Peptidoglycan: Definition, Function & Structure

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  • 0:00 What Is Peptidoglycan?
  • 0:52 What Does…
  • 1:24 The Bacterial Cell Wall
  • 2:18 Gram Staining
  • 2:39 Antibacterial Agents
  • 3:47 Lesson Summary
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Lesson Transcript
Instructor: Catherine Paul

Catherine has taught high school science and has a master's degree in biology.

In this lesson, discover the role that the peptidoglycan layer of the bacterial cell plays in protecting bacteria. Learn what comprises the peptidoglycan layer and how the cell wall differs between gram-negative and gram-positive bacteria.

What Is Peptidoglycan?

Envision a cold rainy day. You're sitting in your home, gazing out the window. But despite the bad weather, you're warm and dry thanks to the roof over your head. Bacteria have something very similar to a 'roof over their head' called a cell wall. The cell wall surrounds the entire bacteria, holding the cell together and offering protection. It also maintains osmotic pressure, meaning it lets in just the right amount of water and ions that the cell needs.

Similar to the roof on our home, the cell wall is rigid to help secure the shape of the bacteria. The cell wall contains a layer of peptidoglycan, a molecule naturally found only in bacteria. The peptidoglycan layer acts as the cell wall's backbone, offering strength to the cell wall. The peptidoglycan layer is able to allow sugars, amino acids, and other ions into the cell as needed.

What Does Peptidoglycan Look Like?

Peptidoglycan is made of chains of alternating molecules called N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM). When these two molecules are covalently bonded together, it is called a glycan chain. Like the shingles on our roof, there can be many layers of glycan chains in the peptidoglycan layer. Gram-positive bacteria can have upwards of 30 sheets of glycan chains. These glycan chains are held together by branches of four amino acids called the tetrapeptide chain.

The Bacterial Cell Wall

The most easily identifiable feature of the gram-positive bacteria is its thick peptidoglycan layer. As mentioned previously, this layer may be up to 30 sheets of glycan chains thick. Just beneath the peptidoglycan layer, like the frame beneath our roof, is the plasma membrane. This is a lipid bilayer, or two fatty sheets, which functions to hold the cell cytoplasm together. The space between the outer membrane and the plasma membrane is filled with a gel-like fluid called periplasm.

The gram-negative bacterial cell wall is more intricate than the gram-positive cell wall. On the outermost surface of the cell, lies the outer membrane. The outer membrane is a lipid bilayer, or two fatty sheets. Just inside the outer membrane is the peptidoglycan layer. But unlike gram-positive bacteria, the gram-negative peptidoglycan layer is only one or two sheets thick. Finally, further into the cell wall is the plasma membrane.

Gram Staining

Gram staining is a common test used to determine whether a bacterium is of the gram-positive or gram-negative category. It is due to the difference in the cell wall composition in the gram-negative and gram-positive bacteria that they display different colors when gram stained. As a result, gram-positive bacteria stain purple while gram-negative bacteria stain reddish-pink.

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