Problem 1: Antibiotic Selectivity
Problem: A patient has a bacterial infection and is given streptomycin. The doctor is concerned about side effects on the patient's tissues. At what point would streptomycin begin to affect eukaryotic cells, and why?
Solution:
- Step 1: Streptomycin targets the 30S subunit of 70S prokaryotic ribosomes.
- Step 2: Human cytoplasmic ribosomes are 80S (40S + 60S) → not affected by streptomycin.
- Step 3: However, mitochondria in human cells contain 70S ribosomes (same as prokaryotes).
- Step 4: At very high doses, streptomycin can affect mitochondrial protein synthesis in human cells.
- Step 5: This explains why high-dose aminoglycosides cause ototoxicity and nephrotoxicity — hair cells of the inner ear and kidney tubule cells have high mitochondrial content.
- Answer: Streptomycin is selectively toxic to bacteria at therapeutic doses; at high doses, mitochondrial (70S) ribosomes in human cells can be affected.
Problem 2: Identifying Cell Type
Problem: An electron micrograph shows a cell with: double membrane with cristae, stacked thylakoid membranes, circular DNA, and 70S ribosomes. Is this a mitochondrion, a chloroplast, or a prokaryotic cell?
Solution:
- Feature 1: Double membrane with cristae → suggests mitochondria
- Feature 2: Stacked thylakoid membranes (grana) → characteristic of chloroplasts
- Feature 3: Circular DNA + 70S ribosomes → present in both organelles AND in prokaryotes
- Resolution: A cell cannot be both mitochondria and chloroplast. This question describes two different organelles within a plant cell (which has both). If it's a single organelle with both cristae AND thylakoids — this is a trick question; such an organelle does not exist. In reality, the plant cell contains both organelles separately.
Problem 3: Plasmolysis Calculation (Conceptual)
Problem: A plant cell placed in 1M NaCl solution loses turgor and becomes plasmolyzed. What happens if it is then placed back in distilled water?
Solution:
- Step 1: In 1M NaCl: hypertonic solution → water exits cell by osmosis → vacuole shrinks → plasma membrane separates from cell wall (plasmolysis).
- Step 2: Cell wall remains intact (rigid; does not shrink).
- Step 3: Returned to distilled water: hypotonic solution relative to cell sap → water enters cell by osmosis.
- Step 4: Vacuole expands → pushes plasma membrane back against cell wall → cell regains turgor (deplasmolysis).
- Key point: Plasmolysis is reversible if damage is not severe. This demonstrates the physical separation of the plasma membrane from the cell wall.