A scientist observes some cells under a compound microscope and needs to determine if they are bacteria or yeast. Aside from size, what chemical compounds (not organelles!) would positively identify the cell as either a bacterium or a yeast?

To identify whether a cell is a bacterium or yeast, a scientist can look for the presence of certain chemical compounds.

1. Peptidoglycan: This is a compound found in the cell walls of bacteria but not in yeast. It is a polymer consisting of sugars and amino acids that forms a mesh-like layer outside the plasma membrane of most bacteria, providing them with structural support and protection. If peptidoglycan is detected, the cell is likely a bacterium.

2. Chitin: This is a compound found in the cell walls of yeast but not in bacteria. It is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is usually linked to proteins. If chitin is detected, the cell is likely a yeast.

3. Lipopolysaccharides (LPS): These are large molecules consisting of a lipid and a polysaccharide joined by a covalent bond; they are found in the outer membrane of Gram-negative bacteria, but not in yeast. If LPS is detected, the cell is likely a bacterium.

4. Mannans: These are polysaccharides that make up a significant portion of the yeast cell wall. They are not found in bacteria. If mannans are detected, the cell is likely a yeast.

5. Ergosterol: This is a sterol found in the cell membranes of fungi and protozoa, including yeast, but not in bacteria. If ergosterol is detected, the cell is likely a yeast.

6. Hopanoids: These are pentacyclic compounds that are structurally similar to sterols and are found in bacteria but not in yeast. If hopanoids are detected, the cell is likely a bacterium.

By testing for these compounds, a scientist can determine whether a cell is a bacterium or a yeast.