AP Biology Unit 2: Transport Mechanics & Osmoregulation

Selective Permeability

The plasma membrane's property that allows it to maintain internal conditions different from the external environment.

Fluid Mosaic Model

A model describing the structure of the plasma membrane as a mix of lipids and proteins.

Passive Transport

Movement of molecules from high concentration to low concentration that does not require metabolic energy.

Active Transport

Movement of molecules from low concentration to high concentration that requires metabolic energy.

Diffusion

The spontaneous movement of particles from an area of higher concentration to an area of lower concentration.

Dynamic Equilibrium

A state when molecules cross the membrane in both directions at equal rates, resulting in no net change in concentration.

Facilitated Diffusion

Passive movement of molecules across the membrane via transport proteins.

Channel Proteins

Transport proteins that provide hydrophilic corridors for molecules to pass through the membrane.

Carrier Proteins

Transport proteins that undergo a change in shape to translocate solutes across the membrane.

Osmosis

The diffusion of free water across a selectively permeable membrane.

Tonicity

The ability of a surrounding solution to affect the movement of water into or out of a cell.

Hypotonic Solution

A solution where solute concentration is less than that inside the cell, causing it to lyse.

Isotonic Solution

A solution where solute concentration is equal to that inside the cell, resulting in a normal cell shape.

Hypertonic Solution

A solution with a higher solute concentration than inside the cell, causing the cell to shrivel.

Water Potential

The tendency of water to move from high water potential to low water potential.

Solute Potential ($S$)

A measure of the effect of dissolved solutes on the water potential.

Sodium-Potassium Pump

An active transport mechanism that pumps 3 Na^+ out of the cell and 2 K^+ into the cell.

Cotransport

The coupling of 'downhill' diffusion of one substance to the 'uphill' transport of another substance.

Exocytosis

The process by which a cell secretes biological molecules by the fusion of vesicles with the plasma membrane.

Endocytosis

The process through which cells take in biological molecules by forming new vesicles from the plasma membrane.

Phagocytosis

A type of endocytosis where a cell engulfs a particle in a vacuole.

Pinocytosis

A type of endocytosis where molecules are taken up when extracellular fluid is engulfed into tiny vesicles.

Receptor-Mediated Endocytosis

Endocytosis triggered by the binding of specific solutes to receptors, resulting in vesicle formation.

Ionization Constant ($i$)

The number of particles the solute breaks into; e.g., Sucrose = 1, NaCl = 2.

Pressure Constant ($R$)

A constant used in the water potential equation, valued at 0.0831 liter bars/mole K.

Temperature in Kelvin ($T$)

The temperature used in calculations for water potential, defined as degrees Celsius + 273.