Pane barrier
Question: The two outer panes of a passenger aircraft cabin window have a
tiny piece of cylindrical metal separating them. It is always near the base of
the panes, not in the centre, and is frequently surrounded by condensation. What
purpose does it serve and what is it made of?
Answer: Airline windows typically comprise three or more layers of glass (or
acrylic) to provide insulation from the very cold atmosphere at altitude. The
tiny silvery cylinder is really the edge of a small hole drilled in the middle
layer to allow the pressure to equalise between the layers while minimising
convection.
The condensation around the hole is due to the inner airspace cooling. Ice
often forms here. The position of the hole is chosen to maintain the best clear
viewing area when condensation forms, to minimise the likelihood of a crack
forming between the hole and the edge of the window, and to avoid excessive
condensation pooling over the hole, which could freeze and block it.
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Fred Parkinson
Sandiacre, Nottinghamshire
Fighting dirty
Question: Why do most natural antibiotics come from soil fungi and bacteria?
Why do they produce them?
Answer: The question of why fungi and soil-dwelling bacteria such as
Streptomyces produce antibiotics has been debated for many years. A simple,
though possibly simplistic, explanation is that these organisms use antibiotics
to protect their food supply.
Soil bacteria and fungi live by digesting and recycling dead plant material
such as leaves and seed cases. Obviously it is impossible for the bacteria to
carry away their prized food supply and therefore it is argued that they lace
surrounding food with compounds that are toxic to those of other species. While
there is some lab-based experimental evidence to support this hypothesis, it has
been difficult to prove this theory in the wild. Antibiotics that work just fine
in the lab may be absorbed or diluted by different soils and clays to the point
of being rendered useless, while organisms that are typically found in the soil
may have robust resistance to antibiotics.
Then there are the vast majority, perhaps 90 per cent, of soil organisms that
produce complex and intriguing compounds which have apparently no antibiotic
activity. Why do they bother? Is it simply that our screening procedures, which
are geared to human need, overlook their actual role?
A second explanation is that antibiotic production is rooted in the plant
material that is the food source. This material is typically carbon-rich and
nitrogen-poor. Rather like a human who is given a 2-kilogram portion of chips
with every meal he orders, the soil organism has the problem of achieving a
balanced diet. Most common antibiotics are carbon-rich polymers made by enzymes
that strongly resemble those that normally make saturated fats. The building
blocks of these polymers are often exactly the same as those used to make
saturated fats. So perhaps we are seeing a form of clever bacterial bulimia:
faced with a situation in which the bacteria literally swim in a soup of
fat-producing carbon compounds, the bacteria turn these compounds not into fatty
lipids but into their structurally close relations, the antibiotics.
These are then excreted and, should they prove to have a useful, coincidental
effect, the bacteria thrive. According to this theory, the myriad useful
antibiotics, anti-cancer agents and anti-parasitic agents that soil organisms
produce is just a by-product of a bad diet.
Howard Cooper
Medical Microbiology Section
Division of Investigative Science
Imperial College of Medicine
London
Answer: Most antibiotics stem from research programmes that were inspired by
the discovery of penicillin. It was simpler to screen for antibiotics made by
microbes than for antibiotics made by other organisms. Furthermore, it was
easier to produce chemically complex antibiotics by batch culture of the microbe
than it was to attempt a synthesis of a molecule made by a plant or animal. So
for purely practical reasons, microbes were the best route for discovering new
antibiotics.
It can be argued that most soil microbes share their habitat with many
microbial competitors, so many of them will have evolved antibiotics as a way of
enhancing their fitness. However, a little caution is needed because microbes
produce a huge range of chemicals for which there is no obvious role, and any
large collection of chemicals will, by chance, contain molecules with high
biological activity.
Actually, antibiotics are not that easy to find in microbes. One study
screened 400,000 microbial cultures over a 10-year period and yielded only three
useful compounds. It can be argued that the ability to produce and retain a rich
chemical diversity enhances the chances of an organism producing the very rare
compound that gives it enhanced fitness
(see www-users.york.ac.uk/~drf1/rdf_sp1.htm).
This means that individual chemicals with potent
antibiotic properties will be made by many organisms but maybe only some of
these chemicals will give the maker increased fitness because of that antibiotic
property.
Richard Firn
Biology Department
University of York
This week鈥檚 questions
Grey matter: The surfaces of the incandescent light bulbs where I work become
progressively greyer over time. Why?
Kirsty Rhode
Manchester
Flying high: I was recently aboard a transatlantic flight which had
television monitors displaying information about the flight and a map of our
position. The altitude of the flight puzzled me: we were flying higher than
Mount Everest but there are no mountains in the way between the US and Europe.
What is the point of flying at this height?
Edward King
Derby
Big rock: Why does our Moon not have a name? Can we call it Eeyore?
Andrew Dymock (aged 3陆)
Bath, Somerset
The Moon has got away with being called 鈥渢he Moon鈥� for a very long time. But
perhaps someone could let us know the correct procedure for registering the
names of astronomical objects and whether there is any chance of renaming it
贰别测辞谤别鈥抬诲