Glossary of Key Terms
Glossary of Key Terms
| Term | Definition |
| α-Actinin | A protein in nonerythroid cells that cross-links actin filaments to form a meshwork. |
| Active Transport | An energy-requiring process that transports a molecule against an electrochemical gradient via carrier proteins. |
| Amphipathic | A term describing molecules, such as phospholipids, that consist of both a polar (hydrophilic) part and a nonpolar (hydrophobic) part. |
| Ankyrin | A protein that links band 3 proteins to spectrin tetramers in red blood cells, attaching the cytoskeleton to the membrane. |
| Annexins | A group of anionic, calcium-dependent, lipid-binding peripheral proteins that function in membrane trafficking and ion channel formation. |
| Antiport | A transport process that moves two different molecules across a membrane in opposite directions. |
| Aquaporins | Channels designed for the rapid transport of water across the cell membrane. |
| ATP-binding cassette transporters (ABC-transporters) | Transmembrane proteins with intracellular nucleotide-binding domains and membrane-spanning domains that export materials from the cytoplasm using ATP. |
| Autocrine hormones | Signaling molecules released into the intercellular space that act on the releasing cell itself. |
| Band 3 proteins | Integrins located in the plasmalemma of red blood cells. |
| Band 4.1 protein | A protein that binds to and stabilizes spectrin–actin complexes in red blood cells. |
| Carrier proteins | Multipass transmembrane proteins that undergo conformational changes to transport specific molecules; they function in both passive and active transport. |
| Catalytic receptors | Single-pass transmembrane proteins whose extracellular moiety is a receptor and whose cytoplasmic component is a protein kinase. |
| Channel-linked receptors | Receptors that bind a signaling molecule, causing a gate to temporarily open or close, thereby permitting or inhibiting ion movement. |
| Cholesterol | A lipid component present in both leaflets of the plasma membrane that helps maintain structural integrity and decreases membrane fluidity. |
| Cystinuria | A hereditary condition caused by abnormal carrier proteins unable to remove cystine from the urine, leading to kidney stones. |
| E-face | The internal surface of the outer leaflet, exposed during freeze-fracturing. |
| Endocrine hormones | Signaling molecules carried in the bloodstream that act on distant target cells. |
| Facilitated Diffusion | A type of passive transport that occurs via specific ion channels and/or carrier proteins. |
| Fluid Mosaic Model | The model describing the plasma membrane as a lipid bilayer in which proteins are embedded or associated. |
| G protein-linked receptors | Transmembrane proteins that interact with a G protein after binding a signaling molecule to activate intracellular second messengers. |
| Gated ion channels | Channels that open only transiently in response to stimuli such as voltage changes, mechanical force, or ligand binding. |
| Glycocalyx (cell coat) | A layer on the outer surface of the plasmalemma consisting of oligosaccharide chains linked to proteins and lipids, along with proteoglycans. |
| Glycolipids | Lipids with attached polar carbohydrate residues, restricted to the outer leaflet of the plasma membrane, that form part of the glycocalyx. |
| Hereditary spherocytosis | A disease resulting from defective spectrin, characterized by fragile, misshapen red blood cells (spherocytes). |
| Hydrophilic | “Water-loving”; refers to polar molecules or parts of molecules that interact with water. |
| Hydrophobic | “Water-fearing”; refers to nonpolar molecules or parts of molecules that do not interact with water. |
| Integral proteins | Proteins that are dissolved in the lipid bilayer, some of which span the entire membrane. |
| Integrins | Proteins that associate the plasmalemma and cytoskeleton by binding to extracellular matrix components and intracellular cytoskeletal components. |
| Ion channels | Multipass transmembrane proteins that form small aqueous pores for specific ions and small molecules to pass through. |
| Ionophores | Lipid-miscible molecules that complex with ions to transport them across the membrane, either by enclosing the ion or by forming a channel. |
| K⁺ leak channels | The most common ion channels; they are ungated and primarily responsible for establishing the potential difference across the plasmalemma. |
| Ligand-gated channels | A type of gated channel that opens in response to the binding of a signaling molecule or ion. |
| Lipid bilayer | The fundamental structure of the plasma membrane, composed of two leaflets of phospholipids, glycolipids, and cholesterol. |
| Lipid rafts | Microdomains formed by cholesterol and phospholipids that can affect the movement of integral proteins. |
| Multidrug-resistant (MDR) proteins | ABC-transporters in some cancer cells that can transport cytotoxic drugs out of the cell. |
| Multipass proteins | Transmembrane proteins that are folded to pass back and forth across the plasmalemma multiple times. |
| Na⁺–K⁺ pump | An active transport system mediated by Na⁺–K⁺ ATPase that pumps three Na⁺ ions out of the cell and two K⁺ ions into the cell for each ATP hydrolyzed. |
| P-face | The protoplasmic face, or the outer surface of the inner leaflet, exposed during freeze-fracturing, to which integral proteins preferentially attach. |
| Paracrine hormones | Signaling molecules released into the intercellular space that act on nearby cells. |
| Passive Transport | Transport processes, including simple and facilitated diffusion, that do not require energy because molecules move down a concentration gradient. |
| Peripheral proteins | Proteins located on the cytoplasmic aspect of the inner leaflet that bind to integral proteins or phospholipid polar groups via noncovalent interactions. |
| Phospholipids | Amphipathic molecules with one polar head and two nonpolar fatty acyl tails that are the primary component of the lipid bilayer. |
| Plasma Membrane (Plasmalemma) | The semipermeable membrane that envelops the cell, maintains its integrity, and consists of a lipid bilayer with associated proteins. It is approximately 7.5 nm thick. |
| Proteoglycans | Glycosaminoglycans bound to integral proteins, found within the glycocalyx. |
| Second messengers | Intracellular signaling molecules, such as cAMP and Ca²⁺, that are activated in response to an extracellular signal. |
| Simple Diffusion | A type of passive transport for small nonpolar molecules and small, uncharged polar molecules across the plasma membrane. |
| Spectrin | A long, flexible protein in red blood cells that forms tetramers, providing a scaffold for structural reinforcement of the cytoskeleton. |
| Symport | A transport process that moves two different molecules across a membrane in the same direction. |
| Synapsin I | A peripheral protein that binds synaptic vesicles to the cytoskeleton. |
| Transmembrane proteins | Integral proteins that span the entire thickness of the plasma membrane. |
| Trilaminar (unit membrane) | The three-layered appearance of the plasma membrane when viewed with a transmission electron microscope. |
| Uniport | A transport process that moves a single molecule across a membrane. |
| Vinculin | A protein in nonerythroid cells that binds to α-actinin and talin, helping to link the cytoskeleton to the plasma membrane. |
| Voltage-gated channels | A type of gated channel that opens when the potential difference across the membrane changes. |
| Xenobiotics | Macromolecules, such as antibiotics, that are not manufactured by the cells of an organism. |