Membrane Structure and Selective Permeability (AP Biology Unit 2)

Plasma membrane

Thin, flexible boundary separating a cell’s interior from the external environment; controls what enters/exits, enables communication, and helps maintain homeostasis.

Fluid mosaic model

Model describing membranes as a dynamic (fluid) bilayer with a diverse mix (mosaic) of lipids, proteins, and some attached carbohydrates; components can move laterally but not all move freely or equally.

Phospholipid bilayer

Two-layer arrangement of phospholipids with hydrophilic heads facing aqueous environments and hydrophobic tails facing inward; forms the membrane’s basic barrier.

Amphipathic

Having both hydrophilic (polar) and hydrophobic (nonpolar) regions, as in phospholipids that self-assemble into bilayers in water.

Integral protein

Membrane protein embedded in the lipid bilayer (often with hydrophobic regions interacting with the bilayer’s interior); many function in transport or signaling.

Peripheral protein

Membrane protein attached to the membrane surface (often to integral proteins) but not embedded in the hydrophobic core.

Glycoprotein

Protein with attached carbohydrate chain(s), typically projecting on the extracellular side; important for cell recognition and communication.

Glycolipid

Lipid with attached carbohydrate chain(s), typically on the extracellular side; contributes to cell recognition and membrane identity.

Glycocalyx

“Sugar coat” of carbohydrates on the extracellular surface of the membrane (from glycoproteins/glycolipids) used in cell-cell recognition and communication.

Cholesterol

Lipid in animal cell membranes that sits among phospholipid tails and helps regulate membrane fluidity and stability across temperature changes.

Membrane fluidity

How easily membrane components (especially lipids and some proteins) move within the bilayer; influenced by temperature, cholesterol, and fatty-acid saturation.

Selective permeability

Property of membranes that allows some substances to cross more easily than others; depends on the bilayer’s nonpolar interior and specific transport proteins.

Simple diffusion

Passive net movement of molecules from higher to lower concentration directly through the membrane (common for small nonpolar molecules like O₂ and CO₂).

Concentration gradient

Difference in concentration across space (or a membrane) that drives net diffusion from high concentration to low concentration.

Dynamic equilibrium

State where concentrations are equalized so net movement is zero, even though molecules continue random motion in both directions.

Osmosis

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

Tonicity

Measure of how a solution affects water movement and cell volume (hypotonic, hypertonic, isotonic).

Hypotonic solution

Solution with lower solute concentration than the cell; water enters the cell, which may swell (and potentially lyse in animals).

Hypertonic solution

Solution with higher solute concentration than the cell; water leaves the cell, causing the cell to shrink.

Isotonic solution

Solution with equal solute concentration to the cell; no net movement of water across the membrane.

Facilitated diffusion

Passive transport down a concentration gradient using membrane proteins; specific and can be regulated; no ATP required.

Channel protein

Transport protein forming a hydrophilic pathway across the membrane (often gated) for ions or water to move down their gradient.

Carrier protein

Transport protein that binds a solute and changes shape to move it across the membrane; can show saturation when all carriers are occupied.

Primary active transport

Movement of substances against their gradient using energy directly (often ATP hydrolysis) to power a pump (e.g., ion pumps).

Secondary active transport

Movement of a substance against its gradient using energy stored in an existing gradient; “downhill” movement of one solute drives “uphill” movement of another (coupled transport).