06413_21 - U V diffraction-ring systems, appearing as...

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U ubiquinone (coenzyme Q) Any of a group of related quinone-derived com- pounds that serve as electron carriers in the * electron transport chain reactions of cellular respiration. Ubiquinone mol- ecules have side chains of different lengths in different types of organisms but function in similar ways. ulna The larger of the two bones in the forearm of vertebrates ( compare radius ). It articulates with the outer carpals at the wrist and with the humerus at the elbow. ultracentrifuge A high-speed cen- trifuge used to measure the rate of sedi- mentation of colloidal particles or to separate macromolecules, such as pro- teins or nucleic acids, from solutions. Ultracentrifuges are electrically driven and capable of speeds up to 60 000 rpm. ultradian rhythm See biorhythm . ultra Ü ltration The process in which hydrostatic pressure causes water and small dissolved molecules and ions to move across a membrane against a * con- centration gradient. Ultra Ü ltration is re- sponsible for the formation of * tissue Û uid and * glomerular Ü ltrate from blood. In both these processes the ultra Ü ltered Û uid has the same composition as the plasma except that it does not contain blood cells or large protein molecules. ultrahigh frequency (UHF) A radio fre- quency in the range 3 × 10 9 – 0.3 × 10 9 Hz; i.e. having a wavelength in the range 10 cm to 1 m. ultramicroscope A form of microscope that uses the * Tyndall effect to reveal the presence of particles that cannot be seen with a normal optical microscope. Col- loidal particles, smoke particles, etc., are suspended in a liquid or gas in a cell with a black background and illuminated by an intense cone of light that enters the cell from the side and has its apex in the Ü eld of view. The particles then produce diffraction-ring systems, appearing as bright specks on the dark background. ultramicrotome See microtome . ultrasonics The study and use of pres- sure waves that have a frequency in ex- cess of 20 000 Hz and are therefore inaudible to the human ear. Ultrasonic generators make use of the * piezoelectric effect, * ferroelectric materials, or * mag- netostriction to act as transducers in con- verting electrical energy into mechanical energy. Ultrasonics are used in medicine for diagnosis, particularly in conditions such as pregnancy, in which X-rays could have a harmful effect, and for treatment, the vibratory effect being used to break up kidney stones, etc. Ultrasonic tech- niques are also used industrially to test for Û aws in metals, to clean surfaces, to test the thickness of parts, and to form colloids. ultrastructure The submicroscopic, al- most molecular, structure of living cells, which is revealed by the use of an elec- tron microscope. ultraviolet microscope
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06413_21 - U V diffraction-ring systems, appearing as...

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