Cell Biology: Cellular Architecture, Membranes, and Transport

A comprehensive vocabulary review set covering lipid classification, membrane structure and permeability, transport mechanisms, organelle systems, endomembrane trafficking, and cytoskeletal components from Chapters 6 and 7.

Hydrocarbon

A nonpolar, hydrophobic molecule containing only carbon and hydrogen atoms, in which electrons are shared equally in carbon-carbon and carbon-hydrogen bonds.

Isoprenoid

A branched hydrocarbon chain formed from linked isoprene subunits that functions as pigments, scents, vitamins, and sex hormone precursors, serving as a building block for more complex lipids.

Fatty Acid

A lipid composed of an unbranched hydrocarbon chain containing $14\text{ to }20$ carbon atoms bonded to a polar carboxyl functional group ($-\text{COOH}$).

Saturated Fatty Acid

A hydrocarbon chain that contains only single covalent bonds between carbon atoms, holding the maximum possible number of hydrogen atoms and packing tightly into a solid at room temperature.

Unsaturated Fatty Acid

A hydrocarbon chain containing one or more carbon-carbon double bonds, which produce a kink in the chain that increases fluidity and keeps the lipid liquid at room temperature.

Steroid

A family of lipids characterized by a bulky, four-ring hydrocarbon skeleton with varying functional groups attached to the rings, exemplified by cholesterol, estrogen, and testosterone.

Fat (Triacylglycerol)

A non-polymeric lipid composed of three fatty acids linked to a glycerol molecule via ester linkages, functioning primarily for cellular energy storage.

Ester Linkage

The covalent bond formed through a dehydration reaction joining the hydroxyl group of glycerol to the carboxyl group of a fatty acid.

Phospholipid

An amphipathic lipid consisting of a glycerol molecule linked to a negatively charged phosphate group and two hydrocarbon tails (fatty acids in Bacteria and Eukarya; branched isoprenoids in Archaea).

Amphipathic

The property of a substance containing both a hydrophilic (polar or charged) region and a hydrophobic (nonpolar) region.

Micelle

A tiny spherical lipid aggregate formed spontaneously in water by free fatty acids, in which hydrophilic heads face outward toward water and hydrophobic tails face inward.

Liposome

An artificial, spherical, membrane-bound vesicle created in the laboratory by an enclosed phospholipid bilayer surrounding an aqueous interior.

Planar Bilayer

An artificial lipid bilayer constructed across an opening in a partition separating two aqueous chambers, used experimentally to measure membrane permeability.

Selective Permeability

The property of a phospholipid bilayer allowing small, nonpolar molecules ($\text{O}_2$, $\text{CO}_2$, $\text{N}_2$) to pass rapidly while resisting small uncharged polar molecules ($\text{H}_2\text{O}$, glycerol), large uncharged polar molecules (glucose, sucrose), and blocking small ions ($\text{Cl}^-$, $\text{K}^+$, $\text{Na}^+$).

Diffusion

The spontaneous net movement of solutes down their concentration gradient from high-concentration regions to low-concentration regions, driven by thermal energy and increasing entropy.

Osmosis

The special case of passive diffusion involving the net movement of water across a selectively permeable membrane from a region of low solute concentration to high solute concentration.

Hypertonic Solution

An external solution with a higher solute concentration relative to the cell interior, causing net water loss from the cell resulting in cell shrinkage, animal cell crenation, or plant cell plasmolysis.

Hypotonic Solution

An external solution with a lower solute concentration relative to the cell interior, driving net water entry into the cell and resulting in swelling, potential animal cell lysis, or plant cell turgor.

Protocell

A simple vesicle-like structure harboring nucleic acids, hypothesized to have served as an intermediate in the chemical evolution of the first living cells.

Fluid-Mosaic Model

The accepted model of biological membranes proposing that integral proteins span a dynamic, fluid bilayer of phospholipids rather than forming sandwich-like outer coats.

Freeze-Fracture Electron Microscopy

A preparation method that strikes a frozen cell with a knife to split the lipid bilayer along its hydrophobic interior, exposing mounds and pits that represent membrane-spanning proteins under a scanning electron microscope.

Integral Membrane Protein

A transmembrane amphipathic protein that spans the lipid bilayer and possesses peptide segments exposed to both the interior and exterior environments of the cell.

Peripheral Membrane Protein

A protein that binds to membrane lipids or integral proteins at either the interior or exterior surface without inserting into the hydrophobic interior of the bilayer.

Detergent

A small, water-soluble, amphipathic molecule that forms micelles in water and is used experimentally to disrupt phospholipid bilayers and isolate membrane proteins.

Electrochemical Gradient

The combined concentration gradient and electrical (charge) gradient that determines the direction and rate of passive ion movement across a membrane.

Aquaporin

A specialized channel protein with key pore-lining residues that selectively facilitates the rapid passive transport of water across the plasma membrane while blocking ions.

Gated Channel

A membrane channel protein that opens or closes in response to a specific signal, such as the binding of a particular regulatory molecule or a change in electrical voltage.

CFTR

Cystic fibrosis transmembrane conductance regulator; an integral membrane protein that functions as a chloride ion ($\text{Cl}^-$) channel, defects in which produce thick secretions in cystic fibrosis.

GLUT-1

A transmembrane glucose carrier protein that binds glucose from outside the cell and undergoes a conformational change to release it into the cell interior.

Active Transport

The energy-requiring process that moves solutes against their electrochemical or concentration gradients via transmembrane transport proteins known as pumps.

Sodium–Potassium Pump

An active transport protein ($\text{Na}^+/\text{K}^+\text{-ATPase}$) that hydrolyzes ATP to export three $\text{Na}^+$ ions and import two $\text{K}^+$ ions against their concentration gradients.

Secondary Active Transport

A cotransport mechanism where the movement of a substance against its gradient is driven by an electrochemical gradient previously established by an ATP-consuming primary pump.

Nucleoid

The localized region in a prokaryotic cell where the single, circular chromosome of DNA is bundled and organized with structural proteins.

Plasmid

A small, circular, supercoiled DNA molecule in prokaryotes that carries non-essential genes and exists independently of the main chromosome.

Peptidoglycan

A tough, fibrous polysaccharide polymer that forms the primary structural component of the bacterial cell wall.

Fimbriae

Needlelike surface appendages found on prokaryotic cells that promote attachment to other cells or surfaces.

Nuclear Lamina

A lattice-like sheet of intermediate filament proteins lining the inner nuclear membrane that provides mechanical support, shapes the nucleus, and anchors chromosomes.

Nucleolus

A dense, distinct subregion of the eukaryotic nucleus responsible for the synthesis of ribosomal RNA (rRNA) and the assembly of ribosomal subunits.

Rough Endoplasmic Reticulum (RER)

A network of membrane-bound sacs studded with ribosomes that synthesizes, processes, and folds proteins destined for membranes, other organelles, or secretion.

Smooth Endoplasmic Reticulum (SER)

A network of membrane sacs lacking ribosomes that contains enzymes for lipid synthesis and breakdown, detoxifies poisonous compounds, and acts as a reservoir for $\text{Ca}^{2+}$ ions.

Golgi Apparatus

An endomembrane organelle composed of stacked, flat membranous sacs called cisternae with distinct polarity (cis face receiving products from the RER and trans face shipping them).

Acid Hydrolases

A collection of approximately $40$ hydrolytic digestive enzymes located in animal lysosomes that function optimally at an acidic pH of approximately $5.0$.

Endomembrane System

An integrated internal membrane system comprising the rough ER, smooth ER, Golgi apparatus, and lysosomes that synthesizes, processes, packages, and sorts cargo.

Peroxisome

A globular eukaryotic organelle that serves as a center for reduction–oxidation reactions and uses the enzyme catalase to detoxify hydrogen peroxide.

Cristae

Sac-like invaginations of the inner mitochondrial membrane that expand surface area for ATP production.

Thylakoid

Flattened, sac-like internal membranes within chloroplasts that are stacked into grana and convert light energy to chemical energy.

Endosymbiosis Theory

The evolutionary theory stating that mitochondria and chloroplasts originated when free-living prokaryotes were engulfed by ancestral eukaryotic cells and developed mutualistic relationships.

Nuclear Localization Signal (NLS)

A specific $17$-amino-acid-long sequence on cytosolic proteins that acts as a molecular zip code directing entry into the nucleus through nuclear pore complexes.

Pulse–Chase Experiment

An experimental technique tracking molecule movement over time by exposing cells to a brief period of labeled amino acids (pulse) followed by excess unlabeled amino acids (chase).

Signal Recognition Particle (SRP)

A cytosolic ribonucleoprotein complex that binds the $20$-amino-acid ER signal sequence on a nascent polypeptide, halting translation until bound to the ER membrane SRP receptor.

Translocon

A membrane channel protein in the rough ER membrane through which a nascent polypeptide enters the ER lumen during protein synthesis.

Autophagy

A lysosomal degradation pathway ('same-eating') in which a damaged organelle is wrapped by an internal membrane to form an autophagosome that fuses with a lysosome.

Phagocytosis

A form of endocytosis ('cell-eating') where the plasma membrane envelops an extracellular cell or food particle to form a phagosome, which delivers its cargo to a lysosome for digestion.

Receptor-Mediated Endocytosis

An endocytic process where extracellular ligands bind to cell-surface receptors, triggering vesicle budding to form an early endosome, which acidifies and matures into a late endosome and lysosome.

Actin Filaments (Microfilaments)

Cytoskeletal elements composed of two coiled strands of polymerized actin subunits with plus and minus ends, functioning to resist pull, move cells, divide animal cells, and drive cytoplasmic streaming.

Myosin

A motor protein that binds to actin filaments and hydrolyzes ATP to induce mechanical sliding, powering cytokinesis, cytoplasmic streaming, and muscle contraction.

Centrosome

The primary microtubule-organizing center (MTOC) in animal cells, consisting of a protein matrix and two orthogonally oriented centrioles composed of microtubule triplets.

Microtubule

A hollow cylinder measuring approximately $25\,\text{nm}$ in diameter formed from polarized polymerization of $\alpha$- and $\beta$-tubulin dimers, serving as structural support and tracks for vesicle movement.

Kinesin

A motor protein with a head region that binds microtubules and a tail that binds transport vesicles, converting ATP chemical energy into mechanical walking steps toward the plus end.

Axoneme

The characteristic '$9 + 2$' structural core of eukaryotic cilia and flagella, consisting of nine outer doublet microtubules surrounding two central singlet microtubules connected by dynein arms.

Plasmodesmata

Microscopic, membrane-lined cytoplasmic channels that perforate the cell walls of adjacent plant cells, allowing direct intercellular transport and communication.

Tight Junction

A specialized cell-cell junction in animal epithelial sheets where membrane proteins bind tightly to one another, creating a barrier that prevents leakage of extracellular fluid between cells.

Desmosome

An anchoring cell-cell junction in animal epithelial tissue that fastens adjacent cells together via intermediate filaments to form mechanically resilient sheets.

Gap Junction

A communicating intercellular junction in animal cells formed by channel-forming proteins, providing direct cytoplasmic passageways for ions, water, and small signaling molecules.