**Question:** True regarding transpo across a cell membrane is:

A. Active transport is always associated with the expenditure of energy in the form of ATP.
B. Passive transport only occurs in the absence of a concentration gradient.
C. Osmosis is the movement of water across a semipermeable membrane.
D. Facilitated transport is a type of passive transport.

**Correct Answer:**

**Core Concept:**
Transport of molecules across a cell membrane can be classified into various types based on the energy requirement, direction of movement, and mechanism involved.

**Why the Correct Answer is Right:**
Active transport is indeed associated with the expenditure of energy in the form of ATP (Adenosine Triphosphate), which is used to move molecules against their concentration gradient. Passive transport, on the other hand, does not require energy and involves molecules moving down their concentration gradient. Passive transport can be further categorized into two types:

1. **Osmosis (Option C):** Osmosis is the movement of water across a semipermeable membrane, driven by the difference in water concentration on either side of the membrane. Water molecules move from the area of low water concentration to the area of high water concentration, following the principle of osmosis.
2. **Facilitated transport (Option D):** Facilitated transport is a type of passive transport that occurs when a carrier protein binds to the substrate molecule and moves across the membrane, driven by the concentration gradient. This transport process does not require energy.

**Why Each Wrong Option is Incorrect:**

**Option A (False):** While ATP is used in active transport, it is not always associated with the expenditure of energy. For example, in the sodium-potassium pump, ATP is used to actively transport sodium ions out of the cell, but it does not necessarily require energy (ATP) in all cases of active transport.

**Option B (False):** Passive transport can occur without a concentration gradient, unlike what the question suggests. Examples of passive transport include osmosis and facilitated transport.

**Option C (True):** Osmosis is a specific type of passive transport, driven by the difference in water concentration across a semipermeable membrane. Water moves from an area of low water concentration to an area of high water concentration.

**Option D (True):** Facilitated transport is a type of passive transport that does not require energy. Carrier proteins play a crucial role in facilitating the movement of molecules across the membrane.