Cell Biology Terms Starting With O
Cell Biology Glossary: O
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Organelle
/ or-guh-NEL / · Latin: organum (tool) + -elle (small)
Organelle is a discrete, structurally defined component inside a cell that carries out a specific biochemical function, analogous to how organs carry out specific functions in a body.
Membrane-bound organelles, including mitochondria, endoplasmic reticulum, Golgi apparatus, and lysosomes, compartmentalize metabolic pathways to improve catalytic efficiency and prevent incompatible reactions from interfering with one another. Non-membrane-bound organelles such as ribosomes consist of protein and RNA subunits that assemble into functional complexes capable of catalyzing peptide bond formation without requiring a lipid boundary. Eukaryotic cells contain between 1000 and 10,000 mitochondria per cell depending on energy demand, while prokaryotes lack membrane-bound organelles but concentrate enzymes in specific cytoplasmic regions or membrane domains for functional organization.
Compartmentalization by organelles lets eukaryotic cells achieve greater size and metabolic complexity than prokaryotes by reducing diffusion distances and allowing simultaneous biochemical reactions that would otherwise be mutually inhibitory.
Mitochondria and chloroplasts contain their own circular DNA and divide by binary fission, leading Lynn Margulis to propose the endosymbiotic theory in 1967. Genetic and structural evidence since confirmed that both organelles descended from free-living prokaryotes engulfed by ancestral eukaryotic cells more than 1.5 billion years ago.
Only eukaryotic cells have organized internal structures. Prokaryotes lack membrane-bound organelles, but many species concentrate specific enzymes into protein-bounded microcompartments, such as the carboxysome in cyanobacteria, which confines carbon-fixation enzymes to increase their local concentration and efficiency.
Difference Between Prokaryotic and Eukaryotic Cells →In the leaves of spinach (Spinacia oleracea), each mesophyll cell contains between 20 and 100 chloroplasts, each roughly 4 to 6 micrometers long. These organelles migrate toward the cell surface under low light and reorient parallel to the surface under intense light, a movement driven by actin-based motor activity that optimizes photon capture without causing photodamage.
Osmosis
/ oz-MOH-sis / · Greek: osmos (pushing)
Osmosis is the net movement of water molecules across a selectively permeable membrane from a region of lower solute concentration to a region of higher solute concentration.
Osmosis moves water down its water potential gradient, driven by differences in solute concentration; water potential equals osmotic potential plus pressure potential, and water flows toward the side where water potential is lower. Water molecules form hydrogen bonds with dissolved solutes, reducing the effective concentration of free water on the solute-rich side and creating a net flux toward that side. The rate of osmotic water movement depends on the magnitude of the solute concentration difference, the permeability of the membrane to water, and temperature, with aquaporin channel proteins accelerating water flux by up to 40-fold compared with diffusion through the bare lipid bilayer.
In hypotonic solutions, water enters plant cells until internal turgor pressure balances the osmotic gradient, while in hypertonic solutions water leaves cells, causing plasmolysis in which the plasma membrane pulls away from the cell wall.
Aquaporins, the protein channels that accelerate osmotic water movement, were discovered by Peter Agre, who shared the 2003 Nobel Prize in Chemistry for the finding. Red blood cells express the aquaporin AQP1 at roughly 200,000 copies per cell, allowing them to equilibrate water rapidly as they pass through tissues with varying solute concentrations.
Osmosis describes the movement of any molecule across a membrane. Osmosis refers exclusively to water movement; the net movement of dissolved solutes across membranes is called diffusion or, when energy is required, active transport.
In the cells of freshwater green algae such as Spirogyra, water continuously enters by osmosis because the surrounding pond water is hypotonic relative to the cell contents. These algae maintain a central vacuole that can occupy more than 90 percent of cell volume, and contractile vacuoles in related freshwater protists expel the excess water at rates of one full vacuole volume every few minutes to prevent cell rupture.
Osmotic Pressure
/ oz-MOT-ik PRESH-er / · Greek: osmos (pushing) + Latin: pressura
Osmotic pressure is the minimum pressure that must be applied to a solution to prevent water from moving into it by osmosis across a semipermeable membrane.
Osmotic pressure can be calculated using the van’t Hoff equation, where osmotic pressure equals the molar concentration of solute multiplied by the gas constant and absolute temperature, predicting that a one-molar solution of an ideal non-electrolyte exerts roughly 22.4 atmospheres of osmotic pressure at 0 degrees Celsius. In plant cells, dissolved sugars and ions in the central vacuole generate osmotic pressure that draws water inward until the cell wall exerts an equal and opposite turgor pressure, creating the rigidity that supports herbaceous stems and leaves. Typical turgor pressure in well-hydrated plant cells ranges from 3 to 10 atmospheres, but cells in very dilute external solutions can reach 20 atmospheres or higher.
When external solute concentration rises above that of the cell interior, the osmotic pressure gradient reverses, water leaves the cell, and turgor pressure drops to zero, causing wilting or plasmolysis.
Marine fish face a constant osmotic challenge because seawater has an osmotic pressure of approximately 25 atmospheres, far exceeding that of their body fluids. Bony fish such as Atlantic cod (Gadus morhua) counteract this by drinking seawater continuously and actively excreting salt through specialized chloride cells in their gills, expending significant metabolic energy to maintain osmotic balance.
Osmotic pressure is caused by water pushing randomly against a membrane. Osmotic pressure arises specifically from differences in solute concentration across a selectively permeable membrane; the greater the solute concentration difference, the higher the osmotic pressure, regardless of which solutes are present.
In red blood cells placed in a saline solution with a sodium chloride concentration below 0.9 percent, the osmotic pressure of the cell interior exceeds that of the surrounding fluid. Water enters the cells by osmosis, and at concentrations below roughly 0.4 percent sodium chloride, the cells swell until the membrane ruptures in a process called hemolysis.
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