Cell Biology: Cell Membrane, Cytoskeleton, Organelles, and Endomembrane System

Vocabulary practice flashcards covering cell membrane dynamics, cytoskeletal structures, cytoplasmic inclusions, mitochondria, peroxisomes, and the endomembrane system based on the provided biology lecture notes.

Fluid Mosaic Model

A structural model proposed by Singer and Nicholson describing the plasma membrane as a dynamic lipid bilayer where embedded proteins move laterally in a mosaic-like arrangement.

Flip-Flop Movement

The transverse diffusion process in which a lipid molecule flips from one monolayer (leaflet) of the membrane bilayer to the opposite monolayer.

Lipid Rafts

Denser microdomains of the cell membrane enriched in cholesterol and sphingolipids that move laterally and participate in cell signaling, cell movement, and endocytosis.

Glycocalyx

A sugar-rich coat on the outer surface of the plasma membrane composed of glycoproteins, glycolipids, and proteoglycans that functions in cell protection, recognition, and adhesion.

Coupled Transport Modes

Transporter mechanisms classified by cargo direction: uniport (single molecule), symport (co-transport of two molecules in the same direction), and antiport (transport of two molecules in opposite directions).

$\text{Na}^+/\text{K}^+$ Pump

An ATPase-mediated antiport pump that uses $1\,\text{ATP}$ to pump $3\,\text{Na}^+$ ions out of the cell and $2\,\text{K}^+$ ions into the cell, maintaining cell volume and membrane potential.

Caveolae

Bulb-shaped invaginations ($50\,\text{nm}$) of the plasma membrane formed by caveolin dimers that represent a specialized form of lipid raft involved in micropinocytosis.

Anterograde Transport

Vesicular movement of proteins and lipids forward from the rough endoplasmic reticulum to the Golgi apparatus and between Golgi cisternae via COP-II coated vesicles.

Transcytosis

The intracellular transport of molecules from one extracellular space to another through sequential endocytosis and exocytosis, such as the transport of IgA dimers across epithelial cells.

Hyaloplasm

The watery matrix or cytosol of the cytoplasm containing water-soluble salts, metabolites, enzymes, and signaling molecules.

Microtubule Structure

A hollow cylindrical cytoskeletal filament with a diameter of $20\text{--}25\,\text{nm}$ composed of $13$ parallel protofilaments formed by repeating $\alpha/\beta$-tubulin heterodimers.

Kinesin and Dynein Motor Proteins

Microtubule motor proteins that use ATP-derived energy to transport cargoes along microtubules: kinesin moves toward the plus end, whereas dynein moves toward the minus end.

Axoneme

The structural cytoskeletal core of motile cilia and flagella consisting of $9$ peripheral microtubule doublets surrounding $2$ central microtubule singlets ($9+2$ pattern).

Intermediate Filaments

Non-polar, stable cytoskeletal fibers ($8\text{--}12\,\text{nm}$ in diameter) composed of fibrous proteins (such as keratins, vimentins, and lamins) that provide mechanical strength to cells.

G-Actin

The globular monomeric subunit of actin that polymerizes into a double-helical polymer known as F-actin to form flexible microfilaments ($6\text{--}8\,\text{nm}$ in diameter).

Lipofuscin

Granular, electron-dense brown cytoplasmic inclusions composed of undigested lipid residues and cell debris that accumulate with age in quiescent cells like neurons and heart muscle.

Cardiolipin

A unique phospholipid containing four fatty acid tails located in the inner mitochondrial membrane that lowers ion permeability and supports oxidative phosphorylation.

Electron Transport Chain

A series of four enzyme complexes (I--IV) located in the inner mitochondrial membrane that transfer electrons and pump protons ($\text{H}^+$) into the intermembrane space to generate a proton gradient.

Peroxisome

A single-membrane organelle containing catalase to break down hydrogen peroxide ($\text{H}_2\text{O}_2$) and enzymes responsible for the $\beta$-oxidation of very long-chain fatty acids.

KDEL Signal Sequence

A specific four-amino-acid sequence (Lys-Asp-Glu-Leu) located at the carboxyl terminus of proteins that retains them as resident proteins in the lumen of the endoplasmic reticulum.

ERAD System

Endoplasmic Reticulum Associated Degradation; a quality control mechanism where chaperones detect misfolded proteins in the RER lumen and export them to the cytosol for proteasomal degradation.

Dictyosome

A structural stack of $4\text{--}6$ flattened membranous cisternae that forms the core unit of the Golgi apparatus.

Acid Hydrolases

A group of hydrolytic enzymes (including proteases, nucleases, and lipases) housed within lysosomes that degrade macromolecules optimally at an acidic internal $\text{pH} < 5.0$.

Proteasome

A barrel-shaped non-membranous protein complex composed of a $19\text{S}$ regulatory particle and a $20\text{S}$ core particle that degrades misfolded or tagged proteins.

Lys48 Polyubiquitination

A specific polyubiquitination linkage pattern on target proteins that serves as the explicit signal for recognition and degradation by the proteasome.

Autophagy

A lysosome-dependent catabolic pathway where double-membrane vesicles (autophagosomes) sequester damaged cellular components or organelles and fuse with lysosomes for degradation and recycling.