Lipids, Membranes, and Cellular Transport

Vocabulary flashcards covering lipid classes, membrane structure, permeability, fluidity, and membrane transport proteins.

Plasma membrane

A selectively permeable cellular boundary that separates life from nonlife by regulating entry and exit of substances and sequestering chemicals necessary for life reactions.

Lipids

A diverse class of carbon-containing organic compounds that are insoluble in water due to having a high proportion of nonpolar $\text{C-C}$ and $\text{C-H}$ bonds.

Hydrocarbons

Nonpolar, hydrophobic organic molecules consisting exclusively of carbon and hydrogen atoms.

Isoprenoids

Branched hydrocarbon chains constructed by linking five-carbon isoprene monomer subunits together.

Fatty acid

A lipid building block consisting of an unbranched hydrocarbon chain joined at one end to a carboxyl functional group ($\text{-COOH}$).

Saturated lipid

A lipid in which all carbon atoms in the hydrocarbon tail are linked by single bonds, allowing straight chains to pack tightly and remain solid at room temperature.

Unsaturated lipid

A lipid possessing one or more carbon-carbon double bonds ($\text{C=C}$) that introduce kinks into the hydrocarbon chain, preventing tight packing and making it liquid at room temperature.

Hydrogenation

A chemical process that artificially converts liquid, unsaturated lipids into solid, saturated fats by breaking double bonds and adding hydrogen atoms across the hydrocarbon chain.

Steroids

A family of lipids characterized by a bulky structural core of four interconnected carbon rings, distinguished by their attached functional groups.

Fats (Triacylglycerols)

Non-polymeric lipids consisting of three fatty acids linked to a glycerol backbone, functioning primarily to store roughly twice as much energy as carbohydrates.

Ester linkage

The covalent bond formed via a dehydration reaction between the hydroxyl group ($\text{-OH}$) of glycerol and the carboxyl group ($\text{-COOH}$) of a fatty acid.

Phospholipids

Amphipathic lipids consisting of a glycerol molecule linked to a charged phosphate group (with a polar head group) and two hydrophobic hydrocarbon tails; the primary structural component of biological membranes.

Archaeal phospholipids

Membrane phospholipids distinctive to domain Archaea that feature branched isoprenoid tails rather than the fatty acid tails found in Bacteria and Eukarya.

Amphipathic

Having both hydrophilic (polar or charged) and hydrophobic (nonpolar) regions within the same molecule.

Micelles

Tiny spherical aggregates formed spontaneously by single-tailed amphipathic lipids in water, where hydrophilic heads face the surrounding water and hydrophobic tails cluster inward.

Lipid bilayer

A two-molecule-thick sheet formed by phospholipids where hydrophobic hydrocarbon tails interact in the interior and hydrophilic heads face aqueous solutions on both outer surfaces.

Planar bilayer

An experimental artificial lipid bilayer spanning a hole between two water compartments, used to measure solute permeability across membranes.

Selective permeability

The property of a lipid bilayer that permits certain solutes to cross more easily than others; influenced by hydrocarbon tail length, saturation state, and cholesterol presence.

Membrane fluidity

The physical dynamic of a lipid bilayer where phospholipids are in constant lateral motion within each leaflet but rarely flip from one side of the bilayer to the other.

Fluid-mosaic model

The accepted model of membrane structure stating that integral membrane proteins span across and move dynamically within a fluid phospholipid bilayer.

Freeze-fracture electron microscopy

A specialized technique where frozen cells are cleaved with a knife along the hydrophobic interior of the lipid bilayer, producing pits and mounds that demonstrate proteins span the membrane.

Integral membrane proteins

Transmembrane proteins that span the lipid bilayer, having segments exposed to both the interior and exterior environments of the cell.

Peripheral membrane proteins

Membrane proteins that bind to the exterior or interior surfaces of the lipid bilayer or to integral proteins without inserting into the hydrophobic interior.

Channel proteins

Selective transmembrane proteins with hydrophilic interiors that form pores across biological membranes, permitting specific ions or small molecules to pass by passive diffusion.

Aquaporins

Channel proteins lined with key amino acid residues that facilitate the rapid, selective passage of water molecules across biological membranes while blocking ions.

Electrochemical gradient

The combined effect of a concentration gradient and an electrical charge gradient across a membrane that drives the directional movement of ions.

Gated channels

Pore-forming channel proteins that open or close in response to specific regulatory signals, such as changes in the electrical charge across the membrane.

Carrier proteins (e.g., GLUT-1)

Membrane transport proteins that selectively bind solute molecules and undergo reversible conformational changes to facilitate their diffusion across the bilayer.

Sodium-potassium pump ($\text{Na}^+/\text{K}^+\text{-ATPase}$)

An active transport transmembrane protein that hydrolyzes ATP to export three $\text{Na}^+$ ions and import two $\text{K}^+$ ions against their electrochemical gradients.