Cell Transport Mechanisms: Passive vs. Active Movement

Passive Transport Mechanisms

Diffusion: Movement Down the Concentration Gradient

Diffusion is the passive movement of particles from a region of high concentration to a region of low concentration, aiming to equalize solute concentration. Diffusion occurs naturally because more particles move from the region of higher concentration toward the region of lower concentration.

Simple Diffusion

Simple diffusion enables substances to passively cross cell membranes from a region of higher concentration to a region of lower concentration without using metabolic energy (ATP). Once the concentration of the substance is equal on both sides of the cell membrane, a state of equilibrium is reached.

Facilitated Diffusion

Large, charged, hydrophilic, and polar substances are unable to pass directly through the phospholipid bilayer.

Facilitated diffusion is the passive movement of these substances down their concentration gradient with the assistance of a membrane-bound transport protein that "escorts" materials across cell membranes.

This process is mediated by two distinct types of transport proteins:

• Carrier proteins: Generally result in a slower rate of transport.
• Channel proteins: Generally result in a faster rate of transport.

Osmosis

Osmosis is the passive movement of water molecules across a semi-permeable membrane from a region of low solute concentration to a region of high solute concentration, until equilibrium is reached. Attractions between solute particles and water molecules drive water movement toward regions with a higher solute concentration.

Osmotic Effects on Cells

• Hypotonic Solution: The concentration of solute is lower outside than inside the cell. This results in a net water movement into the cell.
• Hypertonic Solution: The concentration of solute is higher outside than inside the cell. This results in a net water movement out of the cell to attain equilibrium, causing the cell to shrink.

Active Transport Mechanisms

Active transport is the movement of substances across cell membranes against a concentration gradient (from a region of low concentration to a region of high concentration) using metabolic energy derived from ATP.

Carrier-Mediated Active Transport

Carrier-mediated active transport requires both energy (ATP) and the assistance of a specific transport protein, often called a pump. Since each pump transports only particular substances, cells maintain strict control over what is absorbed and what is expelled.

Vesicular Active Transport

Vesicular active transport is an energy-requiring mechanism used to move large molecules or bulk quantities of substances into or out of cells via enclosed vesicles.

Types of Vesicular Transport

• Endocytosis: Moves substances from the extracellular environment to the intracellular environment. A portion of the cell membrane surrounds an extracellular particle, enclosing it in a vesicle.
• Exocytosis: The process by which large substances (or bulk amounts of small substances) exit the cell. They are packaged in vesicles that fuse with the plasma membrane, expelling their contents into the extracellular environment.