The bacterial cell wall has to be strong to prevent cell lysis but also porous to allow transport across the cell membrane.
In this lesson, we will examine the structure of the bacterial cell wall and how it accomplishes both of these crucial tasks.
Bacterial cells appear simple, but building these cells represents quite an engineering challenge. To understand this challenge, we first have to understand osmotic pressure and its relationship to bacterial cells.Osmosis is defined as the movement of water through a semipermeable membrane from areas of low solute concentration to areas of high solute concentration. A solute is simply any substance dissolved in a solvent liquid, which is typically water. The water is able to pass through the membrane, but the solute cannot.
If one side has more solute (a hypertonic solution) than the other side (a hypotonic solution), the water will move to the more concentrated side in an attempt to balance the solute concentrations. If the hypertonic solution is in an enclosed vessel, the inflow of water can create significant pressure, possibly enough to rupture the vessel.So what does this have to do with bacteria? Bacteria are covered by a cell membrane, which is semipermeable. Water is able to freely diffuse into or out of the cell via transport proteins, depending on the solute concentrations. Generally, on the inside of the cell, there’s a considerable amount of solute.
The bacteria have DNA, proteins, enzymes, salts, nutrients, and ions in solution in the cytoplasm that are held in by the semipermeable nature of the membrane. It is crucial for the survival of the bacterium that these molecules stay in the cytoplasm. Inside the cell, the cytoplasmic solution is hypertonic compared to the environment. What we have just learned about water is that it strives for balance and should diffuse into the cell. Unfortunately for the cell, the resulting osmotic pressure can be up to 2 atmospheres, which is about the same pressure as your car tires!So, going back to that engineering challenge, the bacteria need to develop some mechanism that will prevent the cell from rupturing but still be porous enough for materials like nutrients or wastes to get into and out of the cell.
Bacterial cells are covered by a cell envelope that is composed of a cell membrane and a cell wall. The cell membrane is a phospholipid bilayer that regulates the transport of molecules into and out of the cell.
This is the weak structure that would burst from the osmotic pressure without reinforcement. The cell wall is the component of the envelope that provides that reinforcement.
Nearly every genus of bacteria has a cell wall, which is a rigid, carbohydrate-containing structure that surrounds the bacterial cell. As is always the case in biology, there are a few oddballs, like the genus Mycoplasma, that have lost their cell walls, but since they are a minority, having a cell wall must be a major advantage to the bacteria.This cell wall exoskeleton provides the bacteria with several benefits. The cell wall protects the bacterium from damage by encircling it with a tough, rigid structure. This structure is also porous.
Small molecules are able to freely pass through the cell wall to the membrane, but large molecules are excluded. In this way, the cell wall acts as a coarse filter. The primary function of the cell wall, however, is to maintain the cell shape and prevent bursting from osmotic pressure (called lysis).Not surprisingly, nature seems to have solved the challenge perfectly. The cell wall maintains its shape and its permeability! By digging into the details, we can understand how the cell wall accomplishes this.
The major structural component of the cell wall is peptidoglycan, which is a complex molecule composed of alternating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) cross-linked by short peptides.
What results is a flat, crosshatch pattern that is very strong and rigid, yet open enough for movement of particles. As you can see, peptidoglycan resembles a chain-link fence.Nearly all bacteria have cell walls made of peptidoglycan. But there is more to a cell wall than just peptidoglycan. In nature, there are two major types of cell walls, Gram-positive and Gram-negative, each with very different structures. The term ‘Gram’ refers to the Gram-staining technique that differentiates bacteria with the two different cell walls. For this lesson, it’s not important to know the exact staining procedure.
The Gram-positive bacteria have multiple peptidoglycan layers forming very thick, rigid cell walls. The flat, crosshatched layers of peptidoglycan are stacked on top of each other, creating a relatively thick cell wall. Spanning the stack of peptidoglycan is teichoic acid. This long molecule has a negative charge and helps move ions through the thick cell wall.
It is important to note that teichoic acid is only found in Gram-positive bacteria.
The Gram-negative bacteria differ from Gram-positive in two major ways. The Gram-negative cell wall is composed of only one or two layers of peptidoglycan that is covered by an outer membrane.Gram-negative bacteria have a typical cell membrane that covers the entire cell. Just outside of this membrane is the periplasm, which is a jelly-like layer between the outer membrane and the cell membrane. The periplasmic space includes the layers of peptidoglycan as well as enzymes and additional transport proteins. In Gram-negative bacteria, there are only one or two layers of the peptidoglycan mesh.
Immediately outside the periplasm is the outer membrane. The inner surface of the outer membrane is made of phospholipids just like the cell membrane. The outer surface of the outer membrane has a different composition.
Instead of being phospholipids, this half of the membrane is composed of lipopolysaccharides (LPS) attached to Lipid A molecules. These LPS are primarily structural but are also bacterial endotoxins. If a Gram-negative bacterium dies, the LPS are released and can be toxic to the host organisms.Throughout the outer membrane are porins.
These proteins form pores in the outer membrane that allow small, hydrophilic molecules like sugars and amino acids into the periplasmic space but keep larger, hydrophobic molecules out. Dyes, antimicrobials, and disinfectants are often prevented from getting into the cell by these porins.The last important thing to remember about Gram-negative bacteria is that they are considered double-membraned bacteria, with an outer membrane in addition to the regular cell membrane. The Gram-positive bacteria only have the cell membrane.
We have learned that nearly all bacteria have a cell wall.
The primary function of the cell wall is to maintain the shape and integrity of the cell in the face of high osmotic pressure. The pressure results from the high concentration of dissolved molecules inside the cell relative to the environment.In addition to preventing the cell from rupturing, the cell wall is porous, allowing transport of nutrients and wastes into and out of the cell.The strength and porosity of the cell wall is a result of peptidoglycan. NAG and NAM molecules linked together by peptides form a crosshatch pattern, resembling a chain-link fence.If bacteria have a thick wall of many peptidoglycan layers, the bacteria is Gram-positive. Throughout the cell wall is teichoic acid that facilitates ion transfer.
If bacteria have a thin cell wall of only one or two peptidoglycan layers surrounded by a second, outer membrane, it is Gram-negative. These double-membraned bacteria have porins and lipopolysaccharides in their outer membranes.
When you have finished this lesson, you should be able to:
- Recognize the purpose of cell walls and bacteria
- Describe why cell walls are porous
- Compare and contrast why bacterial walls may be Gram-positive or Gram-negative