The Plasma Membrane

The one structure common to all cells, prokaryotes and eukaryotes, is a plasma membrane. We have already talked about the macromolecules that are the foundation of this plasma membrane, the phospholipids. The membrane functions to separate the cell from its external environment. When you think about a single-celled organism, such as a bacteria or a protist living with its cell exposed to the environment, there must be something which regulates how that cell interacts with the environment and which allows the cell to determine what enters the cell and what will be excluded. The plasma membrane provides protection, functioning somewhat like a gatekeeper. The structure of phospholipids, with both hydrophobic and hydrophilic regions, creates the lipid bilayer that separates the internal environment of the cell from the external environment. We covered this in Lesson 2. Figure 3.1 shows this type of lipid, with the glycerol and two fatty acids attached.

The phosphorylated alcohol portion is hydrophilic or water loving.  The fatty acid tails make up the remainder of the molecule which is hydrophobic.

Figure 3.1. The Structure of Phospholipids
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Remember, these bonds are formed by dehydration synthesis, the loss of a water molecule. Instead of having a third fatty acid, as in a triacylglycerol, there is a phosphate group. This group is polar or charged, so it gives this the molecule two different characteristics. It has a polar region and a nonpolar region. The nonpolar region is not water soluble. It's called hydrophobic or water-fearing. When you get a double layer of these phospholipids, the hydrophobic tails point in toward each other. This is how the structure is determined by the chemical nature of these molecules. These tails point in and then the polar water-loving heads point out, because the inside of the cell is aqueous, as is the fluid outside the cell. These hydrophobic tails maintain the integrity of the plasma membrane. If at any time they flip out of the membrane, they turn right back in again. The basic structure of the plasma membrane (Figure 3.2) is very similar in prokaryotic and eukaryotic cells.

The arrangement of phospholipids to form a bilayer; the graphic shows three layers: Polar hydrophilic heads (top layer), nonpolar hydrophobic tails (middle layer), and  polar hydrophilic heads (bottom layer).

Figure 3.2. The Arrangement of Phospholipids to Form a Bilayer
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The plasma membrane of a cell is a very complex structure. The lipid bilayer is the basic structure, but there are also proteins that are embedded in the membrane (Figure 3.3). These proteins help to transport material into and out of the cell. There are some proteins on the cell surface that act as receptors, helping to identify the cell. These are particularly important in multicellular organisms, where there are many different types of cells. They function like antennas, receiving signals; hormonal signals, for example. All our cells have estrogen receptors on them, and estrogen is an important hormone that regulates a number of things, including the growth of bones. Cells have these receptors so they can receive messages from other cells that help to determine how they function. There are also carbohydrates attached to the surface of cells. The ABO blood groups are determined by different carbohydrates on the surface of red blood cells. There is still much to learn about the many proteins in cell membranes.

The plasma membrane contains proteins.  Some of these simply stick out of the plasma membrane while other pass completely through the lipid bilayer.

Figure 3.3. Proteins in the Plasma Membrane
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You can't think of cells as having a hard surface. This plasma membrane is a permeable, almost fluid, layer around the cell; think of the texture of a soap bubble. The surface of a soap bubble, when gently prodded, moves but does not burst. The lipid bilayer is strong and not easily disrupted, but proteins are moving freely through this dynamic membrane. Because cells can't be completely isolated from their environment, this membrane must act like a gatekeeper, allowing some substances to enter and leave the cell, while preventing others from crossing through. This is why the membrane is called “selectively permeable.” Some substances are free to enter the cell and can come and go as needed. But many other substances are prevented from crossing. Some are completely prevented, while others are regulated by proteins embedded in the membranes. These proteins allow the membrane to be selective since each type of protein is specific to a certain substance, allowing only that substance to cross under certain conditions. One reason the cell membrane is such a complex and active area of study is because it has different proteins for different substances . . . and there is still much to learn.