Question

Given below are two statements, one is labelled as Assertion A and the other is labelled as Reason R.
Assertion A: Outer membrane in Gram negative bacteria is more permeable than the inner membrane.
Reason R: Porin proteins are present in the outer membrane.

In light of the above statements, choose the correct answer from the options given below:

• A. Both A and R are true and R is the correct explanation of A
• B. Both A and R are true but R is NOT the correct explanation of A
• C. A is true but R is false
• D. A is false but R is true

Hint:

- Outer Membrane = Contains Porins = More permeable (allows small hydrophilic solutes).
- Inner Membrane = Lacks Porins = Less permeable (highly selective phospholipid bilayer).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Gram-negative bacteria possess a complex cell envelope consisting of an inner cytoplasmic membrane, a thin peptidoglycan layer, and an asymmetric outer membrane.
These layers function together as a selective physical barrier against environmental stressors and harmful substances.
Detailed Explanation:
Let us analyze both statements:
Assertion A:
The outer membrane of Gram-negative bacteria is more permeable than the inner membrane.
The inner (cytoplasmic) membrane is a highly selective phospholipid bilayer that requires active transport systems or specific permeases for almost all solutes.
In contrast, the outer membrane allows passive diffusion of small, water-soluble molecules.
Thus, Assertion A is true.
Reason R:
This higher permeability of the outer membrane is due to the presence of transmembrane channel proteins called porins.
Porins form open, water-filled channels across the outer membrane.
These channels allow the passive, non-specific diffusion of small hydrophilic nutrients, ions, sugars, and amino acids (typically molecular weight $<$ 600 Daltons) into the periplasmic space.
Thus, Reason R is true and explains the physical mechanism behind the outer membrane's high permeability.

Step 2: Final Answer:
Both A and R are true, and R is the correct explanation of A.
Therefore, the correct option is (A).