AP Biology Unit 2 Notes: How Cells Control Exchange Across Membranes

Selective permeability

Property of a membrane that allows some substances to cross more easily than others based on size, polarity/charge, and the presence of transport proteins.

Phospholipid bilayer

Double layer of phospholipids with hydrophilic heads facing water and hydrophobic tails facing inward; forms a hydrophobic barrier to many polar/charged substances.

Amphipathic

Having both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions, as in phospholipids.

Concentration gradient

Difference in solute concentration across a membrane; drives net movement from higher to lower concentration (down the gradient).

Electrochemical gradient

Combined effect of an ion’s concentration gradient and the electrical gradient (membrane potential) on its movement across a membrane.

Passive transport

Movement across a membrane that requires no cellular ATP input; occurs down a concentration or electrochemical gradient (e.g., diffusion, osmosis, facilitated diffusion).

Active transport

Movement across a membrane that requires energy (directly or indirectly) to move substances against their concentration or electrochemical gradients.

Simple diffusion

Passive movement of small nonpolar molecules directly through the lipid bilayer down their concentration gradient (e.g., O2, CO2).

Osmosis

Diffusion of water across a selectively permeable membrane; water moves toward the side with higher nonpenetrating solute concentration (lower free water availability).

Facilitated diffusion

Passive transport in which membrane proteins help polar molecules or ions move down their concentration or electrochemical gradient without ATP.

Channel protein

Membrane protein that forms a hydrophilic tunnel for specific ions or water; many channels are gated (open/close in response to stimuli).

Aquaporin

Water channel protein that greatly increases a membrane’s permeability to water, enabling rapid osmosis.

Carrier protein

Transport protein that binds a specific solute and changes shape to move it across the membrane; transport is specific and can plateau when carriers are all occupied.

Dynamic equilibrium

State in which molecules continue moving randomly in both directions, but there is no net movement because rates of movement are equal.

Tonicity

Ability of a surrounding solution to cause a cell to gain or lose water by osmosis; depends on the concentration of nonpenetrating solutes.

Isotonic solution

Solution with the same effective concentration of nonpenetrating solutes as the cell; no net water movement and cell volume remains stable.

Hypotonic solution

Solution with lower nonpenetrating solute concentration than the cell; water enters the cell, causing swelling (and possible lysis in animal cells).

Hypertonic solution

Solution with higher nonpenetrating solute concentration than the cell; water leaves the cell, causing shrinking (crenation in some animal cells).

Turgor pressure

Pressure of a plant cell’s contents against the cell wall in a hypotonic environment; helps support the plant and makes cells turgid.

Plasmolysis

In plant cells, loss of water in a hypertonic environment causing the plasma membrane to pull away from the cell wall.

Osmoregulation

Physiological control of internal water balance and solute concentration to maintain conditions necessary for cell function.

Primary active transport

Active transport that uses ATP hydrolysis directly to move solutes against their gradients via pumps (e.g., sodium-potassium pump, proton pump).

Secondary active transport (cotransport)

Transport that uses energy stored in an ion gradient (built by primary active transport) to move another solute against its gradient (symport or antiport).

Endocytosis

Bulk (vesicular) transport into the cell in which the membrane invaginates to form a vesicle; includes phagocytosis, pinocytosis, and receptor-mediated endocytosis.

Exocytosis

Bulk transport out of the cell in which vesicles fuse with the plasma membrane to release contents outside; important for secretion and membrane delivery.