Membrane Structure 1: Study with Video Lessons, Practice Problems & Examples

Membranes are primarily composed of proteins, which facilitate various biological functions, alongside phospholipids and cholesterol. Cholesterol reduces membrane fluidity, crucial for structural integrity in plasma membranes. Phospholipids exhibit lateral movement, with enzymes like flippase and scramblase managing their distribution. Membrane composition varies with temperature; saturated fatty acids dominate in warmer environments, while unsaturated fatty acids are prevalent in cooler conditions. Additionally, lipid rafts, dense pockets of sphingolipids and cholesterol, contribute to membrane organization and function.

Membrane Structure 1

Even though we tend to think of lipids as the main components of membranes, proteins actually tend to make up the majority of membranes, and that's because proteins allow membranes to have functions. Proteins are the doers in biology. Now, in addition to proteins and phospholipids, membranes also contain sterols, and chief among these is cholesterol, which you can see in this molecule right here. Cholesterol tends to fill in the gaps left by the fatty acids of the phospholipids. So, it actually decreases the fluidity of the membrane. As a result, the outer membrane, the plasma membrane, tends to have a large amount of cholesterol because this membrane is in part responsible for the structural integrity of the cell, whereas internal membranes tend to have more because they're not as responsible for the structure of the cell, and they tend to be more fluid. Now, when we say the membrane is fluid, we literally mean the membrane is a fluid substance. There are no covalent bonds between the membrane components, and phospholipids tend to move around a lot. In fact, when you track the motion of a phospholipid, you find that phospholipids tend to move around a lot laterally. They'll go this way or that way, but it's rare for them to flip-flop between the outer and inner monolayers of the membrane. However, there are enzymes whose job it is to flip phospholipids between the monolayers, and their names are, I kid you not, flippase, floppase, and scramblase. In this image, we have the outer monolayer and the inner monolayer. Flippase's job is to take a phospholipid from the inner layer to the outer layer. Scramblase does a combined job; it moves phospholipids from the outer layer to the inner layer and also vice versa. It's important to note that there's an asymmetric distribution of phospholipids in the membrane. Phosphatidylethanolamine, phosphatidylserine, and phosphatidylinositol tend to be found on the inner monolayer, whereas phosphatidylcholine and sphingomyelin tend to be found on the outer monolayer. The distribution of phospholipids is different in internal membranes; the total composition and the monolayer in which they're found tend to differ. If you think of the outer monolayer being made up of blue and the inner monolayer being red, this symbolizes the difference in the composition of lipids. An internal membrane basically has the opposite appearance; its outer monolayer is going to be like the red one, whereas the inner monolayer is going to be more like the blue one. This is the inside here. The reason for this is because of how the internal and plasma membranes interact. The internal membrane often forms vesicles that merge with the plasma membrane. This process occurs in reverse when the plasma membrane pinches inward, forming a vesicle that goes into the cell. This difference in distribution is because of how they interact, making sense that they have an opposite distribution of phospholipids. The composition of membrane lipids is related to the temperature of the organism's environment. Organisms want to maintain a plasma membrane with a certain level of rigidity. If you live at higher temperatures, you're going to want more saturated fatty acids because your membrane's components are subjected to more energy. You don't want them to become so fluid and move around quickly that they fall apart. Conversely, if you live in colder temperatures, you're going to want more unsaturated fatty acids, as unsaturated fatty acids have cis double bonds causing kinks, allowing them to remain more fluid at lower temperatures instead of hardening into a crystal-like structure. Within the membrane, the distribution of phospholipids isn't even within one monolayer. There are high-density pockets formed by concentrations of sphingolipids and cholesterol molecules, appearing like rafts in an ocean of other phospholipids.

Here’s what students ask on this topic:

What are the main components of cell membranes?

Cell membranes are primarily composed of proteins, phospholipids, and cholesterol. Proteins are the main functional components, enabling various biological activities. Phospholipids form the basic structural framework, creating a bilayer with hydrophilic heads facing outward and hydrophobic tails inward. Cholesterol, a type of sterol, fills gaps between phospholipids, reducing membrane fluidity and contributing to structural integrity, especially in the plasma membrane. This combination of components allows the membrane to be both flexible and selectively permeable, essential for maintaining cellular homeostasis.

How does cholesterol affect membrane fluidity?

Cholesterol plays a crucial role in modulating membrane fluidity. It inserts itself between the fatty acid tails of phospholipids, filling gaps and thereby reducing the movement of these tails. This results in decreased fluidity of the membrane. In the plasma membrane, which requires structural integrity, a higher concentration of cholesterol is present. Conversely, internal membranes, which are more fluid and less structurally demanding, contain less cholesterol. This balance ensures that the membrane remains functional under various conditions.

What is the role of flippase, floppase, and scramblase in membrane dynamics?

Flippase, floppase, and scramblase are enzymes that manage the distribution of phospholipids between the inner and outer monolayers of the membrane. Flippase moves phospholipids from the outer to the inner monolayer, while floppase does the reverse. Scramblase facilitates the bidirectional movement of phospholipids, maintaining an asymmetric distribution. This dynamic movement is essential for various cellular processes, including membrane repair, apoptosis, and vesicle formation.

How does temperature affect membrane composition?

Temperature significantly influences membrane composition. Organisms in higher temperatures have membranes with more saturated fatty acids, which have stronger Van der Waals interactions and higher melting points, preventing the membrane from becoming too fluid. Conversely, organisms in colder environments have more unsaturated fatty acids with cis double bonds, creating kinks that maintain fluidity at lower temperatures. This adaptation ensures that the membrane retains its functional integrity across different thermal conditions.

What are lipid rafts and their function in the membrane?

Lipid rafts are microdomains within the membrane, enriched with sphingolipids and cholesterol, making them more densely packed than surrounding areas. These rafts serve as organizing centers for the assembly of signaling molecules, influencing membrane fluidity and protein trafficking. They play a crucial role in various cellular processes, including signal transduction, protein sorting, and membrane fluidity regulation. By concentrating specific lipids and proteins, lipid rafts facilitate efficient cellular communication and function.