Question

Transport of molecules smaller than about 600 daltons across the outer membrane of bacteria is facilitated by trimeric integral membrane proteins called:

• A. Siderophores
• B. ABC transporters
• C. Porins
• D. Hopanoids

Hint:

Remember:
• Porins \(\rightarrow\) Channel proteins in gram-negative outer membrane
• ABC transporters \(\rightarrow\) ATP-driven active transport
• Siderophores \(\rightarrow\) Iron acquisition molecules \[ \text{Porins enable passive diffusion of small hydrophilic molecules} \]

Answer 1

The Correct Option is C

Concept: Gram-negative bacteria possess an outer membrane that acts as a protective barrier. Small hydrophilic molecules generally cannot diffuse freely through the lipid bilayer. Transport of molecules smaller than approximately: \[ 600 \text{ daltons} \] occurs through special channel-forming proteins called: \[ \text{Porins} \] Porins are trimeric integral membrane proteins that form water-filled channels allowing passive diffusion of small molecules.

Step 1: Understanding the role of porins. Porins:
• Are located in the outer membrane of gram-negative bacteria
• Form aqueous channels
• Permit diffusion of small solutes such as sugars, ions, and amino acids These proteins are typically arranged as trimers. Therefore: \[ \boxed{\text{Porins facilitate transport of small molecules across bacterial outer membrane}} \]

Step 2: Eliminating the incorrect options.
• Option (A): Siderophores are iron-chelating compounds used for iron uptake.
• Option (B): ABC transporters are ATP-dependent transport proteins mainly associated with active transport across plasma membranes.
• Option (D): Hopanoids are membrane lipids that help stabilize bacterial membranes. Hence, the correct answer is Porins. \[ \boxed{\text{Porins} \Rightarrow \text{Channels for molecules } < 600 \text{ daltons}} \]