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The Plasma Membrane

Cell Membrane Structure and Function

All biological membranes are made of two main components:

  1. Phosphorus, or phosphate groups (–H2PO4R, where R is a functional group) 
  2. Lipids
These components combine to create a molecule called a phospholipid.

Well-named, don’t you agree?

You may have read about lipids in relation to fats in an earlier unit. Lipids are constituents of fats. So yes, these lipids fall under the same category as the lipids we discussed in the other unit. Their aversion to water even makes them greasy or oily in consistency.

Phospholipids line up next to each other because part of the molecule is attracted to water (the "phospho" part), and the other part is repelled by water (the "lipid," or oily, part). But, because the lipid part is repelled by water, it cannot be left out in the open; another line, or layer, of phosopholipids will line up next the first layer, with the "lipid" parts facing each other at the line where the two layers meet, and the "phospho" parts facing outward toward the water on both sides of the line. As more and more "lines" of phospholipids are added adjacently to the first line, but in the same plane, a phospholipid bilayer is born!

Did we just confuse you? A picture is worth a thousand words.

Transmission electron micrograph. Image source.

Because phospholipid bilayers, also called "membranes," are so important and so common to cells—meaning, every single cell on Earth has at least one—that it is important to look at this cellular structure in a little more detail.

All prokaryotic cells have a phospholipid bilayer, or sometimes multiple bilayers, called the plasma membrane. This structure marks the boundary between the inside and outside of the cell even though it is found on the inside of the prokaryotic cell wall. The cytoplasm and other prokaryotic cellular contents are found inside the plasma membrane.

On the other hand, all eukaryotic cells have a plasma membrane as well, but they also have additional phospholipid membranes surrounding internal structures like the nucleus, the mitochondria, and the chloroplasts. Each of these internal membranes plays a vital and unique role in the growth, survival, and development of the cell.

The Plasma Membrane

The plasma membrane marks the boundary between life and nonlife.

It's a well-oiled machine, if you ask us:

This structure keeps the contents of a cell separate from the environment surrounding it. In addition to phospholipids, the plasma membrane has cholesterol molecules and proteins that allow the membrane to function properly. Cholesterol molecules are primarily responsible for giving the membrane the rigidity it needs to hold the cell’s shape. Without cholesterol, cells would be nothing more than useless, flattened bags of mush. Balloons with all of the air let out. All gross and wilty-looking. If you look at the chemical shape of cholesterol, you will notice that is has a lot of ring-like structures, which tend to be very rigid.

Proteins embedded in the membrane play important roles in helping the cell communicate with its environment, including with other cells, and with transporting materials into and out of the cell.

There are two main types of membrane proteins:

  1. Those that transverse the membrane, called integral proteins
  2. Those that are stuck on the inside or outside of the membrane, called peripheral proteins
Integral proteins are often involved in the transport of materials while peripheral proteins generally function in cellular communication. All of these membrane components, lipids, cholesterol molecules, and proteins can move laterally, or side-to-side, through the membrane, causing biologists to consider the membrane as a kind of fluid. Also, because the membrane is made up of a number of different components, it is also considered to be a mosaic. Yes, like a piece of art. These two ideas come together in what is known as the "Fluid-Mosaic Model" of the plasma membrane7.

Biologists have also recently learned that the plasma membrane and the cytoskeleton interact very closely with one another8. Because the phospholipid bilayer is somewhat fluid, proteins embedded in the membrane could easily move around willy-nilly. So that this doesn’t happen, the cytoskeleton attaches to the membrane proteins and anchors them in place. You guys aren't going anywhere.

The interior of the phospholipid bilayer is hydrophobic, so only very small neutrally charged molecules like oxygen (O2), carbon dioxide (CO2), and water (H2O) can pass freely through the membrane. Everything else must pass through a transmembrane protein, meaning the cell has general control over what gets in and what gets out. The transmembrane protein can be thought of as the security guard at the door of the Sublime with Rome concert. He steals your plastic bottles and confiscates your food in the event that you get a little frisky and start chucking items at other concertgoers or, heaven forbid, at Rome because you still have pent-up aggression over Bradley's death, despite the fact that it was 15+ years ago. Poor Rome. Sorry, where were we again?

In short, the plasma membrane plays the very same roles for a cell as your skin, eyes, ears, mouth, and nose play for your body. The phospholipids provide protection to the cell while membrane proteins allow it to "eat," "drink," and "breathe," as well as "feel," "see," and "hear" the outside environment. Without the membrane proteins, a cell would be blind, deaf, and dumb. It would also quickly starve to death. It would be as functionless as a human who lacks the five major senses. We wouldn't want that, now would we? The plasma membrane is a very important part of the cell indeed.

Brain Snack

Your plasma membrane is the place where your cells meet all foreign material, including pathogens. Because of this fact, it's also the place where the cells of your immune system communicate with other cells. 

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