Cell Structure: Question 4

Worked solution

Part (a): Identifying the eukaryotic and prokaryotic cell

The clearest distinguishing features are:

• Presence of a nucleus. The eukaryotic Chlorella cell has a nucleus, in which its DNA is enclosed by a nuclear envelope. The prokaryotic Bacillus subtilis cell has no nucleus. Its DNA lies free in the cytoplasm, in a region called the nucleoid.
• Presence of membrane-bound organelles. The eukaryotic cell contains membrane-bound organelles, such as mitochondria, an endoplasmic reticulum and a Golgi body. The prokaryotic cell has none of these; it has no internal membrane systems dividing its cytoplasm into compartments.

(The form of the DNA is a further valid difference: linear DNA associated with histone proteins in the eukaryotic cell, versus a single circular DNA molecule with no histones in the prokaryotic cell.)

Part (b): Why a light microscope cannot show the ribosome size difference

Ribosomes are extremely small structures, only around 20-30 nanometres across. The resolution limit of a light microscope is roughly 0.2 μm, which is 200 nanometres, far larger than an individual ribosome. Since resolution is the smallest distance between two points that can still be seen as separate, structures smaller than this limit cannot be seen as distinct objects at all, no matter how much the image is enlarged.

Enlarging a light microscope image beyond its resolution limit therefore only produces a bigger, blurred image; it does not reveal genuine new detail. To see individual ribosomes, and to compare their sizes, an electron microscope is needed, because its electron beams have a much shorter wavelength than light and so give a far better resolution, fine enough to resolve structures just a few nanometres apart.

Part (c): Bacterial cell wall vs plant cell wall

• Chemical composition. The wall of Bacillus subtilis is made of murein (peptidoglycan), a polymer built from sugar chains cross-linked by short chains of amino acids. A plant cell wall is made of cellulose, a polysaccharide made only of glucose units, with no amino acids involved.
• Molecular architecture. The peptidoglycan wall is a single, continuous cross-linked mesh that surrounds the whole bacterial cell, with the peptide bridges joining adjacent sugar chains together. The plant cell wall, in contrast, is built from separate, parallel cellulose microfibrils embedded within a matrix of other polysaccharides. The microfibrils are not joined to one another by peptide cross-links.

Final answers

• (a) Presence/absence of a nucleus (DNA enclosed by a nuclear envelope vs free in the cytoplasm), and presence/absence of membrane-bound organelles.
• (b) Ribosomes (20-30 nm) are far smaller than the ~0.2 μm resolution limit of a light microscope, so they cannot be resolved by light at any magnification; only an electron microscope’s much shorter wavelength gives sufficient resolution.
• (c) The bacterial wall is made of murein/peptidoglycan (sugar chains cross-linked by peptide bridges), not cellulose (separate microfibrils in a matrix) like a plant cell wall, different chemical building blocks and a different molecular architecture.