ScienceWeek

SCIENCE-WEEK

A Weekly Email Digest of the News of Science

A journal devoted to the improvement of communication
between the scientific disciplines, and between scientists,
science educators, and science policy makers.

March 19, 1999 -- Vol. 3 Number 12

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At the last dim horizon, we search among ghostly
errors of observations for landmarks that are
scarcely more substantial. The search will continue.
The urge is older than history. It is not satisfied
and it will not be oppressed.
-- Edwin Hubble (1889-1953)

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Contents of This Issue:

1. On Brown Dwarf Stars
2. Optical Rotation and Molecular Chirality
3. Observation of Transient Structures in Chemical Reactions
4. Electroluminescence in Conjugated Polymers
5. Extreme Climate Warmth in the Late Cretaceous Period
6. On Apoptosis Inhibitor Proteins

-- In Focus: On the Brains of Mice and Humans

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FYI: A new-book page has been introduced at the SW website, the
page an annotated list of selected recent titles of interest in
science and technology. The titles on this frequently updated
list are selected by the Editors of SW. The URL of the page at
the SW website is: [http://scienceweek.com/books.htm].
-- The Editors.

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1. ON BROWN DWARF STARS
Brown dwarf stars are formed by the contraction of a lump of gas
with a mass too small for nuclear reactions to begin in the core
[*Note #1]. Such a star has a relatively short-lived luminosity
(with estimates ranging from approximately 100 million years to
several billion years) as the result of conversion of
gravitational energy to radiation. The surface temperature of a
brown dwarf is estimated to range from below 2500 degrees kelvin
to less than 1000 degrees kelvin. As recently as 1994, brown
dwarf stars were "theoretical" stars, with no brown dwarf stars
considered to be unambiguously observed, but in recent years a
number of brown dwarf stars have been identified.
... ... C.G. Tinney (Anglo-Australian Observatory Epping, AU)
presents a review of recent observations of brown dwarf stars,
the author making the following points: 1) Most stars spend most
of their lives in a state of pressure balance maintained between
gravitational contraction and the energy generated by nuclear
reactions. In 1963, Kumar suggested there may exist a class of
star-like bodies with masses too low to create the central
temperature and densities required to ignite nuclear fusion
reactions. These "failed stars" became known to astronomers as
"brown dwarfs". 2) The lowest-mass ordinary stars can
theoretically maintain a quasi-equilibrium luminosity for almost
6000 billion years. Brown dwarf stars, in contrast, are expected
to fade throughout their lifetime, cooling to temperatures below
1000 degrees kelvin and becoming undetectable by direct
observation after just a few billion years. This has engendered
considerable interest in brown dwarf stars as possible candidates
for the *dark matter which apparently composes more than 90
percent of the mass our Galaxy. 3) The past 4 years have seen
success finally achieved in the hunt for brown dwarf stars. These
detections have confirmed predictions that both methane and dust
play an important role in determining the spectral behavior of
these objects. But the detection of brown dwarf stars in
significant numbers, when combined with results for the space
density of low-mass stars and *gravitational microlensing
results, allows us to conclude that brown dwarf stars do not make
a significant contribution to the dark matter of our Galaxy. The
author concludes: "No matter how nicely brown dwarfs would solve
the *baryonic dark matter problem, it appears we must look
elsewhere for a solution to this long-standing astronomical
quandary."
-----------
Editor's note: In addition to the background material below, see
SW Focus Reports "The Birth of Galaxies and Stars" and
"Cosmology: Dark Matter", both available at URL
[http://scienceweek.com/swfr.htm].
-----------
C.G. Tinney: Brown Dwarfs: The stars that failed.
(Nature 7 Jan 99 397:37)
QY: C.G. Tinney, Anglo-Australian Observatory, PO Box 296, Epping
NSW 1710, AU.
-----------
Text Notes:
... ... *Note #1: Present theoretical models predict a lower
mass-limit for fusion burning stars with the same element mix as
the Sun of 0.07 solar-mass, equivalent to 74 times the mass of
Jupiter.
... ... *dark matter: In general, in this context, the term "dark
matter" refers to material whose presence can be inferred from
its effects on the motions of stars and galaxies, but which
cannot be seen directly because it emits little or no radiation.
It is believed that at least 90 percent of the mass in the
Universe exists as some form or dark matter.
... ... *gravitational microlensing: Gravitational lensing is the
bending of light and other radiation by a massive gravitational
entity such as a star, a black hole, a galaxy, or a cluster of
galaxies. The effect is predicted by Einstein's theory of
relativity and was first detected during a total solar eclipse by
Eddington in 1919. Large-scale gravitational lensing causes
multiple images of an object, the type and arrangement of the
images determined by the specifics of the lensing entity.
Gravitational "microlensing" is a small-scale lensing effect, the
gravitational field of the lensing object not strong enough to
form distinct images of the background source, but instead
causing an apparent brightening of the source. Stars are expected
to vary in brightness in a characteristic manner if low-mass
stars pass in front of them.
... ... *baryonic dark matter: Ordinary matter too dim to be
observed. A baryon is a nuclear particle, e.g., a proton, built
from 3 quarks (fundamental particles that combine to make up
protons, neutrons, and mesons).
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 19Mar99
-------------------
Related Background:
THE DUSTY ATMOSPHERE OF A BROWN DWARF STAR
... C.A. Griffith et al (3 authors at 3 installations, US) now
report observations of the brown dwarf star Gliese 229B, which
exhibits certain unique characteristics. At 900 degrees kelvin,
the atmosphere of this object is too warm to contain ice clouds
like those on Jupiter and too cool to contain silicate clouds
like those on low-mass stars. These unique conditions (high
gravity and the lack of high clouds) permit spectroscopic
visibility of the atmosphere down to higher pressures (i.e.,
closer to the surface) than possible in cool stars or planets.
The authors investigated the structure of the atmosphere of
Gliese 229B by analyzing its optical spectrum in the interval
0.85 to 1.0 micron, the spectrum obtained at the *Keck 1
telescope. The authors report that the spectrum of Gliese 229B
indicates deep-atmosphere particulate matter with the optical
properties of neither ice nor silicates. The authors suggest the
reddish color of the particles indicates an organic composition
characteristic of aerosols in planetary stratospheres, and that
the *mass fraction of the particles agrees with a photochemical
origin involving incident radiation from its companion primary
star (Gliese 229A).
-----------
C.A. Griffith et al: The dusty atmosphere of the brown dwarf
Gliese 229B.
(Science 11 Dec 98 282:2063)
QY: Caitlin A. Griffith, Northern Arizona University 520-523-5511
-----------
Text Notes:
... ... *Keck 1 telescope: The Keck telescopes are a pair of twin
telescopes at the W. M. Keck Observatory on Mauna Kea, HI US,
each with a 10 meter mirror, the pair constructed 1992-1996. The
installation is managed by the University of California (US) and
the California Institute of Technology (US).
... ... *mass fraction: The mass fraction of aerosols is related
to the *eddy diffusion coefficient k, the mass density of the
atmosphere d, the net mass flux f, and the scale height of the
atmosphere h according to F = fh/kd.
... ... *eddy diffusion coefficient: (turbulent diffusion
coefficient) The exchange coefficient for the diffusion of a
conserved property by eddies in a turbulent flow. In general, an
"eddy" is a vortex-like motion of fluid running contrary to the
main current.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 12Feb99
-------------------
Related Background:
GIANT PLANETS VS. BROWN DWARFS
Filipe D. Santos (Centro de Fisica da Universidade de Lisboa, PT)
presents a short review of current ideas concerning giant
extrasolar planets and *brown dwarf stars. The author makes the
following points: 1) The recent discoveries of planets orbiting
nearby Sun-like stars have revealed that planetary systems can be
surprisingly diverse. The initial discovery in 1995 of the planet
around the star 51 Pegasi was a surprise because it is apparently
a planet with mass about that of Jupiter (at least 0.44 Jupiter-
mass) and an orbital period of only 4.2 days, which implies that
the planet is 20 times closer to its star than Earth is to the
sun. 2) Seven additional planets around solar-type stars have
since been discovered, with Jupiter-mass values ranging from 0.44
to 6.84. 3) Two critical questions are, a) Where should we set
the dividing line that distinguishes massive planets from brown
dwarfs? and, b) What are the mechanisms leading to the formation
of massive planets and brown dwarfs? 4) Brown dwarfs are expected
to have masses smaller than the hydrogen-burning limit of
approximately 0.075 solar-mass (approximately 75 Jupiter-mass),
but probably larger than the deuterium-burning limit of 0.013
solar-mass (approximately 13 Jupiter-mass). 5) Like the companion
massive planets mentioned, several companion brown dwarfs to
solar-type stars have also been identified. One method of
investigating brown dwarfs involves *astrometric measurements,
and in all cases of brown dwarfs investigated by the astrometric
method, the masses are above or very close to the hydrogen-
burning limit. The extant data thus suggest that the distribution
of mass of brown dwarfs does not extend to masses as small as
giant planets. Also, the new measurements indicate that brown
dwarfs orbiting solar-type stars are very rare. 6) The discovery
of Jupiter-mass planets with orbits very close to their stars
poses a considerable problem, because it is difficult to
understand how such planets could form in place. (Five known
Jupiter-mass planets have orbital radii smaller than the distance
from Mercury to the Sun.) The suggestion has been made that these
planets formed at larger distances and migrated inward, but the
proposed migration mechanisms are not yet empirically
distinguishable. The author concludes: "Clearly the discovery of
planetary systems outside our solar system has opened a Pandora's
box of startling phenomena and new questions."
QY: Filipe D. Santos [fdsantos@milkyway.cii.fc.ul.pt]
(Science 17 Jul 98 281:359) (Science-Week 31 Jul 98)
-----------
Text Notes:
... ... *brown dwarf stars: See previous report.
... ... *astrometric measurements: This method of detection
infers the presence of a companion to a star by measuring the
position of the star as it orbits the center of mass of the
entire system. From the orbital inclination, the real mass of the
companion can be derived.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 31Jul99
-------------------
Related Background:
ON THE NATURE OF DARK MATTER
Joel R. Primack (University of California Santa Cruz, US)
presents a commentary on a paper by E. Gawiser and J. Silk
(University of California Berkeley, US) ((Science 29 May 98
280:1405), Primack making the following points: 1) One of the
fundamental issues facing cosmologists concerns the evidence that
observable matter in the universe makes up only a fraction of
what is needed to explain the properties of the universe. A large
portion of matter in the universe must therefore be unobserved,
or "dark matter". 2) In current cosmology, "hot" dark matter is
defined as particles that were still moving at nearly the speed
of light at about a year after the big bang. "Cold" dark matter
is defined as particles that were moving sluggishly at that time.
Neutrinos are the standard example of hot dark matter, although
other more exotic possibilities have been discussed. 3) Gawiser
and Silk (ref. cited above) conclude that of all the currently
popular cosmological models, the only one whose predictions agree
with the data on the cosmic microwave background anisotropies and
the large-scale distribution of galaxies is the cold + hot dark
matter model, with 70% of the matter cold dark, 20% hot dark, and
10% ordinary matter (baryonic). 3) There are 3 species of
neutrinos, and there are mounting astrophysical and laboratory
data suggesting that neutrinos oscillate from one species to
another, which can only happen if they have nonzero mass. As
dark-matter candidates, neutrinos are entities with masses that
may be 10^(-5) of the mass of the electron, but with an expected
density more than 8 orders of magnitude greater than the density
of electrons and protons in the universe. Neutrinos, therefore,
can provide a substantial fraction of dark matter. 4) The success
of the cold + hot dark matter model in fitting the cosmic
microwave background and galaxy distribution data indicates that
this type of model should be investigated in more detail.
QY: Joel R. Primack [joel@physics.ucsc.edu]
(Science 29 May 98 280:1398) (Science-Week 19 Jun 98)


2. OPTICAL ROTATION AND MOLECULAR CHIRALITY
In chemistry, chirality is a property of certain asymmetric
molecules, the property being that the mirror images of the
molecules cannot be superimposed one on the other while facing in
the same direction. Chiral molecules are characterized by a
specific rotation angle, the angle through which *plane-polarized
light is rotated on passing through an *enantiomerically enriched
solution. Recent developments in methodology allow computation of
both the sign and magnitude of these rotation angles, but what
remains elusive is a general strategy for assigning the
individual contributions that atoms and functional groups make to
the optical rotation angle and to the molecular chirality in
general. Linking the rotation angle to the molecular structure is
a challenge of fundamental as well as practical importance. For
example, the chemical bonding pattern is often known for natural
and synthetic products of potential use in medicine, but
determination of the absolute handedness of every chiral center
can be a formidable challenge. When N *stereocenters are present,
there are 2^(N) possible *stereoisomers for a structure.
... ... R.K. Kondru et al (University of Pittsburgh, US) present
a quantitative method to dissect the optical rotation angle into
its individual atomic contributions. The authors suggest this
atomic mapping protocol provides a foundation for establishing
fundamental relations between chemical structure and optical
rotation angles in molecules and links modern quantitatively
reliable computation to numerous empirical models developed over
the past 100 years. The authors also suggest that atomic analyses
of optical rotation may help to establish new quantitative
definitions of molecular asymmetry and the nature of its
propagation through bonds and through space.
-----------
R.K. Kondru et al: Atomic contributions to the optical rotation
angle as a quantitative probe of molecular chirality.
(Science 18 Dec 98 282:2247)
QY: Rama K. Kondru, Department of Chemistry, University of
Pittsburgh, Pittsburgh, PA 15260 US.
-----------
Text Notes:
... ... *plane-polarized light: Electromagnetic radiation
involves the propagation of both electric and magnetic forces,
and at each point in a light beam, there is a component electric
field and a component magnetic field, both of which oscillate in
all directions perpendicular to each other and to the direction
in which the beam is propagated. In plane-polarized light, the
component electric field oscillates as in ordinary light except
that the direction of oscillation is contained within a plane.
Likewise, in plane-polarized light, the component magnetic field
oscillates within a plane, the planes in question being
perpendicular. Circularly polarized light has a component
electric field that varies in direction but not in magnitude, so
that the field traverses a spiral path in either a clockwise or
counterclockwise direction.
... ... *enantiomerically enriched solution: In chemistry, an
enantiomer is a compound whose structure is not superimposable on
its mirror image, the compound being one of a pair of optical
isomers, each of which interacts differently with polarized light
(i.e., shows optical activity). A mixture of two optical isomers
in equal amounts is called a racemic mixture, and racemic
mixtures do not show optical activity. A reactant or process that
produces or selects an enantiomeric excess is simply a reactant
or process that produces or selects one enantiomer in excess over
the other enantiomer. The phrase "enantiomerically enriched
solution" thus refers to a solution with an excess of one
enantiomer over another.
... ... *stereocenters: (chiral center) Any atom (in a molecule)
attached to 4 different groups.
... ... *stereoisomers: In general, stereoisomers are compounds
whose molecules have the same number and kinds of atoms and the
same atomic arrangement, but whose molecules differ in their
spatial arrangements.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 19Mar99
-------------------
Related Background:
CHIRAL SEPARATIONS WITH APOENZYME MEMBRANES
Chirality is a property of certain asymmetric molecules such that
their mirror images cannot be superimposed. Such molecules
usually exist in two mirror image forms (enantiomorphs), and in
processes involving the binding of such molecules with receptors,
it is usually only one of the forms that binds. For this reason,
and other reasons, laboratory separation methods for enantio-
morphs are of great interest. This week Brinda B. Lakshmi and
Charles R. Martin (Colorado State University, US) report a method
of chiral separation involving the embedding of apoenzymes in
porous polymer membranes. An apoenzyme is an enzyme with its co-
factor removed (and therefore its catalytic action obliterated).
The essential idea is that the apoenzyme still binds to one
enantiomorph, but now without altering it, and by proper
arrangement of the apoenzyme in the pores of the polymer
membrane, the binding enantiomorph diffuses through the membrane,
and the separation of the enantiomorphs can be effected.
QY: Charles R. Martin 
(Nature 21 August) (Science-Report 29 Aug 97)
-------------------
Related Background:
DESIGN AND SYNTHESIS OF A MOLECULE WITH CHIRALITY MEMORY
In chemistry, chirality is a property of certain asymmetric
molecules, the property being that the mirror images of the
molecules cannot be superimposed one on the other while facing in
the same direction. A pair of gloves, for example, has this
property. The two forms of the molecule are called enantiomorphs,
and beginning chemistry students are often taught that
enantiomorphs are chemically identical. This may be true in
ordinary solutions, but it is definitely not true when the
reaction of significance involves, for example, the binding of
one enantiomorph to a receptor which itself exhibits chirality.
In that case, one enantiomorph binds and the other does not, a
situation quite prevalent in biological systems, where any carbon
atom with four different bonds (and there are many) exhibits
chirality. One of the interesting properties of molecules that
possess chirality is that the enantiomorphs are optically active,
rotating the plane of polarized light equally but in opposite
directions, and this optical activity is often used as a means of
analysis. Recently, Takuzo Aida et al (University of Tokyo, JP)
reported the synthesis of organic molecules with what they call
"chirality memory". The group of compounds are derivatives of
porphyrin, an interesting molecule having a fully conjugated
cyclic structure of four pyrrole rings. Although porphyrin itself
does not exist in nature, the porphyrin ring system is found in
several quite important natural products such as hemoglobin and
chlorophyll. What Aida and his group have done is design a fully
substituted porphyrin that adopts nonplanar saddle shapes that
alternately tilt up and down. On its own, this compound exists as
a mixture of the two possible conformations, as many tilted one
way as the other way. The essence of this little story is that
when an optically active reactant is introduced, all of this new
porphyrin entity flips into one form or the other, depending on
the reactant (not surprising, so far), but when the optically
active reactant is replaced by one not optically active, the
porphyrin molecules retain their flipped configuration,
remembering the chirality of the previous reactant, and that
memory can be erased by visible light, and then caused to
reappear when the light is switched off. The experts in porphyrin
chemistry are calling this work unique. The idea is that such
organic molecules might one day form the basis for nanoscale
optical and electronic devices. Meanwhile, they will be used as
chirality sensors.
(J. Amer. Chem. Soc. 119:5267 1997) (Science-Week 25 July 97)


3. OBSERVATION OF TRANSIENT STRUCTURES IN CHEMICAL REACTIONS
The term "femtochemistry" refers to the probing of chemical
reactions on the femtosecond [10^(-15) sec] time scale. In such
experiments, nuclear motions on the time scale of bond breaking
and bond making are monitored by using an initiation pulse to
establish time-zero and probing pulses to view the motion.
Typical probes are optical and infrared spectroscopy,
*photoelectron spectroscopy, *mass spectroscopy, *nonlinear
optical techniques, and *diffraction methods. Ultrafast *electron
diffraction is considered a unique method for studies of the
structural changes of complex molecular systems.
... ... J. Cao et al (California Institute of Technology, US) now
report a direct ultrafast electron diffraction study of the
evolution of short-lived intermediates in the course of a
chemical change. The authors observed the transient intermediate
in the *elimination reaction of 1,2-diiodotetrafluoroethane
[C(sub2)F(sub4)I(sub2)] to produce the corresponding ethylene
derivative by the breakage of 2 carbon-iodine C-I bonds. The
authors report that the evolution of the *ground-state
intermediate [C(sub2)F(sub4)I radical] is directly revealed in
the population change of a single chemical bond, namely the
second C-I bond. The elimination of 2 iodine atoms was shown to
be nonconcerted, with a reaction time of the second C-I bond
breakage of 17 +- 2 picoseconds. The authors suggest this
apparent leap in the ability to record structural changes on the
picosecond and shorter timescales bodes well for many future
applications in complex molecular systems.
-----------
J. Cao et al: Ultrafast electron diffraction and direct
observation of transient structures in a chemical reaction.
(Proc. Natl. Acad. Sci. US 19 Jan 99 96:338)
QY: Ahmed H. Zewail [zewail@cco.caltech.edu]
-----------
Text Notes:
... ... *photoelectron spectroscopy: This is a technique for
determining the *ionization potentials of molecules, the sample a
gas or vapor irradiated with a narrow beam of ultraviolet
radiation. The photoelectrons produced in accordance with the
"*photoelectric effect" (and Einstein's photoelectric equation)
are passed through a slit into a vacuum region to be deflected by
magnetic or electrostatic fields to exhibit an energy spectrum
with peaks corresponding to the ionization potential of the
molecule. The technique also provides information about the
vibrational energy level of the formed ions.
... ... *ionization potentials: In general, the ionization
potential is the energy required to produce an ionization of a
molecule. Specifically, the minimum energy required to remove an
electron from a specified atom or molecule to such a distance
that there is no electrostatic interaction between ion and
electron.
... ... *photoelectric effect: In general, the liberation of an
electric charge by electromagnetic radiation incident on a
substance.
... ... *mass spectroscopy: The mass spectrometer is a
device in which molecules are ionized and the accelerated ions
are separated according to their mass to charge ratio. The
relative abundance of isotopes or other ionized species can thus
be determined by measuring positive or negative ion currents.
... ... *nonlinear optical techniques: In general, nonlinear
optics is a branch of optics concerned with the optical
properties of matter subjected to intense electromagnetic fields.
In order for nonlinearity to be exhibited, the external field
must not be negligible with respect to the internal fields of the
atoms or molecules constituting the substance. The nonlinearity
concerns the relation between induced polarization and the
strength of the external electromagnetic radiation. In linear
optics, this relation is linear. The methods of experimental
nonlinear optics are highly dependent on the use of lasers to
generate sufficiently intense external fields.
... ... *diffraction methods: In general, in this context,
diffraction methods are any methods that utilize the wave
properties of radiation to analyze structure (e.g., x-ray
diffraction).
... ... *electron diffraction: In general, electron diffraction
is diffraction of a beam of electrons by atoms or molecules. The
fact that electrons can be diffracted in a manner similar to the
diffraction of x-rays and light is a demonstration that particles
can act as waves. Electrons show diffraction effects with
molecules and crystals in which the interatomic spacing is
comparable to the wavelength of the beam of electrons. The
advantage is that the electron beam wavelength can be set by
adjusting the voltage. Due to the fact that electron beams have
very low penetrating power, electron diffraction is not useful
for investigations of crystal structure but is instead used to
measure bond lengths and angles of molecules in gases.
... ... *elimination reaction: In general, an "elimination
reaction" is a chemical reaction involving elimination of some
portion of a reactant compound, with the production of a second
compound.
... ... *ground-state intermediate: In general, in this context,
the "ground state" is the lowest energy electronic, vibrational,
or rotational state of an atom, molecule, or ion.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 19Mar99
-------------------
Related Background:
NEW LASER STUDIES OF ULTRAFAST REACTION DYNAMICS
Most chemical reactions involving two or more reactants occur in
the presence of a solvent, a significant presence, since the
solvent molecules are always interacting with the reactants
themselves in some important manner. To fully understand what
occurs between the main reactants, one must understand the
participation of the solvent in the reaction sequence. The
problem has been that these solvent-reactant events are of
extremely short duration, on the order of femtoseconds
[10exp(-15) sec], and it is only recently, with the availability
of laser pulse techniques, that physical chemists can observe
chemical bonds break and reform in real time. B. Jefferys
Greenblatt et al (University of California Berkeley, US) have now
described the use of such techniques to examine the influence of
a solvent on the photodissociation of a molecular cluster ion
(Iodine-Argon). Apparently, in a single experiment, they have
presented "a comprehensive picture of the recombination,
relaxation, and evaporation processes that follow photodissocia-
tion of a diatomic molecular ion in a cluster."
(Science 13 June 97) (Science-Week 19 Jun 97)


4. ELECTROLUMINESCENCE IN CONJUGATED POLYMERS
In general, a "diode" is an electronic device with two electrodes
in a physical configuration such that the device permits flow of
electric current in one direction only. In general, a
"semiconductor" is a crystalline solid, such as silicon or
germanium, with an electrical conductivity intermediate between
that of a conductor and an insulator. In general, an "orbital" is
a space region in which an electron may be found in an atom or
molecule, the space region essentially a probability distribution
for the electron described by the equations of quantum mechanics.
Since the atoms in a crystalline solid are in close proximity,
the orbitals of their electrons overlap and possible the electron
energy levels are effectively spread into energy "bands".
Electric conduction occurs in semiconductors as the result of a
net movement under the influence of an applied electric field, a
movement of electrons in the "conduction band" and a movement of
empty states (called "holes") in the "valence band". A hole
effectively behaves as an electron with a positive charge, and
electrons and holes are known as the "charge carriers" in a
semiconductor. In general, "electroluminescence" is the emission
of light by a substance as the result of an applied electric
field, and in general, a "light-emitting diode" (denoted as LED)
is a semiconductor device that converts electrical energy into
light or infrared radiation, a radiation wavelength span from 550
nanometers (green light) to 1300 nanometers (infrared radiation).
Research in the use of organic *conjugated polymers as the active
semiconductors in light-emitting diodes has advanced rapidly in
recent years, and prototype devices now meet what are viewed as
realistic specifications for applications. Conjugated polymers
derive their semiconducting properties by having delocalized ã-
electron bonding along the polymer chain, with *bonding and
antibonding orbitals effectively forming delocalized valence and
conduction bands which support mobile charge carriers.
... ... R.H. Friend et al (11 authors at 6 installations, UK IT
BE SE) present a review of electroluminescence in conjugated
polymers, the authors making the following points: 1) The
phenomenon of electroluminescence has been seen in a wide range
of semiconductors, and for organic semiconductors was first
reported for anthracene single crystals in the 1960s. These early
studies established that the process responsible for
electroluminescence requires injection of electrons from one
electrode and holes from the other, the capture of oppositely
charged carriers (called "recombination"), and the radiative
decay of the excited electron-hole state (called an "exciton")
produced by this recombination process. 2) Development of organic
thin-film electroluminescence was accelerated in the 1980s
through the work of Tang and Van Slyke, who demonstrated
efficient electroluminescence in 2-layer *sublimed molecular film
devices. These early devices consisted of a hole-transporting
layer of an aromatic diamine and an emissive layer of 8-
hydroxyquinoline aluminum. 3) Since the first report of metallic
conductivities in "*doped" polyacetylene in 1977, the science of
electrically conducting polymers has advanced rapidly. As high
purity polymers have become available, a range of semiconductor
devices have been investigated, including *transistors,
*photodiodes, and light-emitting diodes. Much progress has been
made in understanding the basic science that controls the
properties of these devices, but at the present time, in
comparison with inorganic semiconductors, relatively little is
known about the electronic properties of polymeric
electroluminescent semiconductors, and even the nature of the
semiconductor excitations remains controversial.
-----------
R.H. Friend et al: Electroluminescence in conjugated polymers.
(Nature 14 Jan 99 397:121)
R.H. Friend [rhf10@cam.ac.uk]
-----------
Text Notes:
... ... *conjugated: In this context, the term "conjugated"
refers to alternating double (or triple) and single bonds.
... ... *bonding and antibonding orbitals: A "bonding orbital" is
a molecular orbital formed by a bonding electron whose energy
decreases as the nuclei of the bonding atoms are brought closer
together, resulting in a net attraction and a chemical bonding.
In contrast, an "antibonding orbital" is a molecular orbital
whose energy increases as the nuclei are brought closer together,
indicating a net repulsion rather than a net attraction.
... ... *sublimed molecular film: Sublimation is the
transformation of system directly from the vapor state to the
solid state (or from the solid state to the vapor state) without
passing through an intermediary liquid state. A "sublimed"
molecular film is thus a solid molecular film formed directly
from the vapor state.
... ... *doped: In general, in this context, "doping" is the
addition of impurities to a semiconductor to achieve desired
conductivity characteristics. Only a very small addition of
impurity is needed to produce material suitable for use in the
manufacture of semiconductor devices. For example, an impurity
content of 1 part in 10^(8) increases the conductivity of pure
germanium and silicon 16 times. As a corollary, production
control of the electrical characteristics of semiconductors
requires extremely pure germanium and silicon at the start, and
this initial purification is one of the most difficult processes
in the manufacture of semiconductor devices.
... ... *transistors: In general, a transistor is a semiconductor
device in which it is possible to control voltage or current in
such a way as to achieve gain or switching action.
... ... *photodiodes: In general, a photodiode is a diode device
whose electrical characteristics are dependent on incident light.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 19Mar99
-------------------
Related Background:
SEMICONDUCTOR-POLYMER INTERFACE TUNABLE DIODE
In physics, the term "tuning" refers to an adjustment of the
response (for example, oscillatory behavior) of a system to an
applied force by changing one or more of the system parameters.
In general, a diode is any two-electrode electrical system, and a
tunable diode is any such system whose voltage-current relation
(force-response characteristic) can be quantitatively tuned by
parameter variation. In physics, the term "work function" is the
minimum energy required to release an electron as it passes
through the surface of a conductor or semiconductor. A conjugated
polymer is a polymer with alternating double and single bonds in
its structure, the conjugation often producing enough electron
delocalization to make the polymer a conductor of electricity.
The term "doping" refers to the incorporation of impurities in a
substance to alter its physical properties. M. Lonergan (Univ. of
Oregon, US) reports experimental observations and analysis of a
tunable diode based on a hybrid inorganic-organic n-indium
phosphide/poly-(pyrrole)/nonaqueous electrolyte architecture.
Electrochemical manipulation of the work function of the conjug-
ated poly(pyrrole) enables continuous and active tuning of the
diode through a range from 0.0 to 0.6 volts. The author suggests
this type of tunable diode represents a new type of device archi-
tecture that allows the electrochemical control available with
doped conjugated polymers, and that a general path to diodes with
specific properties for specialized applications is now open.
QY: Mark C. Lonergan, Univ. of Oregon, Dept. of Chemistry
541-346-4789.
(Science 19 Dec 97) (Science-Week 9 Jan 98)


5. EXTREME CLIMATE WARMTH IN THE LATE CRETACEOUS PERIOD
Reconstructions of the history of climate change on Earth are a
significant challenge for Earth science in general, but the
results of such reconstructions are also of particular importance
to biologists concerned with the impact of environment on the
evolution of various species of life forms. One geological period
of great interest is the Cretaceous period, the name given to the
approximate time-frame 140 to 65 million years ago. The evidence
indicates this period was characterized by the first emergence of
flowering plants, the appearance of many groups of insects, and
the evolution of many varieties of large dinosaurs. The climate
was apparently tropical to subtropical throughout most of the
globe, with a high concentration of carbon dioxide in the
atmosphere. Finally, at the end of the Cretaceous period, a
widespread and dramatic extinction of large numbers of both
vertebrate and invertebrate species occurred, possibly as the
result of an asteroid impact. One aspect of the study of this
period concerns the apparent prevalent volcanic activity and the
contribution of this volcanic activity to the extant climate.
Another aspect concerns the identification of fossil forms in
various geographical regions, and the use of these fossil forms
as data for climate reconstruction. In general, the term "magma"
refers to molten rock generated by partial melting of the Earth's
crust, "igneous rocks" are rocks that have congealed from magma,
and the term "magmatism" refers to the formation of igneous rocks
from magma. In the context of Earth science, a "province" is any
region or area of large extent with similar features throughout.
... ... J.A. Tarduno et al (6 authors at 4 installations, US CA)
now report the discovery of a fossil vertebrate assemblage from
the high Canadian Arctic (Axel Heiberg Island). The authors
suggest this Late Cretaceous (92 to 86 million years ago) group
of vertebrate fossils indicates that polar climates were warm
(mean annual temperature exceeding 14 degrees centigrade) rather
than near freezing as some studies have implied. The discovered
fossil assemblage includes large (2.4 meters long) champsosaurs,
which are extinct crocodile-like reptiles. The authors further
suggest that magmatism at 6 large igneous provinces during this
time period indicates that volcanic carbon dioxide emissions
helped cause the global warmth, and that a "greenhouse Earth"
analog may be found in this time interval.
-----------
J.A. Tarduno et al: Evidence for extreme climatic warmth from
Late Cretaceous Arctic vertebrates.
(Science 18 Dec 99 282:2241)
QY: J.A. Tarduno [john@earth.rochester.edu]
-------------------
Summary by SCIENCE-WEEK [http://scienceweek.com] 19Mar99
-------------------
Related Background:
ISOTOPIC EVIDENCE FOR THE CRETACEOUS-TERTIARY IMPACTOR
The Cretaceous period is the geological period ranging approxi-
mately from 146 million years ago to 65 million years ago, and
was apparently characterized towards its end by the rapid
extinction of a number of species, including the dinosaurs. There
have been five major extinctions according to the fossil record,
the Cretaceous extinction one of them, and the consensus is that
these extinctions were related to violent geophysical events,
perhaps asteroid impacts. The Chicxulub impact crater in the
Yucatan peninsula of Mexico is a large impact crater apparently
caused by a 10 kilometer-diameter asteroid, the impact area
extending at least 100 kilometers from the impact center. Using
Argon(40)/Argon(39) isotope dating methods, this impact crater
has been dated with high precision at 64.98 million years ago,
which places the impact at the end of the Cretaceous, and the
most popular current hypothesis to explain the Cretaceous
extinction is the global effect of the Chicxulub impact on the
extant life forms. This hypothesis was first proposed by Luis and
Walter Alvarez in the 1970s on the basis of non-terrestrial dust
of presumed cosmic origin in deposits at the K/T boundary, but
the Yucatan crater was unknown at that time and was not
discovered until the 1990s. But direct isotope evidence of an
impactor is still missing, and some researchers have argued that
high concentrations of iridium and other noble metals in K/T
boundary sediments, the basis for the K/T impactor hypothesis,
can be explained by enhanced volcanic activity that occurred near
the end of the Cretaceous, bringing up noble metals from Earth's
mantle, which similar to meteorites has high concentrations of
noble metals. ... ... A. Shukolyukov and G.W. Lugmair now report
a high-precision *mass spectrometric analysis of chromium in
sediment samples from the K/T boundary confirms the cosmic
origins of the K/T phenomenon. The authors report that the
isotopic composition of chromium in K/T boundary samples from
Stevns Klint, Denmark, and Caravaca, Spain, is different from
that of Earth and indicates its extraterrestrial source. The
authors suggest the chromium isotope signature is consistent with
a *carbonaceous chondrite-type impactor, and that the observed
differences in the chromium isotopic composition among the
various meteorite classes can serve as a diagnostic tool for
deciphering the nature of impactors that have collided with Earth
during its history.
-----------
A. Shukolyukov and G.W. Lugmair (Univ. of Calif. San Diego, US)
Isotopic evidence for the Cretaceous-Tertiary Impactor and its
Type.
(Science 30 Oct 98 282:927)
QY: A. Shukolyukov, Univ. of Calif. San Diego 619-534-2230.
-----------
Text Notes:
... ... *mass spectrometric analysis: The mass spectrometer is a
device in which molecules are ionized and the accelerated ions
are separated according to their mass to charge ratio. The
relative abundance of isotopes or other ionized species can thus
be determined by measuring positive or negative ion currents.
... ... *carbonaceous chondrite: "Stony" meteorites (aerolites)
are meteorites formed solely of rock-forming silicates, and
chondrites are a type of stony meteorite consisting of an
agglomeration of millimeter-sized globules (chondrules) that are
thought to be unchanged since the original condensation out of
the nebula from which the sun and solar system formed. A
carbonaceous chondrite is a chondritic meteorite that contains a
relatively large amount of carbon, with a resultant dark
appearance.
-------------------
Summary & Notes by SCIENCE-WEEK  27Nov98
-------------------
Related Background:
ON METEORITE IMPACT AND THE K/T MASS EXTINCTION
... In a short review of the meteorite impact hypothesis and the
K/T extinction, K.O Pope et al make the following points: 1)
Confirmation of the impact portion of the Alvarez hypothesis
marks a turning point in the study of the K/T mass extinction, a
turning point away from speculations about possible causes and
toward linking the extinctions to a single catastrophic event. 2)
Advances in computer modeling of the impact, coupled with
knowledge of the target rocks and their behavior under the
high-pressure shock, have shed light on what happened during the
first few seconds after impact. A key aspect of the Yucatan site
is that the upper 3 kilometers of rock were rich in water,
carbonate, and sulfate, which upon impact produced about 200
gigatons each of SO(sub2) and H(sub2)O vapor and other gases that
greatly altered the properties of the stratosphere. 3) Early work
predicted that smoke and dust from the impact plunged the Earth
into a freezing blackout. Recent computer simulations and
atmospheric models indicate that within a few weeks to months
temperatures and light levels would have begun to rebound due to
the release of heat stored in the oceans and the coagulation and
fall of the dust and soot. The major effects of the dust and soot
would last about 1 year or less, but SO(sub2) and water vapors
would remain in the stratosphere and ultimately produce sulfuric
acid aerosols. Models indicate that a global aerosol cloud would
be continuously produced for approximately 12 years, blocking out
over 50 percent of the sunlight during the first 10 years. The
authors conclude: "Now that we have a better understanding of the
dynamics of the impact, gleaned from the discovery of the crater
and the studies that followed, we can begin to address a wide
range of complex global effects. There is much work ahead, but
the course is clear."
-----------
K.O. Pope et al (3 authors at 3 installations, US)
Meteorite impact and the mass extinction of species at the
Cretaceous/Tertiary boundary.
(Proc. Natl. Acad. Sci. US 15 Sep 98 95:11028)
QY: Kevin O. Pope, Geo Eco Arc Research, 2222 Foothill Blvd., La
Canada, CA 91011 US.
-------------------
Summary by SCIENCE-WEEK  23Oct98
-------------------
Related Background:
ANALYSIS OF THE CHICXULUB IMPACT CRATER
... An important parameter of the [Chicxlulub] impact is the
total area of the impact crater, since that area would be related
to the amount of debris thrown into the atmosphere. Until now,
the usual figure for the largest dimension of the impact crater
has been approximately 300 kilometers. Morgan et al (20 authors
at 8 installations, UK US MX CA) now report an analysis of
seismic data of the Chicxulub impact, determining the diameter of
the transient cavity at about 100 kilometers. The authors suggest
this parameter is critical for constraining any proposed
impact-related effects on the Cretaceous environment, and that
the seismic data indicate the morphology of the crater is similar
to large impact structures observed on other planets such as
Venus.
QY: Mike Warner 
(Nature 4 Dec 97) (Science-Week 26 Dec 97)


6. ON APOPTOSIS INHIBITOR PROTEINS
In general, the term "apoptosis" refers to programmed cell death,
whether as a part of normal tissue differentiation and
development, or as a program activated in a defective cell. In
the molecular biology of cancer, apoptosis is the name given to
the programmed cell death provoked by the proteins expressed by
so-called "tumor suppressor genes". Thus, malignant cells are
defective cells with a deactivated apoptosis program, and this
allows malignant cells to survive and replicate. Caspase is a
type of protease, an enzyme that degrades protein by hydrolysis,
and it has been implicated in the sequence of events denoted as
apoptosis. ... ... Q.L. Deveraux and J.C. Reed (The Burnham
Institute La Jolla US) present an extensive review of current
research concerning a family of proteins that apparently inhibit
apoptosis, the authors making the following points: 1) Disrupted
regulation of apoptosis biochemical pathways occurs in cancer,
autoimmune and immunodeficiency diseases, and in certain
neurodegenerative disorders, and proteins involved in apoptosis
regulation are therefore of intense biological interest, and many
of these proteins are potential therapeutic targets. 2) The
family of proteins that inhibit apoptosis was first discovered in
*baculoviruses, where these proteins were shown to be involved in
suppressing the apoptosis response of the host cell to viral
infection. *Ectopic expression of certain baculoviral protein
inhibitors of apoptosis blocks apoptosis in mammalian cells,
which suggests evolutionary conservation of the apoptosis program
among diverse species and commonalities in the mechanism used by
these proteins to block apoptosis. 3) Although the mechanism used
by apoptosis inhibitor proteins remains unclear, several studies
have provided insights into the biochemical functions of these
proteins, and a variety of reports have suggested an important
role for these proteins in some human diseases. 4) Apoptosis
inhibitor proteins are characterized by a novel domain of
approximately 70 amino acids (called the "baculoviral inhibitor
of apoptosis [IAP] repeat"). Up to 3 tandem copies of this domain
can occur within this family of proteins, and the presence and
spacing of cysteine and histidine amino acid residues in these
repeats suggests that this structure may represent a *zinc-
binding fold in the protein structure. Proteins containing these
repeats have been identified in a wide range of *eukaryotic
species, including yeasts, *nematode worms, fruit flies, mice,
rats, chickens, pigs, and humans, but not all proteins possessing
these repeats are apparent apoptosis inhibitors. 5) One important
question is how do the apoptosis inhibitor proteins suppress
apoptosis? A possible answer is provided by recent studies
indicating that several of the human varieties of these proteins
directly inhibit caspases, the family of cysteine proteases with
substrate specificity for aspartic acid. Caspase enzymes are
apparently important components of apoptosis programs throughout
the animal kingdom, and the apoptosis inhibitor proteins are
evidently the first identified family of endogenous cellular
inhibitors of caspases in mammals.
-----------
Editor's note: In addition to the background material below, see
the SW Focus Report "Molecular Biology: Apoptosis" available at
URL [http://scienceweek.com/swfr.htm].
-----------
Q.L. Deveraux and J.C. Reed: IAP family proteins -- suppressors
of apoptosis.
(Genes & Development 1 Feb 99 13:239)
QY: John C. Reed [jreed@burnham-institute.org]
-----------
Text Notes:
... ... *baculoviruses: These are a group of rod-shaped DNA
viruses that apparently replicate only in invertebrates. They
have been genetically engineered to synthesize foreign proteins. 
... ... *Ectopic expression: In general, the term "ectopic
expression" refers to the occurrence of gene expression in a
tissue in which such expression normally does not occur. The term
"gene expression" refers to the ultimate synthesis of the
particular protein encoded by a gene.
... ... *zinc-binding fold: In general, in zinc-binding protein
folds, cysteine and histidine amino acid residues in a
polypeptide motif coordinate a zinc ion, the motif known as a
"zinc finger" because the loop is finger-like. Such zinc-binding
sites are present in a number of DNA-binding proteins, and they
are apparently important in gene expression.
... ... *eukaryotic species: Any species whose cells contain
membrane-bound organelles such as a cell nucleus.
... ... *nematode worms: An abundant and ubiquitous phylum of
unsegmented roundworms.
-------------------
Summary & Notes by SCIENCE-WEEK [http://scienceweek.com] 19Mar99
-------------------
Related Background:
MOLECULAR BIOLOGY: APOPTOSIS, MITOCHONDRIA, AND CASPASES
Apoptosis (programmed cell death) is a rapid and specific process
involving the production of a number of enzymes in the cell
programmed to be destroyed. This programmed destruction is not
always harmful, or always the result of cellular damage of one
sort or another. In humans, for example, the lack of webbing
between fingers and toes is a result of apoptosis of cells of
webbing tissue occurring during embryological development, the
apoptosis in this case being a normal part of the larger
embryological program. In the mature organism, apoptosis is the
usual method of removing damaged cells after these cells are
recognized to be damaged by one mechanism or another. It is known
that normal cells carry an apoptosis receptor on their surfaces,
called CD95, and that when this surface receptor is cross-linked
by its specific ligand, this triggers the sequence of events
known as apoptosis. In the apoptosis sequence, certain
*proteolytic enzymes inside the cell are activated, and in
addition a variety of lipids that cause cell dysfunction are
synthesized. ... ... D.R. Green and J.C. Reed review the
involvement of *mitochondria with apoptosis in *metazoan cells,
and the authors make the following points: 1) The current
consensus among biologists is that approximately 2 billion years
ago the cells destined to become the ancestors of all *eukaryotes
entered into a partnership with an ancestor of today's *purple
bacteria, an ancestor that subsequently became the mitochondria
of today. 2) It has been hypothesized by several investigators
that the *endosymbiotic origins of mitochondria and the evolution
of aerobic metabolism in eukaryotes formed the basis for the
evolution of active cell death, which in metazoans is manifested
predominantly as apoptosis. Central roles for mitochondria as the
orchestrators  of apoptosis have been firmly established in many
systems. 3) In recent years it has become apparent that the
effectors of apoptosis are a family of intracellular proteases
known as caspases, although inhibiting these enzymes does not
always prevent apoptosis. 4) At least 3 general mechanisms have
been proposed for the involvement of mitochondria in the control
of cell life and death: a) disruption of *electron transport,
*oxidative phosphorylation, and adenosine triphosphate (ATP)
production; b) release of proteins that trigger activation of the
caspases family of proteases; c) alteration of cellular *redox
potentials. 5) In many apoptosis scenarios, the mitochondrial
inner electrical transmembrane potential collapses, indicating
the opening of large conductance channels through the inner
membrane. In contrast, certain stimuli can induce rupture of the
outer membrane of mitochondria and release of caspase-activation
proteins. The authors conclude: "Perhaps a few hundred million
years ago, either convergent or divergent evolutionary processes
allowed the ... fundamental framework for bacterial warfare to be
incorporated into the cell death mechanisms used by animal cells,
thereby establishing mitochondria as important participants not
only in animal cell life but also in active cell death." ... ...
In a companion and contiguous review of caspases and apoptosis,
N.A. Thornberry and Y. Lazebnik point out the following: 1)
Proteolysis is irreversible, which implies that regulation of
proteases is limited to control of their activity and
availability of substrate -- the only known way of "correcting" a
cleaved protein is to make it afresh. 2) Most proteases are
synthesized as precursors that have little if any catalytic
activity. The precursor is usually converted to the active enzyme
by proteolytic processing mediated either by another protease or
by autocatalysis. Thus large amounts of precursor can be
accumulated in advance and activated on demand. 3) Proteases can
regulate their own activation, resulting in an exponential rate
of activation. 4) Where there are proteases there are inhibitors,
and these inhibitors regulate the concentration of active
protease in the cell. 5) Proteolytic reactions can be specific,
determined by a combination of primary, secondary, or tertiary
structures of protein substrates. Proteolysis that governs
critical biological processes such as the cell cycle or cell
death is highly specific and involves a restricted set of
substrates. 6) The various caspases share similarities in amino
acid sequence, structure, and substrate specificity. 7) Caspases
are among the most specific of proteases with an unusual and
absolute requirement for cleavage after aspartic acid and
recognition of at least 4 amino acids terminal to the cleavage
site. 8) The strict specificity of caspases is consistent with
the observation that apoptosis is not accompanied by
indiscriminate protein digestion, but rather a select set of
proteins is cleaved in a coordinated manner, usually at a single
site, resulting in a loss or change in function. 9) Apoptotic
events include DNA fragmentation, *chromatin condensation,
*membrane blebbing, cell shrinkage, and disassembly into
membrane-enclosed vesicles (apoptotic bodies). In vivo, this
process culminates with the engulfment of apoptotic bodies by
other cells, preventing complications that would result from a
release of intracellular contents. In apoptosis, these changes
occur in a predictable reproducible sequence and can be completed
with 30 to 60 minutes. The authors conclude: "Substantial
progress has been made in understanding the structural and
catalytic properties of active caspases and their contribution to
apoptosis. The goal for future research is to understand the
regulation of these enzymes. This should facilitate efforts to
rationally manipulate the apoptotic machinery for therapeutic
gain."
-----------
D.R. Green and J.C. Reed (2 installations, US): Mitochondria and
apoptosis.
(Science 28 Aug 98 281:1309)
QY: Douglas R. Green, La Jolla Institute for Allergy and
Immunology, 10355 Science Center Drive, San Diego, CA 92121 US.
N.A. Thornberry and Y. Lazebnik (2 installations, US): Caspases:
enemies within.
(Science 28 Aug 98 281:1312)
QY: Nancy A. Thornberry, Merck Research Laboratories, Rahway, NJ
07065 US.
-----------
Text Notes:
... ... *proteolytic enzymes: These enzymes, also called
"proteases", split proteins and thereby degrade them. The enzymes
catalyze the hydrolysis of peptide bonds, fragmenting proteins
into polypeptide chains, and fragmenting polypeptide chains into
constituent amino acids. Sometimes proteolytic enzymes and
proteases are distinguished, with the term "proteases" reserved
for proteolytic enzymes with high specificity for peptide bonds
between particular amino acids.
... ... *mitochondria: Mitochondria are double-membrane enclosed
organelles of cells that are involved with several important
biochemical pathways, including electron transport and oxidative
metabolism, and across the membrane of the mitochondrion there
exists a potential difference apparently due primarily to a
concentration gradient of hydrogen ions. Various types of
eukaryotic cells may contain from a few to several thousand
mitochondria in each cell type. The mitochondria are relatively
large cylindrical structures up to 10 microns long and up to 2
microns in diameter.
... ... *metazoan cells: Metazoans are multicellular animals.
... ... *eukaryotes: Cells (and organisms consisting of such
cells) that contain intracellular membrane-bound compartments
such as a nucleus (membrane-bound "organelles").
... ... *purple bacteria: Specifically, any of the various
photosynthetic bacteria that contain bacteriochlorophyll, and are
thus distinguished by purplish or reddish-brown pigments. But the
term "purple bacteria" is sometimes used as a synonym for the
phylum Proteobacteria, a general category comprising a large
number of diverse forms.
... ... *endosymbiotic: Endosymbiosis is an arrangement in which
one organism lives inside another organism, but the term is
usually restricted to arrangements of mutual benefit, thus not
including parasite-host relationships. A number of eukaryotic
cell organelles (including mitochondria) are believed to have
originated from endosymbiotic relationships between eukaryotic
cells and simpler cells.
... ... *electron transport: Refers to a sequence of steps in the
final stage of the aerobic respiration biochemical pathway in
which high energy electrons are effectively passed through a
series of membrane-bound carrier molecules to support a proton
gradient involved in energy storage. The term "transport" here
refers essentially to a chemical flow diagram and not necessarily
to an actual spatial translocation of electrons.
... ... *oxidative phosphorylation: Production of ATP during
aerobic respiration. It takes place in the mitochondria of
eukaryotic cells and requires molecular oxygen as a terminal
electron acceptor.
... ... *redox potentials: Chemical potentials in a chemical
reaction involving the simultaneous reduction and oxidation of
two compounds by a transfer of electrons between them.
... ... *chromatin: The entire complex of a eukaryotic
chromosome, including DNA, chromosomal proteins, and chromosomal
RNA.
... ... *membrane blebbing: Refers to the macroscopic blistering
of the surfaces of cells when they die under certain conditions.
-------------------
Summary & Notes by SCIENCE-WEEK  2Oct98


=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
 
IN FOCUS: ON THE BRAINS OF MICE AND HUMANS

"No category of cell, no particular type of circuit is specific
to the human cerebral cortex. The components of our cerebral
machinery derive from a stock very similar, if not identical, to
that of the mouse. The major event in the evolution of the
mammalian brain is the expansion of the neocortex. This growth is
accompanied by an increase in the total number of neurons, and
thus in the number and complexity of the operations which the
cortex can perform. The number of cellular elements per unit of
surface area has not changed. The cortical thickness varies, but
much less than its surface area. On average, the cortex of man is
only three times thicker than that of the mouse, although the
increase is not uniform in all layers... The more the surface
area of the cortex expands, the more the number of neurons
capable of establishing association connections increases... This
translates, finally, into an increase in the mean number of
connections per neuron, with a consequent burgeoning of the
dendritic and axonal trees, reaching a maximum in man.
Nevertheless, the increase in the mean number of synapses per
neuron is not directly proportional to the increase in cortical
area. Far from it. The density of synapses per cubic millimeter
of cortex is of the same order in the rat as in man... At the
levels of both the macroscopic anatomy of the cortex and its
microscopic architecture, no sudden qualitative reorganization
marks the passage from the "animal" brain to the human brain.
There is, on the contrary, a continuous _quantitative_ evolution
in the total number of neurons, the diversity of areas, the
number of possible connections between neurons, and, therefore,
the complexity of the neuronal networks that make up the cerebral
machine."

-- Jean-Pierre Changeux: _Neuronal Man: The Biology of Mind_
   (Oxford University Press, Oxford 1985, p.66)


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