Foundations of cell structure — Which cellular component is present in both prokaryotic and eukaryotic cells and defines the cell boundary?

Introduction / Context:
Despite profound differences, prokaryotes and eukaryotes share core features required for life. Identifying universal structures clarifies what is fundamental versus specialized in cellular evolution.

Given Data / Assumptions:

• All cells must separate internal chemistry from the environment.
• Some organelles are exclusive to eukaryotes.
• Prokaryotes lack membrane-bound organelles but do have a boundary membrane.

Concept / Approach:
The plasma membrane is a phospholipid bilayer with embedded proteins that controls transport, signaling, and energy transduction. Every cell, whether bacterial, archaeal, or eukaryotic, possesses a plasma membrane. Organelles such as lysosomes, vacuoles, chloroplasts, and Golgi apparatus are eukaryote-specific.

Step-by-Step Solution:

Verification / Alternative check:
Introductory biology resources define cells by having a plasma membrane; this holds across all domains of life.

Why Other Options Are Wrong:

• Lysosome, vacuole, and Golgi are not found in prokaryotes.
• Chloroplasts are specific to plants and some protists.

Common Pitfalls:
Confusing “cell wall” (common in bacteria, plants, fungi) with “plasma membrane” (universal). Cell walls are extracellular and not universal.

Final Answer:
Plasma membrane