Cell Biology Terms Starting With M
Cell Biology Glossary: M
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Membrane Protein
/ MEM-brayn PROH-teen / · Latin: membrana + Greek: proteios (primary)
Membrane protein is a protein that associates with a biological membrane either by spanning the lipid bilayer one or more times or by attaching to the membrane surface, where it carries out transport, signaling, adhesion, or enzymatic functions.
Membrane proteins are classified as integral proteins that cross the bilayer, typically as alpha-helical bundles or beta-barrel structures, or as peripheral proteins attached to the membrane surface through lipid anchors or noncovalent interactions with other proteins. Integral membrane proteins include channels and transporters such as aquaporins and glucose carriers that facilitate movement of specific solutes, as well as receptors that bind extracellular signals including hormones and growth factors. Approximately 25 to 30 percent of all protein-coding genes in most sequenced genomes encode membrane proteins, reflecting their broad involvement in communication, transport, and cell identity.
Because their hydrophobic transmembrane segments make them difficult to crystallize, membrane proteins remained structurally undercharacterized until Hartmut Michel, Johann Deisenhofer, and Robert Huber resolved the first membrane protein crystal structure, that of a bacterial photosynthetic reaction center, in 1985.
The human genome encodes roughly 800 G protein-coupled receptors, all of which are integral membrane proteins with seven transmembrane helices. These receptors detect stimuli ranging from photons in retinal rod cells to odorant molecules in nasal epithelium, making them the largest single family of drug targets in clinical pharmacology.
Are Enzymes Proteins? →The membrane is mostly lipid and proteins are minor additions. Proteins typically make up about 50 percent of membrane mass by weight, and in energy-transducing membranes such as the inner mitochondrial membrane that fraction can exceed 75 percent.
Phospholipid Bilayer →The Band 3 anion exchanger in human red blood cell membranes spans the lipid bilayer 14 times and exchanges one bicarbonate ion for one chloride ion, cycling approximately 5 × 10^4 times per second per protein copy to transport CO2 from tissues to the lungs. Each red blood cell carries roughly 1.2 million Band 3 copies, making it the most abundant membrane protein in human erythrocytes and collectively exchanging approximately 70 millimoles of bicarbonate per minute across the entire circulating red cell mass. Band 3 also anchors the spectrin-actin cytoskeletal network through ankyrin and protein 4.2, linking membrane transport function to structural integrity.
Respiratory System Fun Facts →Metaphase
/ MET-uh-fayz / · Greek meta, between; phasis, appearance
Metaphase is the stage of mitosis or meiosis in which chromosomes reach maximum condensation and align along the cell's equatorial plane, called the metaphase plate, with each kinetochore attached to spindle fibers from opposite poles.
During metaphase, the spindle assembly checkpoint monitors kinetochore-microtubule attachments and arrests cell division until every chromosome has achieved bipolar attachment, with kinetochores connected to spindle fibers from opposite poles. Alignment at the metaphase plate is not a resting state but a dynamic equilibrium in which chromosomes experience equal tension from poleward and antipoleward forces, and any chromosome that loses proper attachment rapidly triggers checkpoint arrest through the mitotic arrest deficiency protein MAD2. Chromosomes are most condensed at this stage, compacting each DNA molecule to roughly one ten-thousandth of its extended length, which makes them most easily photographed and measured.
This condensation is the basis for karyotyping, in which metaphase spreads are used to count and characterize chromosomes for clinical diagnosis of conditions such as trisomy 21.
The spindle assembly checkpoint protein MAD2 was first identified in budding yeast (Saccharomyces cerevisiae) through screens for mutants that failed to arrest in mitosis when spindle assembly was disrupted; a single unattached kinetochore produces enough MAD2 signal to delay division of the entire cell.
Cell Cycle →Chromosomes split apart during metaphase. Separation of sister chromatids occurs only in anaphase, after the spindle checkpoint confirms that all kinetochores are under proper bipolar tension.
In onion (Allium cepa) root tip cells, metaphase chromosomes align visibly along the cell equator and can be observed under a light microscope at 400x magnification. Onion has 16 chromosomes per diploid cell, making individual chromosomes relatively easy to count in well-prepared squash preparations.
Microfilament
/ my-kroh-FIL-uh-ment / · Greek: mikros (small) + Latin: filamentum (thread)
Microfilament is a cytoskeletal fiber approximately 7 nanometers in diameter, composed of two intertwined strands of polymerized actin, that supports cell shape, drives cell movement, and forms the contractile ring during cell division.
Microfilaments polymerize from globular actin monomers in a polarized manner, with fast-growing barbed ends and slow-growing pointed ends, and individual filaments turn over within seconds to minutes depending on the activity of regulatory proteins such as cofilin and profilin. Cross-linking proteins including alpha-actinin and spectrin organize filaments into bundles and networks that resist deformation at the cell cortex. During cell migration, myosin II motors pull on actin filaments to generate contractile force that retracts the cell rear while actin polymerization at the leading edge pushes out new protrusions.
At cytokinesis, a contractile ring of actin and myosin II, roughly 0.2 micrometers thick, constricts the cell equator to pinch the two daughter cells apart.
The drug cytochalasin D, originally isolated from the fungus Helminthosporium dematioideum, caps actin barbed ends and blocks polymerization at concentrations as low as 1 micromolar, causing cultured cells to round up and lose their shape within minutes. Researchers use it routinely to distinguish actin-dependent processes from those driven by microtubules.
Microfilaments and microtubules are the same structure. Microfilaments are 7-nanometer actin fibers, while microtubules are hollow tubulin cylinders about 25 nanometers in outer diameter, and the two systems perform distinct mechanical and transport functions.
In crawling neutrophils responding to a bacterial infection, actin filament polymerization at the leading edge produces protrusive lamellipodia extending at 0.1 to 0.3 micrometers per second, with Arp2/3 complex generating branched filament networks at angles of approximately 70 degrees. The Arp2/3-nucleated branches turn over within 20 to 30 seconds through the action of cofilin, which severs older ADP-actin segments near the branch point and accelerates monomer recycling to the barbed-end pool at the cell front. In a typical neutrophil lamellipodium measuring 10 micrometers wide and 1 micrometer thick, roughly 4,000 actin filaments are actively growing simultaneously.
Microtubule
/ my-kroh-TOO-byool / · Greek: mikros (small) + Latin: tubulus (small pipe)
Microtubule is a hollow cylindrical polymer approximately 25 nanometers in outer diameter, assembled from alpha-tubulin and beta-tubulin heterodimers, that supports cell shape, directs intracellular transport, and separates chromosomes during cell division.
Microtubules consist of 13 protofilament strands arranged in a ring, with tubulin subunits adding preferentially at the plus end and dissociating at the minus end. GTP bound to beta-tubulin is hydrolyzed to GDP after incorporation, destabilizing the filament and causing rapid depolymerization if the GTP cap at the plus end is lost, a behavior called dynamic instability. This switching between growth and shrinkage phases lets microtubules probe the cell interior and reorganize rapidly during interphase or assemble into the mitotic spindle during cell division.
Taxol, a compound first isolated from the Pacific yew tree (Taxus brevifolia) in 1967, stabilizes microtubules by preventing depolymerization and is now used clinically to treat breast, ovarian, and lung cancers by blocking mitotic spindle dynamics.
The drug nocodazole depolymerizes microtubules within minutes at concentrations of 10 micromolar in cultured cells, and upon washout the entire microtubule network reassembles from centrosomes within roughly 10 minutes, demonstrating how rapidly tubulin subunits can repolymerize from the soluble pool.
Microtubules are rigid, permanent rods inside the cell. Each microtubule undergoes dynamic instability, switching between growth and rapid shrinkage on a timescale of seconds to minutes, so the network continuously remodels itself.
In the flagella of Chlamydomonas reinhardtii, a single-celled green alga, microtubules are arranged in the 9+2 axonemal pattern , nine peripheral doublets surrounding two central singlets , and dynein arms on each doublet produce the sliding forces that drive the flagellar beat at 50 strokes per second. Each doublet consists of a complete A-tubule with 13 protofilaments and a partial B-tubule with 10, joined at a seam. Outer dynein arms spaced 24 nanometers apart along each A-tubule can generate up to 6 piconewtons of force per arm, and coordinated inhibition of alternate arms by the central-pair projection apparatus converts uniform sliding into the asymmetric waveform needed for steering.
Differences Between Plant and Animal Cells →Microvilli
/ my-kroh-VIL-ee / · Greek: mikros (small) + Latin: villus (tuft of hair)
Microvilli are densely packed, finger-like projections on the apical surface of epithelial cells, each supported by a core bundle of actin filaments, that increase the membrane surface area available for absorption and secretion.
Each microvillus is approximately 1 micrometer long and 0.1 micrometer in diameter, supported internally by a bundle of 20 to 30 actin filaments crosslinked by villin and fimbrin. A single intestinal epithelial cell carries 1,000 to 3,000 microvilli, increasing its absorptive surface area roughly 20-fold compared to a flat cell surface. Digestive enzymes including lactase and sucrase-isomaltase are anchored to the outer surface of microvilli in the brush border membrane, positioning them directly at the site of nutrient absorption.
Villin, the primary actin-bundling protein in microvilli, can sever actin filaments when calcium concentrations rise, allowing rapid remodeling of the brush border in response to injury or infection.
Kidney proximal tubule cells carry microvilli that are taller and more densely packed than those in the intestine, forming a brush border that recovers more than 99 percent of the glucose and amino acids filtered from blood each day, amounting to roughly 180 liters of filtrate processed in 24 hours.
Microvilli and cilia are the same structure. Microvilli are short, actin-supported projections that increase surface area, while motile cilia are longer, microtubule-based structures that beat rhythmically to move fluid or mucus across a cell surface.
In the brush border of human small intestinal enterocytes, microvilli are packed at a density of roughly 90,000 per square millimeter of cell surface. Combined with the folding of the intestinal wall into villi and circular folds, this microvillar density contributes to a total absorptive surface area estimated at 30 to 40 square meters in a living adult.
Fun Facts About Digestive System →Mitochondria
/ my-toh-KON-dree-uh / · Greek: mitos (thread) + chondrion (granule)
Mitochondria are double-membrane organelles found in nearly all eukaryotic cells that generate most of the cell's ATP through oxidative phosphorylation and also regulate calcium signaling, apoptosis, and several biosynthetic pathways.
Mitochondria oxidize pyruvate and fatty acids through the citric acid cycle and electron transport chain, generating approximately 30 ATP molecules per glucose molecule under typical cellular conditions. Electron carriers in complexes I through IV of the inner mitochondrial membrane transfer electrons to oxygen while pumping protons into the intermembrane space, creating an electrochemical gradient that ATP synthase harnesses to phosphorylate ADP. Beyond ATP production, mitochondria buffer cytosolic calcium, synthesize heme and iron-sulfur clusters, and release cytochrome c into the cytosol to activate caspases during apoptosis.
Each mitochondrion carries its own circular genome of approximately 16,600 base pairs in humans, encoding 13 proteins, 22 transfer RNAs, and 2 ribosomal RNAs, a remnant of the ancient proteobacterial ancestor that Lynn Margulis championed in her endosymbiotic theory.
Brown adipose tissue mitochondria in hibernating ground squirrels (Spermophilus lateralis) express high levels of uncoupling protein 1, which short-circuits the proton gradient to generate heat rather than ATP, raising body temperature from near 0 degrees Celsius to roughly 37 degrees Celsius during arousal from hibernation in a matter of hours.
Mitochondria only make ATP. They also regulate intracellular calcium levels, initiate apoptosis through cytochrome c release, produce heat through uncoupling in brown adipose tissue, and synthesize key metabolites including heme and certain amino acid precursors.
Cell Death →Hummingbird flight muscles contain mitochondria occupying roughly 30 percent of muscle cell volume, a density achieved by stacking cristae at 3 to 6 per micrometer of inner membrane length, creating total inner membrane surface areas up to 40 square meters per gram of tissue. Mitochondria in hummingbird flight muscle also contain one of the highest known densities of cytochrome oxidase complexes, enabling oxygen consumption rates of approximately 200 milliliters O2 per gram of muscle per hour during hovering flight. At rest, the same mitochondria reduce ATP synthesis rate by over 90 percent through feedback inhibition of the F1-Fo ATP synthase, conserving substrate for the next burst of wing activity.
Motor Protein
/ MOH-ter PROH-teen / · Latin motor, mover; Greek protos, first
Motor protein is a protein that converts ATP energy into directed mechanical movement along cytoskeletal filaments to transport cargo or generate force within cells.
Motor proteins hydrolyze ATP to produce conformational changes that drive stepwise movement along cytoskeletal tracks, generating forces in the piconewton range at speeds from nanometers to micrometers per second. Kinesin and dynein move along microtubules, with kinesin typically carrying cargo toward the cell periphery and dynein moving cargo toward the cell center, while myosin motors walk along actin filaments. Each motor family contains multiple classes adapted for specific cargo types, from membrane vesicles to organelles to chromosomes.
Multiple motors often coordinate on a single cargo, allowing bidirectional transport and precise positioning within the cell.
In 1985, Ron Vale and colleagues at the University of California, San Francisco first isolated kinesin from squid (Doryteuthis pealeii) giant axons, revealing that a single motor protein could move cargo along microtubules using ATP, a finding that opened the field of intracellular transport biology.
Motor proteins only transport cargo from one place to another. Myosin II, for example, does not transport vesicles at all; instead, it generates contractile force by sliding actin filaments past one another, which drives muscle contraction and cell division.
In axons of nerve cells, kinesin motors transport vesicles at approximately 0.5 to 2 micrometers per second over distances of up to 1 meter in the longest human motor neurons, covering the full axon length in roughly 6 to 23 days of continuous processive movement. Dynein motors simultaneously carry cargo in the retrograde direction at comparable speeds, creating bidirectional traffic that a single axon supports using only 26 microtubule tracks. Cargo switching between anterograde kinesin and retrograde dynein occurs through scaffold proteins of the JIP family that simultaneously bind both motors and regulate which one dominates by sensing local mechanical tension and biochemical signals.
Fun Facts About the Nervous System →