**Question:** True about active transport are all except

A. It is passive transport
B. It requires energy
C. It transports molecules against their concentration gradient
D. It is limited to the basolateral membrane of polarized cells

**Core Concept:**
Active transport is a cellular process in which molecules are transported across biological membranes against their concentration gradient. This process requires energy in the form of ATP and can be divided into two types: primary active transport and secondary active transport. Primary active transport directly uses ATP as energy source, while secondary active transport relies on the energy from another process, such as ion gradients.

**Why the Correct Answer is Right:**
Active transport is a crucial mechanism allowing cells to maintain specific concentrations of molecules within their compartments. A passive transport process, like diffusion, would not require energy input and follow concentration gradients. Option A is incorrect because active transport is not passive.

**Why Each Wrong Option is Incorrect:**
Option B is incorrect because ATP is used to drive active transport, not hinder it. While some processes like primary active transport may directly utilize ATP, secondary active transport relies on energy from other sources.

Option C is incorrect because active transport can occur without necessarily transporting molecules against their concentration gradient. While it is true that molecules are transported against gradients in some cases, this is not always the case. For example, secondary active transport relies on energy from another process like ion gradients.

Option D is incorrect because active transport can occur on both the apical and basolateral membranes of polarized cells. Polarized cells have specialized membranes, but active transport can still occur on both sides.

**Clinical Pearl:**
Understanding active transport is essential for understanding cellular processes and transport mechanisms in medicine. It is crucial in various physiological and pathological conditions, including ion transport in the kidney, drug transport in the gastrointestinal tract, and glucose uptake in muscle cells.

**Correct Answer:** A. It is passive transport

Active transport is distinct from passive transport, which relies on concentration gradients to drive molecule movement across a membrane. Active transport, on the other hand, is an active process that requires energy input, usually in the form of ATP. This distinction is essential for understanding cellular processes and transport mechanisms in medicine, particularly in physiological and pathological conditions.