**Question:** Out of the following steps the one which is passive and does not depend on ATP distribution is:

A. Active transport
B. Passive transport
C. Active diffusion
D. Facilitated diffusion

**Correct Answer:** B. Passive transport

**Core Concept:**

Passive transport is a process of transporting substances across biological membranes without requiring an external source of energy. This process relies on the concentration gradient of the substance present on either side of the membrane, allowing substances to move from an area of high concentration to an area of low concentration. There are two types of passive transport: simple diffusion and facilitated diffusion.

**Why the Correct Answer is Right:**

Passive transport is right because it does not depend on ATP distribution, which is a characteristic of active transport. In passive transport, substances move according to their concentration gradient, unlike active transport, which requires the hydrolysis of ATP for energy.

**Why Each Wrong Option is Incorrect:**

**A. Active transport:** This option is incorrect because active transport requires ATP for energy, which is the primary distinction between active and passive transport.

**B. Active diffusion:** This option is incorrect because, while it is a type of transport, active diffusion specifically refers to a form of active transport that requires energy from ATP.

**C. Active diffusion:** This option is incorrect as it is a type of active transport and not a passive transport mechanism.

**D. Facilitated diffusion:** This option is incorrect because facilitated diffusion is a type of passive transport that occurs through carrier proteins that transport substrates in a down-gradient. Although it does not require ATP, it is still a form of passive transport, making it incorrect in comparison to passive transport.

**Clinical Pearl:**

Understanding passive transport is crucial for medical professionals, as it helps explain the movement of substances across cell membranes without requiring energy input, such as in the process of osmosis, which is essential for maintaining fluid balance within and between body fluids. Additionally, passive transport mechanisms like facilitated diffusion are vital for drug distribution and medication administration.