In this lesson, we will discuss the components of the cell membrane and why the fluid mosaic model paints the best picture of its structure. We’ll learn about the roles of the phospholipid bilayer, cholesterol, proteins and carbohydrates.
The Fluid Mosaic Model
A cell is the basic fundamental unit of all life on Earth. It contains numerous important structures and components that function together, with all the parts interacting with each other.
The cytoplasm is the intracellular space within the cell containing all these structures and components.Cells are often likened to a tiny metropolis. Like any town, a cell needs to create boundaries and to protect itself.
Not much inside it is static or stationary, including the cell membrane that surrounds it. All cells have a fluid but defining cell membrane that encircles the cytoplasm like an old city would have a fluid protective moat. In this way, a cell membrane provides a barrier to separate the cytoplasm from the extracellular space. Things are brought in and things are shipped out, but it would be dangerous to let just anything pass through it without the right credentials.
Therefore, we also describe a cell membrane as selectively permeable or only allowing certain substances to pass through the cell membrane.A cell membrane, however, is made up of a few more sophisticated parts than a moat and is constructed from entirely different materials. This cell membrane is primarily composed of a phospholipid bilayer, cholesterol, proteins, and carbohydrates.
Though the parts are different, it’s easier to think about how the membrane functions by again comparing it to that protective moat around your city. The composition of the cell membrane is best described by the fluid mosaic model, where different parts of the membrane can float in a fluid-like space. Like it sounds, this model suggests that the cell membrane is dynamic or moving and asymmetrically scattered with different pieces, such as phospholipids, cholesterol, proteins, and carbohydrates, and that these different parts of the cell membrane float in this fluid-like space, moving laterally throughout the membrane.
Okay, so let’s start to build what the mosaic looks like around a cell. We’ll start with one of the most popular components of the cell membrane – the phospholipids. These phospholipids form the phospholipid bilayer.
As a refresher, this is two layers of phospholipids that give us a membrane with hydrophilic heads oriented towards the outside and inside of the cell, where there is an aqueous environment. The fatty acid hydrophobic tails of the phospholipids don’t really like to get their tails wet, so they face each other, creating this hydrophobic region inside the bilayer. A function of this bilayer is to create a distinct hydrophobic region which is essential in both keeping fluidity and moderating who can get by the selectively permeable membrane and enter our miniature city.
In later lessons, we’ll talk about who’s allowed through and when.
Now we’re going to start adding molecules to the phospholipid bilayer that’s really going to turn this membrane into a biologically beautiful piece of mosaic art. We’ll start with cholesterol, which is found floating throughout the membrane. Remember that not only is there a lot of cholesterol in a double cheeseburger, there’s also a good amount in our cells.
Here, it does a lot more good than clogging our arteries. Like phospholipids, cholesterol is also a hydrophobic molecule. So, where do you think it would want to spend its time? Because alike molecules typically hang out together, hydrophobic cholesterol is also located within the hydrophobic region of the cell membrane created by the phospholipid tails.
Cholesterol resides within this hydrophobic space, and helps maintain membrane fluidity. It both provides structure and keeps the membrane from freezing. In addition, cholesterol helps the membrane act as a protective barrier to unwanted molecules.
Let’s keep adding to our magnificent mosaic. Proteins are another important part of the cell membrane. There are two types of membrane proteins, integral and peripheral.
These proteins are so named for the space they occupy relative to the cell membrane. Integral proteins are membrane proteins that actually ‘integrate’ themselves throughout the bilayer, where they can even cross both sides to touch the inside and outside of the cell. In order to have this special power, these proteins also have hydrophobic domains that span the hydrophobic regions, and hydrophilic domains that touch the hydrophilic areas inside and outside of the cell.
They provide structure to the cell membrane, act as recognition sites for other proteins, participate in cell-cell communication, and help transport substances across the membrane.
Now, we have one last piece to add to our picture – carbohydrates. Not only are carbohydrates delicious in some fresh-baked bread, but they are an important part of the cell membrane, too.
Carbohydrate chains can be found on the outside of a cell membrane. They are molecules attached to lipids or proteins and they act as binding sites or can help cells adhere to other cells.
Now let’s take a step back and summarize how we put together this beautiful cell membrane mosaic. This cell membrane encloses the cytoplasm of the cell. Remember that the cytoplasm is the inside of the cell and all its structures enclosed by a cell membrane.
It is a dynamic structure best described by the fluid mosaic model, which states that cell membrane components float in a fluid-like space.The cell membrane is composed of a phospholipid bilayer as the background canvas for this mosaic, which is then embedded with cholesterol, proteins and carbohydrates that float and move laterally within this bilayer like objects in water. These four components aid in keeping a fluid-like structure to the cell membrane, maintaining a barrier between the cytoplasm and extracellular environment, recognizing cell-cell communication signals, and providing transport.
At the end of this video, you will be able to explain how the phospholipid bilayer, cholesterol, proteins and carbohydrates are integrated into the Fluid Mosaic Model of the cell.