In cell envelope anatomy, porin proteins are found in which bacterial structure?

Introduction / Context:
Porins are trimeric channel proteins that facilitate diffusion of small hydrophilic molecules. Their correct localization is critical to understanding antibiotic entry, nutrient uptake, and intrinsic resistance in Gram-negative organisms.

Given Data / Assumptions:

• Gram-negative bacteria possess an outer membrane external to a thin peptidoglycan layer.
• Gram-positive bacteria lack an outer membrane and instead have a thick peptidoglycan wall with teichoic acids.
• Porins create water-filled channels across the outer membrane.

Concept / Approach:
Porins reside in the outer membrane of Gram-negative bacteria. They regulate entry of nutrients and certain antibiotics (e.g., beta-lactams, fluoroquinolones) into the periplasmic space and ultimately to the cytoplasmic membrane transporters or targets.

Step-by-Step Solution:

Verification / Alternative check:
Biochemical isolation of outer membranes yields abundant porins (e.g., OmpF, OmpC in Escherichia coli), confirming localization.

Why Other Options Are Wrong:
Peptidoglycan of Gram-positives: Contains no porins; there is no outer membrane. Cytoplasmic membrane: Contains transporters and channels but not porins as defined in Gram-negative outer membranes. Periplasmic space: A compartment, not a membrane; porins span a membrane.

Common Pitfalls:
Assuming any membrane channel is a porin; porins specifically refer to outer membrane channels in Gram-negatives.

Final Answer:
the outer membrane of gram-negative bacteria