Cell Biology Terms Starting With H
Cell Biology Glossary: H
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Hemidesmosome
/ hem-ee-DEZ-moh-sohm / · Greek hemi, half; desmos, bond; soma, body
Hemidesmosome is a multiprotein adhesion structure on the basal surface of epithelial cells that anchors the cell to the underlying basement membrane through connections between intracellular keratin filaments and extracellular matrix proteins.
Hemidesmosomes are built around integrin heterodimers, particularly alpha-6-beta-4 integrin, which bind extracellular laminin-332 in the basement membrane and connect intracellularly to plectin and BP230 adaptor proteins that link to keratin intermediate filaments. This arrangement creates a continuous mechanical linkage from the extracellular matrix through the plasma membrane to the cytoskeleton, distributing tensile forces across the cell. Collagen XVII, a transmembrane collagen also called BP180, spans the hemidesmosome and contributes to its structural integrity.
Autoimmune destruction of collagen XVII or BP230 causes bullous pemphigoid, a blistering disease in which hemidesmosome failure allows skin layers to separate under normal mechanical stress.
Mutations in the ITGB4 gene encoding beta-4 integrin cause a severe inherited blistering condition called junctional epidermolysis bullosa. Affected newborns can lose large areas of skin and mucous membrane with minimal trauma, illustrating how a single hemidesmosome protein sustains the mechanical integrity of the entire epithelial surface.
Hemidesmosomes do not connect two cells to each other as classical desmosomes do. Each hemidesmosome anchors a single epithelial cell to the underlying basement membrane through alpha-6-beta-4 integrin heterodimers, making it a cell-to-matrix junction rather than a cell-to-cell junction.
In the corneal epithelium of the human eye, hemidesmosomes anchor the basal cell layer to Bowman's layer of the underlying stroma through alpha-6-beta-4 integrin and laminin-332, providing the mechanical stability needed to withstand shear forces during blinking. Each hemidesmosome contains roughly 100 integrin heterodimers clustered within a plaque approximately 200 nanometers in diameter, connected internally to keratin filaments through the cytoplasmic linker protein BP230. Mutations in BP180, a transmembrane component of the hemidesmosome outer plaque, cause bullous pemphigoid, an autoimmune blistering disease affecting roughly 20 per million people annually.
Cells of the Epidermis →Homeostasis
/ hoh-mee-oh-STAY-sis / · Greek: homoios (similar) + stasis (standing still)
Homeostasis is the process by which a cell or organism detects deviations from a stable internal condition and activates corrective mechanisms to return that condition to its normal operating range.
Cells maintain homeostasis through negative feedback loops that detect deviations from set-point values and trigger corrective responses. When cytoplasmic calcium concentration rises above roughly 100 nanomolar, calcium release from the endoplasmic reticulum stops and plasma membrane pumps export excess calcium, restoring normal levels of 50 to 100 nanomolar. Temperature, pH, osmotic pressure, and oxygen levels are all homeostatic variables regulated by specific cellular mechanisms.
These regulatory systems keep thousands of simultaneous chemical reactions proceeding at consistent rates despite fluctuations in the external environment.
The concept of homeostasis was formalized by American physiologist Walter Cannon in 1926, building on Claude Bernard's 19th-century idea of the milieu intérieur. Cannon coined the term from the Greek words for "same" and "steady," though he emphasized that the regulated state is dynamic rather than fixed.
Homeostasis is not a static, unchanging state. Regulated variables constantly fluctuate within tolerated ranges as feedback mechanisms make continuous small adjustments rather than locking conditions at a single fixed value.
In freshwater paramecia (Paramecium caudatum) placed in hypotonic water, contractile vacuoles collect excess water entering by osmosis and expel it through a pore every 6 to 10 seconds, pumping a volume equal to the cell's own body volume roughly every 20 minutes. ATP-powered proton pumps acidify the vacuole lumen, driving secondary active transport of ions into the vacuole and drawing water in osmotically; blocking these pumps with bafilomycin causes cells to swell and lyse within minutes. The rate of vacuole discharge scales precisely with external osmolarity, halving when external solute concentration doubles, demonstrating tight homeostatic regulation.
Hypertonic
/ hy-per-TON-ik / · Greek: hyper (above) + tonos (tension)
Hypertonic describes a solution whose total solute concentration is higher than that of a reference cell's cytoplasm, causing water to move out of the cell by osmosis when the two are separated by a semipermeable membrane.
A hypertonic solution contains dissolved solutes at a concentration higher than the cell’s internal solute concentration, creating a water potential gradient that drives water out of the cell. Animal cells in hypertonic solutions lose water and shrink; when the loss is severe enough to wrinkle the plasma membrane, the process is called crenation. Plant cells in hypertonic solutions undergo plasmolysis, where the cytoplasm pulls away from the rigid cell wall as water exits the vacuole.
Water movement continues until solute concentrations equalize across the membrane or the cell becomes too dehydrated to sustain normal function.
Marine cartilaginous fishes such as sharks maintain blood solute concentrations slightly above seawater by retaining high levels of urea and trimethylamine oxide, keeping their tissues in a mildly hypertonic state relative to the surrounding ocean. This strategy eliminates the osmotic water loss that would otherwise occur if their tissues were hypotonic to seawater.
Hypertonic refers to solute concentration, not water concentration directly. A higher solute concentration corresponds to lower water concentration, which is why water exits a cell placed in a hypertonic solution.
Human red blood cells immersed in a 5 percent sodium chloride solution undergo visible crenation within seconds as water exits by osmosis, shrinking cell volume by approximately 30 percent and producing characteristic spiculated projections called echinocytes. The crenation is fully reversible if cells are returned to isotonic saline within a few minutes, but prolonged exposure above 2 percent sodium chloride causes irreversible membrane damage and hemolysis. Medical hyperosmolar agents such as 3 percent saline solution exploit this principle to draw water out of swollen brain tissue and reduce intracranial pressure in patients with cerebral edema.
Hypotonic
/ hy-poh-TON-ik / · Greek: hypo (below) + tonos (tension)
Hypotonic describes a solution whose total solute concentration is lower than that of a reference cell's cytoplasm, causing water to move into the cell by osmosis when the two are separated by a semipermeable membrane.
A hypotonic solution has a solute concentration lower than the cell’s internal concentration, creating a water potential gradient that drives water into the cell. Animal cells in hypotonic solutions swell as water enters, and severe hypotonic exposure causes hemolysis, where the plasma membrane ruptures and the cell contents spill into the surrounding fluid. Plant cells in hypotonic solutions become turgid as the vacuole expands and presses the cytoplasm against the cell wall, generating turgor pressure that can reach 0.5 to 1.0 megapascals in many species.
Water influx continues until solute concentrations equalize across the membrane or the cell reaches its structural limit.
Medical saline solutions used for intravenous infusion are carefully formulated to be isotonic with human blood plasma at approximately 0.9 percent sodium chloride. Accidental infusion of pure water or a strongly hypotonic solution can cause red blood cells to lyse rapidly, a potentially life-threatening complication.
Hypotonic conditions do not always benefit animal cells. Excessive water uptake causes swelling and membrane rupture, which is why cells lacking rigid walls, such as red blood cells, burst when placed in distilled water.
Differences Between Plant and Animal Cells →Freshwater protists such as Paramecium caudatum survive in hypotonic pond water because contractile vacuoles continuously expel excess water entering by osmosis, contracting roughly every 10 seconds to discharge approximately 2 picoliters per cycle. Without functional contractile vacuoles, Paramecium cells placed in distilled water swell and rupture within 3 to 5 minutes. Plant cells handle hypotonic stress differently , their rigid cell walls resist osmotic expansion and build turgor pressure up to 10 atmospheres, the force that keeps leaves erect.