Defining Active and Passive Transport

Active and passive transport processes are two ways molecules and other materials move in and out of cells and across intracellular membranes. Active transport is the movement of molecules or ions against a concentration gradient (from an area of lower to higher concentration), which does not ordinarily occur, so enzymes and energy are required.

Passive transport is the movement of molecules or ions from an area of higher to lower concentration. There are multiple forms of passive transport: simple diffusion, facilitated diffusion, filtration, and osmosis. Passive transport occurs because of the entropy of the system, so additional energy isn't required for it to occur.

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Differences Between Osmosis and Diffusion

By Anne Marie Helmenstine, Ph.D.

• Both active and passive transport move materials and can cross biological membranes.

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• Active transport moves materials from lower to a higher concentration, while passive transport moves materials from higher to lower concentration.
• Active transport requires energy to proceed, while passive transport does not require the input of extra energy to occur.

Active Transport

Solutes move from a region of low concentration to high concentration. In a biological system, a membrane is crossed using enzymes and energy (ATP).

Passive Transport

• Simple Diffusion: Solutes move from a region of higher concentration to lower concentration.
• Facilitated Diffusion: Solutes move across a membrane from higher to lower concentration with the aid of transmembrane proteins.
• Filtration: Solute and solvent molecules and ions cross a membrane because of hydrostatic pressure. Molecules small enough to pass through the filter may pass.
• Osmosis: Solvent molecules move from lower to higher solute concentration across a semipermeable membrane. Note this makes the solute molecules more dilute.
• Note: Simple diffusion and osmosis are similar, except in simple diffusion, it is the solute particles that move. In osmosis, the solvent (usually water) moves across a membrane to dilute the solute particles.