Lipids and Membranes

A set of vocabulary flashcards covering lipid structures, membrane composition, selective permeability, and transport mechanisms based on the lecture transcript.

Lipids

A heterogeneous group of highly hydrophobic molecules composed mostly of nonpolar hydrocarbon ($\text{C-H}$) bonds that do not form polymers, but assemble into large non-covalent structures in water.

Steroids

A class of lipids characterized by a four-ring carbon structure, including cholesterol and long-distance signaling hormones such as testosterone, estrogen, cortisol, and aldosterone.

Fats

Lipid molecules composed of glycerol bonded to three fatty acids via ester linkages, whose primary biological function is energy storage.

Esterification

A dehydration reaction that forms an ester bond between a hydroxyl group of glycerol and a carboxyl group of a free fatty acid.

Deesterification

The reverse reaction of esterification, involving the hydrolysis of an ester bond to separate a fatty acid from glycerol.

Saturated Fatty Acid

A fatty acid chain containing single carbon-carbon bonds only and the maximum possible number of hydrogen atoms, producing a straight structure with a higher melting temperature.

Unsaturated Fatty Acid

A fatty acid chain containing one or more $cis$-double bonds, which produce kinks in the chain and decrease the melting temperature.

Phospholipids

Amphipathic membrane lipids consisting of a hydrophilic head (glycerol, phosphate, and a polar or charged molecule) attached to a hydrophobic tail of two hydrocarbon chains.

Amphipathic

Describing a molecule or protein that possesses both hydrophilic (polar/charged) and hydrophobic (nonpolar) properties.

Micelles

Spherical assemblies formed by single-chained lipids like free fatty acids in aqueous solution, where hydrophilic heads face outward toward water and hydrophobic tails face inward.

Lipid Bilayer

A two-layered structure formed by phospholipids in solution, where hydrophilic heads interact with water and hydrophobic tails interact with each other in the interior.

Membrane Polarity

The structural asymmetry of natural membranes caused by restricted phospholipid diffusion between leaflets, resulting in distinct lipid compositions in inner and outer layers.

Selective Permeability

The property of lipid bilayers that allows small nonpolar molecules to cross rapidly while restricting large uncharged polar molecules and small ions.

Diffusion

The non-directional movement of solute molecules down their concentration gradient (from high to low concentration) leading to equilibrium.

Osmosis

The diffusion of solvent (water) molecules across a semi-permeable membrane toward a higher solute concentration to equalize solute concentration on both sides.

Sandwich Model

An early, disproved model of membrane structure proposing that a phospholipid bilayer was sandwiched between two continuous outer layers of membrane proteins.

Fluid-Mosaic Model

The accepted structural model depicting biological membranes as a fluid phospholipid bilayer studded with a dynamic mosaic of embedded amphipathic proteins.

Freeze-Fracture Preparation

An experimental preparation method that strikes frozen cells with a knife to split the lipid bilayer, allowing visualization of integral membrane proteins by electron microscopy.

Electrochemical Gradient

The combined gradient of concentration difference and electrical charge difference across a membrane that determines the net driving force for ion diffusion.

Channels

Transmembrane protein pores that facilitate passive diffusion by opening simultaneously to both sides of the membrane for selective passage of specific ions or molecules.

Aquaporins

Specialized channel proteins ("water-pores") that selectively permit water molecules to cross the plasma membrane rapidly.

Carrier Proteins

Transmembrane transport proteins that undergo conformational changes to selectively transport solutes across a membrane by opening to only one side at a time.

GLUT-1

A specific glucose carrier protein that undergoes conformational changes to facilitate the passive diffusion of glucose across cell membranes.

Primary Active Transport

Membrane transport that pumps molecules against their electrochemical gradient using direct energy input, such as ATP hydrolysis.

Sodium-Potassium Pump ($\text{Na}^+/\text{K}^+\text{-ATPase}$)

A primary active transport protein that uses ATP to export $3\,\text{Na}^+$ and import $2\,\text{K}^+$, generating an electrochemical gradient and negative membrane potential.

Secondary Active Transport

Cotransport mechanism that uses the energy stored in an ion gradient (established by primary active transport) to move another molecule against its gradient.

Sodium-Glucose Cotransporter (SGLT)

A secondary active transporter that uses the energy of $\text{Na}^+$ diffusing down its electrochemical gradient to actively transport glucose into cells.