Cell Biology Terms Starting With L
Cell Biology Glossary: L
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Lipid Bilayer
/ LIP-id BY-lay-er / · Greek: lipos (fat) + bi (two) + layer
Lipid bilayer is the two-layered sheet of phospholipid molecules that forms the structural foundation of all biological membranes, with hydrophobic fatty acid tails facing inward and hydrophilic phosphate heads facing the aqueous environments on each side.
The arrangement of phospholipids in a bilayer is thermodynamically favorable because it buries nonpolar fatty acid chains away from water while exposing polar phosphate heads to the aqueous phase on both sides. Small nonpolar molecules such as oxygen and carbon dioxide diffuse through the hydrophobic core relatively easily, while charged ions and large polar molecules require protein channels or pumps to cross. The bilayer is not rigid; at physiological temperature, individual phospholipid molecules exchange positions with neighbors roughly ten million times per second, giving the membrane a fluid, two-dimensional quality.
This fluidity, first described in the fluid mosaic model proposed by Singer and Nicolson in 1972, lets membrane proteins diffuse laterally and cluster into functional domains.
Archaeal membranes contain ether-linked isoprenoid lipids rather than the ester-linked fatty acids found in bacterial and eukaryotic bilayers, and these archaeal lipids can span the entire membrane as a monolayer, giving archaea extraordinary stability at temperatures above 80 degrees Celsius.
Phospholipid Bilayer →The lipid bilayer is a static, fixed structure. Its phospholipid molecules move continuously within each leaflet, and the bilayer composition can shift rapidly in response to temperature changes or cellular signals.
In the myelin sheath wrapped around axons of the peripheral nervous system, the lipid bilayer is compacted into a multilayer stack averaging 70 nanometer repeats, with the cytoplasmic leaflets of adjacent membranes fused by myelin basic protein to reduce electrical capacitance. A single Schwann cell can wrap up to 100 membrane layers around an axon segment, creating a sheath with up to 200 stacked bilayers over a length of 1 to 1.5 millimeters. This compact insulation raises the action potential conduction velocity from roughly 1 meter per second in unmyelinated C fibers to over 70 meters per second in the largest myelinated A-alpha fibers.
Lumen
/ LOO-men / · Latin lumen, light or opening
Lumen is the enclosed interior space of a hollow biological structure, whether an organelle such as the endoplasmic reticulum or a tubular organ such as a blood vessel or the intestine.
A lumen is separated from the surrounding cytosol or tissue by a membrane or epithelial wall, creating a chemically distinct compartment where specialized processing occurs. Proteins destined for secretion enter the endoplasmic reticulum lumen through the translocon complex and then traverse the Golgi lumen, where enzymes add sugar chains, sulfate groups, and other modifications before sorting the proteins toward their final destinations. The lumen of the small intestine, by contrast, is a macroscopic space roughly 2.5 centimeters in diameter in adult humans, where digestive enzymes and bile salts act on ingested food.
Despite the scale difference, both compartments share the defining feature of being enclosed spaces whose chemical environment differs sharply from the surrounding cellular or tissue context.
The diameter of the lumen of a coronary artery can narrow by more than 70 percent before blood flow becomes restricted enough to cause angina, because flow resistance increases with the fourth power of radius reduction as described by the Hagen-Poiseuille equation.
Lumen always refers to a unit of light measurement. In physics that is true, but in biology the term denotes the interior space of any hollow structure, from a capillary to an organelle.
In the rough endoplasmic reticulum of pancreatic acinar cells, the lumen fills with newly synthesized digestive enzyme precursors at a concentration of several milligrams per milliliter, accumulating zymogen proteins destined for secretion into the duodenum. The ER lumen maintains an oxidizing environment with a glutathione redox potential near minus 180 millivolts, roughly 100 millivolts more oxidizing than the cytoplasm, which allows protein disulfide isomerase to catalyze disulfide bond formation at rates sufficient to process the 1 to 2 million insulin molecules a single beta cell secretes per minute. Calcium concentration in the ER lumen also reaches 400 to 600 micromolar, driving chaperone function and triggering inositol trisphosphate-gated calcium release when signaling demands it.
Lysosome
/ LY-soh-sohm / · Greek: lysis (loosening) + soma (body)
Lysosome is a membrane-bound organelle in eukaryotic cells that contains acid hydrolases capable of breaking down proteins, lipids, nucleic acids, and carbohydrates at an acidic internal pH.
Lysosomes maintain an internal pH of approximately 4.5 to 5.0 through vacuolar-type proton pumps embedded in their membrane, creating optimal conditions for more than 50 distinct hydrolytic enzymes including cathepsin proteases, lipases, nucleases, and sulfatases. When lysosomes fuse with endocytic compartments or autophagosomes, their enzymes break down captured bacteria, worn organelles, and macromolecules into amino acids, fatty acids, and nucleotides that the cell recycles. Christian de Duve first identified lysosomes in 1955 using differential centrifugation of rat liver homogenates, work that contributed to his Nobel Prize in Physiology or Medicine in 1974.
Defects in lysosomal enzyme function underlie more than 50 inherited storage diseases, including Tay-Sachs disease, in which GM2 ganglioside accumulates to toxic levels in neurons.
Lysosomes can fuse with the plasma membrane and release their contents outside the cell, a process called lysosomal exocytosis. Osteoclasts use this mechanism to secrete acid and cathepsin K directly onto bone mineral, dissolving roughly 100 micrometers of bone matrix per day during normal bone remodeling.
Are Enzymes Proteins? →Lysosomes are simply garbage cans that destroy cellular waste. They recycle useful molecular building blocks from worn-out organelles, pathogens, and macromolecules, returning amino acids, fatty acids, and sugars to the cytosol for reuse.
In macrophages, lysosomes fuse with phagosomes containing engulfed bacteria within minutes of phagocytosis, dropping the phagolysosome pH to approximately 4.5 and activating cathepsin proteases that degrade bacterial proteins within 30 minutes. A single macrophage lysosome contains over 60 distinct hydrolase enzymes and maintains its acidic pH through 5 to 10 copies of the vacuolar-type proton ATPase embedded in the membrane, each pumping roughly 650 protons per second. Mycobacterium tuberculosis evades destruction by secreting lipoarabinomannan, a lipoglycan that blocks phagosome-lysosome fusion and prevents pH from dropping below 6.5.