Practice - Membrane Structure 1 - Online Tutor, Practice Problems & Exam Prep

Phospholipase A2 is crucial for removing unsaturated fatty acids from phospholipids, typically found at the 2 position. Integral membrane proteins, characterized by hydrophobic and hydrophilic regions, can be extracted using detergents. Membrane transport involves flipases, floppases, and scramblases. The plasma membrane of animal cells contains 45% phospholipid and 55% protein, yielding a lipid-to-protein mole ratio of approximately 65, calculated using average molecular weights of 750 Daltons for phospholipids and 60,000 Daltons for proteins.

Practice - Membrane Structure 1

Moving on to membrane structure, if you haven't done question 22 yet, pause the video now. Alright. So the phospholipase that is most likely to remove an unsaturated fatty acid from a phospholipid is phospholipase A2. And the reason for this is, well, first, let's just draw our phospholipid. Here we have a phosphate group and this is attached to the head. Right. And here we have our ester bond. And here we have another ester bond and whatever fatty acid is there. Okay. So, what I was kinda like what we're just looking at before. Here's the one position, 2, and the 3. So the head's on 3. Fatty acids are in 1 and 2. And if there is only 1 unsaturated fatty acid like we were just saying, it's always going to be at the 2 position. And that's where phospholipase A2 cuts. It cuts right here. A1 cuts up here. And C cuts right here. And D cuts right there. So you can see that A2 would be the most likely to remove an unsaturated fatty acid because, if there's only 1 unsaturated fatty acid, it's going to appear at the 2 position and that's A2 is cutting there. Great.

So the shortest helix segment that could span a membrane is about 20 amino acids long. This is just a fact. Not much else to say about this. 5 isn't long enough. 50 could certainly do the job. 100, 200...probably getting a little excessive there. Alright.

An integral membrane protein can be extracted with a solution containing detergent. And the reason for this is integral membrane proteins have hydrophobic and hydrophilic regions. Right? They're going to have these hydrophilic regions in the head area or in like the head-tail. That's sort of how I think of it anyways. And then this hydrophobic membrane-spanning region. Right? So I've drawn the hydrophobic region in red. The hydrophilic regions are in blue. And you need to use detergent because that aids in the interaction with the hydrophobic portion of the protein. So the detergent molecules will help extract the integral membrane protein by stabilizing that hydrophobic portion which is normally embedded in the membrane but now you want to remove it.

Looking at question 25, which of the following are not enzymes involved in moving phospholipids from one leaflet to another? That's like from one side of the membrane to the other. The answer is C, Flip Flop Bases. That's not a real thing. You have flippases, floppases, and scramblases. So if we pretend this here is our membrane and here is the inside and here is the outside, you have flippases that move something from the outer leaflet to the inner leaflet. So that's a flip. A floppase is the opposite direction. Floppase brings something from the inner leaflet or you could call it the cytosolic leaflet because the inside is going to be the cytosolic side as opposed to the outside being the extracellular fluid, the external fluid. So, a floppase will move something from the inside to the outside and scramblase does the flip-flop. Right? It scramblase is kind of like the flip-flop ace but it's of course called scramblase. Alright.

Last questions. Take a look at 26 here. The plasma membrane of animal cells contains 45% by weight phospholipid and 55% of protein. What is the mole ratio of lipid to protein assuming the average molecular weight of phospholipid is 750 Daltons and the average molecular weight of protein is 60000 Daltons? The easiest way to answer this question, you're given these percentages. Let's just pretend that you have 100 total grams of your membrane. So that would mean that you'd have 45 grams of phospholipid and you would have 55 grams of protein. And you'd want to divide those by their respective weights. So dividing by 750 Daltons for the lipids and 60000 Daltons for the proteins gives you 0.06 moles of lipid and 0.00092 moles of protein. To find the ratio of lipid to protein, divide these two numbers and what you get is about 65, approximately.

Here’s what students ask on this topic:

What is the role of phospholipase A2 in membrane structure?

Phospholipase A2 plays a crucial role in membrane structure by removing unsaturated fatty acids from phospholipids. Typically, these unsaturated fatty acids are found at the 2 position of the glycerol backbone in phospholipids. Phospholipase A2 specifically cleaves the ester bond at this position, facilitating the removal of the unsaturated fatty acid. This process is essential for maintaining the fluidity and functionality of cellular membranes, as the presence of unsaturated fatty acids affects membrane dynamics and interactions with proteins.

How can integral membrane proteins be extracted from the membrane?

Integral membrane proteins can be extracted from the membrane using detergents. These proteins have both hydrophobic and hydrophilic regions. The hydrophobic regions are embedded within the lipid bilayer, while the hydrophilic regions interact with the aqueous environment. Detergents, which have hydrophobic tails and hydrophilic heads, interact with the hydrophobic regions of the integral membrane proteins. This interaction helps to solubilize the proteins, allowing them to be extracted from the membrane while maintaining their structural integrity.

What are flipases, floppases, and scramblases in membrane transport?

Flipases, floppases, and scramblases are enzymes involved in the movement of phospholipids across the lipid bilayer of cellular membranes. Flipases move phospholipids from the outer leaflet to the inner leaflet (cytosolic side), while floppases move them in the opposite direction, from the inner leaflet to the outer leaflet. Scramblases facilitate the bidirectional movement of phospholipids, effectively scrambling their distribution between the two leaflets. These enzymes are essential for maintaining membrane asymmetry and facilitating various cellular processes.

How is the lipid-to-protein mole ratio in the plasma membrane of animal cells calculated?

The lipid-to-protein mole ratio in the plasma membrane of animal cells is calculated using the weight percentages and average molecular weights of phospholipids and proteins. Given that the membrane contains 45% phospholipid and 55% protein by weight, and assuming average molecular weights of 750 Daltons for phospholipids and 60,000 Daltons for proteins, the calculation is as follows:

1. Assume 100 grams of membrane: 45 grams of phospholipid and 55 grams of protein.

2. Convert to moles: 45 grams / 750 Daltons = 0.06 moles of lipid, 55 grams / 60,000 Daltons = 9.2 × 10^-4 moles of protein.

3. Calculate the ratio: 0.06 moles lipid / 9.2 × 10^-4 moles protein ≈ 65.

Thus, the lipid-to-protein mole ratio is approximately 65.

What is the shortest helix segment that can span a membrane?

The shortest helix segment that can span a membrane is about 20 amino acids long. This length is sufficient to traverse the hydrophobic core of the lipid bilayer, which is typically around 30 Å thick. The helical structure allows the amino acid side chains to interact favorably with the hydrophobic environment of the membrane, stabilizing the protein's position within the bilayer. Segments shorter than 20 amino acids are generally not long enough to span the membrane, while longer segments can also span the membrane but may be excessive for this purpose.