U
ubiquinone Any of a group of related quinone-derived compounds
that serve as electron carriers in the
*electron transport chain reactions
of cellular respiration. Coenzyme Q
molecules have side chains of different lengths in different types of organisms but function in similar ways.
ultracentrifuge A high-speed centrifuge used to measure the rate of
sedimentation of colloidal particles
or to separate macromolecules, such
as proteins or nucleic acids, from solutions. Ultracentrifuges are electrically driven and are capable of
speeds up to 60 000 rpm.
ultrahigh frequency (UHF) A
radio frequency 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. Colloidal 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 Üeld of view. The particles then produce diffraction-ring systems, appearing as bright specks on
the dark background.
ultrasonics The study and use of
pressure waves that have a frequency
in excess of 20 000 Hz and are therefore inaudible to the human ear.
Ultrasonics are used in medical diagnosis, particularly in conditions such
as pregnancy, in which X-rays could
have a harmful effect. Ultrasonic
techniques are also used industrially
to test for Ûaws in metals, to clean
surfaces, to test the thickness of
parts, and to form colloids.
ultraviolet radiation (UV) Electromagnetic radiation having wavelengths between that of violet light
and long X-rays, i.e. between 400
nanometres and 4 nm. In the range
400–300 nm the radiation is known
as the near ultraviolet. In the range
300–200 nm it is known as the far ultraviolet. Below 200 nm it is known
as the extreme ultraviolet or the vacuum ultraviolet, as absorption by the
oxygen in the air makes the use of
evacuated apparatus essential. The
sun is a strong emitter of UV radiation but only the near UV reaches
the surface of the earth as the
*ozone layer of the atmosphere absorbs all wavelengths below 290 nm.
Ultraviolet radiation is classiÜed in
three ranges according to its effect
on the skin. The ranges are:
UV-A (320–400 nm);
UV-B (290–320 nm);
UV-C (230–290 nm).
The longest-wavelength range, UV-A,
is not harmful in normal doses and is
used clinically in the treatment of
certain skin complaints, such as psoriasis. It is also used to induce *vitamin D formation in patients that are
allergic to vitamin D preparations.
UV-B causes reddening of the skin
followed by pigmentation (tanning).
Excessive exposure can cause severe
blistering. UV-C, with the shortest
wavelengths, is particularly damaging. It is thought that short-wavelength ultraviolet radiation causes
skin cancer and that the risk of conwww.AzShimi.ir www.AzShimi.com
ubiquinone Any of a group of related quinone-derived compounds
that serve as electron carriers in the
*electron transport chain reactions
of cellular respiration. Coenzyme Q
molecules have side chains of different lengths in different types of organisms but function in similar ways.
ultracentrifuge A high-speed centrifuge used to measure the rate of
sedimentation of colloidal particles
or to separate macromolecules, such
as proteins or nucleic acids, from solutions. Ultracentrifuges are electrically driven and are capable of
speeds up to 60 000 rpm.
ultrahigh frequency (UHF) A
radio frequency 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. Colloidal 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 Üeld of view. The particles then produce diffraction-ring systems, appearing as bright specks on
the dark background.
ultrasonics The study and use of
pressure waves that have a frequency
in excess of 20 000 Hz and are therefore inaudible to the human ear.
Ultrasonics are used in medical diagnosis, particularly in conditions such
as pregnancy, in which X-rays could
have a harmful effect. Ultrasonic
techniques are also used industrially
to test for Ûaws in metals, to clean
surfaces, to test the thickness of
parts, and to form colloids.
ultraviolet radiation (UV) Electromagnetic radiation having wavelengths between that of violet light
and long X-rays, i.e. between 400
nanometres and 4 nm. In the range
400–300 nm the radiation is known
as the near ultraviolet. In the range
300–200 nm it is known as the far ultraviolet. Below 200 nm it is known
as the extreme ultraviolet or the vacuum ultraviolet, as absorption by the
oxygen in the air makes the use of
evacuated apparatus essential. The
sun is a strong emitter of UV radiation but only the near UV reaches
the surface of the earth as the
*ozone layer of the atmosphere absorbs all wavelengths below 290 nm.
Ultraviolet radiation is classiÜed in
three ranges according to its effect
on the skin. The ranges are:
UV-A (320–400 nm);
UV-B (290–320 nm);
UV-C (230–290 nm).
The longest-wavelength range, UV-A,
is not harmful in normal doses and is
used clinically in the treatment of
certain skin complaints, such as psoriasis. It is also used to induce *vitamin D formation in patients that are
allergic to vitamin D preparations.
UV-B causes reddening of the skin
followed by pigmentation (tanning).
Excessive exposure can cause severe
blistering. UV-C, with the shortest
wavelengths, is particularly damaging. It is thought that short-wavelength ultraviolet radiation causes
skin cancer and that the risk of conwww.AzShimi.ir www.AzShimi.com