The construction of the plasma membrane is examined in detail in Section 2.4 of the AP Biology curriculum. Phospholipids, membrane proteins, and other components essential for the formation of functioning cell membranes are discussed in this section. The “fluid mosaic model,” which explains the fluid nature of these components as they operate, illustrates how these components interact together. Take a look!
Plasma Membrane Overview
We’ll go over the most crucial elements of plasma membranes in this section. We’ll begin by looking at phospholipids and how they form a lipid bilayer. Then we’ll look at membrane proteins and how they contribute to the formation of a live membrane. Finally, we’ll look at some more membrane components and how they all fit together to produce the Fluid Mosaic Model. These are important things to know before taking the AP exam!
Let’s start with the biological macromolecules that allow for the formation of the plasma membrane. A plasma membrane is nothing more than a sheet made up of numerous phospholipid molecules lined up in a row. We need to look at the chemical structure of phospholipids to understand why they may act in this manner.
A phospholipid molecule contains a hydrophilic head and a hydrophobic tail, as may be seen by looking at its chemical components. We can understand why these areas have polar and nonpolar characteristics in various portions of the molecules if we look at them more closely.
Both the phosphate and positively charged nitrogen groups in the polar head group are polar and may interact with water molecules. Furthermore, these molecules are attached to glycerol, a somewhat polar molecule capable of grouping fatty acids.
When we examine the hydrophobic tail, we can tell that it is a typical hydrocarbon chain. The tail portion of phospholipid molecules may be made of saturated or unsaturated fatty acids, depending on the organism and the environment it lives in. Because all the carbon atoms have four links to other carbons and hydrogens, fatty acids are nonpolar compounds. These atoms share electrons uniformly, so no polar sides or dipoles are formed. As a result, these hydrophobic tails work hard to keep water out and only interact with nonpolar molecules!
These polar and nonpolar regions of each phospholipid molecule interact when several of them are together. The polar groups keep together, but the hydrophobic tails prefer to orient themselves in the same direction. Phospholipids form what is known as a “micelle” at the smallest scale. Because the hydrophobic tails meet and prohibit any polar molecules, there is no place in the interior structure for any water at this size. It gets increasingly difficult to exclude water when more phospholipids are added.
Water enters the micelle and is followed by polar head groups, forming a tiny chamber. Liposomes are little structures that your body uses to transport fats and other chemicals through the circulation. Finally, a lipid bilayer is formed when enough phospholipids are combined. Though cells are simply giant liposomes, they contain a lot of room on the inside—this bilayer sheet is the foundation of the cell membrane in every cell!
This phospholipid bilayer efficiently divides the cytoplasm from the extracellular space. In eukaryotes, a phospholipid bilayer also forms the endomembrane system, which essentially divides the cell into several compartments that may perform distinct activities. Because the bilayer’s hydrophobic core keeps water and ions out, each side might have an entirely distinct chemical makeup.
As we shall show, the phospholipid bilayer should not be considered a solid structure. In reality, each phospholipid is totally self-contained, communicating only via weak hydrogen bonds and nonpolar interactions with the others. As a result, these phospholipid molecules are always flowing and moving.
Certain ions, macromolecules, and other substances must be able to leave or enter the cell in order to establish cells or compartments within cells with a different chemical composition than the aqueous solution on the other side of the phospholipid bilayer. Because these chemicals cannot readily pass through the lipid bilayer, cells contain a variety of proteins specialised for transporting them across cells.
Protein channels are just big protein molecules folded into a straw-like shape. Due to size or chemical qualities, the hollow interior is precisely the perfect size to enable particular ions or molecules to pass through the cell membrane while preventing other substances. These protein channels often have a hydrophobic region that aids in the retention of the protein in the lipid bilayer’s hydrophobic core.
There are several protein channels, each with its own mode of operation. Some need energy to open, while others only function when certain chemicals attach to their active site. Carrier proteins are the name for these proteins.
Other membrane proteins are required for cellular communication, interaction with the external and internal surroundings, and the attachment of essential molecules to the cell membrane. These proteins must often float about the cell membrane in order to perform their functions. These proteins are made up of hydrophilic amino acids that easily establish hydrogen bonds with the hydrophilic heads of phospholipid molecules, rather than hydrophobic amino acids that attach to the centre of the lipid bilayer.
While phospholipids and membrane proteins are critical components in the creation of a functioning plasma membrane, a cell’s ability to properly integrate and interact with its surroundings requires numerous additional components. Steroids, glycolipids, and glycoproteins are some of the other components.
Let’s start with hormones like cholesterol, which are important in influencing the fluidity of the plasma membrane. Cholesterol is a kind of steroid lipid that may be found in the plasma membrane. Cholesterol forms several nonpolar interactions with the hydrophobic tails of phospholipid molecules due to its structure. As a result, the cellular membrane becomes more stiff and impermeable.
Cells often add carbohydrate chains to proteins or lipids in addition to cholesterol. If the chain is attached to a phospholipid, it is termed a glycolipid, and if it is attached to a protein, it is called a glycoprotein. These molecules play a wide range of roles in cells. They’re employed for cell-to-cell communication, linking to the extracellular matrix, and a variety of other functions that proteins or lipids aren’t equipped to do.
Let’s talk about how the plasma membrane works now that we’ve gone over all the components that go into its construction. The “fluid mosaic model” is a model that scientists use to explain the cell membrane.
Fluid Mosaic Model
A mosaic is an artistic method in which numerous tiny pieces are assembled to form a bigger picture. Because all of these tiny components are permitted to flow freely and nothing is held in place, scientists call this the fluid mosaic model. This is feasible since nothing is truly bound together in a cell membrane. The only forces that hold the membrane together are hydrogen bonds and interactions between nonpolar molecules. Proteins may bind to the membrane, but they are not chemically bound to it.
You’re probably asking whether there’s any proof that the plasma membrane works like the fluid mosaic model that scientists use to explain it. In 1970, two Johns Hopkins University researchers set out to discover the nature of the cell membrane. Two cells were infected with a virus that induced their cell membranes to fuse in this Frye-Edidin experiment.
The researchers monitored as the cells joined using a labelling approach that enabled them to trace proteins from each original source. Each half of the new cell initially contained solely the proteins from the original cell. However, throughout time, these proteins were randomly dispersed throughout the new cell, indicating that the cell membrane was more fluid than solid, changing and shifting positions fast!