**Core Concept**
The transport system in cell membranes refers to the mechanisms by which molecules move across the lipid bilayer. This includes selective transport via channels and carriers, energy-dependent processes, and the regulation of permeability through specific gate mechanisms. Receptor activation is not a core structural or functional component of the transport system itself.

**Why the Correct Answer is Right**
Receptor activation typically occurs in signal transduction pathways, where extracellular ligands bind to membrane receptors to initiate intracellular responses. It is not part of the actual transport mechanism across the membrane. In contrast, a selective gate (like ion channels or transporters), a non-selective channel (allowing passive diffusion of ions), and an energy coupling system (e.g., ATP-driven pumps like Na⁺/K⁺-ATPase) are all integral parts of membrane transport systems. Receptor activation governs signaling, not direct transmembrane transport.

**Why Each Wrong Option is Incorrect**
Option B: A selective gate (e.g., voltage-gated or ligand-gated ion channels) is a key component of transport systems, allowing controlled passage of specific ions.
Option C: Non-selective channels (e.g., leak channels) permit passive diffusion of ions without specificity, playing a vital role in maintaining membrane potential.
Option D: Energy coupling systems (e.g., ATP-driven transporters) provide the energy required for active transport, such as in the Na⁺/K⁺-ATPase or proton pumps.

**Clinical Pearl / High-Yield Fact**
Membrane transport systems rely on physical gates and energy inputs, not receptor signaling. Always distinguish between **signal transduction** (receptor activation) and **membrane transport** (channels, carriers, pumps).

✓ Correct Answer: A. Receptor activation