Ultrastructural View
The detailed structure of a biological specimen, such as a cell, tissue, or organ, that can be
observed only by electron microscopy. Also called fine structure.
Unicellular
Having or consisting of one cell
Unicellular prtozoans
Any of a large group of single-celled, usually microscopic, eukaryotic organisms, such as
amoebas, ciliates, flagellates, and sporozoans.
Unisexual
Having only one type of sexual organ.
Unsaturated fatty acid
A fatty acid possessing one or more double bonds between the carbons in the hydrocarbon
tail. Such bonding reduces the number of hydrogen atoms attached to the carbon skeleton.
Unreactive
Not reacting chemically.
Urea
A soluble form of nitrogenous waste excreted by mammals and most adult amphibians.
Urea cycle
The urea cycle is a cycle of biochemical reactions occurring in many animals that produces
urea from ammonia (NH 3 ). This cycle was the first metabolic cycle didcovered (Kerbs and
Kurt Henseleit, 1932). In mammals, the urea cycle takes place only in the liver.
Ureter
A duct leading from the kidney to the urinary bladder.
Urethra
A tube that releases urine from the body near the vagina in females or through the penis in
males; also serves in males as the exit tube for the reproductive system.
Uric acid
An insoluble precipitate of nitrogenous waste excreted by land snails, insects, birds, and
some reptiles.
Urinary bladder
An elastic, muscular sac situated in the anterior part of the pelvic cavity in which urine
collects before excretion.
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