**Question:** A factor favouring the transport across a cell membrane

**Core Concept:** Transporters and facilitators are proteins embedded in the cell membrane that help transport molecules across it. They can be passive or active transporters. Passive transporters are facilitated by factors such as concentration gradients, pH, and ionic strength.

**Why the Correct Answer is Right:** Amino acids are transported into cells via passive transport mechanisms like the Amino Acid Transporters (AATs), which are facilitated by the concentration gradient of the amino acid inside and outside the cell. The correct answer (A) is correct because it refers to the concentration gradient as a factor that favors the transport of amino acids across the cell membrane.

**Why Each Wrong Option is Incorrect:**

A. **Option B (Sodium Ion Gradient):** While sodium ions play a role in active transport, the correct answer (A) is specific to passive transport facilitated by the concentration gradient.

C. **Option C (Electrolyte Gradient):** Similar to option B, electrochemical gradients are relevant for active transport, not passive transport facilitated by concentration gradients.

D. **Option D (Proton Gradient):** Like option C, proton gradients are associated with active transport mechanisms, not passive transport facilitated by concentration gradients.

**Clinical Pearl / High-Yield Fact:** The cell membrane has selective permeability, meaning substances can pass through it due to their properties like size, charge, and hydrophobicity. This selective permeability ensures the cell maintains its internal environment and prevents unwanted substances from entering the cell. Understanding passive transport mechanisms and their facilitating factors is crucial for understanding cell biology, pharmacology, and drug absorption, distribution, and elimination.

**Correct Answer:** A (Concentration Gradient)