Microbiology, part 11: Prokaryotes - Plasma Membrane & Cell Wall

Hi, I'm Cathy with Level Up RN. In this video, I'm going to continue my coverage of prokaryotic cells. Specifically, I'll be talking about the plasma membrane and the cell wall. At the end of the video, I'm going to give you guys a quiz to test your understanding of some of the key facts I'll be presenting. So definitely stay tuned for that. And if you have our Level Up RN microbiology flashcards, go ahead and pull out your flashcards on the plasma membrane and cell wall so you can follow along with me, and pay close attention to the bold red text on the back of the cards because those are the things that you are likely to get tested on in your micro class.

The plasma membrane is a phospholipid bilayer that encloses the cell's cytoplasm. It provides selected permeability, which means it only allows certain materials to enter or exit the cell. Smaller nonpolar molecules, such as oxygen or carbon dioxide, are able to freely diffuse across the membrane. However, the plasma membrane would be impermeable to larger polar molecules. The plasma membrane has the consistency of oil, so it is not a solid structure. And it is often described in terms of the fluid mosaic model, which means that components of the membrane are able to move freely within the membrane.

Let's take a look at the structure of the plasma membrane using an illustration from our microbiology flashcard deck. As I mentioned before, the plasma membrane is composed of a phospholipid bilayer, so two rows of phospholipids. Each phospholipid molecule has a polar head, which is hydrophilic or water-loving, and two nonpolar tails, which are hydrophobic or water-fearing. Proteins are embedded in the plasma membrane and carry out various functions. Transmembrane proteins span the entire plasma membrane. An example of a transmembrane protein is a protein channel, which allows the transport of specific molecules across the membrane. Peripheral proteins are proteins that do not extend through the entire plasma membrane, and they function in a number of ways. They provide support for the cell, and some act as receptors or enzymes. You will also find glycoproteins and glycolipids that extend out from the plasma membrane into the extracellular environment, so outside the cell. Glycoproteins are proteins attached to carbohydrates, whereas glycolipids are lipids attached to carbohydrates. The key function of these structures is cell-to-cell communication.

Surrounding the plasma membrane, we have our cell wall. Most but not all prokaryotes have a cell wall, which protects the cell from the outside environment. The major component of bacterial cell walls is peptidoglycan. Peptidoglycan is composed of alternating NAM and NAG sugar molecules that are cross-linked by peptide bridges. Let's take a look at an illustration from our microbiology flashcard deck, which shows the differences between gram-positive cell walls and gram-negative cell walls. On the left here, we have a gram-positive cell wall, which consists of many layers of peptidoglycan, whereas a gram-negative cell wall, which is shown here on the right, contains a thin layer of peptidoglycan.

Gram-positive cell walls are embedded with teichoic acid as well as lipoteichoic acid. The main purpose of these acids is to bind cations, which are positive ions for use by the bacterial cell. Gram-negative cell walls do not contain teichoic acid. In gram-negative cells, we have an outer membrane, which is a second phospholipid bilayer external to the peptidoglycan layer. The gel-like matrix between the inner and outer membrane is the periplasmic space. The outer membrane contains porins, which are channels that permit certain substances into the cell. The outer membrane also contains lipopolysaccharide, or LPS, which consists of lipid A, which is an endotoxin, core polysaccharides, and O polysaccharides.

The other type of cell wall that was not shown in the previous illustration is the acid-fast cell wall. So acid-fast cell walls contain an external layer of mycolic acid, which are long fatty acids. These types of cells are called acid-fast, because once they are stained, they resist decolorization by acids. The most well-known acid-fast bacterium is mycobacterium tuberculosis, which is the causative agent for tuberculosis or TB.

All right. It's quiz time, and I have five questions for you. Question number one, the plasma membrane is made up of a blank bilayer. The answer is phospholipid. Question number two, blank is a major component of bacterial cell walls composed of long chains of NAM and NAG sugar molecules. The answer is peptidoglycan. Number three, which type of cell wall contains a thick layer of peptidoglycan, gram-positive or gram-negative? The answer is gram-positive. Number four, which type of cell wall contains mycolic acid? The answer is an acid-fast cell wall. Number five, which type of cell wall contains an endotoxin, gram-positive or gram-negative? The answer is gram-negative. All right. That's it for this video. I hope you did great with that quiz. Thank you so much for watching, and good luck with studying.