Which is the best definition of active transport?

Options

A. Movement of molecules from an area of their higher concentration to an area of their lower concentration
B. Movement of water across a semi-permeable membrane from an area of high water concentration to an area of lower water concentration
C. Movement of molecules from an area of their lower concentration to an area of their higher concentration
D. Movement of a substance against its concentration through the release of energy from ATP

Explanation

### Step-by-Step Solution:

1. **Understanding Active Transport**: Active transport is a biological process that moves substances across a cell membrane against their concentration gradient. This means that molecules move from an area of lower concentration to an area of higher concentration.

2. **Energy Requirement**: Unlike passive transport processes such as diffusion and osmosis, which do not require energy, active transport requires energy to occur. This energy is typically derived from ATP (adenosine triphosphate).

3. **Concentration Gradient**: In active transport, the movement of molecules is against the concentration gradient. This is the opposite of what occurs in diffusion, where molecules move from an area of higher concentration to an area of lower concentration.

4. **Correct Definition**: The best definition of active transport is: "The movement of substances against their concentration gradient, requiring energy in the form of ATP."

5. **Examples of Active Transport**: A common example of active transport is the sodium-potassium pump, which moves sodium ions out of the cell and potassium ions into the cell, both against their respective concentration gradients.