition temperature.

• A. A decrease in the fatty acid tail length.
• B. An increase in the number of double bonds in the fatty acid chains.
• C. Loose packing of fatty acid tails.
• D. Free fatty acids in the environment.
• E. None of the above would increase the transition temperature.

Here’s what students ask on this topic:

What is the difference between lateral and transverse diffusion in biological membranes?

Lateral diffusion refers to the rapid, uncatalyzed side-to-side movement of lipids within the same layer of the lipid bilayer. This process is extremely fast and allows lipids to move freely along the plane of the membrane. In contrast, transverse diffusion, also known as flip-flop diffusion, involves the movement of lipids from one layer of the bilayer to the opposite layer. This process is slow and requires the presence of specific enzymes, such as flippase, floppase, and scramblase, to occur at a significant rate. Without these enzymes, transverse diffusion can take days to complete.

What enzymes are involved in catalyzing transverse diffusion in biological membranes?

Three main types of enzymes catalyze transverse diffusion in biological membranes: flippase, floppase, and scramblase. Flippase moves specific lipids from the outer sheet to the inner sheet of the membrane and requires ATP. Floppase does the opposite, moving lipids from the inner sheet to the outer sheet, also requiring ATP. Scramblase, on the other hand, moves lipids in both directions across the bilayer down their concentration gradients and does not require ATP. These enzymes help maintain the lipid composition and diversity between the inner and outer sheets of the membrane.

How does the transition temperature (Tm) affect the fluidity and permeability of biological membranes?

The transition temperature (Tm), also known as the melting temperature, is the specific temperature at which a membrane transitions between a gel-like, viscous state and a fluid-like state. At temperatures above the Tm, the membrane becomes more fluid and permeable, allowing substances to cross more easily. Below the Tm, the membrane is more rigid and less permeable. Factors affecting the Tm include the length and saturation of fatty acid chains, with longer and more saturated chains increasing the Tm. Additionally, the presence of cholesterol and proteins in the membrane can also influence its fluidity and permeability.

What factors influence the transition temperature (Tm) of lipid bilayers?

The transition temperature (Tm) of lipid bilayers is influenced by two primary factors: the length of the hydrocarbon chains and the degree of saturation of the fatty acids. Longer hydrocarbon chains increase the Tm, making the membrane more rigid at lower temperatures. The degree of saturation also plays a role; more double bonds (unsaturation) lower the Tm, making the membrane more fluid at lower temperatures. Additionally, other molecules such as cholesterol and proteins embedded in the membrane can affect its fluidity and permeability, thereby influencing the Tm.

Why is transverse diffusion slower than lateral diffusion in biological membranes?

Transverse diffusion is slower than lateral diffusion because it involves the movement of lipids from one layer of the bilayer to the opposite layer, a process that is energetically unfavorable and requires the assistance of specific enzymes. Without these enzymes, such as flippase, floppase, and scramblase, transverse diffusion can take days to occur. In contrast, lateral diffusion is an uncatalyzed process that involves the side-to-side movement of lipids within the same layer of the bilayer, which is energetically more favorable and occurs rapidly.