02 Cell Transport

Selective Permeability

Only small, neutral molecules (like O2, CO2) and hydrophobic lipids can easily cross the phospholipid bilayer directly. Water crosses slowly on its own but rapidly via aquaporin channels.

Concentration Gradient

The difference in solute concentration between areas. Molecules naturally move down the gradient (from high to low concentration).

Diffusion

Movement of molecules down their concentration gradient until equilibrium is reached. Requires no energy.

Passive Transport

Diffusion across a membrane without energy investment. Includes simple diffusion and facilitated diffusion.

Osmosis

Passive transport (diffusion) of free water across a selectively permeable membrane. Water moves from an area of lower solute concentration (higher free water) to an area of higher solute concentration (lower free water).

Tonicity

Describes how a surrounding solution affects cell volume based on solute concentration.

Isotonic

Solution has the same solute concentration as the cell; no net water movement.

Hypertonic

Solution has higher solute concentration; water leaves the cell, causing it to shrink/shrivel.

Hypotonic

Solution has lower solute concentration; water enters the cell, causing it to swell and potentially burst.

Facilitated Diffusion

Passive transport across a membrane aided by transport proteins (channel or carrier proteins). Used for polar or charged molecules that cannot cross the bilayer directly. Still requires no energy.

Active Transport

Movement of molecules against their concentration gradient. Requires a carrier protein and energy (usually ATP).

Bulk Transport (Large Molecules)

Endocytosis: Moving large molecules into the cell using vesicles.

Exocytosis

Moving large molecules out of the cell using vesicles (e.g., exporting proteins).