Section 2.4 of the AP Biology curriculum takes a close look at the structure of the Plasma Membrane. This section looks at the components necessary to create functional cell membranes – such as phospholipids, membrane proteins, and other components. These components work together in a theory known as the “fluid mosaic model” that describes the fluid nature of these components as they function. Check it out!

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Resources for this Standard

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ENDURING UNDERSTANDING
ENE-2
Cells have membranes that allow them to establish and maintain internal environments that are different from their external environments.

LEARNING OBJECTIVE
ENE-2.A
Describe the roles of each of the components of the cell membrane in maintaining the internal environment of the cell.

ENE-2.B
Describe the Fluid Mosaic Model of cell membranes.

ESSENTIAL KNOWLEDGE
ENE-2.A.1
Phospholipids have both hydrophilic and hydrophobic regions. The hydrophilic phosphate regions of the phospholipids are oriented toward the aqueous external or internal environments, while the hydrophobic fatty acid regions face each other within the interior of the membrane.

ENE-2.A.2
Embedded proteins can be hydrophilic, with charged polar side groups, or hydrophobic, with nonpolar side groups.

ENE-2.B.1
Cell membranes consist of a structural framework of phospholipid molecules that is embedded with proteins, steroids (such as cholesterol in eukaryotes), glycoproteins, and glycolipids that can flow around the surface of the cell within the membrane.

2.4 Plasma Membrane Overview

In this section, we will cover the most important aspects of plasma membranes. We will start with the phospholipids and how they create a lipid bilayer. Then, we’ll move on to membrane proteins and how they create a living membrane. Finally, we’ll look at some other membrane components and how all these pieces come together to form the Fluid Mosaic Model. These are things you need to know before you take the AP test!

Let’s start with the biological macromolecules that make the plasma membrane possible. A plasma membrane is simply a sheet formed by many phospholipid molecules lined up together. To understand why phospholipids can function in this way, we need to look at their chemical structure.

If we look at the chemical components of a phospholipid molecule, we will see that the molecule has a hydrophilic head and a hydrophobic tail. If we take a closer look at these regions, we can see why they have polar and nonpolar properties in different parts of the molecules.

The polar head group contains a phosphate group and positively charged nitrogen group – both of which are polar and can interact with water molecules. Further, these molecules are bound to glycerol – a slightly polar molecule that can bind fatty acids together into groups.

Then, if we look at the hydrophobic tail, we can see that this tail is a standard hydrocarbon chain. Depending on the organism and the conditions it lives in, phospholipid molecules may use saturated or unsaturated fatty acids to create the tail region. Fatty acids are nonpolar molecules since all of the carbon atoms have 4 bonds to other carbons and hydrogens. These atoms share electrons evenly, which does not form polar sides or dipoles. Therefore, these hydrophobic tails do their best to exclude water and only interact with other nonpolar substances!

When you start putting many phospholipid molecules together, these polar and nonpolar areas of each molecule interact. The polar groups stick to other polar groups, but the hydrophobic tails like to orient in the same direction. At the smallest level, phospholipids will form what is known as a “micelle”. At this size, there is no room in the internal structure for any water, as the hydrophobic tails touch and exclude any polar molecules. As more phospholipids are added, it becomes harder to exclude water. As water enters the micelle, polar head groups follow it to make a small chamber. These structures – known as liposomes – are often used by your body to transfer fats and other substances through the bloodstream. Finally, if you put enough phospholipids together you get a lipid bilayer. Though cells are essentially just very large liposomes, they have a huge amount of space on the inside – this bilayer sheet is what makes the basis of the cell membrane in all cells!

This phospholipid bilayer effectively separates the extracellular space from the cytoplasm within. In eukaryotes, the endomembrane system is also created by a phospholipid bilayer and effectively separates the cell into multiple compartments that can carry out different tasks. The hydrophobic core of the bilayer excludes water and ions, so each side of the bilayer can have a completely different chemical composition. However, as we will see, you should not consider the phospholipid bilayer a solid structure. In fact, each phospholipid remains completely independent of the other phospholipids, interacting only through weak hydrogen bonds and nonpolar interactions. Therefore, these phospholipid molecules are constantly flowing and moving around.

In order to create cells or compartments within cells that have a different chemical composition than the aqueous solution on the other side of the phospholipid bilayer certain ions, macromolecules, and other chemicals must be able to either leave or enter the cell. Since they cannot easily cross the lipid bilayer, cells have a number of proteins dedicated to moving these substances across cells.

Protein channels are simply large protein molecules that fold into a structure similar to a straw. The hollow inside is just the right size to allow specific ions or molecules through the cell membrane while inhibiting other substances due to size or chemical properties. These protein channels are often formed with a hydrophobic section that helps hold the protein in the hydrophobic core of the lipid bilayer.

There are many different protein channels that operate through different mechanisms. Some need energy in order to open or only operate when specific substances bind with their active site. These proteins are known as carrier proteins.

Other membrane proteins are needed to communicate with other cells, interact with the external and internal environments, or attach important substances to the cell membrane. These proteins most often need to float around the cell membrane as they complete various tasks. So, instead of being created with hydrophobic amino acids that bind to the core of the lipid bilayer, these proteins are formed with hydrophilic amino acids that readily form hydrogen bonds with the hydrophilic heads of phospholipid molecules.

While phospholipids and membrane proteins are very important components when creating a functional plasma membrane, several other components are required for a cell to fully integrate and interact with its environment. Some of these other components include steroids, glycolipids, and glycoproteins.

First, let’s take a look at steroids like cholesterol, which play an integral role in determining how fluid the plasma membrane is. Cholesterol is a steroid lipid molecule that can be embedded in the plasma membrane. Because of its shape, cholesterol creates many nonpolar interactions with the hydrophobic tails of phospholipid molecules. In turn, this helps create a more rigid and impermeable cellular membrane.

In addition to cholesterol, cells often add carbohydrate chains to either proteins or lipids. These are called glycolipids if the chain is added to a phospholipid, and a glycoprotein if the chain is added to a protein. These molecules have an enormous number of functions for cells. In general, they are used in cell-to-cell communication, connecting to the extracellular matrix, and many other tasks that proteins or lipids are not suited for on their own.

Now that we have discussed all of the pieces that go into building the plasma membrane, let’s talk about how this membrane actually functions. The model that scientists use to understand the cell membrane is called the “fluid mosaic model”.

A mosaic is an artistic technique where many small parts are put together to create a larger image. Scientists call this the fluid mosaic model because all of these smaller components are allowed to flow freely and nothing is bound in place. This is possible because, in a cell membrane, nothing is actually bonded together. Hydrogen bonds and interactions between nonpolar substances are the only forces keeping the membrane together. Proteins can embed within the membrane, but they are not actually chemically bonded to it in any way.

You’re probably wondering what evidence we have that the plasma membrane actually functions like the fluid mosaic model that scientists use to describe it. Well, in 1970 two researchers from Johns Hopkins University set out to determine the nature of the cell membrane. In this Frye-Edidin experiment, two cells were infected with a virus that caused them to merge their cell membranes. By using a staining technique that allowed the researchers to track proteins from each original source, the researchers watched as the cells merged. At first, each half of the new cell contained only the proteins from each original cell. But, over time these proteins became randomly distributed across the new cell – evidence that the cell membrane was more of a fluid than a solid and rapidly changed and shifted position over time!