**Question:** Most permeable to pure phospholipid bilayer:-

A. Water
B. Sodium ions
C. Hydrochloric acid (HCl)
D. Lactose

**Core Concept:** A pure phospholipid bilayer is a simple model of a cell membrane, made up of phospholipids, which are amphipathic molecules. They form a semi-permeable barrier due to their hydrophobic nature, allowing only small, non-polar molecules to pass through.

**Why the Correct Answer is Right:** A phospholipid bilayer is most permeable to small, non-polar molecules due to its hydrophobic nature. Lactose (a large, polar molecule) is unable to pass through the bilayer. In contrast, water, sodium ions, and hydrochloric acid (HCl) are small, non-polar molecules that can easily diffuse through the hydrophobic core of the phospholipid bilayer.

**Why Each Wrong Option is Incorrect:**

A. **Water:** While water is smaller than lactose, it is polar and cannot pass through the hydrophobic core of the phospholipid bilayer. Water molecules can only diffuse across the membrane through specific transporters or channels.

B. **Sodium ions (Na+):** Sodium ions are charged particles and cannot passively diffuse through the phospholipid bilayer due to their size and charge. They require active transport mechanisms, such as Na+/K+-ATPase, to maintain their concentration gradient across the membrane.

C. **Hydrochloric acid (HCl):** Similar to water, HCl is polar and cannot pass through the hydrophobic core of the phospholipid bilayer. It requires specific transporters or channels to cross the membrane.

**Clinical Pearl:** Understanding the permeability of the phospholipid bilayer is crucial for understanding the role of membrane transport proteins and active transport mechanisms in maintaining cellular homeostasis.

**Correct Answer:** D. Lactose is large, polar, and cannot passively diffuse through the phospholipid bilayer due to its size and polarity. Small, non-polar molecules like D-glucose (glucose is the molecule that makes up lactose) can easily pass through the membrane without the need for transporters or channels.

**Why Each Wrong Option is Incorrect:**

A. **D-glucose:** Unlike lactose, glucose is small, non-polar, and can easily diffuse through the hydrophobic core of the phospholipid bilayer. However, this passage of glucose across the membrane does not require specific transporters or channels.

B. **D-galactose:** Similar to lactose, D-galactose is a sugar and cannot pass through the phospholipid bilayer without transporters or channels.

C. **Large, polar molecules:** Larger, polar molecules cannot pass through the hydrophobic core of the phospholipid bilayer due to their size and polarity. These molecules require specific transporters or channels for transport across the membrane.

D. **Small, non-polar molecules:** Small, non-polar