Prokaryotic cell hallmarks — Which structure is expected in a typical bacterium?

Introduction:
Although bacteria lack many eukaryotic organelles, they are fully functional cells. This question checks your ability to separate universal cellular features from those specific to eukaryotes.

Given Data / Assumptions:

• Organelles listed include nucleus, Golgi, ER, mitochondrion (eukaryotic hallmarks) and plasma membrane (universal).
• Target organism: bacterium (prokaryote).

Concept / Approach:
All cells are bounded by a plasma membrane composed primarily of phospholipids and proteins. Prokaryotes do not possess membrane-bound organelles such as a nucleus, ER, Golgi, or mitochondria; their DNA resides in a nucleoid region.

Step-by-Step Solution:

Verification / Alternative check:
Protoplast formation experiments and transport assays in bacteria directly demonstrate an intact selective plasma membrane controlling solute flux and energy coupling (e.g., proton motive force).

Why Other Options Are Wrong:

• Nucleus: absent in prokaryotes.
• Golgi apparatus: eukaryotic trafficking organelle; absent in bacteria.
• Endoplasmic reticulum: eukaryotic membrane network; absent in bacteria.
• Mitochondrion: absent in bacteria; aerobic respiration occurs at the plasma membrane.

Common Pitfalls:
Assuming that complex functions require the same organelles across all life; bacteria achieve respiration and secretion with specialized membrane systems without organelles like mitochondria or Golgi.

Final Answer:
Plasma membrane.