**Question:** Movement of substances prevented across intercellular space by-

A. Active Transport
B. Passive Transport
C. Facilitated Diffusion
D. Active Diffusion

**Correct Answer:** A. Active Transport

**Core Concept:**
Active transport is a cellular process that moves substances across cell membranes against their concentration gradient, requiring energy in the form of ATP (Adenosine Triphosphate). This process helps maintain the concentration gradients of essential ions and molecules within cells, ensuring proper cellular function and maintaining homeostasis in the body.

**Why the Correct Answer is Right:**
Active transport involves the action of specific proteins called transporters, which use energy from ATP hydrolysis to move substances from an area of low concentration to an area of high concentration, thus counteracting the natural tendency of substances to move passively from high concentration to low concentration (based on Fick's first law of diffusion). This ensures substances do not disseminate randomly across the cell membrane, as passive transport would require, and thus maintains homeostasis.

**Why Each Wrong Option is Incorrect:**

A. **Passive Transport (B)**: Passive transport relies on the concentration gradient and does not require energy. This process allows substances to move from an area of high concentration to an area of low concentration, without affecting the concentration gradient.

B. **Facilitated Diffusion (C)**: Facilitated diffusion is a passive transport mechanism that does not require energy. It involves the assistance of carrier proteins that help transport molecules across the cell membrane. However, it does not require energy, unlike active transport.

D. **Active Diffusion (D)**: Active diffusion is a term used to describe passive transport, not active transport. It refers to any transport that occurs against the concentration gradient, which is a characteristic of both active and passive transport mechanisms.

**Clinical Pearl:** Understanding the distinction between passive and active transport is crucial for maintaining cellular homeostasis and understanding various physiological processes, such as transport of ions, glucose, and hormones across cell membranes. This distinction is particularly important in understanding how medications and therapeutic interventions affect cellular processes.