Q1.
Which of the cells given in Fig. 2.10 fall under the categories of prokayotic and eukaryotic cells?
Answer
The bacterial cell (Fig. 2.10a) is prokaryotic. The plant cell (Fig. 2.10b) and the animal cell (Fig. 2.10c) are eukaryotic.
pro = primitive, karyon = nucleus → prokaryotic = primitive nucleus
eu = true, karyon = nucleus → eukaryotic = true nucleus
eu = true, karyon = nucleus → eukaryotic = true nucleus
What you can actually see in the figure that settles it:
- Fig. 2.10a shows a nucleoid — genetic material lying naked in the cytoplasm — with ribosomes scattered around and an appendage for locomotion. There is no nuclear membrane and no organelle bounded by its own membrane.
- Fig. 2.10b and 2.10c both show a nucleus with a nucleolus inside a nuclear membrane, together with mitochondria, endoplasmic reticulum, Golgi body, lysosome and vacuole — every one of them membrane-bound.
| Characteristic | Prokaryotic cell | Eukaryotic cell |
|---|---|---|
| Diameter of a typical cell | 1 to 10 µm | 10 to 100 µm |
| Membrane-bound nucleus | Absent | Present |
| Membrane-bound organelles | Absent | Present |
| Number of cells in an organism | Usually unicellular | Can be unicellular or multicellular |
Why it happens: internal membranes are what make a eukaryotic cell possible. They divide the cell into rooms, so incompatible reactions can run side by side at the same time — digestive enzymes stay sealed in lysosomes instead of dissolving the cell, energy release is confined to mitochondria, protein processing to the ER and Golgi. A prokaryote has one open compartment, so all its chemistry must share a single space; that is one reason it stays small. The size figures follow from the same logic: 1–10 µm against 10–100 µm is a tenfold difference in length, which is a thousandfold difference in volume.
Note on Table 2.2: the book's first row reads “Primitive nucleus — Present in prokaryotic, Absent in eukaryotic”. Read it as “primitive nucleus (nucleoid)”, which is indeed present only in prokaryotic cells; the last two rows then correctly say that a membrane-bound nucleus is present only in eukaryotic cells.