1. Channel proteins form open pores, through which suitably sized molecules/ions can cross the membrane.
2. Carrier proteins selectively bind specific molecules, and transport them across the membrane by undergoing a conformation change.
3. Transport of molecules across membranes, through channel proteins and carrier proteins, always involves active transport.
4. Small uncharged molecules can diffuse freely through the phospholipid bilayers.

Choose the correct answer from the options given below:

1. (A) and (B) only.
2. (A), (B) and (C) only.
3. (A), (B) and (D) only.
4. (B), (C) and (D) only.

   Channel proteins and carrier proteins are key to membrane transport, but not all statements about them are accurate. The correct option is (A), (B), and (D) only.

   Option Analysis

   Statement (A)

   Channel proteins form open pores, through which suitably sized molecules/ions can cross the membrane.
   This is correct, as channel proteins create hydrophilic pores that allow passive diffusion of specific ions or small polar molecules based on size and charge, without energy input.

   Statement (B)

   Carrier proteins selectively bind specific molecules, and transport them across the membrane by undergoing a conformation change.
   This is correct; carriers bind substrates on one side, change shape, and release them on the other, enabling facilitated diffusion or active transport.

   Statement (C)

   Transport of molecules across membranes, through channel proteins and carrier proteins, always involves active transport.
   This is incorrect. Both channels and carriers often mediate passive transport (facilitated diffusion down gradients without energy), though some carriers use active transport.

   Statement (D)

   Small uncharged molecules can diffuse freely through the phospholipid bilayers.
   This is correct; nonpolar molecules like O₂ and CO₂ pass directly via simple diffusion through the lipid bilayer, bypassing proteins.

   Membrane transport channel proteins and carrier proteins regulate how molecules cross the cell membrane, crucial for biology exams like GATE Life Sciences. This guide breaks down the mechanisms, verifies statements on open pores and conformation changes, and clarifies passive diffusion for small uncharged molecules.

   Core Mechanisms

   • Channel Proteins: Form hydrophilic pores for passive flow of ions/molecules matching size/charge; no binding or energy needed.

   • Carrier Proteins: Bind specific molecules, flip conformation to shuttle across; supports facilitated diffusion or active transport.

   • Simple Diffusion: Small uncharged/nonpolar molecules (e.g., O₂, CO₂) slip through lipid bilayers freely.

   Common Misconception

   Channel and carrier-mediated transport isn’t always active—most is passive (no ATP), unlike pumps.

   Exam Relevance

   For competitive exams, remember: (A), (B), (D) are true; (C) fails due to passive options. Focus on selectivity and energy use for PYQs.