Endocytosis types—pinocytosis Pinocytosis (“cell drinking”) is typically observed in which cells or compartments?

Introduction / Context:
Pinocytosis is a form of endocytosis whereby cells internalize extracellular fluid and solutes via small vesicles. It is essential for nutrient uptake, turnover of membrane, and regulation of surface receptors.

Given Data / Assumptions:

• Endocytosis requires an internal membrane system (endomembrane) and cytoskeletal machinery.
• Eukaryotes possess such systems; bacteria lack classical endocytosis.
• Organelles like mitochondria are not independently performing endocytosis.

Concept / Approach:
All eukaryotic cells can perform pinocytosis to varying extents—some constitutively (for example, endothelial cells), others in response to signals. Prokaryotes do not use classical clathrin- or caveolin-mediated endocytosis due to their cell envelope architecture and absence of an endomembrane system.

Step-by-Step Solution:
Define pinocytosis as endocytic uptake of fluid/solutes via vesicles.Map this process to cells with endomembrane systems: eukaryotes.Exclude prokaryotes and organelles (mitochondria) as sites of endocytic vesicle formation.Therefore, pinocytosis occurs in all eukaryotic cells.

Verification / Alternative check:
Live-cell imaging shows macropinocytosis and clathrin/caveolae-mediated pinocytosis across diverse eukaryotic lineages.

Why Other Options Are Wrong:

• All prokaryotes: Lack canonical endocytosis.
• Mitochondria: Do not bud endocytic vesicles from plasma membrane.
• Only chloroplasts: Organelles do not conduct plasma-membrane endocytosis.

Common Pitfalls:
Confusing secretion/import via transporters with vesicular endocytosis.

Final Answer:
All eukaryotic cells.