Cell Biology Test 1 Terms

Alpha helix

most common structure in plasma membrane. secondary structure of proteins where the polypeptide backbone coils into a right‑handed spiral, stabilized by hydrogen bonds between the carbonyl oxygen of residue i and the amide hydrogen of residue i+4

Amphipathic

molecule that contains both a hydrophilic (water‑loving) region and a hydrophobic (water‑fearing) region, such as phospholipids in a membrane

Beta barrel

cylindrical protein fold formed by β‑strands that hydrogen‑bond to each other, creating a pore‑like structure often found in outer‑membrane protein

Cell cortex

thin network of actin filaments and associated proteins just beneath the plasma membrane that helps maintain cell shape and mechanical stability

Cholesterol

sterol that inserts into lipid bilayers, modulating membrane fluidity by filling gaps between phospholipid tails at high temperatures and preventing tight packing at low temperatures.

Fatty acid

hydrocarbon chain ending with a carboxyl group; saturated fatty acids have no double bonds, while unsaturated ones contain one or more double bonds

Flip-flop

rare, spontaneous movement of a lipid molecule from one leaflet of a bilayer to the opposite leaflet

Flippase

ATP‑dependent enzymes that selectively transport specific lipids from the outer leaflet to the inner leaflet, establishing membrane asymmetry

Glycocalyx

dense layer of carbohydrate‑rich molecules (glycoproteins and glycolipids) coating the exterior of the plasma membrane, involved in protection and cell‑cell recognition

Glycolipid

Lipids with carbohydrate head groups attached; they reside in the outer leaflet of the membrane and contribute to cell‑surface identity

Glycoprotein

protein bearing covalently attached carbohydrate chains, often functioning as receptors or adhesion molecules on the cell surface

Hydrophilic

“Water‑loving”; describes molecules or regions that readily interact with aqueous environments via polar or charged groups

Hydrophobic

“Water‑fearing”; describes molecules or regions that avoid water, typically non‑polar and lipid‑soluble

Inositol phospholipid

phospholipid whose head group contains an inositol ring, such as phosphatidylinositol 4,5‑bisphosphate (PIP₂), crucial for signaling pathways

Integral membrane protein

protein that spans the lipid bilayer (single‑pass or multi‑pass) and is embedded within the membrane, often containing hydrophobic transmembrane helices

Lectin

carbohydrate‑binding protein that can recognize specific sugar motifs on glycoproteins or glycolipids, frequently involved in cell adhesion and immune responses

Peripheral protein

protein loosely attached to one side of the membrane, typically via interactions with lipids or integral membrane proteins, without spanning the bilayer

Phospholipids

Amphipathic lipids composed of a glycerol backbone, two fatty‑acid tails, and a phosphate‑containing head group; they form the basic bilayer structure of membranes

Porin

β‑barrel protein that creates a water‑filled channel in the outer membrane of Gram‑negative bacteria, allowing passive diffusion of small molecules

Proteoglycan

core protein heavily decorated with long carbohydrate chains (glycosaminoglycans), forming a major component of the extracellular matrix and cell surface

Saturated

Refers to fatty acids or phospholipids whose hydrocarbon chains contain no double bonds, leading to tighter packing and reduced fluidity

Scramblase

enzyme that catalyzes the bidirectional, energy‑independent randomization of lipids between the two leaflets of a membrane, disrupting asymmetry

Spectrin

cytoskeletal protein that forms a flexible, lattice‑like network beneath the plasma membrane, providing mechanical support and maintaining cell shape

Tight junction

seal between neighboring epithelial cells formed by claudins and occludins, preventing free passage of molecules through the intercellular space

Unsaturated

Describes fatty acids or phospholipids that contain one or more cis double bonds, introducing kinks that increase membrane fluidity

Active transport

the movement of ions or molecules across the membrane against their electrochemical gradient, uses ATP

Antiport

type of active transport where two different substances move in opposite directions (one in, one out) simultaneously, each driven by the gradient of the other

Aquaporin

specialized membrane channel that allows water molecules to pass rapidly while excluding ions and solutes, facilitating osmotic balance

Calcium pump

ATP‑dependent pump (often a SERCA or PMCA) that expels Ca²⁺ from the cytosol to keep intracellular calcium concentrations low

Channel

a membrane protein that forms a pore allowing passive diffusion of specific ions or molecules down their electrochemical gradient

Electrochemical gradient

the combined effect of concentration differences and electrical charge across the membrane that drives the movement of charged particles