Relative sizes in biology and chemistry: which listed structure is the smallest?

Introduction / Context:
Comparing scales helps connect biology and chemistry. Understanding the approximate sizes of atoms, subcellular structures, acellular agents (like viroids), and cells is valuable for microscopy, filtration, and molecular biology.

Given Data / Assumptions:

• Candidates: viroid (tiny infectious RNA), hydrogen atom, bacterium (prokaryotic cell), mitochondrion (organelle).
• We compare typical diameters/length scales.

Concept / Approach:
An individual atom is far smaller than biological macromolecular assemblies. A hydrogen atom has an approximate diameter near 0.1 nm (1 angstrom). Biological entities, even minimal ones like viroids (small circular RNA), are orders of magnitude larger in physical extent than a single atom.

Step-by-Step Solution:

Verification / Alternative check:
Unit conversion: 1 µm = 1000 nm. Thus even the smallest cellular structures are thousands to tens of thousands of times larger than atoms.

Why Other Options Are Wrong:

• A: Viroids are extremely small biological agents but still vastly larger than an atom.
• C: Bacteria are whole cells, orders of magnitude larger than molecular-scale entities.
• D: Mitochondria are large organelles compared with bacteria’s subcomponents.

Common Pitfalls:
Assuming “viroid” (very small) must be the smallest item overall; forgetting that atomic dimensions are far below biological macromolecules.

Final Answer:
Hydrogen atom