Membrane Structure and Transport – Vocabulary Review

Flashcards covering key vocabulary for membrane structure, function, and transport mechanisms.

Phospholipid Bilayer

Double layer of phospholipids that forms the foundation of all cell membranes and creates a selectively permeable barrier.

Selective Permeability

Property of membranes that allows some substances to cross more easily than others while blocking the rest.

Fluid Mosaic Model

Concept that a membrane is a dynamic, fluid combination of phospholipids, proteins, and other molecules drifting laterally.

Membrane Fluidity

Extent to which lipid molecules and proteins move laterally within the bilayer; influenced by lipid type and cholesterol content.

Cholesterol

Steroid embedded in animal membranes that buffers fluidity—preventing excess rigidity at low temperatures and excess fluidity at high temperatures.

Integral Membrane Protein

Protein that penetrates the hydrophobic core of the bilayer; most span the entire membrane and are permanently attached.

Transmembrane Protein

Type of integral protein that extends completely across the lipid bilayer from one side to the other.

Peripheral Membrane Protein

Protein loosely bound to the membrane surface or to exposed parts of integral proteins; can detach and reattach.

Semipermeable Membrane

Alternative term for selectively permeable membrane, highlighting its ability to let certain molecules pass while restricting others.

Concentration Gradient

Difference in solute concentration across a space or membrane; substances tend to move from high to low concentrations.

Passive Transport

Movement of substances across a membrane without energy input, down their concentration gradient.

Simple Diffusion

Passive movement of small, non-polar molecules (e.g., O₂, CO₂, lipids) directly through the phospholipid bilayer.

Facilitated Diffusion

Passive transport in which polar molecules or ions cross the membrane with the aid of channel or carrier proteins.

Channel Protein

Transmembrane protein forming a hydrophilic pore that allows specific ions or water to pass; may be gated.

Aquaporin

Specialized water channel protein that dramatically increases membrane permeability to H₂O.

Carrier Protein

Membrane protein that binds a specific solute, undergoes a shape change, and transports the solute across the membrane.

Active Transport

Energy-requiring movement of substances across a membrane against their concentration gradient, often using ATP.

Proton Pump

Active transport protein that moves H⁺ ions out of a cell or organelle, creating an electrochemical gradient.

Sodium-Potassium Pump (Na⁺/K⁺ Pump)

Active transport protein that exchanges three Na⁺ out for two K⁺ into the cell, maintaining vital ion gradients.

Osmosis

Passive diffusion of water across a selectively permeable membrane from low solute concentration to high solute concentration.

Tonicity

Ability of an external solution to cause a cell to gain or lose water.

Isotonic Solution

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

Hypertonic Solution

External solution with higher solute concentration than the cell; water leaves the cell, causing it to shrink.

Hypotonic Solution

External solution with lower solute concentration than the cell; water enters the cell, causing it to swell.

Osmoregulation

Process by which organisms control internal water and solute balance in environments of varying tonicity.

Endocytosis

Bulk import of substances into the cell by forming vesicles from the plasma membrane.

Phagocytosis

“Cellular eating” form of endocytosis in which large particles or cells are engulfed into food vacuoles.

Pinocytosis

“Cellular drinking” form of endocytosis in which the cell ingests extracellular fluid and dissolved solutes.

Receptor-Mediated Endocytosis

Selective form of endocytosis where specific molecules bind to membrane receptors and are internalized in coated vesicles.

Exocytosis

Bulk export of materials from the cell as vesicles fuse with the plasma membrane and release their contents.

Integral Protein Functions

Roles including transport, enzymatic activity, signal reception, cell-to-cell recognition, intercellular joining, and cytoskeleton attachment.

Fluidity Adaptation

Adjustments in membrane lipid composition (e.g., more unsaturated fats in cold environments) that maintain optimal fluidity.

Facilitated Diffusion Specificity

Characteristic whereby each channel or carrier protein permits passage of only a particular ion or molecule.

Semipermeable vs. Selectively Permeable

Terms often used interchangeably; both describe membranes that allow some substances to cross while restricting others.